The effect of strontium modification on casting/chill interfacial heat flux and casting surface profile during solidification of Al-7Si alloy

The effect of Sr modification of Al-7%Si alloy on casting/chill interfacial heat flux transients was investigated. The alloy was solidified against polished surfaces of copper, brass and cast iron chills. The heat flux across the casting/chill interface was estimated using inverse modeling technique. On modification melt treatment, the peak heat flux increased by about 70%, 24% and 22% for copper, brass and cast iron chills respectively. The effect of Sr modification on casting surface texture was analyzed using a surface profilometer. The surface profile of the casting and the chill surfaces clearly indicated the formation of anair gap at the periphery of the casting. The arithmetic average value of the profile departure from the mean line, within a sampling length(Ra) and arithmetical mean of the absolute departures of the waviness profile from the centre line within a sampling length(Wa) were reduced by 50% on modification. The enhancement in heat transfer achieved due to modification was attributed to the decrease in gap width formed at the interface. The width of the gap formed for the unmodified and modified casting surfaces at the periphery were found to be about 35μm and 19μm respectively. The gap width was used to determine the variation of heat transfer coefficient (HTC) acrossthe chill surface after the formation of stable solid shell.It was found that the HTC decreased along the radial direction.