Study On Preparation And Squeeze Casting Of Nano-Al₂O_3p/A356 Composites Based On Al-calcined Kaolin System

A356 alloy is widely used in industry because of its good casting properties and comprehensive mechanical properties.However,in order to meet the requirements of lightweight,the mechanical properties of A356 alloy can be further improved by introducing in-situ reinforcing particles.In order to control the preparation cost of composite materials effectively,this study systematically studies the reaction process and mechanism of the Al-calcined kaolin system using a wide-source and low-cost calcined kaolin as the reactant materials.The high-strength nano-Al2O3p/A356 composite materials are prepared by the melt direct reaction method successfully.The influence of the process parameters on the microstructure and mechanical properties of the composites are analyzed,and the fracture and strengthening mechanism of the composites are discussed.Conclusions are as follow:The free energy of reaction of the Al-calcined kaolin system ΔGT0=-2093230+166.41T,let ΔGT0=0,then T0=12578.75K.When T<12578.75K,ΔGT0<0,the in-situ reaction can take place successfully.The DSC analysis has an exothermic reaction at 873℃,and the reaction activation energies at the initial,peak and end stages of the reaction are 84.5kJ/mol,87.6kJ/mol and 301.5kJ/mol,respectively.With the help of Kissinger equation,the DTA data with heating rates of 10℃/min,15℃/min,20℃/min,25℃/min and 30℃/min are calculated.The results show that the activation energy of the system increased with the reaction.The chemical reaction of the Al-calcined kaolin system is studied through the combination of ball milling powder and vacuum reaction sintering.The results are that the Al-calcined kaolin system achieved local reaction through the diffusion and infiltration of liquid aluminum on the surface of calcined kaolin,and generated γ-Al2O3 and Si.When the holding time is 30 min,the higher the heating temperature,the more complete the reaction and the larger the size of γ-Al2O3.When the heating temperature is 800℃ and the holding time is 0 min,the particle size of γ-Al2O3 is about 1μm.When the holding time is 50 min,the reaction is basically completed,and a small amount of nano-particles appeare on the basis of micro γ-Al2O3 particles,forming γ-Al2O3p/Al composites.With the help of high-energy ultrasonic treatment,nano-Al2O3p/A356 composites with different volume fractions are prepared by melt direct reaction method successfully.The results show that after ultrasonic treatment for 6 min,the particles in 0.6vol.%nano-Al2O3p/A356 composite are uniformly dispersed without obvious pores.Al2O3 particles have the best effect on the refinement of α-Al and Si phases.The α-Al secondary dendrite arm spacing is 14～17μm and the longest Si phase is 15 μm.The T6 heat treatment has an obvious refinement effect on the Si phase,but has no obvious effect on the dispersion of nano-particles and the spacing of the secondary dendrite arms of α-Al.The squeeze casting study of 0.6vol.%nano-Al2O3p/A356 composite materials show that under the conditions of pouring temperature 720℃,injection speed 0.1 m/s,holding pressure time 10 s and injection specific pressure 80 MPa,the α-Al and Si phase are further refined,and defects such as voids and porosity are eliminated,and the compactness of the material structure is improved.T6 heat treatment has obvious effect on the spheroidization of Si phase in the composite material,and the dispersion is uniform.The hardness,tensile strength and elongation of 0.6vol.%nano-Al2O3p/A356 composite material in the T6 state after 80 MPa squeeze casting reach 143.5 HV,326 MPa and 6.33%,respectively.Compared with the casting T6 state,the increase is 24.57%,17.27%and 32.43%respectively,and compared with T6 of the squeeze cast A356 alloy increased by 27.9%,42.36%and 161.57%,Fracture analysis shows that the A356 matrix is brittle fracture under gravity casting,and the 0.6vol.%nanoAl2O3p/A356 composite materials are mainly brittle fracture.The squeeze cast A356 alloy and composite materials are brittle-ductile mixed fracture.The fracture modes of the squeeze cast matrix and composite after T6 heat treatment tend to be ductile fracture.