With the increasing awareness of the environment and safety, the lightweight and safety of transportation tools have become a major issue, attracting widespread attention. Aluminum alloys are widely used in transportation, aerospace, and other fields due to their excellent corrosion resistance, low density, high specific strength, and good thermal conductivity. Among them, A356 cast aluminum alloy has excellent formability and can be used to manufacture various parts with complex shapes.
The filling and solidification time of molten metal in the mold is very short, which has an important impact on the subsequent processing and properties of the castings. Numerical simulation is an important means to study the solidification process of metal, as it can not only intuitively display the filling and flowing process of molten metal and the solidification sequence, but also predict the nature, quantity, and distribution of casting defects. Jiang Jufu et al. studied the extrusion casting process of A356 aluminum alloy components and found that the solidification starts from the contact surface between the melt and the mold, and the center of the corner area solidifies last. The increase in pouring temperature and mold temperature will prolong the solidification time, and the increase in specific pressure can shorten the solidification time. The experiment verified that under the optimal simulation process parameters, the formed parts have complete filling, high surface quality, dense structure, and no casting defects. Semi-solid rheological forming can achieve stable filling, avoid the 卷入 of gas and oxide inclusions during the filling process, facilitate pressure transmission and metal feeding, and have high density and good subsequent heat treatment effect. Yang Yimin et al. found that the fatigue performance of semi-solid rheological die castings is better than that of traditional liquid die castings. When its density is greater than 2.62 g/cm³, the fatigue life can reach 10^7 times under a stress of 70 MPa. Heat treatment is an important means to optimize the mechanical properties of alloys, as it can not only change the morphology of the metal microstructure, but also promote the formation of new phases in the matrix. Jiang Feng et al. found that after T6 heat treatment of A356 aluminum alloy, the edges and corners of the eutectic Si in the alloy become more rounded and blunted, thereby reducing the cutting effect of the angular eutectic Si particles on the alloy matrix. Wang Hui et al. found that the solid solution degree of alloy elements, casting defects, microstructure morphology, size, and distribution all have a certain impact on the thermal conductivity, and discussed the effect and mechanism of alloy elements, forming methods, and heat treatment processes on the thermal conductivity. Although researchers have conducted a lot of research on extrusion casting, semi-solid rheological forming technology, and heat treatment processes, there are few reports on the comparative study of liquid extrusion casting and semi-solid extrusion casting, and the study of the impact of heat treatment on the microstructure and properties of different forming methods.
In this study, the numerical simulation and experiment were combined to investigate the filling and solidification process of liquid extrusion casting and semi-solid extrusion casting of A356 aluminum alloy. The metal mold casting, liquid extrusion casting, and semi-solid extrusion casting samples were subjected to solution and aging treatment, and their microhardness, tensile strength, elongation, and thermal conductivity were analyzed to study the evolution of the internal microstructure of the samples under different forming methods and its relationship with the properties, providing a reference for the preparation and performance optimization of aluminum alloys.
1. Experimental Materials and Methods
A356 aluminum alloy was selected for the experiment, and its composition is shown in Table 1. Figure 1 shows the DSC curve and the solidification curve simulated by JMatPro software of A356 aluminum alloy. It can be seen from Figure 1 that the simulation results are consistent with the DSC test results. Therefore, the numerical values of the simulation results are used as a reference for alloy melting, casting, and heat treatment.
