Study On Microstructures And Mechanical Properties Of Automotive High Stength Steel DP780 Rsw Joints

In this thesis, the DP780 welding problems of the narrow welding lobe, the serious electrode wear and the softening heat affected zone(HAZ) were easy to occur. The micro-structure and mechanical properties of welded joints were studied by resistance spots welding(RSW) test, tensile shear(TS) test and cross tension(CT) test, with a dissimilar thickness of 1.6 mm and 2 mm DP780 specimens.The results showed that the micro-structure of the fusion zone in the dual phase steel 780(DP780) RSW joints was the martensite and ferrite. The grain was coarse if more close to the center region,and the main micro-structure was strip martensite, near the critical region of the HAZ was small crystal needle martensite and ferrite. With the increasing of welding current, the size of nugget increased, the martensite structure in the fusion zone was coarse, the HAZ decreased and the grain structure of the HAZ became coarse. When the electrode force increased, the size of the fusion zone became wide, the size of martensite in the fusion zone had no obvious transformation, the HAZ decreased and the grain structure became fine. With the increasing of welding time, the size of the welding nugget and the width of the HAZ had no obvious exchangment,but the grain structure of the fusion zone was wider.The micro-hardness distribution of dissimilar thickness DP780 RSW joints was that micro-hardness of the fusion zone and heat affected zone were similar,the average micro-hardness was 400 HV, the micro-hardness of the base material was the lowest,and the average micro-hardness was 320. However,the micro-hardness of the HAZ reduced sharply, and the softening phenomenon was widespread, the micro-hardness was lower than 250 HV. With the welding current and electrode force increasing, the micro-hardness of the fusion zone increased, and the welding time was not relevant about the welding joints.RSW joints had two failure modes, which were interfacial fracture and nugget pullout. The interfacial fracture often occured in the fusion zone. The DP780 RSW joints of the dissimilar thickness dual phase steel occured to offset to the 2 mm side, and the indentation rate, the penetration rate, the weld penetration rate and the degree of electrode wear on both sides of the 1.6 mm side and 2.0mm side were different. The failure energy of the tensile shear test was more than the result of cross tension test. The optimal resistance spot welding process parameters were as follows: the welding current was 12 k A,the welding time was 10 cycle,the electrode force was 3.5 k N. In this RSW process parameters,the maximum tensile-shear stress of the joint was 32.70 k N and the maximum cross-tension stress was 19.51 k N. In the end, through the research of dissimilar thickness DP780, accumulated experiences and provided reference for further research.