DISSIMILAR STEELS LASER WELDING: EXPERIMENTAL AND NUMERICAL ASSESSMENT OF WELD MIXING

摘要

Upcoming strict CO_2 regulations leads car manufacturers to look for mass savings solutions. The use of advanced high strength steels (AHSS) solutions enable to optimize both crash performances and mass saving. Particularly, the use of Laser Welded Blanks made of dissimilar high strength steels is an efficient weight optimisation solution. To support the joining of AHSS in car body design, a 3D model of heat transfer, turbulent flow and transport of species in the laser weld pool has been developed. It aims at providing a better understanding of diffusive-convective mixing in the weld, and its influence on the weld mechanical properties. The presented model allows predicting the weld geometry and the element distribution. To validate the model, experimental tests were carried out. Welding of two dissimilar steels with different laser beam offset from joint line was performed. Numerical and experimental investigations of dissimilar butt laser welding between high Mn (TWIP) and Dual Phase (DP) steels were carried out. The cross sections of the welds were characterized by SEM with EDX elemental analysis. Quantitative mappings of Mn distribution in the melted zone offer an overview of mixing intensity. The results of the simulation have been found in good agreement with the experimental data. To go further, and to assess the effect of weld mixing on mechanical performances, tensile tests were done. It was found that tensile behaviour of the welds is determined by level of Mn and C dilutions. For attaining maximal joint performances, it is necessary to comprehend the elements distribution in the melted zone and to be able to control it through an accurate choice of operational parameters.