Final Report Summary - CLAIM (Customised laser-assisted plasma arc welding of light alloys and steels)
Executive summary:
CLAIM provides a new sheet metal welding technology to enhance and strengthen the ability of European small and medium-sized enterprises (SMEs) to increase their research and innovation (R&I) capacities. The project brought the participating industry partners closer to the research and academia community by developing and integrating a new welding process. The new process is based on a two common tungsten inert gas (TIG) welding process which will be stabilised by low-power laser radiation (< 400 W). Because of the stabilisation effect from the emitted laser radiation the SME users will be able to achieve the requirements of the industry to faster production. Also the laser radiation allows to better control the actual position of the weld, leading to higher quality welded parts. The produced work pieces during the project show clearly that the new process can weld steel and aluminium parts with a thickness of 2 mm at an outstanding precision and in a very short time frame. A traditional TIG welding process can produce as well the same parts but when compared to the samples these are evidently of lower precision / accuracy and need twice the time to be produced.
Furthermore, as a key component for the new welding process a prototype laser enhanced double TIG arc welding head was produced including all electronics and accessories needed. The prototype welding head allowed for accelerated welding speed and improved quality of the weld line. The working of the head and process were demonstrated and thus proven during the project review meeting of 19 September 2011. Novel quality control equipment has also been prepared, including specific electronic hardware and software, thus also showing excellent progress in this area as well.
A gadget and a video that proof the concept of 'good quality welding' were made for dissemination purposes.
The new technology developed in the CLAIM project will tackle common and complementary plasma welding problems and will demonstrate a clear exploitation potential for the SMEs. Therefore they will become more competitive to compete in the global market.
Project context and objectives:
The new CLAIM process is based on a common electrical arc welding process which will be stabilised by laser radiation. The results of a previously finished research project clearly show that by overlapping the electric arc with a low-power laser beam the feed rate can be increased. In this kind of process, the laser beam does not serve as an additional energy provider, as it does in hybrid laser arc welding processes. Instead, the laser stabilises the welding process which at least enables the increase in speed. The physics underlying this effect are still objects of current research.
Based on the promising research results on laser assisted gas metal arc welding, the methods transfer to TIG or plasma welding seems to be obvious. Based on a market analysis the consortium decided to develop a laser assisted TIG welding process. Using the laser for stabilisation seems to be very promising to increase the welding speed. One of the basic ideas of the technical realisation of the welding process in CLAIM is to have a significant increase of the overlapping zone of the laser beam and the electrical arc, compared to previous research results. The amount the laser influences the electrical arc is measurably raised which surely allows attaining CLAIM objectives verifiably:
- to increase the maximum achievable welding speed by 50 %,
- to raise the welding penetration depth to at least 30 %.
For improving their competitiveness in their working area the SME participants of the project consider the exploitation of the economic advantage of the laser assisted TIG welding to be mandatory. To attain this goal, application-oriented research efforts are required. The outsourcing of research activities to specialised research institutes providing the required devices, enables the participating SMEs to conduct an efficient and result oriented development of their product portfolio, concerning the process knowledge and the technical requirements. Therefore the objectives of CLAIM are specified and timed as follows:
- Construction of an industrial prototype: M6
- Development of the laser assisted welding process: M12
- Verification of economic efficiency: M22
Attaining these goals, the SME participants are provided with knowledge about the process technology developed and its application. They are therewith enabled to implement the results of the project in their production or to commercialise them in products, respectively. This way the SME participants of CLAIM contribute to a strengthening of the European economic area.
Development of laser assisted TIG welding process
Based on parameter sets for the conventional TIG welding process, parameter sets for the laser assisted arc welding process newly invented in CLAIM shall be derived for the welding of high tensile stainless steel or aluminium. The CLAIM analysis programme is designed to achieve maximum welding speeds, whereas the quality of the weld seams is kept constant or is improved. The analysis programme will account for typical laser parameters like the wavelength, the laser beam's intensity or its operational mode.
Construction of an industrial prototype
The industrial application of the laser-assisted TIG welding process developed in CLAIM requires an advancement of the conventional welding system architecture. Especially for beam guidance coaxial to the welding arc, an adoption of the welding head has to be considered. In industrial applications, the robustness of the system developed in CLAIM has to be proven by a process control system. For that reason, a process diagnostic unit will be developed which will be capable of being integrated in common industrial quality control systems.
Verification of economic efficiency
Opposed to the process the developed in CLAIM, a more complex machine technology is required. In turn, the enhanced technology requires elevated financial expenses for their acquisition and operation. For a profitable usage of the process, the expenses have to be compensated by an increased productivity which is ensured by higher welding speeds and / or by improvement of welding seam properties. In CLAIM, a direct comparison of the conventional welding process and laser assisted TIG process under SME shop floor conditions shall demonstrate the advantageous economic efficiency of the laser assisted TIG process.
