Laser hybrid welding of tailored tubes with integrated quality analysis and closed loop process control

In car-body construction, the use of high strength steel grades and adapted lightweight construction technologies is increasing steadily. Therefore, more and more Tailored Blanks and other laser welded parts are used. Further development can lead to a combination of the "Tailoring"-idea with hydroforming processes to so called "Tailored Tubes". As joining process, laser beam welding becomes more evident because of its low thermal impact on the workpieces and the high production speed. As the beam interaction zone in the laser focus has a diameter below 0.6 mm, the positioning of the sheet-edges is very important. Additionally, small gap size is required because of the low gap bridging ability of the small laser induced melting pool. The requirements of exact positioning and small gap size is met by expensive clamping technology. Specially for small series and prototyping, the expenses for adapted clamping usually inhibits the use of laser welding technology. The gap bridging ability of laser welding can be increased by the use of filler material. For joining, powder and mostly wire is used. The use of powder is limited to those applications where the required filler metal composition can not be delivered as wire. As the powder efficiency is low, an important contamination of the machine results. Therefore, cored wires can be used. Filler wire is usually used as cold wire. An alternative is a conductive preheating of the wire by electric current. In both cases, the expensive laser power has to melt the filler wire and the welding speed is comparatively low. If the wire is molten in an electric arc process, the laser power is used for deep welding only. Laser-GMA hybrid welding enables higher welding speed or a deeper penetration at constant of even lower laser power. In addition, heat input is low and prevents thermal influences on the base materials. The productivity is mainly influenced by the welding speed and the investment cost. The mechanical properties of the welded tube can be improved by a narrow seam. For high strength steel grades, a controllable heat input gives further improvement of the formability due to decreased microhardness. This leads to the objective of this project which is a welding technology for small series and prototyping of lightweight construction parts such as tubes and tailored tubes as well as linear and non-linear tailored blanks. As the clamping technology has to be less costly, the zero-gap necessary for laser welding can not be assured. Therefore, the laser-GMA-hybrid welding process will be developed for zinc coated sheet metal for higher gap bridging. The properties of both laser and hybrid welded parts have to be compared in order to assure the replaceability between both processes. The properties of the welded seams will be investigated concerning weld defects, metallurgical modifications, strength and formability as well as corrosion resistance. Literature study at the beginning of the project has shown no previous work on hybrid welding of zinc coated steel. All previous investigations on steel were focused on thick plates where the coating was removed prior to welding. One source was found during the work proving the limited process stability when welding zinc coated sheet metal. As the assurance of stable process results is essential for the use of a process, an automatic process monitoring system was build for laser-GMA-hybrid welding. Additionally, a control method was found that can stabilise the process in case of process failures. In this project, the laser-GMA hybrid welding process was developed and qualified for zinc coated sheet metal. Compared to uncoated sheets, the process window is significantly smaller and the achievable welding speed is lower. The combination of process monitoring technology commonly used for the single processes (laser and GMA) allows an automated quality monitoring and the detection of instabilities. With the temporal decrease of welding speed, a met