Effect Of Rare Earth Sc On Microstructure And Mechanical Properties Of A356 Alloy

In this paper, the effect of rare earth Sc on microstructure and mechanical properties of A356 alloy was studied. By adding different amounts of rare earth Sc to the A356 alloy for modification, modification mechanism of rare earth Sc on the primary α-Al phase and eutectic Si of A356 alloy was researched. The mechanism of Sc on improving the mechanical properties of A356 alloy was investigated. The main results and conclusions are as follows:1. Through adding 0.1-0.3wt% rare earth Sc to the A356 alloy, the effect of rare earth Sc on the primary α-Al phase of A356 alloy was studied. The results show that the primary α-Al phase of A356 is changed from coarse dendrite without modification to fine dendrite, equiaxed grain when adding 0.2wt%Sc, the size of a-Al grain is significantly reduced. When the Sc content is 0.3 wt%, the shape factor of the primary α-Al phase is increased from 0.49 without modification to 0.8, the shape of the grains become gradually rounded. The reason why rare earth Sc can refine a-Al grain is that A356 alloy added rare earth Sc generates Al3Sc, which form coherent relationship with the α-Al matrix, it can be used as heterogeneous core of a-Al phase, refining of α-Al grains.2. The influence of rare earth Sc on eutectic Si of A356 alloy was studied by adding 0.1-0.3wt% rare earth Sc to A356 alloy. The results show that the addition of rare earth Sc makes the morphology of eutectic Si significantly change, which changes from needle flake without modification to fibrous, worm-like when adding 0.2wt% Sc. DSC thermal analysis shows that eutectic temperature is decreased when the addition of rare earth Sc to A356 alloy, the eutectic temperature of A356 alloy added 0.1% Sc and 0.2% Sc decreases from 590.2℃ without modification to 587.8 ℃,584.5 ℃ respectively. The addition of rare earth Sc increases eutectic undercooling, which improves the eutectic nucleation rate. While the larger undercooling degree inhibites the growth of the eutectic Si. The rare earth Sc is enriched in the eutectic Si solid liquid interface at the time of eutectic Si nucleation, changing the way of growth mode of eutectic Si.3. The impact of rare earth Sc on the mechanical properties of as-cast A356 alloy was studied. The results show that when adding 0.2wt% rare earth Sc, the tensile strength of A356 alloy is increased from 170.43MPa without modification to 210.58MPa, elongation is rose from 4.9% to 6.5%, hardness is increased from 58HB to 67.5HB. Continue to improve the Sc content to 0.3wt%, the mechanical properties of A356 alloy begins to decline. A356 alloy with 0.2wt% Sc has the best mechanical properties.4. The effect of heat treatment on microstructure and mechanical properties of A356 alloy was studied. The results show that A356 alloy containing 0.2wt%Sc after solution and aging heat treatment, the morphology of eutectic Si is changed from as-cast fibrous tissue into granules. A356 alloy with 0.2wt% Sc after solution and aging heat treatment, the tensile strength is increased from as-cast 210.58MPa to 292.21MPa, the elongation is increased from 6.5% to 7.1%. A356 alloy with 0.3wt%Sc after heat treatment, the tensile strength and elongation are both improved compared with the as-cast By analysis:the A356 alloy containing rare earth Sc after solution aging heat treatment, the microstructure precipitates some Mg2Si, AbSc and other strengthening phase, eutectic Si becomes passive and sphere, which improves the mechanical properties of A356 alloy.5. The tensile fracture morphology of A356 alloy after heat treatment was observed and studied. It is found that A356 alloy added rare earth Sc, the fracture appear some ductile fracture characteristics, in the tensile fracture surface appear a great number of tough nest morphology, show its good plasticity. While A356 alloy without modification, the tensile fracture presents brittle fracture characteristics, and has a large number of rivers cleavage plane.