Laser welding of high strength steels

The usage of high strength steels is of increasing interest in the automobile industry for tailor welded blanks, as is the application of lasers in place of resistance spot welding for construction purposes. Considerable experience has already been gained in the use of laser welding, but there is relatively little information available concerning the welding of high strength materials such as HSLA and DP600 steels. In order to utilise these materials in tailor welded blanks it is necessary to investigate the influence of welding on these steels to answer questions such as how the increase in hardness for DP600 affects the formability? The research presented in this paper deals with the microstructural development of the weld metal and the HAZ. Results to date indicate that a martensitic structure is formed in the weld pool and grain boundary cementite in the heat affected zone in DP600, leading to a decrease in formability and impact resistance for these materials. Also included in this investigation is the laser weldability of HSLA steel. It has been found that the hardness increase is low suggesting that no significant problems are to be expected during forming of the laser welded tailored blanks, even though the formed weld of the HSLA microstructure contains martensite. It can be concluded that DP600 steel cannot be used without taking special precautions to reduce the hardness (DP600) and to find process variables or steel compositions that can minimise the loss of formability for these materials.