Effect of heat input on microstructure and properties of dissimilar laser welded joints between TWIP and TRIP steels

Abstract

In this present, the experimental steels were produced by different rolling and heat treatment processes. After that, dissimilar laser tailor welding was carried out with different heat inputs (40, 60 and 100 J/mm2) using IPG YLR 6000 fiber laser with Ar gas shielding. Hitachi SU5000 scanning electron microscope (SEM) and Zeiss Axio Vert.Al optical microscope were used to observe the microstructure of the welded joints after being polished and etched with 4% nital. HV-1000 microhardness tester was used to measure the microhardness of welded joints. Tensile test was carried out using DNS-100 universal material testing equipment, and Erichsen test was conducted using CTM604 Erichsen test machine. Five groups of repeated experiments were selected for the performance test. Tensile fracture surface morphology of welded joint was observed by SEM. The effect of heat input on microstructure and properties of dissimilar welded joints between high manganese TWIP steels and low manganese TRIP steels was studied. The results showed that weld width and penetration were increased and complete penetration was obtained when the heat input increased to 60 J/mm2. With heat input increased, the microstructure of welded seam was more inhomogeneous. The distribution of Mn content in welded seam was inhomogeneous such that columnar/equiaxed austenite was formed in rich Mn zone and lath martensite was formed in barren Mn zone. Heat input had no influence on the microstructure and hardness on the HAZ. The mechanical properties of welded joints of 60 J/mm2 were superior and its strength-ductility balance was greater than TRIP BM (23 GPa%) and was 82% of TWIP BM (36 GPa%). The formability of 60 and 100 J/mm2 was similar.