Simulation of forging processes for titanium alloys and typical forgings

With the rapid advancement of aviation technology, aerospace metal materials face pressing demands for lightweight performance, long service life, and excellent environmental adaptability. Titanium alloys, as a novel lightweight material developed over recent decades, exhibit superior properties such as high specific strength, excellent toughness, high damage tolerance, strong corrosion resistance, and good weldability. These characteristics have made them crucial in reducing aircraft structural weight, enhancing structural efficiency, and improving overall reliability, establishing them as one of the primary structural materials for advanced aircraft [1–5]. Titanium alloys have evolved from low-to-medium strength to high-strength varieties, with high-strength and high-toughness alloys becoming a major global trend in aviation applications—ideal materials for critical load-bearing components like aircraft frames, landing gear parts, and joint connectors. Representative high-strength and high-toughness titanium alloys such as Ti-1023, Ti-15-3, β-C, β-21S, BT22, TC21, and Ti-5553 [6–10] have been widely adopted. However, when applied to actual aerospace components, these alloys still fail to exceed a strength level of 1250 MPa.

In recent years, driven by the growing demand for lightweight materials in the aerospace industry, research institutions have conducted studies on ultra-high-strength titanium alloys with strength levels of 1300 MPa and above [11–15], aiming to further enhance structural efficiency, achieve greater weight reduction while ensuring safety and reliability, and deliver superior technical and economic benefits.

1. Establishment of Material Models

This study employs a newly developed Ti-Al-Mo-VCr-Zr-based multi-element strengthened metastable β-type ultra-high-strength titanium alloy, Ti-46531, with the nominal composition (by mass fraction) of 4% Al, 5% Mo, 6% V, 3% Cr, and 1% Zr. Accurate determination of the physical parameters of Ti-46531 is essential for numerical simulation studies; thus, key parameters—including specific heat capacity, thermal conductivity, and linear expansion coefficient—at various temperatures were measured experimentally, as shown in Table 1. The table indicates that while the specific heat capacity remains relatively constant with increasing temperature, both the thermal conductivity and linear expansion coefficient show significant increases. Additionally, the material exhibits a phase transition point at 820°C, a density of 4690 kg·m⁻³ at room temperature, and a Poisson's ratio of 0.32.

Constant temperature and constant strain rate compression tests were conducted on the Gleeble-3800 thermal simulation testing machine, yielding a constitutive model for Ti-46531 titanium alloy based on temperature, strain rate, and deformation magnitude. β-type titanium alloys are typically forged at temperatures 20–50 °C below their phase transition point, with strain rates ranging from 0.001 to 1.0 s⁻¹; consequently, the thermal compression tests were performed at temperatures of 720,750,780,810,840,870, and 900 °C with strain rates of 0.001,0.01,0.1, and 1.0 s⁻¹.

The true stress–true strain curves of Ti-46531 titanium alloy under various deformation conditions are shown in Figure 1. As illustrated, the rheological stress decreases with increasing temperature and increases with rising strain rate; both strain rate and deformation temperature significantly influence the rheological stress. At the same strain rate, the rheological stress at 700 °C is 2–3 times that at 900 °C; at the same temperature, the flow stress at a strain rate of 1.0 s⁻¹ is 3–5 times that at 0.001 s⁻¹. The high sensitivity of Ti-46531's flow stress to deformation temperature and strain rate substantially increases the difficulty of forging processability.

2. Forging Process Design

The typical forging structure of Ti-46531 titanium alloy is shown in Figure 2. The forging weighs 8.0 kg and has external dimensions of 447 mm × 117 mm × 74 mm. It exhibits a top-bottom symmetrical configuration, with the symmetry plane serving as the maximum projection surface of its contour; thus, this plane was selected as the parting line. The allowance dimensions are as follows: 5 mm on the outer side of the annular structures at both ends, 10.5 mm for the central web, and 8 mm for all other areas. The ejection angle is 7°; the chamfer radius for the central groove is 20 mm, while that for the protruding section is 8 mm; for all other areas, the chamfer radius is 5 mm.

The forging process plan developed based on the structure and material properties of the forging is shown in Figure 3. The specific procedure is as follows: First, cut the circular bar into a blank and chamfer both ends; then rapidly form the blank using a hammer forging machine, followed by grinding to remove surface defects; finally, perform single-pass pressing on either an 8000-ton electric screw press or a 5000-ton hydraulic press.

3. Forging Simulation and Analysis

3.1 Simulation Parameter Settings

Electric screw presses and hydraulic presses are commonly used for forming complex forgings from difficult-to-deform metals, each offering distinct advantages in forming characteristics, control methods, and process adaptability. To determine the optimal forming equipment for typical Ti-46531 titanium alloy forgings, numerical simulation techniques widely employed in metal plastic forming were applied to model the forming processes of these forgings on both hydraulic presses and electric screw presses using computer simulations. A comparative analysis was conducted on key technical parameters—including filling performance, strain fields, and temperature fields—to identify the most suitable forming equipment.

The forging simulation parameter settings are shown in Table 2. The simulation parameters for the electric screw press and hydraulic press are essentially identical: after blanking, the blank is reheated with an initial forging temperature of 780°C; the die preheating temperature is 350°C, and the friction coefficient between the blank and die is 0.3. The upper die pressing speed on the hydraulic press is approximately 10 mm·s⁻¹. To prevent excessive temperature drop of the blank, it is typically wrapped with insulating material, resulting in a heat transfer coefficient between the blank and die of 1 N·(s·mm·°C)⁻¹. For the electric screw press, the heat transfer coefficient between the die and blank during forming is 5 N·(s·mm·°C)⁻¹, and the forming speed is controlled by energy input; specific motion parameter settings are presented in Table 3.

