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COaxially Laser Assisted cold spray

Final Report Summary - COLA (COaxially Laser Assisted cold spray)

Executive Summary:
Background
Metal components used in industrial applications are often coated, to increase their resistance to wear, corrosion, high temperatures in service etc. Coating processes can also rebuild damaged high value parts, for re-use. As such, a spectrum of coating processes has developed, including weld deposition, thermal spray, laser cladding etc. Nevertheless, and depending on which process is used, these can be disadvantages, including slow build rate, poor quality or poor adherence to the substrate, under-performance in service etc.

Cold spray does not involve melting the coating material, but instead adheres a spray of fine powder particles in a carrier gas at high speed against a substrate. Accelerating these particles, particularly if they or the substrate they are coating is a harder, higher performance material, can require large amounts of helium (as the accelerant). Helium is both a costly and a finite resource. This limits the wider spread application of this process. Nevertheless, a promising variant uses laser heating of the substrate, which can soften it, and assist deposition. With laser heating, even harder particles or harder substrates have the potential to be processed using more affordable nitrogen gas. However, to date, this technology has yet to be made available commercially.

The COLA project has created novel laser-assisted cold-spray equipment which can be retro-fitted around existing cold spray plant, with proven integrated process control, and with demonstrated capability in depositing industrially relevant coatings under what would normally be unfavourable conditions, without having to resort to expensive helium.

Objectives
The COLA project met the following objectives in its delivery:
• Design and manufacture of a working prototype laser assisted cold spraying head.
• Development and demonstration of non-contact process temperature control.
• Computational fluid dynamic (CFD) modelling of new nozzle designs suited to laser assisted cold spray, followed by manufacture and validation by particle velocimetry (PIV).
• Determination of suitable laser assisted cold spraying parameters, with characterisation of the coatings produced.
• Industry-based demonstrations of laser assisted cold spraying using the COLA equipment, and a techno-economic analysis of the results.

Results
Modelling of laser substrate heating, CFD for nozzle design, experimentation using pre-existing equipment, and background processing know-how guided specification requirements for a preliminary design of the new equipment. This was manufactured as a first prototype. Requirements and a design for a process monitoring and control system were also developed, then built and integrated. Following testing, the overall system (accessed by an operator via a GUI on a PC) was then used in laser-assisted cold spray trials on seven powder-substrate combinations of industrial relevance. These identified suitable laser assisted spraying parameters, as well as settings for effective on-line control. In particular, Ni alloys or Cu bronzes could be deposited on to steels, or Al alloys on to Al alloys, using lower kinetic energy conditions.

Selected coating deposition efficiencies, microstructures, porosity and oxygen contents and cohesive strengths were characterised, as were hardness profiles and adhesive strengths across the coating/substrate interfaces. Laser assistance was observed to increase deposition efficiency (up to four-fold), reduce porosity contents (down to 0.1%), not affect oxygen contents (held at or reduced to ≤0.2%), produce hardness values up ~60-80% harder than the substrate being coated (depending on the materials used), increase cohesive strengths up to five-fold and at least double adhesive strengths.

An improved second prototype was manufactured and used in industrial validation testing. Disc- and tube-shaped components were coated, and the quality and characteristics of the coatings achieved evaluated. Coating hardnesses ranging from ~100HV0.1 (e.g. for an Al alloy) to ~350HV0.1 (e.g. for a bronze) were achieved, with coating porosity contents <1%.

For more information about the project, a website (www.cola-project.eu) has been set up that contains public information on the project. A video about the project has also been put together and can be accessed via YouTube: http://youtu.be/p1tTiPdh5JY.

Project Context and Objectives:

Project Context
Metal components used in industrial applications are often coated, to increase their resistance to wear, corrosion, high temperatures in service etc, whilst keeping down overall component cost. Coating processes can also be used to rebuild damaged high value parts, for component life extension and re-use. As such, a spectrum of coating processes has developed, including weld deposition, thermal spray, laser cladding etc. Nevertheless, and depending on which process is used, these can be disadvantages, including slow build rate, poor quality or poor adherence to the substrate, under-performance in service etc.

Potential disadvantages of these existing methods are summarised in the Table below.

Weld deposition:
Process features
• Established process.
• Cost-effective. Manual or automatic process.
• Electric arc melts a consumable wire to add material.
Potential disadvantages
• Properties of deposit can be below parent material.
• Post weld heat treatment can be needed.
• Hot cracking can occur in some materials.
• Distortion and stress can be introduced.
• Range of wire consumables can be limited.
• Quality of manual operations is very skill dependent.

Thermal spray:
Process features
• Established processes.
• Wide range of variants.
• Wide range of consumables.
• Jet of molten or semi-molten powder particles accelerated at substrate, adhering to it.
Potential disadvantages
• Mechanical bond between coating and substrate can be of low strength.
• Coatings can be porous.
• Coatings can contain oxides.
• Oxidation-sensitive powers require protective atmosphere.
• Properties of deposit can be below parent material.
• Post weld heat treatment can be needed.
• Hot cracking can occur in some materials.
• Distortion and stress can be introduced.
Laser cladding:
Process features
• Laser beam melts wires or powders to add material.
• Fine scales possible.
• 3D build-up also possible.
Potential disadvantages
• Thermal process, with some disadvantages in common with fusion welding.
• Deposition rate can be low.
• Equipment costs can be high with certain lasers.

A contender to these processes, which could overcome many of these disadvantages, as it does not involve either the melting of the material being coated nor the substrate, is cold spray (also known as cold gas dynamic spraying of CGDS). Cold spray is a powder-based spray process capable of depositing thick, high quality coatings, which do not necessarily require any further treatment (other than finish machining) before use. Unlike its competitors, cold spray employs a combination of high particle velocities and low temperatures. This is summarised schematically in Figure 1. As Figure 1 shows, in the cold spray process, powder particles with very high speeds are used. This is accomplished by introducing the particles in to a jet of high pressure, often heated, expanding gas, exiting a gun like nozzle. Example cold spray equipment is shown in Figure 2. Particles injected in to this gas get swept up by it and reach the high speeds necessary for deposition. As shown schematically in Figure 3, when a fast moving particle collides with the substrate it is going to coat, it experiences very rapid and extreme plastic deformation. This deformation disrupts any oxide films which may be present on the surfaces of either the particle or the substrate, and leads to an intimate, conformal contact between the deformed particle and the substrate. The resulting fine scale intermixing and adhesion can result in a solid-state bond between the two materials. This can enhance the properties of the deposit, as detrimental high temperatures or melting is avoided, and some work hardening (introduced during the collision) is involved.

Nevertheless, for all of this to happen successfully, the particle must exceed a certain critical velocity (ref. Figure 4), otherwise insufficient deformation will take place, and the particle will not adhere or, in the worst case, act to abrade the substrate. High values for hardness or strength of either the particle to be deposited or the substrate to be coated can raise the value of this critical velocity. In practical terms, achieving higher particle velocities (above this critical velocity requirement) can be accomplished by heating the gas, or changing the gas species. In its most economical form, the cold spray can be carried out in air, but nitrogen is commonly used (e.g. to avoid oxygen pick-up by the particles). However, higher particle velocities can be achieved in helium, so helium has to be used if the highest quality and best performance coatings are required of harder materials systems. However, helium is significantly more expensive than nitrogen. It is also a globally finite resource, being predicted by some to run out in the next 30 years unless steps are taken to conserve and recycle it. These concerns, short term of cost, and longer term of availability, act to limit the wider spread application of the cold spray process for performance coatings.

Nevertheless, arguably the simplest and most promising variant of the cold spray process to address this problem, as already demonstrated academically, uses laser beam heating of the substrate to soften it and assist the deposition process. With this step, affordable nitrogen can probably continue to be used, for an even wider range of powders and substrates than at present. Furthermore, the broad diameter of the cold spray powder jet, compared with many finer scale laser cladding operations, can mean that a highly focused, more expensive laser source is not necessary, as laser heating is needed over a wide area. In addition, the cold spray process itself is often carried out in a sound-proof booth, and converting such a booth in to a safe enclosure for working with laser light need not be an expensive exercise.

However, in spite of these advantages, to date this variant of the process has yet to be made available commercially.

The COLA project has succeeded in creating novel proprietary laser-assisted cold-spray hardware, which can be retro-fitted to existing equipment already used for carrying out the cold spray coating process. These new equipment developments also include bespoke, fast-response, proven integrated process control, for quality control and assurance purposes.

The combination of this new hardware and process control has been shown capable, of depositing new high-quality coatings. Developing new coating process know-how with this equipment has been demonstrated to bring the key benefits of:
• Improved deposit characteristics, with lower porosity and oxygen contents, and a reduced level of defects, when compared with other methods, such as conventional cold spray or thermal spray processes.
• Deposits which can be considered for higher performance applications, having higher strength and hardness values, and indicators of improved corrosion resistance.
• Improved cost effectiveness in operation when compared with conventional cold spray giving comparable quality, with projected running cost savings of >€400k/yr/system, depending on the level of equipment utilisation.
• The potential for this equipment to deliver these benefits across a range of applications and industries.

A specific project team of SMEs was put together, covering key points in a potential future supply chain for the project, covering equipment manufacture, consumables supply and service provision. This team was underpinned in its developments by the work of key research players in their respective fields. Further information about the project team is summarised in Figure 5.

Project Objectives
The COLA project had a total of five global objectives:

1. Carry out computational fluid dynamic (CFD) modelling of different nozzle designs, so as to produce new designs which, when validated by experiments, could be proven to give exit flows 20% narrower than conventional cold spray nozzles, better size-matched to the laser beam diameter used for laser-assisted cold spraying.
2. Determine coaxially laser-assisted cold spraying parameters producing deposits with <1% porosity and an/or an open cell potentiometric value within 10% of coating material in bulk form for at least 2 powder/substrate combinations materials, and independently measure the corresponding laser heating induced optimum temperatures to an accuracy of +/-10K.
3. Design and build a working prototype COLA spray head, incorporating laser assistance and the bespoke nozzle design developed.
4. Develop and demonstrate a non-contact substrate temperature control concept using one or more thermal sensors, for control of process temperature to an accuracy of +/-10% or +/-20K (whichever the smaller), capable of communication at a frequency of ≥20Hz with, and subsequent change of (as a function of that measured temperature), via a PLC or similar device, at least two different elements of the COLA spray equipment (e.g. including laser power or manipulation device controllers).
5. Carry out industrially-based demonstrations of coaxially laser assisted cold spraying using the COLA head with its in-built substrate temperature controls, and a techno-economic analysis of the results, in terms of deposit quality and properties, exceeding at least three different end-use requirements (minimum hardness, maximum ductility, minimum porosity etc), for at least two cases of industrial application, with operating cost estimates.

