| Lightweight of vehicles represents an important trend in the current development of automobile industry.Weight reduction of brake drum as an important part of the wheel end is an effective way to achieve the purpose.Aluminum matrix composites are very suitable for the preparation of brake drums due to its advantages in high strength,wear resistance and high thermal conductivity.To give full play to the advantages of aluminum matrix composites,and offset its poor processing performance,aluminum matrix composite brake drums of layered structure were designed in this paper by using particle-fiber hybrid on the inner wear surface to enable high strength and wear resistance of reinforced aluminum matrix composites,and using single aluminum alloy material on the outside to give good toughness and facilitate machine manufacturing.The thermosetting phenolic resin-based binder,graphite-based pore-forming agent,and aluminum dihydrogen phosphate were selected as ingredients of high-temperature binder.The flexible and adjustable mixed perform of silicon carbon particulate(14?m in particle size)and alumina short fiber(10?m in diameter,length-diameter ratio 2-5)with a porosity within the range of 50%-70%was prepared by stamping process.The green body was subject to sintering in air at 1000?,which would not only completely burn out the graphite in the perform,but also form a 0.2 ?m oxide film on the silicon carbide surface,so that the wettability between the particles and the aluminum matrix is improved.Compared with paraffin-based binder and polyvinyl alcohol-based binder,phenolic resin-based binder enables higher high-temperature strength and porosity of the green body whose dimensional stability will also be higher.Heat-treatment strengthening 2024 deformation aluminum alloy was selected for base material of the composite.The alloy composition contained in the aluminum alloy is mainly Cu and Mg,which improves the wettability between the matrix and the reinforcing phase while enhancing mechanical properties,corrosion resistance,and oxidation resistance of the aluminum matrix.The preform infiltration was performed by vacuum pressure infiltration process,and near-net-shape preparation of the layered structured composite was done through rational mold design.Study has been made on the effects of temperature,pressure and pressure maintaining mechanism on structure properties of the layered structured composites in the infiltration process.Finally,the best infiltration process was determined to maintain temperature at 720? for 10 minutes and then pressurize to 15 MPa,maintain for 10 minutes before cooling during which pressurization continues until solidification.The effects of the ratio of particles and fibers in the reinforcement system on material microstructure and properties were explored by observing material microstructure and testing mechanical properties.The tensile property of(20 vol.%SiCp+10 vol.%Al2O3sf)/A12024 was the highest among the aluminum matrix composites with total volume fraction of reinforcement phase at 30%,which was 391.34 MPa at room temperature and 164 MPa at 300?.The finite element modeling of stress distribution within the material under 300 MPa tensile stress was performed for the composite materials with four reinforcement phases,finding that the maximum stress values distributed on the reinforcement phase were 786.27 MPa,376.03 MPa,526.33 MPa,and 1441.1 MPa,respectively and the maximum stress values in the matrix were 1301.8 MPa,1276.2 MPa,1373.6 MPa,and 1441.1 MPa,respectively when the volume ratio of particles to fibers was 4:1,2:1,1:2,and 1:4.It can be seen that the volume ratio of particles to fibers at 2:1 can improve the stress distribution in the reinforcement phase and the matrix.Under this ratio,the stress distributed in the reinforcement phase and the matrix was the lowest among the four ratios.Thus,composite has the best tensile properties under this ratio.Stress analysis of the layered structure of brake drum under actual working conditions was made using Ansys finite element modeling.The thickness of inner composite material layer should be not less than 0.4mm when it is assumed that the thickness of outer aluminum alloy layer is 2mm.The pin-disk friction test was conducted on the composite and HT400 at 4.2m/s and 0.7MPa,finding that the average friction coefficient of the composite decreases with the increasing proportion of short fiber alumina in the reinforcement phase,and(30vol.%A12O3sf+l0vol.%SiCp)/A12024 has the lowest average friction coefficient of 0.37.As the volume ratio of A12O3sf and SiCp in the reinforcement phase increased from 0 to 1,the wear rate of the composite decreased from 0.57 kg/m3 to 0.39 kg/m3 in wear mechanism of abrasive wear and furrow wear;as the volume ratio of A12O3sf and SiCp in the reinforcement phase increased from 1 to 3,the wear rate of the composite increased to 0.59 kg/m3 in the wear mechanism of adhesive wear.With the increase of Al2O3sf content in the reinforcement phase,abrasive dust gradually decreased in size,with diameter gradually transiting from 6?m to about 1.5?m.Due to the lower thermal conductivity of short fiber alumina,the thermal conductivity of the composite decreased from 108W·m-1·K-1 to 86W·m-1·K-1 with the increasing ratio of short fiber alumina in reinforcement phase.In this paper,thermal conductivity calculation model was constructed for particle-fiber hybrid reinforced metal matrix composites using approximate method based on the generalized self-consistent theory.The silicon carbide particles containing boundary phases were equivalent to "primary equivalent enhanced particles",the matrix containing short fiber alumina was equivalent to "primary equivalent matrix",and the "primary equivalent reinforcement particles" were embedded in the"primary equivalent matrix" as the "secondary equivalent particles" together.Afterwards,the "secondary equivalent particles" were embedded in uniform isotropic medium with the same thermal conductivity as A12024 to calculate the thermal conductivity of particle-fiber hybrid reinforced aluminum matrix composites containing particles,fibers,and interfaces,which means a certain guiding significance for material design. |
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