3.2 Analysis of Simulation Results

The forming process of the die-cast part is illustrated in Figure 4, with Figures 4a and 4b showing the forming processes of the short sword under a hydraulic press and an electric screw press, respectively.

As shown in Figure 4, the blank is accurately positioned within the die; as the upper die moves downward, both ends of the blank simultaneously contact the die without rigid displacement, ensuring a stable forming process; as the upper die continues to move downward, the upper surface of the blank gradually conforms to the die cavity, with all parts of the forging being formed almost simultaneously, at which point flash begins to form; when the pressure on the upper die is reduced by 2 mm, the forging is fully filled with minimal flash and exhibits high material utilization. A comprehensive analysis of the forging forming process reveals that there are minimal differences between the forming processes on hydraulic presses and electric screw presses.

The temperature distribution of the forged part after forging is shown in Figure 5 (the straight line indicates the cross-sectional position; Figures 5a and 5b depict the temperature distributions during hydraulic press forming and electric screw press forming, respectively). As shown in Figure 5, the flash area exhibits higher temperatures, while the surface temperature is lower with minimal variation at the center. During hydraulic press forming, the minimum temperature reaches 456°C and the maximum reaches 845°C; whereas during electric screw press forming, the minimum temperature is 783°C and the maximum reaches 1090°C. Comparison reveals distinct temperature patterns: in hydraulic press forming, the body temperature decreases progressively from interior to exterior, with surface temperatures dropping below 732°C and central temperatures exceeding 796°C, primarily concentrated between 700–812°C; in electric screw press forming, temperatures rise across different regions—with flash areas exceeding 885°C and the main body temperature range being 780–840°C. Overall, electric screw press forming results in a more uniform temperature distribution throughout the forged part.

The equivalent strain distribution of the forging after forging is shown in Figure 6 (the straight line in Figure 6 indicates the cross-sectional position). Figures 6a and 6b illustrate the equivalent strain distributions during hydraulic press forming and electric screw press forming, respectively. As shown in Figure 6, the minimum equivalent strain during hydraulic press forming is 0.08, while the maximum is 19.900; during electric screw press forming, the minimum equivalent strain is 0.12 and the maximum is 9.070. The equivalent strain is higher at the flash area and lower on the surface, with values ranging from 0.25 to 0.5 in the central region. The equivalent strain field distribution during electric screw press forming is more uniform than that during hydraulic press forming.

The temperature distribution of the lower die is shown in Figure 7, with Figures 7a and 7b depicting the temperature distributions during hydraulic press forming and electric screw pressure forming, respectively. As illustrated in Figure 7, the minimum temperature of the lower die during hydraulic press forming ranges from 349 °C to 619 °C, while that during electric screw pressure forming ranges from 349 °C to 448 °C. Based on the temperature distribution trends, the lower die cavity exhibits similar characteristics under both forming methods.

Temperatures increased to varying degrees, with an average rise of approximately 150 °C. During electric screw press forming, the temperature rise in the lower mold cavity was limited, with only localized temperature increases of about 100 °C at both ends of the cavity. Mold temperature is a critical factor affecting service life; excessively high temperatures can cause material tempering and softening, accelerate creep deformation, and consequently shorten service life. Therefore, it can be concluded that using an electric screw press results in relatively lower mold temperatures, thereby extending mold service life.

The forming loads applied to the forgings are illustrated in Figure 8, where Figures 8a and 8b show the load-time curves for hydraulic press and electric screw press forming, respectively. As shown in Figure 8, the hydraulic press process lasted 5.83 seconds with a final forming load of 4.0 × 10⁴ kN, whereas the electric screw press required only 0.16 seconds with a maximum forming load of 6.1 × 10⁴ kN. Although the forming time for the electric screw press is significantly shorter, its forming load is approximately 50% higher than that of the hydraulic press.

A comparison of the forming processes of typical Ti-46531 titanium alloy forgings on hydraulic presses and electric screw presses reveals: (1) The filling processes are nearly identical, with all forgings achieving full filling; (2) During forming on an electric screw press, the temperature field and isotropic strain field distribution are more uniform; (3) Due to its higher forming speed and lower mold temperature rise, the electric screw press offers longer mold service life. Considering these factors, an 8000-ton electric screw press was selected as the testing equipment for forging Ti-46531 titanium alloy forgings.

4. Experimental Results

The typical forging was tested on an 8000-ton electric screw press, with the results shown in Figure 9. As evidenced by the physical specimen in Figure 9, the forging was fully filled without any folding cracks, confirming the validity of the simulation results.

Conclusion

(1) Through experiments, the physical parameters of Ti-46531 titanium alloy—including specific heat capacity, thermal conductivity, and linear expansion coefficient—were determined, and a constitutive model for Ti-46531 based on temperature, strain rate, and deformation was established.

(2) A typical forging part and its forging process for Ti-46531 titanium alloy were designed. Using numerical simulation analysis, the forming processes of this forging part on hydraulic presses and electric screw presses were compared, and the optimal forging equipment was identified as an 8000-ton electric screw press.

(3) Final forging tests were conducted on the 8000-ton electric screw press, with results meeting design specifications; the forgings exhibited full filling without defects such as folding or cracks.

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