In turn, these five global objectives were met by carrying out a series of supporting actions, including:
1a. Model, design, build and test novel nozzles for COLA spraying.
2a. Carry out laser-assisted cold spray trials (with the two sets of processing equipment specified, designed and built as per action 3a below).
2b. On the basis of the results from action 2a, and their industrial relevance to the SMEs, select two powder/substrate materials combinations for further development.
2c. As a part of the trials in action 2a, also provide examples of processing data, in particular thermal data, for the purposes of monitoring and control software and hardware development.
2d. Develop the laser-assisted cold spraying process for the two combinations selected by action 2b.
2e. Evaluate the characteristics of the materials deposited for the two combinations developed by action 2d.
3a. Specify, design and build two prototype COLA laser-assisted cold spraying heads.
4a. Design hardware and software for COLA process monitoring and control.
4b. Build and test the hardware and software designed in action 4a, in conjunction with both prototypes delivered by action 3a.
4c. Demonstrate the function of the control system from action 4b for the closed-loop substrate temperature control during COLA spraying to +/-10% or +/-20K (whichever is the smaller) with at least one of the prototypes from action 3a.

Further details on these actions and their results are given in the following section.

Project Results:
Introduction
The overview of the main scientific and technological results of the COLA project, and thus foreground generated, that follows, covers:
• The issues that were considered in the preliminary design of the first prototype COLA laser assisted cold spray head.
• Subsequent manufacture of that head.
• Basic operational testing of that head.
• The later manufacture of an improved second prototype head.
• The issues that were considered in the preliminary design of a laser assisted cold spray process monitoring and control system, to be integrated with both COLA prototypes.
• Subsequent manufacture of that monitoring and control system.
• Testing, developments and verification of that system, in conjunction with both COLA prototypes.
• The issues that were considered in cold spray nozzles designed for the COLA process, including:
o Modelling and experimentation associated with laser substrate heating.
o Modelling and experimentation associated with nozzle design and performance.
• Novel nozzle designs considered based on the above work.
• Subsequent nozzle testing.
• Subsequent set up of COLA equipment for laser assisted cold spray trials.
• Results of preliminary trials.
• Effect of powder grade on results.
• Hardnesses of laser assisted cold spray deposits.
• Down-selection of powder/substrate combinations investigated in the remainder of the project.
• Results of further development trials.
• Characteristics and properties of coatings produced in further trials.
• Specifications and requirements for industrial validation.
• Industrial installation at Putzier Oberflächentechnik.
• Validation trials at Putzier Oberflächentechnik.
• Validation trials at TWI.
• Comparison of validation trial results against competing processes.
• First-order approximation of process running cost savings.
• Multi-functional uses of the COLA prototypes.
• Overall conclusions of the project, with respect to its objectives.

It should be noted that in some instances it has been decided by the project consortium to maintain this information as proprietary, to protect potential exploitation interests, and as such full details cannot be given in this public report.

Design considerations for first COLA head prototype
The design of the first (of two) COLA head prototypes made in the project was required to have all the same functionality of the second (and final) prototype, but with more adjustability options, as were anticipated to be required for processing developments. This design decision was made as many of the requirements for optimum laser assisted cold spraying remained unknown at that point. Due to these adjustability requirements the first prototype head design was anticipated to be larger and heavier than the final design, and thus less suitable for immediate industrial use.

For easier assembly and good modularity the first head was based mainly on commercial opto-mechanical components. This modularity facilitated manufacture, and made it easier to introduce modifications to the head when needed.

The first (and second) heads were design to be compatible with/connect to a number of different commercial cold spray systems and guns in use by the project consortium. The largest of these guns used in the COLA project was a Oerlikon Metco Kinetiks™ 4000 Cold Spray Gun.

In addition to the three design requirements outlined above (namely adjustability, commercial sub-component modularity and cold spray gun compatibility), further proprietary requirements were compiled by the project consortium, led by TUT, including those relating to:
• Laser wavelength.
• Laser power, and its real-time control.
• Laser beam focal length.
• Laser focusing optic stand-off from substrate being cold sprayed.
• Laser beam angle off of substrate being cold sprayed.
• Range of laser beam diameters on the substrate being cold sprayed.
• Integration of process temperature monitoring and control in to head.
• Range of cold spray gun nozzle stand-off distances from substrate.
• Protection of laser optics from process dust.
• Prevention of laser casing overheating owing to proximity to hot gun body.
• Prevention of nozzle end overheating owing to inadvertent laser heating of nozzle.

Based on these requirements a preliminary design was put forward for the mounting of the laser assistance to the cold spray process with respect to the cold spray gun position, including orthogonal manipulators which would allow for some relative adjustment (for alignment purposes) between the two. A screenshot of an early design to satisfy these requirements is shown as an example in Figure 6.

Adaptors were also designed which would allow the attachment of cold spray guns of different dimensions.

A mounting device was also designed to enable the attachment of the whole assembly to a manipulator, e.g. the wrist of an articulated robot arm.

Manufacture of first COLA head prototype
The first prototype was manufactured by TUT and Cavitar based on the design considerations presented above. However, some modifications to the preliminary design were carried out, to facilitate manufacturing and accommodate additional design requirements that arose subsequently.

The laser element of the COLA prototype was manufactured by Cavitar. On supply to TUT, this element was evaluated using a laser beam analyser. The analysis results were used to estimate the quality of this part of the COLA prototype, and guide its installation into the overall head design.

These other components of the head were either manufactured by TUT or procured from commercially available sources, and included:
• A common base plate to which the laser and cold spray elements of the head were both connected.
• A modular mounting bracket to connect different designs of cold spray guns to this base plate.
• A device supplied with compressed air to protect the optics of the laser element from contamination and damage by dust during or after cold spraying.
• Manipulators for axial, Cartesian and rotational adjustments to the position of the laser element, for alignment/set-up purposes.
• Mounting points for the connection of any thermal sensors selected for process monitoring and control (see below).

Basic testing of first COLA head prototype
Following the assembly of all of these subcomponents, the outline function of the assembled first prototype was also then demonstrated by TUT, using temporary electrical and water cooling connections, in conjunction with a Kinetics® 3000 cold spraying system at TUT. Solely for equipment demonstration purposes, and without any process optimisation, a limited amount of laser assisted cold spraying of a NiCu alloy on to mild steel was carried. A short YouTube video was released to mark this event: https://www.youtube.com/watch?v=Ig0tiJe1jHk.

Manufacture of second COLA head prototype
Later in the project the second prototype was constructed by TUT. A number of important design modifications were carried out between the first and second prototypes, in an effort to make the second simpler and more robust in use, and reduce its size and weight, including:
• A reduction in the volume occupied by the laser heating element of 50%.
• A reduction in weight of the laser heating element by 40%.
• An increase in the range over which the position of the cold spray gun could be adjusted, with respect to the laser heating element.
• Simplification, and increased rigidity, in the way the laser heating element is connected to and aligned with respect to the cold spray gun.
• A reduction in the weight of the brackets used to configure the laser heating element with respect to the cold spray gun by over 50%.
• A reduction in the overall head weight by 33%.
• Simplification in the way the whole head is connected to a manipulating device, e.g. a robot.
• A reduction in volume and an improvement associated with the way the laser heating element is protected from process dust.

This second prototype was used principally for industrial validation at Putzier Oberflachentechnik, and in some additional trials carried out at TUT (see below).

Design considerations for process monitoring and control
Proprietary requirements of the design for process monitoring and control hardware and software were also compiled by the project consortium, led by LTU, including those relating to:
• Number of processing-related parameters to control.
• Size and cost of temperature monitoring sensors to be integrated in to the COLA heads.
• Any operating environment restrictions of temperature monitoring sensors.
• Range of temperature detected.
• Sensitivity of temperature measured to surface conditions.
• Avoidance of interferences between temperature measurements and laser wavelength used for substrate heating.
• Size of area monitored by temperature sensors.
• Stand-off of temperature monitoring sensors.
• Simplicity of sensor signals interpretation.
• Simplicity and speed/frequency of interfacing between sensors, process control unit and other equipment used in processing.
• User-friendliness of GUI operation to set up and monitor behaviour of control unit (e.g. see Figure 7).
• Ease of software programming of GUI.
• Flexibility in design and use of GUI.

To address these requirements, and in agreement with the SMEs in the project that using a single thermal sensor mounted inside the COLA processing head would present the most compact and cost-effective monitoring solution, a number of pyrometers and thermal cameras were assessed and ranked for their suitability. Subsequent to this ranking exercise a suitable thermal sensor was selected and procured. This sensor analysed suitable areas of the substrate surface (through the use of different sets of interchangeable optics), at a wavelength that would not be affected by the laser wavelength used for substrate heating, was of compact size and was the most cost-efficient from comparable alternatives.

Manufacture of COLA process monitoring and control system
Before being incorporated in to the first COLA prototype (see above), both LTU and TUT independently verified that this sensor was functioning correctly and working with its own software. Communication between the sensor and the bespoke LabView program being specifically developed in the project as the GUI for process monitoring and control was also verified by LTU.

The writing of the LabView program itself by LTU had begun at this point in the project and already incorporated several functions, including:
• Real-time graphing of the temperature measured by the thermal sensor.
• The capability for the user to set and control the power of the laser assistance.
• The filtering of the signal from the thermal sensor in one of two different ways (with separate setting of filtering parameters for each approach).
• The capability to change control settings depending on gun traverse direction, by I/O communication with a robot manipulator.

The hardware connection protocol/pin configurations for the control were also agreed and an analogue unit to control laser power was set up to run under LabView. An I/O control box was assembled (Figure 8) and made ready for testing.


Testing of COLA process monitoring and control system
The testing of the monitoring and control hardware and software itself was scheduled in to the laser assisted cold spraying trials that took place subsequently, both at TWI (with the first prototype) and TUT (principally with the second).

Initial trials of the control system were carried out by LTU during processing trials at TWI (see below), using different thermal sensor signal filtering settings, to determine settings given a sufficiently smooth yet responsive enough signal from which to control the processing equipment. Using these settings, TWI then examined control to different target temperature ranges. These ranges were selected for each powder-substrate combination being sprayed, depending on factors such as the melting point of the substrate being sprayed, a running analysis (e.g. visual examination and metallographic sections) of the quality of deposits being achieved, the heat sink of the substrate, and the other thermal properties of both that substrate (e.g. thermal conductivity) and powder being sprayed.

Figures 9-14, and Figure 16 (all described below) show examples of screen dumps from the Labview GUI, albeit to different time and temperature scales.

Figure 9 shows an example of a screen dump from the LabView GUI, of the monitored thermal sensor signal, during a series of laser assisted cold spray tracks being made from side to side across (and over either end of) a rectangular coupon. This Figure shows how the monitoring temperature could gradually increase, for a constant laser heat input, if that heat input was not controlled to prevent the substrate from overheating.

Figure 10 shows a second example of a screen dump. As Figure 10 shows, even with constant processing conditions, the raw pyrometer signal (the brighter yellow line in the Figure) can be somewhat noisy, and thus unsuitable as a closed loop feedback control signal.

