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Adaptive laser cladding system with variable spot sizes

Final Report Summary - ALAS (Adaptive laser cladding system with variable spot sizes.)

Executive Summary:
ALAS project (Adaptive laser cladding system with variable spot sizes) developed an innovative laser cladding head, focused on the repairing of complex geometries, controlling the effect of heat accumulation, yielding a new fully automated cladding system. The ALAS project develops three technological progresses with the complementary idea to make the laser cladding process more flexible, self adapted and easy to use:
- Variable optical path: ALAS project developed a variable optical system that ensures the performing of clad tracks with variable width simplifying the programming cladding strategy.
- Heat accumulation control: A RT-control system modifies laser power values in order to guarantee the characteristics of the laser cladding process, avoiding heat accumulation effects due to external factors (workpiece geometry, undesired speed variations ...).
- High Level Control System. It has been developed to be intuitive for the cladding operator; the operator can set-up the adaptive cladding process, only “sketching” the design of the part to repair, and setting the main parameters of the process. The HLCS interpolates laser beam widths at any point of a tool path and sends the obtained time-based table to the FPGA controller.
The project resulted in the realization of a fully functional prototype that represents a significant step forward into the industrial implementation of a new automated industrial laser cladding process specially adapted to laser repair and refurbishment industries, in which the problems of adaptation to variable surfaces and control of the heat accumulation have been the key-factors to solve. Thus, ALAS project has developed 3 technological progresses with the complementary idea to make the laser cladding process more flexible, self adapted and easy to use: Variable optical path, RT-monitoring and control of the laser power during the cladding and an user-friendly graphical interface.
Project Context and Objectives:
The ALAS project aims to shorten the gap between research and economy with latest research activities in the fields of optics, measurement and control, by providing an innovative, adaptive laser cladding system with variable spot size to end users. Thus, Europe’s SMEs will be empowered to strengthen their global market position as the productivity will increase due to shortened setup times, flexibility gain and controlled quality.
The adaptive laser cladding system has potential to replace the actual cladding process in most real life applications. Currently, to process a cladding track with complex geometry, it is necessary to program several cladding paths Using the adaptive ALAS cladding head, developed in this project, this tedious programming process will be eliminated.

The project objectives as included in GA-Annex I are as follows:
- O1. Design and development of the optical path system. (Related with WP2 and D2.1 D2.2 D2.3 and D2.4). To be achieved in Month 12.
- O2. Design and development of the Real-Time control system. (Related with Milestone 2 MS2, WP3 and D3.1 D3.2). To be achieved in Month 14.
- O3. Construction of a prototype of the adaptive cladding head. (Related with Milestone 3 MS3, WP4 and D4.1 D4.2). To be achieved in Month 16.
- O4. Integration and assembly of the system. (Related with Milestone 4 MS4, WP5 and D5.1 D5.2 and D5.3). To be achieved in Month 20.
- O5. Validation of the system. (Related with Milestone 5 MS5, WP6 and D6.1 D6.2 and D6.3). To be achieved in Month 24.

Thanks to the work developed by the consortium as a whole, these objectives have been achieved at 100%.
- Definition of user requirements and system performance, WP1, related to T1.2 T1.2 , D1.1
- Definition the specifications of validate procedures, WP1, related to T1.3 ,D1.1
- Definition of performance requirements for variable optical path WP2, related to T2.1 D1.1 D2.1.
- Optical design of the laser cladding head WP2, related to T2.3 D2.1 D2.2
- Opto-mechanical design of the laser cladding head WP2 related to T2.1 T2.2 D2.2
- Selection of the optical and coating material for laser cladding head WP2 related to T2.4 D2.1 D2.2
- To design and evaluate the monitoring system WP3 related to T3.1 T3.2 D3.1
- To design and evaluate the adaptive power control system WP3 related to T3.3 T3.4 T3.5 D3.2 D3.3
- To assembly and alignment the opto-mechanical system WP4, related to T4.1 T4.2 D4.1
- To assembly and integration between the opto-mechanical design of the cladding head and the RT-power control system developedWP4, related to D4.2 T 4.3 T4.4 D4.3
- To develop and integrate the High level control system WP5, related to T5.1 T5.2 T5.3 D5.1 D5.2
- Laboratory validation of the ALAS prototypeWP5, related to T5.4 D5.1 D5.2
- To characterize the system performanceWP6, related to T 6.3 D6.2 D6.3
- To transfer the knowledge from RTD to SMEsWP7 related to, T7.1 T7.2 D7.5
- To disseminate the results of the projectWP7, related to T 7.4 D7.1 D7.3 D7.5 D7.6 D7.7
- To maximize the exploitation opportunities for SME partners WP7, related to T7.3 D7.2 D7.4

