Study On Preparation, Micro Structure And Properties Of Medium Volume Fraction SiCp/Al Composites

In the present paper, material design on the background of optical/inertial instrument composite application is carried out with the aim of acquiring high special strength,high special elastic modulus, high thermal conductivity, low thermal expansion properties and good dimension stability. For this purpose, Si C particle(volume fraction of 30%-40% and particle size of 3μm-40μm) were added to 2024 alloy and Si Cp/Al composite were acquired. Effects of Si C particles on microstructure, mechanical properties, thermal physics properties, thermal and machinability were studied by using transmission electron microscope(TEM), scanning electron microscope(SEM), optical microscope(OM),thermal conductivity tester, tensile testing machine, laser flash diffusivity apparatus, Brinell hardness, X-ray diffraction and Gleeble-1500 thermo-simulation machine and influencing mechanism were also discussed. Research work and results are as follows:(1) The effects of hot pressing process parameters, such as pressing temperature, pressing pressure and holding time, on properties and microstructure of Si Cp/Al composites were investigated, the optimized process program benefiting for the interlace bonding and mechanical properties of composite were detected, the influencing factor of the sintering parameters on the mechanical properties of composite were acquired. The optimal preparation parameters of Si Cp/Al composites are hot pressing temperature at 580 ℃, hot pressure at 70~80MPa, holding time is 3h. The influencing factor of the heat treatment process parameters on mechanical properties were proved, and the reasonable parameters of heat treatment are solution at 505 ℃ for two hours, water quenching and artificial aging treatment at 190 ℃ for 7 to 10 hours.(2) By changing the Si C particle size, the mechanical properties of Si Cp/Al composites were improved, the high overall performance of Si Cp/Al composites were acquired. For example, 30%Si Cp(3μm)/Al(σb=409MPa, E=219GP), 30%Si Cp(8μm)/Al(σb=367MPa, E=195GPa), 35%Si Cp(3μm)/Al(σb=385MPa, E=209GPa) and35% Si Cp(8μm)/Al(σb=403MPa,E=215GPa). Four composites have specific strength more than 120Nm/kg and have specific modulus greater than 60 m, All of these techniques provide technical support for industrial production. The effects of microstructure changes caused by Si C particle size on tensile deformation behavior and mechanism were studied, the strengthen mechanism and fracture mode of Si Cp/Al composites were obtained. The influence rule of particle size on composite properties are: the yield strength, tensile strength and hardness of the composites increase with decreasing Si C particle size; the relative density is closely related to the ratio of the Si C particle size and matrix particles size ratio(d/D). Fracture mechanism is also affected by Si C particle size, when Si C particle size becomes larger, the fracture mechanism of composite change from mainly matrix toughness fracture to Si C particles cleavage fracture.(3) The microstructures of the interface between Si C and Al were studied, The study results shows that the interface is clean and smooth, have good combination with matrix. Three kinds of Si C/Al interfaces were present in the composites, which include vast majority of clean planer interfaces, few slight reaction interfaces and tiny amorphous interfaces. The combination mechanism of Si C and Al in the clean planer interface is the formation of a half coherent interface by closely matching of atoms. There are no fixed or preferential crystallographic orientation relationships between Si C and Al. The combination mechanism of Mg Al2O4 and Si C or Al is also the formation of a half coherent interface by closely matching of atoms. Mg Al2O4 spinel particles act as an intermediate to form stable interlace structures in the slight reaction interfaces.(4) The precipitates evolution after heat treatment were characterized and described from such aspects as size, composition and interface. The study results shows that the precipitates are nanosized plate-shaped Al2 Cu and rod-shaped Al2 Cu Mg particles, these precipitates dispersed in the matrix and play a precipitation hardening effect by means of pinning effect on dislocation. With an increase of aging time, the size of both nanosized plate-shaped Al2 Cu and rod-shaped Al2 Cu Mg particles are increased gradually. The transformation law of interface between precipitate and Al matrix is: coherency → semi coherency(aged 8h).