Study On The Preparation And Hot Extrusion Of In-situ Submicron MgAl₂o_4p/6061Al Composites

Compared with the traditional micro-particle reinforced aluminum matrix composites,in-situ sub-micron/nano particle reinforced aluminum matrix composites,not only present the advantages of high specific modulus,high specific strength,and good wear resistance,but maintain the plasticity of the matrix alloy,makes them have a broad application prospects in the automotive,aircraft,aerospace and other fields.However,the preparation of such aluminum matrix composites is complicated and difficult due to the physical and chemical properties of submicron and nanoparticles.One difficulty in the preparation of such composites is that submicron/nano particles have poor wettability with the matrix alloy.The another is the dispersion of submicron/nano reinforcement phases.In order to solve the above problems,the in-situ submicron MgAl₂O₄ particle reinforced 6061 matrix composites have been successfully prepared by melt direct reaction method combined with high energy ball milling,hot rolling,and high-intensity ultrasonic treatment.The composites were deformed by hot extrusion technique.The effects of hot extrusion and heat treatment on the microstructure and mechanical properties of the composites were investigated.The strengthening and toughening mechanisms of the hot extruded and the heat treated composites were analyzed.In the preparation process of SiO₂-Al preform,submicron SiO₂ particles was uniformly distributed into the surface of pure aluminum powders by high-energy ball milling,and then the dense SiO₂-Al preform was prepared by hot rolling.During the addition of reaction particles,the gas adsorbed on the surface of the submicron SiO₂ reaction particles was eliminated and the wettability between the reaction particles and the aluminum melt was improved,therefore the particles were smoothly fed into the aluminum melt.The melt with SiO₂ reaction particles was then insulated and subjected to high-intensity ultrasonic treatment and the in-situ submicron MgAl₂O₄ particle reinforced 6061 Al matrix composites were obtained.The microstructure analysis of the as-cast composites shows that in-situ submicron and nano-MgAl₂O₄ particles have formed by the reaction of submicron SiO₂ particles,which mainly distribute at the grain boundary of the matrix alloy.In addition,the interface between these in-situ particles and the matrix alloy is clean and these particles are well bonded to the aluminium matrix.As the MgAl₂O₄ content increases,the more particles are distributed at the grain boundary due to the solid-liquid interface pushing during solidification.Compared to the matrix alloy,the grain size of the composite is significantly reduced.As the particle content increases,the grains of the composites change from coarse dendrites to fine equiaxed crystals,and the grain size decreases from 272.3?m of the material without MgAl₂O₄particles to 74.4?m of that with 2.0 wt.%MgAl₂O₄ particles.Among them,the grain size decreases with the increase of the content of reinforcement particles.The microstructure analysis of the hot extruded composites shows that the effects of hot extrusion deformation treatment are as follows:?1?reducing or eliminating defects such as casting porosity and increasing the density of the composites;?2?fragmenting and refining the brittle second phases to reduce their splitting effect on the matrix;?3?causing the matrix to undergo dynamic recrystallization and refine the grains;?4?redistributing the particles along the extrusion direction to improve the dispersion of the particles.The hot extrusion process has a significant effect on the microstructure of the composite.It is found that the relatively good hot extrusion process is with the extrusion ratio of 16 and the extrusion temperature of450°C.Under this process,the casting defects can be eliminated effectively,the brittle second phase can be refined,and a uniform fine equiaxed grain structure can be obtained.The microstructure analysis of the heat-treated composites shows that the well-dispersed nano-precipitates are formed in the alloy matrix after heat treatment,which can promote the composite performance.The tensile properties of the composites have been greatly improved compared to those of the 6061 alloy matrix.The tensile strength,yield strength and elongation of the as-cast in-situ 1.5wt.%MgAl₂O₄/6061Al composites are 254 MPa,151 MPa and 20.8%,respectively.Compared with the 6061Al matrix,the corresponding increase ratios are 34.3%,62.4%and26.8%,respectively.The tensile strength and yield strength of the hot-extruded in-situ 1.5 wt.%MgAl₂O₄/6061 Al composite?the extrusion ratio is 16,the extrusion temperature is 450°C?are 282 MPa and 171 MPa,respectively.Compared with the as-cast,the increase ratios are11.0%and 11.7%,respectively.In addition,the elongation of the hot extruded composite increases with the increase of the deformation amount,and increases first and then decreases with the increase of the extrusion temperature.The tensile strength of?T6+extrusion?composites has been significantly improved.The tensile strength and yield strength of in-situ1.5wt.%MgAl₂O₄/6061Al composites increase from 282 MPa and 171 MPa to 384 MPa and277 MPa,the increase ratios are 36.2%and 59.2%,respectively.In addition,the hardness of the composites increase with the increasing particle content,and the hardness of the?T6+extrusion?state composites increases most significantly.The strengthening mechanism of the composites in the as-cast,the extruded or the?T6+extrusion?state was analyzed.The conclusions show that the comprehensive properties of the composites are improved due to the synergistic effect of Orowan strengthening,fine grain strengthening,dislocation strengthening,and load-bearing strengthening.