Claus Bagger

Review of laser hybrid welding

In this article, an overview of the hybrid welding process is given. After a short historic overview, a review of the fundamental phenomenon taking place when a laser (CO2 or Nd:YAG) interacts in the same molten pool as a more conventional source of energy, e.g. tungsten in-active gas, plasma, or metal inactive gas/metal active gas. This is followed by reports of how the many process parameters governing the hybrid welding process can be set and how the choice of secondary energy source, shielding gas, etc. can affect the overall welding process. An overview of the benefits and drawbacks of hybrid welding is presented, including reports on gap bridging ability, changes in welding speed and weld penetration, overall weld quality, and changes in heat input to the material being welded. This overview is followed by a few examples of industrial applications of hybrid welding. Finally, a section is devoted to explain about further work required in order to understand and tackle the hybrid welding process more efficiently in the future.In this article, an overview of the hybrid welding process is given. After a short historic overview, a review of the fundamental phenomenon taking place when a laser (CO2 or Nd:YAG) interacts in the same molten pool as a more conventional source of energy, e.g. tungsten in-active gas, plasma, or metal inactive gas/metal active gas. This is followed by reports of how the many process parameters governing the hybrid welding process can be set and how the choice of secondary energy source, shielding gas, etc. can affect the overall welding process. An overview of the benefits and drawbacks of hybrid welding is presented, including reports on gap bridging ability, changes in welding speed and weld penetration, overall weld quality, and changes in heat input to the material being welded. This overview is followed by a few examples of industrial applications of hybrid welding. Finally, a section is devoted to explain about further work required in order to understand and tackle the hybrid welding process more ...

Laser welding closed-loop power control

A closed-loop control system is developed to maintain an even seam width on the root side of a laser weld by continually controlling the output laser power of a 1500 W CO2 laser. Quality control is done by a photodiode monitoring the root-side light emission from the process. The control system is successfully demonstrated to work in the bead-on-plate configuration on 2 mm sheets, both at constant speeds and at speeds ranging from 0.32 to more than 0.72 m/min in one welding trial. The power control system is able to deal with sheets of variable thickness in one welding trial, with the following thickness changes made as partly lap welds: 1.25 to 1.25 mm, 0.5 to 2 mm, and 1.25 to 2 mm. Sheets of 0.5 mm lap joined to 2 mm sheets displayed even root seams along the entire seam, except at the immediate edges of thickness change.

Comparison of plasma, metal inactive gas (MIG) and tungsten inactive gas (TIG) processes for laser hybrid welding

In this paper, TIG, plasma, and MIG processes have been individually combined with a 2.6 kW CO2 laser. In a number of systematic laboratory tests, the general benefits and drawbacks of each process have been individually assessed and compared. Aspects such as ease of integration with a CO2 laser source, ignition and running torch stability, weld phase transformation and change in ductility and overall weld quality are described.The results show that all three processes can successfully be integrated with a CO2 laser beam for hybrid welding. Due to the pilot arc in plasma welding, this process enables a more stable ignition and running process than both TIG and MIG hybrid welding. Because of the delivery of extra material from a hot wire, the MIG hybrid process is well suited for bridging gaps of up to 0.6 mm in butt-welding of 2 mm steel. But because of the constant delivery of new material, the MIG process is more difficult to control than laser/plasma and laser/TIG processes.All three types of secondary heat sources enable an increased ductility of the weld as compared to pure laser welding when welding 1.8 mm GA 260 with a TIG torch and 2.13 mm CMn steel with a plasma arc or MIG. For the TIG, plasma, and MIG the reductions in hardness are 19, 27 and 33 %, respectively.In this paper, TIG, plasma, and MIG processes have been individually combined with a 2.6 kW CO2 laser. In a number of systematic laboratory tests, the general benefits and drawbacks of each process have been individually assessed and compared. Aspects such as ease of integration with a CO2 laser source, ignition and running torch stability, weld phase transformation and change in ductility and overall weld quality are described.The results show that all three processes can successfully be integrated with a CO2 laser beam for hybrid welding. Due to the pilot arc in plasma welding, this process enables a more stable ignition and running process than both TIG and MIG hybrid welding. Because of the delivery of extra material from a hot wire, the MIG hybrid process is well suited for bridging gaps of up to 0.6 mm in butt-welding of 2 mm steel. But because of the constant delivery of new material, the MIG process is more difficult to control than laser/plasma and laser/TIG processes.All three types of secondary...

