Tensile and fatigue properties of fiber laser welded high strength low alloy and DP980 dual-phase steel joints

The study was aimed at evaluating the microstructure and mechanical properties of high-speed fiber laser welded high strength low alloy (HSLA) and DP980 dual-phase steel joints with varying weld geometries. Fusion zone (FZ) consisted of martensitic structure, and heat-affected zone (HAZ) contained newly-formed martensite in both steels and partially tempered martensite in DP980. While HAZ-softening was present in DP980, it was absent in HSLA. A distinctive “suspension bridge”-like hardness profile with the FZ hardness as a “pylon” appeared in the fiber laser welded joints. Both HSLA and DP980 joints showed a superior tensile strength, with a joint efficiency of 94–96% and 96–97%, respectively, despite a reduced elongation in DP980 joints. Fatigue strength was higher in DP980 joints than in HSLA joints at higher stress amplitudes, but had no obvious difference at lower stress amplitudes. DP980 multiple linear welds exhibited a larger scatter and lower fatigue strength. Fatigue failure of HSLA joints occurred in the base metal at a stress amplitude above 250MPa, and at weld concavity at a lower stress amplitude below 250MPa. Fatigue crack in DP980 joints initiated predominantly from the weld concavity at both high and low levels of stress amplitudes.