Such results confirmed both that control was necessary, but that the raw signal from the thermal sensor required suitable filtering/smoothing before it would prove suitable as a control signal.

Suitable filter settings were developed carrying out both laser heating trials traversing a beam from side to side (and over the ends of) a rectangular coupon, and then repeating this action but with a cold spray gas and powder jet in action. Such trials allowed the determination of a preliminary set of filtering parameters which went on to prove sufficient for laser assisted cold spraying. These parameters could be entered readily by the system user, via a dedicated Filter Tuning tab in the GUI.

As Figure 10 also shows this real-time filtering of the signal (cyan line) when carried out led to some smoothing of the raw signal from the thermal sensor. This was sufficient to avoid the sharpest or shortest time changes in the control signal. Using this smoothed signal for control purposes, real-time changes to processing parameters could then be affected, e.g. laser power (lower darker yellow line in Figure). Carrying out such changes then appeared to allow the process to be controlled within a narrow temperature range between a user-set lower limit (green line) and upper limit (red line): in this example 700±20ºC (i.e. ±3%). This already met or exceeded the control targets of the project of ±20ºC or ±10% (whichever happened to be the smaller). This meant that the process could be controlled so that a certain minimum critical temperature would be reached for laser assisted cold spray, whilst at the same time avoiding a build-up of heat and overheating of the substrate. It should be noted, however, that this control could only be achieved if some forethought had been given to the balance between laser heat input being used, the materials being cold sprayed, heat sink effects etc. Figures 11 and 12 show two examples. In Figure 11, control inside a lower temperature range is easily achieved and, in fact, the laser control signal has to be gradually reduced, to avoid substrate overheating. Conversely, in Figure 12, in an example aiming for control inside much higher temperature range, the laser heat input has proved insufficient with respect to the heat sink being processed to reach temperatures within that range (with or without control). In this instance, control was not achieved therefore.

The target temperature ranges themselves could be easily specified and set-up by the system user, via a dedicated Materials tab in the GUI. This allowed a name for the settings to be stored (e.g. “Ni alloy on Ni alloy”), the lower and upper limits in between which temperature control was required, and additional parameters associated with the laser assistance settings which could also therefore be controlled via the LTU equipment, either before the start of a laser assisted cold spray run or in real time.

Following early establishment of the capabilities of the control system, the system was disconnected from the first prototype (at TWI) by TWI, and temporarily integrated by LTU with the second (at that stage in the project at TUT), in order to confirm that the system was compatible with both prototypes, and carry out a formal demonstration of control capability.

Before this second set of control proving experiments started, improvements and bug fixes were implemented in to the software, based on feedback from its use by TWI in the trials with the first prototype. These changes included:
• A new tab with a separate history graph was inserted, so that the long time base behaviour of the thermal sensor signal could be tracked by the system user.
• A separate power control % indicator was also added, to make the control response of the system more visible to the system user.
• The possibility to start with a (reduced) laser heat input on entry on to a coupon, to avoid the entry edge of the coupon from overheating (see below), was also made available.
• The ability to select a given Materials program (i.e. a given set of control settings – see above) using a 2-bit digital input to the control system was also introduced, e.g. from a robot.

Different filtering approaches were once again compared in this second set of experiments. The parameters for both approaches were fine-tuned to improve both the robustness and response of the filtering. Based on tests carried out, the approach with the fastest response was then selected. Figure 13 shows the level of control achieved with this new filtering approach, and an appropriate temperature range set, in this second set of trials with the second prototype.

The usefulness of starting an individual laser heating condition at a reduced laser heat input to avoid coupon entry edge overheating (see also below) was also confirmed. Figure 14 shows the thermal sensor responses when running on to, and then off of, coupon edges, for different amounts of reduction of starting power when running on, compared with the power that had been pre-programmed to be used during steady state heating in the bulk of the coupon. As Figure 14 shows, reducing this entrance power reduces the size of the short-time temperature fluctuations when running on to the coupons (example arrowed). Perhaps more importantly, as shown in Figure 15, an appropriate reduction in entrance power can also avoid the entry edge from inadvertently melting.

The ability to select a given Materials program in real time, during spraying, was also confirmed. This is demonstrated by Figure 16.

Control was also verified independently using a pyrometer external and unrelated to the thermal sensor (driving the response of the control system) situated inside the COLA head. Figure 17 compares the signals from these two different sensors when running the same test as that shown in the previous Figure. As Figure 17 shows, the precision of control to ±20K was independently confirmed.

A short YouTube video was released to mark this event: https://www.youtube.com/watch?v=hBee9L7rSJk.

Design considerations for novel nozzles
Two activities were carried out by TWI, to provide inputs in to the design of nozzles suited to laser assisted cold spray:
1. Estimating the likely amount (in terms of temperature rise) and extent (in terms of heated area) of substrate heating for a given set of laser heating conditions. These estimates were based on both experimentation and thermal modelling. This activity, in turn, consisted of:
o Use of one, and development of a second, conduction-based thermal model, to predict substrate temperature development during laser heating.
o Generation of offline laser heating experimental data, and its comparison with the modelling predictions for laser heating.
2. CFD modelling of the performance of existing cold spray nozzle designs, to identify areas for design improvement. In particular, continuing to produce particle exit velocities compatible with laser-assisted cold spray deposition, whilst at the same time devising means by which as wide a range as possible of exit trajectories are constrained to match the area over which the substrate is likely to be plasticised by the laser heating effect. This activity, in turn, consisted of:
o Development of a CFD process model for cold spray.
o Generation of offline cold spray experimental data, and its comparison with CFD model predictions.

Further details on these activities are given below.


Laser substrate heating modelling and experiments
Using a commercially available code for temperature prediction, Thermal Source Optimiser (TSO) (http://www.abingtonconsultants.co.uk/tso.htm) available at TWI, predictions of substrate heating using both axi-symmetric and non-axi-symmetric laser beam heating were made. The means by which these different symmetries were to be achieved by the COLA prototypes cannot be divulged publically as this information is proprietary. Nevertheless, this modelling exercise confirmed that for omni-directional processing axi-symmetric beam heating was preferable, so as to avoid any ‘leading vs. trailing’ effects. Figure 18 shows one example of these predictions. As Figure 18 shows, the maximum temperatures reached and temperature distributions differ significantly.

Further thermal conduction modelling development was then carried out specifically within the COLA project to predict the maximum temperature likely to be reached when using laser beam substrate heating, as a function of substrate material and processing parameters, and the extent over the substrate surface of that heating. FEA was carried out, using the commercially available ANSYS software. A suitable substrate volume was defined and an appropriate mesh set up. A laser beam heat source was also modelled, with characteristics including its power (fixed), area of action (e.g. depending on beam optics) and surface absorption (depending on substrate material and its surface condition: initially literature values were used, but this parameter was then fitted by comparison to experimental results, see below). The substrate volume was traversed with respect to a fixed source and mesh. The initial temperature of the substrate and its environment were assumed to be 23ºC (i.e. room temperature). Convective heat losses to this environment, once the substrate was being laser beam heated, were also accounted for. Radiative heat losses were negligible, and were thus ignored from the model.

Before reading too much in to the predictions of this model, however, and in order to support and validate this modelling activity, laser beam heating experiments were also carried out using different beam powers and power densities, at different traverse speeds, on different substrates, with different heat sinking capacities. The extents of heating in each case were observed using a thermal camera, and the maximum temperatures reached were estimated, first using values for substrate emissivity taken from the literature and then from experimental measurements, correlating either the solidus or liquidus temperatures for the substrate with the apparent temperature estimated by the thermal camera. These correlations were used to give upper and lower bound estimates for the emissivity of each substrate material being laser heated. Figure 19 summarises the measurements made of maximum temperature for two different substrates (a Ni alloy and an Al alloy), for each using one of two different emissivity values, and for each sitting on one of two different heat sinks, at different beam traverse speeds. Furthermore, this Figure is specific to a certain laser power and power density.

As Figure 19 shows, temperatures in the range ~100-150ºC were estimated when beam heating the Al alloy, owing to the low absorption of the heating beam and the high thermal conductivity of the alloy. Conversely, maximum temperatures up to 500-700ºC could be reached when beam heating the Ni alloy, owing to its lower thermal conductivity and greater absorption of the beam. Furthermore, these results were achieved on shot-blasted surfaces, which in all cases assisted absorption of the heating beam, which also (in later trials) would be the standard surface preparation used to assist the adherence of a cold sprayed deposit. These experiments also indicated the range of beam areas on the substrate which would likely prove useful for substrate heating. Knowing the size of these areas then served as an input in to nozzle designs. One example of the spatial extent of substrate heating calculated from thermal camera measurement data is shown in Figure 20.

Armed with all of these experimental findings, a reasonably accurate value for the surface absorption parameter used in the thermal conduction model could be arrived at by best-fitting the model predictions to the experimental data. Figure 21 show two examples of the model predictions for beam heating of the Ni alloy, with an 8% surface absorption. Higher absorption values resulted in unrealistically high temperatures.

As Figure 21b shows, fitting of modelled isotherms to thermal camera data also allowed a reasonable estimate of surface emissivity to be made.

The combination of thermal modelling and supporting experiments therefore allowed suitable starting conditions for laser beam substrate heating experiments to be identified, albeit the actual process temperatures involved also proved to be affected by the gas/powder jet from the cold spray gun (see below), and these affects were outside the scope of the modelling activities in this project. Nevertheless, this exercise also provided an input in to the design of nozzles specifically for laser-assisted cold spray, by giving information on the likely spatial extent of that heating on the substrate.

Cold spray nozzle behaviour modelling and experiments
As well as thermal modelling of laser substrate heating being used as one tool to guide laser assisted cold spray nozzle design, CFD modelling of the behaviour of gas and powder particle flows through the nozzles themselves, using ANSYS FLUENT, was also carried out.

Firstly, an accurate geometric model of a conventional spray nozzle was constructed. As a guide for this, MicrosetTM synthetic rubber replicating compound was injected in to a Type 24 cold spray nozzle, allowed to set, removed and then measured accurately. These measurements were used to generate an FEA model based mesh file. The CFD model itself was based on a discrete control volume, using a computational grid, integrated of the governing equations, to form fully coupled (i.e. simultaneously) algebraic equations for discrete dependent variables, such as velocity, pressure, temperature etc. These discretised equations were then linearised implicitly (i.e. using both existing and unknown values), and solved, to predict updated values of the dependent variables. The flow was calculated using Reynolds averaged Navier-Stokes equations with a realisable κ-ε turbulence model. All flows were treated as a steady-state, fully coupled (i.e. including heat transfer), compressible flow problems, using an ideal gas law. CFD models were computed assuming nitrogen was the spraying gas, but for at least two different spraying gas inlet pressures, and for four gas inlet temperatures. A discrete phase model with continuous phase interaction (ie both momentum and heat transfer coupling between spraying gas and powder particles was modelled) was chosen to simulate particle flow. Non-spherical particles, with a Rosin-Rammler logarithmic diameter distribution, were assumed. A particle shape factor of 0.8 and a size distribution common to cold spray processing, comprising of minimum, mean and maximum diameters of 7, 22 and 45µm, respectively, were assumed. A fully coupled solver was initiated, including modelling of the heat transfer from the spray gas to the surrounding nozzle walls and the substrate modelled. This took into account conduction and convection, but neglected radiative heat transfer, as this was assumed negligible. A similar calculation procedure was also used for a discrete phase model between gas and particle flows. In all cases, temperature-dependent polynomial fits to thermophysical properties were used, such as specific heat capacity and thermal conductivity, in simulations of the temperature rises on the nozzle walls, substrate and particles, respectively. The density of each material modelled was set as constant, however. Literature values were used for these physical parameters.

