Modeling crack initiation in Al-Si coating during heating/quenching phase of hot stamping process

In hot-stamping processes, Al-Si coating is generally applied on the steel substrate to avoid decarburization and to enhance corrosion resistance of the hot-stamped parts. However, during hot stamping, the AlSi coating fractures due to thermal and mechanical loads. This deteriorates the surface quality of the stamped parts, increasing tool wear and friction between the stamping tool and coated sheet metal. These cracks are generally initiated during the heating and/or quenching phase due to phase transformations and thermal loads. The initiation of the cracks in the coating can be largely influenced by the evolution of coating microstructure, i.e. intermetallic compounds- FexAly, each of which has different thermal and mechanical properties. These intermetallic compounds are formed during the heating phase and grow in a natural order of increasing iron content in the layers close to the substrate-coating interface.

The goal of this study is to investigate the initiation of cracks in the coating during quenching stage due to thermal loads only. Heat treatment experiments are conducted on the Al-Si coated hot-stamping steel at different austenitization temperatures, dwell times and cooling rates. The distribution of voids/micro-cracks and intermetallic compounds in the coating are examined via digital microscopy and SEM/EDX measurements, respectively. A thermal-structural finite-element model is built to predict the crack initiation in Al-Si coating during quenching; the model accounts for the spatial distribution and mechanical properties of different intermetallic compounds. The results show large strain localization around the voids due to thermal loads during quenching, leading to micro-cracks towards the surface.