Preparation,Microstructure And Properties Of SiCw Reinforced High Alloying 7000 Series Aluminum Matrix Composites

The comprehensive properties of silicon carbide whisker(SiCw)reinforced aluminum matrix composites are excellent,which can meet the special requirements of aerospace,military and other fields,having a significant application background.In this thesis,the SiCw and self-designed high alloying 7000 series aluminum alloy(Al-11.1Zn-3.0Mg-1.1Cu-1.4Ti-0.2Zr)were used as the reinforcement and matrix to prepare aluminium matrix composites.The powder metallurgy preparation process,microstructure and properties of SiCw reinforced aluminium composites were studied.The main research contents and conclusions are as follows:(1)The microstructure and properties of aluminium matrix prepared by cold pressing-argon protected sintering-hot extrusion were studied.Three kinds of multi-stage solid solution treatments were designed according to DSC analysis: S1(450oC×2h+460oC×2h);S2(450oC×2h+460oC×2h+470oC×2h);S3(450oC×2h+460oC× 2h+470oC×2h+480oC×2h).The results show that a large number of particle precipitation phases formed by hot extrusion of alloy are basically dissolved in the matrix under S2 solid solution process,and alloy matrix has the best solid solution strengthening effect and the strength of dislocation.The alloy matrix has the maximum hardness of 220.7HV under S2 solid solution-121oC×18h aging treatment.With the increase of solid solution temperature and solid solution time,the compressive strength decreases gradually,but the fracture strain increases slowly.The alloy matrix has great comprehensive properties with compressive strength of 643.1MPa and fracture strain of 7.0% under S2 solid solution process.(2)The preparation process(ball milling mixed powder-cold pressing-argon protected sintering-hot extrusion-soild solution and aging treatment)of composite was studied.The results show that wet milling SiCw-wet milling SiCw and matrix mixed powder-dry milling SiCw and matrix mixed powder process can obtain the composite powder with uniform distribution of SiCw in the matrix.The increase of pressing pressure can improve the microstructure and increase the density and hardness of composites.However,the pressure reaches 740 MPa,having little effect on the microstructure and properties of the composites.The composite is not completely sintered when the sintering temperature is less than 550oC.The relative density of composite decreases slightly when the sintering temperature reaches 600oC.But the hardness of composite can be improved owing to the appearance of the liquid phase.The relative density and hardness of composites subjected to hot extrusion increase greatly.The microstructure and properties of the composites are the optimal under solid solution process(450oC×2h+460oC×2h+470oC×2h+480oC×2h).When the sintering temperature is 550oC,owing to the weak interfacial bonding ability of the composites,the age hardening phenomenon of composite does not occur,and the hardness value starts to decrease.(3)The microstructure and properties of aluminium matrix composites prepared by argon protected sintering at 620oC-hot extrusion were studied.The results show that the performance of the composites prepared by argon protected sintering at 620°C is significantly improved compared to sintering at 550°C.With the increase of SiCw content,the internal particle size of composite becomes nonuniform,and the dislocation density and contribution of dislocation to strength increase.2.5vol.% and 5vol.% SiCw composites have the relative density of 98.9% and 99.2% respectively,the compressive strength of 617.1MPa and 658.9MPa respectively,and the fracture strain of 11.4% and 13.9% respectively.(4)The microstructure and properties of composites prepared by HIP-hot pressing were preliminarily studied.The results show that the performance of composites prepared by HIP-hot pressing is further improved compared to argon protected sintering.With the increase of SiCw content,the internal particle size of the material becomes smaller,and the dislocation density and contribution of dislocation to strength increase.The relative densities of 2.5vol.% and 5vol.% SiCw composites are 99.5% and 98.6% respectively.2.5vol.% SiCw composite has the best comprehensive performance with hardness of 217.7HV,compressive strength of 674.4MPa,and the fracture strain of 9.0%.This thesis creatively used the self-designed high alloying 7000 series aluminum alloy as the matrix and SiCw as the reinforcement to get SiCw reinforced aluminium composites with better comprehensive performance.The preparation was powder metallurgy(argon protected sintering,HIP)-plastic deformation(hot extrusion,hot pressing)-strengthening heat treatment.Meanwhile,the thesis effectively controlled the relationship between process parameters of powder metallurgy and properties of composites.