| Rare-earth element is effective to refine the microstructure of the alloy andimprove its mechanical properties. Moreover rare-earth is very useful to modify theeutectic Si of the Al-Si alloy, and the degree of the modification induced by rare-earthelements is different with each other. Sm is one of rare-earth elements withorthorhombic crystal structure, in addition, the price of Sm is not high which will notincrease the cost. However, it has not be confirmed that the addition of Sm canmodify the eutectic Si, in addition, the effect of Sm addition on α-Al andFe-intermetallic is not clearly. Thus, in this paper we study the effect of Sm on α-Al,eutectic Si and Fe-intermetallic by adding Sm into ADC12alloy. The results showthat the addition of Sm can significantly refine the secondary dendrite armspacing(SDAS), and its value decrease with increasing Sm content, as the Sm contentincreases to1.0%, the lowest SDAS (20μm) is obtained. In addition, Sm is effectiveto modify the coarse acicular eutectic Si into a fine particle or short rod structure, andthe degree of the modification is improved with the increasing of the Sm content.Simultaneously, Sm (about1.0wt.%) has marked effects in shortening the length ofneedle-like β-Al5FeSi phase without morphology changing, as the Sm contentincreases to1.0%, the lowest length (18.9μm) is obtained. Furthermore, the tensileproperties were improved by the addition of Sm, with1.0%Sm addition, the tensilestrength and elongation reach220MPa and3.1%, increased by22%and210%,respectively, compared with those of ADC12alloy.Ultrasonic vibration treatment (UVT) is thought as an attractive method toprepare semi-solid alloy while the strong impact coupled with local high temperaturesintroduced by UVT can disperse the reinforcement phases in composite. Thus, in thispaper, semi-solid Mg2Si/AM60composite was prepared by combine in situ methodwith UVT. The effects of UVT on the morphology and the size of the primary α-Mgparticles in semi-solid Mg2Si/AM60composite slurry were investigated. In addition,the apparent viscosity of the semi-solid composite was also measured. The resultsshow that it is more effective in controlling the morphology and size of α-Mg particles when applied just a few degrees above liquidus temperature. A goodsemi-solid slurry with average grain sizes of65m and shape coefficient of0.49could be obtained by applying UVT at622°C for about60s. The morphological ofα-Mg particles is confirmed to evolution into dendrite or rosette when the melt treatedat lower temperature for longer time, this may be due to the intensity of convection inthe slurry is weak at this moment. In addition, the α-Mg particle is becoming finerand rounder with the increasing of UVT power. It should be noted that the Mg2Si andMg17Al12intermetallics are mainly located along the grain boundaries or disperseduniformly in the liquid matrix with network morphology. The microstructure of theliquid matrix also can be identified as a distribution of “smaller grains” together witha network of Mg2Si and Mg17Al12intermetallics.In addition, the apparent viscosity of semi-solid Mg2Si/AM60compositeincreases with the increasing of the volume of reinforcing phase and solid α-Mgparticles. The higher the solid fraction is, the higher the increasing amplitude of theapparent viscosity is. The higher the UVT power, the lower the apparent viscosity.And the reduction amplitude of the apparent viscosity decreases with the increasingof the power is influenced by the solid fraction, the higher the solid fraction is, thehigher the reduction amplitude of the apparent viscosity is. According to theexperimental data, an empirical equation that shows the effect of UVT temperature,power and the volume of reinforcing phase on the apparent viscosity of semi-solidMg2Si/AM60composite was built:... |
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