Project results:
At the beginning of the project the specific end user requirements such as the definition of the work pieces, process technology and system integration were defined by the SME partners. Furthermore, the evaluation criteria and testing methods were specified. The consortium validated the developed welding process and its results. All SME partners performed a separate market analysis to assess the potential market for the newly developed welding process, system technology and software. The result of the market analysis determined a need for all partners to develop an innovative laser assisted TIG process with higher productivity and the potential for a patentable laser TIG welding head. The process development of the laser assisted TIG (WP2) started earlier than scheduled. The main process limit for TIG welding is the welding speed. Slow speed leads to high heat affects and work piece distortion. To overcome this limit it was decided to develop a double TIG laser process. It was expected that the laser radiation will guide the arcs on one foot point and this will lead to a higher process speed.
Before starting the first welding test a schematic model of the double TIG laser welding head was designed. In the schematic model the interaction of both electrical arcs with the laser radiation was realised. To realise a function model MERKLE delivered the needed equipment (D4.1). All parts were adapted to a CNC axis system. The completion of the function model is pictured in figure 2.
First trails were done to prove the stabilisation of two arcs on one point at the headquarters of MERKLE. LZH together with MERKLE showed for the first time that, with 400 W laser low-power and a beam diameter of 1.6 mm on the material, the guiding of two arcs on aluminium and stainless steel is possible. This was even possible with a welding speed of 3 m/min for aluminium and 1 m / min for stainless steel.
After the function model was successfully tested, the LZH searched through a literature and found an existing triple TIG welding head. For using this head modifications were planed so that the head can fulfil all industrial end user requirements. In particular the following aspects were respected:
(a) hardware of the welding head must be industry proven;
(b) gas protection must be guaranteed for high welding speeds;
(c) easy exchange of the TIG electrodes;
(d) industrial standard components shall be used;
(e) compact size of the welding head;
(e) low investment costs.
Before testing the prototype welding head, MERKLE attached the hose packages to their welding machines. After this the laser beam was aligned through the head. The test phase took 3 weeks, in which time, aluminium and stainless steel test pieces were welded. Different welding and laser power were used. In close cooperation with MERKLE it was decided that the prototype fulfils the industrial requirements and that the process development will be carried out with the prototype.
After the double TIG welding head was thoroughly tested, it was used for the further process development experiments. These were performed on 2 mm thick AISI 304 steel and 2 mm aluminium sheet material in coordination with the needs of the project partners. For both materials the parameters for butt welds and full penetration were found and the new process was compared with the process without laser enhancement.
AISI 304 steel
In order to find the parameters for a full penetration at different welding speeds and compare them with the process without laser enhancement over 200 tests were performed. Table 3 shows the welding speed gain of the new process compared to the standard double TIG. The best results were obtained at low welding currents with a speed gain of over 60 %. With these results the requirements of the project partners have been met.
The applications have already been extended to tailored blanks. Parts with different sheet thicknesses were welded for dissemination purpose. They consist of two parts, a 1.5 mm thick inner part with the smiley face and a 2 mm thick outer ring. Tests without laser stabilisation led to an unstable arc process. This was overcome by the new technique. The workpiece could be welded without any warp due to heat input.
Aluminium
The first welding tests on aluminium were performed without feeding wire. The target was to develop basic welding parameters for full penetration welding with a maximum welding speed. Moreover the energy input from the alternating current (AC) arc should be much smaller than the energy input from the direct current (DC) arc.
Best results of the bead on plate welding were achieved with the electrical energy of 60 A AC and 85 A DC. The complete weld showed an undercut in the seam centre and good penetration. The reached welding speed was 950 mm/min. Compared to conventional TIG welding this welding speed is 2-3 times higher. The next step was to integrate a feed wire in order to prevent an undercut in the seam. Therefore an aluminium alloy (AlSi 12) wire with a diameter of 1.2 mm fulfilled this requirement. Using AlSi 12 decreases the probability of weld cracking that is an undesirable characteristic specific to aluminium welding. The wire feed was mounted directly in front of the AC needle so the wire would be fed in line with the weld seam. Test results were conclusive that a proper wire feed was important. Ideally, the wire should contact the surface of the specimen directly below the AC needle. Having the wire touch directly below the AC needle also permits ample cleaning of the wire before melting.
A speed of 600 mm/min that is 2 times faster than basic TIG welding yields a dependable weld with a wide seam and sufficient penetration. 60 A AC and 85 A DC currents were utilised for this weld. This process was further optimised regarding 200 mm / min speed increase and reduction of both DC and AC currents to 50 A AC and 80 A DC. The relationship between speed, current and appearance is evident. Depending on the use and requirements of the user, both specimens were quality welds. Due to the implementation of the wire feed, more energy was required so the speeds were slightly lower than the without the wire. The desired results were obtained so the next stage of experimentation could commence.
