Study On Microstructure And Properties Of In-situ TiB₂ Particle Reinforced 7075 Aluminum Matrix Composites

Particle reinforced aluminum matrix composites have great application potential in many fields such as automotive,aerospace and manufacturing due to their advantages of low density,high specific stiffness and specific strength,good thermal stability and excellent wear resistance.However,there are few studies on the mechanical properties of 7075 aluminum matrix composites at different temperatures,and the studies on its high temperature flow stress behavior are mostly limited to the traditional constitutive equations.Therefore,the prediction accuracy of the flow stress of 7075 aluminum matrix composites under different deformation conditions needs to be improved.In this paper,Al-K2TiF6-KBF4 mixed salt reaction system was used to prepare in-situ TiB2 particle reinforced 7075 aluminum matrix composites,and extrusion,solution aging and other procedures were carried out on the prepared materials.The microstructure,mechanical properties,wear properties,flow stress behavior at high temperature,constitutive model and heat treatment process of the composites were studied by means of advanced characterization test.The thermal deformation process of the composite ingot after homogenization was simulated by axisymmetric isothermal compression experiment,and the flow stress behavior and the evolution of microstructure under different deformation conditions were analyzed.The results show that the composite has the characterist/ics of near steady flow stress during high temperature deformation.The flow stress is negatively correlated with the deformation temperature and positively correlated with the strain rate.The critical temperature for dynamic recrystallization of the composite is 400℃.Based on the stress-strain data of thermal simulation experiments,the material parameters of the classical Arrhenius constitutive model were obtained.The Arrhenius constitutive model coupled with strain variables and BP artificial neural network constitutive model were established,and the prediction accuracy of the two models was compared,which provided a theoretical basis for optimizing the forming process of the composite.The effects of homogenization,hot extrusion,solution temperature,solution time and aging time on the microstructure and properties of the composites were studied by extrusion and heat treatment of the composites.Finally,the optimal solution temperature of 500℃ the optimal solution time of 1.5h and the peak aging time of 26h were determined,and the reasonable coordination of Matrix precipitates(MPt),grain boundary precipitates(GBP)and TiB2 was realized.In addition,the effects of TiB2 particles on the homogenization,solution and aging process of the matrix were also discussed.The effect of TiB2 particles on the homogenization process depends on its morphology and distribution.TiB2 particles can inhibit the solution process of the matrix while promote the aging precipitation.The evolution of microstructure,mechanical properties and wear resistance of in-situ TiB2/7075 composites were studied by observation of microstructure,high temperature tensile test and friction and wear test.The strengthening mechanism,influence of high temperature on mechanical properties and wear mechanism of in-situ TiB2/7075 composites were analyzed.The results show that TiB2 particles can refine the grain size.The recrystallization proportion in composite at T6 state is small,and a large amount of aging precipitates are dispersed.In the temperature range from room temperature to 200℃,the strength of the composites with both mass fractions is higher than that of the alloy.Although the strength of the alloy and the composites decreased to a certain extent after 100℃,the composites still maintained a high high temperature strength.The plasticity of the composites is slightly lower than that of the matrix alloy.With the increase of temperature,the ratio of toughness in the fracture mechanism of composites increases gradually.In addition,the composites showed better wear resistance.The wear mechanism of 7075 alloy and TiB2/7075 was determined by the wear morphology as adhesive wear and abrasive wear.