1. Introduction
With the global climate becoming warmer and energy crisis intensifying, environmental protection and energy conservation have become major global challenges. In the automotive industry, reducing total vehicle weight can significantly alleviate fuel consumption pressure and achieve energy conservation and emission reduction goals. Therefore, lightweight automotive manufacturing and minimizing the weight of automotive components have become the primary themes in the automotive manufacturing sector. Replacing steel with aluminum is currently one of the most actively researched approaches. Aluminum alloys possess excellent characteristics such as low density and high specific strength, gradually replacing steel as the preferred alloy material in the manufacturing industry. The application of aluminum alloys brings significant economic and social benefits in terms of energy conservation and emission reduction, which is of great significance for the development of the automotive industry.
However, due to the strong demand for lightweight solutions, the requirements for product structural design and production methods have accordingly increased. Automotive components have become structurally complex due to weight reduction efforts, bringing tremendous pressure to traditional production methods. In recent years, the use of lightweight aluminum alloy components has been favored by numerous automobile manufacturers. The production methods for automotive lightweight aluminum alloy components mainly include Low Pressure Die Casting (LPDC), High Pressure Die Casting (HPDC), and Forging technologies, among which HPDC is widely applied owing to its high precision and superior product performance.
Through HPDC, not only can castings with relatively thin walls, complex shapes, and smooth surfaces be obtained, but the precision, dimensions, and structural density of the resulting castings are also significantly improved, leading to better mechanical properties. A die-casting factory primarily engaged in the production of automotive engine bracket products cooperates with well-known domestic and international enterprises such as GM, VOLVO, Ford, and SAIC-GM. With the ever-increasing demands for low emission, lightweight, high efficiency, and safety in today’s society, the requirements for the air-tightness and strength of engine bracket products have correspondingly increased, exposing the limitations of existing die-casting production methods.
The engine bracket products produced by this factory generally have relatively thick walls and complex structures. During the filling process, gas entrapment, gas locking, and uneven shrinkage phenomena are prone to occur. The die-cast parts exhibit defects that affect product quality, mainly manifested as gas and shrinkage porosity. The presence of gas and shrinkage porosity defects affects the strength and fatigue performance of the die-cast components to some extent. To enhance material mechanical properties, reducing porosity defects is the core objective. However, many factors during the die-casting process influence porosity characteristics, making it difficult to systematically and accurately predict the factors affecting product porosity, and also challenging to identify effective ways to improve bracket mechanical properties. Therefore, how to quickly and correctly select reasonable methods to reduce the high scrap rate caused by gas pores has become a significant concern for die-casting manufacturers.
Various sand foundry defects in high pressure die casting, such as gas porosity and shrinkage porosity, significantly degrade the structural integrity and fatigue resistance of aluminum components. Understanding and mitigating these sand foundry defects through optimized gating design, process parameter control, and advanced local squeeze techniques constitute the central theme of this research. The purpose of this study is to employ Magmasoft software to numerically simulate the defects formed during the HPDC process of engine brackets, comparing the simulation results with actual production. This study seeks reasonable methods to reduce the porosity defects of engine brackets. By analyzing the structural characteristics of special bracket positions, the causes, and locations of gas pore formation, a local squeeze device will be developed. The local squeeze technology will assist the HPDC process to reduce sand foundry defects at special positions, while X-ray real-time imaging technology will be used for analytical verification. The aim is to seek ways to reduce engine bracket gas porosity, improve bracket mechanical properties, identify the main factors causing porosity, and establish efficient bracket die-casting processes and production methods, providing certain guiding significance for actual production.
2. Theoretical Analysis of HPDC of Engine Bracket
2.1 Introduction
High Pressure Die Casting (HPDC) technology is a special precision casting method. It involves filling molten metal into a shot sleeve and, under high pressure, rapidly injecting it into the die cavity of a die-casting mold. The die-cast product crystallizes and solidifies under high pressure to form a finished part or blank. Therefore, high pressure and high-speed cavity filling are the most significant characteristics of die casting. Before formally predicting and optimizing gas porosity defects, it is essential to analyze the relevant HPDC theories, especially filling theories, pore formation mechanisms, process analysis, and CAE simulation theories. These theoretical analyses will directly provide a foundation for subsequent product defect prediction and the proposal and implementation of new processes.
