In the automotive industry, lightweighting has become a critical strategy to reduce environmental pollution and enhance fuel efficiency. With over 28 million vehicles produced annually in China alone, the demand for components that offer high strength-to-weight ratios is ever-increasing. Hollow shafts, particularly hollow gear shafts, present a significant advantage over solid shafts due to their reduced mass and superior torsional resistance. In advanced markets like Europe and Japan, hollow shafts already account for approximately 10% of automotive applications. Traditional manufacturing of gear shaft blanks often involves forging followed by machining, but near-net-shape processes like rotary forging offer higher precision, material efficiency, and flexibility. Rotary forging, also known as radial forging, is a precision forming technique suitable for bars, tubes, and stepped shafts. This process involves simultaneous radial compression and axial feeding, resulting in refined microstructures and improved mechanical properties. However, optimizing the rotary forging parameters for complex geometries like hollow gear shafts is essential to avoid defects such as folding and surface spiraling. In this study, I employ orthogonal experimental design coupled with Forge finite element simulation to identify the optimal process parameters for hollow gear shaft rotary forging. Physical validations through microstructure and microhardness analyses are conducted to confirm the simulation predictions. The goal is to establish a robust manufacturing route for high-quality hollow gear shafts, contributing to automotive lightweighting initiatives.
The material selected for the hollow gear shaft is 27MnCr5 steel, a common alloy for automotive components due to its balance of strength and toughness. The chemical composition (in wt.%) is: C 0.38–0.45%, Si 0.17–0.37%, Mn 0.50–0.80%, P ≤ 0.035%, S ≤ 0.035%, Cr 0.90–1.20%, Mo 0.15–0.25%, and Fe balance. This steel provides good hardenability and wear resistance, making it suitable for gear applications. The rotary forging trials were performed on a GFM SKK-10 precision forging machine, which is capable of multi-axis radial forging. For simulation, the Forge software was utilized to model the thermo-mechanical behavior during rotary forging. The simulation parameters were calibrated based on experimental data to ensure accuracy. Physical characterization included scanning electron microscopy (SEM) using a JSM-6460LV microscope and microhardness testing with an HVS-1000 Vickers hardness tester. Microhardness measurements were taken at 100 g load and 10 s dwell time, with points sampled along the radial direction to assess uniformity.
To systematically investigate the influence of key process parameters on the quality of the hollow gear shaft, an orthogonal experimental design was adopted. The factors considered were: initial billet temperature (A), axial relative feed rate (B), billet rotation speed (C), hammer radial reduction (D), and hammer radial velocity (E). Each factor was set at four levels, as summarized in Table 1. The response variables were the number of folds (n) and the depth of spiral ridges (h), both of which are critical defects in rotary forging. Folds occur due to improper metal flow, while spiral ridges are surface imperfections that affect fatigue life. In the simulation, folds and ridges were quantified using a scale: 0 for minor or negligible defects, 1 for moderate defects, and 2 for severe defects. This scoring system allowed for a relative comparison of process conditions. The orthogonal array L16(4^5) was used, resulting in 16 simulation runs. The experimental scheme and results are presented in Table 2.