Application of wire fed double TIG but welds were the next pursuit. As with the other stages, high speeds and low power consumption were desired. Speed values around 1 000 mm / min showing sufficient penetration were accomplished. The associated currents were 85 A DC and 50 A AC. Parameters were then manipulated to determine consistency and sensitivity of the process. A number of tests exhibiting similar weld properties were produced, concluding the stability of the process. During regular testing only a quick wipe on the surface of the material using ethanol was practiced. Pores or other abnormalities in the weld can occasionally be a result of pre-process preparation where the specimen becomes contaminated with foreign materials.
The complete process development (WP2) was supported by mechanical and metallographic evaluations, spectral analysis of the welding process and diagnostic of the welding process based on a high-speed camera. Furthermore WP 3 leads to a better process understanding. To realise the industrial requirements defined in WP1, the welding results were proven through taken cross-sections. The cross-sections have information about the penetration depth, heat affected zone and the seam geometry. This information aids in defining optimal process parameters in WP2.
In the spectral analysis of the welding process the wavelength of the diode laser was observed. The wavelength of 811 nm is well absorbed by the plasma, leading to a stabilisation affect. However, the wavelength of a diode laser depends on the temperature of the diodes. It is well known that a change of 3 K leads to a wavelength shift of 1 nm. To keep the process conditions constant, the LZH observed the laser temperature visually and detected the laser wavelength with a spectroscope during the welding process.
In cooperation with MERKLE and the LZH worked diagnostic of the welding process based on a high-speed-camera. The made several videos were made to show the influence of the laser radiation on the electrical arcs. Upon evaluation of the videos it became clear that the laser radiation leads both arcs to one foot point.
The main part in WP3 was the development of a process monitoring system (Task 3.4). The system will supervise most of the critical parameters in the double TIG laser process. The following working steps were defined:
(1) Arc welding sensors selection to monitor and supervise welding process.
(2) Development of a system for acquisition and conditioning of sensor signals.
(3) Execution of experimental test plan:
(a) find most critical parameters;
(b) define process windows;
(c) define supervision algorithms.
(4) Development of supervision system.
According to these definitions, the following actions have been carried out:
(1) Arc welding sensors selection to monitor and supervise welding process
All sensors for the supervision system have been selected, purchased and received. End user process requirement have been specified in order to choose the right sensors. The number of sensors has been doubled in order to fully sense the double TIG torch.
2. Development of a system for acquisition and conditioning of sensor signals
A supervision system architecture has been designed in order to monitor and supervise the double TIG laser assisted welding process.
A compact system has been developed in order to acquire and condition the sensor signals.
This system will also be used for supervision tasks. This system is comprised of the following components:
(a) data acquisition device from national instruments: NI USB-6211 bus powered M series;
(b) power supplies, voltage converters, terminals, electronic components;
(c) specific software to acquire and register all the sensor signals.
(3) Execution of experimental test plan
The data acquisition system has been installed within the LZH facilities as part of the double TIG laser assisted welding machine. The following actions have been developed:
(a) Sensor installation in welding machine: current, gas flow and arc voltage. Two sets of sensors have been installed for double TIG welding process.
(b) Data acquisition software installation in supervision PC at LZH facilities.
(c) Data acquisition software configuration concerning data sampling, filters, trigger levels and so on. Fine tuning has been developed for all configuration parameters related data acquisition software.
(d) Training to LZH personnel on data acquisition software use. A user manual has been developed which, including the use of the software, has been explained to the LZH personnel in situ at the LZH facilities.
(4) Development of supervision system
Before starting to develop the supervision system the verification of the developed process monitoring system was performed (Task 3.4). Therefore an exhaustive welding test plan with the double TIG laser assisted machine has been executed in order to:
(a) Find optimum welding parameters (nominal parameters) for the selected materials and thicknesses. The objective has been to find a good setup by means of a fine tuning of the welding parameters.
(b) Define the process windows for the analysed signals.
(c) Find correlations between the supervised signals and the welding defects.
(d) Identify the relevant process parameters for the supervision system to assure a good product quality. Not only sensor signals have been analysed, but also calculated parameters such as the heat input, the electrical power and the electrical energy used in the welding process.
(e) Define the welding supervision algorithms and techniques to be used by the supervision system.
(f) Stainless steel 304L with 2mm thickness has been used for all these welding tests.
After the verification of the process monitoring system the supervision system has been developed with the capability to detect and identify welding defects in real time. This supervision system has been implemented on a PC which communicates with the data acquisition system by means of a USB-cable. The main features of the supervision system are:
(a) friendly user interface developed with LabView on Windows OS;
(b) automatic detection, supervision and recording of welding signals;
(c) critical welding parameters monitoring and supervision by means of intelligent control techniques;
(d) real time supervision of welding beads.
The supervision system has been developed using the following supervision techniques:
(a) process windows on critical welding parameters: current, voltage, gas flow, heat input, etc.;
(b) fulfilment of welding procedure specifications. A welding procedure data sheet has been developed for this specific welding process. The welding procedure is a document that contains all the information to describe how welding has to be carried out in double TIG laser assisted welding beads;
(c) intelligent-supervision techniques with fuzzy control algorithms to analyse correlations between welding parameters. These fuzzy control algorithms are based on welding engineer expertise acquired during welding experimentation with double TIG laser assisted process.
