Effect of workpiece size on microstructure evolution of different regions for TA15 Ti-alloy isothermal near-β forging by local loading

The microstructure evolution and mechanical properties in different loading regions during isothermal near-β local forging were investigated using three TA15 Ti-alloy workpieces of different sizes, namely, a cylindrical sample, a quadrate billet, and a bulkhead component. Reasons for differences in the microstructure and mechanical properties were determined. Fine primary equiaxed α and clustered acicular secondary α were obtained for the small cylindrical sample, and the different loading regions exhibited good uniformity in microstructural morphology and size. Significant differences in microstructure existed between the first and second loading regions for the quadrate billet and bulkhead component, especially in the secondary lamellar α, which resulted in different mechanical properties. Specifically, coarse primary equiaxed α and straight and thin clustered lamellar α existed in the first loading region, and less coarse primary equiaxed α and thickened and globularized secondary lamellar α with short rod-like or equiaxed shape existed in the second loading region. The holding time and different cooling rates caused by the workpiece sizes resulted in differences in their primary equiaxed α and secondary lamellar α size. The combined effects of strain history, holding time, and cooling rate led to microstructural differences in different regions for the quadrate billet and bulkhead component, whereas the effect of strain history was reduced for the cylindrical sample because of the short holding time and fast cooling rate, and little difference existed between the first and second loading regions.