Unstable aluminum segregation phenomenon in blind keyhole laser welds of Al-Si coated hot stamping steel

Abstract Al-Si coated hot stamping steel is widely used in weight reduction of vehicles, due to its ultra-high strength and good formability. Aluminum coating promotes the formation of aluminum-rich brittle phase segregated in laser welds, such as delta ferrite, or Fe-Al intermetallics, which severely deteriorates the mechanical properties of tailor-welded blanks. A thorough understanding of segregation mechanism is beneficial for the chemical heterogeneity in macro scale. In this work, a three-dimension model is proposed to describe keyhole mode laser welding of coated Al-Si coating. The statistical analysis indicates aluminum element is uniformly distributed at top regions with a average concentration of 20 vol.% and fluid flow time of 25-38 ms whereas several random segregation regions appear at the bottom, and the keyhole opening at the bottom is unstable. The initial element distribution determines the segregation characteristics that coating element mainly comes from the upper surface, circular fluid flowing around the keyhole, but few from the front wall of the keyhole at the partial penetration stage.