A Comprehensive Assessment of Al-Si Coating Growth at Various Heating Rates, Soaking Temperatures, and Times

In conventional hot stamping, an Al-Si-coated blank is first heated above the austenitization temperature and then soaked for a period of time within a furnace, prior to the stamping operation. In this work, the impacts of furnace heating rate, soaking temperature, and soaking time on the Al-Si coating evolution were investigated for two commercial coating weights, 80 and 150 g/m2. These heat treatment parameters during heating and soaking affect the coating microstructure and the thickness of the interdiffusion layer, which affect the properties of the as-formed coatings. The transformation and growth of binary Fe-Al and ternary Fe-Al-Si intermetallic layers were characterized and quantified for soak times up to 240 s. The results show that the effect of the heating rate on the Al-Si intermetallic distribution and ternary phase morphology was more severe than the soaking time and soaking temperature. The Fe2Al5 (η) phase was the dominant layer at the beginning of the soaking stage with a Fe3Al2Si3 (τ1) layer formed within it, and then the Fe3Al2Si3 layer transformed into FeAl (β2) as the soaking time increased due to the interdiffusion of Fe and Al. The transformation of Fe3Al2Si3 to FeAl occurred at a higher rate for elevated soaking temperatures due to the greater diffusivity of Al and Fe. The interdiffusion layer (IDL) consisted of FeAl,Fe3Al(β1) and α−Fe. Higher soaking temperatures of 1000 °C resulted in a thicker IDL for the same soak time when compared with 900 °C and 950 °C, but when the heating rate was lower, the IDL was thicker than that at the higher heating rate since a longer heating time was required to reach the soaking temperature of 900 °C, which prolonged the diffusion time during the heating stage. The findings were similar for AS80.