Numerical and experimental studies of AlSi coating microstructure and its fracture at high temperatures

As AlSi-coated press hardening steel is heated to austenitization temperatures, various Fe$-$Al intermetallic compounds (e.g. FeAl, Fe₂Al₅ etc.) and voids are generated throughout the coating, increasing also the surface roughness. The goal of this study is to investigate the effects of coating surface roughness, voids and intermetallic distribution on AlSi coating fracture during its deformation at elevated temperatures. For this purpose, hot tensile experiments and finite element (FE) analyses are conducted to understand crack initiation and propagation in the coating. The coating$-$substrate FE model is built, taking the realistic distributions of intermetallics, voids and surface profile into account. The FE model is calibrated to experiments and the sensitivity of coating fracture to the distributions of intermetallics, voids and surface profile is analyzed. According to FE simulation results, coating fracture is minimized either by increasing the content of FeAl intermetallic or by reducing the void fraction in AlSi coating. Furthermore, to validate the aforementioned numerical prediction, the heating stage parameters are modified to reproduce coating micro-structure from the FE model. Hot tensile experiments on the samples with modified heating parameters confirm the FE simulation results, showing a similar decline in coating crack density. In conclusion, the AlSi coating fracture during hot tensile deformation depends on its micro-structure, which is mainly generated during the heating stage. Furthermore, the results also suggest that coating$-$substrate FE simulations can be utilized as a tool to achieve a suitable coating micro-structure which minimizes coating fracture.