Study On The Preparation And Squeeze Casting Of In-situ ?-Al₂O_3p/ZL109 Composites

In-situ particle reinforced aluminum matrix composites have the advantages of high specific strength and specific modulus,good heat resistance and corrosion resistance,low linear expansion coefficient,and good particle/aluminum matrix interface bonding,etc.,which has a broad application prospect in automotive,aerospace,electronic devices and other fields.However,there are still some technical problems such as the dispersion of in-situ particles,need to be solved for the existing preparation methods of such composites,which limits their wide application.In this paper,the 2?m?-Al₂O_3p/ZL109 composites were successfully fabricated from Al-SiO₂ system by melt direct reaction method with the aid of electromagnetic stirring.Room and high temperature tensile properties,Brinell hardness and linear expansion coefficient of the composites were systematically studied.The relevant mechanisms were discussed,and the accuracy of the linear expansion coefficient prediction models was analyzed.Thermodynamics and kinetics analysis showed that the Al-SiO₂ reaction was a spontaneous exothermic reaction process and relied on the diffusion of elements Al,Si,and O,which could be divided into three stages:contact reaction,infiltration reaction,and complete reaction.Metastable?-Al₂O₃ was firstly formed by the Al-SiO₂ reaction at 755.8 ^oC,and then transformed into the steady-state?-Al₂O₃ at 1045.6 ^o C.Due to the density differences between SiO₂ and?-Al₂O₃ as well as?-Al₂O₃ and?-Al₂O₃,both the above two processes underwent a certain degree of volume shrinkage,but the size of the formed?-Al₂O₃ particle was basically consistent with that of SiO₂ particle.With semi-solid mechanical stirring,the Al-SiO₂ ball-milled mixture powders were added into the A356 alloy and followed by the reaction between the aluminum melt and SiO₂to form?-Al₂O₃ reinforcement particles.Then,Si,Cu,Ni,and Mg elements were added to obtain ZL109 alloy matrix,and finally the?-Al₂O_3p/ZL109 composites with different volume fractions of reinforcement particles were fabricated.The results of microstrcture observation showed that with the increase of?-Al₂O₃ particle volume fraction,particle agglomerations and porosity defects were aggravated.After a further treatment for 10 min under an electromagnetic field with a current of 250 A,a frequency of 7.5 Hz,and a magnetic induction intensity of 0.039 T,in 5 vol.%?-Al₂O_3p/ZL109 composite slurry,?-Al₂O₃ particle agglomerations were substantially eliminated and?-Al₂O₃ particles were well dispersed,but the porosity defects were not significantly improved.Influenced by the interfacial pushing mechanism,?-Al₂O₃ particles were mainly distributed at the?-Al grain boundary and around the eutectic structure,and the?-Al grains were refined.As the volume fraction of?-Al₂O₃particles increased,the refining effect was gradually strengthened.Under the squeeze casting conditions with a pouring temperature of 720 ^oC,an injection specific pressure of 80 MPa,an injection speed of 0.1 m/s,and a holding time of 6 s,in remelted 5 vol.%?-Al₂O_3p/ZL109 composite,the porosities caused by semi-solid mechanical stirring were completely eliminated,the?-Al dendrites and coral-like eutectic silicon were further refined,and?-Al grains were obviously transformed from dendrites to equiaxed crystals.However,squeeze casting had no significant effect on the dispersion of?-Al₂O₃particles.Room temperature tensile tests showed that the tensile strength,elongation and Brinell hardness of the T6-treated remelted 5 vol.%?-Al₂O_3p/ZL109 composite under a squeeze pressure of 80 MPa were 347 MPa,3.69%and 136 HB,respectively,which increased by21.3%,29.5%and 20.9%compared with those of the remelted composite in gravity casting.Compared with the ZL109 alloy in squeeze casting,which increased by 5.8%,decreased by5.9%and increased by 5.4%,respectively.High temperature tensile tests showed that the strengths of the above composite at 150^oC,200 ^oC,250 ^oC,300 ^oC and 350 ^oC were 336 MPa,280 MPa,228 MPa,168 MPa and 99MPa,respectively,which were higher than those of the ZL109 alloy.The contribution of?-Al₂O₃ particles to strength?K?got a maximum of 23.6%at 300 ^oC.Linear expansion coefficient tests showed that at the range of 30⁴00 ^oC,the linear expansion coefficients of the T6-treated remelted 5 vol.%?-Al₂O_3p/ZL109 composite under a squeeze pressure of 80 MPa were lower than those of ZL109 alloy.At 30 ^oC and 400 ^oC,the linear expansion coefficients of the composite were 13.99×10^{-6 o}C^-11 and 23.33×10^{-6 o}C^-1,respectively,which decreased by 7.97%and 3.79%compared with those of ZL109 alloy.The analysis showed that Kerner model was more accurate for predicting the linear expansion coefficient of?-Al₂O_3p/ZL109 composites.