Dendrite coarsening during directional solidification of Al–Cu–Mn alloys

Abstract

Steady state directional solidification of Al–3.75%Cu–1.5%Mn and Al–1%Cu–1.5%Mn dendritic monocrystals was interrupted for different lengths of time prior to quenching and dendrite coarsening kinetics were evaluated versus temperature. For a given alloy composition, the isothermal coarsening rate, expressed as the time variation of the normalized specific dendrite surface area, increased with temperature and decreased with time. It also decreased with increasing copper concentration. The secondary dendrite arm spacing increased with time and for shorter times, with decreasing temperature. For Al–3.75%Cu–1.5%Mn alloy this trend was reversed for times exceeding about 1.5 min. For all times the rate of increase of the spacing increased with temperature. Secondary dendrite arm spacing increased with time during solidification and with decreasing growth rate. The dependence of specific surface area on time was predicted reasonably well by combining a coarsening model which is valid for the earlier stages of coarsening with a model which is valid for later stages. Copper diffusion through the liquid was found to be the rate controlling step in dendrite coarsening. Experimental measurements of secondary arm spacing during isothermal coarsening, as well as during solidification agreed reasonably well with model predictions.