Microstructure And Properties Of Laser Welded Joints Between Duplex Steel And Austenitic Stainless Steels

Due to the obvious differences between dissimilar steels in the aspects of microstructure, chemical composition, materials properties and so on, the final performance of dissimilar steel welded joint was worse than the welded joint formed by same steel. But the welded joints of dissimilar steel could not be avoided in the automobile manufacturing industry. In this paper, duplex steel DP590 and austenitic stainless steel 304 were used to study the microstructure and the properties of three laser welded joints(DP590-DP590, 304-304, DP590-304). The main research contents and conclusions are as follows:(1) The influence law of laser process(average power, pulse width, pulse frequency,defocus) on the quality of welded joints(surface morphology and cross-sectional dimension) were researched. The results show that: when the other laser parameters were set constant, the weld penetration, weld width and splashing increased along with the increased average power and pulse width. The increase of pulse frequency led to the increase of weld width and the decrease of splashing. The weld width and splashing decreased with the increased focus. Laser parameters had an effect on the quality of welded joints mainly by changing the laser energy, heat accumulation and laser peak power density.(2) The rules of microstructure evolution of three different laser welded joints were researched. The results show that: the microstructure in the weld area of DP590-DP590 and DP590-304 were martensite and that in 304-304 was columnar and equiaxed austenite.The place where was close to the DP590 of DP590-DP590 and DP590-304 welded joints had an obvious heat-affected zone and the microstructure were martensite and ferrite. The place where was close to the 304 of 304-304 and DP590-304 welded joints had no obvious heat-affected zone and demonstrated a melted line.(3) The mechanical properties of three different laser welded joints were studied. Theresults show that: the hardness of DP590-DP590 welded seam and DP590-304 welded seam were higher than the base metal and heat affected zone, in these two joints the hardness of the base material of DP590 was lower, which led to the position of stretched failure appearing in the base material of DP590, the fracture had obvious necking and it was ductile fracture. The hardness of 304-304 welded seam was slightly higher than that in the base metal and the position of stretched failure appeared in the welded seam. The tensile deformation was no necking, which was ductile fracture.The disappearance of twins significantly affect the mechanical properties of 304-304 welded joint. Compared with the base material, the elongation of the three different welded joints were reduced.(4) The formability of three laser welded joints was researched. The results show that:there were two kinds of cupping fracture locations on DP590-DP590 laser welded joint and the cupping value could be amounted to 83.4% than that of the base material. The cupping fracture location was perpendicular to the welded zone for 304-304 laser welded joint and the cupping value could be amounted to the 95.9% than that of the base material. The cupping fracture location was perpendicular to the weld for DP590-304 laser welded joint and the cupping value was below to DP590-DP590 and 304-304 laser welded joints. The highest cupping value of DP590-304 can be amounted to 73.18% than DP590 base material and up to 61.53% than 304 base material.(5) The fatigue fracture of three laser welded joints was researched. The results show that: The fatigue limit of DP590-DP590 was 400 MPa, 304-304 was 340 MPa, DP590-304 was 380MPa. The fatigue fracture of three laser welded joints was all located in the welded zone and the crack sources also were pores and depressions.