Effect of Stress and Strain Path on Cavity Closure during Hot Working of an Alpha/Beta Titanium Alloy
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INTRODUCTION
THE development of successful manufacturing techniques for metallic materials requires reliable information regarding hot-working characteristics. The proper hot working temperature and deformation rate must be established to produce high-quality wrought products of complex geometry. In addition to the avoidance of surface defects that reduce product yield, the control of internal cavitation is also very important, because such damage may lead to poor service properties as well as premature failure.[1–4] Because of the industrial significance of damage phenomena, a large amount of research has been conducted to quantify cavitation for various metals and alloys and to develop predictive models. Most of these efforts have focused on the determination of the conditions under which cavitation can be fully suppressed (or at least minimized) or on quantifying cavitygrowth kinetics as a function of a stress state. In the latter regard, a number of testing techniques have been employed to investigate cavitation as a function of stress state; these methods include uniaxial tension, biaxial tension, simple shear/torsion, and uniaxial tension with superimposed hydrostatic pressure.[4–9] Multistep cogging and open-die forging are the industrial processes usually employed for the breakdown of the lamellar colony microstructure during the conversion of titanium ingot materials into wrought mill products. Such processes typically involve complex states of stress and strain. Therefore, the effect of strain and strain-rate path P.D. NICOLAOU, Consultant, is with El. Venizelou 31, 191 00, Megara, Greece. Contact e-mail: [email protected] R.L. GOETZ, Research Engineer, is with UES Inc., Dayton, OH 45432. S.L. SEMIATIN, 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 July 31, 2007. Article published online January 23, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
on the flow and cavitation behavior has been attracting increasing attention in order to provide better insights into the industrial processes.[10,11] The present work is a continuation of earlier efforts on cavitation during monotonic and reversed-torsion* *The term reverse torsion denotes torsion testing in which the twist direction is reversed during the experiment.
testing. The specific objective of the present research was to establish and model the effect of a different strain path (torsion followed by compression) on the closure of cavities and to compare the closure kinetics to those found for reversed torsion. To this end, torsion-compression experiments were conducted on the same material used previously (Ti-6Al-4V with a colonyalpha microstructure), and the results were interpreted using a model originally formulated to treat the consolidation behavior of powder-metal compacts.
II.
MATERIALS AND PROCEDURES
A. Material The material used in the present work was Ti-6Al-4V, which was receive
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