Achieving excellent properties of laser tailor-welded thin Al-Si coated press-hardened steel joint via controlling dilution rate

Abstract

In laser tailor welded traditional 25 μm thick Al-Si coated press-hardened steel (PHS), a lot of ferrite was easily formed in the weld, so filling with expensive high-alloys wire is usually necessary. In this work, a new thin Al-Si coated PHS with 18 μm thickness was laser tailor welded, and a cheap low alloy wire was attempted to fill into the weld to via control the dilution rate. At a high dilution rate of 94%, the weld contained a ferrite fraction of 38%, resulting in the fracture occurring in the weld during tension and bending. At a medium dilution rate of 85%, the ferrite fraction in the weld decreased to 18%, obtaining excellent tensile and bending properties. The tensile strength and elongation of the joint reached 1528 MPa and 5.6%, respectively, with fracture being in the base metal (BM). The maximum bending load and displacement were 1291 N and 7.0 mm, respectively, and cracks initiated and propagated in the BM. This is attributed to the small amount of ferrite and stress concentration in the narrow weld. At a low dilution rate of 76%, the ferrite reduced to 6%, and the bending cracks initiated and propagated in the BM. However, the tensile specimen fractured in the weld, which should be related to the relatively low C concentration and the stress concentration at the weld toe due to the weld reinforcements.