Producing Mg₂Si(Si)/Al Gradient Composites By Centrifugal Casting And Study On Wear Ability

Functionally gradient materials (FGM) are new kind of composites. They are produced by advanced compound technique, the elements (like constitutes, structure and so on) are continuously varied along the thickness direction, so the properties and functions change gradually. The materials attract interests of scholars at home and broad, and become the hot spot of material research. The principle of FGM produced by centrifugal casting is that solid-liquid two phases with different densities are separated in centrifugal field; solid phase is segregated and deposited at a certain location in the sample and the contents are distributed gradually, so FGM are made when liquid is solidified. The key of this method is designing the compositions, which ensure added particles or primary, secondary phase generated at the initial stage of solidification are presented gradient along the radial in the centrifugal field. The component gradient distribution is controlled by centrifugal velocity, the size of particles, the time of produced, temperature and density. It is well known that centrifugal casting has many advantages in producing FGM: simple device, high efficiency and low cost, moreover, it can make FGM with high relative density and large size, and the production is stable and so on. So centrifugal casting is deemed to be the most efficiency method to produce FGM.Due to the reinforced phase of Mg2Si is thermodynamically stable phase produced during solidification process, Mg2Si/Al composites exhibit a simple prepare technology, low cost, uniform distributing reinforced phase, optimized interface and excellent thermodynamically stable systems. In addition, Mg2Si/Al composites have a good machinability in comparison with ceramic particulate reinforced Al composite. Therefore, the Mg2Si/Al composites have high potential and applied prospects as automobile piston-cylinder etc. lightweight component. So it has significance to study on Mg2Si/Al gradient composites.In the present thesis, the microstructure of hypereutectic Al-Si alloy and Mg2Si/Al gradient composites produced by centrifugal casting, and the microstructure when alloying element P added were studied, the growth mechanisms of primary Mg2Si and Si, and the mechanism of P effects on primary phase were discussed. The motion state of second phase is analyzed by studying the effect of mold rotation on gradient sate. The dry sliding wear capability of hypereutectic Al-Si alloy and Mg2Si/Al gradient composites are studied. The major research efforts of the present study are as follows:(1) The centrifugal casting was used to produce Mg2Si/Al functionally gradient composites with Mg2Si, Si particulates reinforcement in both inside and outside wall and alloy matrix in the centre. The primary Si grow in the mode of facets, and the morphology is hexahedron; the primary Mg2Si is mixed interface, under the condition of negative temperature gradient, primary Mg2Si grows in the way of dendrite, but at the surface of dendrite crystal have facets characteristic.(2) According to phase diagram and probative experiment, the solidification process of Mg2Si/Al gradient composites is as follows: the concretionary approach of (Al-20Mg2Si)-9Si:L→L1+Mg2Sip→L2+Mg2Sip+(Al+Mg2Si)e→Mg2Sip+(Al+Mg2Si)e+(Al+Mg2Si+Si)e;the concretionary approach of (Al-20Mg2Si)-14Si: L→L1 + Sip→L2 + Sip +Mg2Sip→L3+Sip+Mg2Sip+(Si+ Mg2Si)e→Sip+Mg2Sip +(Si+Mg2Si)e+(Al+Mg2Si+Si)e. It is found that the secondary Mg2Si of (Al-20Mg2Si)-14Si can attach to the surface of primary Si, nucleating and growing in the way of semi-coherent growth.(3) The modification of Al-Si alloys by adding P is refining primary phase, refining primary Si from 100μm to 20μm.The influence of P on Mg2Si/Al mainly is refining the dendrite morphology of primary Mg2Si to little polygon .(4) The force condition along radial direction of the reinforcements in the centrifugal field is analyzed theoretically, and found that the displacement of the reinforcements changes exponentially with increasing time; the relation between the motion and the time is deduced, which provides theoretical support for producing gradient composites by centrifugal casting. With the increasing rotate speed of mold, the microstructure of gradient composites becomes much compacter, gradient distribution is more obvious, but the bubbles distribution is more wide and thinner. At the experiment condition, 1000r/min is the best rotate speed.(5) The results of dry sliding wear tests show that: under the condition of variable load and rotate speed, the wear capacity of Mg2Si/Al gradient composites are better than hypereutectic Al-Si alloy; the wear capacity of modified composites are better than unmodified ones, because the fined, dense and uniformly distributed primary Mg2Si reinforced phase could restrain the occurring of plastic deformation, enhancing the ability of plastic flow resistance; so the wearability of modified and high rotate seed produced composites are more better. Due to metal intersolubility, Mg2Si/Al gradient comoposites wearabilities are much better than HT200, when they wear against 45 steel.