Application of Mechanically Alloyed Al-Ti-B P/M Materials to Grain Refining of Aluminum Castings

将机械合金化粉末经冷挤压和热挤压工艺制备的Al-Ti和Al-Ti-B合金粉末冶金材料用作铝铸件的晶粒细化剂。在Al-Ti-B合金中，Ti的组成相为金属Ti、TiAl_3和TiB_2，B的组成相为金属B、TiB_2和AlB_2，以探讨Ti和B的加入细化铝合金晶粒的机理。将晶粒细化剂加入到99.9%的铝熔体中，并浇铸到金属型和隔热型中。结果表明，直接加入机械合金化粉末作为晶粒细化剂时，由于氧化损失，晶粒细化效果不如加入相同成分的粉末冶金材料。粉末冶金Al-TiAl_3合金比普通Al-Ti母合金具有更好的晶粒细化效果，这是由于粉末冶金材料中的TiAl_3弥散分布较细。含TiB_2的粉末冶金Al-Ti-B材料的形核效果较差，TiB_2不是铝的有效形核剂。添加晶粒细化剂后，在熔体保温时间短、保温温度较低的条件下，含TiAl_3和AlB_2的粉末冶金材料的效果最好。然而，含有金属Ti和B的那些与常规Al-Ti-B合金硬化剂一样有效。与工业铝合金相比，TiB_2或TiC弥散分布的粉末冶金材料具有更高的高温强度。TiAl_3和TiAl经机械合金化后均为非晶，但经热挤压加热后均为晶化。

P/M materials of Al-Ti and Al-Ti-B alloys, which were processed from mechanically alloyed powders by cold pressing and hot extrusion, were used as grain refiner for aluminum castings. In Al-Ti-B alloys, constituent phase of Ti was varied as metallic Ti, TiAl_3 and TiB_2, and that of B as metallic B, TiB_2 and AlB_2, in order to discuss the mechanisms of grain refining in aluminum castings by the addition of Ti and B. Grain refiner was added to 99.9% aluminum melt,and cast into both metal mold and thermally insulated mold. The macro structures of the ingots were examined and the extent of grain refinement was evaluated.When mechanically alloyed powder was directly added as grain refiner, attained grain refining was not as much as in case of addition of P/M material of the same composition, due to oxidation loss occurred in the former. The P/M Al-TiAl_3 alloy is more effective as the grain refiner than conventional Al-Ti mother alloy, due to finer TiAl_3 dispersion in P/M material. The P/M Al-Ti-B materials containing TiB_2 are less effective, and thus, TiB_2 is not an effective nucleant for aluminum. P/M materials containing TiAl_3 and AlB_2 are most effective under the condition of short time and lower temperature holding of the melt after addition of grain refiner. However, those containing metallic Ti and B are as effective as conventional Al-Ti-B alloy hardners. P/M materials with fine dispersion of TiB_2 or TiC showed higher strength at high temperatures than commercial aluminum alloys. Both TiAl_3 and TiAl were found to become amorphous aer mechanical alloying, but they became crystalized after heating for hot extrusion.