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

将Al-Ti和Al-Ti- b合金粉末经冷压和热挤压加工而成的粉末冶金材料用作铝铸件的晶粒细化剂。在Al-Ti-B合金中，将Ti的组成相转变为金属Ti、TiAl_3和TiB_2，将B的组成相转变为金属B、TiB_2和AlB_2，探讨了添加Ti和B后铝铸件晶粒细化的机理。检查了铸锭的宏观组织，评价了晶粒细化程度。当直接加入机械合金粉作为晶粒细化剂时，由于机械合金粉存在氧化损失，其晶粒细化效果不如添加相同成分的P/M材料。P/M Al-TiAl_3合金作为晶粒细化剂比常规Al-Ti母合金更有效，这是由于P/M材料中TiAl_3弥散更细。含TiB_2的P/M Al-Ti-B材料的成核效果较差，因此，TiB_2不是铝的有效成核剂。在添加晶粒细化剂后，熔体保温时间短、保温温度低的条件下，含TiAl_3和AlB_2的粉末冶金材料效果最好。然而，那些含有金属Ti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分散较好的P/M材料在高温下的强度高于工业铝合金。TiAl_3和TiAl在机械合金化过程中均呈非晶态，但在热挤压加热后均呈结晶态。

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.