In modern automotive manufacturing, the demand for lightweight components with superior mechanical performance has driven the widespread adoption of aluminum alloy die castings. Among these, engine blocks and cylinder heads are commonly produced by high-pressure die casting (HPDC) due to its high productivity and excellent surface finish. However, the inherent high-speed filling and turbulent flow during HPDC often lead to gas entrapment, resulting in porosity defects that significantly impair the mechanical properties and preclude subsequent heat treatment. Vacuum-assisted high-pressure die casting (vacuum die casting) has emerged as an effective technique to mitigate such defects by evacuating the die cavity before and during melt injection. Compared with traditional sand casting, where sand foundry defect such as sand inclusion, blowholes, and rough surfaces are frequently observed, vacuum die casting offers a cleaner and denser microstructure; nevertheless, porosity remains a critical concern. This study focuses on the influence of cavity vacuum degree and heat treatment parameters on porosity and mechanical properties of a vacuum die-cast aluminum engine block. Particular attention is paid to the evolution of pore morphology and distribution, as well as to the possibility of applying post-casting heat treatment without causing blistering or excessive void growth. Throughout this paper, the term sand foundry defect is used as a benchmark for conventional casting imperfections, highlighting the advantages and remaining challenges of vacuum die casting.
1. Introduction
The automotive industry is under increasing pressure to reduce vehicle weight and emissions. Aluminum alloys, especially Al-Si-Cu systems such as ADC12, are widely utilized for complex structural components like engine blocks. Conventional high-pressure die casting offers high production rates and near-net-shape capability, but the turbulent filling of the die cavity at high speeds inevitably entrains gas, producing porosity in the solidified casting. These internal defects not only reduce load-bearing cross-sections but also act as stress concentrators, substantially lowering tensile strength, ductility, and fatigue life. Moreover, the presence of entrapped air makes conventional T4/T6 heat treatment impossible, as the expansion of pressurized gas at elevated temperatures leads to surface blistering and dimensional distortion. To overcome this limitation, vacuum die casting was developed. By evacuating the die cavity to a predetermined absolute pressure before and during injection, the amount of residual gas in the mold is minimized, thereby reducing gas porosity and enabling heat treatment in many cases. The effectiveness of vacuum die casting depends strongly on the degree of vacuum achieved, the timing of evacuation, and the sealing quality of the die. Several studies have shown that a lower absolute cavity pressure yields fewer and smaller pores, which directly improves mechanical performance. In contrast, traditional sand casting inherently contains sand foundry defect such as sand fusion, gas holes from mold moisture, and poor surface quality, although these defects may be controlled by proper mold design and pouring practice. For vacuum die-cast components, even with high vacuum, some residual gas remains, and solidification shrinkage may still create micro-porosity. Therefore, a thorough understanding of the relationship between vacuum level, pore characteristics, and mechanical behavior is essential. Additionally, the potential of applying heat treatment to vacuum die-cast parts is of great industrial interest, provided that porosity remains within acceptable limits. The purpose of this research is to systematically investigate the effects of three different vacuum degrees (50 kPa, 20 kPa, and 10 kPa absolute) on pore density, pore morphology, porosity fraction, and tensile properties of an ADC12 engine block. Furthermore, the influence of solution treatment duration (0.5 h and 2 h at 500 °C) on porosity and mechanical performance is studied, aiming to identify a heat treatment window that improves strength without severe porosity growth. The results contribute to optimization of vacuum die casting and heat treatment processes for high-integrity aluminum castings.
