In the automotive industry, the gear shaft plays a critical role in transmitting power and adjusting speed within various mechanisms such as drive axles, differentials, transmissions, and steering systems. The performance and durability of these gear shaft components are paramount, as failures can lead to severe consequences like cracking in the shaft or teeth, and excessive wear, ultimately compromising vehicle safety and stability. Traditional manufacturing methods, including machining and hot forging, often fall short in terms of efficiency, material utilization, and product quality. Machining involves multiple steps, low production rates, and cuts through fiber structures, reducing load-bearing capacity. Hot forging, while improving material usage, suffers from oxidation and decarburization, leading to inferior mechanical properties and surface quality. In contrast, cold heading offers a high-quality, efficient, and cost-effective precision forming technique that enhances metal flow lines, strength, and service life. This study focuses on developing a cold heading process for a complex automotive gear shaft, utilizing finite element simulation with Deform-3D to optimize the工艺 and validate it through experimental trials.
The gear shaft under consideration features intricate structures such as grooves, tooth profiles, concave pits, and through-holes, making it unsuitable for single-step cold heading. The material used is 25CrMo4 alloy steel, which exhibits poor plasticity and high deformation resistance at room temperature, accompanied by work-hardening tendencies that increase the risk of cracking. To address this, annealing is applied to soften the material, reducing its deformation resistance and allowing a maximum deformation degree of approximately 65%. Based on this, a multi-station cold heading process is designed, consisting of seven stages to ensure uniform deformation and avoid defects. The process begins with station 1 for blank shaping to prevent eccentricity and uneven deformation, followed by stations 2 to 7 for forming various features like lower teeth, upper teeth, arcs, pre-formed holes, and through-holes. Stations 1 and 7 are standard in cold heading and are not deeply analyzed in simulations due to their low risk of defects. The overall process aims to achieve complete filling of die cavities, minimal material damage, and high production rates.
To accurately simulate the cold heading process, a material model for 25CrMo4 alloy steel is essential. Compression tests are conducted at room temperature to obtain true stress-true strain curves under different strain rates. The data is fitted to the Norton-Hoff constitutive model, which accounts for strain hardening, strain rate sensitivity, and temperature effects. The model is expressed as:
$$ \sigma = K (\varepsilon_0 + \varepsilon)^n \cdot \dot{\varepsilon}^m \cdot \exp\left(\frac{\beta}{T}\right) $$
where $\sigma$ is the true stress, $\varepsilon$ is the true strain, $\dot{\varepsilon}$ is the strain rate, $n$ is the hardening exponent, $m$ is the strain rate sensitivity index, $T$ is the temperature, and $K$, $\varepsilon_0$, and $\beta$ are material constants. By linear regression of experimental data, the parameters are determined as $n = -4.821$, $K = 8130.4$, $m = 0.0196$, $\beta = 14823.9$, and $\varepsilon_0 = 55530.2$. Thus, the specific model for 25CrMo4 alloy steel is:
$$ \sigma = 8130.4 \times (55530.2 + \varepsilon)^{-4.821} \times \dot{\varepsilon}^{0.0196} \times \exp\left(\frac{14823.9}{T}\right) $$
This model is implemented in Deform-3D to simulate the material’s flow stress during cold heading. Additionally, a critical damage model based on the Normalized Cockcroft & Latham criterion is used to predict cracking tendencies. The damage value $D$ is given by:
$$ D = \int_0^{\varepsilon_f} \frac{\sigma^*}{\sigma} d\varepsilon $$
where $\varepsilon_f$ is the strain at fracture, and $\sigma^*$ is the maximum principal stress. Through compression tests and simulations, the critical damage value for 25CrMo4 alloy steel is found to be 0.62. If the damage exceeds this threshold in four consecutive elements, fracture is assumed to occur, allowing visual detection of cracks in simulations.
