Research On Softening And Properties Of DP1000 Steel Laser Welded Joints

DP1000 steel is one of the important lightweight materials for automotive lightweight technology.The laser welding process,the microstructures and properties of DP1000 steel welded joints were systematically researched in this paper to provide a theoretical basis and reference for its applications in lightweight automobile.This paper focused on a parametric study of DP1000 steel laser welding butt joint to obtain the responding DP1000 steel laser welding process window.The width of the weld bead and softening zone decreased with decreasing laser heat input at the beginning and then increased with further decreasing laser heat input.Mechanical properties also had the same trend.This is related to the change of laser welding mode from deep penetration welding to heat conduction welding with the reduction of laser heat input.The microstructure and properties of the DP1000 steel welded joints were systematic analyzed.The softening phenomenon was observed in the welded joints.The softening area was located in the tempering zone and incomplete phase transformation zone of heat affected zone(HAZ),and the most serious zone was located in the incomplete phase transformation zone.The existence of softening zone leads to the dramatically decreasing of overall uniformity and the plastic of the welded joint.A thermal simulator was used to simulate the thermal cycle of weld heat affected zone based on the measured thermal cycle curve in welding process.The thermal simulated samples have been analyzed in order to research the softening mechanism of the laser welded joints of DP1000 steel.The reasons leading to softening are divided into two types: One was the carbide precipitation in the martensite and the dislocation density in original microstructure decreased in martensite tempering temperature range.Another was the volume percent of the martensite in incomplete phase transformation zone reduced obviously under the welding thermal cycle so that the hardness of this zone is significantly decreased and lead to softening.Tensile test,Erichsen cupping test and fatigue test were conducted to DP1000 steel laser welded joints.The corresponding results show that the tensile strength of welded joints falls slightly while the plasticity,formability and fatigue strength fall significantly.In this research,tensile strength of the welded joints were 86% ~ 91% of the base metal,elongation was 20% ~ 38% of base metal and Erichsen cupping index was 53.5% ~ 77% of base metal.The fatigue strength of welded joints under the optimized welding parameters was only 45.5% of base metal.The decrease of the tensile strength,plastic properties and fatigue strength was caused by the inhomogeneity of microstructure and mechanical properties caused by softening.An enhanced cooling condition in the process of laser welding could help to solve the HAZ softening problem.The residual stress of specimens were also analyzed in this paper through the keyhole test method and finite element simulation.The result showed that,whether the residual stress was transverse or longitudinal,its maximum value appeared in the tempering zone of heat affected zone which was about 4 mm from the weld center.