An Al-8B-2C Master Alloy And Its Application In A356Alloy

Lightweight development requirement has promoted the replacement of rolled steel with hypoeutectic cast aluminum alloy A356, making it the most widely used automobile, motorcycle wheel hub material. To improve the comprehensive mechanical properties of A356alloy, grain refinement has become an indispensable craft in the production of wheel hubs.Because of Si "poisoning" phenomenon to Ti, the widely used Al-5Ti-1B master alloy in industry and Al-Ti-C master alloy could not effectively perform on the grain refinement of A356alloy. Using A1-3B alloy as boron source and graphite powder as carbon source, our research team has developed a highly efficient Al-Ti-B-C master alloy earlier, which shows an excellent grain refining effect on A356alloy. Fluoride salt method is widely adopted in industry to produce A1-3B alloy or A1-5Ti-1B master alloy. This process will release large amount of harmful gas and erode furnace; on the other hand, the poor wettability of graphite with aluminum melt leads to a low C absorption rate and the difficulty in synthesizing TiC.Therefore, a novel kind of boron and carbon source is urgently needed. In this work, an A1-8B-2C master alloy has been developed and directly used to reinforce A356alloy. Besides, it is used as boron and carbon source to prepare Al-Ti-B-C master alloy, which is finally applied in the grain refinement of A356alloy. This method is clean, environmeenvironmentally friendly and tends to be easier for the addition of carbon.In this paper, the microstucture evolution patterns and three-dimensional morphologies of particles in A1-8B-2C master alloy have been systematically studied. The growth mechanism of its main phase Al3BC is revealed; the strengthening effect of this master alloy on A356alloy is studied; the melt reaction mechanism of Al-8B-2C/Al-Ti system is studied, while a new technique to prepare Al-Ti-B-C master which could be applied in the grain refinement of A356alloy is presented.The major research results are as follows:(1) Microstructure of AI-8B-2C master alloy and its evolution patternInfluence of temperature on the microstructure of A1-8B-2C master alloy and its evolution pattern are studied in this article. It is found that Al3BC is the main phase in A1-8B-2C master alloy prepared at800°C and the size is300nm-μm, with the three-dimensional morphology to be tetrakaidecahedron or hexagonal prism;In addition, AlB2phase is also generated and the size is4-15μm with a tetrakaidecahedron morphology. When prepared at1300°C,the morphology of Al3BC remains unchanged with its size slightly increasing to800nm-4μm.As the temperature rises, AlB2transforms to polyhedral β-AIB12phase, whose size is30～60μm.(2) Analysis of the growth mechanism of Al3BCAl3BC has a close-packed hexagonal structure. Since{1011} facets have the highest growth rate, Al3BC crystal tends to form the hexagonal prism (equilibrium crystal shape), with{1010} and{0001}(or{0003}) facets being exposed. However, Al3BC crystal also appears in tetrakaidecahedron (non-equilibrium shape), caused by the melt environment.Since Al3BC is investigated to have the characteristics of small size, low density, high stability and high elastic modulus, A1-8B-2C master alloy which mainly consists of AI3BC phase was applied to strengthen A356alloy. Research results indicate that this master alloy can obviously improve the hardness and tensile strength of A356alloy.(3) Melt reaction mechanism of A1-8B-2C/Al-Ti systemIn A1-8B-2C/Al-Ti system, the reaction of AlB2or soluble B atoms with Al-Ti melt leads to integral hexagonal or dendritic TiB2particles, while the reaction of Al3BC particles with Al-Ti melt leads to the formation of macroporous lamellar TiB2and TiC particles. Various reaction mechanisms result in different particle shapes.During the solid-liquid reaction of Al3BC with Al-Ti melts, Ti atoms diffuse to the surface of Al3BC.As a result, TiB2and TiC fragments are produced encompassing Al3BC and grow bidirectionally as the reaction proceeds. Since the size of Al3BC is less than2μm, TiB2and TiC grow with great deficits in B and C. Therefore, TiB2and TiC tend to develop into particles with irregular fringe and incomplete or macroporous centers. Besides, some TiB2or TiC particles are produced simultaneously and grow with mutual restriction, which is another reason why their morphologies are macroporous.Further, as B and C elements consist in one compound, TiC and TiB2are prone to be doped mutually in the reacting process, which engenders TiC to grow from polyhedral morphologies to lamellas.(4) Application of Al-8B-2C master alloy in A356alloyBased on the melt reaction mechanism of Al-8B-2C/Al-Ti system, a new technique to prepare Al-Ti-B-C master alloy by using Al-8B-2C master alloy as boron and carbon sources is put forward, which is environmentally friendly, simple and can fabricate a master alloy of high cleanliness. The amount of potent nucleants for a-Al in this Al-5Ti-0.8B-0.2C master alloy is numerous and their distribution is dispersive. The grain refining effect of this master alloy on A356alloy is better than that of Al-5Ti-1B.