2. Experimental Procedures
2.1 Materials and Casting
The material used in this investigation was ADC12 aluminum alloy (AlSi9Cu3), whose chemical composition is provided in Table 1. The melt was processed and cast into an engine block using a Bühler cold-chamber die casting machine with a locking force of 27,000 kN. The die cavity was evacuated with a vacuum system to three different absolute pressure levels: 50 kPa, 20 kPa, and 10 kPa. Two blocks were produced for each condition. Table 2 summarizes the key process parameters.
| Zn | Fe | Cu | Mn | Mg | Si | Ti | Al |
|---|---|---|---|---|---|---|---|
| 1.0 | 1.2 | 3.0 | 0.1–0.5 | 0.1–0.5 | 9.0 | 0.15 | Bal. |
| Parameter | Value |
|---|---|
| Shot sleeve length (including biscuit) | 1200 mm |
| Plunger diameter | 140 mm |
| Filling ratio | 42.85% |
| Low speed | 0.2 m/s |
| High speed | 5.0 m/s |
| Slow-to-fast switch position | 575 mm |
| Cavity fill time | 5.82 ms |
| Pouring temperature | 640 °C |
| Die temperature | 150 °C |
The automatic pouring line used in this work is illustrated in the figure above. The melt was handled under controlled conditions to minimize hydrogen absorption, yet the inherent gas content of the melt remained a contributing factor to porosity.
2.2 Sample Preparation
Specimens were extracted from the cylinder bore region of the engine block using electric discharge machining. Metallographic samples (10 × 10 × 10 mm) were ground, polished, and examined using an optical microscope (OLYMPUS GX41). For pore quantification, images at 50× magnification were stitched to cover the entire specimen cross-section. Pore size and number were analyzed with Nano Measurer 1.2 software. For scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), a TESCAN VEGA 3 LMH SEM was employed. Tensile specimens were machined according to GB/T 228.1-2010 with rectangular cross-section, and tensile tests were performed using a 10 T universal testing machine at a crosshead speed of 1 mm/min. For each condition, at least five specimens were tested to obtain average values and standard deviations.
2.3 Heat Treatment
To examine the influence of solution treatment time, specimens from the 10 kPa vacuum condition (which exhibited the lowest as-cast porosity) were subjected to two heat treatments: (a) 500 °C × 0.5 h + 150 °C × 5 h and (b) 500 °C × 2 h + 150 °C × 5 h. Solution treatment was performed in an MXQ14000-30 box furnace. After solutionizing, specimens were quenched in water at 60 °C for 2 min, followed by artificial aging at 150 °C for 5 h and air cooling.
2.4 Porosity Measurement
Density of specimens was measured by Archimedes’ principle using a precision balance with a density kit. The density \( \rho_p \) was calculated from the mass in air (\( m_1 \)) and the mass in water (\( m_2 \)) using:
$$ \rho_p = \frac{m_1}{m_1 – m_2} \rho_w \tag{1} $$
where \( \rho_w \) is the density of water (≈0.998 g/cm³). The porosity fraction \( P \) was obtained from the ratio of measured density to the theoretical density of ADC12 (\( \rho_{wz} = 2.76 \, \text{g/cm}^3 \)):
$$ P = \left( 1 – \frac{\rho_p}{\rho_{wz}} \right) \times 100\% \tag{2} $$
3. Results and Discussion
3.1 Effect of Vacuum Degree on Pore Characteristics
Optical micrographs of specimens cast under three vacuum levels are shown in Figure 1 (not displayed here). At 50 kPa, large irregular pores with complex shapes were observed, some exceeding 140 μm in diameter. When the vacuum was increased to 20 kPa, the pore size and number decreased significantly, and the pores became more rounded. At 10 kPa, the microstructure appeared dense with only fine dispersed micro-porosity. This trend confirms that higher vacuum levels reduce the entrapment of air during mold filling. The pore size distributions, obtained from image analysis, are presented in Figure 2 and summarized in Table 3.
