Influence of stress state on cavitation during hot working of Ti-6Al-4V
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1/8/04
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Influence of Stress State on Cavitation during Hot Working of Ti-6Al-4V P.D. NICOLAOU, R.L. GOETZ, and S.L. SEMIATIN Notched-tension tests were conducted on Ti-6Al-4V samples with a colony-alpha microstructure to determine the effect of the stress state on cavitation during hot working of alpha/beta titanium alloys. The experiments were complemented by finite-element-method analyses to establish the local stress state, strain, and damage factor for different areas within each sample. A critical damage factor, defined by a modified Cockcroft-and-Latham criterion, was found to be applicable for the prediction of cavity initiation for the different notch geometries. Measured cavity growth rates were also correlated to the stress state (i.e., ratio of mean-to-effective stress, M /e) and compared to predictions of prior models. Model predictions showed reasonable agreement with measurements at low levels of stress triaxiality, but exhibited some deviations at higher values of M/e. The differences were attributed to differences in the properties of the present material and those assumed in deriving the models as well as the neglect of cavity interaction in one of the models. The results were summarized in terms of a processing map, which delineates the initiation of cavities and their size as a function of the stress state and effective strain.
I. 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 complicated geometry. In addition to the avoidance of surface defects that reduce product yield, the control of internal cavitation is very important because such damage may lead to poor service properties as well as premature failure.[1,2,3] The development of internal cavitation depends on the nature of the material (e.g., its microstructure) and the externally applied processing conditions. Examples of materials exhibiting cavitation include aluminum, conventional titanium, titanium aluminide, copper, lead, and iron alloys.[4,5,6] With respect to processing conditions, the extent of cavitation is influenced by working temperature, the imposed strain rate, and the local stress state. The stress triaxiality, i.e., ratio of mean-to-effective stress, is perhaps the most important stress-state-dependent parameter.[7,8] A substantial amount of research has focused on the effect of stress state on cavity growth. Early analytical and experimental studies showed that the cavity-growth rate increases as the ratio of the mean-to-effective stress. For example, the cavity growth rate during equibiaxial expansion is noticeably greater than that during uniaxial tension. On the other hand, the cavity-growth rate decreases if a compressive hydrostatic pressure is superimposed during deformation.[9,10,11] Moderate to high levels of stress triaxiality typical of industrial hot working p
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