In this paper, the numerical simulation of the solidification microstructure of a new type of non-heat-treated Al-Mg-Si alloy during squeeze casting was carried out. The results of macroscopic numerical simulation show that the solidification sequence of the casting with variable cross-section is the middle lower part, the middle upper part, the top part, and the bottom part. With the increase of pouring temperature or mold temperature, the shrinkage porosity volume fraction first decreases and then increases. When the pouring temperature is 700°C and the mold temperature is 475°C, the shrinkage porosity volume fraction is the smallest. With the increase of squeeze casting pressure, the shrinkage porosity volume fraction decreases, and the shrinkage porosity volume fraction is the smallest at 100MPa. The microstructure numerical simulation results show that the microstructure at the bottom and middle and upper parts of the casting is coarser, and the microstructure at the middle and lower parts and the top part is finer, which is related to the wall thickness and the degree of supercooling. The experimental results are in good agreement with the simulation results, which verifies the accuracy of the established model. This study provides a theoretical basis and technical support for optimizing the squeeze casting process and improving the quality of Al-Mg-Si alloy castings.
1. Introduction
Al-Mg-Si alloys have high strength and good corrosion resistance and are widely used in the automotive and aerospace industries. However, the traditional heat treatment process of Al-Mg-Si alloys is complex and energy-consuming, which restricts its further application. In recent years, non-heat-treated Al-Mg-Si alloys have attracted increasing attention due to their simple manufacturing process and low cost. Squeeze casting is a new type of casting process that combines the advantages of casting and forging. It can effectively improve the density and mechanical properties of castings and is suitable for the production of high-quality Al-Mg-Si alloy parts. Numerical simulation is an important means to study the solidification process of squeeze casting. By simulating the temperature field, flow field, and stress field during the solidification process, the distribution of shrinkage porosity and the microstructure of the casting can be predicted, and the process parameters can be optimized to improve the quality of the casting. In this paper, the numerical simulation and experimental study of the solidification microstructure of non-heat-treated Al-Mg-Si alloy by squeeze casting were carried out to provide a theoretical basis and technical support for the industrial application of this alloy.
2. Experimental Materials and Methods
2.1 Experimental Materials
The chemical composition of the Al-Mg-Si alloy used in this experiment is shown in Table 1. The raw materials used in the alloy smelting are pure aluminum, Al-10Mn, Si, Fe, Zn, Cu, Al-5Ti, Al-4Zr, and Al-2Sc. The magnesium block is wrapped in aluminum foil and added to the melt at the end of the smelting process.
| Element | Mg | Si | Mn | Fe | Ti | Zn | Cu | Zr | Sc | Al |
|---|---|---|---|---|---|---|---|---|---|---|
| Content (wt%) | 5.500 | 2.610 | 0.630 | 0.190 | 0.068 | 0.110 | – | – | – | Balance |
2.2 Experimental Methods
The alloy was melted in a graphite clay crucible in a resistance furnace at a smelting temperature of 750°C. After the pure aluminum was melted, the alloying elements were added in turn. After melting for 30 minutes, hexachloroethane was added for melt degassing. After the melt temperature reached the pouring temperature, it was held for 10 minutes and then poured into a preheated metal mold for squeeze casting. After the casting was completely cooled, a 10mm parallel section sample was taken from the bottom center for grinding, polishing, and etching with a 0.5% hydrofluoric acid (HF) aqueous solution. The microstructure of the alloy was observed using an optical microscope (Zeiss Axio Observer.Z1m), a scanning electron microscope (JSM-6301F), and an electron backscatter diffraction detector (EBSD) equipped with a Zeiss Merlin Compact field emission scanning electron microscope.
The shrinkage porosity in the squeeze casting was counted using the following method:
- The sample was meshed with a grid size of 5mm×5mm. A two-dimensional coordinate system was established, and the zero point of the coordinate system was set at the intersection of the outer wall extension line and the bottom extension line. The grid coordinates were the center points of each grid.
- The total shrinkage porosity area and its proportion in each grid were counted using ImageProPlus 6.0 software by taking pictures of all the shrinkage porosity in each grid in the BSE mode of the scanning electron microscope. The shrinkage porosity area fraction in each grid was calculated.
- The data set of the shrinkage porosity area fraction and distribution was created using Origin software. The data was converted into a matrix in a grid manner, and the contour cloud map of the shrinkage porosity area fraction and distribution in the casting was made.
3. Numerical Simulation Model
3.1 Macroscopic Numerical Simulation Model
- Physical and geometric model: The three-dimensional model of the casting and mold was established using software and imported into the ProCAST software. The element mesh size of the finite element model was set to 2mm (casting) and 10mm (mold), and the total number of volume meshes generated was 306,422.
