Multi-pass Hot Deformation Behavior and Microstructure Evolution of Spark Plasma-Sintered Ti-6Al-4V Alloy

The spark plasma-sintered Ti-6Al-4V alloy was used as experimental specimens to perform the multi-pass hot compression. The deformation temperature was 950, 900, 850 °C, and the compressed amount was 20, 30, 20% corresponding to one pass, two passes and three passes, respectively. The flow behaviors revealed that the curve with the strain rate of 5 s −1 appeared obvious oscillatory phenomenon after peak. Additionally, the curve with strain rate of 0.1 and 1 s −1 almost kept constant after peak stress during the process of two-pass compression. The microstructures showed that the sintered voids with the average diameter of 7.53 μm could be observed in initial Widmanstatten structures with the aspect ratio of 11.49 ± 0.5. The deformed inhomogeneity had prominent influence on the microstructures. For three-pass deformation with the strain rate of 0.1 s −1 , the microstructures of severe plastic deformation area were equiaxial microstructures with the grain diameter of 3.82 μm, while the microstructures for medial and small plastic deformation area were dual and Widmanstatten, respectively. With the strain rate of 0.1 s −1 , the fraction of dynamic recrystallization ( f DRX ) increased from 0.034 through 0.112 to 0.440 with the passes increased, and the variation tendency toward average diameter of grain size versus compressed passes shifted reversely. The β→α phase transformation was the main reason for the misorientation angle nearby 60° for one pass with the strain rate of 0.1 s −1 . The f DRX declined from 0.440 through 0.225 to 0.297 with raising the strain rate for three-pass deformation, but the grain size changed oppositely. The CDRX and DDRX were both appeared during multi-pass hot deformation.