Project Results:
The work plan followed in ALAS project is structured into eight work packages (WP) divided into different tasks, each one having a distinctive role towards the accomplishment of the project objectives.
During the first period of the project, the main research activities have been focusing on defining process specifications and system development that includes the user requirements and system performance, the work piece definition and the specifications of validation procedures.
The concept of the innovative laser cladding head, developed in ALAS project includes an adaptive optical path that control the laser focus distance on the fly, and I has a user friendly interface in order to simplify the programming of this variable tracks. Coevally, a monitoring system which controls the laser parameters depending on the process status will ensure a steady process quality.
The end users have defined two different geometries to validate the ALAS system prototype; a cylinder piece to validate the control system in order to avoid the heat accumulation during laser cladding and complex geometries, to be reconstructed in order to validate the optical path system.
The ALAS system has been designed and validated for High Power Diode Laser, HPDL-users with these specifications: wavelength optimized for 940nm and optics system compatible with 808 and 980mm and diameter of spot size between 2.7-5.5mm.
The type of zoom optics selected for ALAS cladding head is based on a mechanically compensated system. It is formed by the combination of two optical systems, the variator and the compensator, each one being a group of two lenses, one divergent and one convergent. In addition, the whole system remains afocal, i.e. the effective focal length is infinitum. In this case, the rear focal point of the compensator is coincident with the rear focal point of the variator. In order to change the ALAS zoom optics magnification the two lenses of the variator are moveable, while those of the compensator are kept fixed.
The first-order requirements imposed on the system are:
Ø spot : 2,7 – 5,5 mm/ Focal length to collimation: 100 mm / Ø beam at collimation: 44,7 mm/ Focal length to focalized: 150 mm/ Ø beam at focalized lens: 32-48,4 mm/ Zoom system magnification: 1-2,5X
The arrangement of the components of the optical zoom system is a combination of two convergent lenses and two divergent lenses, with two fixed lenses and two movable
Zoom equation describes the lens movement of the variator and compensator, L3 and L4 respectively, in order to obtain the magnification range. The movement of the variator is compensated by means of the movement of the compensator element, in order to keep the whole system afocal.
Deviation of position on compensator has a higher weight than deviation on variator. As a consequence the most critical parameter to take into account is the deviation of position for L4 lens. Specially, system is more sensitive to position deviation of the lenses in configuration ranges from 1,6x to 2x magnification.
The minimum requirements for the engines responsible for the lenses movement is, for L3, travel range 80,02mm with an accuracy of 0,1mm and for L4 the ravel range is 29,1mm with an accuracy of 0,1mm.
The heat sources inside of a high power laser system as ALAS are due to light absorption in each component in the optical path. The most important parameter for the absorption at wavelength of 940nm is the OH content. The selected substrate to build the zoom optics system is the Suprasil 3001, with low absorption at working wavelengths due to its lower content in OH and other impurities. This absorption produces a very low warm on substrate, implying that there is no change in the focusing behaviour and, therefore, it is no necessary to refrigerate the lenses.
CAD modelling and assembling of the parts of the cladding head have been developed, including housing for the optical system and mobile parts, motors and actuators for controlling the ALAS system.
The Zoom-Module has the dimension of 325 mm x 187 mm x 155 mm and a weight of about 10 kg.

REAL-TIME CONTROLLER
To manage in Real-Time condition all the parameters involved in the development of a variable cladding head, a new real-time control system based on machine vision and hardware control has been developed. This new controller ensures an efficient control of laser cladding process, focussing in two main technological developments:
- Variable optical path: A RT-control ALAS ensures the performing of variable clad tracks managing the position of the lenses of the optical zoom and simplifying the programming cladding strategy.
- Heat accumulation control: A RT-control system modifies laser power values in order to guarantee the characteristics of the laser cladding process, avoiding heat accumulation effects due to external factors (workpiece geometry, undesired speed variations ...).
The combination of the variable optical path and the laser power control enables the operation of the system in several modes.
The Real-Time control system is based on a FPGA board. Specifically, the Real-Time control system is based on ALTERA DE2-115 Development and Education Board through the use of Cyclone IV FPGA. This device supports the industrial camera connection through the CameraLink interface, and offers others communication interfaces like Ethernet to connect FPGA to external Human-Machine Interfaces (operator interfaces).
The architecture of the ALAS controller has been designed taking into account performance and development effort. A hybrid soft-hard architecture has been developed, where parts are in hardware and other parts are in software.
The FPGA system has been developed using ALTERA tools as SDK, and VERILOG as hardware description language and ANSI C as micro processor language. Besides a RT preemptive multitasking operating system (µCOS-II) has been used to guarantee a hard real time operation of the overall system.