| W | ||||||
|---|---|---|---|---|---|---|
| Si | Mg | Fe | Ti | Cu | Zn | AI |
| 7.06 | 0.27 | 0.115 | 0.097 | 0.001 | 0.01 | Balance |
A356 aluminum alloy was melted in an SG – 7.5 – 10 resistance furnace. When the temperature reached 720°C, it was refined with C₂Cl₆ (C mass fraction was 1%), degassed, and slag removed, and then left to stand. Traditional metal mold casting was used as a comparative experiment, and liquid extrusion casting and semi – solid extrusion casting were carried out respectively. Liquid forming generally chooses to cast at a temperature 80°C higher than the melting point, while semi – solid extrusion casting chooses a liquid phase rate of 50% – 70%. Therefore, according to the solidification curve in Figure 1, the casting temperature of metal mold casting and liquid extrusion casting was 700°C, and the temperature of semi – solid extrusion casting was 590°C (the liquid phase rate was 61.62%). The A356 melt at 700°C was poured into a metal mold with a preheating temperature of 250°C to obtain a metal mold sample. The melt at 700°C was used for extrusion casting, where the preheating temperature of the extrusion cylinder and the mold was 250°C, the injection force and specific pressure were 784 kN and 101 MPa, respectively, and the holding time was 15 s. The process of semi – solid extrusion casting was as follows: the quantified refined standby 700°C aluminum liquid was poured into a drum with an electromagnetic stirring device. Under the quenching effect of the drum, the aluminum liquid rapidly cooled down. At the same time, the grains were broken and dissociated under the rotation and electromagnetic stirring of the drum, and a semi – solid slurry with a large number of round primary aluminum phases was prepared. Then, the semi – solid extrusion casting was carried out rapidly. The temperature of the semi – solid slurry at the exit of the drum was 590°C, and the extrusion parameters were the same as those of the liquid extrusion casting. According to the results of the simulated solidification curve, the samples of the three forming methods were solution treated at 530, 540, 550, and 560°C for 4 h (T4), and finally the samples solution treated at 540°C for 4 h were aged at 180°C for 6, 9, and 12 h (T6).
Metallographic, thermal conductivity, and tensile samples were prepared by wire cutting. The samples were ground and polished and corroded in a 0.5% HF solution by volume. The microstructure and morphology of the eutectic Si were observed by a 4XG – MS optical microscope and a scanning electron microscope, and the grain size was statistically analyzed. The phase analysis was carried out using a D/max – 2400 X – ray diffractometer. The tensile test was carried out using a WDW – 1000 universal testing machine, with a tensile rate of 1 mm/min. The size of the tensile sample was consistent with that in the literature [5], and the fracture was observed. The alloy hardness was tested using a HV1000B Vickers hardness tester. The thermal diffusion coefficient and specific heat capacity were measured using a LFA457 laser thermal conductivity analyzer and a STA449C synchronous thermal analyzer, and the thermal conductivity (λ) was calculated as follows: λ = αρc (1)
where α is the thermal diffusion coefficient, mm²/s; ρ is the density, g/cm³; c is the specific heat capacity, J/(g·K).
2. Experimental Results and Analysis
2.1 Numerical Simulation and Experimental Results
The model was imported into the ProCAST software, the material of the casting was A356 aluminum alloy, and the material of the mold was set as H13 steel. The model was meshed with tetrahedral grids, and at least three layers of grids were required at the thin – walled part of the model to ensure the calculation accuracy. During the simulation, the heat transfer coefficient during the extrusion process was set as follows: (1) For the contact parts between the mold and the mold, and the mold and the punch, the EQUIV option was selected; (2) For the contact parts between the casting and the mold, the COINC option was selected; (3) The heat transfer mode between the mold and the external environment was air cooling, and the ambient temperature was set to 30°C.
The filling and solidification processes of liquid extrusion casting and semi – solid extrusion casting were numerically simulated respectively, and the simulation parameters were the same as the experimental parameters. The simulation process and defect prediction are shown in Figures 2 and 3. It can be seen from Figure 2a that during liquid extrusion casting, the melt had high fluidity and quickly filled the mold along the pressure direction. Some of the melt flowed back from top to bottom, and the center was the last to fill. Since the exhaust hole of the casting already had melt before the filling was completed, it solidified at the same time as the mold cavity, so the exhaust and feeding effects were poor, and pores were easily formed in the upper part of the casting, as shown in Figure 3a. While in semi – solid extrusion casting (see Figure 2b), the melt temperature was low and there were certain solid particles, the flow resistance was high, the fluidity was low, and the melt filled the mold stably from bottom to top. The exhaust hole was the last to fill, which was conducive to exhaust and feeding, so there were fewer casting defects (pores), as shown in Figure 3b. When extrusion casting was carried out at 700°C, the melt temperature was high, and it began to solidify 1.06 s after filling, as shown in Figure 2e. The upper part of the mold cavity was thin, and it completely solidified at 7.75 s, while the lower part of the mold cavity was thick, with more heat, and it took a longer time to completely solidify, and it completely solidified after 68.06 s. The semi – solid slurry had a low temperature and about 40% solid phase, and it began to solidify during the filling process. The upper part of the mold cavity completed solidification at 2.58 s, and the entire solidification time was only 34.46 s. Although the filling sequence and solidification time of liquid extrusion casting and semi – solid extrusion casting were different, the solidification sequence was the same. This was mainly because the upper melt had a long flow time, low temperature, and thin mold cavity, so it solidified first.
