| Nano-size particle reinforced aluminum matrix composite, compared with the traditional micron particles reinforced aluminum matrix composite, presents the higher specific strength and specific modulus, corrosion and wear resistance, electrical conductivity, thermal conductivity due to the larger specific surface area and strong interface interaction of nanoparticles, which make them have a broader application prospects in the aerospace, automotive industry and other fields. However, the preparation of nano-particle reinforced aluminum matrix composite is more complex and difficult than the traditional aluminum composites due to the inherent physical and chemical properties of nanoparticles. The main difficulties of the preparation of nano-particle reinforced aluminum matrix composite lies in the uniform distribution of nanoparticle and the wetting bettwen the matrix and ceramic nanoparticle. Up to now, the technology is still less for the scaled-up preparation of bulk aluminum matrix nanocomposite.In this dissertation, nano-Al2O3p/2024aluminum matrix composite was prepared by solid-liquid mixed casting process, which effectively solve the key problems associated with poor wettability between the nanoparticle reinforcement and the matrix melt, and nanoparticle clustering in the melt during the preparation of metal matrix nanocomposite. The solidified microstructure and particle re-distribution behavior of nano-Al2O3p/2024aluminum matrix composite melt processed by the high-intensity ultrasonic were investigated. The main deformation process performed on the nanocomposite was repeated upsetting and extrusion (RUE), and the microstructure evolution and tensile properties during the multi-pass upsetting and extrusion process were studied, and the refinement mechanism of the microstructure and the deformation mechanism of RUE process were indicated. The strengthening mechanism of as-RUE processed nano-Al2O3p/2024aluminum matrix composite was analyzed. The main conclusions were drawn as follows:The flowing field of the fluid in the crucible during solid-liquid mixed process was simulated using finite element method (FEM). The effects of the stirrer geometry and some stirring process parameters on the flowing characteristics of the fluid were analyzed. The results showed that the stirrer geometry and processing parameters had significant effects on the flowing behavior of the fluid in the crucible. The multistage stirrer, lower blade angle and bigger diameter of the impeller were beneficial to reduce the mixing inefficient zone and "dead" zone in the crucible. Under the mixing action of multistage stirrer, a strong turbulence formed in the central region of the crucible. When the blade angle was30°, the turbulences generated by each impeller were like a "spiral" vortex. Increasing in the rotating speed, the flowing characteristics of the fluid in the crucible was gradually changing from "self-feeding" generated by each impeller to "the whole cycle", and the strong shear zone formed in the upper and lower regions of each impeller.During the preparation of nano-Al2O3p/Al composite powders, with the extension of ball time, the distribution homogeneity of Al2O3nanoparticle on the surface of Al powder was obviously improved. With the increase of the content of Al2O3nanoparticle, the clustering tendency of Al2O3particles in the composite powder gradually increased. When the content of Al2O3nanoparticle was4wt.%, the as-received nanopowder of Al2O3particle was uniformly distributed on the surface of Al powder after the ball time of12h. During the solid-liquid mixed process, Al2O3nanoparticles were effectively incorporated into the matrix melt and well dispersed in the melt under the action of the Al powders carrier. Based on the microstructure analysis, a model about the dispersion of Al2O3nanoparticles in the melt during the solid-liquid mixed process was established.When nano-Al2O3p/2024aluminum matrix composite melt prepared by the solid-liquid mixed process was processed by high-intensity ultrasonic, the solidified microstructure was obviously refined. The refinement efficiency was weaken on the too high or too low ultrasonic temperature. With the extension of ultrasonic time, the refinement efficiency gradually decreased. When the ultrasonic temperature was650～670℃, ultrasonic power was300W, and ultrasonic time was60s, the composite presented a fine uniform microstructure, which was composed of fine grains with an average size of25μm The Al2O3nanoparticles were pushed to grain boundary and final solidification regions in the normal casting condition. Under the condition of ultrasonic treatment, the transient cavitations could remove the gas layer from the nanoparticle surface, improving the wettability between nanoparticles and the matrix melt. Some Al2O3nanoparticulates were captured by the growing grain. As a result, the resulting distribution of Al2O3nanoparticles within the matrix was improved with the cooperation of ultrasound refinement of the microstructure.Before the RUE deformation of nano-Al2O3p/2024aluminum matrix composite, the distribution characteristics and their evolution laws of flowing field, strain field, temperature field and stress field during RUE process were analyzed using FEM. And the releated experiments were carried out to further verify the simulation. The results indicated that the traditional RUE process easily led to the processing defects, such as the axile hole and the fold, and the strain distribution inside the material was very uneven. The processing defects were effectively avoided using the modified RUE, and the uniform region of equivalent strain inside the processed sample increased with the increase of the billet length. A bigger corner radius and smaller friction were helpful to improve the strain homogeneity inside the processed sample. The strong shear region formed in the each stage of modified RUE process. Flowing fields of modified RUE showed that the grid flow line on the surface of as-processed sample flowed backward due to the friction after the odd number pass, and after the even pass RUE deformation, the grid characteristics reverted to the initial state. With the increase of the odd and even deformation passes, the relevant flowing characteristics of the grid lines presented no significant change.The microstructure evolution of nano-Al2O3p/2024aluminum matrix composite during modified RUE process was investigated by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results indicated that the matrix was effectively refined after modified RUE processing, and the grain refining efficiency decreased with the increase of RUE pass number. Lower deformation temperature and more deformation pass, the microstructure was more refine and homogeneous. After6pass RUE deformation at350℃, the average grain size of the matrix was5μm. The grain refining mechanisms of the matrix were consist of recrystallization refining mechanism and alternate shear refining mechanism. For the second phase, its refining mechanism was the mechanical cracking mechanism.Room-temperature tensile test results showed that after T6heat treatment, the tensile strength of as-RUE processed nano-Al2O3p/2024aluminum matrix composite increased initially, reached a peak and there after decreased to attain a constant value with the increase of the RUE pass. The ultimate tensile strength (UTS) and yield strength (YS) of as-extruded nano-Al2O3p/2024aluminum matrix composite processed by T6heat treatment was485MPa,382MPa and8.5%, respectively. The UTS and YS of as-T6nano-Al2O3p/2024aluminum matrix composite processed after1RUE pass was492MPa and391MPa, respectively. The UTS and YS of as-T6nano-Al2O3p/2024aluminum matrix composite processed after4RUE pass was477MPa and375MPa, respectively. After that, the tensile property presented no significant change with the increase of RUE pass. With the increase of RUE pass, the elongation (δ) was obviously improved. After5RUE pass, the8increased from8.5%to13.5%. The improvement on the comprehensive tensile properties of as-processed nano-Al2O3p/2024aluminum matrix composite resulted from the combined action of grain refinement strengthening mechanism, the strengthening mechanism of the matrix precipitated phase and Al2O3nanoparticle. |