The outputs of the CFD models of the conventional (Type 24) spray nozzle included predictions of:
• Spray gas velocity through the nozzle, e.g. Figure 22.
• Spray gas, nozzle and substrate temperatures, e.g. also Figure 22.
• Gas jet diameter.
• Particle temperature within the gas jet, e.g. Figure 23.
• Particle velocity within the gas jet, e.g. also Figure 23.

As Figure 23 shows, rapid expansion (acceleration) and cooling of the spray gas is predicted to occur from the throat (smallest internal diameter length) of the nozzle, onwards.

Modelling indicated that a spraying gas inlet temperature of ≥400ºC was required to ensure that on expansion the gas temperature at no point along its axis was then <0ºC. Temperatures just prior to and at the substrate surface however were predicted to rise again, owing to the blunt (or bow) shock of the gas jet on impacting the substrate. This shock structure was also predicted to affect and then slow the gas velocity immediately prior to the substrate surface.

As Figure 23 also shows, particles are predicted to heat up in the heated spray gas, but then cool again, as that gas itself expands and cools. As the gas expands, the particles are also predicted to be accelerated within it.

Although not shown, these behaviours were also modelled for particles of different diameters. Larger particles were predicted to have their temperatures and velocities affected less than smaller particles.

CFD model predictions were then compared against the results of comparable cold spray experiments, at this stage in the project being carried out without laser substrate heating, in order to:
• Determine the ‘minimum conditions’ necessary for the onset of deposition.
• From model predictions, estimate what the corresponding gas and particle characteristics were under these conditions.
• Use these predictions as subsequent criteria for the ‘minimum’ requirements for any nozzles to be designed specifically for laser assisted cold spray.

TWI sprayed Ni alloy powder in nitrogen on to Ni alloy substrates using its Kinetiks® 4000/47 cold spray system, mounted on a 6-axis robot. A standard tungsten carbide (WC) Type 24 nozzle was used in these spray runs. A spraying gas inlet pressure of 40bar was used, but with different inlet temperatures, in the range 27-800ºC. A gun traverse speed of ~0.5m/s was also used. Figure 24 shows an example of some of the results achieved.

Such results were used to identify the minimum inlet gas temperature necessary for the beginnings of particle deposition (in the absence of substrate laser heating). The predictions of the CFD modelling at this temperature could then be used to define the critical criteria (minimum particle speed and minimum particle temperature for deposition etc) that would have to be met by any new nozzle designs.

The widths of the cold spray tracks deposited were also measured and, at 3-4mm when deposition clearly took place, were found to be in reasonable agreement with the dimensions of the diameter of the fastest moving ‘core’ of the gas and particle jet predicted by CFD modelling. These observations were also used to guide nozzle design for laser assisted cold spray.

Novel nozzle designs
Given the modelling and experimental activities described in the previous section to assist in the design of new nozzles for laser assisted cold spray, further CFD modelling activities then took place at TWI to investigate the effects of different possible nozzle designs on predicted nozzle behaviour.

The effects on particle temperatures and velocities, and particle jet diameters at the substrate, of different parameters of nozzle design that were modelled included:
• Overall internal profile.
• Nozzle throat diameter.
• Length of the divergent section beyond the nozzle throat.
• Introduction of a proprietary additional gas flow.

On the basis of the simulations carried out, key design features for a spray nozzle suitable for laser assisted cold spray were revealed. Four CAD models of selected proprietary nozzle designs were then produced, each incorporating at least one of these features.

Three nozzles were then manufactured from either tungsten carbide or tool steel, under subcontract, for cold spray testing purposes (both with and without laser substrate heating).

Nozzle testing
Following manufacture, a series of tests and trials were carried out by TWI on the novel nozzle, as follows:
1. A particle image velocimetry (PIV) system was used, as available at TWI, to measure the velocity and positions of powder particles as they exited the nozzle.
2. Deposition of cold spray tracks without laser assistance.
3. Deposition of cold spray tracks with laser assistance.

Further details on these tests and trials are given below.

1. Particle image velocimetry
An example of the set-up used for PIV is shown in Figure 25. The equipment was arranged so as to detect, using a high speed camera, shadow images of particles as they travelled through an illuminated field-of-view.

A series of PIV measurements were carried out on the three manufactured nozzles, as well as a Type 24 standard nozzle (for reference). A number of different spraying gas inlet pressures and temperatures were used. Spraying of Ni alloy particles took place in to free space. Nevertheless, the average particle velocities were measured in the jet at a position centred around a stand-off from the end of the nozzle representative of where the substrate would usually be located during deposition.

These measurements indicated that, in terms of particle speeds, the performances of COLA nozzles manufactured from tool steel were broadly comparable with those achieved in a standard Type 24 nozzle. Nevertheless, the tool steel nozzles very quickly clogged.

Manufacturing COLA nozzles from tungsten carbide appeared more suitable. PIV measurements from such nozzles then indicated that particle velocities were once again broadly comparable with those achieved by the standard nozzle, with some spread in values, but being ≥80% the average value from the standard nozzle. This was in line with earlier modelling predictions, where significant differences in particle speeds with the changes in nozzle design considered in this project had not been predicted, albeit the CFD model appeared to consistently over-predict, by ~20%, the particle speeds that would be achieved.

2. Deposition of cold spray tracks without laser assistance
In addition to comparing particle velocities, a comparison of the tracks produced by different nozzle designs was also carried out. This comparison was carried out firstly without laser assistance, whilst TWI awaited the arrival of the first COLA prototype from TUT. In particular, it was anticipated that a narrow track of deposition could be better matched to the area on the substrate which would be subsequently laser heated when using the COLA prototype with a cold spray system. Some idea of the size of this area was already in mind from preceding thermal modelling and off-line experimentation of laser substrate heating.

Cold spray tracks were deposited therefore using different nozzles, and different gas inlet pressures and temperatures. A Ni alloy powder was sprayed on to a substrate of matching grade.

Overall, for a given gas inlet temperature, powder flow rate, gun traverse speed etc, the tracks became both wider and thicker as gas inlet pressure increased. Using the standard Type 24 nozzle, typical deposited track widths were in the range ~4-6.5mm. Typical track thicknesses were in the range ~40-140µm (after one pass of the spray gun). As shown in Figure 26, tracks ~40% narrower could be achieved using a COLA design of nozzle manufactured from tungsten carbide.

The COLA design of nozzle had therefore exceeded its specification target, of producing a flow of particles at least 20% narrower than a standard nozzle.

However, comparisons of track thickness between the two nozzles indicated that although these were comparable at gas inlet pressures of 20 or 30bar, a COLA design of nozzle actually resulted in tracks up to 50% thinner at 40bar. This result suggested that the COLA design may be better suited to finer scale work (e.g. repairs) than coarser scale (e.g. area coverage).

Without laser substrate heating, the adherence of the powder track to the substrate was as equally poor as that of the standard nozzle. This also indicates that the powder particle velocities achieved by both the COLA nozzle design and the standard design were similar (both being too low to achieve adherence of the Ni alloy to its matching substrate with the very low kinetic energy cold spray conditions used).

3. Deposition of cold spray tracks with laser assistance
In addition to comparing tracks dimensions produced by a COLA nozzle designs and a standard design, the dimensions of tracks deposited using tungsten carbide nozzles of a COLA design and of the standard design were also compared when laser assistance was present.

A Ni alloy powder was sprayed on to a steel substrate in these trials.

Figures 27 and 28 provide information on the track dimensions achieved. Overall, for a given (fixed) set of spraying conditions, the tracks narrowed and thinned as gun traverse speed increased, as anticipated. For a given speed, the introduction of laser assistance widened the track and, at least in the case of the COLA nozzle, thickened it also. In all cases, the COLA tracks were narrower than their counterparts made using the standard nozzle, as was the design intention. Nevertheless, this came at the expense of that deposit also being thinner. These results confirmed those from trials carried out without laser assistance, in that the COLA design of nozzle appeared better suited to finer scale deposition. Nevertheless, it should be noted that the laser assistance conditions used in these trials, which ran in parallel to processing developments (see below), were not optimised at this stage in the project.

Work will continue outside of the COLA project to address optimising the efficiency of the nozzle design for laser assisted cold spray.



Set up of COLA equipment for laser assisted cold spray trials
Following the design, build and test of the three elements of the first COLA prototype (i.e. head from TUT, controls from LTU and nozzle from TWI), up to thirteen industrially relevant powder-substrate materials combinations were considered by the project consortium as the subjects of laser assisted cold spray processing trials. Of these thirteen, seven were selected on the basis of their industrial application, difficulty in processing, and the likelihood that laser substrate heating would enhance the deposition process. These seven combinations included the cold spraying of dissimilar materials, i.e. of one metal alloy on to another, or of a metal alloy on to a ceramic, to coat and enhance the properties of the substrate, as well as the cold spraying of similar materials, i.e. of one metal alloy on to a very similar base material, to simulate the repair of that base material. The majority of these powders were supplied to the project by the project partner, TLS Technik.

