Microstructure evolution and dynamic mechanical behavior of laser welded dissimilar joint between QP1180 and TRIP780

The present work investigates dissimilar fiber laser joints between QP1180 and TRIP780 steels. The scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD) characterizations were conducted to study the detailed microstructures of each region of the joints. The result showed that a large volume fraction of tempered martensite (TM) is responsible for the softened zone in the inter-critical heat-affected zone at the QP1180 side of the joints. The digital image correlation (DIC) technique demonstrated that the softened zone triggered by TM in the heat-affected zone (HAZ) at the QP1180 side has little effect on the tensile mechanical properties because of the higher hardness than at the TRIP780 side. The dimples on the fracture surface and higher elongation of the tensile samples show good ductility, which is due to the TRIP effect. The formability of dissimilar weld joints was characterized by the Erichsen cupping test. The Erichsen values varied at different punch pole locations due to the softening zone and non-uniform microstructure on the two sides of HAZ.