In the commercial vehicle industry, the V-shape thrust rod is a critical component of the balanced suspension system, responsible for transmitting longitudinal and lateral loads along with corresponding moments between the axle and frame to maintain vehicle stability. Traditional manufacturing methods for these rods, such as forging combined with hot riveting or friction welding, often involve numerous processes, extended production cycles, and high costs. These techniques can lead to issues like internal stress concentrations, reduced precision, and complex quality control. To address these challenges, we propose a technical solution of substituting steel with iron, specifically through the development of an as-cast QT700-6 ductile iron casting for the V-shape thrust rod. This approach leverages the advantages of ductile iron castings, including high strength, toughness, excellent castability, wear resistance, damping capacity, and lower production costs. Moreover, the lower density of ductile iron castings (approximately 7.1 g/cm³ compared to 7.85 g/cm³ for steel) contributes to lightweighting goals, which is essential for modern commercial vehicles. In this study, we detail the structural design, finite element analysis, manufacturing, and experimental validation of this innovative ductile iron casting, demonstrating its reliability and performance superiority over traditional steel structures.
The structural design of the V-shape thrust rod was reimagined using an integrated, one-piece casting approach. Instead of assembling separate components like tubular sections and forged steel ball seats, we designed a monolithic H-shaped cross-section that extends continuously from the rod body to the ball seats. This H-section configuration, with symmetrical upper and lower flanges and a connecting web, enhances bending and compression resistance while facilitating the casting process. The design principles for the H-section were informed by standards for axial compression members, ensuring structural integrity. Key parameters of the rod body cross-section are summarized in Table 1, which outlines dimensions optimized for performance and weight reduction. By eliminating welded or riveted joints, this ductile iron casting inherently avoids stress concentrations and internal stresses associated with traditional methods. Comparative mass analysis shows that the ductile iron casting reduces weight by approximately 9% compared to its steel counterpart, primarily due to the elimination of solid ball ends and overlapping sections in steel designs. The streamlined production process—replacing multiple fabrication steps with a single casting operation—lowers costs and improves efficiency, making ductile iron castings a compelling alternative for automotive applications.
| Parameter | Symbol | Value (mm) |
|---|---|---|
| Left Flange Height | H1 | 43.8 |
| Right Flange Height | H2 | 28.3 |
| Flange Top Thickness | m | 7.7 |
| Web Thickness | n | 7.0 |
| Section Width | B | 51.1 |
To validate the structural integrity of the ductile iron casting, we conducted comprehensive finite element analysis (FEA) using ABAQUS software. The assembly model included the thrust rod casting, spherical joint pins, end caps, retaining rings, and rubber elements, with meshing performed in HyperMesh. The mesh consisted of C3D10M elements for the ductile iron casting and pins, and C3D8R elements for rubber and other components, totaling over 1.9 million elements. Material properties were defined as per Table 2, with the ductile iron casting modeled using QT700-6 parameters and steel components using alloy steel properties. The rubber behavior was characterized by a third-order Ogden hyperelastic model to accurately simulate its response under load. Boundary conditions and loading scenarios were established to replicate real-world operating conditions: radial tension (150 kN), radial compression (150 kN), combined loading (150 kN axial plus radial tension), and torsion (400 N·m). Constraints were applied at the bolt holes of the straight ball seats, and loads were coupled to a reference point at the spherical joint of the upper ball seat, as illustrated in the simulation setup.
