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.
Microstructure of Joints Made in High-Strength Steels Using the Robotic Laser Beam Welding Process
