The occurrence of shear bands in isothermal, hot forging
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INTRODUCTION
INCREASING emphasis is being placed on materials conservation. This emphasis translates into tighter controls in manufacturing practice in areas such as metalworking, powder metallurgy, and casting. For example, there is a strong interest in forging of parts to near-net shape, a goal which, if realized, would minimize subsequent machining time and material waste. ~ Success in near-net-shape-metalworking operations such as forging will result only when accurate control of metal flow via preform design, lubrication, and so forth can be achieved without introducing defects in the worked parts. In cold forging, these defects include freesurface fractures, die-contact fractures, and central bursts." In warm and hot forging, defects such as brittle intergranular fractures, hot shortness, cavitation, and shear bands must be avoided. 3.4.5 Besides promoting better metal flow and die filling in hot forging, isothermal or hot-die forging (dies and workpiece at or near the same temperature) 6 eliminates the source of many shear band defects, namely, die chilling. During forging, chilling leads to the development of dead metal zones (regions of limited metal flow in the workpiece) whose extent depends on die temperature, working speed, lubrication, and the material's flow stress dependence on temperature. However, material properties and geometry may cause shear bands to occur even in isothermal or nearisothermal-forging situations. 7 In these instances, techniques must be developed to predict shear band occurrence. The prediction of shear bands in isothermal, hot forging can be studied from a gross viewpoint 5"8"9(in which only the event of failure onset is usually considered) or from a viewpoint employing process simulation ~~ (in which failure is studied as a total process). In the former case, it is typical S.L. SEM1ATIN, Principal Research Scientist, and G.D. LAHOTI, Senior Research Scientist, are both with the Metalworking Section, Battelle's Columbus Laboratories, Columbus, OH 43201. Manuscript submitted March 25, 1981. METALLURGICAL TRANSACTIONS A
that some sort of instability criterion is examined in a manner very analogous to the prediction of tensile uniform elongation or sheet metal forming limits using maximum load conditions, tL~: On the other hand, process simulation to predict failure is much more complex and has received limited attention in cold metalworking operations. ~~ let alone hot-working operations. The present paper is a continuation of previously reported work on the deformation and unstable flow of Ti-6AI-2Sn4Zr-2Mo-0. I Si (Ti-6242) in hot forging 7 and hot torsion.~7 In the previous work, an instability criterion was used to gage flow localization tendencies and shear band occurrence during isothermal deformation. The predictions compared favorably to observations. In the present work, the instability criterion is again employed to interpret data and shear band observations from a more complete test matrix of isothermal, hot forging variables. In addition, results from computer
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