Cracking and interfacial debonding of the Al-Si coating in hot stamping of pre-coated boron steel

This study is focused on the mechanisms of cracks initiation, propagation and interfacial debonding of the Al-Si coating in hot stamping of the pre-coated boron steel. The investigation was performed isothermally at three deformation temperatures (700, 750, 800 °C) at a strain rate of 0.1/s. Cracking and interfacial debonding of the coating were observed with optical and scanning electron microscope, to reveal the damage evolution under applied tensile strains. Microstructures and phase inside the coating before and after austenitization were determined by energy dispersive spectroscopy and X-ray diffraction. The results indicate that austenitization led to micro-cracks and Kirkendall voids initiation inside the Al-Si coating because of thermal loading, and the cracks were arrested by α-Fe diffusion layer. When the coating on substrate system was submitted to the uniaxial tensile test, the surface coating exhibited multiple cracking normal to the tensile direction. The Kirkendall voids seemed to promote the macro-crack growth through the diffusion layer. The macro-cracks followed a Mode I path, leading to the coating deteriorates to cracked segments. The macro-cracks then continued to propagate following a Mode II path that along the diffusion layer/substrate interface because of shear stress transferred from the deformed substrate, resulting in the interfacial debonding of the coating segments. The crack density firstly increased with the increasing tensile strain and then reached saturation. Decreasing deformation temperature caused an increase in the crack density visibly. Furthermore, the coating cracking correlated to the Fe-Al intermetallic compounds in it.