Study On Microstructures And Mechanical Properties Of Dissimilar Thickness High Strength Steel HC660/DP780 Laser Welded Joints

The important means to achieve lightweight, save energy and improve safety are adopting multi-materials structure and high strength steels. The automobile structure parts are designed with multi-materials structure, which are welded high strength steels in different thickness and different strength grade by laser, and then are plastic formed. Based on structure characteristics and process of the manufacturing, The dissimilar thickness high strength steel HC660/DP780 welded joints were investigated. this paper carried out the dissimilar thickness high strength steel HC660/DP780 welded research. In the research, the weld thermal cycles, microstructures and properties of joints were analysised. At the same time, effects of process parameters on weld, and the mechanical properties of welded joints were also reviealed.The results showed that HC660 weld thermal cycles peak temperatures were measured to be 1229℃, 806℃ and 463℃, and DP780 side weld thermal cycles peak temperatures were 1295℃, 849℃ and 488℃. Compared with the different substrates, the weld thermal cycles were different on HC660 and DP780 welded joint. According to the microstructure observation, HC660/DP780 laser welded joint was composed of weld beam, heat affected zone(HAZ) and base metal. The weld beam constituted by coarse martensite and ferrite. The microstructure of HAZ was uneven, the grain size of which was larger close to the weld beam. Compared with DP780, the HAZ width of the HC660 side was higher. As for the mechanical properties of joints, it was observed that the average microhardnesses of weld beam, HAZ and base metal of the HC660 side were 419.8 HV, 389.5 HV and 33.96 HV. The average microhardnesses of weld beam, HAZ and base metal of DP780 side were 427.9HV, 333.18 HV and 290.9 HV. Both of HC660/DP780 laser welded HAZ occurred softening, and softening rate of HC660 and DP780 was 0.90 and 0.73, respectively. The tensile strength of the HC660/DP780 joint was 997.5MPa and elongation was 7.8%. The junction of HC660 and its HAZ occurred to neck during the tensile testing, and the fracture surface revealed a large number of dimples. During the cupping test, the value of laser welded joints of similar steels HC660/HC660, DP780/DP780 and dissimilar steels HC660/DP780 were 10.33 mm, 8.88 mm and 9.35 mm, respectively. Crack of HC660/HC660 joint was started from the HAZ expansion along direction paralleled to the weld beam. The crack of DP780/DP780 joint was started in the weld beam, and then extended along the direction which was vertical to the weld beam. Differently from the similar joints, the crack of HC660/DP780 joint generated from HAZ of HC660, developed along the direction which was paralleled to the HAZ of HC660 side, and then vertical weld extension to DP780 substrate.The effects of welding parameters on the microstructures and properties of HC660/DP780 laser welded joint were studied. The results showed that the width of weld beam was increased with increasing of laser power, and fracture position of static tensile offset from HC660 to the softening region of HAZ. The width of weld beam and size of martensite was decreased with increasing of welding speed. Fracture position of static tensile offset was from the softening region to HC660. The window of process parameters of dissimilar thickness high strength steel HC660/DP780 laser welded was narrower, and the optimum laser welded parameters laser power and welding speed was 1.2k W and 2000mm/min, respectively, with defocusing distance of-0.5mm.