In recent years, driven by national strategies and the increasing demand for improved living standards, pure electric vehicles and hybrid electric vehicles have experienced rapid development. The weight of a vehicle plays a decisive role in the fuel economy of hybrid vehicles and the driving range of pure electric vehicles. For instance, a reduction of 100 kg in vehicle weight can decrease fuel consumption by 0.7 L/100 km. According to industry consensus, reducing unsprung mass is significantly more effective in lowering fuel consumption than reducing sprung mass. In the design process of powertrain and chassis components, product engineers often thin down parts as much as possible based on CAE analysis results to reduce weight, such as thinning the wall thickness of a transmission housing in a hybrid vehicle. However, this approach can lead to new issues.
Beyond optimizing product structure, the primary pathway for reducing automotive energy consumption is the use of new lightweight materials. Die-cast aluminum alloys have become a popular material for automotive lightweighting due to their excellent material properties, processing stability, and high specific strength. High-strength and high-toughness aluminum alloy die castings have been mass-produced since the 1990s. In recent years, aluminum alloy die castings have gradually replaced iron castings, with their usage increasing annually. They are widely applied in automotive transmission housings, engine components, and wheels, among other shell castings.
With advancements in automotive industrial technology, most aluminum alloy automotive components, including shell castings, are evolving towards thinner walls, higher strength, improved quality, and enhanced reliability. For example, German Rhein Aluminum Company developed new die-cast aluminum alloys like Magsimal-59, Silafont-36, and Castasil-37. By controlling the iron content, these alloys achieve a post-fracture elongation of up to 17% in as-cast standard round bar samples, successfully used in car door manufacturing. Research by Wang Haidong et al. showed that adding trace elements like titanium to Al-Si-Mg series alloys can effectively refine grains, significantly enhancing tensile strength and yield strength. Adding trace zirconium or strontium also markedly improves the mechanical properties of aluminum alloys, providing solutions for manufacturing aluminum alloy car bodies and energy-absorbing components.
The most widely researched and applied aluminum alloys are mainly the Al-Mg, Al-Si, Al-Si-Cu, and Al-Si-Mg series. The transmission housing in question is made of an Al-Si-Cu series die-cast aluminum alloy, grade ADC12, which is a typical material for shell castings.
During road testing of a hybrid vehicle transmission, the housing cracked at the bolt installation area. The crack originated at the root of the third thread under the bolt mounting surface, a location identified by CAE simulation as a stress concentration zone. Due to vehicle vibration and load, the crack propagated further. Since the crack was close to the surface, it preferentially extended toward the surface. Subsequently, the bolt preload was lost, increasing stress at the crack site and causing rapid crack extension, leading to fracture and detachment of the housing section.
To investigate the failure, a comprehensive detection and analysis plan was implemented on the cracked transmission housing shell castings. The housing was made of ADC12, meeting the Japanese standard JIS H5302-2006, subjected to T1 treatment, and produced via low-pressure casting. Analyses included chemical composition, surface hardness, metallography, scanning electron microscopy (SEM), pinhole degree assessment, and iron inclusion detection.
Chemical composition analysis showed that the cracked shell castings met the required specifications. The results are summarized in the table below.
| Element | Cu | Si | Mg | Zn | Fe | Mn | Ni |
|---|---|---|---|---|---|---|---|
| Requirement (wt%) | 1.5-3.5 | 9.6-12 | ≤0.3 | ≤1.0 | ≤1.3 | ≤0.5 | ≤0.5 |
| Measured (wt%) | 1.75 | 10.63 | 0.16 | 0.8 | 0.83 | 0.19 | 0.06 |
Surface Brinell hardness measurements also indicated compliance with standards, as shown below.
| Requirement (HB) | Measured Values |
|---|---|
| ≥80 | 108, 109, 109 |
Metallographic examination revealed a microstructure consisting of α-solid solution and short needle-like eutectic silicon, with some small blocky primary silicon. Iron inclusions were minimal and appeared as fine, fragmented particles, indicating acceptable metallographic structure for shell castings.
