Microstructure and fatigue performance of single and multiple linear fiber laser welded DP980 dual-phase steel

The aim of this study was to identify the effect of fiber laser welding (FLW) on the microstructure, hardness, tensile properties and fatigue performance of high-strength (UTS ≥ 980 MPa) DP980 dual-phase steel with single linear and multiple linear joint geometry. While FLWed joints showed tempered martensite at the outer heat-affected zone (HAZ) which caused softening in the welds, a lower extent of martensite tempering and higher hardness value were observed in the narrower HAZ of the FLWed joint than in the wider HAZ of the diode laser welded (DLWed) joint due to a higher power density, faster welding speed, lower heat input, and faster cooling rate. A characteristic “suspension bridge”-like hardness profile with the fusion zone (FZ) hardness as a “pylon” appeared in the FLW due to the formation of almost fully martensitic structure in the FZ. Despite the lower ductility after FLW, a joint efficiency of about 96–97% was achieved with the yield strength essentially unchanged. At higher stress amplitudes, fatigue life of the FLWed joints was equivalent to that of BM, but at lower stress amplitudes no direct improvement in the fatigue resistance was observed due to the presence of soft zone and weld concavity. Multiple linear welds appeared to increase the probability of premature dynamic fatigue failure at lower stress amplitudes as a result of increasing number of weld concavities and soft zones. Fatigue crack initiation occurred from the specimen surface where the weld concavity met with a soft zone, and crack propagation was mainly characterized by the typical fatigue striations along with secondary cracks.