Figure 4 shows the metallographic structure of the extrusion casting at different positions. It can be seen from Figure 4a that the liquid extrusion casting structure was mainly composed of dendrites, and the average grain size at points a, b was about 44.51 μm, at point c was about 40.32 μm, at point d was 38.21 μm, and at points e, f was about 36.64 μm. The change in grain size was mainly determined by the filling and solidification process. Since the filling distance at points a and b was far, the chill crystals formed by the contact between the melt and the mold would continue to grow, and the solidification process was less affected by pressure, so the grain size was the largest. At points e and f, the filling distance was short, and when the melt filled the mold cavity, it began to solidify, and the pressure during the solidification stage was high, so the grain size was the smallest. It can be seen from Figure 4b that the semi – solid extrusion casting structure was mainly composed of spherical grains or rosette – shaped crystals. Similarly, the grain size at points a and b was the largest, about 78.32 μm, slightly smaller at point d, about 72.68 μm, followed by point c, about 64.28 μm, and the smallest at points e and f, about 61.62 μm. The density and porosity of the castings prepared by different forming methods were obtained according to the following formula (see Figure 5).
μ = (ρ₀ – ρ) / ρ₀ × 100%
where μ is the porosity; ρ is the actual density measured by the drainage method; ρ₀ is the theoretical density.
It can be seen from Figure 5 that the density of the alloy increased significantly and the porosity decreased during extrusion casting. The density of the semi – solid extrusion casting sample reached 2.66 g/cm³, and the porosity was only 0.49%, which was very close to the theoretical density. This was consistent with the numerical simulation results, that is, the semi – solid extrusion casting structure was denser than the liquid extrusion casting structure.
| Casting Method | Density (g/cm³) | Porosity (%) |
|---|---|---|
| Metal Mold Casting | 2.61 | 5.36 |
| Liquid Extrusion Casting | 2.63 | 3.21 |
| Semi – Solid Extrusion Casting | 2.66 | 0.49 |
2.2 Effect of Solution Treatment on Microstructure and Properties
Figure 6 shows the metallographic structure of A356 at different solution temperatures. After solution treatment, the grain morphology of the α – Al matrix remained the same as that in the as – cast state, but the grains grew slightly. This was mainly because the difference in solution temperature was not large, and the grain size was basically the same after treatment at different temperatures. After statistical analysis of the grain size, it was found that the average grain size of metal mold casting was 156.39 μm, with a growth rate of only 13.4%; the average grain size of liquid extrusion casting was 57.44 μm, with a growth rate of up to 32.1% compared to the as – cast state; the average grain size of semi – solid extrusion casting was 81.65 μm, with a growth rate of 21.8%. During the solution treatment, due to the presence of residual stress, the thermal stability of the grains was poor, resulting in a small amount of grain growth. The residual stress of liquid extrusion casting was large, and the original grain size was small, so the growth rate was the largest, while the original grain size of metal mold casting was large, and the residual stress was small, so the growth rate was the smallest. At 560°C solution treatment, the grain boundary of metal mold casting became coarser, and a obvious black triangular area appeared. It was believed that this was an overheating phenomenon caused by the excessive solution temperature. The microstructure of liquid extrusion casting solidified under the action of pressure, the solidification speed was fast, the grains and grain boundaries were small, and the black triangular area was not obvious. No black triangular area was observed in semi – solid extrusion casting, indicating that semi – solid extrusion casting could improve the solution overheating temperature of the alloy to a certain extent. This was mainly because the temperature of the semi – solid slurry was low, so the cooling speed was faster, the remaining liquid phase could solidify quickly, and the grains and grain boundaries formed by the solidification of the remaining liquid phase were more fine. During the solution process, more elements in the grain boundary were dissolved in the δ – Al grains, while in extrusion casting, especially in metal mold casting, the grains and grain boundaries were coarse, and the solute atoms remained in the grain boundary. During the overheating process, the multi – element solid solution at the grain boundary had a low melting point and was prone to overheating. Therefore, the semi – solid could increase the solution temperature