Pinhole degree assessment was conducted by sectioning the cracked housing along the crack, corroding it in a 15% NaOH solution for 10 minutes, cleaning with a 20% nitric acid solution, and rinsing with water before rating. The断面 showed numerous肉眼可见的小孔. According to the standard “JB/T 7946.3-2017 Casting Aluminum Alloy Pinholes,” which is typically used for ordinary gravity-cast aluminum alloys, the required pinhole degree is ≤ Grade 2. However, the measured pinhole degree was Grade 5, indicating non-compliance. This suggests that the pinhole defect in these die-cast shell castings is more severe than in gravity-cast counterparts.
Scanning electron microscopy (SEM) analysis of the fracture surface showed a quasi-cleavage morphology, characteristic of an overload-type fracture. The surface contained pore defects of varying sizes, with typical pinholes around 0.1 mm. In some areas, pinholes aggregated into honeycomb-like cavities measuring approximately 1.7 mm × 0.6 mm. These pores exhibited clear contours and smooth inner walls, indicating the presence of hydrogen gas that had not been released during solidification, trapped within the shell castings matrix.
The formation of pinholes in aluminum alloy shell castings is primarily due to hydrogen absorption during melting. Various sources, such as raw materials, equipment, auxiliary materials, and air, can introduce moisture that reacts with aluminum. The chemical reactions at different temperatures are as follows:
Below 250°C:
$$2\text{Al} + 6\text{H}_2\text{O} \rightarrow 2\text{Al(OH)}_3 + 3\text{H}_2 \uparrow$$
Above 400°C:
$$2\text{Al(OH)}_3 \rightarrow \text{Al}_2\text{O}_3 + 3\text{H}_2\text{O}$$
$$2\text{Al} + 3\text{H}_2\text{O} \rightarrow \text{Al}_2\text{O}_3 + 3\text{H}_2 \uparrow$$
The generated hydrogen dissolves in the molten aluminum. At 660°C, the melting point of pure aluminum, the solubility of hydrogen in liquid aluminum is 0.7 cm³/100 g, while in solid aluminum at the same temperature, it is 0.037 cm³/100 g. Thus, the solubility of hydrogen in liquid aluminum is approximately 19 times that in solid aluminum. This significant difference can be expressed as:
$$S_{\text{liquid}} \approx 19 \times S_{\text{solid}} \quad \text{at} \quad 660^\circ\text{C}$$
During casting of shell castings, hydrogen has high solubility in liquid aluminum alloy. As the alloy cools after pouring into the mold, hydrogen solubility decreases, causing hydrogen to precipitate and form gas bubbles. In thick sections of shell castings, which remain in the liquid-solid two-phase zone longer, the outer regions solidify first, trapping the hydrogen gas released from the thick areas. This leads to pinhole formation, especially in regions with substantial wall thickness.
The presence of pinholes compromises the density of the microstructure, reducing mechanical properties. In critical shell castings like transmission housings, thick sections often coincide with stress concentration areas. Pinholes significantly diminish the mechanical performance and reliability life of these components.
To mitigate the cracking risk in aluminum alloy transmission shell castings, improvement strategies are proposed from three perspectives. First, optimize the vehicle’s general layout to reduce stress levels at high-stress locations. Second, enhance the casting process for shell castings to minimize pinhole defects and improve mechanical properties. Third, redesign the housing structure to improve castability and reduce pinhole formation during casting.
CAE simulation indicates that increasing the number of mounting bolts can lower stress levels at the cracking site. Currently, the housing has three mounting bolts; adding a fourth bolt could reduce the maximum stress from 149 MPa to 136 MPa, alleviating stress concentration in the shell castings.
