Post-weld cold working for fatigue strength improvement of resistance spot welded joint of advanced high-strength steel

[Display omitted] •Post-weld cold working generates large compressive stress in DP980 spot welded joint.•The fatigue strength of DP980 joint is doubled after PWCW experiment with 10 mm tool.•The effect of PWCW experiment is enhanced continuously with a larger tool diameter. Advanced high-strength steel (AHSS) has been widely used for lightweight body-in-white. However, the fatigue strength of welded joints is not improved even employed AHSS. In this research, the advanced post-weld cold working (PWCW) experiment was proposed to improve the fatigue strength of DP980 spot welded joints. X-ray diffraction (XRD) method was employed to measure the welding residual stress. A numerical model was established to analyze the PWCW induced residual stress as the first try. The influence of the diameter of the cylindric tool used in PWCW on the residual stress of welded joints was also discussed in detail. The hardness distribution on weld joint was increased after PWCW, which represented the plastic strain induced work hardening in DP980 steel. The predicted PWCW induced residual stress had a reasonable agreement with XRD measurement. The PWCW introduced a compressive stress in the weld nugget and its heat affected zone with a value of more than -400 MPa at the edge of joined zone. The strengthened material and compressive stress were beneficial to suppress the generation and propagation of fatigue crack, thereby enhance the fatigue strength of welded joints. Besides, the effect of PWCW continuously strengthened with a larger diameter of tool, especially when the tool diameter was larger than 10 mm. The fatigue strength was almost doubled compared with the as-welded condition. The smaller tool provided a critical requirement for precise positioning. The operation of PWCW was also simple enough to realize the automation, which indicated its potential in manufacturing automotive units.