2.2 Pressure in Die Casting Process
For the HPDC process, the most distinctive feature compared to other casting methods is the high pressure applied during casting, which is utilized to ensure casting quality. The pressures involved in HPDC mainly include injection pressure and intensification pressure (casting pressure). The injection pressure is the driving force generated by the injection mechanism of the die-casting machine, i.e., the injection rod pushing the injection piston in the shot sleeve. Its magnitude is determined by the hydraulic pressure of the accumulator and the cross-sectional area of the piston. The intensification pressure is the pressure per unit area exerted on the molten metal within the die cavity. It can also be calculated as the ratio of the injection force to the cross-sectional area of the shot chamber. The calculation formulas are as follows:
$$F_{injection} = P_{cylinder} \times \frac{\pi D_{cylinder}^2}{4}$$ (2-1)
$$P_{intensification} = \frac{F_{injection}}{S_{chamber}}$$ (2-2)
where \(F_{injection}\) is the injection force (N), \(P_{intensification}\) is the intensification pressure (Pa), \(S_{chamber}\) is the cross-sectional area of the shot chamber (m²), \(P_{cylinder}\) is the working fluid pressure in the injection cylinder (Pa), and \(D_{cylinder}\) is the diameter of the injection cylinder (m).
The role of injection pressure is to push the molten metal within the shot sleeve, filling the cavity at high speed, and maintaining a certain intensification ratio during solidification after filling is complete. The magnitude of the intensification pressure directly affects the internal structural density and surface roughness of the die-cast product. As the intensification pressure increases, the internal crystals of the casting become finer, the fine crystal layer gradually thickens, and the surface quality improves. The size and distribution of gas pores also significantly decrease. However, an excessive increase in intensification pressure can reduce the elongation of the product. The recommended intensification pressure values for different types of castings are shown in Table 2-1.
| Casting type | Aluminum alloy / MPa | Zinc alloy / MPa | Brass / MPa |
|---|---|---|---|
| Light load, low strength castings | 30-40 | 13-20 | 30-40 |
| Heavy load, high strength castings | 40-80 | 20-30 | 40-60 |
| High air-tightness castings | 80-120 | 25-40 | 80-100 |
2.3 Velocity in Die Casting Process
The filling speed of the die casting is directly influenced by the injection force, and together with the intensification pressure, the filling speed plays a crucial role in the internal and surface quality of die-cast products. The main velocities involved in actual die-casting production are the plunger velocity, gate velocity, and cavity filling velocity.
According to the continuity principle, the relationship between gate velocity and plunger velocity is given by:
$$V_{gate} = \frac{S_{plunger} \times V_{plunger}}{S_{gate}}$$ (2-3)
where \(V_{gate}\) is the gate velocity (m/s), \(S_{plunger}\) is the cross-sectional area of the plunger or shot sleeve (cm²), \(V_{plunger}\) is the plunger velocity (m/s), and \(S_{gate}\) is the gate cross-sectional area (cm²).
The recommended values for the filling velocity in different sections of the gating system are shown in Table 2-2, and the recommended gate velocities for different wall thicknesses are shown in Table 2-3.
| Section | Vertical runner | Horizontal runner | Gate |
|---|---|---|---|
| Filling velocity (m/s) | 15-25 | 20-25 | 30-60 |
| Average wall thickness (mm) | Gate velocity (m/s) | Average wall thickness (mm) | Gate velocity (m/s) |
|---|---|---|---|
| 1 | 46-55 | 5 | 32-40 |
| 1.5 | 44-53 | 6 | 30-37 |
| 2 | 42-50 | 7 | 28-34 |
| 2.5 | 40-48 | 8 | 26-32 |
| 3 | 38-46 | 9 | 24-29 |
| 3.5 | 36-44 | 10 | 24-29 |
| 4 | 34-42 | – | – |
2.4 Formation Mechanism of Porosity Defects
Gas porosity is a defect characterized by regularly shaped, smooth-surfaced cavities remaining inside the casting after solidification. The formation causes are mainly twofold: one is the inability to discharge gas from the mold cavity smoothly, causing gas accumulation (gas locking); the other is the turbulent flow of molten metal during the filling process, causing gas to be drawn (entrapped) into the molten metal. Shrinkage porosity forms during the solidification process of the casting in the die cavity due to an imbalance between internal volume contraction and so-called feeding compensation. This results in relatively rough irregular holes on the casting surface.
