Study On Three-direction Constrained Defomation Technics, Microstructure And Properties Of High Volume Fraction SiCp/Al Composites

For particle reinforced aluminum matrix composites, plastic molding is an idealmethod to eliminate the microscopic defects, improve performance and reducemachining allowance. However, with the Volume fraction of particle increasing, thecontinuity of aluminum matrix becomes bad. Then it is so difficult for composites toachieve second plastic molding. So far it is not found the reports about sever plasticdeformation of high volume fraction particle reinforced aluminum matrix composites.This paper is to explore the plastic processing deformation of45%volume fractionof SiCp/Al composites. A new deformation method—three-direction constraineddeformation was used to achieve severe plastic deformation of SiCp/Al composites. Andthen the effects of three-direction constrained deformation on microstructure andmechanical properties were investigated.At first, high temperature compression deformation tests were taken. The resultsshowed that at the temperature ranging from300℃to610℃and strain rates rangingfrom0.0005/s to0.1/s, the compressive stress-strain curves of SiCp/Al composites hadthree stages: elastic deformation stage, work hardening stage and strain softening stage.As the two main influence factors, the deformation temperature and rates affect thecompression maximum deformation resistance and the critical compression reduction.As the temperature increased, the maximum deformation resistance decreased withexponential relationship. The greater the strain rate, the greater the deformationresistances. However, with temperature increasing, the influence of Strain rate on thedeformation resistance became smaller. The critical compression reduction reflects theplasticity of composites. Then it can be identified that the best deformation processparameter is:505℃,0.005s/s.According to the analysis results of DTA and effects of the temperature ondeformation resistance, the deformation activation energy and the flow stress equationwere obtained respectively in Low temperature area (300~400℃), medium temperaturearea (425~505℃), high temperature area (530~610℃). The deformation activationenergy is328.6kJ/mol，385.9kJ/mol and842.5kJ/mol respectively. While liquid phaseexisted, the deformation activation energy is exceptional high. This means thedeformation mechanism of composite under different temperatures is different.In this paper, a new method named three-direction constrained deformation wasproposed and designed. By generating constraint in the deformation process, thecomposites got three-direction compressive stress from beginning to end. Then the earlycrack production was suppressed and severe plastic deformation was achieved. Using the parameter got from high temperature compression deformation tests, perfectspecimen with smooth surface were produced in the deformation tests and themaximum deformation amount was70%. During the three-direction constraineddeformation tests, different constraint materials were used and the effect of constraintforce on deformation results was analyzed. The result showed that the formation ofcomposites was more uniform and pressure was less when using the pure aluminum asconstraint. While using the2024aluminum alloy as constraint, the pressure was greaterand composites became more densification after deformation.Compared the microstructure and properties of the SiCp/Al composites before andafter three-direction constrained deformation, it was found that the density ofcomposites has greatly improved after deformation and reached nearly fully dense. Asubstantial increase in the strength of the composites was obtained after three-directionconstrained deformation. In aging state，the maximum bending strength of compositeafter deformation is968MPa, increasing72.0%and the maximum tensile strength is597MPa, increasing72.0%. The plasticity and elastic modulus of composites afterdeformation also increased. The elongation is0.28, increasing155%.Transmission electron microscopy (TEM) observations indicate that, comparedwith composites before deformation, the dislocation densities of composites increasedobviously and the reversion and recrystallization also happened after deformation. Theexist of large number of dislocation made the precipitated phase more than before.SiCp/Al composites have been strengthened after three-direction constraineddeformation. The mechanism of strengthen is the result of dislocation strengthening,dispersion strengthening, grain refinement strengthening, solution treatment and agingstrengthening.