| In order to satisfy the energy saving, environmental protection and security principles of new generation cars, the advanced high-strength steels (AHSS) have been widely used in automobile industry. Resistance spot welding is the primary joining method in automobile manufacturing. In general, the spot weld undergoes fatigue damage during the service period, thus, it is of significant importance to investigate the fatigue performance of spot welds for achieving a safe and reliable design. The materials used in this study were DP780, DP980, TRIP780, HS and Q&P980 steels. The micro structure change and micro-hardness distribution of spot welds for the five materials were observed; then, the fatigue performance and fatigue fracture modes of tensile-shear and cross-tension specimens were investigated; finally, the factors influence the fatigue properties of spot welds were discussed.Owing to the formation of dense martensite in the nugget, the hardness value for nugget is the highest. The hardness values of nugget for HS, Q&P980, TRIP780, DP980 and DP780 spot welds are 515 HV,497 HV,477 HV,393 HV and 385 HV, respectively. For Q&P980, DP980 and DP780 spot welds, the occurrence of "soft zone" was resulted from the martensite tempering in sub-critical heat affected zone. As for HS spot weld, the formation of ferrite and bainite phase in inter-critical heat affected zone and martensite tempering in sub-critical heat affected zone is the main reason for the appearance of "soft zone".The fatigue performances of DP780, TRIP780 and Q&P980 spot welds with similar geometry are approximately the same for both tensile-shear and cross-tension specimens. The fatigue failure modes involve pullout, and fracture along the circumference or along the direction of width for tensile-shear specimens. As for cross-tension specimens, the main fracture mode is pullout. It was found that the fatigue cracks mainly initiated at the interface between two sheets, and propagated through the thickness in heat affected zone. For pullout failure mode, the fatigue cracks propagated through the thickness of sheets up to fracture. As for fracture along the circumference or along the direction of width, the fatigue cracks firstly penetrated through the thickness of sheets, and then the through-thickness cracks could be considered as the secondary fatigue crack source, which grew into the base material along the direction of width up to the final fracture.For the same material spot welds with different sheet thickness, it is noted that the fatigue property of spot welds with thick sheets is better than that of spot welds with thin sheets. For the same material spot welds with same sheet thickness, it is found that fatigue strength of spot weld do not increases as the nugget size increases. Choosing a suitable nugget size (5t0.5) can achieve the optimum fatigue property for spot welds. It is also observed that the fatigue strength for 1.2 mm thick spot welds is influenced by the properties of base material. However, it is detected that fatigue strength for 1.6 mm thick spot welds is independent of base material. The rules found in this study can provide guidance for the selection of materials, which are used in automobile spot welding. |