For gas porosity caused by poor exhaust or turbulent flow, the main solutions include optimizing mold design, increasing the cross-sectional area of overflow wells at exhaust locations, adjusting filling velocity, and increasing casting pressure. For shrinkage porosity, common solutions include appropriately lowering the pouring temperature, adding cores or chillers, adjusting cooling water flow, and applying spot cooling or intensification pressure.
3. Analysis of Porosity Defects via HPDC of Engine Bracket
3.1 Product Structure Analysis
The researched component is a right engine bracket for a specific vehicle. Due to the lightweight design philosophy of this vehicle, many recesses are present on the surface of the bracket. To ensure strength, numerous reinforcing ribs are designed on the bracket, making its structure relatively complex. The shape is irregular with uneven wall thickness, ranging from a maximum of 17 mm to a minimum of 3 mm. The mold cavity has significant high-low parting lines, and the farthest end hole position protrudes above the parting line, making it impossible to set up exhaust there.
3.2 Mold Structure Design Analysis
Based on the shape of the engine bracket product, the mold design scheme is determined. The die casting uses three gates for feeding to ensure stable filling and sufficient pressure application. Overflow wells are set at the farthest end of the bracket to collect cold material, and exhaust is provided for each overflow well to facilitate the smooth escape of gas from the cavity. Figure 3-1 shows the gating and overflow system design for the bracket. During the initial HPDC process, the casting pressure cannot effectively reach the location of the gas porosity defect due to the complex geometry and early gate solidification, leading to local porosity exposure after pressure holding and solidification, which directly affects the performance of the bracket.
3.3 Selection of Initial Process Parameters
According to the theoretical analysis and empirical formulas, the initial HPDC numerical simulation parameters were determined. To ensure the fluidity of the aluminum alloy melt, the pouring temperature was set at 650°C. The mold initial temperature was set to 180°C to allow the mold to quickly reach thermal equilibrium. The HPDC process involves three main stages: low-speed stage, high-speed stage, and intensification stage. In the low-speed stage, the plunger velocity was set to 0.35 m/s. For AlSi9Cu3 alloy with a casting weight of 2722.3 g, plunger cross-section area of 50.3 cm², and liquid density of 2.6 g/cm³, the high-speed range is calculated as:
$$H_{high-speed} = \frac{G_{product+biscuit}}{\rho \times S_{chamber}} + 10 = \frac{2722.3}{2.6 \times 50.3} + 10 \approx 218.16 \text{ mm}$$ (3-1)
Therefore, the high-speed range was initially set to 220 mm, the high-speed velocity to 3.5 m/s, and the intensification pressure to 80 MPa. These parameters are listed in Table 3-1.
| Parameter | Value | Parameter | Value |
|---|---|---|---|
| Initial pouring temperature GT (°C) | 650 | Low-speed velocity (m/s) | 0.35 |
| Initial mold temperature MT (°C) | 180 | High-speed velocity (m/s) | 3.5 |
| Machine tonnage (tonnes) | 840 | High-speed range (mm) | 220 |
| Closing time (s) | 12 | Intensification pressure (MPa) | 80 |
3.4 CAE Simulation Analysis of HPDC Process
The three-dimensional model of the bracket was converted to STL format using UG software and imported into the Magmasoft preprocessor module. The material groups were imported in sequence: Cover (fixed die), Ejector (moving die), Inlet (plunger), Biscuit, Gating (runner), Ingate, Cool (cooling channels), Cast (casting), and Special (exhaust). Exhaust must be fully connected to the corresponding overflow wells and extended outside the mold to ensure the simulation matches the actual trial. Cooling tracking points were set to monitor the temperature at specific locations during the simulation. The analysis was performed using the material thermal-physical properties listed in Tables 3-2 and 3-3.
