Influence of martensite content on tension-compression fatigue life of spot-weld joints of dual-phase steels

The fatigue performance of spot-weld joints is an important factor to determine the safety and reliability of the car body. The welded joints of DP780, DP980 and DP1180 were inspected. Load-life curves of the welded joints were obtained by the fatigue test on fatigue test machine. The relationship between microstructure, micro-hardness distribution of spot welds and fatigue properties of spot-weld joints was analyzed. And then, the fatigue failure of solder joint was analyzed. The results showed that spot-weld joints fatigue performance was affected by base material properties. The higher martensite content, the better fatigue property. All of the three fatigue failure modes were fracture along the circumference. It was found that the fatigue cracks mainly initiated at the heat affected zone between the fusion zone and the base material, and firstly penetrated through the sheets along thickness. Finally, 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 fracture of weld joints.