Induction heat treatment of laser welds

In this article, a new approach based on induction heat treatment of flat laser welded sheets is presented. With this new concept, the ductility of high strength steels GA260 with a thickness of 1.8 mm and CMn with a thickness of 2.13 mm is believed to be improved by prolonging the cooling time from 750 to 450 °C. Initially, a simple analytical model was used to calculate the ideal energy contributions from a CO2 high power laser source together with an induction heat source such that the temperature can be kept at 600 °C for 2.5 s. This knowledge was then used for the design of an induction coil. A number of systematic laboratory tests were then performed in order to study the effects of the coil on bead-on-plate laser welded samples. In these tests, important parameters such as coil current and distance between coil and sample were varied. Temperature measurements were made at distances of 0, 2, and 4 mm away from the virtual weld line for the induction heating process and at distances of 2 and 4 mm for...

Comparison of a pulsed CO2 laser and a pulsed Nd:YAG laser for welding

In a series of systematic laboratory experiments, a pulsed 400 W Nd:YAG laser is compared with a 1150 W slow flow CO2 laser, operated pulsed at 400 W average power. The pulse capabilities of the two lasers are shown. Five types of materials are welded with the two lasers, AISI 304 stainless steel, AISI 316 stainless steel, SS2377 Dublex stainless steel, AlMgSi1 (DIN 1725) and pure copper (DIN SFCu).For both laser types it is shown that a minimum pulse width exists at a fixed pulse energy that gives the deepest weld penetration while maintaining a sound weld. When the pulse energy is varied, a near linear correlation is found between the pulse energy and the optimum pulse width resulting in sound welds. The pulse energy/pulse width correlations for the two laser types are compared, and the differences between the obtained welds in the materials are discussed. The welds are quantified by the penetration depths, the weld surface conditions and the occurrence of undercut, pores and cracks in the material.In a series of systematic laboratory experiments, a pulsed 400 W Nd:YAG laser is compared with a 1150 W slow flow CO2 laser, operated pulsed at 400 W average power. The pulse capabilities of the two lasers are shown. Five types of materials are welded with the two lasers, AISI 304 stainless steel, AISI 316 stainless steel, SS2377 Dublex stainless steel, AlMgSi1 (DIN 1725) and pure copper (DIN SFCu).For both laser types it is shown that a minimum pulse width exists at a fixed pulse energy that gives the deepest weld penetration while maintaining a sound weld. When the pulse energy is varied, a near linear correlation is found between the pulse energy and the optimum pulse width resulting in sound welds. The pulse energy/pulse width correlations for the two laser types are compared, and the differences between the obtained welds in the materials are discussed. The welds are quantified by the penetration depths, the weld surface conditions and the occurrence of undercut, pores and cracks in the material.

Investigations in CO2 laser beam caustics measuring techniques

The performance of an industrial laser is very much depending upon the characteristics of the laser beam. The ISO standards 11146 and 11154 which are describing test methods for laser beam parameters have been approved. To implement these methods in industry is difficult and especially for the infrared laser sources, such as the CO2-laser, the available analysing systems are slow, difficult to apply and are having limited reliability due to the nature of the detection methods. In some RTD projects 3 different laser beam analysing systems dedicated to high power CO2-lasers have been developed. One of the projects was aiming towards developing a beam analysing system meeting the strict demands of the standards. The other project aimed towards developing built-in detectors for laser cutting systems. In this paper these detecting systems will be described.The performance of an industrial laser is very much depending upon the characteristics of the laser beam. The ISO standards 11146 and 11154 which are describing test methods for laser beam parameters have been approved. To implement these methods in industry is difficult and especially for the infrared laser sources, such as the CO2-laser, the available analysing systems are slow, difficult to apply and are having limited reliability due to the nature of the detection methods. In some RTD projects 3 different laser beam analysing systems dedicated to high power CO2-lasers have been developed. One of the projects was aiming towards developing a beam analysing system meeting the strict demands of the standards. The other project aimed towards developing built-in detectors for laser cutting systems. In this paper these detecting systems will be described.