| Temperature (°C) | Thermal conductivity (W·m⁻¹·K⁻¹) | Specific heat (J·kg⁻¹·K⁻¹) | Density (kg·m⁻³) |
|---|---|---|---|
| 1 | 140.0 | 820 | 2650 |
| 50 | 142.5 | 860 | 2643 |
| 100 | 144.5 | 900 | 2637 |
| 200 | 147.5 | 950 | 2619 |
| 300 | 152.5 | 980 | 2597 |
| 400 | 148.0 | 1050 | 2575 |
| 479 | 141.5 | 1150 | 2555 |
| 578 | 70.0 | 1000 | 2530 |
| 2000 | 70.0 | 1000 | 2095 |
| Temperature (°C) | Thermal conductivity (W·m⁻¹·K⁻¹) | Temperature (°C) | Specific heat (J·kg⁻¹·K⁻¹) |
|---|---|---|---|
| 20 | 24.60 | 23 | 458.8 |
| 200 | 26.25 | 200 | 518.5 |
| 500 | 27.30 | 400 | 587.8 |
| 600 | 27.76 | 600 | 726.2 |
| 800 | 28.07 | 700 | 905.4 |
| 850 | 28.39 | 760 | 1151.1 |
The heat transfer coefficients between material groups are given in Table 3-4. Figure 3-2 shows the specific HTC values for AlSi9Cu3-HPDC and the Default.spray.
| Parameter | Value |
|---|---|
| Casting-mold heat transfer (W/m²·K) | AlSi9Cu3-HPDC |
| Mold-mold heat transfer (W/m²·K) | 3500 |
| Mold-cooling heat transfer (W/m²·K) | 2000 |
| Mold-coating heat transfer (W/m²·K) | Default.spray |
| Ambient temperature (°C) | 20 |
Mathematical Equations for Filling Simulation
During the die-casting filling simulation, the molten metal is regarded as an incompressible fluid. Its flow follows the conservation of mass and momentum, and the mathematical equations can be expressed via the continuity equation and the Navier-Stokes equations:
$$\frac{\partial u}{\partial x} + \frac{\partial v}{\partial y} + \frac{\partial w}{\partial z} = 0$$ (3-2)
$$\rho\left(\frac{\partial u}{\partial t} + u\frac{\partial u}{\partial x} + v\frac{\partial u}{\partial y} + w\frac{\partial u}{\partial z}\right) = -\frac{\partial p}{\partial x} + \rho g_x + \mu \nabla^2 u$$ (3-3a)
$$\rho\left(\frac{\partial v}{\partial t} + u\frac{\partial v}{\partial x} + v\frac{\partial v}{\partial y} + w\frac{\partial v}{\partial z}\right) = -\frac{\partial p}{\partial y} + \rho g_y + \mu \nabla^2 v$$ (3-3b)
$$\rho\left(\frac{\partial w}{\partial t} + u\frac{\partial w}{\partial x} + v\frac{\partial w}{\partial y} + w\frac{\partial w}{\partial z}\right) = -\frac{\partial p}{\partial z} + \rho g_z + \mu \nabla^2 w$$ (3-3c)
The energy conservation equation is:
$$\rho c \left(\frac{\partial T}{\partial t} + u\frac{\partial T}{\partial x} + v\frac{\partial T}{\partial y} + w\frac{\partial T}{\partial z}\right) = \frac{\partial}{\partial x}\left(k\frac{\partial T}{\partial x}\right) + \frac{\partial}{\partial y}\left(k\frac{\partial T}{\partial y}\right) + \frac{\partial}{\partial z}\left(k\frac{\partial T}{\partial z}\right) + S$$ (3-4)
Mathematical Equations for Solidification Simulation
For the solidification simulation, the three-dimensional transient heat conduction equation is:
$$\rho c \frac{\partial T}{\partial t} = \lambda \left(\frac{\partial^2 T}{\partial x^2} + \frac{\partial^2 T}{\partial y^2} + \frac{\partial^2 T}{\partial z^2}\right) + \Delta G$$ (3-5)
where \(\Delta G = \rho L \frac{\partial f_s}{\partial T}\), \(L\) is the latent heat of fusion, and \(f_s\) is the solid fraction.
3.5 Simulation Results and Defect Prediction
From the material trace analysis (Figure 3-3), the molten aluminum reached the gate at approximately 1.943 s and began filling the casting cavity. The filling reached the farthest end at approximately 1.997 s, with a total cavity filling time of about 54 ms. The temperature analysis indicated that the temperature at the gate during filling was around 648°C with good fluidity. The temperature at the middle sections of the cavity ranged between 642°C and 646°C. However, at the farthest end bearing hole position (P8, P9, P10), the filling temperature dropped to about 636°C. This lower temperature led to worse fluidity of the aluminum melt at the far end, preventing rapid mold filling and promoting loose internal structure. The air pressure analysis showed that most regions of the bracket had gas pressure close to the atmospheric pressure of 1013 MPa. However, around the farthest end bearing hole, the gas pressure locally reached 3000 MPa, indicating that gas could not be efficiently discharged and was highly prone to forming gas porosity defects.
