Microstructural mechanisms during multidirectional isothermal forging of as-cast Ti-6Al-4V alloy with an initial lamellar microstructure

Abstract

Microstructural evolution and tensile properties of Ti-6Al-4V alloy with an initial lamellar microstructure during the multidirectional isothermal forging (MDIF) were investigated. After three steps isothermal forging, a homogeneous equiaxed grained microstructure with an average grain size of 1.9 μm was achieved. The grain refinement mechanism included both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). The necklaces of new DDRX grains with high angle grain boundaries (HAGBs) were formed along the initial β grain boundaries. Grain subdivision was through CDRX. The fraction of recrystallization and the homogeneity of microstructure were improved with the isothermal forging steps increasing, the fractions of recrystallization increased from 43% to 63% and the fractions of HAGBs increased from 48% to 71%, respectively. The tensile properties of as-cast Ti-6Al-4V alloy were significantly improved at both room temperature and 400 °C, respectively. The yield strength, ultimate tensile strength and elongation increased 21%, 23%, and 210% at room temperature, respectively. And at 400 °C, the yield strength, ultimate tensile strength and elongation increased 46%, 48%, and 21%, respectively. The fracture mechanism changed from brittle fracture to ductile fracture after the MDIF process.