An approach for Room-Temperature Multi-Directional Forging of Pure
Titanium for Strengthening

H. Miura*, M. Kobayashia*, T. Aoba*, H. Aoyama**, T. Benjanarasuth***
*Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan
**Department of Mechanical Engineering and Intelligent Systems, The University of Electro-Communications, Tokyo
182-8585, Japan
***Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand

Commercially pure Ti grade 2 was multi-directionally forged (MDFed) MDFed at room temperature on an Amsler-type universal mechanical testing machine at an initial strain rate of 5.010-2 s-1 up to a cumulative strain of ΣΔε = 2.0 at maximum, i.e., 20 passes of forging (Fig. 1). Pass strains of Δε = 0.1 for each forging pass were employed referring to reduce sharp texture evolution, which prevents large straining at room temperature.

Typical microstructures evolved during MDFing are displayed in Fig. 2. As can be seen in Figs. 2(b), (c), the coarse initial grains with an average diameter of 33 µm (Fig. 2(a)) were promptly subdivided by mechanical twinning after MDFing to ΣΔε = 1.0. After MDFing to ΣΔε = 2.0, the microstructure became more homogeneous and relatively equi-axed because of multiple twinning to have an average grain size of approximately 400 nm (Fig. 2(e)). Such multiple twinning, having a completely different feature from the lamellar twins (Fig. 2(d)), must be formed at the areas remained still coarse at ΣΔε = 1.0.

Some samples MDFed to a cumulative strain of ΣΔε = 2.0 were further subjected to cold rolling up to 86% reduction at maximum, i.e., equivalent strain of 4.0 in total. Figure 3 shows a series of true stress vs. nominal strain curves obtained by tensile tests of the samples MDFed to various cumulative strains and additionally 86% cold rolled after MDFing to ΣΔε = 2.0. The initial Ti sample, i.e., before MDFing, shows large ductility over 40% while the yield strength and ultimate tensile strength (UTS) are only 200 MPa and 410 MPa, respectively. The yield strength and UTS, however, rapidly increased with increasing cumulative strain of MDFing and 630 MPa and 710 MPa respectively were achieved at ΣΔε = 2.0. The yield strength and UTS were more significantly increased by the additional cold rolling of 86% up to 810 MPa and 930 MPa, respectively. From the above experimental results, the way for room-temperature MDFing of Ti to attain superior balance of mechanical properties has been successfully suggested.

[Published in Materials Science & Engineering A 731 (2018) 603-608]

Fig. 1 A schematic illustration of multi-directional forging with pass strains of Δε = 0.1. Forging axis was changed for 90 degrees, x→y→z→x · ·, pass by pass of forging.

Fig. 2 Microstructural change during multi-directional forging (MDFing): (a) before MDFing and after MDFing to (b) ΣΔε = 0.1 and (c) ΣΔε = 1.0 observed by orientation-imaging microscopy. Transmission-electron micrographs after MDFing to (d) ΣΔε = 1.0 and (e) ΣΔε = 2.0, respectively. Component unit cells are shown in (b) as a sample of {101–2} twin. Some of kinks and double twins are indicated by arrows and circles in (c). F.A. indicates final forging axis.

Fig. 3 Change in tensile behavior by multi-directional forging (MDFing) and followed by additional cold rolling (CR). R.D. and T.D. indicate that tensile tests were carried out along rolling and transverse directions, respectively.