The solidification analysis showed that the gate region fully solidified at approximately 3 s. After this time, the casting pressure could no longer be transmitted through the gate to the casting. Since the far-end bearing hole area solidifies later and cannot receive the beneficial effect of pressure, sand foundry defects in the form of gas porosity are prone to form in this region. This simulation prediction was confirmed by actual production tests, which showed a maximum gas pore diameter of 3.2 mm and a local porosity ratio of 9.0% in the far-end bearing hole area, far exceeding the acceptable limits according to standard ASTM E505.
3.6 Taguchi Optimization of HPDC Process Parameters
The main process parameters affecting the porosity defects of the casting were identified as intensification pressure, high-speed velocity, and high-speed range. A Taguchi L9(3³) orthogonal experiment was designed to optimize these parameters. The variable ranges are shown in Table 3-5, and the experimental design in Table 3-6.
| Parameter | Range |
|---|---|
| Intensification pressure (MPa) | 80-100 |
| High-speed velocity (m/s) | 3.0-4.0 |
| High-speed range (mm) | 190-210 |
| No. | Intensification pressure (MPa) | High-speed velocity (m/s) | High-speed range (mm) |
|---|---|---|---|
| A1 | 80 | 3.0 | 190 |
| A2 | 80 | 3.5 | 200 |
| A3 | 80 | 4.0 | 210 |
| B1 | 90 | 3.0 | 190 |
| B2 | 90 | 3.5 | 200 |
| B3 | 90 | 4.0 | 210 |
| C1 | 100 | 3.0 | 190 |
| C2 | 100 | 3.5 | 200 |
| C3 | 100 | 4.0 | 210 |
The production trials were conducted on a Bühler 840T cold-chamber die-casting machine. For each process group, three castings were taken for X-ray analysis. In process group C2 (intensification pressure 100 MPa, high-speed velocity 3.5 m/s, high-speed range 200 mm), the best results were achieved, with a maximum gas pore diameter of 1.8 mm and a local porosity ratio of 4.2%. Although the porosity was improved compared to the initial state, the local gas pores remained significant. Therefore, conventional process parameter optimization alone was insufficient to reduce the gas porosity defect to an acceptable level.
4. Local Squeeze-Assisted HPDC Process
4.1 Introduction to Local Squeeze Theory
When internal gas pores in die-castings cannot be effectively improved through injection process parameters and mold structure optimization, one available method is to compress the gas pores by directly applying pressure to a portion of the mold cavity after the molten metal is injected. This local pressing method cannot remove the already formed gas pores. Still, it can force the molten metal to enter the pores during solidification through the application of pressure, reducing the size of gas pores and improving the density of the local structure.
During the entire die-casting production process, the role of casting pressure is crucial, directly affecting the quality of the die-casting and the density of the microstructure. However, due to the limitations of the casting and mold structure, some local special parts of the casting cannot receive the full effect of the casting pressure, resulting in the appearance of defects such as gas pores and cold shuts. Local squeeze technology is a special processing method that improves the internal gas porosity defects of the casting by applying pressure to thick-walled portions or structurally complex local positions, forming a casting with a smooth surface and a sound internal structure. As shown in Figure 4-1, the formation of gas pores is mainly caused by gas bubbles formed due to poor exhaust during high-speed filling. When the gas fraction is high, these bubbles are exposed as defects after solidification, appearing nearly spherical. A thin oxide film forms at the gas/aluminum interface under high temperature and pressure. The local squeeze process works by directly applying pressure to the pore-prone area when the aluminum melt is semi-solid. The high pressure breaks the thin film between the pore and the melt, allowing the surrounding aluminum melt to fill the void, disperse the pore, and compress it.
