Numerical Simulation Of Solidification Structure Formation Of TiAl Alloy Ingots

The grain structure formation and evolution during solidification processes of Ti-(45~48)at%Al alloy ingots are simulated, based on a finite differential method (FDM) for macroscopic field calculation and a cellular automaton technique (CA) for microscopic modeling of grain nucleation and growth. A new stochastic model is developed by considering the peritectic growth kinetics for calculating the evolution of solid fraction of a cell. The shrinkage cavity formation during solidification is taken into account for the first time. Based on the CA cell, the conventional method is modified to release the constraint on the selection of FDM mesh size and therefore, the shrinkage formation is coupled with grain structure formation in a real sense. A moving allocation technique (MA), characterized as simple-programe-making, is designed to minimize the computation time and memory size associated with a large number of cells. And by the MA technique, in the present study, 66.7 percent of CPU time is saved at best. A columnar-to-equiaxed transition (CET) criterion is developed for the case of solidification of ingots.By means of the above-mentioned mathematical models and numerical techniques, the influences of processing parameters, such as alloy composition, mould materials, mould-preheating temperature, superheat, size of ingot and heat transfer coefficient at the metal/mould interface, on the CET, are presented and systematically investigated. A larger and finer equiaxed region is formed by increasing the alloy composition. Compared to the solidification of Ti-(45~48)at%Al binary alloys, with additions of the third and the forth elements, the refined equiaxed grains are more easily obtained for Ti-(45~48)at%Al-2Cr-2Nb alloy systems. Decreasing the thermal conductivity of metal mould yields a larger equiaxed zone and the extent of variation is different depending on the selections of heat transfer coefficient at metal/mould interface and the size of ingot. For casting in the sand mould, a coarser equiaxed structure is easily generated. The higher the mould temperature, the larger the equiaxed zone accompanied by a coarser grain size. A larger equiaxed region is favored by a lower melt superheat. However, changing superheat in a special range has a negligible influence on grain structure formation. The ingot with a larger section...