Deformation and unstable flow in hot forging of Ti-6Ai-2Sn-4Zr-2Mo-0.1Si
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developing sound processing sequences and avoiding defects in finished parts. These defects include brittle failures due to hot shortness, 6 grain boundary and triple point cracks, 7 and void generation (cavitation)? Another possible defect, in hot forging in particular, is that of shear bands, or regions of localized deformation crossing many grains. 9 During deformation processing, these regions of intense deformation and heat develop at the interface between adjacent areas of discontinuous deformation such as that between regions of deforming and nondeforming (i.e., dead) metal? ~ Because the width of the interface can be infinitesmal, even small bulk strains can produce shear strains of the order of several thousand percent. Once initiated, a shear band may become self-propagating because the essentially adiabatic heating at the interface produces a marked drop of the local flow stress. As a result, localized phase changes or fracture may occur. Depending on the alloy, various combinations of process variables (tool and workpiece geometry, deformation rate, preheat temperature, die temperature, lubrication) and material parameters (strain-rate sensitivity of the flow stress, temperature dependence of the flow stress, strain-hardening rate, thermal conductivity, specific heat~ phase transformation kinetics) have been observed to lead to the development of shear bands. This problem has been observed in both cold and hot forging of alloys such as aluminum alloys, 11-13 carbon and alloy steels, la-17titanium alloys, is-E~and nickel-base alloys.E~Forged parts of these materials are found frequently in service applications requiring high performance. Hence, it is critical that the development of shear bands during metalworking and their possible effects on service properties be understood. Two-phase (0~//3) titanium alloys are particularly susceptible to shear bands during hot forging.2~The sharp dependence of flow stress on temperature, low strain-hardening rate, and poor thermal conductivity
ISSN 0360-2133/81/1012-1705500.7510 9 1981 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME lEA, OCTOBER 1981--1705
characteristic of these alloys 2~-~4contribute to the occurrence of shear bands. The investigation reported here was undertaken to establish the occurrence of shear bands in a typical o~/B titanium alloy--Ti6A1-2Sn-4Zr-2Mo-0.1Si (Ti-6242), an alloy which is receiving increasing attention in.jet-engine applications. 25 To characterize the deformation behavior of the alloy, isothermal, hot-compression tests were conducted over a range of temperatures and strain rates. The deformation mechanisms involved were inferred from the flow stress curves and metallography. These results were employed in the interpretation of observations of shear bands developed in both nonisothermallysidepressed cylinders as well as isothermally-sidepressed cylinders. EXPERIMENTAL PROCEDURES
tooling design is based on one originally employed by K u h n ? 6 Using a 5 kW Applied Test Systems resistance furnace, the
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