The occurrence of shear bands in nonisothermal,hot forging of Ti-6AI-2Sn-4Zr-2Mo-0.1Si

The occurrence of shear bands in nonisothermal, hot forging of Ti-6Al-2Sn-4Zr-2Mo-0. ISi (Ti-6242) was investigated in order to establish the material properties and process parameters which generally lead to shear bands and shear cracks in conventional hot forging of metals. Upset compression tests on cylindrical samples and lateral sidepressing tests on long, round bars were performed to determine the modes by which flow localizes in deformation states ranging from axisymmetric to plane strain. In axisymmetric deformation, it was found that nonisothermal, hot compression leads to chill zones and bands of intense deformation separating the chill zones from the deforming bulk. For plane strain deformation, shear bands were found to initiate along zero extension directions and subsequently localize flow in a manner analogous to the formation and propagation of shear bands in isothermal, hot forging. For both deformation states, it was found that material properties, such as the flow stress dependence on temperature, and process parameters, such as forging speed and die temperature which strongly influence the amount of heat transfer, play critical roles in the flow localization process. A simple model quantifying these effects was developed to predict the occurrence and severity of the shear bands observed in the Ti-6242 alloy hot forged at various temperatures and rates. In addition, the occurrence of shear cracking under certain forging conditions was rationalized in terms of the chilling brought about by nonisothermal, hot forging conditions and the inferior workability of Ti-6242 at temperatures far below the transus temperature.