| Absolute pressure (kPa) | Number of pores | Average size (μm) | Max diameter (μm) | Min diameter (μm) | Porosity (%) | Density (g/cm³) |
|---|---|---|---|---|---|---|
| 50 | 3298 | 8.65 | 142.3 | 2.24 | 4.8 | 2.627 |
| 20 | 2956 | 8.12 | 127.6 | 1.98 | 4.4 | 2.638 |
| 10 | 1832 | 5.61 | 111.9 | 1.44 | 2.8 | 2.682 |
It is evident that decreasing the absolute cavity pressure from 50 kPa to 10 kPa reduced the total pore count by about 44%, and the average pore size was lowered from 8.65 μm to 5.61 μm. The porosity fraction decreased from 4.8% to 2.8%. These results are consistent with the expectation that a stronger vacuum minimizes the amount of residual gas in the cavity, thereby reducing gas entrapment. The pore morphology also became progressively more globular with increasing vacuum. At 50 kPa, many pores exhibited irregular contours and sharp corners, which are detrimental to mechanical performance because they introduce severe stress concentration. In contrast, at 10 kPa, pores were mostly spherical or elliptical, evenly distributed throughout the matrix. This observation underlines that not only the total porosity but also pore shape and distribution are critical, especially when considering fatigue and fracture behavior. In conventional sand casting, sand foundry defect such as sand erosion, gas cavities due to low permeability, and inclusions are common; vacuum die casting, on the other hand, can significantly reduce such gas-related defects, but the residual porosity is still a concern.
3.2 Pore Types and Microstructure
SEM observation revealed four distinct types of porosity: gas pores, shrinkage pores, gas-shrinkage pores, and micro-porosity. Gas pores displayed smooth, bright inner walls and a near-circular shape (Figure 3a). EDS analysis detected oxygen in these pores, confirming their origin as entrapped air. Shrinkage pores exhibited irregular, dark, rough internal surfaces, often associated with dendritic arms. Gas-shrinkage pores showed a complex morphology with a spherical part and an elongated tail. Compared with typical sand foundry defect such as sand holes or gas blowholes, vacuum die-cast pores are generally finer and less numerous, but their morphology still depends on local solidification conditions. The microstructure in all three vacuum conditions consisted of α-Al dendrites, eutectic silicon, and Fe-rich intermetallic compounds. The secondary dendrite arm spacing (SDAS) was visibly smaller at higher vacuum levels, as shown in Figure 3, suggesting improved feeding and more uniform solidification. This refinement contributed to the enhanced tensile strength observed in the higher vacuum specimens.
3.3 Mechanical Properties under Different Vacuum Levels
Tensile tests were conducted on specimens from each vacuum condition. The mean ultimate tensile strength (UTS) and elongation are plotted in Figure 4 and listed in Table 4. The standard deviation was calculated using:
$$ s = \sqrt{\frac{\sum_{i=1}^{n} (x_i – \bar{x})^2}{n}} \tag{3} $$
where \( x_i \) are individual values and \( \bar{x} \) is the mean.
| Condition | UTS (MPa) | Elongation (%) | σ_UTS (MPa) | σ_elongation (%) |
|---|---|---|---|---|
| 50 kPa | 205 | 1.76 | 25 | 0.11 |
| 20 kPa | 236 | 2.19 | 14.5 | 0.07 |
| 10 kPa | 258 | 2.25 | 12.8 | 0.06 |
The tensile strength increased from 205 MPa at 50 kPa to 258 MPa at 10 kPa, an improvement of 25%. Similarly, elongation increased from 1.76% to 2.25%, a relative increase of 27.8%. The standard deviation of UTS decreased significantly from 25 MPa at 50 kPa to 12.8 MPa at 10 kPa, indicating improved consistency and reliability. This improvement can be attributed to the combined effects of reduced porosity, finer pore size, more spherical pore shape, and slightly finer SDAS. The presence of large irregular pores, especially those clustered in bands, acts as crack initiation sites and reduces the effective load-bearing area. In contrast, a uniform distribution of small spherical pores is less harmful. This aligns with the known beneficial effect of eliminating sand foundry defect by vacuum casting, although the latter still contains some micro-porosity that limits ductility.
3.4 Fracture Surface Analysis
Macroscopic examination of the fractured tensile specimens revealed a flat, brittle fracture without necking for all vacuum levels. However, microscopic examination showed a transition from cleavage fracture to quasi-cleavage fracture as vacuum increased. At 50 kPa, the fracture surface exhibited large, irregular pores and secondary cracks, consistent with the low ductility and strength. At 10 kPa, the fracture surface displayed more tear ridges, and some dimples were observed, indicative of enhanced local plasticity. The pore distribution on the fracture surface mirrored the metallographic observations: pores were clustered and large at 50 kPa, while they were small and well dispersed at 10 kPa. This qualitative correlation confirms that pore characteristics strongly influence the fracture process. In comparison, sand casting often suffers from sand foundry defect that can cause similar stress concentrations and premature failure, but vacuum die casting offers a more controllable and reproducible microstructure.
