Study On Technology And Properties Of In-situ Particles Reinforced Aluminum Matrix Composites Piston

Aluminum matrix composites as an advanced engineering material are being increasingly used in diesel engine pistons because of their superior properties (high service temperature, good wear resistance, low thermal-expansion and excellent mechanical properties). During the last three decades, to meet the requirements for high-power, high-speed and low energy consumption, short fibers and SiC particulates reinforced aluminum matrix composites are fabricated by squeeze casting for enhancing the whole or local piston performance. But the ceramic reinforcement phase is directly added into its matrix, a series of problems, such as interface reaction between matrix and reinforcement, intricacy fabrication processes, high costs, etc., were existed in these technologies and greatly limited its application. However, in-situ composites involve the production of reinforcements within the matrix during the fabrication process, and they posse clean interface between reinforced phase and matrix, simple fabrication technology, small investment and adaptability to a large-scale industry production. Thus, in-situ particles reinforced aluminum matrix composites piston has attracted a great attention in the academic community and industrial sector.Si and Mg₂Si are very suitable as reinforced phase in aluminum matrix composites due to its low density, high melting point, high hardness, low thermal expanding coefficient (TEC) and high elastic modulus. In this paper, through centrifugally casting Al-18Si and Al-18Si-5Mg alloys, using the difference in density between particles and the molten metal ,we have obtained two kinds of aluminum matrix composites pistons reinforced by in situ primary Si particles alone and primary Si/Mg₂Si particles jointly. The microstructure characteristics, hardness and wear resistance of these two composite pistons were investigated and the motion characteristic of the in-situ particles in the centrifugal field was analyzed. The results of the experiments showed that the primary particles segregated at the regions of the piston top and piston ring grooves under the effect of centrifugal force. And particulate reinforced regions had a higher hardness and better wear resistance compared with the unreinforced regions. The hardness and wear resistance of Al-18Si-4Mg material in the particulate reinforced regions are greatly higher than Al-18Si material and in the unreinforced regions their hardness and wear resistance are close to each other. The results show that mixing enhancement than single particle reinforced. The effects of centrifugal casting process parameters: rotating rate, mold temperature, pouring temperature and gate size on the primary Si and primary Mg₂Si particles locally reinforced aluminum matrix composites piston have been investigated and the internal relationships between shrinkage and gas hole and the technological parameters of centrifugal casting were analyzed. The appropriate technological parameters are that: the rotational speed is 800r/min,point of pouring is 770℃,the preheating temperature of the mold is 400℃,the ingate size is 8mm.The microstructures and mechanical properties of Al-18Si-5Mg material piston after T6 heat treatment were investigated. The results show that the shape of primary Si and primary Mg₂Si particles has changed little, but its angular start passivation. Eutectic Si and eutectic Mg₂Si are changed to global or short bar shape and well distributed inα-Al matrix. The mechanical properties were increased by heat treating. And the mechanical properties of particles reinforced zone were significantly higher than that of a piston.