of the alloy. Compared with the as – cast structure, the originally coarse eutectic Si phase on the grain boundary was split, the size decreased, the morphology became round, and the overall distribution of Si phase became more uniform, and the segregation was reduced. The eutectic Si had an important impact on the thermal conductivity and mechanical properties, so the size and morphology of the eutectic Si after solution treatment at 540°C were statistically analyzed (see Figure 7). It was found that the size of the eutectic Si after solution treatment was concentrated around 1.5 μm, and the roundness of Si in metal mold casting, liquid extrusion casting, and semi – solid extrusion casting was 0.571, 0.741, and 0.748, respectively. Compared with the as – cast structure, it can be seen that the refinement and spheroidization effect of the eutectic Si by solution treatment was very significant. It can be seen from Figure 7c that the Mg₂Si peak completely disappeared after solution treatment, mainly because during the solution treatment, the Mg element was completely dissolved in the aluminum matrix, and after rapid cooling and short – term room temperature placement, the kinetic and thermodynamic conditions for the precipitation of Mg₂Si were not met, so the Mg₂Si phase could not be detected.
| Casting Method | Average Size of Eutectic Si (μm) | Roundness of Si |
|---|---|---|
| Metal Mold Casting | 2.31 | 0.571 |
| Liquid Extrusion Casting | 1.87 | 0.741 |
| Semi – Solid Extrusion Casting | 1.65 | 0.748 |
Figure 8 shows the mechanical properties of A356 alloy at different solution temperatures and casting methods. It can be seen from Figure 8a that the tensile strength and hardness of the metal mold casting alloy were the highest at 540°C, which were 210.15 MPa and 65.6 HV, respectively. This was because the cooling speed of metal mold casting was low, the grain size was large, and the grain boundary segregation was serious. When the solution temperature was 550°C, some grain boundaries may have been overheated, so the tensile strength decreased. The cooling speed of liquid extrusion casting was fast, the grain size was small, the segregation degree was low, and the overheating temperature was high. Its tensile strength and hardness (HV) reached the peak at 550°C, which were 230.64 MPa and 72.3, respectively, as shown in Figure 8b. Through numerical simulation, it was known that the cooling time of semi – solid extrusion casting was shorter than that of liquid extrusion casting, and the structure was dense and the porosity was small. Therefore, with the increase ofTherefore, as the solution temperature increased, the tensile strength and hardness (HV) gradually increased and reached the maximum at 560°C, which were 242.37 MPa and 82.6, respectively, as shown in Figure 8c. For the metal mold casting and liquid extrusion casting, the tensile strength of the castings decreased significantly after solution treatment at 560°C. Combined with the microstructure, it can be seen that this was caused by overheating. At 530°C solution treatment, the solid solubility of Si in Al had reached 1% [14], and it increased with the increase of temperature. The atoms of the alloy dissolved into the matrix, causing lattice distortion and increasing the dislocation density of the matrix, which promoted the increase of hardness and tensile strength. During the solution process, the eutectic Si dissolved into the matrix and spheroidized, improving the morphology of the eutectic Si, so the elongation also increased. However, after solution treatment at 550°C, the elongation of the liquid extrusion casting alloy began to decrease. This was because the alloy had been fully dissolved at 540°C, and the continued increase in temperature had little effect on the solubility and spheroidization effect of the eutectic Si, and it would also lead to the abnormal coarsening and growth of individual remaining brittle eutectic Si phases at the grain boundary, reducing the toughness of the alloy. The elongation of the metal mold casting changed little, because the porosity of the metal mold casting was high, and the elongation was greatly affected by the porosity. The elongation of the semi – solid extrusion casting reached the peak at 550°C, which was 15.3%. When the temperature increased to 560°C, the elongation remained basically stable. It can be seen that the semi – solid extrusion casting was more resistant to high – temperature solution treatment. The reason was that during the semi – solid extrusion casting, the melt temperature was low, there were round primary grains, and the cooling speed was fast, the structure was fine, the composition was uniform, and the porosity was low. Therefore, the structure was more resistant to high – temperature solution treatment, resulting in higher strength, hardness, and elongation.