Prior to the commencement of any laser assisted cold spray trials, equipment set up consisted of the configuring of the COLA head and monitoring thermal sensor with the cold spray gun. For set up, this required:
• Determination of the location of the beam waist. As a first example, beam profilometry was carried out at TUT to examine and characterise the beam caustic, an example of this being shown in Figure 29. A simpler approach carried out, of particular use to locate the beam waist, was to carry out a series of timed burn marks/beam releases on to a substrate, the beam waist then corresponding to the stand-off distance at which the size of the burn mark was the smallest.
• As a starting point, setting of the plane of that beam waist to be coincident with the plane of the substrate surface.
• Setting of a suitable stand-off distance from the substrate surface of the cold spray nozzle (and thus with respect to the plane of that beam waist, also).
• Alignment of the centre of the projected axis of that cold spray nozzle with that beam waist. This could be accomplished by temporarily inserting a centred pointer of appropriate length in to the end of the cold spray nozzle and aligning it with the centre of a burn mark made on the substrate surface.
• Setting of a suitable stand-off distance from the substrate surface of the thermal sensor used for process monitoring.
• Alignment of the centre of the field-of-view of that thermal sensor with the impingement point of the centre of the powder jet on to the substrate sensor (and thus with the centre of that beam waist, also). Again, this could be accomplished by centring the field-of-view of the thermal sensor on a burn mark made on the substrate surface (once it was known that the nozzle was also centred to this same burn mark).
• Following alignment of the thermal sensor, a check was run to observe the type of readings that would be returned in the absence of laser heating. Figure 30 shows an example of a trace returned when cold spraying a substrate using a powder initially at room temperature (as it enters the cold spray gun) and a gas with an inlet temperature of 400ºC. As Figure 30 shows, the substrate did appear to be heated by the combined powder/gas jet, but all readings were <400ºC. In fact, at no time were readings achieved greater than this value in the work programme, unless laser heating was used (see below).
• Determination of a suitable laser stand-off or range of stand-off distances for substrate heating. In order to determine this, trials were carried out at different traverse speeds using different laser stand-off distances, laser heating different substrates but without any gas or powder being sprayed out of the end of the cold spray nozzle. Figure 31 shows some examples of the thermal sensor readings achieved (between 300-1000ºC) when a Ni alloy was the substrate. Traverse speeds are expressed as percentages of the maximum speed investigated in these trials. Two different laser power densities were also investigated. As Figure 31 shows, and as would be anticipated, using a higher heating power density and a higher heat input (lower traverse speed) results in higher readings from the thermal sensor. In some instances these maxima exceeded the maximum temperature that could be detected by the thermal sensor. In others, temperatures of ~500ºC were recorded. In addition, Figure 31 also shows that, commonly, momentarily higher readings were detected as the beam ran on to, and off of, the coupons being heated. These could be owing to spurious reflections, but more likely represent true temperature rises at the coupon edges, where the heat sink available for the energy from the beam to conduct away in to would be less than in the bulk of the coupon.
• Figure 32 shows the average thermal sensor reading from a series of trials on different substrate types, neglecting these edge effects. As Figure 32 shows, and as would be anticipated, higher power densities and/or heat inputs resulted in higher temperature readings for laser heating of steel, as well as Ni alloy. However, for a given heating condition, the temperature read from the Ni alloy appeared to be higher than that from the steel. This was presumably owing to the lower thermal conductivity of the Ni alloy than the steel. Similarly, the ceramic coated surface of the second steel would be expected to have an even lower thermal conductivity than the Ni alloy, and thus heat up even more when spot heated by the laser beam.
• Figure 33 shows examples of the average thermal sensor readings from trials on different substrate types combining laser heating with a powder and gas jet. In some instances a given set of trials was repeated. As Figure 33 implies, there was a complex inter-play between the heating, the gas, the powder and the substrate. The net result of this inter-play could be an increase in the thermal sensor reading compared with that achieved when the gas and powder jet were absent, or a decrease, depending on the beam power density being used and the powder being sprayed. With a higher power density:
o When spraying Ni alloy powder on to laser heated Ni alloy, there was a marked reduction in the thermal sensor reading. This may have indicated that the powder itself was absorbing some of the laser energy, and that then not all of that powder was adhering to the substrate. The net result would be the loss of some heat from the overall system.
o When spraying Ni alloy powder on to laser heated steel there appeared to be the same effect taking place.
• With a lower power density:
o When spraying Ni alloy powder on to laser heated Ni alloy a slight increase in thermal sensor reading appeared possible. This may have resulted from the combined heating of the substrate by the beam and the hot spray gas jet being used (see above). If correct, this in turn implies that the Ni alloy powder particles did not absorb the beam’s energy as effectively when using a less concentrated beam, and that the net effect on the substrate temperature of this could in fact be beneficial. An alternative explanation would be that the powder coated surface of the Ni alloy substrate itself became a more effective absorber of the beam’s energy. This would not explain why when using a more concentrated beam (see above) that that same surface did not have the same net effect, however.
o When spraying Ni alloy powder on to laser heated steel the same net effect seemed to occur as when spraying Ni alloy powder on to laser heated Ni alloy.
o Similar thermal sensor readings were reported when spraying a bronze powder on to laser heated steel as when not. This may imply that the bronze powder itself was not an efficient absorber of the beam, perhaps owing to different absorption characteristics when compared with the Ni alloy powder.
o When spraying Cu powder on to a ceramic coated steel there was a marked reduction in the thermal sensor readings. The most likely explanation for this is that the adhering Cu built up a surface layer of high thermal conductivity, helping the heat input from the beam to dissipate more quickly. In particular, at slower gun traverse speeds, where that layer was thicker, lower and lower thermal sensor readings were achieved, in spite of the laser beam heat input becoming higher and higher.
• Although complex in their interpretation, the net results of all of these experiments was to highlight those laser assisted cold spraying conditions which were most likely to result in higher substrate temperatures, which was then be anticipated to be most beneficial to the adherence of the coating to the substrate and its own internal cohesion.

Results of preliminary laser assisted cold spray trials
Armed with the know-how from the set-up trials (see above), preliminary trials were carried out on the seven powder/substrate materials combinations selected by the project consortium. Trials on two of the seven were carried out at TUT (without process monitoring and control), and on the remaining five at TWI (with process monitoring and control). Standard nozzles were used at this stage in the project for both sets of trials.

Further details of these trials are given in the “COLA project team release preliminary details of equipment capability” report (see www.cola-project.eu) but can be summarised as follows:
• Laser assistance of the cold spray process using the COLA equipment for Ni alloy deposited on to Ni alloy was only partially successful. Metallography indicated that the quality of the coating/substrate interface and the internal structure of the Ni alloy coating could be improved significantly, but lack of adherence and voids remained. This is illustrated in Figure 34. Without laser assistance the coating essentially floats as a ‘pancake’ on top of the substrate surface, without being adhered to it. The coating itself contains many voids and remnants of powder particle boundaries. With substrate heating, the coating has fewer internal defects. The largest problem is probably that adherence is improved, lacks of adherence at the coating/substrate interface are still present. Closer examination of these suggested that they may be associated with the presence of oxide on the substrate surface. Although the substrate surface was grit-blasted immediately prior to spraying, this may indicate that the residence time of that surface at high temperature when laser heated has been too long, and/or that the temperatures themselves required for plasticising the substrate are too high and result in oxidation.
• Laser assistance for Ni alloy on to steel appeared more successful. Metallography indicated significant deformation of the (softer steel) substrate was taking place when the Ni alloy powder particles collided with it. The quality of the Ni alloy coating itself also appeared improved. This is shown in Figure 35.
• A more detailed analysis of the structure of a second Ni alloy coating deposited on to a second steel was carried out using SEM. Figure 36 summarises the difference in structure between the coating made with and without laser substrate heating (coating/substrate interfaces not shown). As Figure 36 shows, the laser assisted coating has significantly fewer defects.
• Laser substrate heating also resulted in a significant improvement in the quality and apparent adhesion of bronze coatings cold sprayed on to steel, as shown in Figure 37.
• A series of a second set of bronze coated steels, prepared by TUT by cold spraying with different laser assistance settings, were subjected to 3.5wt% NaCl aqueous solution corrosion resistance testing, also at TUT. Figure 37 shows the open cell potential behaviour of this set of materials over time. As Figure 37 shows, the quality (i.e. adherence and soundness) of the optimum coatings deposited using this bronze appear to be such that the open cell potential approaches that of bulk copper. This implies that the net corrosion resistance of the bronze coated steel may be comparable also. This suggests that laser assisted cold spraying of bronze on to steel using the COLA equipment can be a suitable means of depositing a corrosion resistant coating.
• Laser substrate heating also resulted in a significant improvement in the quality and apparent adhesion of Al alloy coatings cold sprayed on to similar substrates. An example of the coating quality achieved is shown in Figure 38.
• Laser assistance of the cold spray process using the COLA equipment for Cu alloy deposited on to a ceramic coated steel (that ceramic coating having been previously applied using a separate thermal spray process) was not successful. Although laser assistance led to significant improvements in the consolidation and resulting cohesion of the Cu powder particles, adherence to the underlying ceramic layer could not be achieved. This is illustrated in Figure 39.

Effect of laser assistance on grade of powder that can be used in cold spray process
Powders typically used for cold spray require tight controls both on the diameter range of particles they contain and on the shape (spherical) of those particles. These properties are required to ensure a consistently high velocity of the particles in the gas jet, a consistently high level of adhesion of the particles to the substrate, and a consistently high level of cohesion between the particles in the coating, Producing a powder to these requirements adds cost, and previous work has shown that often the cost of the powder is one factor that contributes to the running costs of any cold spraying process.

For this reason, the COLA project compared the coating qualities achieved when using both a standard (high) grade of powder and a more economical lower grade of powder, with less control in particular on particle shape.

Unfortunately, this comparison was made for the laser assisted cold spraying of Ni alloy powder on to Ni alloy, before the baseline results (using the standard powder – see above) had been fully evaluated. Using the standard powder, intermittent lacks of coating adherence to the Ni alloy substrate underneath were detected (see earlier results), even with laser assistance. These defects also continued to be detected when using the lower grade powder, suggesting they were associated with the condition of the substrate. Nevertheless, the quality of the coating itself when using the lower grade powder appeared equivalent to that achieved when using the higher grade. This result suggests that with laser assistance lower grade powders may be able to be used, saving on processing costs, for those powders where laser assistance leads to more significant improvements in the coating process. For reference, Figure 40 compares the coating qualities achieved with the two harder to deposit Ni alloy powder grades.

Coating and substrate hardnesses of laser assisted deposits
TWI carried out four sets of further trials (with work on Cu deposition on to ceramic coated steel being discontinued at this stage in the project), making overlapping tracks to form layers, and then, in some cases, making overlapping layers to form thicker deposits.

These layers and deposits were then micro-hardness surveyed, through thickness, to characterise the resulting hardness of the coatings, and examine the hardness of the substrates immediately underneath those coatings. In summary, these surveys showed that:
• A Ni alloy coating could be made that was harder than the Ni alloy substrate on to which it was deposited. The substrate itself hardened slightly immediately underneath the coating, possibly representing a work hardened zone, but only to a depth of <0.5mm.
• A second (only solution hardened) Ni alloy coating made on to steel was softer than that steel substrate. Once again there appeared to be a work hardened zone, again to the depth of 0.5mm in the substrate immediately beneath the coating.
• A bronze coating was harder than the steel substrate on to which it was deposited. Once again, there was a presumably work hardened zone in the steel beneath this coating, but again only to a depth of <0.5mm.
• An Al alloy coating had a hardness comparable to the similar Al alloy substrate on to which it was deposited. However, the laser assisted coating process softened the substrate itself, albeit only to a depth <0.5mm immediately beneath the coating. This was presumably a heat affected zone.

