Microstructure Characterization and Mechanical Properties of Laser Welded Super High Strength QP980 Automotive Steel

Abstract

The Ultrahigh strength Q&P automotive steel, i.e. QP980, has a broad application prospect in lightweight due to its high strength and good plasticity. In this study, the range of heat input (30 ~ 40 J/mm) was selected by controlling laser power in laser welding of QP980 in order to investigate the microstructure and properties of welded joint. At the heat input of 30 ~ 40 J/mm, the joint of QP980 had acceptable penetration. The weld widths were 417.93 mm, 582.02 mm and 521.56 mm, respectively. The macroscopic morphology of the joint is hourglass type. The microhardness of the welded joint hardening zone is higher than that of the base metal, and the maximum value is 519 HV0.5. When the heat input is 35 J/mm, the tensile strength of the welded joint is 1109 MPa. The maximum joint factor is 91.88 %. The fracture is close to the base metal. A large number of dimples are observed on the fracture surface, implying as ductile fracture. Based on the EBSD results, the proportion of low angle grain boundary was consistent with mechanical properties. A large number of deformation twins are formed in the 35 J/mm sample through deformation, which has a great contribution to the strength of the weld.