Influence of heat-pretreatments on the microstructural and mechanical properties of galfan-coated metal bonds

In the present study, heat-treatment assisted bonding of galfan-coated low-carbon steel sheets was investigated. Steel sheets were bonded by cold rolling subsequently to a heat treatment in the temperature range from 400 °C to 550°C. The reduction ratio during cold rolling was varied in the range from 50% to 80%. Such high reduction ratios were achieved by splitting the bonding process into three stages. By employing heat-treatments, the mechanical properties of the bonds were improved. The heat-pretreatment allowed the formation of brittle intermetallic phases that were easily fractured in the rolling gap during the bonding process. Thus, juvenile non-oxidized surfaces were formed, which facilitated the bonding between the steel layers, and thus increased the bond strength. The intermetallic phases were actively formed at temperatures of 450 °C and above; however increasing temperatures resulted in decreasing mechanical properties due to oxidation processes. The local microstructure was analyzed by scanning electron microscopy in order to characterize the contact zone on the micro level with a focus on the formation of intermetallic phases. The mechanical properties were determined in tensile shear tests. Interestingly, it was found that the galfan coating allowed for bonding at room temperature, and the aluminum fraction was primarily responsible for the enhanced oxide formation during the heat-pretreatment.