Down-selection to two powder/substrate material combinations
Given the results of the trials using the first prototype on the seven powder/substrate combinations outlined above, the SMEs in the project consortium down-selected these seven to two, for further process development, towards industrial validation. These two combinations were:
• A bronze coating on a steel substrate.
• An Al alloy coating on a similar substrate.

Results of further trials to investigate process sensitivity and develop the most suitable conditions
Further laser assisted cold spraying trials were carried out on these two combinations to investigate the sensitivity of the results achieved to changes in, and determine the most suitable, processing conditions.

The sensitivity of the processing results with respect to small changes in processing conditions examined the effects of a number of factors, including the effect of:
• Making no changes (i.e. a check on reproducibility).
• Changing the offset between the laser assisted cold spray tracks in a given layer of deposit.
• Changing the laser stand-off position, and hence incident power density, used for laser assistance.
• Changing the cold spray gun angle (and with it, the incident angle of laser assistance).
• Changing the filtering approach applied to raw signal processing from the thermal sensor used for control purposes.
• Increasing the laser power (in selected cases), raising both laser heat input and incident power density (simultaneously).

These trials indicated that:
• Conditions developed in the preliminary trials were broadly reproducible. Figure 41 shows metallographic sections of example deposits achieved in the sensitivity trials.
• These trials also confirmed that in some instances either edge (across the width) of the coatings were inadequately adhered. It was presumed this was due to the broader width of the powder jet (from the standard nozzle used in these trials) leading to a small amount of powder particles at the edges of the coating impacting on substrate which was not being directly laser heated. Figure 42 shows metallographic sections through the edges of two coatings. Practically, this could probably be solved by spraying parallel to but half over the edge to be coated, or spraying on to scrap material either side of the region to be coated, and then machining that scrap material off.
• When depositing bronze on to steel, using too large a track offset appeared to result in the introduction of localized lack of adherence defects at the coating/substrate interface. Using too small a track offset appeared to result in excessive (grossly visible) deformation of both the substrate and the coating being deposited on to it.
• When depositing Al alloy on to Al alloy, using too large a track offset appeared to result in the introduction of inhomogeneities (presumably between track centres) in the deposit. Using too small a track offset appeared to introduce lack of adherence defects at the coating/substrate interface.
• When depositing bronze on to steel, using too short a laser stand-off (and hence higher power density) resulted in a reduction in the temperature readings from the thermal sensor. This affect had also been seen in earlier work (see above) when depositing Ni alloy powder, and could equally be attributed to absorption by, and removal from the system of, laser heating energy by the powder particles that do not adhere to the substrate. These possible changes in the process appeared to result in the introduction of lack of adherence defects at the coating/substrate interface.
• A similar net result occurred when depositing Al alloy on to Al alloy using too short a laser stand-off.
• Although only examined when depositing bronze on to steel, a 90º gun angle resulted in the best overall deposit. Gun angles off of vertical appeared to lead to thinner coatings, containing lack of cohesion defects within them.
• Changing the filtering approach applied to raw signal processing, whilst controlling within the same target temperature, appeared to be too subtle a change to affect the process overall, with sound, well adhered coatings continuing to be achieved.
• Although only examined when depositing Al alloy on to Al alloy, increasing the laser heat input (and power density) available, by up to ~20%, did not appear to affect the process overall either. This indicates that in the case of depositing Al alloy on to Al alloy, the lower heat input was already sufficient, and any unnecessary increases were probably successfully ‘controlled back out’ by the control system.

Characteristics and properties of coatings produced in further trials
Following the findings from the further development trials detailed above, coated coupons were prepared for an analysis of the characteristics and properties of those coatings. Coatings were deposited both with and without laser assistance, for comparison purposes. The Table below details the coating coupons prepared.

Flat rectangle, 4 layer coating:
Characteristic or property measured
• Detailed check on quality of deposit
• Apparent porosity content in coating
• Oxygen content of coating
• Vickers micro-hardness through coating
• Coating:substrate adhesion/bond strength
Measurement method
• SEM of metallographic section
• Image analysis of SEM micrographs1
• Wet chemical analysis
• HV0.1 survey on metallographic section
• Tensile testing of a coated sample adhesively bonded to steel studs2

Abutting tubes, 4 layer coating applied over joint:
Characteristic or property measured
• Cohesive strength within coating
Measurement method
• Tensile testing of two abutting tubes held together only by a cylinder of coating sprayed on to their outer diameters3

Notes:
1Using a method based on ASTM E2109-01 and BS 7590.
2Using a method based on ASTM C633-01.
3Using a method based on that developed by Schmidt et al. (ref: Schmidt T., Gartner F., Assadi H. and Kreye H., Acta. Mat. 54 (2006), pp. 729-742, and Schmidt T., Gartner F. and Kreye H., J. Thermal Spray Tech., 15 (4), 2006, S. 488-494.

Figures 43 and 44 show examples of the flat rectangular and tube samples after coating, whereby coatings were applied with and without laser assistance. As these Figures show, in most instances the laser assisted coatings were already visually better than those made without laser substrate heating.

SEM detected that some voids and entrapped oxide particles were nevertheless still present in the laser assisted bronze coating. The levels of these defects were then quantified, however, using image analysis and wet chemical analysis of the coating, respectively. These quantifications indicated that:
• Although some voids were still present, the porosity content was ~0.3±0.2% (mean±1 standard deviation).
• The porosity content of the coating made without laser assistance that it could not be quantified using an image analysis method.
• The mean oxygen contents of both the coatings made with and without laser assistance were the same, at 0.2%.
• Coatings with a low porosity content can be made using laser assistance, without the higher deposition temperature raising the oxygen content of the coating.

SEM detected that an even lower level of voids and entrapped oxide particles appeared to be present in the laser assisted Al alloy coating. Quantifications of these levels indicated that:
• The porosity content was ~0.1±0.1% (mean±1 standard deviation).
• The porosity content of the coating made without laser assistance by comparison was ~0.5±0.5%.
• The mean oxygen contents of both the coatings made with and without laser assistance were similar, at ~0.2-0.3%.
• As was the case with bronze coatings on steel, Al alloy coatings with a low porosity content can also be made using laser assistance, with again the higher deposition temperature not affecting the oxygen content of the coating.

In terms of coating hardness, it was already anticipated from earlier work (see above) that bronze coatings harder than their steel substrate could be produced, with some probable work hardening of the steel immediately underneath the coating. Re-measuring the hardnesses through the thickness of the new coatings broadly confirmed these anticipations, with the following conclusions:
• The bronze coating made with laser assistance was up to nearly twice as hard as the parent steel substrate.
• Nevertheless, and perhaps owing to the heat introduced from the laser assistance, this coating was in turn ~15% softer than that which could be achieved (although of very poor quality) without laser assistance.
• The steel beneath the laser assisted coating was hardened, to a depth of ~0.2mm by up to ~40% compared with hardness values measured remote from the coating.
• Conversely, although the steel beneath the coating made without laser assistance was also hardened, this was only by ~20%, compared with its hardness remote from the coating.

It was similarly anticipated that Al alloy coatings of the same hardness as their similar substrate could be produced, albeit with some probable HAZ softening of that substrate immediately beneath the coating. Re-measuring the hardnesses through the thickness of the new coatings also broadly confirmed these anticipations, with the following conclusions:
• The Al alloy coating made with laser assistance could be up to ~30% harder than the substrate.
• Nevertheless, and perhaps owing to the heat introduced from the laser assistance, this coating was in turn ~25% softer than that which could be achieved (although of inferior quality) without laser assistance.
• The substrate immediately underneath the laser assisted coating was softened, to a similar depth of ~0.2mm by up to ~30% compared with hardness values measured remote from the coating.
• Conversely, this softening did not appear to take place in the substrate which had not been laser heated.

Adhesion/bond strength measurements carried out on the coatings indicated that:
• The adhesion of the bronze coating on steel made without laser assistance was very poor, so poor that it could be picked/flaked off with a fingernail.
• The adhesion strength of the laser assisted bronze coating, by comparison, was at least ~57MPa, with failure occuring in the adhesive used to bond the coated sample to the steel studs instead.
• The adhesion strength of the Al alloy coating on the similar substrate made without laser assistance was ~8MPa, with failure initiating by delamination.
• Conversely, the adhesion strength of the Al alloy coating made with laser assistance was in the range ~20-40MPa, albeit failure again appeared to initiate by delamination.

Cohesive strength measurements carried out on the Al alloy coatings indicated that:
• The cohesive strength of the Al alloy coating made without laser assistance was ~120MPa.
• Conversely, the cohesive strength of the Al alloy coating made with laser assistance was even higher, at ~180MPa.

The cohesive strengths of the bronze coatings proved more difficult to measure, but nevertheless a limited number of measurements appeared to indicate that:
• The cohesive strength of the bronze coating made without laser assistance was <10MPa (with the exception of one, potentially spurious, result).
• The cohesive strength of the laser assisted bronze coating, by comparison, was in the range ~20-35MPa.

Taking the adhesive and cohesive strengths together:
• With laser assistance, a well adhering bronze coating can be deposited on to steel, with some internal strength to it.
• Without laser assistance, bronze coating is essentially not possible, using the same cold spraying conditions at least.
• With laser assistance, a reasonably well adhering, and internally very strong, Al alloy coating can be deposited on to a like substrate.
• Without that assistance, adherence to the substrate becomes poorer, albeit a good fraction of the strength within the coating remains.

Specifications and requirements for industrial validation
Given the results demonstrated in the preceding work, specifications and requirements for validation of the industrial performance of the COLA prototypes were then developed in discussion with the SMEs in the project consortium, including:
• Defining the components to be coated using the laser assisted cold spray process with the COLA equipment. These are illustrated schematically in Figure 45.
• Trials using the second prototype following its integration with cold spraying facilities at Putzier Oberflachentechnik, with the COLA process monitoring and control system.
• Trials using the first prototype at TWI, with the same process monitoring and control system and same model of cold spraying equipment.
• Evaluating the results of both sets of trials, in terms of the qualities of microstructures achieved, hardness values reached and oxygen contents in the coatings, and comparing these with the results of the preceding trials.

