Study On Shot Peening Of TiB₂/Al Composite And Its Characterization

Metal matrix composites have wide application prospects because of their good combination of excellent properties. Shot peening is an effective method to improve the surface properties of metallic materials. In order to full excavate property potentialities of the metal matrix composites, the shot peening method of the TiB₂/Al composite was improved in this dissertation. The residual stresses and microstructures induced by shot peening were investigated, and the relaxation behaviors of the residual stresses as well as the thermal stabilities of the microstructures were studied. Furthermore, the surface mechanical properties of the peened composite were also investigated and the mechanism of shot peening on composite was discussed.The results reveal that the values of compressive residual stresses for the peened composite increase with the depth to the peak value, then decrease and finally become low level tensile stresses at the subsurface layer. Under 0.15 mmA peening intensity, the surface compressive residual stresses of the peened sample is -123 MPa. The maximum compressive residual stresses and its depth is -175 MPa and 60μm, respectively. And the whole depth with compressive residual stresses is 290μm. Warm peening, stress peening and compound peening had further improved the residual stresses field and the microstructures of the TiB₂/Al composite. Compound peening combines the positive effects of warm peening and stress peening, and has the most strengthening effects.The relaxation behaviors of the residual stresses induced by shot peening under applied loadings and at high temperatures were investigated. It showed that the higher the applied loadings or temperatures, the more relaxations were. Under cyclic loading, residual stresses had a linear relationship with the logarithm of the cycle number. While under high temperatures, the residual stresses relaxation can be described by Zener-Wert-Avrami function. The activation energy of the residual stresses relaxation is 158.2 kJ/mol, and relaxation exponent is 0.2363. Thus the relaxation of residual stresses at elevated temperature was a creep process.The simulation results of the peening process showed that the depth distribution of the residual stresses is similar to the results obtained through XRD technique. There are high level compressive residual stresses in the matrix nearby the surface. The distribution of the residual stresses in the deformation layer is inhomogeneous. And the maximum compressive residual stresses were generated nearby the reinforcements.The X-ray diffraction profiles of the composite were fully investigated. It was found that the microstrain in the matrix was increased by the reinforcements, and then the Gaussian and Lorentzian components of a structurally profile were changed. Taking account of the Gaussian and Lorentzian components of each reflection profiles and introducing a power exponentωi, an exponential Modified Williamson-Hall plot method was proposed. Using the proposed exponential Modified Williamson-Hall plot method and Modified Warren-Averbach method, the size distribution in the peened composite can be obtained. Then the microstructures of the peened composite can be described objectively. Furthermore, the investigation of the profiles of the TiB₂ in the peened surface showed that shot peening had little influence on their size, while the microstrain increased slightly after shot peening.The microstructures of the peened composite were investigated by XRD line profile analysis. The results showed that fine domain, high microstrain and high value of dislocation density were introduced to the deformation layer, and the pre-exist texture were almost eliminated especially in the upmost surface. With the depth increasing, domain size increased and microstrain/dislocation density decreased, respectively. During annealing, the peening microstructures were recovered and recrystallized. Comparing to the monolithic alloy, during the whole annealing process, the grain growth of the composite was lower, while the declining of dislocation density and microstrain was a little faster in the early annealing stage. The activation energy of the grain boundary migration, for the TiB₂/Al and its monolithic alloy, is 240 and 220 kJ/mol. And their activation energy of microstrain relaxation is 217 and 211 kJ/mol, respectively. The movements of grain or subgrain boundaries were impeded by the pinning role of the reinforcements. Therefore, the recrystallization activation energy of the composite is higher than the alloy's one.The surface mechanical properties of the TiB₂/Al composite were improved by shot peening. The microhardness increased by at least 81% directly at the surface for the traditional peened sample at 0.15 mmA peening intensity. While under 0.23 mmA peening intensity, the proof stress of the matrix in the upmost surface layer was increased by 27%, and the whole proof stress of the top deformation layer was increased by 21%. The corresponding contribution of reinforcements on the strength increased after shot peening. Then the whole surface strength of the peened composite was improved significantly.The strengthening mechanism of shot peening on the metal matrix composite can be separated to stress strengthening and microstructure strengthening. The strengthening effects mainly took place in the matrix. The stress strengthening refers to the high level compressive residual stresses introduced to the matrix in the deformation layer. And for the microstructure strengthening, the microstructures are improved by shot peening, then the mechanical properties are enhanced. The reinforcements play important roles in the two strengthening effects. They can prevent stress relaxation under applied loadings and optimized the microstructures of the metal matrix during shot peening.