The effect of stress state on cavity initiation during hot working of Ti-6Al-4V
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11/9/03
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Communications The Effect of Stress State on Cavity Initiation during Hot Working of Ti-6Al-4V P.D. NICOLAOU, R.L. GOETZ, J.D. MILLER, and S.L. SEMIATIN The development of internal cavitation during hot working may lead to premature failure or significantly reduce mechanical properties and thus subsequent service performance.[1,2] Hence, the control of internal damage in formed parts is of significant industrial interest. One of the key parameters that influences cavitation is the stress state. Analytical as well as experimental studies have shown that the cavity growth rate increases as the ratio of the mean to effective stress increases.[3,4,5] By contrast, the effect of stress state on cavity initiation has not been investigated to as great an extent. For this reason, the present work was undertaken to delineate the effect of stress state on cavity initiation by means of notched-tension tests.[6,7] Notched hot-tension tests and corresponding finite-element-method (FEM) simulations were conducted on Ti-6Al-4V samples with a colony-alpha microstructure to establish the effect of stress state on cavityinitiation behavior. The as-received material comprised 22.2-mm-diameter hot-rolled bar with a fine equiaxed-alpha microstructure. Its measured composition (in weight percent) was 6.20 aluminum, 4.04 vanadium, 0.052 carbon, 0.0025 hydrogen, 0.17 oxygen, 0.026 nitrogen, 0.18 iron, and balance titanium. The material was heat treated at 955 °C for 30 minutes followed by 1040 °C for 6 minutes, then cooled at an average rate* of 100 °C/min to 815 °C, soaked *Because we did not thermocouple the samples, this cooling rate has been estimated. Assuming a one-dimensional heat-transfer problem for a cylinder of diameter d and length , with heat losses from the lateral surface only, the cooling rate is estimated by the following relationship: dT/dt (4e/cd) (T 4 TF4), in which e is the emissivity (0.65), denotes the Stefan– Boltmann constant, T is the average temperature of the bar in Kelvin, and TF 815 273 1088 K, is the density of Ti-6-4 (4.43 g/cm3), and c is the specific heat (0.2 cal/g K at hot working temperatures).
for 15 minutes, and finally air cooled. The microstructure developed consisted of a mixed colony, basketweave structure with a prior-beta grain size (and comparable colony size) of 100 m and a grain-boundary alpha layer approximately 3-m thick. The heat-treated Ti-6Al-4V bars were machined into tension specimens with notches of various geometries to vary the stress state during hot deformation (Figure 1). All of the specimens had the same radius at the
P.D. NICOLAOU, R&D Scientist, is with S&B, S.A, Athens, 10672, Greece. Contact e-mail: [email protected] R.L. GOETZ, Research Engineer, is with UES Inc., Dayton, OH 45432. J.D. MILLER, Undergraduate Student, is with the Chemical Engineering Department, University of Dayton, Dayton, OH 45409. S.L. SEMIATIN, Senor Scientist, Materials Processing/Processing Science, is with the Air Force Research Laboratory, Mat
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