Thomas Harrer

Influence of ambient pressure on spattering and weld seam quality in laser beam welding with the solid-state laser

Presently, a research project with respect to laser beam welding in a vacuum is carried out at the Institute of Joining and Welding in Braunschweig, together with the two industrial enterprises Trumpf Laser-und Systemtechnik and pro-beam. For this purpose, the 1.5 m³ vacuum chamber of the former electron beam plant was reconstructed so that the laser beam may be coupled into the chamber via quartz glass. By means of this test set-up, reproducible pressures of up to 10-3 hPa could be set. The research was done with a 6 kW disc laser on mild steel of the quality S355 in thick sheet metal up to 15 mm. The result showed that a reduction of the ambient pressure causes major changes in the generation of the metal vapor plume above the joining zone as well as a major impact on the welding seam geometry. In the case of identical welding parameters, a narrower welding seam develops and the achievable penetration depth rises significantly. Thus, by a slight decrease of the ambient pressure by approximately 10 hPa, spattering could be suppressed in a way that optically excellent welding seams could be achieved which, in this seam depth, qualitatively equal those of an electron beam weld seam.With metal sheets of 10 mm thickness, penetration welds were achieved at a feed rate of 2.0 m/min which, in this excellent quality of atmosphere, cannot even be obtained with a CO2 laser. An excellent seam surface with regular flaking and a slight over-fill without weld seam collapse was formed. At the same time, an excellent seam root without any irregularities occurred. Due to this tremendous improvement in quality of laser beam welding in a vacuum, new fields of application and markets may be opened to this procedure which so far has not been industrially established.Presently, a research project with respect to laser beam welding in a vacuum is carried out at the Institute of Joining and Welding in Braunschweig, together with the two industrial enterprises Trumpf Laser-und Systemtechnik and pro-beam. For this purpose, the 1.5 m³ vacuum chamber of the former electron beam plant was reconstructed so that the laser beam may be coupled into the chamber via quartz glass. By means of this test set-up, reproducible pressures of up to 10-3 hPa could be set. The research was done with a 6 kW disc laser on mild steel of the quality S355 in thick sheet metal up to 15 mm. The result showed that a reduction of the ambient pressure causes major changes in the generation of the metal vapor plume above the joining zone as well as a major impact on the welding seam geometry. In the case of identical welding parameters, a narrower welding seam develops and the achievable penetration depth rises significantly. Thus, by a slight decrease of the ambient pressure by approximately 10 hPa, ...

Hot cracking during laser welding of steel: influence of the welding parameters and prevention of cracks

In continuous wave keyhole-mode laser welding of high strength steel alloys hot cracking can occur. The hot crack susceptibility depends on the mutual interaction of several factors like the welding parameters, the alloy composition and the weld fixturing. In this paper we focus on the influence of the welding parameters and investigate the dependency of the laser power, the welding speed and the laser wavelength on the crack formation. X-ray images are used to visualize the hot crack patterns, which exhibit a pronounced periodicity. To influence the hot crack formation, the incident energy input into the process was adapted. For specific welding parameters, we show exemplarily the prevention of hot cracking by the use of a twin-spot optics.

Formation mechanism of process instabilities and strategies to improve welding quality

The deep penetration welding process with CO2 lasers has been employed successfully for many years in industry. It generates little spatter on the work piece surface and therefore produces excellent seam quality at high process speed with good process reliability over a wide range of parameters. With a wavelength around 1 µm, solid-state lasers are being increasingly used in industrial production thanks to simple beam guidance by means of laser light cable and their high electrical efficiency. Disk and fiber lasers have advanced into the domain of CO2 lasers by way of power and beam parameter product. However the seam quality is highly dependent on the focusing conditions used, whereby the mechanisms that cause process instabilities are still not properly understood. Also, at high intensities with high feed rates, considerable spatter is generated on the work piece surface, reducing productivity in applications where the demands on surface quality are high. In this publication, based on online X-ray observation and high-speed imaging, the suitability for welding and the formation mechanism of process instabilities like spattering and humping were compared at different feed rates. Based on a better process understanding strategies which can improve welding quality are presented.The deep penetration welding process with CO2 lasers has been employed successfully for many years in industry. It generates little spatter on the work piece surface and therefore produces excellent seam quality at high process speed with good process reliability over a wide range of parameters. With a wavelength around 1 µm, solid-state lasers are being increasingly used in industrial production thanks to simple beam guidance by means of laser light cable and their high electrical efficiency. Disk and fiber lasers have advanced into the domain of CO2 lasers by way of power and beam parameter product. However the seam quality is highly dependent on the focusing conditions used, whereby the mechanisms that cause process instabilities are still not properly understood. Also, at high intensities with high feed rates, considerable spatter is generated on the work piece surface, reducing productivity in applications where the demands on surface quality are high. In this publication, based on online X-ray obser...

