Semi-solid Compression Deformation Mechanism Of Particle-rein-forced Aluminum-cooper Composites

Aluminum matrix composites have many applications in aerospace,rail transit,automo-bile due to their excellent mechanical properties.In the process of solidification during casting and semi-solid processing,the defects of materials such as the formation of micropores and hot-tearing caused by the thermal shrinkage stress have puzzled the industrial manufacturing and scientific research.Studying the semi-solid deformation of aluminum-based composite materi-als can help researchers deeply understand the microstructure evolution and deformation mech-anism of semi-solid deformation,which is of great importance for casting and semi-solid pro-cessing.In this thesis,in order to study the semi-solid deformation mechanism,the mechanical stirring and ultrasonic vibration were used to prepare nano/micro-Al₂O₃ particle reinforced Al-10wt.%Cu composites with different contents and the Al-10wt.%Cu base alloy.Then,fast syn-chrotron computed tomography was used to quantitatively study the semi-solid compression process of three materials.Through combination of qualitative and quantitative analysis of mi-crostructure evolution,this thesis investigates the deformation behavior of the three-phase structure??-Al dendrite,Cu-rich interdendrite liquid channel and voids?in semi-solid state,and thus revealing the semi-solid deformation microstructure evolution mechanism of the three ma-terials.The results showed that the addition of Al₂O₃ particles in Al-10wt.%Cu alloy refined grains.The grain refinement effect is more obvious and the distribution of interdendritic region is more homogeneous in the nanoparticle reinforced Al-10wt.%Cu composite than in the mi-croparticle reinforced Al-10wt.%Cu composite.The addition of particles improved mechanical properties of as-cast alloys.With the addition of particles at the same content level,nanoparticle Al-10wt.%Cu composite has a higher tensile strength at room temperature than the micronpar-ticle Al-10wt.%Cu composite.Meanwhile,the addition of nano-particles increased the fracture toughness of material,while the addition of microparticles caused detrimental effect to the frac-ture roughness.Through synchrotron in situ imaging experiments,the void evolution process of nano Al₂O₃ particle-reinforced Al-10wt.%Cu composite under semi-solid deformation at different strain rates includes three stages:void closure,incubation period,and rapid growth period.This was consistent with the void evolution of particle-free alloy during semi-solid compression.The shape of a single void underwent the process of transforming from irregular shape to spher-ical morphology.However,voids of micro Al₂O₃ particle reinforced Al-10wt.%Cu composite showed a continual growth trend with increasing strain.With the growth of voids,dilation is a common phenomenon in three material compositions during semi-solid compression.The cross section of sample expanded locally at the location where a large number of voids gathered and grew.Some of the local interdendritic liquid channels gradually expanded with increasing strain.The results show that the local deformation mechanism of nano/micro-particle reinforced Al-10wt.%Cu composite is similar to that of particle-free alloy,such as the movement of den-dritic grains,the expansion of liquid-phase channels,and the nucleation and growth of pores.However,the particles with different size affect the morphology of the crystal grains,the dis-tribution of eutectic phases and the position of the particles and thus cause the change of hot-tearing sensitivity.By comparison,this thesis reveals that nanoparticle-reinforced Al-10wt.%Cu composite has a finer liquid phase channel,improved liquid phase channel distribu-tion,finer grains,lower viscosity and fewer initial voids,and showing the best hot-tearing re-sistance.In this thesis,the 4D?3D+time?study of aluminum matrix composites under load using synchrotron in situ imaging technique can provide a reference for exploring the semi-solid deformation behavior of aluminum matrix composites.