An analysis of deformation, temperature, and microstructure for hot extruded titanium alloy
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I.
INTRODUCTION
C O M M E R C I A L L Y pure titanium and its alloys have excellent corrosion resistance and high specific strength. Therefore, they are mainly used as structural materials, such as airframe structures, which need durability. The hot extrusion process receives a lot of attention because it can potentially provide near net shape products, m The extrusion process involves large deformations at high rates; therefore, large temperature changes can be produced by the conversion of plastic work to heat, which results in microstructural inhomogeneities. In order to improve homogeneity in the microstructure, and hence properties, of extrusions, it is essential to understand cross-sectional distributions of strain, strain rate, and temperature under various extrusion conditions. Shabaik et al. ~21analyzed deformation in symmetric extrusions by using visioplasticity method. Altan and Nagpal t3] used flow function methods and Kiuchi et al. TM used upperbound elemental technique (UBET) for deformations in nonsymmetric exlrusions. In these analyses, material flow is considered but not its impact on microstructural changes. In this work, strain and temperature distributions, as well as microstructure, of hot extruded Ti-6A1-4V alloy were studied using visioplasticity methods, thermal calculation, tS] and optical microscopy.
II.
Before hot extrusion, billets were machined to 62 m m in diameter and 155 m m in length. These billets were then cut in half along the center line. A grid of 0.3 m m line thickness with a distance of 5 m m was printed on one surface of the split billet using electric discharging. The billet halves were then reassembled and extruded. The billets were extruded after heating to 950 ~ or 1100 ~ in air and then extruded at a ram speed of 70 m m / s through a conical die with a half cone angle of 45 deg. The extrusion ratios were 6 and 12. A silicatebase lubricant was used. The cross-sectional temperature distribution at the beginning of the extrusion was measured with thermocouples embedded at various depths in the billet. The extrusion process was stopped when a sufficient length of billet had been extruded to establish steady-state motion. On separation of the two billet halves, the deformed grid was measured to calculate strain and strain rate along flow lines. Microstructural observations were made on the midplane with respect to phase transformations and the morphology of the primary a phase. The computations included determination of the flow function by using the visioplasticity method I2] and thermal calculations using two-dimensional ordinary differential equations. The coordinate axes and flow lines are shown in Figure 1. It is known that plain strain flow of an incompressible material can be represented by using a flow function ~ defined by
EXPERIMENTAL PROCEDURE
A Ti-6A1-4V alloy billet, finish forged in a + 13 region, was used in this study. Its chemical composition is shown in Table I. K. KIMURA, Senior Researcher, and H. YOSH1MURA, Chief Researcher, are with the Hikari Rese
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