| Level | A: Initial Temperature (°C) | B: Axial Feed Rate (mm) | C: Rotation Speed (rpm) | D: Radial Reduction (mm) | E: Radial Velocity (mm/s) |
|---|---|---|---|---|---|
| 1 | 750 | 0.05 | 50 | 2 | 7.5 |
| 2 | 780 | 0.10 | 60 | 2.5 | 10 |
| 3 | 800 | 0.30 | 70 | 3 | 12 |
| 4 | 830 | 0.50 | 76 | 3.5 | 15 |
| Experiment No. | A | B | C | D | E | Simulation Scheme | Folds (n) | Ridge Depth (h) |
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | 1 | 1 | 1 | 1 | A1B1C1D1E1 | 0 | 0 |
| 2 | 1 | 2 | 2 | 2 | 2 | A1B2C2D2E2 | 1 | 1 |
| 3 | 1 | 3 | 3 | 3 | 3 | A1B3C3D3E3 | 2 | 1 |
| 4 | 1 | 4 | 4 | 4 | 4 | A1B4C4D4E4 | 2 | 2 |
| 5 | 2 | 1 | 2 | 3 | 4 | A2B1C2D3E4 | 2 | 0 |
| 6 | 2 | 2 | 1 | 4 | 3 | A2B2C1D4E3 | 2 | 1 |
| 7 | 2 | 3 | 4 | 1 | 2 | A2B3C4D1E2 | 0 | 1 |
| 8 | 2 | 4 | 3 | 2 | 1 | A2B4C3D2E1 | 1 | 2 |
| 9 | 3 | 1 | 3 | 4 | 2 | A3B1C3D4E2 | 2 | 2 |
| 10 | 3 | 2 | 4 | 3 | 1 | A3B2C4D3E1 | 1 | 1 |
| 11 | 3 | 3 | 1 | 2 | 4 | A3B3C1D2E4 | 2 | 1 |
| 12 | 3 | 4 | 2 | 1 | 3 | A3B4C2D1E3 | 2 | 2 |
| 13 | 4 | 1 | 4 | 2 | 3 | A4B1C4D2E3 | 0 | 0 |
| 14 | 4 | 2 | 3 | 1 | 4 | A4B2C3D1E4 | 0 | 0 |
| 15 | 4 | 3 | 2 | 4 | 1 | A4B3C2D4E1 | 2 | 1 |
| 16 | 4 | 4 | 1 | 3 | 2 | A4B4C1D3E2 | 2 | 2 |
The orthogonal experimental results were analyzed using the range analysis method to determine the optimal parameter combination. For each factor, the average response values (k) and ranges (R) were calculated for both folds and ridge depth, as shown in Table 3. The range indicates the influence magnitude: a larger R means the factor has a greater effect on the response. The primary and secondary order of factors was derived from the R values. For folds, the order is D > C > A = B > E, meaning radial reduction has the most significant impact, followed by rotation speed, initial temperature and axial feed rate (equally), and radial velocity. For ridge depth, the order is B > A = D = E > C, indicating axial feed rate is the most critical, with initial temperature, radial reduction, and radial velocity having moderate effects, and rotation speed the least. To select the optimal levels, I applied the comprehensive balance method, prioritizing factors that minimize both defects. The trend analysis revealed that higher initial temperatures (A4: 830°C) and lower radial reductions (D1: 2 mm) reduced both folds and ridges. For axial feed rate, B1 (0.05 mm) minimized ridge depth while keeping folds low. Rotation speed C4 (76 rpm) and radial velocity E1 (7.5 mm/s) were optimal for folds, with acceptable ridge depth. Thus, the optimized parameter set is A4B1C4D1E1: initial temperature 830°C, axial feed rate 0.05 mm, rotation speed 76 rpm, radial reduction 2 mm, and radial velocity 7.5 mm/s. This combination is expected to produce a high-quality hollow gear shaft with minimal defects.
| Response | Factor | k1 | k2 | k3 | k4 | Range R | Factor Order |
|---|---|---|---|---|---|---|---|
| Folds (n) | A | 1.25 | 1.25 | 1.75 | 1.00 | 0.75 | D > C > A = B > E |
| B | 1.00 | 1.00 | 1.50 | 1.75 | 0.75 | ||
| C | 1.50 | 1.75 | 1.25 | 0.75 | 1.00 | ||
| D | 0.50 | 1.00 | 1.75 | 2.00 | 1.50 | ||
| E | 1.00 | 1.25 | 1.50 | 1.50 | 0.50 | ||
| Ridge Depth (h) | A | 1.00 | 1.00 | 1.50 | 0.75 | 0.75 | B > A = D = E > C |
| B | 0.50 | 0.75 | 1.00 | 2.00 | 1.50 | ||
| C | 1.00 | 1.00 | 1.25 | 1.00 | 0.25 | ||
| D | 0.75 | 1.00 | 1.00 | 1.50 | 0.75 | ||
| E | 1.00 | 1.50 | 1.00 | 0.75 | 0.75 |
To validate the simulation results, physical trials were conducted on the GSK SKK-10 forging machine using the optimized parameters. The formed hollow gear shaft exhibited good dimensional accuracy and surface finish. For microstructure examination, cross-sectional samples were extracted from four key locations of the gear shaft, as illustrated in Figure 1. These locations represent regions with varying deformation levels, such as near the surface, at step transitions, and in consolidated zones. SEM analysis revealed significant grain refinement and fibrous microstructure development due to rotary forging. At location 1 (near the outer surface), the microstructure showed a mix of deformed and original grains, with grain size decreasing from the surface inward. The high-magnification image indicated elongated grains along the forging direction, typical of severe plastic deformation. Location 2, at a step region, displayed a pronounced fibrous structure with grains aligned axially, enhancing mechanical