Effect of Al-Si coating on laser spot welding of press hardened steel and process improvement with annular stirring

Laser spot welding method was employed to fabricate Al-Si coated press-hardened steel (PHS) joints in overlapped configuration. Al and Si segregate at the notch root of a laser spot weld of Al-Si coated PHS resulting in pre-mature cracking and weakening of the joint strength. The thickness-oriented delta-ferrite strips induced by the Al and Si segregation was mainly responsible for the mechanical deterioration though the dispersive delta-ferrite in the fusion zone also contributes. A supplemental annular stirring with beam oscillation is proposed to break up the segregation. With the annular stirring, the lap-shear peak load and energy absorption increased 57% and 80% respectively, which is attributed to the removal of delta-ferrite strips and the reduction in the fraction of dispersive delta-ferrite. Thermo-fluid numerical simulations suggest that the annular stirring promoted the melt flow and extended the duration of the melt pool, which accelerated and prolonged the diffusion of Al and Si atoms into the fusion zone, thus preventing the segregation of Al and Si and formation of delta-ferrite strips. Meanwhile, the prolonged melt flow pulled additional dissolved Al onto the molten pool surface forming the Al oxide film, resulting in further reduction of Al content and fraction of dispersive d-ferrite. (c) 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).