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

以机械合金化粉末为原料，采用冷压和热挤压工艺制备Al-Ti和Al-Ti-B合金粉末冶金材料，作为铝铸件的细化剂。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，以探讨添加Ti和B对铝铸件的细化作用机理。直接加入机械合金化粉末作为细化剂，由于存在氧化损失，细化效果不如添加相同成分的粉末冶金材料时的细化效果。粉末冶金Al-TiAl_3合金由于在粉末冶金材料中弥散分布着更细小的TiAl_3而比传统的Al-Ti母合金具有更好的细化效果。含TiB_2的粉末冶金Al-Ti-B材料效果较差，因此TiB_2不是铝的有效形核剂。含有TiAl_3和AlB_2的粉末冶金材料在加入细化剂后的熔体保温时间短、温度较低的条件下效果最好。然而，含有金属钛和B的硬化剂与传统的Al-Ti-B合金硬化剂一样有效。TiB_2或TiC弥散细小的粉末冶金材料比商用铝合金具有更高的高温强度。在机械合金化过程中，TiAl_3和TiAl均为非晶态，但热挤压加热后，TiAl_3和TiAl发生晶化。

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.