Process Research And Optimization Of Refining Primary Silicon By Melt Mixing Treatment In Al-20wt.%Si Alloy

Hypereutectic Al-Si alloy, which works as a kind of wear resistant materials, has a series of good advantages, such as low density, good wear resistance and low coefficient of linear expansion. But its mechanical property, machining property and applications are restricted because of the coarse size and shape edges of primary silicon in microstructure. It has been the target to refine primary silicon and improve its morphology and distribution for a long time in the material field. Owing to no addition of modification and no environment pollution, melt mixing treatment has been taken highly attention. But the effect of this treatment is not satisfying, for example, the primary silicon in Al-20Si alloy usually is refined to30-40um without other treatment, such as modification and rapid cooling. In Al-Si alloy, the higher Si content is, the better wear resistance and craft properties are, the more difficult primary refines, Al-20wt.%Si alloy was selected in this paper. Primary silicon is refined and ameliorated by researching and optimizing melt mixing treatment, and a very significant progress is obtained which the primary silicon is refined to less than18μm. What’s more, shape edges of primary silicon are rounded and its distribution is uniform.Based on melt mixing treatment, which high temperature melt was prepared upon the critical temperature of liquid-liquid transition and low temperature melt was prepared at appropriate temperature in semi-solid zone, the effect and change law of parameter (including mixing ratio, the temperature and holding time of low melt temperature, the holding time of mixing melts) are systematic explored in this paper. The size of primary silicon and morphology are observed under optical microscope and three-dimensional morphology is detected by SEM. The average size of primary silicon is counted by Image pro plus soft, and the uniformity is measured by standard deviation σ.The main achievements and conclusions of this paper are as follows:1. Overall, compared to the traditional process, a significant refining effect on primary silicon is obtain by melt mixing method and the morphology, spatial distribution, size uniformity are also improved obviously.2. The low temperature is very important to the refining effect by melt mixing method. Research results show that the refining effect of primary silicon when the low temperature melt in the two phase zone is better than the low temperature melt above the liquidus temperature. Moreover, selecting appropriate temperature ranges is also very important even if the low temperature melt is in the two phase region, and its temperature is too high or too low will weaken the refinement of primary silicon. For example, the refining effect of primary silicon is optimal when the low temperature melt is about660℃under the mixing ratio of1:1.3. In terms of the influence and the law of specific process parameters, the holding time of low temperature melt under the setting temperature, the holding time of mixed melt under the setting temperature and the mixing ratio have obvious effect on the refining, distribution and morphology of primary silicon. The general rules are:(1) With the extension of holding time of low temperature melt under the setting temperature, the best holding time of mixed melt under the setting temperature is also postponed.(2) When other conditions are remain constant, the effects and relationships between refining primary silicon and the holding time of mixed melt is different when changing the mixing rate of high temperature melt and low temperature melt, and3:1is the worst mixing rate. The refining effect declined gradually with the extension of holing time of mixed melt. When the mixing rate is1:1and1:2, the refining effect is increased firstly and then decreased gradually with the extension of holing time of mixed melt. The holding time is the longest when the mixing rate is1:1, and the corresponding effect is also the best.(3) Statistics show that the smaller the size of primary silicon in solidification structure, the smaller the value of standard variance, namely, the better the uniformity of the silicon particle. At the same time, the uniformity of the spatial distribution of primary silicon and the effect of morphology improvement is best when all process parameters are in optimal conditions.(4)In addition to refining the primary silicon and improving morphology, the eutectic silicon changes from coarse sheet to curved and fine flake with more branches, accompanying by a certain percentage of shot strip and block.4. In terms of the growth morphology of primary silicon, octahedron, polyhedron and long platy shape of primary silicon appears in the untreated samples which shows typical faceted growth and reflects strong anisotropy in the growth process. But after melt mixing treatment, the morphology of primary silicon turns to near spherical or edge obtuse polyhedron which reflects the weak of anisotropy. 5. About the mechanism of refining primary silicon by melt mixing treatment, the author argue that the primary silicon was refined obvious in this paper and which benefits from two aspects. On one hand, the structure of high temperature melt after L-LST is advantageous to the refinement of silicon crystal. On the other hand, silicon dendrite in the semi-solid melt is decomposed for a large number dispersive crystallite in the mixing process, and which can become the homogeneous nucleation of primary silicon in solidification. The influence and optimization of many process parameters can safeguard these homogeneous microcrystalline will not reduce or disappear, and won’t too bulky or not disperse uniform.6. According to the characters of the space stereoscopic morphology of primary silicon, the improvement of morphology is caused by the change of growth pattern. The typical faceted growth with strong anisotropy turns to multiple directions with weak anisotropy, and the possible mechanisms are highly developed screw dislocation steps and concave angle twin.