Finite element modeling in Deform-3D involves creating 3D models of the billet and dies for each station using SolidWorks. The billet is defined as a plastic body with the Norton-Hoff model, meshed with 60,000 tetrahedral elements. Dies are treated as rigid bodies, with moving components assigned appropriate motions. A friction coefficient of 0.12 is set to reflect realistic lubrication conditions, and a thermal conductivity of 11 N/(s·mm·°C) is used to account for heat generation and transfer during deformation. The simulation controls the main die stroke, stopping when the desired position is reached. This setup enables dynamic analysis of metal flow, stress distribution, and defect formation.
The metal deformation behavior across stations is critical for avoiding defects like folding. In station 2, forward extrusion forms the lower teeth, with metal flowing unidirectionally downward, preventing folding. Station 3 combines upsetting for upper teeth and forward extrusion for arcs and lower teeth; metal flows sequentially without interference, ensuring no internal defects. Station 4 employs compound forward and backward extrusion for pre-forming the upper hole and further shaping lower teeth, with independent metal streams avoiding convergence. Stations 5 and 6 use floating die structures to form top grooves and lower holes, promoting downward flow and eliminating backflow risks. Overall, the metal deformation is rational, with smooth flow lines and no folding observed.
| Station | Forming Feature | Maximum Damage Value | Maximum Load (kN) | Temperature Difference (°C) |
|---|---|---|---|---|
| 2 | Lower Teeth Formation | 0.569 | 1314 | 63.8 |
| 3 | Upper Teeth and Arc | 0.411 | 1730 | 157.7 |
| 4 | Pre-formed Hole and Teeth | 0.526 | 1408 | 127.8 |
| 5 | Top Groove and Lower Hole | 0.478 | 1575 | 118.9 |
| 6 | Lower Hole Completion | 0.492 | 1648 | 140.3 |
The forming quality of the gear shaft is assessed through contact analysis, showing complete die cavity filling at all stations without short shots or flashes. The material damage distribution indicates that critical areas, such as the lower teeth and holes, have values below the critical 0.62, ensuring no cracking. Load-time curves reveal increasing trends due to growing contact areas, with peak loads ranging from 1314 kN to 1730 kN, demonstrating balanced deformation distribution and uniform mold life. Temperature fields, influenced by deformation heat, show maximum differences between 63.8°C and 157.7°C, which are within acceptable limits to avoid significant thermal stresses and cracking risks.
Experimental validation involves producing the gear shaft using a seven-station cold heading process on an NF-33B-7S machine. The billet, made of annealed and phosphated 25CrMo4 alloy steel with dimensions Φ27.0 mm × 34.8 mm, is lubricated with MPS02 cold heading oil to prevent surface defects. Dies are fabricated from VA70 tungsten steel for abrasion resistance and SKH51 steel for punches to withstand high axial loads. The resulting gear shaft samples exhibit excellent appearance, with fully formed features like grooves, teeth, and holes, and no visible defects. Internal examination reveals continuous flow lines without folds or cracks, and microstructural analysis shows elongated grains in upset regions and compressed grains in extruded areas, consistent with simulations. The production rate reaches 65 pieces per minute, confirming the process’s feasibility and reliability for complex gear shaft manufacturing.
In conclusion, the cold heading process for the automotive gear shaft, optimized through finite element simulation, successfully addresses production challenges. The material models and damage criteria provide accurate predictions, while the multi-station design ensures uniform deformation and high quality. Experimental results align closely with simulations, validating the approach for efficient and reliable gear shaft production. This methodology can be extended to other complex components, enhancing automotive manufacturing capabilities.
The development of this cold heading process for the gear shaft highlights the importance of integrating simulation and experimentation. By using Deform-3D, potential defects are identified and mitigated early, reducing development time and costs. The gear shaft’s performance benefits from improved metal flow and strength, making it suitable for demanding automotive applications. Future work could explore optimizing die designs or applying similar approaches to other materials and geometries, further advancing cold heading technology for gear shaft production.
Throughout this study, the gear shaft serves as a focal point for demonstrating cold heading advantages. The process not only enhances product quality but also contributes to sustainability by minimizing material waste. As automotive systems evolve, the demand for high-performance gear shaft components will grow, underscoring the value of precision forming techniques like cold heading. By continuing to refine these methods, manufacturers can achieve greater efficiency and reliability in producing critical automotive parts such as the gear shaft.