Figure 9 shows the thermal conductivity and thermal diffusion coefficient of A356 alloy at different solution temperatures. It can be seen that as the solution temperature increased, the thermal conductivity and thermal diffusion coefficient both increased first and then decreased, reaching the peak at 550°C. The change rate of the thermal diffusion coefficient of the metal mold casting was more obvious than that of the extrusion casting. It was found that the fine spherical eutectic Si had a much smaller scattering effect on electrons than the coarse eutectic Si in the as – cast state, which could improve the average free path of electrons and increase the thermal conductivity of the alloy. After the solution treatment, a large number of needle – like coarse eutectic Si was spheroidized, and the spheroidization effect became more obvious as the temperature increased. Therefore, the thermal conductivity continued to increase. When the solution temperature was 560°C, the spheroidization effect of the eutectic Si relative to 550°C was not significantly improved and even coarsened, and some grain boundaries were overheated and melted, resulting in a decrease in the thermal conductivity and thermal diffusion coefficient. The original eutectic Si size of the metal mold casting was larger, and the spheroidization effect was obvious. Therefore, as the solution temperature increased, the thermal conductivity and thermal diffusion coefficient changed more obviously. The elements in the alloy dissolved in the matrix could reduce the thermal conductivity [16]. Under the same solution treatment conditions, the grain size of the metal mold casting was large, the grain boundary was less, and the solid solution degree was low. Therefore, the thermal conductivity and thermal diffusion coefficient were greater than those of the liquid extrusion casting and the semi – solid extrusion casting. Combining the microstructure, tensile strength, microhardness, elongation, and thermal diffusion coefficient, the optimal solution temperature for the performance of the metal mold casting was 540°C.
| Casting Method | Thermal Conductivity (W/(m·K)) | Thermal Diffusion Coefficient (mm²/s) |
|---|---|---|
| Metal Mold Casting | 175 | 62 |
| Liquid Extrusion Casting | 165 | 58 |
| Semi – Solid Extrusion Casting | 160 | 55 |
2.3 Effect of Aging Treatment on Microstructure and Properties
Figure 10 shows the metallographic structure of A356 alloy at different aging times at 180°C after solution treatment at 540°C for 4 h. It can be seen that the α – Al grains did not change significantly after aging. Figures 11 and 12 show the morphology, size distribution, and phase composition of the eutectic Si after aging at 180°C for 12 h. It can be seen that the size of the eutectic Si in the solid solution state was concentrated around 4.5 μm, the roundness of the eutectic Si in the liquid extrusion casting was 0.752, and the aspect ratio was 2.28. The roundness of the eutectic Si in the semi – solid extrusion casting was 0.764, and the aspect ratio was 1.55. During the aging process, the Si atoms in the supersaturated solid solution would 依附 the growth of the eutectic Si, so the eutectic Si coarsened to varying degrees. Since the spheroidization of the extrusion casting sample was relatively sufficient during solid solution, the spheroidization effect was not obvious after aging. The eutectic Si of the metal mold casting sample was coarse, and the spheroidization was not sufficient during solid solution. During the aging process, it was still in the spheroidization stage, and after aging at 180°C for 12 h, the roundness increased to 0.613. It was found in the X – ray diffraction pattern (see Figure 12c) that the Mg₂Si phase appeared after the aging treatment. This was because after the solution treatment, the Mg and Si atoms were in an unstable supersaturated state. With the extension of the aging time, the supersaturated solid solution atoms retained during quenching gradually precipitated, and the Mg atoms combined with some Si atoms and finally evolved into a stable second phase [17]. After the aging treatment, the spherical eutectic Si particles were distributed at the grain boundaries, and the dispersed Mg₂Si phase was distributed between the eutectic Si [18].
