An analysis of cavity growth during open-die hot forging of Ti-6Al-4V
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I. INTRODUCTION
THE bulk hot working of engineering / titanium alloys such as Ti-6Al-4V is commonly used to convert coarse grain, colony- ingot microstructures to fine, equiaxed- billet structures. To this end, open-die hot forging operations (upsetting, cogging) are frequently employed. Despite their apparent simplicity, these processes involve relatively complex states of stress and strain within the workpiece, which may lead to undesirable defects. These defects include gross fracture, shear bands, and internal cavities. The initiation and growth of internal cavities is particularly important because such deleterious defects may eventually lead to gross fracture. Although the macroscopic stress state is generally compressive during open-die forging, cavities may initiate and grow due to the generation of secondary tensile stresses in some particular regions of the workpiece. The magnitude of these stresses and hence the degree of cavitation depend upon friction, workpiece geometry, processing conditions (e.g., temperature and strain rate), and intrinsic microstructural and textural characteristics of the workpiece material.[1–3] In view of the need to control internal damage during primary and secondary hot working, a considerable amount of research has been devoted to the understanding of cavitation. It has been observed that the cavitation process comprises three distinct stages: (1) cavity nucleation, (2) cavity growth, and (3) cavity coalescence. In many cases, these stages occur simultaneously. Nucleation usually occurs preferentially at grain boundaries, triple points, or second-phase particles. Subsequent growth occurs by either plasticity- or diffusioncontrolled mechanisms, or a combination of the two. For a given material, the particular mechanism varies with the imposed deformation conditions. A number of theoretical and physical models have been developed to predict the effect of P.D. NICOLAOU, R&D Scientist, is with S&B S.A., 106 72 Athens, Greece. Contact e-mail: p. [email protected] S.L. SEMIATIN, Senior Scientist, Materials Processing/Processing Science, is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, AFRL/MLLM, Wright-Patterson Air Force Base, OH 45433-7817. Manuscript submitted June 7, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
deformation conditions on cavity size.[4–7] However, only a few of these efforts have considered the effect of local microstructure and texture on cavitation behavior. The objective of the present work was to establish a modeling approach that could be used in process design to predict cavitation behavior during open-die hot forging of materials in which both local deformation and microstructure/ texture conditions play a role. For this purpose, attention was focused on the pancake forging of the / titanium alloy Ti-6Al-4V with a colony- microstructure. The experimentally determined cavity sizes were compared to predictions of two types of models: mesoscale and microscale. II. MATERIALS AND PROCEDURES A. Material The mat
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