- Material parameter calculation: Since there was no relevant data for this alloy in the ProCAST software material library, a new material was established, and its physical properties were calculated. The Scheil diffusion model was used for the squeeze casting simulation due to the fast cooling rate.
- Interface heat transfer coefficient setting: The interface heat transfer coefficient between the mold and the metal liquid was set to 2000W·(m²·K)⁻¹ during the filling stage. During the solidification stage, the heat transfer coefficient was set according to the applied pressure, and the relationship between the heat transfer coefficient and the pressure was h = 2000 + 98.4p (where h is the interface heat transfer coefficient in W·(m²·K)⁻¹ and p is the punch pressure in MPa).
- Pressure application: The pressure was applied in the form of the “mold-metal liquid” interface heat transfer coefficient, mainly considering the thermodynamic effect of the pressure parameter during the squeeze casting process.
- Process conditions: The punch speed was set to 10mm·s⁻¹, and the holding time was set to 30s. The effects of different pouring temperatures, mold temperatures, and casting pressures on the squeeze casting process were studied.
- Running parameters: The GATEFEED was set to ON for pressure holding and feeding, and the PENETRATE was set to ON to activate the interpenetrating grid algorithm.
3.2 Microscopic Structure Numerical Simulation Model
- CAFE mathematical model: The CAFE model was used for the numerical simulation of the solidification microstructure of squeeze casting, which was the coupling of the macroscopic temperature field and the microscopic solidification structure, combining the finite element method (FE) and the cellular automaton method (CA). The nucleation distribution in this model could be described by the Gaussian distribution:
where is the transient undercooling degree during nucleation in K, is the average nucleation undercooling degree in K, is the standard deviation of the nucleation undercooling degree in K, is the maximum nucleation density in m⁻³. - CA model establishment:
- Growth kinetic parameter setting: The chemical composition of the alloy in Table 1 was input into the ProCAST software to calculate the physical properties and then the CAFE parameters. The Gibbs-Thompson coefficient was , was 10, and (fit) was 10. The growth kinetic parameters was ² and was ².
- Nucleation parameter setting: The and were obtained through the experimental metallographic structure and the corresponding mathematical relationship (where , , and represent the number of grains per unit volume, per unit area, and per unit length, respectively). The other parameters were determined by referring to the experimental microstructure diagram. The nucleation parameters used in the simulation are shown in Table 2.
| Parameter | (m⁻²) | (m⁻³) | (K) | (K) | (K) | (K) |
|---|---|---|---|---|---|---|
| Value | 1.0 | 11.0 | 0.1 | 1.0 |
4. Results and Discussion
4.1 Filling Process Analysis
Figure 1 shows the simulation results of the squeeze casting filling process at a pouring temperature of 700°C, a mold temperature of 500°C, and a pressure of 100MPa. The solidus and liquidus temperatures of the alloy are 585.1°C and 622.7°C, respectively. The results show that the melt filling process is relatively stable under this process condition, and there are no defects such as air entrainment, indicating that the filling speed design is reasonable.
4.2 Solidification Process Analysis
Figure 2 shows the simulation results of the solid fraction change at different solidification times at a pouring temperature of 700°C, a mold temperature of 500°C, and a pressure of 100MPa. The results show that at the beginning of solidification (2.28s), the solid fraction in the middle lower part (thin-walled area) is higher, followed by the middle upper part, and the solid fractions at the bottom and top are the smallest, corresponding to different solidification sequences. At 4.31s, the solid fractions on the inner and outer surfaces of the middle lower part have reached 0.85, indicating that the first solidified part is the surface of the middle lower part, and the corner between the middle lower part and the middle upper part has solidified, while the core has not solidified, indicating that the thin-thick wall transition area is prone to cracking. At 8.15s, the middle lower part is completely solidified, and at this time, the solid fractions on the inner and outer surfaces of the middle upper part and the top have reached 0.9, indicating that the second solidified part is the entire sidewall surface. At 13.25s, the sidewall is completely solidified, and the bottom begins to solidify. At 15.06s, the top is completely solidified, and then the bottom is completely solidified. At 21.94s, the bottom is the last to solidify. As the top and bottom are the last to solidify, they have a greater tendency to form shrinkage porosity defects. Therefore, the solidification sequence of the variable cross-section casting is: middle lower part – middle upper part – top part – bottom part.
4.3 Effect of Different Process Parameters on Shrinkage Porosity Volume Fraction and Distribution
- Macroscopic model verification: Figure 3 shows the shrinkage porosity distribution cloud maps of the casting at a mold temperature of 475°C, a pouring temperature of 660°C, and a pressure of 100MPa. Figure 3(a) is the simulation result, and Figure 3(b) is the experimental result (contour cloud map). The results show that the positions of the shrinkage porosity in the simulation and experimental results are relatively consistent (at the top), indicating that the established squeeze casting macroscopic finite element model is reasonable.