Parallel to the process development and the setup of the supervision system a software concept for the implementation of the off-line programming methods was developed and realised. The concept comprises two major elements: using the three-dimensional (3D) Automate software for a 3D welding process simulation and creating the new software tool CLAIM to define the process based on computer-aided design (CAD) files. CLAIM was implemented in C++ language using the open source frameworks Qt and Open Cascade. Its main features are the spatial visual representation of work-pieces, the selection and representation of welding spots and seams in a graphical user interface and the flexible tools for the generation of control programmes for welding machines. To unify machine and material specific parameter settings within the generated control programs the 'process code' concept has been defined and implemented in CLAIM. In short, a process code is an indicative character string representation of a group of machine and material specific parameter sets. It is assigned to a welding seam or a welding spot and represents the parameter set the seam or spot has to be produced with. Beside the material type and the machine type the process code forms the missing partial key to identify the appropriate parameter set in a database. This concept is very flexible when it comes to changes due to a change of the material or the machine type. In CLAIM these changes can easily be done by the user by selecting a different machine type or material type in a combo box in the user interface. Even the process code of a weld seam or spot itself can easily be changed by selecting its graphical representation and choosing a specific menu item. Beside the process code concept further concepts have been implemented in CLAIM like respecting a safety box in in-feed movements, custom welding target approach and departure directions as well as process sequence reordering. These concepts are aiming at an improvement of the user-friendliness of CLAIM by enabling an almost automatic tool path generation. They fulfil the objective of the task by providing a user-friendly off-line method to easily change process parameters and modify the whole welding process itself in a matter of minutes and help reducing down time.
To ensure a proper installation of the complete welding equipment, a manual of the prototype system was written by the LZH. Furthermore in the manual a setup for the use of the equipment was described. The manual was reviewed by all the project partners for assurance of clarity and correctness. It is divided as follows:
1. Equipment overview
2. Equipment Installation of the welding machine
2.1 Front and read
3. Laser
3.1 Laser rear site
4. Welding head
4.1 Rear
5. Cooling system
6. Equipment set-up
7. Equipment use
7.1 Basic procedure
7.2 Procedure for 'Smileys'
In order to obtain an industry accepted weld process, thorough testing was performed on both steel and aluminium 2 mm plates to find an acceptable parameter set. The test results were then reviewed by project partners for visual evaluation of weld quality with respect to pore content, penetration, undercutting, holes and other irregularities. Speed and weld quality were the two most important criteria during experimentation matching industry requirement. Each partner rated the welds from 1 to 5.
In conclusion, the project partners agreed on a set of welds for both aluminium and steel that meet industry standards regarding penetration, pore content, cracks, oxidation, cleaning and other visual defects. In the steel welding process, specimens 111 and 142J ranked the highest with speeds of 290 mm / min and 350 mm / min respectively. The corresponding currents were 40 A for 111 and 60 A for 142J.
In the aluminium process, two welds were also found to meet industry requirement. Test specimens 105 and 106 were rated the highest with speeds of 0.95 m / min and 1 m / min respectively. Their currents were 85 A DC and 50 A AC with a wire feed of 2.5 m / min. The speeds in both experiments displayed substantial increases from standard TIG welding. A standard TIG process speed of 0.15 m / min at 50 A was used for comparison as defined in the specifications of the end user requirements.
Finally, an economic analysis has been made. It was concluded, that double TIG plus laser process is profitable, applied to the production of high-value work pieces (where the cost per part (EUR / part) is higher), and / or for higher production rates (high number of manufactured parts per hour). In detail both the environmental implications (energy consumption) of the new welding process such as recycling processes have been examined.
The laser radiation leads to a voltage reduction of 2 V in the TIG welding process. Consequently, the energy input of the welding TIG sources also reduces about 20 %. This will practically compensate the energy input of the laser with the advantage of higher process speed of about 50 % without compromising the welding depth.
In addition, the process increases the weld quality in terms of reduction of pores, cracks, and improving geometry, external appearance, etc. Consequently, it reduces the rework, scrapped and end customer parts rejections. It follows that the number of defective parts to recycle will be reduced, as the processing scrap costs.
Summary of the CLAIM project
In the CLAIM the main scientific and technological project results are:
(1) stabilisation of two electrical arcs with similar and different polarities;
(2) development of a double-TIG laser welding head:
(a) industrial requirements are fulfilled;
(3) process development for 2 mm of aluminium and stainless steel sheet metal:
(a) doubling of the welding speed for stainless steel;
(b) more than double of the welding speed for aluminium;
(c) process results industrial accepted;
(4) set up of a supervision system for the double TIG welding process;
(5) welding system tested and demonstrated;
(6) creating a new software tool to define the process based on CAD files in combination of a 3D Automate software for a 3D welding process simulation.
