Inferring the surface roughness of Al-Si coated 22MnB5 steel using an in situ laser speckle characterization technique

Abstract

Hot stamping of aluminium-silicon (Al-Si) coated 22MnB5 steel blanks is widely used in the automotive industry to produce light yet crashworthy parts. However, the coating melts at ~577°C and transforms into a rough intermetallic layer as iron from the base steel diffuses towards the surface. The blank surface roughness impacts the radiative properties during heating as well as weldability, paint adhesiveness, and cooling rate during forming and quenching. This study pioneers the use of laser speckle patterns, caused by the constructive and destructive interference of collimated light reflected off the blanks, to infer the evolving surface roughness of Al-Si coated steel coupons in situ. The results reveal a significant increase in surface roughness once intermetallic compounds reach the surface and that higher furnace set-points produce rougher parts.