Study On Preparation And Mechanical Properties Of Al-Cu Matrix Composites Reinforced By SiC Nanoparticles Or Dual-Phase Hybrid With Mg₂Si

Nanoparticles reinforced aluminum matrix composites have both excellent strength and plasticity,which have broad application prospects in aerospace,automotive,electronics,military and other fields.Compared with other ceramic particles such as Ti B₂,Al₂O₃,SiC ceramic particles have the advantages of smaller relative density,lower thermal expansion coefficient,higher thermal conductivity and higher elastic modulus,which is a good reinforcing phase.Among the many preparation methods of nano-SiC_P/Al matrix composites,the liquid casting method has attracted much attention because of its simple equipment,low production cost and easy forming of complex-shaped parts at one time.However,the main difficulties in external addition process include extremely easy agglomeration of nanoparticles,poor wettability of SiC_P and Al liquid,severe interfacial reaction,and limited nano-SiC_P content.Therefore,in view of above difficulties in the preparation of nano-SiC_P/Al matrix composites by liquid casting method,this paper proposes a new technology that combines high-energy ball milling to prepare nano-SiC_p/Al composite granules,smelting casting method,ultrasonic-assisted nano-SiC_p dispersion and squeeze forming process,so that nano-SiC_P can be smoothly added to the melt and the particles distribution in nano-SiC_P/Al-Cu composites has been effectively improved.Meanwhile,based on the In-situ Mg₂Si/Al-Cu composites,the nano-SiC_p+Mg₂Si hybrid reinforced aluminum matrix composites with better comprehensive properties are prepared by the combination of the liquid additive method and the in-situ fusion casting method for the first time.Furthermore,the evolution of microstructure and mechanical properties of the nano-SiC_p+Mg₂Si hybrid reinforced aluminum matrix composites are investigated,and strengthening mechanism are discussed in detail.The dispersion behavior and mechanism of nano-SiC_p in composites treated by ultrasonic treatment have been studied.After applying ultrasonic vibration,the distribution of nano-SiC_p in the nano-SiC_p/Al-Cu composites is significantly improved due to the effects of acoustic cavitation and streaming.With the increase of ultrasonic time,the distribution of nano-SiC_p becomes more uniform,and the size ofα-Al and Al₂Cu phases becomes smaller.After ultrasonic vibration for 5 min,the ultimate tensile strength,yield strength and elongation in as-cast state of composites are 270 MPa,173 MPa and 13.3%,which are increased by 7.6%,6.8%and 29%compared with Al-Cu matrix alloy,respectively,and are increased by 27.4%,9.5%and 303%compared with the untreated composites,respectively.The xnano-SiC_p/Al-Cu（x=0.5,1,1.5,2 wt.%）composites have been prepared through the above optimized process.With the increase of nano-SiC_p content from 0.5 to2.0 wt.%,the primaryα-Al and Al₂Cu phases are refined significantly up to 1.5 wt.%but then become coarser at 2.0 wt.%,and the strength and elongation of composites show the same trend change.The optimal ultimate tensile strength,yield strength and elongation are obtained in 1.5 wt.%nano-SiC_p/Al-Cu composites,which are increased by 18.7%,11.3%and 25.3%,compared with the Al-Cu matrix alloy.According to quantitative calculations,the enhancement of strength is attributed to four strengthening mechanisms,among which theΔand?_（69）are the two most important contributors.In order to further strengthen the material,the heat treatment behavior of nano-SiC_p/Al-Cu composites has been investigated in depth.The presence of nano-SiC_p in the composites will hinder the solution of Al₂Cu phase,and the higher the particle content,the greater the inhibition effect.However,nano-SiC_p will promote the aging precipitation ofθ’’phase during aging.Considering the solution effect of Al₂Cu phase,the optimal heat treatment for 1 wt.%nano-SiC_p/Al-Cu composites is 540°C×10h+180°C×8h.The T6treated composites’hardness,ultimate tensile strength and yield strength are 150HV,500MPa and 350MPa,which are increased by 120%,82%and 106%than that of the as-cast composites respectively,and its elongation is 8.5%.Based on the low content and hypoeutectic In-situ x Mg₂Si/Al-Cu（x=1.5,3,5,10wt.%）composites,a process consisting of the liquid addition method and the melting and casting method has been firstly proposed to prepare 1wt.%nano-SiC_p+10wt.%Mg₂Si/Al-Cu and 1wt.%nano-SiC_p+1.5wt.%Mg₂Si/Al-Cu hybrid composites.The nano-SiC_p can act as heterogeneous nucleus of Mg₂Si phase,or be captured by the Mg₂Si solid front during solidification,due to small lattice mismatch of the two-phase.Nano-SiC_p and Mg₂Si can mutually improve the dispersion uniformity of each other,showing the effect of mutually optimized distribution,thereby improving the comprehensive properties of the composites.Compared with gravity casting,the ultrasonic treatment in the composites formed by squeeze casting has a more significant optimization effect on the distribution of nano-SiC_p and Mg₂Si.The as-cast ultimate tensile strength and elongation of 1wt.%nano-SiC_p+10wt.%Mg₂Si/Al-Cu composites fabricated by combining ultrasonic treatment and squeeze casting are 17.5%and 130.8higher than those of composites without ultrasonic treatment.In addition,the ultimate tensile strength and yield strength of 1wt.%nano-SiC_p+1.5wt.%Mg₂Si/Al-Cu composites are 325 MPa and 210 MPa,which are 3.2%and 28.8%higher than those of 1.5 wt.%Mg₂Si/Al-Cu composites.These properties are also improved by 20.4%and 21.4%compared with 1wt.%nano-SiC_p/Al-Cu composites,while it maintains good ductility.