Study On Compound Casting Of In-situ Al₂O₃ Particle Reinforced Aluminum Matrix Composite For Piston

The piston is the heart of automobile engine.As there is considerable difference of the working conditions for different piston parts,especially the working temperature upto 300°C⁴00°C for the top part,new methods should be applied for the design of the piston material and the research on the casting process.The?-Al₂O_3p/WR004 composites for piston were synthesized by in situ melt reaction method.After studying the feasibility of solid-liquid compound casting of Al-Al system,the?-Al₂O_3p/WR004-WR004 joint was successfully prepared.The X-ray diffractometer?XRD?,opticalmicroscope?OM?andscanningelectron microscopy?SEM?equipped with energy spectrometer?EDS?were used to analyze the phases and microstructure.The effects of external extrusion pressure on the microstructure and properties of composites,as well as the property variation of compound casting with interface structures were studied.The influence of T6 heat treatment on the microstructure and properties of compound casting was also discussed.The?-Al₂O_3p/WR004 composites from Al-SiO₂ system were in situ synthesized by blade cutting in semi-solid A356 alloy.The homogeneous distribution of?-Al₂O₃particles was acquired by shearing at a rate of 900r/min for 8min.The alloy grain size,as well as the defects,such as pores and shrinkage,decreased significantly as an extrusion pressure was applied.After T6 heat treatment,the strip eutectic silicon changed into granular or spherical shape,dispersed at the grain boundaries of?-Al phase,while its size was about 5?m.With the increase of the applied pressure,the tensile strength and elongation of4vol.%?-Al₂O_3p/WR004 composites were improved.When the applied pressure reached 60MPa,the tensile strength and the elongation reached the maximum values,279.3MPa and 1.33%respectively,which increased by 10.6%and 58.3%as compared to those of the as-cast ones,252.6MPa and 0.84%.After T6 heat treatment,the performance of materials obviously increased as applying an extrusion pressure of60MPa.The tensile strength of?-Al₂O_3p/WR004 composites increased to 340.8MPa,increased by 22%,and the elongation rate increased to 1.86%,increased by 39.8%.In Al-Al compound casting system,the wettability of aluminum alloy surface could be effectively improved by copper electroplating process.Using this process,the compound casting process for the insert WR004 alloy and the casting liquid aluminum alloy was investigated.Under different conditions,the interface could be divided into two categories:one was the dissolution and diffusion of the Cu layer and the interface was still dominated by Cu layer,the other was that the Cu layer was completely dissolved and formed a good metallurgical bonding.The WR004-Al99.5interface was mainly composed of?-Al equiaxed grains,and the WR004-WR004interface was mainly composed of?-Al columnar grains.The interface structure was proposed to be the result of the diffusion bonding.The mechanical properties of the Al-Al compound casting joint were between the insert alloy and the casting alloy,and the joint could combine the advantages of both the alloys.After the dissolving of Cu layer,the hardness of the WR004-Al99.5interface ranged from 45HV to 75HV,and the hardness of WR004-WR004 interface ranged from 50HV to 75HV.Al-Al compound casting joint achieved the best performance at a pouring temperature of 720°C:the tensile strength and the elongation of WR004-Al99.5 joint were 144.6MPa and 17.3%,as well as the tensile strength and the elongation of WR004-WR004 joint were 222.6MPa and 0.71%.The process with electroplated copper layer was also suitable for the compound casting for?-Al₂O_3p/WR004 composites and WR004 alloy.At a pouring temperature of 700°C,the interface was mainly composed of copper plating layer,and a thin copper layer remained.At a pouring temperature of 720°C,large-scale local melting of the copper layer appeared at the interface.At a pouring temperature of 740°C,a good metallurgical bonding interface formed,the presence of?-Al₂O₃ phase did not affect the interface formation.The interface was still dominated by?-Al phase,and the interface structure was also the result of the diffusion bonding.After T6 heat treatment,the structure of?-Al₂O_3p/WR004-WR004 interface improved,the needle-like and strip-shaped silicon phase was spheroidized and distributed at?-Al grain boundaries.As a result,the interfacial bonding became stronger.The best mechanical property was obtained for?-Al₂O_3p/WR004-WR004joint poured at 740°C.The hardness at the interface was 76.2HV,higher than 64.2HV of the as-cast one,and the tensile strength was 265.9MPa,which was 7.0%higher than that of the as-cast one?248.6MPa?,while the elongation was 0.95%,which was5.6%higher than that of the as-cast one?0.90%?.