- Effect of pouring temperature on shrinkage porosity volume fraction and distribution: Figure 4 shows the simulation results of the shrinkage porosity volume fraction and distribution at different pouring temperatures at a mold temperature of 475°C and a pressure of 100MPa, and Figure 5 shows the shrinkage porosity volume fraction at different pouring temperatures. The results show that the shrinkage porosity is mainly distributed at the bottom and top of the casting. With the increase of the pouring temperature, the shrinkage porosity volume fraction first decreases and then increases, and the shrinkage porosity volume fraction is the smallest at a pouring temperature of 700°C. The formation of shrinkage porosity is affected by two main factors: the feeding ability of the melt and the density difference between the liquid and solid phases. When the pouring temperature is 660°C, due to the relatively high viscosity of the alloy melt, the feeding ability of the melt between the dendrites is relatively weak, so the shrinkage porosity volume fraction of the casting is relatively large. When the pouring temperature is 740°C, although the melt viscosity decreases, the melt density increases significantly with the increase of temperature, and the density difference between the two phases is more significant, and the tendency to form shrinkage porosity is obvious, so the shrinkage porosity volume fraction in the casting is relatively large. When the pouring temperature is 700°C, it not only retains a good melt feeding ability but also the density difference between the solid and liquid phases is not too large, so the shrinkage porosity volume fraction is the smallest.
- Effect of mold temperature on shrinkage porosity volume fraction and distribution: Figure 6 shows the simulation results of the shrinkage porosity volume fraction and distribution at different mold temperatures at a pouring temperature of 700°C and a pressure of 100MPa, and Figure 7 shows the shrinkage porosity volume fraction at different mold temperatures. The results show that with the increase of the mold temperature, the shrinkage porosity volume fraction first decreases and then increases, and the shrinkage porosity volume fraction is the smallest at a mold temperature of 475°C. For the squeeze casting process, the influence of the mold temperature on the shrinkage porosity mainly comes from the change of the solidification speed. When the mold temperature is relatively low, the solidification speed is relatively fast, and the melt poured into the mold quickly forms a solid shell, and the effect of the pressure is not fully reflected. At a mold temperature of 450°C, the shrinkage porosity volume fraction in the casting is relatively large. With the increase of the mold temperature, the solidification time is longer under the action of the pressure, and the positive effect of the pressure on the feeding is better reflected. At a mold temperature of 475°C, the shrinkage porosity volume fraction decreases. When the temperature is too high, the volume fraction of the entrapped gas also increases, which aggravates the formation of shrinkage porosity. At a mold temperature of 500°C, the shrinkage porosity volume fraction increases. It should be noted that due to the small size of the casting, a relatively high mold temperature is used to achieve the squeeze casting of a small amount of melt. In actual squeeze casting production, with the increase of the casting size, the mold temperature can be appropriately reduced, but the internal mechanism and overall trend of the influence of the mold temperature on the shrinkage porosity are consistent.
- Effect of pressure on shrinkage porosity volume fraction and distribution: Figure 8 shows the simulation results of the shrinkage porosity volume fraction and distribution at different pressures at a pouring temperature of 720°C and a mold temperature of 500°C, and Figure 9 shows the change curve of the shrinkage porosity volume fraction at different pressures. The results show that with the increase of the pressure, the shrinkage porosity volume fraction decreases, and the shrinkage porosity volume fraction is the smallest at a pressure of 100MPa. During the squeeze casting process, the pressure action improves the feeding of the melt on the one hand and promotes the healing of the shrinkage porosity by plastic deformation in the local area on the other hand, thus significantly reducing the shrinkage porosity. In addition, the pressure action in squeeze casting improves the heat transfer between the melt and the mold wall, which is beneficial to refining the solidification structure and also reduces the tendency to form shrinkage porosity.
4.4 Grain Structure Simulation Results
- CA model verification: Figure 10 shows the simulation results (grain structure) and experimental results (metallographic structure) of the squeeze casting cup-shaped casting at a pouring temperature of 700°C, a mold temperature of 450°C, and a pressure of 100MPa. Point 1 in the figure is the top, point 2 is the middle upper part, point 3 is the middle lower part, and point 4 is the bottom. It can be seen from Figure 10(a) that the microstructure at the bottom and middle upper part of the casting is relatively coarse, and the microstructure at the middle lower part and the top is relatively fine. Observing Figure 10(b), it can be seen that the overall microstructure distribution of the casting in the experimental results is relatively consistent with that in the simulation results, indicating that the established squeeze casting grain growth CA model is reasonable. The formation of the grain structure includes nucleation and growth, which are mainly affected by the melt undercooling degree and cooling rate in the process. For squeeze casting, the grain structure at the bottom and middle upper part of the casting is coarse because of its relatively large wall thickness, slow melt cooling rate, and
