Cavitation and failure during hot forging of Ti-6Al-4V
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I. INTRODUCTION
THE understanding of bulk hot working operations for high-temperature aerospace alloys such as those that are titanium or nickel base has seen great advances in the last 15 years. The principal considerations in the design of such processes focus on the prediction and control of metal flow, press loads, and microstructure and the avoidance of fracture or other defects. A suite of mathematical tools, ranging from slab analysis to detailed finite element method (FEM) approaches, is available for estimating working loads, preform designs, and metal flow patterns, and as such is commonly used by the metalworking industry.[1,2] The understanding of microstructure evolution during hot working, in particular during ingot breakdown, is less highly developed. A number of researchers have examined the mechanisms of dynamic recrystallization of single-phase alloys, dynamic globularization of lamellar microstructures, etc.,[3,4,5] as well as the phenomenology of such processes (e.g., recrystallization/globularization kinetics[6,7,8]). However, only relatively little effort has been expended to inter-relate mechanistic and phenomenological knowledge.[9,10] Similarly, the gross features of defect formation and fracture during hot working are well documented,[11,12] but a quantitative description of such failure processes, which can be utilized in process design, is still lacking. One of the most important areas in which our knowledge of failure is limited is the occurrence of cavitation during S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, Wright-Patterson Air Force Base, OH 454337817. R.L. GOETZ, Research Scientist, and V. SEETHARAMAN, Senior Scientist, are with the Materials and Processes Division, UES, Inc., Dayton, OH 45432. E.B. SHELL, Graduate Student, is with the Dept. of Chemical Engineering, the University of Dayton, Dayton, OH 45469. A.K. GHOSH, Professor, is with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Manuscript submitted August 17, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
hot working of alpha/beta titanium alloys such as Ti-6Al4V. Typically, thermomechanical processing of ingots of these materials comprises beta hot working and beta annealing to refine/recrystallize the beta grain structure followed by alpha/beta hot working to break down the transformed microstructure produced during cooling from the beta phase field. During the initial steps of alpha/beta working, it is not uncommon for cavities and/or wedge cracks to form at the prior-beta grain boundaries/triple points.[11] These cavities may be located within or at the surface of hot-worked billets depending on friction and workpiece/tooling geometry. Thus, such damage may not always be readily detected in wrought material and may be deleterious to service properties if not closed during subsequent metalworking operations. The development of models for the initia
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