Effect of Tool Rotational Speed on the Microstructure and Associated Mechanical Properties of Incrementally Formed Commercially Pure Titanium

Single-point incremental forming (SPIF) was conducted on a 1-mm-thick commercially pure titanium grade 2 (Ti-G2) sheet metal in a CNC vertical milling unit. A hardened steel ball of 12 mm diameter was used as forming tool. Frustum cups were formed with varying spindle speeds between 300, 450, and 600 RPM. Other process parameters including the vertical step down and feed rate were kept as 0.2 mm and 300 mm/min, respectively. The metallurgical and mechanical properties of the formed material were investigated by cutting samples from the frustum cup walls. Electron-backscattered diffraction (EBSD) investigation revealed limited change in grain size with an increase in spindle speed. Dislocation density was measured by x-ray diffraction peak broadening analysis. The results indicate that an increase in spindle speed resulted in an increased dislocation density. The EBSD-based textural studies revealed a strong basal texture with near P and B type orientations visible at the maximum spindle speed. The tensile tests demonstrated a proportional increase in tensile strength with an increase in spindle speed along with a significant reduction in total ductility. The enhanced dislocation density and the formation of a strong basal texture were considered as the main drivers for the improvement in the tensile strength. A maximum tensile strength of nearly 550 MPa was obtained for samples extracted from the walls of the frustum cup at the maximum spindle speed of 600 RPM. This translates to an 80% enhancement of the tensile strength when compared to the base metal .