Solidification of metal foams
Phase transformations during solidification, foam evolution during solidification studies in-situ using X-ray radioscopy
In this study, foaming is performed inside X-ray transparent furnaces and foam evolution is continuously monitored using X-ray radioscopy technique. The main focus is to study the expansion stage observed during solidification of metal foams. This expansion stage has been referred as solidification expansion or SE. Measurements are carried out by varying alloy composition, holding time (HT), cooling rate, gas atmosphere and pressure in order to understand the mechanisms responsible for SE.
In binary and ternary alloys EE stage can be subdivided into two parts (SE1 and SE2). When HT increases, SE1 initially increases and then decreases to zero. In contrast, SE2 is observed for all HT.
While SE1 is mainly caused by the production of hydrogen from blowing agent, SE2 is a result of precipitation of dissolved hydrogen from the metallic melt during solidification. Cooling rate has significance influence on SE1: increases with decreasing cooling rate. The kinetic interplay of shrinkage of the gas present inside the foam, the out-diffusion of hydrogen to the surrounding space and the production of hydrogen from the blowing agent is the reason of SE. When SE has high value, it induces defects in the foam structure which is caused by SE-induced ruptures during solidification.