The occurrence of shear bands in nonisothermal, hot forging of Ti-6AI-2Sn-4Zr-2Mo-0.1Si
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INTRODUCTION
I N the deformation processing of metals into useful products, two major materials factors must be considered during the selection of proper processing conditions. These factors are the material's resistance to deformation and the material's workability, t'2 For many pure metals and alloys, the deformation resistance, or flow stress, is a strong function of temperature. Flow stresses for a given material are typically much higher at cold-working temperatures (temperatures less than or equal to approximately one-fourth of the melting or solidus temperature) than at hot-working temperatures (temperatures greater than or equal to threefifths of the melting or solidus temperature). For this reason, a large proportion of the deformation processing of many metals is done in the hot-working regime. Examples of this practice include the hot breakdown and hot rolling of steel and aluminum to obtain sheet metal and the hot forging of many metals to obtain parts used, for instance, in structural applications and internal combustion and jet engines. The second major factor to be considered in selecting processing conditions is the material's workability. At hot working temperatures, defects which limit workability include void formation, triple-point cracking, and shear bands. 2-6 Perhaps the most important of these defects in hot forging is shear bands since these defects can occur without the assistance of tensile stresses which are important in the initiation or propagation of most other hot-working defects.
S. L. SEMIATIN is Principal Research Scientist, Metalworking Section, Battelle's Columbus Laboratories, Columbus, OH 43201. G. D. LAHOTI, formerly Senior Research Scientist, Metalworking Section, Battelle's Columbus Laboratories, is presently Senior Research Specialist, Research Department, Timken Research, The Timken Company, Canton, OH 44706. Manuscript submitted March 29, 1982.
METALLURGICALTRANSACTIONS A
Recent w o r k 7-10 has suggested that shear bands may occur in hot forging under nominally isothermal conditions (dies and workpiece at the same temperature) as well as nonisothermal conditions (dies and workpiece at different temperatures). Under isothermal conditions, it has been shown that flow softening or negative work hardening (a characteristic which is not uncommon for metals deformed in the hot-working regime 11) can lead to the localization of plastic flow, particularly when deformation is done in plane strain. This phenomenon is analogous to the onset of necking after maximum load in tensile deformation of metals. In particular, it has been shown for Ti-6242 that shear bands and other forms of nonuniform flow may be induced depending on preform microstructure, test temperature and strain rate, and deformation mode. When the alloy has an equiaxed alpha microstructure, hot forging deformation is always stable in isothermal, uniaxial compression, but may result in shear banding at low hot-working temperatures and high strain rates under plane-strain modes of deformation.9'1~The latter m
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