3.5 Heat Treatment Effects on Porosity and Properties
The 10 kPa specimens were subjected to two solution treatments. Table 5 presents the density and porosity after heat treatment, while Table 6 lists the tensile properties.
| Condition | Density (g/cm³) | Porosity (%) |
|---|---|---|
| As-cast (10 kPa) | 2.682 | 2.8 |
| 500 °C × 0.5 h + 150 °C × 5 h | 2.64 | 4.3 |
| 500 °C × 2 h + 150 °C × 5 h | 2.60 | 5.7 |
Both heat treatments increased porosity significantly. The 0.5 h solution treatment raised porosity from 2.8% to 4.3%, while the 2 h treatment increased it to 5.7%. This indicates that new pores form during solutionizing, likely due to the precipitation of supersaturated hydrogen and the growth of pre-existing micro-voids. The pore morphology after 0.5 h remained mostly spherical, whereas after 2 h, adjacent pores coalesced into elongated irregular voids, as observed metallographically. The maximum pore size increased from about 120 μm (0.5 h) to 200 μm (2 h). This suggests that prolonged solution treatment promotes pore growth and coalescence, which is undesirable.
| Condition | UTS (MPa) | Elongation (%) | σ_UTS (MPa) | σ_elongation (%) |
|---|---|---|---|---|
| As-cast | 258 | 2.25 | 12.8 | 0.06 |
| 500 °C × 0.5 h + 150 °C × 5 h | 268 | 1.95 | 8.6 | 0.04 |
| 500 °C × 2 h + 150 °C × 5 h | 236 | 1.36 | 29.8 | 0.07 |
Interestingly, the 0.5 h solution treatment increased the average UTS to 268 MPa while reducing elongation to 1.95%. The standard deviation of UTS decreased to 8.6 MPa, indicating better consistency. On the other hand, the 2 h treatment caused a marked drop in both UTS (236 MPa) and elongation (1.36%), with a much larger scatter. The improvement after 0.5 h may be attributed to solid solution strengthening and spheroidization of eutectic silicon, which outweighs the adverse effect of moderate porosity growth. However, the prolonged treatment caused excessive porosity, negating any strengthening benefit. To evaluate the overall mechanical integrity, the quality index \( Q \) proposed by Drouzy et al. was used:
$$ Q = \text{UTS} + 150 \log \delta \tag{4} $$
where \( \delta \) is elongation in percent. The calculated average \( Q \) values were 310 MPa for as-cast, 311 MPa for 0.5 h treated, and 256 MPa for 2 h treated specimens. Thus, the 0.5 h treatment did not compromise the overall quality index, whereas the 2 h treatment caused a significant reduction. This demonstrates that a short solution treatment can be safely applied to vacuum die-cast ADC12 parts, provided the initial vacuum is high enough to keep porosity low. In contrast, long solution treatment times are detrimental and should be avoided, similar to the issues encountered when trying to heat-treat sand castings with sand foundry defect such as entrapped sand causing internal discontinuities.
3.6 Fracture Surface of Heat-Treated Specimens
Fracture surfaces of heat-treated specimens were examined. After 0.5 h treatment, the fracture was predominantly cleavage with some tear ridges and small dimples. Pores on the fracture surface were small and uniformly distributed, with a maximum pore diameter of about 40 μm. After 2 h treatment, the fracture showed an intergranular morphology, indicating weakened grain boundaries, and large pores up to 100 μm were present, often clustered. These large pores acted as stress concentrators and led to early failure. The appearance of intergranular fracture suggests that prolonged exposure at 500 °C may cause grain boundary segregation or liquation, which is exacerbated by pore growth. This again emphasizes the importance of controlling heat treatment duration to avoid the simultaneous degradation of porosity and grain boundary integrity.