HEAT ACCUMULATION CONTROL
The main parameter that defines the quality of the cladding process is the dilution. This parameter cannot be directly measured during the cladding process; however, there is a relationship between the meltpool width and the dilution. The correlation between the laser power and the clad properties (i.e. Dilution and width) has been determined from single images of the monitoring signal.
For Heat accumulation control strategy, the FPGA-based system provides a real-time signal of the melt pool geometry to implement a closed-loop control to manage the laser power in order to avoid heat accumulation in the part to be cladded. For this purpose, a measurement algorithm of the melt-pool is implemented in the FPGA in a three steps algorithm,
1. Data sent by the Photonfocus CMOS camera are binarized (segmentation stage), transforming the image from grayscale to a binary.
2. The binarized image is eroded (filtering stage), erasing the noise with a size less than 3x3 pixels. This algorithm removes the incorrect saturated pixels from the image.
3. Finally, the data stream is processed by the geometrical module (measurement stage). This measurement algorithm is based on the calculation of the moments of inertia in order to estimate the best ellipse approximation for the melt pool footprint.
The result of measuring the melt pool width have been showed, the signals show frame per frame the calculated melt pool width while processing. The melt pool width provided by the monitoring system corresponds strong to the seam width of the cross sections and has also strong correspondence with dilution.
From these experiments, the melt-pool depth (i.e. Dilution) was obtained from the analysis of different macrographs showing the cross-section of each clad track.
To control the heat input into the clad layer avoiding heat accumulation in the base material, it is a closed-loop system adapted for variable spot sizes has been develop. This system measures the geometry (e.g. width) of the melt pool during the cladding process.
The system uses a CMOS camera (in a coaxial setup with the laser beam) in combination with a FPGA development board. Images captured by the CMOS camera are transferred via CameraLink Base communication protocol to the control board. An analog input (0-10 V) of the laser power source is used to command the laser power applied to avoid heat accumulation in the part to clad.

VARIABLE OPTICAL PATH
Together with the Heat accumulation control, ALAS controller is capable to manage the position of the two movable lenses (L3 and L4) covering a magnification range of the zoom values from 1.2x to 2.5x. This movable system requires the synchronized movement of the two movable lenses:
The zoom optics is driven by a set of components from Maxon: gear unit, motor, encoder, brake and control. Two motor sets and two power and control drivers (EPOS2) are used in the ALAS prototype to get a precise control of the moving lenses. Thus, the gear unit is coupled to the motor to get a linear movement, while brakes are included to guarantee a safety operation of the system. A closed-loop control is used for the position commanding of these driver units.
The operation of the motors has to be configured like stepper motors, enabling a precise digital control position from an external controller. This solution is used to get a real-time synchronized movement commanded by the FPGA, through the corresponding digital signals (step-by-step). Step direction mode was configured in the EPOS2 controllers, using step and direction inputs as control signals besides a ready/fault output was defined as a status feedback signal.
In addition, two inductive switches (Schneider XS1-N05PA310) are used to limit the maximum displacement allowed for each motor (lens).The inductive limit switches also act as a reference point to set the start position of each lens.
EPOS2 controllers have to be configured in MAXON EPOS Studio software in step direction mode with the following main parameters: scaling factor 200, polarity positive, position offset 0 qc, max following error 2000 qc, max profile velocity 12100 rpm, max acceleration 200000 rpm/s. In addition, the following I/O signals were also configured in MAXON EPOS Studio software: digital input1 as drive enable, digital input2 as general A, digital input3 as general B, digital output3 as ready/fault.
Besides the previous configuration, the maximum speed enabled for the system was fixed in 200 steps per second. Within this configuration the system is capable to move the lenses with a speed of 20 mm/s at an accuracy of 100 µm. This means that the whole range of the magnification can be achieve in 4 seconds, but a linear zoom evolution requires 7.54 seconds (the slowest zoom variation requires 0.29s).

