Abstract
Summary Research continues on the fatigue strength and fracture mechanisms of laser welded butt joints in A155-Zn coated steel sheets of 0.62 and 1.16 mm thickness. The 2 × 106 cycle fatigue strength for both thicknesses have finally been established at 130 and 175 MPa, respectively. The large ferritic grain (300 μm) produced by solidification symmetrical to the fusion zone centre has been confirmed. Fracture initiation and propagation zones, as well as the microstructures involved, were analysed by a scanning electron microscope (SEM). The former are concerned with high stress concentration zones, mainly undercuts or lack of penetration, from which the fatigue cracks extend in semi-ellipses, longitudinally and through sheet thickness, along the fusion line or the heat affected zone (HAZ). The final fracture is produced by tearing the residual fatigue crack bond and deviation towards the centreline of the weld gross grain.
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