| The as-cast structure is of prime importance to aluminum foundry alloys since their mechanical properties cannot be further improved by deformation treatments. In aluminum industries, control of alloy composition and structure are normally performed by adding aluminum master alloys into the melt. Al-Ti-B master alloys are one of the most important master alloys in aluminum melt treatments. They can be divided into two categories, according to the Ti:B atomic ratio:one is that with Ti:B>2.2:1, i.e. the excess-Ti master alloys, in which TiAl3 and TiB2 are the dominant intermetallic phases, the other one is that with Ti:B no more than 2.2:1, i.e. the excess-B master alloys, in which the borides (AlB2, AlB12, TiB2) dominate. The latter one is termed as Al(-Ti)-B for short and is deemed as a promising master alloys for treating aluminum foundry alloys.In this work, with an aim to solve several versatile issues in the conventional halide salt route, such as lower B pick-up, less narrow particle size distribution, unexpected particle morphologies and poor alloy quality, a modified halide salt route, flux-covered reaction method was developed. This method makes use of the density difference between the molten salt and Al melt to form a layer of reaction interface, the intermetallic compounds thus formed settle down while the lighter by-product floats up, keeping the reactions proceeding. The flux-covered reaction method helps to resolve the drawbacks in the conventional route. In this thesis, based on the flux-covered reaction method, the grain refinement performances and mechanism of Al-3B, Al-3Ti-3B master alloys, and the strengthening performance and mechanism of Al-5TiB2 master alloy were investigated to sovle the key issues encountered with in the research and application of Al(-Ti)-B master alloys. The main points studied were:(1) A B recovery more than 90% can be achieved by using the flux-covered reaction method. This recovery is some 15% higher than that of the master alloy prepared using conventiaonal stir casting method. The AlB2 particles in the Al-3B master alloy, which was prepared at 850 ℃, holding for 5 min, and poured into a permanent mold, were fine and uniformly distributed in the alloy matrix, with a clear interface. The Al-3B master alloy thus produced was confirmed to have grain refining potency on CPA1 (99.7% Al). This result clarifies the discrepancy regarding the grain refinement of Al-B master alloys on pure aluminum. Also established is a CET model on the basis of fundamentals of solidification theories. This model indicates that the discrepancy in the literature is ascribed to:the particle size distribution, the product quality, the aluminum purity level and the grain refinement test method itself. Furthermore, the effects of binary and ternary aluminum alloy systems on the refining efficiency of the improved Al-3B master alloy were investigated. Si was found to be able to enhance the grain refining potency of Al-B master alloy. It is inferred that there may exists certain interactions between Si and AIB2, which enhances its nucleating potency on a-Al. A well-developed hexagonal AIB2 particle has been detected to locate at the center of an α-Al grain. This does not just evidence the nucleating potency of AlB2 on a-Al, but verifie that AlB2 is such a stable phase in aluminum melt that it may not that easily be decomposed.(2) To overcome that poor efficiency of Al-5Ti-B and Al-3B in grain refining Al-Si alloys, a double-melt pouring method was developed to prepare a novel Al-3Ti-3B master alloy. This method employs separate preparation of Al-6Ti and Al-6B using the flux-covered reaction method, at 800 and 1000 ℃, respectively, and then mixing by pouring into crucible of 800 ℃. The Al-3Ti-3B master alloy thus obtained contains four types of intermetallic compounds:TiAl3, AlB2, AlB12, and TiB2. Making use of the sluggish nature of Al-B peritectic reaction (AlB12 + Al → AlB2), the otherwise unstable TiAl3 phase can be preserved upon prolonged holding after inoculation of the Al-3Ti-3B master alloy. AlB2, as a product of the peritectic reaction, supplies additional nucleating sites for grain refinement. Compared with the improved Al-3B master alloy, the novel Al-3Ti-3B master alloy not only exhibits a higher grain refining efficiency in refining Al-Si alloys, but shows a fading-resistant nature to grain coarsening upon prolonged holding of the melt. This alloy may find wide applications.(3) The flux-covered reaction method was also extended to fabricate Al-5TiB2 master alloy of high quality, with fine TiB2 particle and adequately uniform distribution. The Al-5TiB2 master alloy, was used to prepare in situ TiB2 reinforced AlSi7Mg0.3-xTiB2, AlCu4.5Si1.1-xTiB2 and AlZn6Mg0.5-xTiB2 (x=1,2,3,4) through remelting and diluting. This method helps to achieve a tighter control of the composite composition with respect to the conventional route. The tensile test results show that this process is well-suited to enhance the mechanical properties of AlCu4.5Si1.1, the strength and ductility of which can be both improved upon introducing TiB2 into the matrix. An parameter εr, featured by the agglomeration level of TiB2, was introduced to relate the different strengthening mechanisms with particle agglomerating behavior. The modified mechanism demonstrated that, these improvements in strength, are probably attributed to the uniform distribution of TiB2 throughout the matrix, which facilitates the grain refinement strengthening, Orowan strengthening and CTE strengthening. In AlSi7Mg0.3, the distribution of TiB2 after solidification is not comparable to AlCu4.5Si1.1, so that the yield strength of the composite in as-cast state was merely adequately improved. As for AlZn6Mg0.5, severe agglomeration of TiB2 is noticed. Consequently, the Orowan and CTE strengthening are less prominent and hence the improvement in strength is largely ascribed to grain refinement. |
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