Extensive research by engineers and technicians has focused on reducing pinholes in cast aluminum alloys. For instance, adjusting the placement of chills can significantly decrease pinhole defects in aluminum alloy shell castings. Studies on ZL101 cast aluminum alloy show that increasing cooling rates, raising solidification pressure, lowering pouring temperatures, and applying sodium modification treatments can collectively reduce casting pinhole defects. These principles can be adapted for die-cast shell castings to enhance quality.
The structural design of shell castings plays a crucial role in pinhole formation. At the cracking location, the wall thickness at the bolt installation area is 30 mm, while adjacent areas are only about 4 mm thick. This drastic thickness ratio (30/4 = 7.5) creates a severe wall thickness effect. During solidification, the thinner sections solidify first, encapsulating the thicker areas still in the liquid-solid phase. Hydrogen precipitating from the thicker region becomes trapped, forming pinholes. To improve castability, designers should minimize abrupt thickness changes in shell castings. Adding transition ribs or gradual thickness variations can reduce the wall thickness effect, decrease pinhole formation, and lower stress concentrations, thereby enhancing the reliability of shell castings.
In summary, the analysis concludes that excessive pinhole defects due to poor casting quality are the direct cause of cracking in the aluminum alloy shell castings. Three improvement approaches are outlined: optimizing the vehicle layout to reduce stress, refining the casting process to lower pinhole degrees, and redesigning the product structure to enhance castability. These measures collectively aim to strengthen the integrity and performance of shell castings in automotive applications.
The importance of shell castings in modern vehicles cannot be overstated, especially as lightweighting trends accelerate. Aluminum alloy shell castings offer a blend of strength and weight savings, but their manufacturing processes must be meticulously controlled to avoid defects like pinholes. Future advancements may involve novel alloy compositions, real-time monitoring during casting, and advanced simulation tools to predict and prevent defect formation in shell castings.
From a materials science perspective, the behavior of hydrogen in aluminum alloys is key to understanding pinhole formation. The solubility difference between liquid and solid phases drives gas porosity, and this can be modeled using thermodynamic principles. For instance, the equilibrium hydrogen concentration in aluminum can be described by Sieverts’ law:
$$C_{\text{H}} = K \sqrt{P_{\text{H}_2}}$$
where $C_{\text{H}}$ is the hydrogen concentration, $K$ is a temperature-dependent constant, and $P_{\text{H}_2}$ is the partial pressure of hydrogen. During solidification, local pressure changes and cooling rates affect hydrogen precipitation, influencing pinhole formation in shell castings.
Moreover, the mechanical impact of pinholes on shell castings can be quantified through fracture mechanics. The stress intensity factor $K$ at a pore can be approximated as:
$$K \approx \sigma \sqrt{\pi a}$$
where $\sigma$ is the applied stress and $a$ is the pore radius. Aggregated pinholes act as larger flaws, reducing the effective load-bearing area and accelerating crack propagation. This underscores the need for stringent quality control in producing shell castings.
In practice, non-destructive testing methods, such as X-ray radiography or ultrasonic inspection, can be employed to detect pinholes in shell castings before they enter service. However, prevention through process optimization is more cost-effective. For die-cast shell castings, parameters like injection speed, pressure, and die temperature must be optimized to minimize gas entrapment. Vacuum-assisted die casting is one technique that reduces air inclusion, thereby improving the density of shell castings.
The automotive industry’s shift towards electric and hybrid vehicles places greater demands on shell castings for components like transmissions and battery housings. These shell castings must withstand higher stresses and offer longer service life, making defect-free manufacturing paramount. Collaborative efforts between material scientists, design engineers, and foundry specialists are essential to advance the state-of-the-art in shell castings production.
In conclusion, the failure analysis highlights the critical role of manufacturing quality in the performance of aluminum alloy shell castings. By addressing pinhole defects through a multi-faceted approach, the reliability of these components can be significantly enhanced, supporting the broader goals of vehicle lightweighting and sustainability. As technology evolves, continuous improvement in shell castings design and processing will drive innovation in the automotive sector.