Industrial installation at Putzier Oberflächentechnik
Following the definitions of the specifications and requirements for validation, the second prototype and the process control and monitoring system were installed for trials at Putzier Oberflächentechnik. Installation steps consisted of:
• The laser power control, and process monitoring and control unit were installed by TUT and TWI, respectively. Figure 46 shows their relatively compact size.
• The I/O of the spraying robot controller was configured by TUT for communication with the laser power control, the process monitoring and control system, and the laser and robot safety circuits.
• The second COLA prototype was mounted on to the robot, connected to the control devices, and also to its required services (compressed air, cooling water etc) by TUT.
• The pre-existing cold spray booth was upgraded to a Class 1 laser enclosure (including the temporary installation of a CCTV, for convenience of operation) by TUT.
• Tests of the laser element of the COLA prototype were then carried out, including checks on power and focal plane position, by TUT.
• Putzier’s pre-existing Kinetiks™ 4000 Cold Spray Gun was then mounted and aligned with respect to the COLA prototype, using burn marks made on pieces of scrap steel to achieve this alignment and set an appropriate stand-off of the nozzle of the cold spray gun.
• The thermal sensor of the process monitoring and control system was then also mounted and aligned with respect to the COLA prototype.
• The correct operation of the monitoring and control system (as previously demonstrated at TUT) was then re-confirmed by TWI at Putzier Oberflachentechnik. This included system start up, communications with the thermal sensor, observing I/O status in real time, selecting and setting appropriate thermal sensor signal filtering, programming and selecting appropriate process control routines, and confirming that those routines were controlling successfully in real time. As simple preliminary confirmations, two tests were carried out:
o Laser spot heating of a piece of scrap steel. Successful process control controlling the temperature sensor measurement to ±20ºC is shown in the top of Figure 47.
o Laser line-by-line heating of areas of scrap steel were also similarly carried out (Figure 47, bottom).

Validation trials at Putzier Oberflächentechnik
Following equipment installation and confirmation of the correct function of the process monitoring and control system as described above, laser assisted cold spraying trials began, using conditions recommended from laboratory trials carried out previously.

Nevertheless, owing to some improvements in both the design and the performance between the first prototype (on which the bulk of the processing routines had been developed) and the second prototype installed at Putzier Oberflächentechnik, some simple set-up trials were carried out to check for a suitable level of laser assistance, particularly in terms of laser stand-off position and power.

These trials consisted of making a number of different series of adjacent tracks on flat steel coupons, both with and without laser assistance, and visually examining the resulting quality. Figure 48 shows two example results from these trials. Figure 49 shows an example of the process control achieved during laser assisted cold spraying.

Following the determination of a suitable laser assistance set-up, further validation trials took place to deposit a laser-assisted cold sprayed bronze coating on steel discs. Figure 50 shows an example of the results achieved. As Figure 50 shows, the coating once again has the characteristic ridged appearance reported earlier. On the raised lip of the disc, the coating has a less uniform appearance. The laser power density used in this plane was known to be less, and non-optimum. However, this coated lip was machined off of the final component. Figure 51 shows cross-sections through this coating following machining. This indicated that the quality of this coating was comparable with the best produced in preceding laboratory trials. Figure 52 shows the results of a micro-hardness survey carried out on this coating. These results indicated that a coating of hardness comparable with those measured previously was also achieved, albeit that the steel substrate on to which this coating was deposited had (presumably work) hardened to a greater extent immediately underneath the coating that those steels coated previously. Image analysis of SEM micrographs of the coating indicated that the coating porosity was ~0.4±0.3% (mean±1 standard deviation). This was similar to the ~0.3±0.2% estimated previously in preceding trials.

Owing to production pressures at Putzier Oberflächentechnik outside the control of the COLA project at this point, further validation trials had to transfer back to the first (near identical) prototype at TWI, but using the exact same process monitoring and control system.

Validation trials at TWI
Further laser assisted bronze coated steel discs were produced at TWI, albeit of a slightly smaller size and using a different steel stock. Examples of these are shown before and after machining in Figure 53. A micro-hardness survey was also carried out on one of these coatings. The results of this survey were comparable with similar results achieved at TWI previously. Image analysis of SEM micrographs of the coating indicated that the coating porosity content was ~0.6±0.1% (mean±1 standard deviation). This was slightly higher than previous results (including that achieved at Putzier Oberflächentechnik), but still met the global project target, of having a porosity content <1%.

Laser assisted Al alloy coated Al alloy tubes were also produced at TWI. Examples of these are shown before and after machining in Figure 54. Figure 55 shows cross-sections through one of these coatings. This indicated that the quality of this coating was comparable with the best produced in preceding laboratory trials. A micro-hardness survey was also carried out on one of these coatings. The trend in this survey was comparable with those from preceding work, albeit that both the parent substrate, its heat affected zone immediately underneath the coating, and the coating itself, were all ~25-30% softer than had been measured previously. Image analysis of SEM micrographs of the coating indicated that the coating porosity content was ~0.7±0.1% (mean±1 standard deviation). This was higher than previous results, but still met the global project target, of having a porosity content <1%.

Comparison of validation trial results against competing processes
In order to set the technical results of the validation trials, and related preceding trials, in to context, a short literature survey was carried out. This compared the results of the trials against those reported for competing processes (including laser cladding, thermal spray methods and conventional cold spray) for the same or similar powder/substrate materials combinations.

The results of this survey are reported in full in the “Concluding report from current project on equipment and coatings developed” (see www.cola-project.eu). In summary, the COLA process appears to result in, when compared with its competitors:
• Bronze coatings of a quality at least as good as, if not better than, laser clad, thermally sprayed or conventionally cold sprayed deposits without, in the case of cold spray, having to resort to the use of expensive helium as the spray gas.
• Bronze coatings harder than thermal spray deposits, and as hard as conventional cold spray or laser clad deposits. However, there appears to be more scope for adjusting laser cladding parameters to increase hardness levels to even higher values. Cold spraying in helium can also result in harder coatings, but would come with a gas cost penalty.
• Aluminium alloy coatings with better qualities than laser clad, thermally sprayed and conventionally cold sprayed deposits.
• Cold spraying in helium appears to result in lower porosity content coatings, but again would come with a gas cost penalty.
• Aluminium alloy coatings with the same or higher hardnesses than laser clad, thermally sprayed and conventionally cold sprayed deposits.
• Aluminium alloy coatings with better qualities than certain SLM deposits also, depending on the alloy in question.
• Coatings of comparable quality to EBM deposits as well.

First-order approximation of process running cost savings
In addition to the comparison made between the capabilities of the COLA process and its competitors, a comparison of the running costs was also made with respect to conventional cold spray, using helium to achieve coatings with similar characteristics and properties.

The chief saving in running costs from implementing the COLA process would come from its capability to avoid having to use helium. Running cost calculations detailed in the “Concluding report from current project on equipment and coatings developed” (see www.cola-project.eu) indicate that gas costs dominate the difference in running costs, assuming both processes use the same amount of powder. The comparison also suggests that using nitrogen could be at least ten times less expensive than helium. Additional running costs that come with the COLA process (e.g. electricity, water cooling and compressed air requirements) appear to add negligible costs, when compared with the cost savings that could be made foregoing helium. With appropriate intensity of usage, the process has been predicted to lead to running cost savings of upwards of 400k€/yr, following amortisation of associated capital investment costs.

Significant cost savings could therefore be available for existing cold spray operators using helium to achieve certain coating characteristics or properties. Alternatively, for existing cold spray operators using nitrogen to produce components with lower performance requirements, adoption of the COLA process could open up new coating capability, and thus bring new business opportunities.

Multi-functional uses of the COLA equipment
As an overall part of the COLA project delivery, alternative possible operation modes, and laser materials processing uses, of the COLA equipment have also been evaluated.

These evaluations consisted of:
1. Making use of proprietary features of the COLA equipment design, carrying out laser assisted cold spraying but applying laser heating to specific areas of the substrate, i.e. in a non axi-symmetric manner.
2. Using the COLA laser equipment for material surface melting/glazing.
3. Using the COLA laser equipment for conventional laser cladding.

These evaluations are reported in full in “COLA project team release details of the multi-functional capabilities of the COLA equipment” (see www.cola-project.eu) but some further details are given below:

1. Non axi-symmetric laser assisted cold spray
A limited number of trials were carried out using laser assistance in specific areas with respect to the cold spray jet (e.g. ahead, behind, to the sides, combinations of these positions etc). These trials indicated that:
• For spraying operations requiring higher temperatures, i.e. where a high laser heat input was required for high quality coating, failure to maintain this heat input resulted in a degradation in resulting quality of the coating deposited.
• In these cases, laser heat inputs directed to the sides or in front or behind the powder jet all resulted in poorly adhering coatings, if the overall heat input was insufficient for the powder/substrate material combination being processed.
• With higher heat inputs for less temperature-critical deposition operations, biasing that heat input to a position behind the powder jet appeared to be the most detrimental to coating quality.
• Therefore, siting the laser assistance coaxially with the powder jet, or biasing it ahead appear the most suitable approaches.

2. Surface melting/glazing
Figure 56 shows an example of a steel that has been surface glazed, by using the COLA laser equipment, albeit with a heat input sufficiently high enough to cause surface melting. Such a heat input is significantly higher than that which would be required for surface heating, to assist the cold spray process. Note that for this application a gas shielding flow (to prevent oxidation) was accomplished by supplying the cold spray gun with a low pressure (4bar) flow of inert argon.



3. Laser cladding
A limited evaluation of conventional laser cladding was also carried out, using the COLA laser equipment, once again with a high enough heat input for melting, this time not only of the substrate material but also a powder jet introduced on to it.

Nevertheless, preliminary trials indicated that the cold spray system itself, even at low pressure, was too coarse a system to introduce this powder flow: the mass flow rates were too high and the diameter of the jet too large to achieve consistent melting of both the substrate and the powder.

Instead, powder feeding equipment designed for conventional laser cladding, i.e. more precisely delivered, lower flow rates, had to be used with the COLA laser equipment.

With this set-up, a Ni-alloy was laser clad on to steel as a test case.

Firstly, a series of single tracks were clad, to determine a suitable laser heat input, cladding speed and powder feed rate. Figure 57 shows an example of a cross-section through one of these tracks.

Series of adjacently deposited tracks were then made, to determine a suitable track offset. Figure 58 shows a cross-section through one of these series, following laser remelting of the entire deposit, again using the COLA laser equipment, to homogenize it.


Overall Conclusions
The overall conclusions of the project with respect to its objectives can be summarised as follows:
• Two working prototype COLA laser assisted cold spray heads have been designed, built and tested in the project, exceeding the project target of having at least one working prototype.
• CFD modelling of a number of different proprietary cold spray nozzle designs has also been carried out, indicating which design features can result in experimentally proven exit flows up to 40% narrower than those achieved using standard nozzles. This achievement exceeded the 20% narrowing targeted by the project at its outset.
• In a laser assisted cold spray process, such narrow flows could be better suited to the area on the substrate that can be softened using the laser heating, but the current net reduction in deposition rate may require careful consideration before application.
• A non-contact process temperature monitoring and control system has also been designed and built and proven to maintain suitably stable processing temperatures to the required value ±20K, meeting the target of the project in this respect.
• This control system has built-in functionality allowing the control of laser heat input, the use of reduced laser heat input on entry on to a work piece, to avoid edge overheating or melting, and the ability for remote selection of a given set of processing parameters using a 2-bit digital input (e.g. from a robot controller).
• Laser assisted cold spraying trials with the new equipment have been carried out on at least seven different combinations of powders and substrates.
• Laser assisted cold spraying parameters have been developed with the COLA equipment producing two different deposits (a bronze alloy or an aluminium alloy) with a porosity content consistently <1%, meeting the project target.
• Parameters have also been determined which result in an open cell potentiometric behaviour, when evaluated, comparable with bulk material. This suggests that the bronze alloy coating in this case would offer good corrosion resistance.
• Suitable processing temperatures for the laser-assisted deposition of either a certain type of bronze alloy or aluminium alloy have been identified, using the bespoke process monitoring developed.
• Validation trials have been carried out on two different materials systems, using both prototypes with process monitoring and control, in either an industrial environment or in a laboratory environment with comparable equipment, achieving results comparable with preceding development work.
• Estimates of operating cost indicate that using such laser assistance to produce high performance deposits whilst avoiding the use of helium gas could save the operator at least 400k€/yr, depending on system utilisation.