3.7 Discussion of Porosity Formation Mechanisms
The evolution of porosity during vacuum die casting can be understood by considering the competing effects of gas entrapment and solidification shrinkage. In the initial filling stage, the high-velocity metal stream may entrain residual gas from the cavity, leading to spherical gas pores. As solidification progresses, dendrite networks hinder liquid feeding, and the remaining liquid under negative pressure can form shrinkage porosity. The application of vacuum reduces the amount of gas available for entrapment, thus lowering gas pore population. Shrinkage porosity, however, still depends on the local thermal gradient and feeding efficiency. The finer SDAS observed at higher vacuum levels suggests better feeding and more uniform solidification, which also reduces shrinkage porosity. During heat treatment, the solubility of hydrogen in aluminum decreases with temperature, and supersaturated hydrogen atoms diffuse to existing micro-voids or heterogeneous nucleation sites, such as oxide films or intermetallic particles. This leads to the formation and growth of new pores, as evidenced by the increased porosity values. The kinetics of this process depend on time and temperature; longer solutionizing allows more diffusion and coalescence. In conventional sand casting, the initial porosity might be even higher, and sand foundry defect such as sand inclusions further complicate heat treatment. Vacuum die casting with a high vacuum level provides a starting microstructure with low porosity, enabling short heat treatments to be beneficial.
4. Recommendations and Practical Implications
Based on the findings, the following recommendations are made for the production of high-integrity vacuum die-cast aluminum components:
- Maintain the cavity absolute pressure as low as possible (≤10 kPa) to minimize gas porosity and achieve consistent mechanical properties.
- If heat treatment is intended, use solution treatment times not exceeding 0.5 h at 500 °C, followed by a low-temperature aging (e.g., 150 °C for 5 h), to improve strength without excessive porosity growth.
- Avoid long solution treatments (≥2 h) for thin-walled sections where pore coalescence is severe.
- Control melt quality (hydrogen content) and die coating to reduce initial gas sources.
- Inspect the critical regions of the casting for pore clustering, which is more harmful than uniformly distributed fine pores.
These practices help mitigate porosity-related failures and allow the use of vacuum die-cast components in applications that demand both high strength and reasonable ductility. Compared with traditional sand castings, where sand foundry defect such as sand inclusion and poor surface finish are common, vacuum die casting offers a superior alternative, especially when combined with optimized heat treatment.
5. Conclusions
In this study, the effects of vacuum degree and heat treatment on porosity and mechanical properties of ADC12 aluminum alloy engine blocks produced by vacuum die casting were systematically investigated. The main conclusions are:
- Increasing the vacuum degree from 50 kPa to 10 kPa significantly reduced the pore count and average pore size. The porosity fraction decreased from 4.8% to 2.8%, and the pore morphology became more spherical and uniformly distributed.
- The tensile strength and elongation improved with increasing vacuum. At 10 kPa, the UTS and elongation reached 258 MPa and 2.25%, respectively, representing 25% and 27.8% improvements over 50 kPa conditions. The scatter also decreased, indicating better repeatability.
- Vacuum die casting effectively mitigates common sand foundry defect although residual porosity still limits the mechanical properties. Pore morphology and distribution were found to be critical factors affecting fracture behavior.
- Heat treatment at 500 °C for 0.5 h followed by aging at 150 °C for 5 h increased the average UTS to 268 MPa while slightly reducing elongation, and the quality index remained essentially unchanged. This short solution treatment is acceptable for high-vacuum castings.
- Extending the solution treatment to 2 h caused severe porosity growth, with porosity increasing to 5.7%, and reduced both strength and ductility significantly. Pore coalescence led to cracked pores up to 200 μm in size, and the fracture mode transitioned to intergranular.
- The formation of new micropores during heat treatment is attributed to the precipitation of supersaturated hydrogen and the growth of pre-existing microvoids. The longer the treatment, the more pronounced the pore growth.
Overall, vacuum die casting combined with a short solution treatment offers a viable route for producing high-performance aluminum castings, while avoiding the traditional limitations associated with sand foundry defect in sand casting and the inability of conventional die castings to undergo heat treatment.