| As an important component of the engine,the working environment of the piston is very harsh,and it generally works under the conditions of high temperature,high pressure,high speed and poor lubrication.In order to meet the requirements of different working conditions of the piston,it is necessary to develop piston materials with high strength,high thermal conductivity,low thermal expansion,excellent wear reduction and wear resistance.At present,the industrialized piston materials include cast iron,steel and aluminum-silicon alloy.Cast iron pistons and steel pistons have high thermal conductivity and mechanical strength,but their density is high and the wear on the cylinder liner is serious.Aluminum alloy has good thermal conductivity,high strength and low density,but aluminum alloy has a large thermal expansion coefficient and poor wear resistance,which is easy to reduce theworking life of the piston.Due to the extensive use of supercharging enhancement technology in engine design,the requirements for piston performance are becoming more and more stringent.Therefore,it is urgent to study new piston materials.Aluminum matrix composites have the advantages of aluminum alloy matrix and reinforcing phase.Graphite has a low coefficient of thermal expansion and has excellent thermal conductivity,wear reduction and wear resistance.In order to improve the wear resistance of the aluminum alloy piston material and reduce its thermal expansion coefficient,graphite and aluminum alloy can be compounded,but the interface between graphite and aluminum alloy has poor wettability,so copper-coated graphite and nickel-coated graphite are used to replace graphite and aluminum alloy to improve the wettability of the interface.In this thesis,ZL105 aluminum alloy is selected as the matrix,and copper-coated graphite and nickel-coated graphite are used as reinforcements to prepare aluminum-coated graphite composites by vacuum hot-pressing sintering process.The contents of coated graphite are 40 wt%,50 wt%and 60 wt%,and the sintering time is 60 min.The sintering temperature ranges from 500?to 580?,and the results show that when the sintering temperature is 560?,the properties of the aluminum-coated graphite composites reach the best.The study found that the coated graphite in the aluminum-coated graphite composite material is evenly distributed without obvious agglomeration.The coating effect of nickel-coated graphite is better than that of copper-coated graphite,and its interface with the aluminum alloy is better.The mechanical properties of the aluminum-coated graphite composites decreased with the increase of the coating graphite content,and under the same conditions,the mechanical properties of the aluminum-nickel coated graphite composites were better than those of the aluminum-copper coated graphite composites.When the content of coated graphite is 40 wt%,the compressive strength and flexural strength of the aluminum-copper coated graphite composite are 219 MPa and 88 MPa,respectively,and the compressive strength and flexural strength of the aluminum-nickel coated graphite composite are 297.4 MPa and 109.2 MPa.The data obtained by studying the thermal properties of aluminum-coated graphite composites with different coating graphite contents show that the aluminum-coated graphite composites have excellent thermal properties.With the increase of the coating graphite content,the thermal conductivity of the aluminum-coated graphite composite gradually increased,and the thermal expansion coefficient decreased gradually.Because the thermal properties of the composites are affected by the thermal properties of the coated copper and nickel themselves,the thermal conductivity and thermal expansion coefficient of aluminum-copper coated graphite composites are higher than those of aluminum-nickel coated graphite composites.When the content of coated graphite is 60 wt%,the thermal conductivity and thermal expansion coefficient of the aluminum-copper-coated graphite composite are 457 W/(m·k)and 11.8×10-6/K,respectively.The thermal conductivity and thermal expansion coefficient of the material are 401 W/(m·k)and 11×10-6/K,respectively.By studying the friction coefficient and volume wear of aluminum-coated graphite composites at different temperatures,different loads,and different rotational speeds,and observing the wear scar morphology of each material,the wear mechanism of each material under different test conditions was obtained.The experimental results show that the addition of coated graphite greatly improves the wear resistance and wear resistance of the composites.The friction coefficient and volume wear of the two aluminum-coated graphite composites are lower than those of ZL105 aluminum alloy.With the increase of temperature,the friction coefficient of the aluminum-coated graphite composite decreases gradually.When the temperature reaches 300?,the friction coefficient of the aluminum-copper coated graphite composite is 0.23,and the volume wear is 0.34 mm~3.The friction coefficient of the aluminum-nickel coated graphite composite is 0.09,and the volume wear is 0.59 mm~3.At this time,the friction coefficient of ZL105 aluminum alloy is 0.5,and the volume wear amount is as high as 4.52 mm~3.With the increase of load and rotational speed,the friction coefficient of each material gradually decreased,and the volume wear increased gradually.When the load reaches 12 N,the friction coefficient and volume wear of aluminum-copper coated graphite composites are 0.176 and 0.35 mm~3,respectively,and the friction coefficient and volumetric wear of aluminum-nickel coated graphite composites are 0.24 and 0.31 mm~3,respectively.When the rotational speed reaches 750 r/min,the friction coefficient and volume wear of the aluminum-copper coated graphite composite are 0.24 and 0.34 mm~3,respectively,and the friction coefficient and volumetric wear of the aluminum-nickel coated graphite composite are 0.25 and 0.19 mm~3,respectively.The main wear mechanisms of aluminum-coated graphite composites are delaminated wear and abrasive wear,while the main wear mechanisms of ZL105 aluminum alloy are delaminated wear,adhesive wear and abrasive wear. |
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