DESING AND MANUFACTURING OF THE ALAS CLADDING HEAD
According to the variety of applications in the project, the ALAS-prototype is designed and manufacturing as a modular cladding head with a zoom-module as the central element. This concept has the advantage that the module can be adapted to components like fibre-connectors, collimators and beam splitters. The zoom module is adapted via coupling flanges to these components. Due to the modular concept, the zoom module is capable to be adapted to other components by a simple exchange of the coupling flanges. The ALAS-zoom module has the external dimensions of 336mm x 202mm and 187mm. The weight of the ALAS head with lenses, drives, collimator and control is 12 kg. Besides, the weight of the COAX 8 nozzle –The nozzle used for validation and demonstration trials— is 2kg.
Electrical and opto-mechanical elements have been integrated to compound the ALAS head prototype

HIGH-LEVEL CONTROL SYSTEM
High-level control system establishes the link between the operator of the cladding cell and the RT-controller. The High-level control system has been developed to be intuitive for the cladding operator allowing an easy operability of the new cladding system. Communication between the user and the beam path is effected through a number of clear graphic user interfaces (GUI) or windows.
This user-friendly software enables the operator of the cladding cell to program a complex laser path only “sketching” the design of the complex part to repair, and setting the main parameters of the process, like initial laser power, process speed...
The connection between High-Level control system and RT-ALAS controller is based in TCP/IP socket connection, where the ALAS controller acts as the server, while the High-Level control system is the client. It enables the control of the system with a simple telnet connection, by using the human readable command protocol. Therefore, this communication protocol is used to configure and monitor ALAS controller in real time.
The control commands are programmed on-line (while the system is working), it enables having a complex routine with no extension limits (especially suitable for LMD, Laser Metal Deposition). The operator only has to define the path and the speed of the positioning system (e.g. robot), required to obtain the final shape of the track.
The HLCS (High-Level Control System) extracts automatically the routine of the optical lenses to adapt the beam size for a defined sketch and path, but it requires the generation of a correlation table that relates the width value with the parameterization of laser power and zoom value.
Finally the ALAS cladding system was integrated with all the elements that compounds a laser cladding cell (robot, laser, powder feeder, electronic devices,...).

LABORATORY VALIDATION TRIALS
First validation trials are focused in the validation of the beam size range to characterize the laser beam diameter at different magnification values. Thus, ALAS system was configured for different magnification values –i.e. x1.2 x1.5 x2.0 x2.5— and beam shape was analyzed for each magnification.
For 1.2x the beam size obtained is 2,7mm /For 1.5x the beam size obtained is 3,4mm/For 2.0x the beam size obtained is 4,57mm/For 2.5x the beam size obtained is 5,8mm.
Analyzing the graph it can be concluded that the ALAS head is working correctly because the real results of the variable zoom system are in consonance with the predefined in the requirements of ALAS prototype: from 2.7mm of diameter up to 5.8mm of diameter.
Once verified the correct operation of ALAS optical system, next trial was focused to check the correct operation of the variable optical zoom. Thus, it was programmed several trials modifying both laser power and zoom value.
Regarding the validation of the operation of the variable cladding system, several variable cladding tracks were programmed modifying both laser power and zoom position on-the-fly.
All these trials evaluate the correct operation of the variable optical path to adapt the laser beam size to the requirements of the geometrical aspects of a complex shape in an easy way.
Finally, regarding the validation of the operation of the heat accumulation control, several trials with and without control of the laser power were programmed. The assessment of the heat accumulation control application was done by defining a validation experiment by performing seven overlapped cladded tracks with and without power control for a specific zoom value.
Metallographic examinations were done in order to characterize the cladding process and the properly operation of the RT-control system. Cross sections were prepared from samples cladded by using the Real-Time molten pool control strategy with an open-loop control strategy (up) or without laser control system. Analyzing the results obtained it is possible to extract how the RT-control system reacts to maintain the dilution of the laser cladding constant.

