Processing And Characterization Of Short Fiber-reinforced Aluminium Matrix Composites

With rapid development of automobile industry, the demand for material with excellent mechanical and physical properties has increased. Metal matrix composites (MMCs) with good physical and chemical properties which can be designed, has come to the fore quickly. The conventional technology for MMCs is squeeze casting. In addition to the good results obtained with this technique, there are some unfavorable factors in squeeze casting limit the development of MMCs. For example, the procedure is pushing molten metal into preheated preform with high pressure, which need the higher quality of preform and put forward hard requirement to infiltration apparatus, moreover process complex and high cost. In order to overcome these problems and broaden the application in the automotive field, in this work, the method that MMCs were fabricated by infiltrating the liquid metal into the preform by low pressure infiltration was used.Liquid infiltration technology consists of reinforcement preform fabrication and molten metal was pushed into a preheated preform with pressure. The high-quality of preform is a foundation of high-quality MMCs. In this work, the fabrication processing of preform was studied systematically, The process parameters of short fiber preform which was shaped by wet forming were designed and filtrated through orthogonal test way, the microstructure of preform was observed by SEM and XRD. The results show that the void and fibers distributed uniformly and the optimum technology parameters of preparing the preform was obtained: mass ratio of fiber/filler of 9:1, mass ratio of fiber/binder of 1:10, 1050℃sintering temperature and 2.5h soaking time, and the preform can be used to infiltration experiment.First of all, the facility was designed and built for gas pressure infiltration of ceramic performs with molten metal. Secondly, the preparation process, the infiltration process, mechanical properties and microstructure of MMCs were studied. The critical pressure, pre-hearted temperature of preform, the infiltration temperature and holding time have important effect on microstructure and interface of MMCs. They were achieved through combination of theoretical and experimental way, which are 0.5MPa gas pressure and keeping time 1min, 800℃infiltration temperature and 1h holding time. Thirdly, according to temperature field variation of infiltration process, the results that liquid molten flow pattern have influence on fabrication of composites were analyzed.Strengthening mechanism of short fiber reinforced MMCs were investigated. The defects of Al₂O_3sf/Al composites fabrication by gas pressure infiltration were observed and analyzed carefully by XRD, SEM, EDAX. The results indicated that the main defect of the composites which have most important effect on the mechanical properties is not interface reaction between matrix alloys and reinforcement, but the void and micro-pore in the composites, the short fibers dispersed in-homogeneously and Mg2Si produced from matrix alloys in the process, so the composites with better mechanical properties can be gained by adjusting preparation technology.