Investigation On Fatigue Behaviors Of Spot Weld DP980-GMW2 And Q&P980-GMW2 Steels

The Paris motor show in October 2010 reflects that the development direction of future car is environmental protection, and energy saving, Vehicle weight reduction is one of the important ways to reduce fuel consumption. In car with boards, to reduce the body weight, the extensive employing of steels with high strength and high plasticity has been inevitable. DP (Dual Phase) steel has good molding and stamping performance, has been widely used in automobile manufacturing. Q&P steel combines the advantages of Trip steel, DP steel and Martensite steel. High strength and high plasticity, in recent years have attracted the wide attention and research of car manufacturing.This study focuses on BaoShan steel DP980, GMW2 and Q&P980 steel solder joints reliability. Experimental samples include DP980-GMW2 and Q&P 980-GMW2 tensile-shear and cross-tension types. Firstly, the microstructure change and micro-hardness distribution of spot weld for the DP980-GMW2 and Q&P 980-GMW2 are observed; Then, the fatigue performance and fatigue fracture modes of tensile-shear and cross-tension specimens are investigated; Finally, the fatigue performance of the same materials and different materials are compared.Owing to the formation of dense martensite in the nugget, the hardness value for nugget is the highest. The soft zone occurred near the heat affect zone of DP980 and Q&P980 side. The occurrence of soft zone is resulted from the martensite tempering, and the softening ratio of DP980 is higher than Q&P980 side.It is found that the fatigue performances of Q&P980-GMW2 tensile-shear specimens are higher than DP980-GMW2 tensile-shear specimens. The fatigue performances of DP980-GMW2 cross-tensile specimens are higher than Q&P980-GMW2 cross-tensile specimens. As for the tensile-shear samples, the fatigue failure modes involve pullout, fracture along the circumference or along the direction of width. As for the cross-tension specimens, the failure modes involve pullout and along the direction of width. As the GMW2 side is thinner, both tensile-shear and cross-tension specimens fractured on it.It is found that the fatigue performances of DP980-GMW2 and DP980-DP980 samples are approximately the same for both tensile-shear and cross-tension specimens; The nugget size and sheet thickness influence the fatigue performances. The fatigue performances of Q&P980-Q&P980 is better than Q&P980-GMW2 for tensile-shear specimens, however the fatigue performances of Q&P980-Q&P980 is better than Q&P980-GMW2 for cross-tension specimens. The influence of sheet thickness is more important than nugget size for the fatigue performances of spot weld.