Refinement of dendrite arm spacings in aluminum ingots through heat flow control

Abstract

A model for heat flow during solidification of alloys is presented which treats the heat of fusion released during solidification separately for three distinct regions of a casting: portions released isothermally at the liquidus temperature, between the liquidus and solidus in a specified manner and the remainder released at the solidus. The model is solved numerically by a finite difference technique for unidirectional and two-dimensional heat flow in end-chilled thin plates. Effects of heat transfer coefficient at the chill, superheat, heat input, liquid convection and amount of sidewise heat loss are considered. Results are presented in terms of position of liquidus and solidus isotherms as a function of time, width of the mushy zone and local solidification time and secondary dendrite arm spacingvs distance from the chill. Results from experimental castings made under controlled heat flow conditions are compared with computer calculations. The local solidification time and resultant dendrite arm spacing are shown to decrease at a given location as a) the chill heat transfer coefficient increases, b) superheat increases, c) the gradient of temperature at the solidification front increases, and d) the multidimensionality of the heat flow path increases.