List of websites: http://www.claim-project.eu
CLAIM provides a new sheet metal welding technology to enhance and strengthen the ability of European small and medium-sized enterprises (SMEs) to increase their research and innovation (R&I) capacities. The project brought the participating industry partners closer to the research and academia community by developing and integrating a new welding process. The new process is based on a two common tungsten inert gas (TIG) welding process which will be stabilised by low-power laser radiation (< 400 W). Because of the stabilisation effect from the emitted laser radiation the SME users will be able to achieve the requirements of the industry to faster production. Also the laser radiation allows to better control the actual position of the weld, leading to higher quality welded parts. The produced work pieces during the project show clearly that the new process can weld steel and aluminium parts with a thickness of 2 mm at an outstanding precision and in a very short time frame. A traditional TIG welding process can produce as well the same parts but when compared to the samples these are evidently of lower precision / accuracy and need twice the time to be produced.
Furthermore, as a key component for the new welding process a prototype laser enhanced double TIG arc welding head was produced including all electronics and accessories needed. The prototype welding head allowed for accelerated welding speed and improved quality of the weld line. The working of the head and process were demonstrated and thus proven during the project review meeting of 19 September 2011. Novel quality control equipment has also been prepared, including specific electronic hardware and software, thus also showing excellent progress in this area as well.
A gadget and a video that proof the concept of 'good quality welding' were made for dissemination purposes.
The new technology developed in the CLAIM project will tackle common and complementary plasma welding problems and will demonstrate a clear exploitation potential for the SMEs. Therefore they will become more competitive to compete in the global market.
Project context and objectives:
The new CLAIM process is based on a common electrical arc welding process which will be stabilised by laser radiation. The results of a previously finished research project clearly show that by overlapping the electric arc with a low-power laser beam the feed rate can be increased. In this kind of process, the laser beam does not serve as an additional energy provider, as it does in hybrid laser arc welding processes. Instead, the laser stabilises the welding process which at least enables the increase in speed. The physics underlying this effect are still objects of current research.
Based on the promising research results on laser assisted gas metal arc welding, the methods transfer to TIG or plasma welding seems to be obvious. Based on a market analysis the consortium decided to develop a laser assisted TIG welding process. Using the laser for stabilisation seems to be very promising to increase the welding speed. One of the basic ideas of the technical realisation of the welding process in CLAIM is to have a significant increase of the overlapping zone of the laser beam and the electrical arc, compared to previous research results. The amount the laser influences the electrical arc is measurably raised which surely allows attaining CLAIM objectives verifiably:
- to increase the maximum achievable welding speed by 50 %,
- to raise the welding penetration depth to at least 30 %.
For improving their competitiveness in their working area the SME participants of the project consider the exploitation of the economic advantage of the laser assisted TIG welding to be mandatory. To attain this goal, application-oriented research efforts are required. The outsourcing of research activities to specialised research institutes providing the required devices, enables the participating SMEs to conduct an efficient and result oriented development of their product portfolio, concerning the process knowledge and the technical requirements. Therefore the objectives of CLAIM are specified and timed as follows:
- Construction of an industrial prototype: M6
- Development of the laser assisted welding process: M12
- Verification of economic efficiency: M22
Attaining these goals, the SME participants are provided with knowledge about the process technology developed and its application. They are therewith enabled to implement the results of the project in their production or to commercialise them in products, respectively. This way the SME participants of CLAIM contribute to a strengthening of the European economic area.
Development of laser assisted TIG welding process
Based on parameter sets for the conventional TIG welding process, parameter sets for the laser assisted arc welding process newly invented in CLAIM shall be derived for the welding of high tensile stainless steel or aluminium. The CLAIM analysis programme is designed to achieve maximum welding speeds, whereas the quality of the weld seams is kept constant or is improved. The analysis programme will account for typical laser parameters like the wavelength, the laser beam's intensity or its operational mode.
Construction of an industrial prototype
The industrial application of the laser-assisted TIG welding process developed in CLAIM requires an advancement of the conventional welding system architecture. Especially for beam guidance coaxial to the welding arc, an adoption of the welding head has to be considered. In industrial applications, the robustness of the system developed in CLAIM has to be proven by a process control system. For that reason, a process diagnostic unit will be developed which will be capable of being integrated in common industrial quality control systems.
Verification of economic efficiency
Opposed to the process the developed in CLAIM, a more complex machine technology is required. In turn, the enhanced technology requires elevated financial expenses for their acquisition and operation. For a profitable usage of the process, the expenses have to be compensated by an increased productivity which is ensured by higher welding speeds and / or by improvement of welding seam properties. In CLAIM, a direct comparison of the conventional welding process and laser assisted TIG process under SME shop floor conditions shall demonstrate the advantageous economic efficiency of the laser assisted TIG process.