(5) The thermal expansion coefficient of the composite can been effectively regulated by means of changing the volume fraction and size of Si C particles. With the increase of the volume fraction of Si C particles, the thermal expansion coefficient of the composite decreases; the thermal expansion coefficient of the composite decreases with decreasing particle size Si C when the volume fraction of Si C is the same, thermal expansion coefficient increases as the temperature rises. The behavior of Si C particle size constraining on the thermal expansion of composite can been achieved by influencing the deformation behavior of thermoplastic matrix and thermal residual stress. Schapery model can be a very good prediction of changes in the thermal expansion coefficient of Si Cp/Al composites below 300 ℃. Further, volume fraction and size of Si C particles affect the dimensional stability of the composite. The higher the composite volume fraction of Si C particles, the smaller the particle size, the greater the thermal cycling residual strain, the worse the size stability resistant to temperature fluctuations. Thermal expansion coefficient of Si Cp/Al composite in this study ranged from 11.6×10-6/K to 13.3×10-6/K, which can achieve better match with thermal expansion coefficient(11.8~13×10-6/K) of beryllium, bearing steel, etc, the specific modulus of Si Cp/Al composite are about 2 to 4 times of aluminum. All these properties can meet demand of thermal expansion matching and high specific modulus for optical / instrument grade composite.(6) Changes in volume fraction and the size of Si C particles influence the thermal conductivity of the composite. For the same volume fraction Si Cp/Al composites, Si C particle size increases, the thermal conductivity of the composite increases, Hasselman and Johnson model can be used to predict the changes of the thermal conductivity. The effect of Si C particle volume fraction on thermal conductivity of Si Cp/Al composites in connection with Si C particle size and the critical dimension. In the good interlace bond, the minimum critical size of Si Cp/Al composites Si C is 9.6μm, when the particle size is larger than 9.6μm, the enlargement of Si C particle volume fraction can improve the thermal properties of the composites, and the thermal conductivity of the composite is greater than the thermal conductivity of the Al matrix; when the Si C particle size is less than the minimum critical particle size, the enlargement of Si C particle volume fraction reduce the thermal conductivity of the composite, and the thermal conductivity of the composite is less than the thermal conductivity of the Al matrix. The thermal conductivity of Si Cp/Al composites studied in this article is in the range of 120~150 W/(m?K), which can satisfy the requirements of the thermal performance of optical/instrumentation grade material.(7)Thermal processing properties of the composites were studied systematically. Constitutive flow behaviors of composite were investigated using hot compression tests. The Arrhenius-type constitutive equations were established with considered values of material constants as a function of strain, strain rate and temperature. Dynamic material model(DMM) were used to construct the power dissipation efficiency maps, the instability maps and thermal processing maps, the optimum thermal processing parameters of Si Cp/Al composites were identified.The results showed that Si Cp/Al composites is sensitive to deformation temperature and strain rate. The flow stress behavior during hot compression deformation can be represented by a Zener-Hollomon parameter in the hyperbolic sine form, the thermal activation energy increase as the reduction of Si Cp volume fraction and the increase of Si Cp size. When Si C particle sizes reduce and content increase, the peak power dissipation coefficient decrease. The power dissipation maps show dynamic recovery(DRV) and dynamic recrystallization(DRX) happened in thermal deformation process, and were verified by metallographic observation; the main instability form are particle cracking of Si Cp particles and hole at separation of Si C particles.(8) Work hardening rate were calculated using testing data of hot compression true stress-strain curve of Si Cp/Al composites, and using the inflection of lnθ-ε curve and minimum on the(-?(lnq)/?e)-e) curve as a criterion to determine the material dynamic recrystallization critical strain, The critical strain increases with the increase of strain rate and the reduction of deformation temperature, there is linear relationship between critical strain and peak strain, while the critical strain increased with increasing Z parameters, a function of the relationship between the critical strain and Z parameters were obtained. Dynamic recrystallization critical strain model of five Si Cp/Al composites were established.