Fatigue behaviour of coated sheets butt joints welded by laser process

Abstract

The fatigue strength and the fracture mechanism studies of butt laser welded joints in A155-Zn coated steel sheets, 0.62 and 1.16 mm thick, have been continued. The 2 x 10⁶ cycles fatigue strength for both thicknesses has been finally established in 130 and 175 MPa, respectively, and the big ferritic grain - 300 μm - produced by symmetrical solidification from the fusion zone centre has been confirmed. Initiation and extension fracture zones are analysed by SEM and so are the involved microstructures. The first ones are concerned with high stress concentration zones, mainly undercuts or lack of penetration, from which the fatigue cracks extend in semiellipses, longitudinally and through thickness, along the fusion line or the HAZ. The final fracture is produced by tearing of the residual fatigue crack bound and crack deviation towards the central gross grain line of the welds.