4.2 Design of Local Squeeze Device
The local squeeze system mainly comprises a squeeze cylinder, a squeeze pin, a squeeze bush, and related connecting devices. The squeeze mechanism can be installed on the die frame or the die insert according to the design requirements. In this study, the approach of directly forming the bottom hole of the cast product is adopted. The original core-pulling pin was replaced with a squeeze pin, and the original core-pulling structure was modified to have an R angle at the front section, thereby achieving squeeze forming of the far-end hole by utilizing the core-pulling mechanism. Figure 4-2 shows the theoretical state of the hole to be squeezed. Before squeeze, the product surface has no hole; after squeeze, the hole is initially formed; subsequent machining completes the final hole geometry.
Since the squeeze pin tip comes into direct contact with the high-temperature molten aluminum during the squeezing process, SKD61 hot-work die steel is selected as the material for the squeeze pin, heat-treated to HRC40-45. To further enhance hardness performance, the squeeze pin and sleeve are treated with a D+ coating to improve surface hardness. The far-end bearing hole has a diameter of 10.5 mm, so the front diameter of the squeeze pin is designed to be 10.5 mm to assist hole formation. The sleeve’s fit dimension is 10.5 mm with a clearance of 0.02-0.04 mm to ensure there is no aluminum leakage between the pin and sleeve during squeezing. To ensure smooth sliding of the squeeze pin, only the front section (15 mm) of the sleeve contacts the pin; the remaining back section is relieved. The maximum wall thickness at the squeezed position is about 10 mm, so the squeeze pin stroke is set to 20 mm to ensure the maximum squeeze effect.
For the squeeze cylinder, the relationship between the key technical parameters is:
$$F_{squeeze} = P_{system} \times S_{piston}$$ (4-1)
$$S_{piston} = \frac{\pi D_{cylinder}^2}{4}$$ (4-2)
$$P_{squeeze\_pressure} = \frac{F_{squeeze}}{S_{pin}}$$ (4-3)
Combining the above equations, the squeeze pressure can be calculated as:
$$P_{squeeze\_pressure} = P_{system} \times \frac{D_{cylinder}^2}{D_{pin}^2}$$ (4-4)
The Bühler 840T machine’s system pressure \(P_{system}\) is 16 MPa. Given a cylinder diameter \(D_{cylinder}\) of 50 mm and a squeeze pin diameter \(D_{pin}\) of 10.5 mm, the squeeze pressure is calculated as \(P_{squeeze\_pressure} \approx 360\) MPa, providing a sufficient squeeze force to effectively compress the gas pores.
4.3 Design of Local Squeeze Process Scheme
The local squeeze die-casting process comprises the filling stage and the solidification stage, with local pressure applied when the molten metal is semi-solid. The specific implementation steps are as follows:
(1) During the metal melt filling stage, the squeeze cylinder remains stationary with the squeeze pin withdrawn outside the cavity, the front face of the squeeze pin being 1-2 mm from the inner surface of the cavity.
(2) The molten metal fills the mold cavity sequentially under the plunger’s action. When the molten metal fills to the far end and wraps around the front of the squeeze pin, the filling is complete, at which point the squeeze cylinder and pin remain in their initial state.
(3) After filling, the pressure-holding solidification stage begins. When the aluminum melt around the far-end bearing hole has solidified for a certain time (squeeze delay time) and reaches the semi-solid state, the squeeze cylinder drives the squeeze pin forward, starting the hole’s squeeze forming. The front face, side surface, and R angle of the squeeze pin press the front and surrounding metal melt laterally toward both sides, thus compressing the pore-prone positions and reducing the gas pore size.
(4) After the squeeze pin advances a certain distance, due to front resistance and squeeze process factors, the pressure-holding phase begins. The holding time (squeeze duration) is adjusted based on actual process and test results.
(5) After the squeeze is completed and the product is fully solidified, the hydraulic pressure in the squeeze cylinder is released, and the cylinder retracts to its original position. The casting can then be ejected by the ejector mechanism, ready for the next die-casting cycle.
4.4 Analysis of Local Squeeze Process Parameters
The key process parameters of local squeeze are squeeze delay time, squeeze depth, and squeeze duration. Understanding the solidification behavior of AlSi9Cu3 alloy is essential. The solid fraction of the alloy as a function of temperature is shown in Figure 4-3. The liquidus temperature is approximately 580°C, and the solidus temperature is approximately 480°C. The temperature of the aluminum melt around the hole region during solidification was obtained from the CAE simulation. This study analyzes the effect of these key parameters on the squeeze results.
