Research On Preparing And Forming Semi-solid Billet Of Nano-SiC_p Reinforced7075Aluminum Matrix Composites

Lightweight of equipment is playing an important role due to requirements forsaving energy, reducing emission and improving efficiency. Lightweight of structurecomponents provides a technical support to lightweight of equipment. It involves areplacement of steel with aluminum alloy, magnesium alloy and their composite andimprovement of service level. Nano-particle reinforced aluminum matrix composite(NPAMC) has been a preferred material due to high strength and modulus and goodwear resistance. However, it is relatively difficult to prepare and form nano-particlereinforced aluminum matrix composite. For example, homodisperse of nano-particle,optimization of preparing process parameters and optimal control of forming processparameters are relatively difficult. In this research, a novel method calledultrasonic-assisted semisolid stirring (UASS) is proposed firstly to solve the difficultproblem of preparing NPAMC. Moreover, a typical deep cylindrical part is used asobject component to investigate rheoforming of NPAMC. Effect laws of differentprocess parameters on microstructure and mechanical properties of semisolid slurriesand formed cylindrical parts are achieved on the basis of observation of microstructureand analysis of mechanical properties. It will provide a technical support for preparingand forming NPAMC.The experimental scheme was determined according to semi-solid stirring time,stirring temperature, SiCpvolume fraction and time of ultrasonic vibration and appliedpressure. A rheo-extrusion die was designed according to the size and shape of thecylindrical parts. Effect laws of process parameters during the UASS process on qualityof semisolid billets were investigated. The results show that semi-solid stirring time,stirring temperature, SiCpvolume fraction and time of ultrasonic vibration have animportant influence on the preparation of semi-solid slurry, which further affects thequality of the forming cylindrical parts. The pressure also has important effect onquality of the formed cylindrical parts. Multiple sets of contrast test was showed that theoptimal process parameters include the semi-solid mixing time of25min, the stirringtemperature of620°C, carbide volume fraction of1%and the ultrasonic vibration timeof10min.Mechanical properties of the rheoformed cylindrical parts were investigated byusing room temperature tensile test. The mechanical properties of7075aluminum alloy extruded rod involve ultimate tensile strength of310MPa, elongation of10%. Themechanical properties of the rheoformed cylindrical parts under the optimal processparameters involve ultimate tensile strength of310MPa, elongation rate of8%, close tothe mechanical properties of7075extruded aluminum bars. The mechanical propertiesof after T6heat treatment reach ultimate tensile strength of340MPa, elongation of13%, which exceed those of7075extruded aluminum bars without heattreatment.Non-desirable process parameters lead to a microstructure consisting of alarge number of dendrites, entrapments, pores and cracks. It is detrimental tomechanical properties of rheoformed cylindrical parts. A fine,uniform and densemicrostructure without defects is achieved under the optimal process parameters. Theultimate tensile strength and elongation are high and some obvious dimples and tearedges is found in the fracture. The results show that high mechanical properties can beobtained by rheoforming the cylindrical parts of NPAMC under2000kN universepress.