Microstructure and mechanical properties of cup-shaped parts of 15% SiCp reinforced AZ91 magnesium matrix composite processed by thixoforging

Thixoforging experiments on SiC particles (SiC_p) reinforced AZ91 magnesium (Mg) matrix composites (AZ91-SiC_p) were conducted under different forging temperatures and forging loads. The microstructures and mechanical properties of the thixoforged AZ91-SiC_p samples were investigated experimentally by microstructural analyses, hardness tests, tensile tests, and compression tests. AZ91-SiC_p semisolid slurries exhibited different forming behaviors at different temperatures, because the different volume fractions and morphologies of the liquid phase, SiC_p, and α-Mg in them at different temperatures. The outflow of the liquid phase not only inhibited crack formation on the surface of the thixoforged samples but also resulted in segregations of intermetallic compounds and SiC_p in the thixoforged samples. Mechanical properties of the thixoforged samples affected significantly by the volume fraction and distribution of brittle Mg₁₇Al₁₂ intermetallic compounds surrounding α-Mg and SiC_p transformed from liquid phase during cooling. Higher forging temperatures improved the outflow of the liquid phase during thixoforging and resulted in thixoforged samples more inhomogeneous distributions of microstructures and mechanical properties. Forging load did not significantly affect the outflow of the liquid phase during thixoforging or the inhomogeneity of the microstructure of thixoforged samples. Higher forging loads caused higher residual stresses in the bottom regions of the thixoforged samples, which resulted in higher values of hardness in these regions.