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The fatigue resistance and failure behaviour of penetration 1.5 + 0.8-P and non-penetration 0.8 + 1.5-N laser welded lap joints prepared with 0.8 mm and 1.5 mm cold-rolled 301L plates were investigated. The weld beads showed a solidification microstructure of primary ferrite with good thermal cracking resistance, and their hardness was lower than that of the plates. The 1.5 + 0.8-P joint exhibited a better resistance to high-cycle fatigue failure, while the 0.8 + 1.5-N joint showed a higher resistance to low-cycle fracture. The failure modes of 0.8 + 1.5-N and 1.5 + 0.8-P joints were 1.5 mm and 0.8 mm lower lap plate fracture, respectively, and the primary cracks were initiated at welding fusion lines on the lap surface. There were long plastic ribs on the penetration plate fracture, but not on the non-penetration plate fracture. The fatigue resistance stress of the penetration and non-penetration plates in the crack initiation areas calculated based on the mean fatigue limits is 408 MPa and 326 MPa, respectively. The main reason for the difference in fatigue performance between the two laser welded joints was that the asymmetrical heating in the non-penetration plate thickness resulted in higher residual stress near the welding fusion line.

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