## 2 Materials and Methods  
The Ti-6 Al-4V alloy used in this study is an AMS4928 grade Ti alloy, and its composition is Ti-5.50-6.75 Al-3.5-4.5V-0.30 Fe, which is a bulk material forged to \(\Phi\)500 with a forging ratio of 1.5-2 after performing VAR (Vacuum Arc Remelting) twice. For observing the initial microstructure, the as-received material of \(\Phi\)500 was divided into four parts, and high-temperature compression specimens of \(\Phi\)10 \(\times\) 15 mm were machined in the circumferential direction at an intermediate point of its radius, as shown in Figure 1. Then, one of them was divided into XY, YZ, and XZ planes, cut, and hot-mounted. Then, macropolishing was performed using abrasives of #200-#2000, and micropolishing was performed using abrasives of 6\(\sim\)0.04 \(\upmu\)m. After polishing, 2 mL of HNO\({}_{3}\) and 2 mL of HF were mixed with 100 mL of H\({}_{2}\)O to perform an etching process. The microstructure of the etched specimen was observed at \(\times\)50-\(\times\)1000 magnification through an optical microscope.  
Figure 1: Schematic position diagram for the high-temperature compression samples in the as-received Ti-6 Al-4V alloy.  
In addition, Vickers hardness was measured using a Vickers hardness tester (HM-200, Mitutoyo, Japan) in order to evaluate the mechanical properties at room temperature, and a room temperature tensile test was conducted using a universal material tester (UTM, BESTUM-10 MD, Sasul Bestech, Seoul, Korea). The Vickers hardness test measured 12 points in the direction of crossing the specimen by applying a load of 0.5 kgf for a holding time of 15 s at 0.25 mm intervals, and calculated the average hardness and standard deviation excluding the maximum and minimum values. The room temperature tensile test was conducted at a strain rate of \(1\times 10^{-3}\)/s after processing the specimen in the circumferential direction (A\({}_{1}\), B\({}_{1}\)) and the center direction (A\({}_{2}\), B\({}_{2}\)) according to the rod-shaped tensile specimen standard with a grip part of \(\Phi\)10, a marking distance of 24 mm, and a diameter of \(\Phi\)6 for the measurement section based on the ASTM ESM-16a standard.  
To determine the physical properties and deformation behavior of the Ti-6 Al-4V alloy at high temperature, a \(\Phi\)0.5 \(\times\) 1 mm hole for mounting a thermocouple on the high-temperature compressed specimen was additionally processed in the center of the specimen. The high-temperature compression test was conducted using a Gleeble 3500 equipment in the temperature range of 800-1100 \({}^{\circ}\)C with a heating rate of 10 \({}^{\circ}\)C/min, a holding time of 60 s, and air cooling at a reduction rate of 70% maximum according to a total of 7 temperature conditions (800 \({}^{\circ}\)C, 850 \({}^{\circ}\)C, 900 \({}^{\circ}\)C, 950 \({}^{\circ}\)C, 1000 \({}^{\circ}\)C, 1050 \({}^{\circ}\)C, and 1100 \({}^{\circ}\)C) and 3 strain rate conditions (\(1\times 10^{0}\)/s, \(1\times 10^{-1}\)/s, and \(1\times 10^{-2}\)/s) with 50 \({}^{\circ}\)C intervals. Then, the strain rate sensitivity was presented as a gradient by deriving the flow stress, and a polynomial regression analysis was performed on the plastic instability and stability criteria.  
Macrostructures and microstructures were observed after cutting the high-temperature compressed specimen in two parts according to each condition in order to observe these structures in each zone after applying high-temperature compressing to the specimen. In addition, an SEM-EBSD analysis was conducted to determine the difference in the formability caused by the grain refinement according to the dynamic recrystallization.  
The ring-rolling process was performed by applying the optimal process conditions derived from the analysis of the above high-temperature compressive deformation behavior, and the macrostructure and microstructure of a practical ring-rolling forming product were observed.