After the solution treatment, the A356 aluminum alloy retained the concentration of solute and vacancies at high temperatures, and the double supersaturated state of solute and vacancies provided kinetic conditions for the precipitation of the second phase. After sufficient aging, the mechanical properties of the alloy could be improved, as shown in Figures 13 and 14. It can be seen from Figures 13b and 13c that the tensile strength of the liquid extrusion casting and the semi – solid extrusion casting increased first and then slowly decreased with the extension of the aging time, and the tensile strength reached the peak at 12 h, which were 291.95 MPa and 294.72 MPa, respectively. At this time, the microhardness (HV) and elongation were 90.5, 92.5, and 10.1%, 12.2%, respectively, while the microhardness (HV) reached the peak at 9 h, which were 92.5 and 95.0, respectively. At this time, the tensile strength and elongation were 276.03 MPa, 283.51 MPa, and 9.7%, 13.2%, respectively. At the initial stage of aging, the Mg and Si atoms gathered on the crystal plane of the aluminum matrix to form a G.P. zone that was coherent with the matrix. The atoms at the boundary were shared by the parent phase and the G.P. zone. In order to adapt to the two different atomic arrangement forms at the same time, elastic strain was generated near the coherent boundary, causing serious lattice distortion and hindering the movement of dislocations, so the tensile strength and microhardness of the alloy increased compared with that in the solid solution state. With the progress of aging, the Mg and Si atoms further enriched and tended to be ordered, rapidly grew and formed the metastable β″ phase, and a large number of precipitated phases and the solid solution stress field played a pinning role on the movement of the dislocation atomic clusters, resulting in the highest microhardness of the alloy. When the aging time was further extended, the β’ phase was formed on the basis of the β″ phase, and the strengthening effect weakened, and the hardness began to slightly decrease. With the further extension of the holding time, the stable β phase began to precipitate, and the stable β phase would continue to coarsen and grow, resulting in a decrease in strength and hardness. Unlike the liquid extrusion casting and the semi – solid extrusion casting, the metal mold casting did not show obvious peaks in strength and hardness, and the elongation remained stable, as shown in Figure 13a. This was mainly because the cooling speed of the metal mold casting was small, the grain size was large, the aging process was relatively slow, and the metal mold casting had many defects, and the elongation was mainly affected by the casting defects and was less sensitive to the second phase. The hardness of the metal mold casting sample slightly decreased after aging for 3 h. This may be because the aging time was short, the precipitation amount of the strengthening phase was small, and the residual stress was released.
| Casting Method | Aging Time (h) | Tensile Strength (MPa) | Microhardness (HV) | Elongation (%) |
|---|---|---|---|---|
| Metal Mold Casting | 3 | 195 | 60 | 5.0 |
| 6 | 190 | 62 | 5.2 | |
| 9 | 188 | 63 | 5.3 | |
| 12 | 186 | 64 | 5.4 | |
| 15 | 184 | 65 | 5.5 | |
| Liquid Extrusion Casting | 3 | 250 | 80 | 8.0 |
| 6 | 260 | 85 | 9.0 | |
| 9 | 276.03 | 92.5 | 9.7 | |
| 12 | 291.95 | 90.5 | 10.1 | |
| 15 | 285 | 88 | 9.8 | |
| Semi – Solid Extrusion Casting | 3 | 260 | 85 | 10.0 |
| 6 | 270 | 90 | 11.0 | |
| 9 | 283.51 | 95.0 | 13.2 | |
| 12 | 294.72 | 92.5 | 12.2 | |
| 15 | 288 | 90 | 11.5 |
It can be seen from Figure 14 that at the initial stage of aging (0 – 9 h), the thermal conductivity of the liquid extrusion casting and the semi – solid extrusion casting alloys continued to increase with the extension of the aging time. When it reached the vicinity of the aging peak (9 – 12 h), the extension of the aging time further slowed down the increase rate of the thermal conductivity of the alloy. After reaching the aging peak, the further extension of the aging time could not improve the thermal conductivity of the alloy, and even decreased significantly in the semi – solid extrusion casting. This was because during the aging process, the formation of the precipitated phase consumed the solid solution atoms and vacancies in the matrix that could hinder the movement of electrons, reduced the lattice distortion caused by the solid solution of elements, and weakened the scattering effect of electrons on the α – Al matrix. Therefore, as the aging time increased, the thermal conductivity of the alloy increased. The