Flexible laser welding of tailored blanks in small batch series

In a EU Craft project including eight partners in five countries, the most important aspects regarding efficient and low cost manufacture by small and medium sized enterprises (SME) of tailored blanks has been systematically investigated. In this paper, small batch series laser welding of tailored blanks in SME will be described. This includes the design, development and systematic use of a flexible and low cost clamping device as well as the practical experience obtained on the job shop through systematic optimization of welding of tailored blanks with even and uneven thickness (0.75 and 1.25 mm). A clamping device that is able to hold finished parts up to 1 x 1 m is successfully manufactured and tested. A special arrangement with alignment needles along the weld line is used to precisely position the sheets. These needles are turned into the clamping device during welding, where root shielding is employed. Hydraulic presses hold down the sheets, so they move less than 0.01 mm during welding. High quality tailored blank welds are successfully manufactured in ten different combinations, including mild steel and medium strength steel with even and uneven thickness with and without zinc coating.

Focal point position optimization with neural networks in CO2 laser welding

CO2 lasers are increasingly being utilized for quality welding in production. Considering the high equipment cost and high productivity, the start-up time and set-up time for new products should be minimized. Today most parameters involved in laser welding still have to be manually fine-adjusted when initiating welding of a new product. Ideally the parameters should be set and optimized more or less automatically. In this work the feasibility to automatically optimize the focal point position, one of the most critical parameters in laser welding, is analyzed. In a number of systematic laboratory experiments, a 1150 W CO2 laser is used to weld 2 mm sheets of mild steel. In the experiments the focus point position is continuously changed during a welding trial, and the process is simultaneously monitored by two photo diodes, one on either side of the workpiece surface. In a number of systematic investigations, the welding speed and the power level are varied. After welding, a number of artificial neural networks are designed to recognize the optimum focus point position. The efficiency and accuracy of the neural networks are then tested on new welds, performed with similar parameter settings as the first set of welds performed. The results show good agreement between the real position of the optimum focus point and the calculated values. Finally a trained neural network has been implemented into a closed-loop control system with one top side photo diode as a sensor. Preliminary test demonstrate that neural networks can be used to optimize the focus point position with good accuracy in cw CO2 laser welding.

Hot cracking of AISI 316 in pulsed CO2 and pulsed Nd:YAG laser welding

So far, the susceptibility to cracking of AISI 316 has not been modelled under different process conditions, e.g. at different welding speeds or different pulse frequencies. In this paper, the susceptibility to cracking is investigated for a pulsed Nd:YAG laser and a pulsed CO2 laser in a number of systematic laboratory experiments. Both lasers are operated at an average power of 400 W, and in the investigations 5 series of weldings are performed with the CO2 laser and 12 series of weldings are performed with the Nd:YAG laser. In each series the welding speed is varied in order to change the cooling speed and thus the cracking behaviour.For each welding, the penetration depth, the seam width, the total number of cracks, the accumulated crack length, the accumulated crack length divided by the penetration depth, and, finally, the percentage of primary austenite are measured and compared for the various series. A comparison is made of the Nd:YAG laser welds and the CO2 laser welds.So far, the susceptibility to cracking of AISI 316 has not been modelled under different process conditions, e.g. at different welding speeds or different pulse frequencies. In this paper, the susceptibility to cracking is investigated for a pulsed Nd:YAG laser and a pulsed CO2 laser in a number of systematic laboratory experiments. Both lasers are operated at an average power of 400 W, and in the investigations 5 series of weldings are performed with the CO2 laser and 12 series of weldings are performed with the Nd:YAG laser. In each series the welding speed is varied in order to change the cooling speed and thus the cracking behaviour.For each welding, the penetration depth, the seam width, the total number of cracks, the accumulated crack length, the accumulated crack length divided by the penetration depth, and, finally, the percentage of primary austenite are measured and compared for the various series. A comparison is made of the Nd:YAG laser welds and the CO2 laser welds.