Project results:
At the beginning of the project the specific end user requirements such as the definition of the work pieces, process technology and system integration were defined by the SME partners. Furthermore, the evaluation criteria and testing methods were specified. The consortium validated the developed welding process and its results. All SME partners performed a separate market analysis to assess the potential market for the newly developed welding process, system technology and software. The result of the market analysis determined a need for all partners to develop an innovative laser assisted TIG process with higher productivity and the potential for a patentable laser TIG welding head. The process development of the laser assisted TIG (WP2) started earlier than scheduled. The main process limit for TIG welding is the welding speed. Slow speed leads to high heat affects and work piece distortion. To overcome this limit it was decided to develop a double TIG laser process. It was expected that the laser radiation will guide the arcs on one foot point and this will lead to a higher process speed.
Before starting the first welding test a schematic model of the double TIG laser welding head was designed. In the schematic model the interaction of both electrical arcs with the laser radiation was realised. To realise a function model MERKLE delivered the needed equipment (D4.1). All parts were adapted to a CNC axis system. The completion of the function model is pictured in figure 2.
First trails were done to prove the stabilisation of two arcs on one point at the headquarters of MERKLE. LZH together with MERKLE showed for the first time that, with 400 W laser low-power and a beam diameter of 1.6 mm on the material, the guiding of two arcs on aluminium and stainless steel is possible. This was even possible with a welding speed of 3 m/min for aluminium and 1 m / min for stainless steel.
After the function model was successfully tested, the LZH searched through a literature and found an existing triple TIG welding head. For using this head modifications were planed so that the head can fulfil all industrial end user requirements. In particular the following aspects were respected:
(a) hardware of the welding head must be industry proven;
(b) gas protection must be guaranteed for high welding speeds;
(c) easy exchange of the TIG electrodes;
(d) industrial standard components shall be used;
(e) compact size of the welding head;
(e) low investment costs.
Before testing the prototype welding head, MERKLE attached the hose packages to their welding machines. After this the laser beam was aligned through the head. The test phase took 3 weeks, in which time, aluminium and stainless steel test pieces were welded. Different welding and laser power were used. In close cooperation with MERKLE it was decided that the prototype fulfils the industrial requirements and that the process development will be carried out with the prototype.
After the double TIG welding head was thoroughly tested, it was used for the further process development experiments. These were performed on 2 mm thick AISI 304 steel and 2 mm aluminium sheet material in coordination with the needs of the project partners. For both materials the parameters for butt welds and full penetration were found and the new process was compared with the process without laser enhancement.
AISI 304 steel
In order to find the parameters for a full penetration at different welding speeds and compare them with the process without laser enhancement over 200 tests were performed. Table 3 shows the welding speed gain of the new process compared to the standard double TIG. The best results were obtained at low welding currents with a speed gain of over 60 %. With these results the requirements of the project partners have been met.
The applications have already been extended to tailored blanks. Parts with different sheet thicknesses were welded for dissemination purpose. They consist of two parts, a 1.5 mm thick inner part with the smiley face and a 2 mm thick outer ring. Tests without laser stabilisation led to an unstable arc process. This was overcome by the new technique. The workpiece could be welded without any warp due to heat input.
Aluminium
The first welding tests on aluminium were performed without feeding wire. The target was to develop basic welding parameters for full penetration welding with a maximum welding speed. Moreover the energy input from the alternating current (AC) arc should be much smaller than the energy input from the direct current (DC) arc.
Best results of the bead on plate welding were achieved with the electrical energy of 60 A AC and 85 A DC. The complete weld showed an undercut in the seam centre and good penetration. The reached welding speed was 950 mm/min. Compared to conventional TIG welding this welding speed is 2-3 times higher. The next step was to integrate a feed wire in order to prevent an undercut in the seam. Therefore an aluminium alloy (AlSi 12) wire with a diameter of 1.2 mm fulfilled this requirement. Using AlSi 12 decreases the probability of weld cracking that is an undesirable characteristic specific to aluminium welding. The wire feed was mounted directly in front of the AC needle so the wire would be fed in line with the weld seam. Test results were conclusive that a proper wire feed was important. Ideally, the wire should contact the surface of the specimen directly below the AC needle. Having the wire touch directly below the AC needle also permits ample cleaning of the wire before melting.
A speed of 600 mm/min that is 2 times faster than basic TIG welding yields a dependable weld with a wide seam and sufficient penetration. 60 A AC and 85 A DC currents were utilised for this weld. This process was further optimised regarding 200 mm / min speed increase and reduction of both DC and AC currents to 50 A AC and 80 A DC. The relationship between speed, current and appearance is evident. Depending on the use and requirements of the user, both specimens were quality welds. Due to the implementation of the wire feed, more energy was required so the speeds were slightly lower than the without the wire. The desired results were obtained so the next stage of experimentation could commence.
