The occurrence of shear bands in isothermal, hot forging

Abstract

The occurrence of shear bands in isothermal, hot forging of metals has been investigated using a variety of experimental and analytical tools. Isothermal sidepressing tests on Ti-6Al-2Sn-4Zr-2Mo-0. 1Si were conducted at a variety of hot working temperatures and deformation rates to develop a data base on the occurrence of shear bands as a function of material properties and processing conditions. An instability criterion was developed, and the tendency to form shear bands was shown to depend strongly on the ratio of the nondimensional work-hardening (or flow-softening) rate to the strain-rate sensitivity of the flow stress. This analysis and supporting experimental data formed the basis for the development of workability maps, which delineate safe regimes for hot working in terms of strain rate and temperature. Employing a sophisticated computer code, an extensive series of simulations was also run for the sidepressing operation to obtain an idea of the detailed process by which shear bands initiate. These simulations were conducted using actual Ti-6242 material properties as well as hypothetical properties which are representative of those of other metals deformed in the hot working regime. From these simulations, predictions of the influence of flow softening and rate sensitivity on flow localization were made and found to be similar to experimental results and predictions obtained from the instability analysis. However, process simulation gave a more complete insight into the initiation and growth stages in the development of shear bands that was not possible using only the instability analysis.