Sensor assisted (remote) welding processes

Remote welding is widely used in automotive applications like car seats or doors. This technology is able to increase productivity and quality while reducing cost and weight of the parts.Using pathfinding and detection of the position of the welding sample in a remote welding system flange size can be reduced even more. With this technology a fillet weld which requires much higher positioning accuracy than overlap weld is possible. The pathfinding and position detection systems are directly connected to the scanner control unit and therefore the correction of the geometry can be done online in real time. Moreover the gap size can be measured and with this information the user can actively adjust to the new welding situation and prevent a fault. In addition new welding applications with this novel sensor system are shown, e.g. door welding with reduced flange and fillet weldRemote welding is widely used in automotive applications like car seats or doors. This technology is able to increase productivity and quality while reducing cost and weight of the parts.Using pathfinding and detection of the position of the welding sample in a remote welding system flange size can be reduced even more. With this technology a fillet weld which requires much higher positioning accuracy than overlap weld is possible. The pathfinding and position detection systems are directly connected to the scanner control unit and therefore the correction of the geometry can be done online in real time. Moreover the gap size can be measured and with this information the user can actively adjust to the new welding situation and prevent a fault. In addition new welding applications with this novel sensor system are shown, e.g. door welding with reduced flange and fillet weld

Softening of hot formed steel

Hot formed steel is the most successful method in the last few years to reduce weight, save cost and increase safety in the automotive industry. It is still gaining importance for automotive body parts because of its advantageous material properties. Especially its high strength of about 1500MPa makes it possible to manufacture structural parts like B-Pillars.Car manufacturers look into the usage of flaps and cutouts in the design of these parts. Using flaps within the assembly process would be helpful for part-positioning. Cutouts are used to create pull-throughs with smoothed, rounded edges. The high hardness in the press hardened part has to be reduced in those specific areas to fulfill their intended task. Implementing this softening process into the forming and heat treatment is hard. A laser heat treatment process has been developed which is able to reduce hardness in those specific areas of automotive parts. With this process energy is only placed were its needed and hardness can be reduced by more than 50% within seconds with transition zones smaller than 15mm to the non treated material.The softening process can be applied to the parts within the same laser machine system which is used for the cutting process.Hot formed steel is the most successful method in the last few years to reduce weight, save cost and increase safety in the automotive industry. It is still gaining importance for automotive body parts because of its advantageous material properties. Especially its high strength of about 1500MPa makes it possible to manufacture structural parts like B-Pillars.Car manufacturers look into the usage of flaps and cutouts in the design of these parts. Using flaps within the assembly process would be helpful for part-positioning. Cutouts are used to create pull-throughs with smoothed, rounded edges. The high hardness in the press hardened part has to be reduced in those specific areas to fulfill their intended task. Implementing this softening process into the forming and heat treatment is hard. A laser heat treatment process has been developed which is able to reduce hardness in those specific areas of automotive parts. With this process energy is only placed were its needed and hardness can be reduced by more...

Trends and basic investigations in high power laser material processing

Continuous application development in combination with advancements in laser sources and accessories enabled several trends in laser material processing. This paper will show investigations in process understandings based on modern process diagnostics like high speed videos. We will focus on applications related to thick sheet welding, welding with wobbling techniques, hot forming materials and dissimilar materials. Additionally we link the gained process understandings to possibilities how to successfully introduce the knowledge in industrial applications.

Advanced welding techniques with optimized accessories for high brightness 1μm lasers

Within the past couple of years one can see several trends in laser material processing. On the one hand optimized application results necessitate advancements of the equipment such as laser sources, laser light cables and focusing optics. On the other hand the optimization of the application results due to in depth process understanding and optimized accessories is indispensible. This paper will link the increase in process knowledge, especially in high power welding, based on modern process diagnostic methods to an optimization of accessories. We will show an example of the influence of the metal vapor plume on the achieved welding results and present a component allowing more consistent welding results regarding surface quality and penetration depth. The second part of the paper will focus on the latest advances in remote scanner processing.

Advanced welding techniques with optimized accessories for high brightness 1 µm lasers

Within the past couple of years one can see several trends in laser material processing. On the one hand optimized application results necessitate advancements of the equipment as e.g. laser sources, laser light cables and focusing optics. On the other hand the optimization of the application results due to a very good process understanding and optimized accessories is indispensible. This paper will link recent insights into process mechanisms e.g. in high power welding based on high speed video and modern process diagnostic methods to an optimization of accessories. We will show an example on the influence of the metal vapor plume to the achieved welding results and present a device allowing more constant welding results regarding surface quality and penetration depth. The second part of the paper will focus on the latest advances in remote scanner processing and wobbling techniques when processing different joints and in welding of aluminum.Within the past couple of years one can see several trends in laser material processing. On the one hand optimized application results necessitate advancements of the equipment as e.g. laser sources, laser light cables and focusing optics. On the other hand the optimization of the application results due to a very good process understanding and optimized accessories is indispensible. This paper will link recent insights into process mechanisms e.g. in high power welding based on high speed video and modern process diagnostic methods to an optimization of accessories. We will show an example on the influence of the metal vapor plume to the achieved welding results and present a device allowing more constant welding results regarding surface quality and penetration depth. The second part of the paper will focus on the latest advances in remote scanner processing and wobbling techniques when processing different joints and in welding of aluminum.