Application of wire fed double TIG but welds were the next pursuit. As with the other stages, high speeds and low power consumption were desired. Speed values around 1 000 mm / min showing sufficient penetration were accomplished. The associated currents were 85 A DC and 50 A AC. Parameters were then manipulated to determine consistency and sensitivity of the process. A number of tests exhibiting similar weld properties were produced, concluding the stability of the process. During regular testing only a quick wipe on the surface of the material using ethanol was practiced. Pores or other abnormalities in the weld can occasionally be a result of pre-process preparation where the specimen becomes contaminated with foreign materials.
The complete process development (WP2) was supported by mechanical and metallographic evaluations, spectral analysis of the welding process and diagnostic of the welding process based on a high-speed camera. Furthermore WP 3 leads to a better process understanding. To realise the industrial requirements defined in WP1, the welding results were proven through taken cross-sections. The cross-sections have information about the penetration depth, heat affected zone and the seam geometry. This information aids in defining optimal process parameters in WP2.
In the spectral analysis of the welding process the wavelength of the diode laser was observed. The wavelength of 811 nm is well absorbed by the plasma, leading to a stabilisation affect. However, the wavelength of a diode laser depends on the temperature of the diodes. It is well known that a change of 3 K leads to a wavelength shift of 1 nm. To keep the process conditions constant, the LZH observed the laser temperature visually and detected the laser wavelength with a spectroscope during the welding process.
In cooperation with MERKLE and the LZH worked diagnostic of the welding process based on a high-speed-camera. The made several videos were made to show the influence of the laser radiation on the electrical arcs. Upon evaluation of the videos it became clear that the laser radiation leads both arcs to one foot point.
The main part in WP3 was the development of a process monitoring system (Task 3.4). The system will supervise most of the critical parameters in the double TIG laser process. The following working steps were defined:
(1) Arc welding sensors selection to monitor and supervise welding process.
(2) Development of a system for acquisition and conditioning of sensor signals.
(3) Execution of experimental test plan:
(a) find most critical parameters;
(b) define process windows;
(c) define supervision algorithms.
(4) Development of supervision system.
According to these definitions, the following actions have been carried out:
(1) Arc welding sensors selection to monitor and supervise welding process
All sensors for the supervision system have been selected, purchased and received. End user process requirement have been specified in order to choose the right sensors. The number of sensors has been doubled in order to fully sense the double TIG torch.
2. Development of a system for acquisition and conditioning of sensor signals
A supervision system architecture has been designed in order to monitor and supervise the double TIG laser assisted welding process.
A compact system has been developed in order to acquire and condition the sensor signals.
This system will also be used for supervision tasks. This system is comprised of the following components:
(a) data acquisition device from national instruments: NI USB-6211 bus powered M series;
(b) power supplies, voltage converters, terminals, electronic components;
(c) specific software to acquire and register all the sensor signals.
(3) Execution of experimental test plan
The data acquisition system has been installed within the LZH facilities as part of the double TIG laser assisted welding machine. The following actions have been developed:
(a) Sensor installation in welding machine: current, gas flow and arc voltage. Two sets of sensors have been installed for double TIG welding process.
(b) Data acquisition software installation in supervision PC at LZH facilities.
(c) Data acquisition software configuration concerning data sampling, filters, trigger levels and so on. Fine tuning has been developed for all configuration parameters related data acquisition software.
(d) Training to LZH personnel on data acquisition software use. A user manual has been developed which, including the use of the software, has been explained to the LZH personnel in situ at the LZH facilities.
(4) Development of supervision system
Before starting to develop the supervision system the verification of the developed process monitoring system was performed (Task 3.4). Therefore an exhaustive welding test plan with the double TIG laser assisted machine has been executed in order to:
(a) Find optimum welding parameters (nominal parameters) for the selected materials and thicknesses. The objective has been to find a good setup by means of a fine tuning of the welding parameters.
(b) Define the process windows for the analysed signals.
(c) Find correlations between the supervised signals and the welding defects.
(d) Identify the relevant process parameters for the supervision system to assure a good product quality. Not only sensor signals have been analysed, but also calculated parameters such as the heat input, the electrical power and the electrical energy used in the welding process.
(e) Define the welding supervision algorithms and techniques to be used by the supervision system.
(f) Stainless steel 304L with 2mm thickness has been used for all these welding tests.
After the verification of the process monitoring system the supervision system has been developed with the capability to detect and identify welding defects in real time. This supervision system has been implemented on a PC which communicates with the data acquisition system by means of a USB-cable. The main features of the supervision system are:
(a) friendly user interface developed with LabView on Windows OS;
(b) automatic detection, supervision and recording of welding signals;
(c) critical welding parameters monitoring and supervision by means of intelligent control techniques;
(d) real time supervision of welding beads.
The supervision system has been developed using the following supervision techniques:
(a) process windows on critical welding parameters: current, voltage, gas flow, heat input, etc.;
(b) fulfilment of welding procedure specifications. A welding procedure data sheet has been developed for this specific welding process. The welding procedure is a document that contains all the information to describe how welding has to be carried out in double TIG laser assisted welding beads;
(c) intelligent-supervision techniques with fuzzy control algorithms to analyse correlations between welding parameters. These fuzzy control algorithms are based on welding engineer expertise acquired during welding experimentation with double TIG laser assisted process.