properties. Location 3, where metal flow converges, exhibited dense fibrous organization due to high strain rates. Location 4, near inner surfaces, showed refined grains attributed to contact with the mandrel and cooling effects. The microstructural evolution can be described by the strain hardening and dynamic recovery mechanisms during warm forging. The equivalent strain (ε) distribution, which influences grain size, can be approximated by the logarithmic strain formula: $$ \epsilon = \ln \left( \frac{A_0}{A_f} \right) $$ where \( A_0 \) and \( A_f \) are the initial and final cross-sectional areas, respectively. For the hollow gear shaft, the strain varies radially, leading to graded microstructures. The Hall-Petch relationship further links grain size (d) to yield strength (σ_y): $$ \sigma_y = \sigma_0 + \frac{k}{\sqrt{d}} $$ where \( \sigma_0 \) and \( k \) are material constants. Thus, grain refinement via rotary forging improves the strength of the gear shaft.
Microhardness testing was performed on cross-sections of the hollow gear shaft, focusing on two step regions and the sizing zone, as depicted in Figure 2. The hardness profiles, plotted in Figure 3, show variations from the inner core to the outer surface. In general, microhardness increases with deformation, reflecting strain hardening. For the first step and sizing zone, hardness rises from surface to core, with the sizing zone exhibiting higher values due to greater consolidation. The second step region shows higher surface hardness compared to the first step, attributed to larger radial deformation. The average microhardness at the second step is approximately 10-15% higher than at the first step, consistent with the increased effective strain. The hardness distribution can be modeled using the relationship between hardness (H) and equivalent strain (ε): $$ H = H_0 + C \cdot \epsilon^n $$ where \( H_0 \) is the base hardness, and \( C \) and \( n \) are material-dependent coefficients. For 27MnCr5 steel, n typically ranges from 0.2 to 0.3. The non-uniform hardness profile underscores the heterogeneous deformation in rotary forging, which is influenced by factors like friction, temperature gradients, and tool geometry. This heterogeneity must be managed to ensure consistent performance of the gear shaft in service.
The discussion of results highlights the efficacy of rotary forging for manufacturing hollow gear shafts. The optimized parameters derived from orthogonal experiments minimize defects while enhancing microstructural integrity. Compared to conventional forging, rotary forging offers finer grain sizes and better material utilization, critical for lightweight automotive components. The fibrous microstructure observed in the gear shaft improves fatigue resistance and torsional strength, key attributes for drivetrain applications. The microhardness data corroborate the strain distribution predicted by simulations, validating the Forge model as a reliable tool for process optimization. However, challenges remain, such as controlling surface defects in complex geometries. Future work could explore advanced tooling designs or hybrid processes to further improve the quality of hollow gear shafts. Additionally, the integration of real-time monitoring and adaptive control could enhance process stability.
In conclusion, this study demonstrates the successful optimization of rotary forging parameters for hollow gear shaft blanks using Forge simulation and orthogonal experimental design. The optimal parameter set—initial temperature 830°C, axial feed rate 0.05 mm, rotation speed 76 rpm, radial reduction 2 mm, and radial velocity 7.5 mm/s—produces gear shafts with minimal folds and spiral ridges. Physical validation confirms significant grain refinement, fibrous microstructure development, and enhanced microhardness due to strain hardening. The second step region exhibits higher hardness than the first step, reflecting greater deformation. These findings contribute to the advancement of lightweight automotive components, supporting the industry’s shift towards sustainability. The methodology presented here can be extended to other rotary-forged parts, paving the way for more efficient manufacturing processes. Ultimately, the hollow gear shaft produced via optimized rotary forging offers a compelling combination of weight reduction and performance, aligning with global automotive trends.