The squeeze delay time is defined as the time interval between the end of filling and the beginning of the local squeeze operation. To determine the rational squeeze timing, the holding pressure was kept constant while varying the delay time. The delay time values are 0.1s, 0.3s, 0.5s, 0.85s, 1.0s, 1.2s, 1.5s, and 2.0s. The squeeze depth analysis helps judge the rationality of the delay time and provides a basis for the subsequent holding time setting. The squeeze duration is from the start of the squeeze to the time the squeeze pin starts extracting. If the squeeze duration is too short, no effective squeeze is achieved. If too long, the squeeze pin will act as an obstacle during mold opening, causing mechanical interference.
5. Experimental Verification and Discussion
5.1 Experimental Setup
The local squeeze die-casting experiments were conducted on a Bühler 840T die-casting machine, equipped with a local squeeze process control system. The specific experimental design schemes are shown in Tables 5-1 and 5-2. For each parameter set, three castings were produced and tested for squeeze depth, maximum gas pore diameter, and porosity rate. To ensure the independence of the adjacent process parameter groups, at least two products were produced and scrapped between different process groups. First, the squeeze depth of the castings was measured using a micrometer. Then X-ray analysis was performed using an SMX3000M system to assess the maximum gas pore diameter and local porosity rate. Finally, the castings were sectioned for visual comparison of gas pore defects.
| No. | Casting pressure (MPa) | Squeeze pressure (MPa) | Delay time (s) | Duration (s) | Squeeze depth (mm) | Average depth (mm) | ||
|---|---|---|---|---|---|---|---|---|
| a | b | c | ||||||
| 1 | 100 | 360 | 0.10 | 0.5 | 8.05 | 7.90 | 8.03 | 7.99 |
| 2 | 100 | 360 | 0.30 | 0.5 | 6.91 | 7.02 | 7.35 | 7.09 |
| 3 | 100 | 360 | 0.50 | 0.5 | 5.71 | 6.60 | 5.80 | 6.04 |
| 4 | 100 | 360 | 0.75 | 0.5 | 5.20 | 5.34 | 5.22 | 5.25 |
| 5 | 100 | 360 | 1.00 | 0.5 | 5.05 | 4.95 | 3.96 | 4.80 |
| 6 | 100 | 360 | 1.20 | 0.5 | 2.50 | 1.86 | 2.41 | 2.27 |
| 7 | 100 | 360 | 1.50 | 0.5 | 1.21 | 0.68 | 1.10 | 0.99 |
| 8 | 100 | 360 | 2.00 | 0.5 | 0.78 | 0.21 | 0.68 | 0.56 |
| No. | Casting pressure (MPa) | Squeeze pressure (MPa) | Delay time (s) | Duration (s) | Squeeze depth (mm) | Average depth (mm) | Max pore diameter (mm) | Porosity rate (%) | ||
|---|---|---|---|---|---|---|---|---|---|---|
| a | b | c | ||||||||
| 1 | 100 | 360 | 1.0 | 0.5 | 5.05 | 4.95 | 3.96 | 4.80 | 2.35 | 4.3 |
| 2 | 100 | 360 | 1.0 | 1.0 | 5.16 | 5.80 | 4.62 | 4.86 | 2.29 | 3.9 |
| 3 | 100 | 360 | 1.0 | 1.5 | 5.71 | 3.51 | 5.80 | 5.01 | 1.97 | 2.9 |
| 4 | 100 | 360 | 1.0 | 3.0 | 5.54 | 6.11 | 5.12 | 5.59 | 1.41 | 1.2 |
| 5 | 100 | 360 | 1.0 | 4.0 | 6.05 | 5.73 | 5.62 | 5.80 | 0.72 | 1.0 |
| 6 | 100 | 360 | 1.0 | 5.0 | 7.16 | 5.99 | 6.21 | 6.45 | 0.46 | 0.6 |
5.2 Effect of Squeeze Delay Time on Squeeze Depth
The experimental results show that the squeeze depth decreases with increasing delay time. With a delay time of 0.1 s, the alloy around the hole is almost entirely liquid, and the squeeze pin advances with minimal resistance, resulting in a larger squeeze depth. When the delay time reaches 2.0 s, the solid fraction of the alloy around the hole approaches 60%, making the pin unable to advance significantly, resulting in a small squeeze depth of only 0.56 mm. Based on the results, a delay time between 0.85 s and 1.0 s is most suitable. At this time, the temperature of the hole surrounding the melt is between 560°C and 570°C, corresponding to a solid fraction in the range of 19% to 37%, leaving a sufficient amount of molten metal for the squeeze action. Since the solid fraction is still relatively low, the squeeze pin can overcome the resistance and push the semi-solid metal to the sides, effectively compressing the gas pores at both sides.