PRE-INDUSTRIAL VALIDATION TRIALS
Pre-industrial validation trials are focused in the assessment of the ALAS cladding system in a pre-industrial environment. Two different applications, defined by end-users, were validated: Heat accumulation control applied to tubes and cladding of variable geometries.
Regarding the application of ALAS system applied for heat accumulation control in tubes, a spiral shape of the laser cladding track is programmed in the steel tube. The pitch of the spiral movement is programmed to have and overlap of 50% between consecutive laser cladding tracks.
When using a fixed laser power it is possible to observe that the melt pool size is growing when executing the laser cladding process. As the melt pool gets wider, the quality of the laser cladding track will be altered as well, due to different microstructural properties and an increasing in the dilution. Moreover one can observe that the higher heat input in the samples caused extra oxidation of the sample.
When comparing the laser cladding process on the same steel tube as used for the non-controlled experiment described above, now using the ALAS controller, one can observe that the camera image shows a steady size of the melt pool. Moreover one can observe that the applied laser power is decreasing as the process advances, resulting in a steady melt pool.
Finally, regarding the application of ALAS system applied to the reconstruction of variable complex geometries, various multi-layer complex shapes have been manufacturing reconstruct using an additive laser-based manufacturing strategy. They include names as “banana”, “bended banana”, “diamond”, “hourglass” and “big-small” shapes.
All these set of trials at different conditions revels that the ALAS prototype is a robust and versatile, flexible system, easy to implement in existing cladding cells. These set of tests is very valuable to introduce a new product like ALAS in a very competitive market, and show the costumers their potential benefit.
Potential Impact:
The ALAS project aims to shorten the gap between research and economy with latest research activities in the fields of optics, measurement and control, by providing an innovative, adaptive laser cladding system with variable spot size to end users. Thus, Europe’s SMEs will be empowered to strengthen their global market position as the productivity will increase due to shortened setup times, flexibility gain and controlled quality.
The development of this innovative laser cladding head includes the develop of an adaptive optical path that will control the laser focus distance on the fly, it also has a user friendly interface in order to simplify the programming of this variable tracks and to develop a monitoring system which will control the laser parameters depending on the process status will ensure a steady process quality
The main features of the ALAS system are: simple interfacing with the operator, simple design adapted to complex geometries, real-time laser power control to avoid heat accumulation in the part to be processed, compact design that minimize the safety laser requirements of the traditional laser cells and provision to interface with the rest of elements of the laser cells.
The potential markets of application of the project results are:
- Companies working on repairing and coating,
- Different industries (heavy duty machinery, tooling, petrochemical, energy, manufacturing, aeronautical, naval, and transport),
- Laser manufacturers and laser equipment manufacturers.
The main application for laser cladding is repairing of parts with high added value (errors in manufacturing ratios, surface cracks, damages during service, prototype production, modifications, etc.).
To achieve this goal, some requirements need to be defined from the technical point of view, which should be met during the course of ALAS project, including:
- Definition of requirements of the variable zoom optics based on the characteristics of laser source
- Selection material used for coating or fabrication, definition of parameters of complex pieces.
- Develop the real time control system to control the laser power.
- Development a communications protocol between FPGA-Laser-control system
- Automation of the process
- Integration of all subsystems that compound the ALAS prototype (variable optical zoom, Real time control system, Human machine interface).
With the prototype fabricated in this project, the company can reduce process time and save money due the optimization of the cladding tracks and the decreasing, till to cero the non quality pieces. For that reason the profit of laser cladding equipment will be definitively increase.
From technological, business and market viewpoints, significant benefits had been reached for the SMEs, including the following:
- Improvement of their innovation ability, benefitting by an integrate approach to problems thanks to a profound synergy between researchers having different skills and expertise in laser, laser cladding, laser system for material processing, expert in optical system, integrator of systems, expert in machine vision, mechanical design..
- Establishing collaborations with well-recognized and highly-reputed academic partners and RTD performers;
- Increasing their possibility of widening their market share;
- Opening new potential market applications and challenges.
ALAS consortium believes that the ideas and the achievements gained during this project will ensure in significant advances in several areas of the material processing field, especially in repairing complex geometries and re- manufacturing and fabricating complex 3D structures using laser cladding. The developed variable optical zoom together with the RT controls system and the High level control system, make more accessible the laser technology to other markets since not high skilled operator are required.
In order to extend the potential benefits of this idea to other fields in the wide world where laser technology is applied, a robust activity of dissemination and exploitation had been carried out.
Deliverable D7.7 “Dossier with all the publications made during the project “includes a list of these activities.

Main dissemination activities on the red:
- Project website.
- A Wikipedia page on the project and its results.
- A Short project video of ALAS prototype.
- Logo designed for ALAS project.

Other dissemination activities:
- Poster prepared for diffusion of the objectives of the ALAS project.
- Flyers printed to disseminate the main features of the ALAS project.
- Poster showed at ICALEO 2013.
- Oral presentation at Photonic West-2014.

List of Websites:
www.alasproject.eu

https://en.wikipedia.org/wiki/Draft:European_Project_ALAS

https://www.youtube.com/watch?v=X1jLhtK-JnA&feature=youtu.be