Study On Heat Treatment And Thixotropic Compression Deformation Behavior Of Al₂O_3np/7075 Aluminum Matrix Composites

Due to their high specific strength,stiffness and low density,7075 Al alloys are widely used in aerospace,military,and automobile industries.However,the coarse secondary phases are easily formed during solidification of alloy.The coarse secondary phases will decrease the strength and hardness of the alloy,thus greatly limiting its application.The mechanical properties of ally can be greatly improved by the subsequent heat treatment or adding the reinforcing particles in the alloy.The heat treatment process influences the content,morphology and distribution of the coarse secondary phases,thus leading to the change in the mechanical properties of the alloy.While a large number of dislocations are produced to strengthen the properties of alloys for the particle reinforced aluminum matrix composites.However,the the particle reinforced aluminum matrix composites increase the tensile strength,Ultimate yield strength and modulus of elasticity,but decrease its plasticity,which have a bad effect on secondary plastic processing.Semisolid processing technology,a good solution to the difficult issue of aluminum matrix composites secondary processing,allowing the application of aluminum matrix composites have become a broader prospect.In this paper,the effects of solution treatment on the evolution of the second phases and mechanical properties of 7075 Al alloy were studied by scanning electron microscopy（SEM）,energy dispersive X-ray spectrometry（EDS）,differential scanning calorimetry（DSC）,hardness and tensile tests.The results showed that Mg（Zn,Cu,Al）₂ phase gradually dissolved into the matrix,and yet the size and morphology of Al7Cu2 Fe phase exhibited no change with the increase of the solution treatment temperature and time due to its high melting point.When the solution treatment temperature and time continued to increase,the formation of coarse black Mg2 Si particles occurred.Compared to the as-cast alloy,the microhardness,tensile strength,and elongation of the sample under solution heat treatment at 460 oC for 5 h were increased by 55.1%,40.9% and 109.1%,respectively.This was because the eutectic phase Mg（Zn,Cu,Al）₂ were almost completely dissolved and basically no coarse black Mg2 Si particles were formed.The effects of T6 heat treatment on the microstructure and hardness of Al₂O_3np/7075 aluminum matrix composites were investigated by XRD,SEM,EDS and hardness tests.The results showed that the second phase of the composite became discontinuous on grain boundaries after the solution heat treatment,and the second phase was sufficiently dissolved into the matrix.Mg（Zn,Cu,Al）₂ phase completely disappeared and converted into Al2 CuMg phase.While Al7Cu2 Fe phase due to high melting point exhibited no change.After aging heat treatment,the fine MgZn2 phase was precipitated again.After the heat treatment,the hardness of the composite material is improved.The hardness of the composites firstly increased,then decreased with the increase of aging temperature and time.Under the heat treatment condition of 480 oC/5 h +120 oC/24 h,the hardness of the composites reached the maximum value of 173.51 MPa.At the same time,the strengthening mechanism of the composites and aging strengthening mechanism were discussed.Also,simulation testing machine Gleeble 3500 D is employed to carry out semisolid isothermal compression experiments for Al₂O_3np/7075 aluminium matrix composites.The compression deformation behavior of Al₂O_3np/7075 aluminium matrix composites with high solid fraction was investigated and the microstructures after semisolid compression were characterized.The results showed that the true stress decreased with the increase of the deformation temperature and size of Al2O3 np,the decrease of the strain rate and mass fraction of Al2O3 np.After semisolid compression,deformation degree in large deformation zone was larger than that in free deformation zone.Besides,the solid grains in large deformation zone showed evidence of having undergone different degrees of plastic deformation under different deformation conditions.Simultaneously,the deformation mechanisms during the semisolid compression process were discussed.