5.3 Effect of Squeeze Duration on Gas Porosity Defects
The results in Figure 5-1 and Table 5-2 indicate that with a fixed delay time of 1.0 s, the squeeze depth increases with longer squeeze duration. However, the trend of change is relatively small compared to the influence of delay time. The maximum gas pore diameter decreases with increasing squeeze duration. At a duration of 0.5 s, the maximum pore diameter is 2.35 mm, corresponding to ASTM grade 3 porosity; at a duration of 5.0 s, the maximum pore size decreases to 0.46 mm, meeting the ASTM grade 1 standard. The porosity rate decreased from 4.3% to 0.6%. Therefore, a squeeze duration of 5 s is deemed most appropriate, resulting in a squeeze depth of 6.45 mm. Table 5-3 summarizes the effect of squeeze duration on the final quality attributes.
| Duration (s) | Average squeeze depth (mm) | Maximum pore diameter (mm) | Porosity rate (%) |
|---|---|---|---|
| 0.5 | 4.80 | 2.35 | 4.3 |
| 1.0 | 4.86 | 2.29 | 3.9 |
| 1.5 | 5.01 | 1.97 | 2.9 |
| 3.0 | 5.59 | 1.41 | 1.2 |
| 4.0 | 5.80 | 0.72 | 1.0 |
| 5.0 | 6.45 | 0.46 | 0.6 |
5.4 Verification of the Achieved Quality
The final optimized local squeeze-assisted HPDC process parameters are: casting pressure 100 MPa, local squeeze pressure 360 MPa, squeeze delay time 1.0 s, and squeeze duration 5.0 s. Under these conditions, the castings show a smooth surface and a sound internal structure. X-ray inspection confirms the maximum gas pore diameter is 0.46 mm, and the local porosity rate is reduced from 9.0% to 0.6%, meeting the ASTM E505 Grade 1 standard. The macroscopic cross-section reveals no visible gas pores. These results demonstrate that the local squeeze technology assisted HPDC process, combined with CAE simulation and Taguchi optimization, can effectively reduce sand foundry defects in complex lightweight aluminum alloy components. The integration of CAE analysis, advanced die design, process optimization, and local squeeze auxiliary forming establishes an effective method for producing high-quality engine brackets with significantly reduced gas porosity. This comprehensive approach significantly enhances production efficiency, reduces scrap rates, and improves the mechanical performance and reliability of automotive components.
6. Conclusions
In this study, the formation of gas porosity defects in an automotive engine bracket produced by high pressure die casting was systematically investigated. The main findings are as follows:
(1) Magmasoft simulation predicted the causes and locations of gas porosity defects, which were confirmed by actual production experiments. The high porosity in the casting was attributed to the complex mold cavity structure and significant high-low parting lines, which prevented the placement of exhaust at certain locations and hindered pressure transmission. This resulted in gas accumulation and the formation of sand foundry defects at the far-end bearing hole region.
(2) Through Taguchi optimization of process parameters (high-speed velocity 3.5 m/s, high-speed range 200 mm, intensification pressure 100 MPa), the gas porosity defect was improved. However, conventional process optimization alone was insufficient to reduce the porosity to an acceptable level, with the local porosity still being 4.2% after optimization.
(3) A local squeeze device was designed, and the technology was applied to compress the gas pores in the local problematic area. The optimized squeeze process parameters were: casting pressure 100 MPa, squeeze pressure 360 MPa, squeeze delay time 1.0 s, and squeeze duration 5.0 s. The local squeeze process resulted in a maximum gas pore diameter of 0.46 mm and a significantly reduced local porosity rate of 0.6%, meeting the ASTM E505 Grade 1 standard.
(4) The study demonstrated the effectiveness of combining local squeeze technology with HPDC and CAE analysis to reduce sand foundry defects in lightweight aluminum alloy components. The integration of local squeeze with HPDC, guided by CAE, was proven to be an efficient method for producing complex automotive components with high structural integrity and improved production efficiency.