Laser Beam Welding of New High Strength Steels for Auto Body Construction

With the regard to the development of modern car bodies the focus lies on low production costs, environmental sustainability and high security standards. In order to meet these requirements the weight of modern car bodies has to be reduced consistently. Amongst other things, this becomes possible by the use of new high and ultra high strength steels. These materials are characterised by their high strength, good ductility and a high absorption capacity. In addition they have a lower density in comparison to other steels. TRIP and TWIP steel belong to these high and ultra high strength steels as well as iron-manganese steel. The development of new materials also puts new demands on the joining technologies used for producing semi finished products and parts of car bodies. Due to its high flexibility, its good automation and the minor influence on the work piece, laser beam welding is an established procedure in the automotive series production. The high cooling rates in combination with a carbon equivalent of the new materials which is usually higher then 0.4% lead to a martensitic solidification of the weld seam. Martensite is characterized by its low ductility and thus affects the forming capability as well as the absorption capacity of the welded parts. In order to avoid this effect a new process has been developed within the scope of the collaborative research program 362 (SFB 362, 1993-2005) at the Laser Zentrum Hannover. Using that process the weld seam structure is inductively annealed directly after the welding process. Experiments with high strength steel like TRIP700 and H320LA have shown that the tempering leads to an increase of ductility. The process is suitable for butt joints and overlap joints and is to be transferred into industrial usage within the scope of the project “Laser Beam Welding of Car Body Parts Made of High and Ultra High Strength Steel”. Based on the results obtained in the SFB 362 continuous investigations will be made in order to qualify the process for boron alloyed steel and iron-manganese steel.