| Material | Density (g/cm³) | Young’s Modulus (N/mm²) | Poisson’s Ratio |
|---|---|---|---|
| Ductile Iron Casting (QT700-6) | 7.3 | 1.73 × 10⁵ | 0.3 |
| Alloy Steel | 7.85 | 2.1 × 10⁵ | 0.3 |
Static analysis results revealed the stress distribution under various loading conditions. The von Mises stress contours indicated that maximum stresses in the ductile iron casting were consistently located at the retaining ring groove of the upper ball seat, a region with geometric discontinuities causing mild stress concentration. For radial tension, the peak stress was 221.14 MPa; for radial compression, 159.9 MPa; for combined loading, 287.4 MPa; and for torsion, 38.79 MPa. In contrast, an equivalent steel V-shape thrust rod model showed higher maximum stresses: 288.9 MPa (tension), 219.7 MPa (compression), 403.6 MPa (combined), and 72.97 MPa (torsion). These results, summarized in Table 3, demonstrate that the ductile iron casting exhibits lower maximum stresses across all scenarios, validating its structural reliability. The superior performance of ductile iron castings can be attributed to their homogeneous microstructure and absence of weld-induced stress risers. The stress-strain relationship for the material is governed by Hooke’s law for linear elastic behavior:
$$ \sigma = \epsilon E $$
where $\sigma$ is the stress, $\epsilon$ is the strain, and $E$ is the Young’s modulus (178 GPa for the ductile iron casting). This formula was instrumental in converting experimental strain measurements to stress values for validation purposes.
| Loading Condition | Ductile Iron Casting Max Stress (MPa) | Steel Thrust Rod Max Stress (MPa) |
|---|---|---|
| Radial Tension | 221.14 | 288.9 |
| Radial Compression | 159.9 | 219.7 |
| Combined Loading | 287.4 | 403.6 |
| Torsion | 38.79 | 72.97 |
The manufacturing of the ductile iron casting involved green sand molding using a high-pressure molding line, melting in an induction furnace, and nodularization via the sandwich method. Molten iron was treated with magnesium for spheroidization, followed by inoculation during pouring to ensure fine graphite formation. The chemical composition was tightly controlled within ranges specified in Table 4 to achieve the desired QT700-6 grade. Pouring temperatures were maintained between 1,380°C and 1,400°C to optimize fluidity and minimize defects. The resulting casting, as shown in the image below, exhibited a smooth surface free of macroscopic imperfections like gas holes or slag inclusions. Non-destructive testing via ultrasonic inspection confirmed the absence of internal shrinkage or porosity, underscoring the quality achievable with ductile iron castings.
Microstructural analysis of the ductile iron casting revealed a matrix predominantly composed of pearlite (approximately 80%) with a small amount of ferrite, which imparts high strength and hardness. Graphite nodules were uniformly distributed with a nodularity of 85% and a size rating of 6 according to relevant standards, as depicted in the micrographs. This refined microstructure enhances crack resistance and contributes to the high toughness of ductile iron castings. Mechanical properties were evaluated through tensile and hardness tests on specimens extracted from the casting本体. The results, presented in Table 5, meet or exceed the QT700-6 requirements, with an average tensile strength of 764 MPa, yield strength of 439 MPa, elongation up to 7.2%, and Brinell hardness ranging from 249 to 259 HBW. These properties confirm that the as-cast ductile iron casting achieves the necessary performance without heat treatment, reducing energy consumption and production costs while avoiding distortion and oxidation issues.
| Element | Content Range (%) |
|---|---|
| C | 3.7–3.9 |
| Si | 2.3–2.6 |
| Mn | 0.3–0.5 |
| P | < 0.05 |
| S | 0.006–0.02 |
| Mg | 0.04–0.05 |
| Cu | 0.7–0.9 |
| Sn | 0.01–0.02 |
| Cr | < 0.08 |
| Specimen ID | Tensile Strength (MPa) | Yield Strength (MPa) | Elongation (%) | Hardness (HBW) |
|---|---|---|---|---|
| 1 | 765 | 440 | 6.9 | 253 |
| 2 | 780 | 443 | 7.2 | 249 |
| 3 | 748 | 434 | 6.7 | 259 |
| Average | 764 | 439 | 6.9 | 254 |
Static bench testing was conducted to verify the accuracy of the FEA model. Strain gauges were attached at nine locations on the ductile iron casting surface, primarily along the H-section杆体 and side of the upper ball seat. The test setup utilized an MTS six-channel coordinated loading system and an HBM data acquisition unit. Loading sequences involved applying radial tension and compression up to 150 kN at a rate of 5,000 N/s, with hold periods for data recording. Strain-time curves were obtained, and peak strain values were converted to stress using the formula $\sigma = \epsilon E$. Comparison between experimental stresses and FEA-predicted stresses at corresponding points is shown in Tables 6 and 7. Under tensile loading, all measurement points showed errors within 15%; under compressive loading, seven out of nine points had errors within 15%, indicating good agreement and validating the finite element model’s precision for ductile iron castings.