Potential Impact:

Introduction
A number of impacts could arise from the carrying out of the COLA project, including technical advances, cost reductions, enhanced business competitiveness, enhanced academic excellence, increased performance from components and avoidance on reliance on finite global resources. These different areas of potential project impact are outlined below.

Possible technical advances
As noted at the start of this report (see Project Context above), coating processes are often used on metal components as a cost-effective means to increase wear, corrosion, high temperature properties etc or to repair or re-build them to extend life and through extended life and reduced maintenance and inspection burdens, drive down whole-life costs.

Example applications that the results of the COLA project could already begin to address include:
• Reduced weight and lower cost bronze coated steel parts for use in chemical, petrochemical, desalination, marine, offshore, ship-building, power generation, aerospace, automotive, rail, building and iron- and steel-making industries, such as gear and clutch components, drawing dies, casting moulds, propeller parts etc.
• Ni alloy coated steels for applications where a high degree of corrosion resistance is required, e.g. battery and electronics components, automotive components exposed to corrosive fuels etc, or where ductile/low stress repairs are needed (e.g. to cast irons)
• Fabrication or repair of Al alloys in components where electronic, thermal or corrosion resistance properties or high specific mechanical strength and/or stiffness are important factors in design, including fabrication of locally reinforced or tailored structures or frames (e.g. for light-weighting of transport structures), specialised electronic housings, valve bodies, pump components etc, or for repair of high value castings.

As also noted previously in the Project Context, the laser assisted cold spray equipment, controls and process know-how developed in the COLA project could compete, in terms of final component performance, with other processes such as weld deposition, laser deposition and thermal spray, whilst being more cost-effective in implementation than conventional cold spray. This combination of performance at reduced cost could help distinguish COLA coated or repair components in a competitive marketplace.

Reduction in process running costs
The potential for improvements in quality and performance stem from the fact that the COLA process remains a non-fusion ‘cold’ method, albeit laser substrate heating raises the temperature of the process to a considerable fraction of the melting point(s) of the material(s) involved.

Nevertheless, as noted above, this performance of the COLA process appears possible using nitrogen as a spray gas, and without having to resort to the use of costly, and finite, helium, when laser substrate heating is used. Although powder feedstock costs dominate the running costs of the cold spray process, helium has been estimated by the project consortium to be more than twelve times as expensive by volume than nitrogen. The avoidance of the use of helium would therefore also make using the COLA process more affordable. Furthermore, preliminary data generated within the project also suggests that less-than-ideal powder feedstocks may also be able to be used with the COLA process. Such feedstocks could be at least one-third less expensive than their conventional cold spray counterparts.

Enhanced EU SME business competitiveness
From an industrial perspective, the results of project could improve the properties resulting from, and affordability of, the cold spray process for high performance metallic coatings, and the know-how specific to the COLA equipment, its control and its use, is currently uniquely held by the EU SMEs who have participated in the project. The further development and application of this know-how could favour the business development of the SMEs involved.


Maintenance of EU institutional excellence
Similarly, the unique know-how behind the control and application of the laser assisted cold spray process using the COLA equipment, developed by LTU, and by TUT and TWI, currently resides uniquely within Europe.
Positive impacts on all users (intermediaries and end-users) of COLA coated components
By producing more affordable, higher quality coated components and/or repairs of existing components, the application of the results of this project will be able to improve the performance and extend the life of a range of components. These benefits will be felt by the users of those components, either through their integrated performance enhancement in to an assembly (e.g. the placement of a COLA component in to an automotive engine or gas turbine) and by the ultimate end-user(s) of that component.

Security of supply
Helium is fractionally distilled from natural gas deposits associated with slowly decaying radioactive elements. These deposits, located in the US, Algeria, Poland, Russia and Qatar, could give rise to EU security of supply concerns, particularly as US supplies of helium, stockpiled from the 1950s until 1996, have been predicted to run out in the next 2-3 decades unless conservation and recycling steps are not put in place. The COLA process does not require helium to be used in the production of its potentially equivalent quality coatings. This result eliminates any reliance on helium. Instead, nitrogen, available from fractional distillation of air, can be used instead, more cost-effectively and more securely.

Main Dissemination Activities
The COLA project partners have already, and are continuing to plan to, disseminate the project results. The interest of the potential users of the COLA technology and its capabilities is being and will be raised through dissemination activities. These activities have included and will include:
• Information on the public side of the COLA project website (www.cola-project.eu).
• Information on the websites of the project partners. Examples include:
o A public project marketing release (http://www.twi-global.com/news-events/news/2013-08-new-research-into-cost-effective-laser-assisted-cold-spray-for-high-quality-applications/) prepared by TWI, as also published in TWI’s September/October 2013 release of its industrial journal, Connect, distributed to up to 15,000 contacts.
o Publicising of the COLA project on TUT’s website (http://www.tut.fi/en/about-tut/departments/materials-science/current-topics/stronger-coatings-with-laser-assisted-cold-spray-p051667c2).
• Scientific papers in conferences for the (thermal) spray and laser materials processing communities. Examples include:
o Inclusion of information about the COLA project in a “High-pressure and low-pressure cold spraying” conference presentation, Advances in Surface Engineering & TWC International Wear Seminar, 5.-6.11.2013 Tampere, Finland.
o Overview of the COLA project at the International Thermal Spray Conference, 21-23/5/14, Barcelona, Spain.
o Publicising of the COLA project by TUT at 28th International Conference on Surface Modification Technologies (http://www.tut.fi/en/international-conference-on-surface-modification-technologies/index.htm) June 16th – 18th, 2014, Tampere, Finland.
o Presentation at the North American Cold Spray Conference 2014, 16-17/9/14, Bromont, Quebec, Canada.
o One presentation submitted to the Industrial Laser Applications Symposium, 17-18/3/15, Kenilworth, U.K.
o Two presentations submitted to the International Thermal Spray Conference, 11-14/5/15, Long Beach, California, USA.
• Informational videos on YouTube and the partners’ websites. Examples include:
o An overview of the project (http://youtu.be/p1tTiPdh5JY).
o A video of a simple COLA demonstration released through both TUT’s channel and TWI’s (e.g. http://www.youtube.com/watch?v=1OnhXGuz9yU).
o A video demonstrating real-time control of laser power during COLA processing, from closed-loop feedback monitoring of a thermal sensor signal (https://www.youtube.com/watch?v=hBee9L7rSJk).
• Publicising the project at trade fairs, in particular at:
o The Schweissen and Schneiden (Welding and Cutting) fair in Essen, 16-21 September 2013, Germany.
o The Hannover Messe, 7-11 April 2014, Germany.
• Technology demonstrations to both the project team members and to other parties (e.g. at the 28th International Conference on Surface Modification Technologies).
• Information released to existing customers of project partners. As on example of this, a project poster has been prepared and will be disseminated freely by all of the project partners.
• Informational papers in trade journals within the industries of the potential customers (e.g. article in ‘Materia’ (Finnish journal), Jan 2014).

Exploitation of Results
The potentially exploitable results of the COLA project can be summarised as:
• The design of the COLA prototype(s).
• The design of the bespoke nozzles for laser assisted cold spray.
• The design of the control hardware for process control and monitoring during laser assisted cold spray.
• The control software/GUI for that hardware.
• The laser assisted cold spray process know-how developed in the project.
The SMEs in the project have agreed in principle to a division of the exploitation rights around each of the results from the COLA project, including ownership and licensing rights, and revenue sharing arrangements. This division has been outlined in a proprietary plan for the use and dissemination of project foreground.

Three potential exploitation routes for one or more of these results have been considered:
1. Supply, either directly or under process license to a third party, of new and bespoke coating knowhow/services under subcontract to customers, initially using existing prototype designs, until further design and processing advances in mind are developed and incorporated.
2. Manufacture, either directly or via design releases to third parties, and sales of COLA processing and/or monitoring and control equipment to users in the cold spray, thermal spray and laser materials processing (especially laser cladding) communities. The latter could also be an example of a potential market for alternative uses of the COLA control system.
3. Armed with proprietary know-how, related aftermarket sales of services and bespoke consumables to equipment users.

Given competing patent applications currently in process at the time of writing, the project team consider that the exploitation of the results of the COLA project are best served in the short-term by supply of proprietary coated components (route 1 above), sales of associated consumables (route 3) and any related customer support services (also route 3). The new coating capabilities offered by the equipment developments in the project are therefore aiming to be exploited first, rather than the equipment itself.

Actions for the project consortium around these two exploitation routes have also been outlined, including:
• The production or repair of trial or example components, with the COLA equipment and using existing processing conditions developed in project, beginning with the requirements of potential end-customers with who Putzier Oberflachentenick is in discussion.
• Based on project results to date, identification of additional markets and applications, from carrying out industry-led market research.
• Developing further capabilities with the COLA equipment, e.g. by follow on work at one or more of the research institutes supporting the project, working with new powder/substrate combinations of industrial interest not yet studied.
• Advancing new designs with further improvements for the COLA processing head and its accompanying controls. A range of possible design improvements have already been discussed and outlined by the project consortium. Additional research and development activities will be sought that determine the most suitable. These improvements will seek to further increase efficiency/productivity, reduce equipment size and complexity and extend equipment and process capability.

By following these actions up, the partners of the COLA project will continue to work together to identify key markets and key applications for the results of the project, and exploit these, as well as to strive to develop further the COLA technology.

List of Websites:
A COLA project website has been set up with two areas:
• Public facing area
o Home page
o Project Background, with overview, project concept and objectives, and a summary of project activities as they arise.
o The profiles of the project partners involved.
o Project publications, as they arise.
o Project news & events of notes.
o Related links.
o A ‘Contact us’ link.
• Private project partner Area ( with a login required )
o Project notices, e.g. meeting details etc.
o Project Files, e.g. reports, data etc.
o Partner contact details.

An example page is shown in Figure 59.

This website can be accessed via www.cola-project.eu.

A number of videos about the project have also been placed on YouTube. The main video about the project can be accessed via: https://www.youtube.com/watch?v=p1tTiPdh5JY

Further videos about the project can also be found via the playlist: http://www.youtube.com/playlist?list=PLqXlLCsUe6g5xNDLY8vfxCkjXY2t7hbNg