Study On The Preparation And Squeeze Casting Process Of In-situ Al₂O₃ Particle Reinforced Aluminum Matrix Composites For Piston

The piston material requires high tensile strength at high temperature,good wear resistance and less coefficient of thermal expansion.Aiming at the above requirements,direct melt reaction technique combined with semi-solid mechanical stirring process was applied to prepare in-situ Al₂O₃ particle reinforced aluminum matrix composites for piston by the chemical reaction between Si O2 particles and aluminum alloy melt in the present paper.And the squeeze casting of the composites was also been investigated.The phase contents and microstructure of the composites,and the size,morphology as well as the distribution of Al₂O₃ particles in the matrix were analyzed.The room temperature tensile properties,high temperature tensile properties and hardness of the composites were tested.The effects of squeeze casting on the microstructure and properties of the composites were studied.And then the fracture mode and strengthening mechanism of the composites were discussed.Al₂O₃ particles with average sizes of about 5?m and 30?m were generated by the reaction between aluminum alloy melt and SiO2 particles with average sizes of 5?m and 30?m.And then in-situ Al₂O₃ particle reinforced aluminum matrix composites for piston were prepared.When the temperature of the composite melt was 750?,ultrasonic was applied to the melt for 10 minutes.After that,most of the 30?m Al₂O₃ particles with surface cracks were broken into small particles and dispersed gradually,but there still existed a small amount of broken particles which were not completely dispersed.5?m Al₂O₃ particles were distributed uniformly and well bonded with the matrix.The refinement of Al₂O₃ particles to ?-Al dendrites and eutectic silicon was found.Most of the eutectic Si particles became granular and a small part became short rod-like.The surface quality of composites was improved by squeeze casting,the original coarse dendrites in the matrix were refined and tended to spheroidize.When the mold temperature was 250? and the extrusion pressure was 70 MPa,the microstructure of composites reached the best.The dispersion of Al₂O₃ particles were improved in the matrix under high pressure,but the morphology and size had no obvious changecompared with gravity casting,while the defects such as holes in the casting were eliminated.After T6 heat treatment,the morphology of Si phase was spheroidized,and the morphology,size as well as the dispersion of Al₂O₃ particles did not change obviously.The results of room temperature tensile testing showed that,when the extrusion pressure was 70 MPa,the tensile strength of the 5?m-Al₂O₃p/ZL109 composite as-cast and T6 heat-treated were 216 MPa,305MPa,respectively.The tensile strength of the30?m-Al₂O₃p/ZL109 composite as-cast and T6 heat-treated were 204 MPa,290MPa,respectively.The tensile strength of the 5?m-Al₂O₃p/ZL109 composite were higher than that of the matrix alloy,and the tensile strength of the 30?m-Al₂O₃p/ZL109 composite were lower than that of the matrix alloy.The results of high temperature tensile testing showed that,when the extrusion pressure was 70 MPa,the tensile strength of the 5?m-Al₂O₃p/ZL109 composite was higher than that of the matrix.The tensile strength of the matrix and5?m-Al₂O₃p/ZL109 composite decreased with the increase of temperature,and decreased obviously when the temperature was higher than 250?.When the temperature was 250?,300?,350?,the tensile strength of the 5?m-Al₂O₃p/ZL109 composite were 192 MPa,132MPa,75 MPa,respectively.The elongation of the5?m-Al₂O₃p/ZL109 composite was significantly lower than that of the matrix.With the increase of temperature,the gap of elongation increased gradually.The results of hardness testing showed that,when the extrusion pressure was70 MPa,the brinell hardness of the 5?m-Al₂O₃p/ZL109 composite as-cast and T6heat-treated were 110,136,respectively.The brinell hardness of the30?m-Al₂O₃p/ZL109 composite as-cast and T6 heat-treated were 110,136,respectively.And all of them were higher than that of ZL109 alloy.