Parallel to the process development and the setup of the supervision system a software concept for the implementation of the off-line programming methods was developed and realised. The concept comprises two major elements: using the three-dimensional (3D) Automate software for a 3D welding process simulation and creating the new software tool CLAIM to define the process based on computer-aided design (CAD) files. CLAIM was implemented in C++ language using the open source frameworks Qt and Open Cascade. Its main features are the spatial visual representation of work-pieces, the selection and representation of welding spots and seams in a graphical user interface and the flexible tools for the generation of control programmes for welding machines. To unify machine and material specific parameter settings within the generated control programs the 'process code' concept has been defined and implemented in CLAIM. In short, a process code is an indicative character string representation of a group of machine and material specific parameter sets. It is assigned to a welding seam or a welding spot and represents the parameter set the seam or spot has to be produced with. Beside the material type and the machine type the process code forms the missing partial key to identify the appropriate parameter set in a database. This concept is very flexible when it comes to changes due to a change of the material or the machine type. In CLAIM these changes can easily be done by the user by selecting a different machine type or material type in a combo box in the user interface. Even the process code of a weld seam or spot itself can easily be changed by selecting its graphical representation and choosing a specific menu item. Beside the process code concept further concepts have been implemented in CLAIM like respecting a safety box in in-feed movements, custom welding target approach and departure directions as well as process sequence reordering. These concepts are aiming at an improvement of the user-friendliness of CLAIM by enabling an almost automatic tool path generation. They fulfil the objective of the task by providing a user-friendly off-line method to easily change process parameters and modify the whole welding process itself in a matter of minutes and help reducing down time.
To ensure a proper installation of the complete welding equipment, a manual of the prototype system was written by the LZH. Furthermore in the manual a setup for the use of the equipment was described. The manual was reviewed by all the project partners for assurance of clarity and correctness. It is divided as follows:
1. Equipment overview
2. Equipment Installation of the welding machine
2.1 Front and read
3. Laser
3.1 Laser rear site
4. Welding head
4.1 Rear
5. Cooling system
6. Equipment set-up
7. Equipment use
7.1 Basic procedure
7.2 Procedure for 'Smileys'
In order to obtain an industry accepted weld process, thorough testing was performed on both steel and aluminium 2 mm plates to find an acceptable parameter set. The test results were then reviewed by project partners for visual evaluation of weld quality with respect to pore content, penetration, undercutting, holes and other irregularities. Speed and weld quality were the two most important criteria during experimentation matching industry requirement. Each partner rated the welds from 1 to 5.
In conclusion, the project partners agreed on a set of welds for both aluminium and steel that meet industry standards regarding penetration, pore content, cracks, oxidation, cleaning and other visual defects. In the steel welding process, specimens 111 and 142J ranked the highest with speeds of 290 mm / min and 350 mm / min respectively. The corresponding currents were 40 A for 111 and 60 A for 142J.
In the aluminium process, two welds were also found to meet industry requirement. Test specimens 105 and 106 were rated the highest with speeds of 0.95 m / min and 1 m / min respectively. Their currents were 85 A DC and 50 A AC with a wire feed of 2.5 m / min. The speeds in both experiments displayed substantial increases from standard TIG welding. A standard TIG process speed of 0.15 m / min at 50 A was used for comparison as defined in the specifications of the end user requirements.
Finally, an economic analysis has been made. It was concluded, that double TIG plus laser process is profitable, applied to the production of high-value work pieces (where the cost per part (EUR / part) is higher), and / or for higher production rates (high number of manufactured parts per hour). In detail both the environmental implications (energy consumption) of the new welding process such as recycling processes have been examined.
The laser radiation leads to a voltage reduction of 2 V in the TIG welding process. Consequently, the energy input of the welding TIG sources also reduces about 20 %. This will practically compensate the energy input of the laser with the advantage of higher process speed of about 50 % without compromising the welding depth.
In addition, the process increases the weld quality in terms of reduction of pores, cracks, and improving geometry, external appearance, etc. Consequently, it reduces the rework, scrapped and end customer parts rejections. It follows that the number of defective parts to recycle will be reduced, as the processing scrap costs.
Summary of the CLAIM project
In the CLAIM the main scientific and technological project results are:
(1) stabilisation of two electrical arcs with similar and different polarities;
(2) development of a double-TIG laser welding head:
(a) industrial requirements are fulfilled;
(3) process development for 2 mm of aluminium and stainless steel sheet metal:
(a) doubling of the welding speed for stainless steel;
(b) more than double of the welding speed for aluminium;
(c) process results industrial accepted;
(4) set up of a supervision system for the double TIG welding process;
(5) welding system tested and demonstrated;
(6) creating a new software tool to define the process based on CAD files in combination of a 3D Automate software for a 3D welding process simulation.
List of websites: http://www.claim-project.eu
