Preparation Of In-situ （TiB₂,ZrB₂） Particle-reinforced Al-Si Composites And Study Of Creep Properties

The near-eutectic poly-Al-Si piston alloy has become one of the most widely used materials for engine pistons due to its superior high temperature performance.As the"heart"of the engine,the piston is used in high temperature and high burst pressure environment for a long time,and the working environment is extremely harsh,which seriously affects the service life of the piston.Therefore,the development of high-strength and heat-resistant Al-Si piston alloys has become a hot topic of academic research for many scholars in recent years.In this paper,TiB₂/Al-12Si piston alloy and（TiB₂,Zr B₂）/Al-12Si piston alloy were prepared by two reaction systems,namely mixed salt reaction method and intermediate alloy reaction method,using Al-12Si-4Cu-2.7Ni-1Mg piston alloy as the base alloy.The effects of reinforcing particles on the microstructure and mechanical properties of the piston alloys were investigated by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray diffractometer（XRD）,room high temperature tensile and creep properties and other analytical testing methods.The main results of the study are as follows:Most of the TiB₂and Zr B₂reinforcement particles prepared by both fluorine salt mixing method and intermediate alloy method were dispersed in the matrix in the form of clusters,and the size of the prepared particles was about 100 nm on average,and their morphology was mostly in the form of round grains or hexagonal sheets.When TiB₂particles were added singly,theα-Al dendrite backbone in the piston alloy became shorter,and the average secondary dendrite spacing was about 15μm,which was reduced by 8.5%compared with the matrix,and the grain refinement was achieved;meanwhile,the average size of the primary Si phase was reduced by 10.6%compared with the matrix alloy.The size refinement ofα-Al grains and primary Si phase is more significant when TiB₂and Zr B₂act in concert.Compared with the base alloy,the tensile strength of TiB₂/Al-12Si piston alloy and（TiB₂,Zr B₂）/Al-12Si piston alloy prepared by both fluorine salt hybrid and intermediate alloy systems were significantly improved at room temperature and high temperature compared with the base alloy.The room temperature tensile strength of TiB₂/Al-12Si piston alloy under the fluorine salt mixing method system reached 327 MPa compared with the base alloy,which is 16%higher;and the elongation also increased to 2.45%,while the high temperature tensile strength increased to 91 MPa,but the elongation was only 4.05%;while the room temperature tensile performance was 352 MPa,which was improved by 25%compared to the collective alloy,while the elongation was only 1.35%,while the high temperature tensile performance of the material was 110 MPa,which was improved by 23.6%compared to the base alloy,while the elongation decreased to 1.65%.In contrast,the room temperature tensile strength of TiB₂/Al-12Si piston alloy under the intermediate alloy method system reached 328 MPa,an increase of 16.7%;the elongation also increased to 2.75%,while the high temperature tensile strength increased by 22.4%,while the elongation was only 4.65%;the room temperature tensile strength of（TiB₂,Zr B₂）/Al-12Si piston alloy was 364 MPa.compared to the base alloy increased by 29.5%,elongation decreased to 1.46%,high temperature tensile strength reached113 MPa,and elongation decreased to 1.95%.The composite material prepared by the intermediate alloy system has better performance than the fluorine salt mixture method.Since pistons are prone to creep in high temperature and high pressure working environment,it has great influence on the life of pistons.Therefore,in this experiment,the high temperature creep resistance of the piston alloys after the synergistic reinforcement of the two reinforcing particles was tested,and the high temperature creep analysis of the composites was carried out at different temperatures and different loads,respectively,and the results showed that when T=523 K,the creep deformation of both TiB₂/Al-12Si piston alloy and（TiB₂,Zr B₂）/Al-12Si piston alloy within 100 h have been increased,while the steady-state creep rates of the two composites are the lowest at 40 MPa stress,8.15×10-10s-1 and 4.982×10-10s-1,respectively;the overall creep deformation is higher and the steady-state creep rate is lower.Moreover,when the temperature is constant,the higher the applied load is,the steady-state creep rate of the composites increases,while the creep life of the composites decreases,and their creep resistance decreases.The steady-state creep rate of the（TiB₂,Zr B₂）particle-reinforced alloy is smaller than that of the single TiB₂particle-reinforced alloy,and the creep resistance is high and the life time is long.