Mechanical behavior and microstructure of a fiber laser–welded TWIP steel

TWIP (twinning-induced plasticity) steels emerge as promising materials for the automotive industry, non-magnetic applications, and oil-and-gas exploration. The aims of this paper is to study fiber laser welding of TWIP steel thin sheets from mechanical behavior and microstructural points of view. The samples were laser-welded with speeds of 50, 100, and 150 mm/s keeping the power constant at 1200 W. These three conditions resulted in weld bead 0.2 to 0.4 mm wide without cracks or pores. According to the analyses, the austenite (FCC) dendritic growth occurred from the fusion line to the center of the weld where an equiaxed grain region is observed. The fine equiaxed grains gave rise to a hardness peak compared to surrounding regions of the fusion and heat-affected zones in accordance with the Hall-Petch effect. The heat-affected zone presented a drop in hardness due to the annealing of the cold rolled base material. The tensile shear strength of the laser-welded coupons ranged between 6300 and 10,500 N, which is above the standard limit of 4.5 kN established for electric resistance spot welds. The formability analysis, using Erichsen tests, has shown high ductility with the Erichsen Index of approximately 10 for 0.7 tons-force load.