Comparisons of the Effects of Air and Helium on Heat Transfer at the Metal-Mold Interface

The effect of helium on heat transfer at the metal-mold interface was studied using a unique casting apparatus. Temperature, as well as displacement, measurements were carried out. The heat-transfer coefficient (HTC) at the metal-mold interface was deduced using an inverse heat-transfer procedure. Comparisons of heat-transfer characteristics were made between aluminum-based alloys solidified with and without helium at the metal-mold interface. These comparisons indicated that the onset of the metal-mold gap developed faster under helium conditions than without helium. The time decrease for the onset of the metal-mold gap with and without helium conditions can be as large as 34 pct. In addition, the average heat-transfer coefficient, from the beginning of the casting until the onset of the metal-mold gap, was higher by as much as 48 pct in the castings using helium compared to castings without the use of helium. The impact of helium on heat transfer relative to air was found to be largest with higher mold roughness. Equations were developed from the onset of the metal-mold gap, which relate the HTC against the size of the gap. These equations had the form \( h\, = \,\frac{1} {{a*G\, + \,b}}\, + \,c \). It was also found that at the onset of metal-mold gap, the rate of the heat-transfer coefficient decrease is higher at the early stages of gap development than at the later stages of gap formation.