Study On Refinement Mechanism Of Al-Ti-B And Al-Ti-C Master Alloys In Aluminum And Its Alloys

Al-5Ti-1B and Al-5Ti-0.2C are the most commonly used master alloys inaluminum alloys, and they have sound refinement effect on aluminum alloyswhich has been confirmed by lots of application facts. But the refinementmechanism remains to be further studied. The refinement mechanismsproposed usually can only explain some phenomena during refinement, butthose mechanisms sometimes are lack of objective proofs, even there aresometimes contrary to each another. The grain nucleation and growthprocesses under different conditions were investigated. The nucleation andgrowth mechanism, the influences of alloy element and the pouringtemperature were analyzed after the master alloy added into the melt.High purity aluminum was refined by two master alloys, respectively.The grain morphology and the concentration gradient of Ti—“circles” whichwere in grains, as well as TiB2and TiC particles acting as nucleation sustrateswere observed by SEM from the refined samples which were electrolyticpolished and deep etched. The nucleation and growth mechanism wasanalyzed and the result indicated that TiB2and TiC particles were in favor ofthe aggregation of Ti and reduced the nucleation energy here. As temperationdorped the nuclei of α-Al which were rich in Ti formed and growed on TiB2and TiC particles which formed “circles” after solidification. And thenucleation and growth of the rest melt will happen on “circles”. The shapes of“circles” around TiB2and TiC particles were different but it had no effect ongrain size and morphology of high purity aluminum after refinement.Aluminum alloys with different alloy elements were refined by the twomaster alloys, respectively. The refinement mechanism with the addition ofalloy elements was investigated. The result indicated that alloy elementsaffected the grain nucleation and growth mechanism by factors such asunder-cooling, growth limiting factor, the divorced eutectic phase and over-heating, of which the divorced eutectic phase was the most importantfactor. The shapes of “circles” had significent effect on grain morphologywhen alloy elements were added.Commercial purity aluminum was refined by the two master alloys, theeffect of pouring temperature during refinement on the grain nucleation andgrowth mechanism was investigated. The result indicated that the pouringtemperature can influence the grain nucleation and growth by effect thequantity of the TiB2and TiC particles participated in the nucleation and thetime of solidification.