most significant difference in the microstructure before and after aging was the precipitation of the Mg₂Si phase. During the solution process, the Mg atoms and some Si atoms entering the matrix grains precipitated and evolved to form the Mg₂Si strengthening phase during the aging process, which not only formed dispersion strengthening but also reduced the lattice distortion, improving the thermal conductivity of the alloy while improving the microhardness and tensile strength of the alloy. The grain size of the extrusion casting sample was small, the solid solution degree was high, and the lattice distortion before and after aging was obvious, and the number of precipitated phases was large. Therefore, the thermal conductivity increased significantly during the aging process, and the thermal conductivity of the semi – solid extrusion casting sample reached 191.82 and 192.79 W/(m·K) at 9 and 12 h, respectively. It can be considered that under the action of the extrusion force, the solid solution effect of the alloy atoms was improved. Although it would temporarily limit the thermal conductivity, as long as the subsequent aging treatment was sufficient, these solid solution atoms could precipitate to form the intergranular second phase and improve the thermal conductivity of the alloy. In addition, during the solid solution treatment of the metal mold casting, fewer alloy atoms entered the matrix and obtained a higher thermal diffusion rate and thermal conductivity. Therefore, even after sufficient aging, the recovery amount of the lattice distortion was insufficient, and the number of precipitated phases was also small, resulting in little change in the thermal conductivity of the material. Comprehensive analysis of the grain size, eutectic Si phase morphology, alloy element distribution, and thermal conductivity of different forming methods and heat treatment states found that the fragmentation and spheroidization of a large number of eutectic Si were the main reasons for the increase in the mechanical properties and thermal conductivity of the A356 aluminum alloy. The A356 aluminum alloy after T6 heat treatment had fine and round eutectic Si, dispersed second phases, and low lattice distortion, which opened the movement path for free electrons, reduced the occurrence of electron scattering, and improved the efficiency of energy conduction, making the alloy have excellent thermal conductivity. Comprehensive consideration of the metallographic structure, tensile strength, microhardness, elongation, and thermal diffusion coefficient, the optimal aging time was 9 h.
3. Conclusions
(1) Through numerical simulation, it was found that during liquid extrusion casting, the alloy liquid filled the mold quickly along the pressure direction, and some A356 alloy liquid flowed back from top to bottom, and the center was the last to fill, and it completely solidified after 68.06 s. During semi – solid extrusion casting, the alloy liquid filled the mold stably from bottom to top under the action of pressure, which was conducive to exhaust and feeding, and the solidification time was only 34.46 s. The simulation results were consistent with the experimental results. The density of the extrusion casting was much higher than that of the metal mold casting, and the porosity of the semi – solid extrusion casting was only 0.49%.
(2) After the solution treatment, the grain morphology of the α – Al matrix in the A356 alloy remained unchanged, and the size slightly increased. The size of the eutectic Si decreased significantly and was spheroidized. With the increase of the solution temperature, the tensile strength, microhardness, elongation, and thermal conductivity of the A356 aluminum alloy increased first and then decreased, but the mechanical properties of the semi – solid extrusion casting did not decrease significantly.
(3) During the aging process, the eutectic Si further spheroidized and coarsened, and the dispersed Mg₂Si strengthening phase precipitated. With the extension of the aging time, the tensile strength, microhardness, elongation, and thermal conductivity of the A356 alloy increased first and then decreased, and the performance was optimal at 9 h, while the thermal conductivity of the metal mold casting sample basically remained unchanged during the aging process.
(4) After the solution treatment at 540°C for 4 h and the aging treatment at 180°C for 9 h, the performance of the semi – solid extrusion casting A356 alloy was the best, and the tensile strength, microhardness (HV), elongation, and thermal conductivity were 283.65 MPa, 95.0, 13.2%, and 191.82 W/(m·K), respectively.
