The high-pressure and low-speed characteristics of squeeze casting technology refine and densify the alloy structure, while also strengthening the alloy through T6 heat treatment, making it an important technology for manufacturing high-performance aluminum alloy components. The influence of extrusion parameters on the microstructure and properties of A390 alloy was studied, and it was found that pressure and pouring temperature have a significant impact on the alloy microstructure. After extrusion casting, the microstructure was refined and the mechanical properties were significantly improved. The influence of extrusion casting parameters on the tensile strength and elongation of Al-9Si-3Cu alloy was studied, and it was found that the degree of influence of parameters on mechanical properties was ranked from high to low as follows: pressure, injection speed, mold temperature, and pouring temperature. This study used B390 alloy as the material to prepare castings through extrusion casting, optimized the process parameters of B390 aluminum castings, and provided reference for exploring the optimal process parameters for T6 heat treatment in the future.
The casting is formed on the SCV-400 vertical extrusion casting machine, and the mold temperature is controlled at 200 ℃ using a mold temperature machine with a holding time of 10 seconds. The influence of pressure, pouring temperature, and injection speed on the quality and performance of castings is mainly studied through the method of controlling variables.
The influence of pressure on the microstructure and properties of castings
The refinement of B390 structure by pressure is usually due to the fact that pressure makes the melt in closer contact with the mold, increases the contact area, improves the heat transfer coefficient, and thus increases the cooling rate of the melt, resulting in a more uniform structure Confidential; Pressure increases the melting point of molten metal, enhances undercooling during solidification, promotes alloy nucleation, and refines grain size. It is generally believed that when the temperature is above the solidification temperature, heat exchange is the main effect, supplemented by supercooling effect. When the temperature is below the solidification temperature, supercooling effect is the main effect, supplemented by heat exchange. The effect of pouring temperature of 740 ℃, injection speed of 0.05m/s, and different pressures on the porosity and mechanical properties of castings. As the pressure increases, the porosity of the casting decreases significantly. At a pressure of 150 MPa, the microstructure becomes denser and the porosity decreases to 0.46%. Pores usually appear in the form of shrinkage, porosity, and gas pores due to compression caused by pressure The pores in die-casting are mainly air pockets. It is generally believed that gas cannot precipitate or dissolve during the solidification process of molten metal, forming bubbles that wrap around inclusions and oxides to form pores. During the squeeze casting process, pressure can increase the solubility of gas and prevent the nucleation and growth of bubbles, thereby preventing the formation of pores.
The tensile strength and hardness of castings are related to pressure, and the mechanical properties of castings are optimal when the pressure is 150 MPa. The tensile strength and hardness (HB) are 232 MPa and 143.4, respectively. As the pressure increases, the heat transfer coefficient and undercooling of the casting increase, the nucleation rate is higher, the cooling rate is faster, and the alloy structure is refined. At the same time, the increase in pressure hinders the formation of pores in the casting, resulting in a denser structure. Therefore, the tensile strength and hardness are significantly improved.
The influence of pouring temperature on the microstructure and properties of castings
OM diagram, average grain size and shape factor of primary Si at different pouring temperatures for castings with a pressure of 150MPa, injection speed of 0.05m/s. It can be seen that as the pouring temperature increases, the dendritic crystals in the α – Al matrix gradually decrease, and the matrix structure mainly presents a rose like shape. The size and morphology of primary Si were significantly improved with the increase of pouring temperature: at a pouring temperature of 760 ℃, the average grain size of primary Si decreased to 18.8 μ m, and the shape factor increased to 0.57. This is because the metal liquid and The contact position of the mold forms a hard shell layer due to the chilling effect, resulting in pressure loss during the solidification process. The pouring temperature directly affects the thickness of the hard shell layer. When the pouring temperature is low, a larger layer thickness significantly reduces the effective pressure bearing capacity of the molten metal. As the pouring temperature increases, the fluidity of the metal liquid is improved, the pressure loss is reduced, and the grain size is refined.
The porosity and mechanical properties of castings under different pouring temperatures with a pressure of 150MPa, injection speed of 0.05m/s. As the pouring temperature increases, the porosity of the casting increases. When the pouring temperature is 720 ℃, the microstructure of the casting is the densest, and when the pouring temperature reaches 760 ℃, the porosity of the casting is the highest, which is 0.96%. The temperature of molten metal has a significant impact on the solubility of gases, and the solubility of gases is positively correlated with temperature. When the temperature of the molten metal is high, a large amount of H2 and O2 are sucked in. As the temperature decreases, the solubility of the gas significantly decreases, and the gas that cannot be precipitated converges to form bubbles, which form pores during solidification. Therefore, as the pouring temperature increases, the gas content in the casting increases, the porosity of the casting increases, and the density of the microstructure decreases accordingly.
As the pouring temperature increases, the tensile strength and hardness of the castings also increase. When the pouring temperature is 740 ℃, the tensile strength of the casting is 232MPa. When the pouring temperature is increased to 760 ℃, the tensile strength of the casting is 235MPa, which is not significantly improved compared to 740 ℃. As the pouring temperature increases from 720 ℃ to 760 ℃, the hardness of the casting slightly increases. The pouring temperature is too low, and the thick hard layer reduces the effective pressure bearing capacity of the casting, resulting in coarsening of the structure and easy occurrence of defects such as shrinkage and porosity. Excessive pouring temperature can easily generate a large amount of oxide inclusions, forming a large number of defects, leading to the scrapping of castings. Moreover, excessive pouring temperature can make it difficult for castings and molds to separate, resulting in defects such as cracks in molds. Therefore, after comprehensive consideration, it is believed that 740 ℃ is the optimal pouring temperature for castings.
Conclusion
(1) With the increase of pressure, the α – Al matrix and primary Si are significantly refined, the morphology is round, the porosity of the casting is significantly reduced, and the mechanical properties continue to improve. As the pouring temperature increases, the primary Si is significantly refined and the morphology is round, but the porosity of the casting increases, and the mechanical properties do not improve significantly after 740 ℃.
(2) As the injection speed increases, there is no significant change in the α – Al matrix and primary Si, and the porosity increases. The mechanical properties reach their optimum when the injection speed is 0.05m/s. Meanwhile, as the injection speed increases, the cold insulation on the surface of the casting becomes more severe.
(3) After comprehensive consideration, it is believed that the optimal extrusion process parameters for B390 castings are: pressure of 150MPa, pouring temperature of 740 ℃, and injection speed of 0.05m/s. At this time, the tensile strength of B390 castings is 232MPa and the hardness (HB) is 143.4.