| Strain Gauge Point | Experimental Stress (MPa) | Simulated Stress (MPa) | Relative Error (%) |
|---|---|---|---|
| 1 | 134.8 | 115.3 | 14.5 |
| 2 | 83.3 | 75.2 | 9.7 |
| 3 | 89.2 | 81.8 | 8.3 |
| 4 | 16.0 | 14.4 | 10.0 |
| 5 | 81.2 | 75.4 | 7.1 |
| 6 | 96.8 | 85.7 | 11.5 |
| 7 | 80.3 | 75.2 | 6.4 |
| 8 | 10.9 | 12.3 | 13.8 |
| 9 | 92.0 | 83.3 | 9.5 |
| Strain Gauge Point | Experimental Stress (MPa) | Simulated Stress (MPa) | Relative Error (%) |
|---|---|---|---|
| 1 | 101.1 | 78.9 | 21.9 |
| 2 | 74.9 | 79.9 | 6.8 |
| 3 | 62.7 | 71.6 | 14.2 |
| 4 | 27.4 | 24.1 | 12.0 |
| 5 | 84.6 | 76.8 | 9.2 |
| 6 | 95.4 | 85.7 | 10.2 |
| 7 | 79.0 | 85.8 | 8.6 |
| 8 | 8.3 | 9.8 | 18.1 |
| 9 | 71.6 | 80.5 | 12.4 |
Fatigue performance is crucial for thrust rods due to cyclic loading in service. We performed longitudinal cyclic loading tests on the ductile iron casting assembly, applying ±150 kN sinusoidal loads at 1.5 Hz for 200,000 cycles. The stiffness change rate, a key indicator of fatigue damage, was monitored throughout. Results from three tested specimens are presented in Table 8. All ductile iron castings completed the test without failure, and stiffness changes remained below 20%, meeting the fatigue bench test requirements. This endurance demonstrates the durability of ductile iron castings under repetitive stress, which can be partially explained by their inherent damping capacity and homogeneous structure. The fatigue life $N_f$ of such components can be estimated using models like the Basquin equation:
$$ \sigma_a = \sigma_f’ (2N_f)^b $$
where $\sigma_a$ is the stress amplitude, $\sigma_f’$ is the fatigue strength coefficient, and $b$ is the fatigue strength exponent. While not explicitly calculated here, the successful test outcomes imply that the ductile iron casting possesses sufficient fatigue resistance for commercial vehicle applications.
| Specimen ID | Loading Condition (kN) | Stiffness Change Rate (%) | Cycles (×10⁴) | Post-Test Condition |
|---|---|---|---|---|
| 1 | Longitudinal ±150 | 11.8 | 20 | Intact |
| 2 | Longitudinal ±150 | 16.1 | 20 | Intact |
| 3 | Longitudinal ±150 | 9.4 | 20 | Intact |
In conclusion, this study successfully developed and validated a ductile iron casting for V-shape thrust rods in commercial vehicles. The H-shaped monolithic design offers significant advantages over traditional steel structures, including a 9% weight reduction, elimination of internal stresses from welding or riveting, and simplified manufacturing processes. Finite element analysis confirmed that the ductile iron casting exhibits lower maximum stresses under various loading conditions compared to steel counterparts, ensuring structural reliability. Static and fatigue bench tests corroborated the simulation results, with experimental stresses aligning closely with predictions and no failures observed after 200,000 cycles. The as-cast QT700-6 material achieved the required mechanical properties without heat treatment, highlighting the cost-effectiveness and sustainability of ductile iron castings. These findings underscore the potential of ductile iron castings to replace steel in critical automotive components, contributing to lightweighting and performance enhancement. Future work could explore further optimization of the casting geometry or extension of this approach to other vehicle parts, leveraging the versatile benefits of ductile iron castings.