After metal semi-solid forming technology was proposed by FLEMINGS M C and others, semi-solid forming has attracted widespread attention due to its characteristics of smooth mold filling, no turbulence, dense structure, small segregation, and near-final forming. In semi-solid squeeze casting, mold filling speed, gate conditions, etc. have a great impact on performance of casting. Numerical simulation has many advantages, such as shortening product design and trial production cycles.
Anycasting software was used to numerically simulate filling and solidification processes of A356 alloy semi-solid squeeze casting process. Effects of injection speed and pouring temperature on squeeze casting process of semi-solid A356 aluminum alloy were studied, process parameters were optimized, and squeeze casting mold was improved.
Graphical results
Pro/E software is used to establish a three-dimensional model of aluminum alloy castings and gating systems, as shown in Figure 1. Model consists of castings, overflow trough, and runner. Perform preprocessing and meshing in AnyPRE, see Figure 2. Casting material is A356 alloy, its chemical composition is shown in Table 1, and mold material is H13 steel.

 

Figure 1 Casting geometric model

 

Figure 2 Casting mesh model

Table 1 Chemical composition of A356 alloy (%)
During extrusion filling process, the higher liquid temperature, the better fluidity, which can reduce possibility of shrinkage cavities and shrinkage defects, thus improving performance of casting. When pouring temperature is 590, 600, and 610℃, mold preheating temperature is 200℃, and injection speed is 0.5m/s. Pouring temperature is analyzed through simulation results to select appropriate pouring temperature.
Figure 3 is a cloud chart of solidification time at different pouring temperatures. It can be seen that when pouring temperature is 590℃, solid phase rate of A356 alloy is relatively high and viscosity is large. Because pouring temperature is low, casting will solidify quickly. When surrounding casting solidifies, temperature of semi-solid material decreases and cannot conduct good heat conduction, which will cause some parts to solidify first and fail to achieve sequential solidification, resulting in shrinkage cavities and shrinkage porosity, as shown in Figure 3a. When pouring temperature is 600℃ and 610℃, no isolated solid phase or liquid phase region appears, and sequential solidification can be performed well. However, the higher pouring temperature, the greater shrinkage of liquid during solidification, probability of shrinkage cavities and shrinkage porosity will be greatly increased, and accuracy of casting will also be reduced. Therefore, reasonable pouring temperature is determined to be 600℃.

 

Figure 3 Cloud diagram of solidification time at different pouring temperatures

 

Figure 4 A356 alloy filling and solidification simulation at different injection speeds
Thickness of inner gate of mold and injection speed jointly determine filling state of semi-solid metal slurry. When injection speed is relatively high and inner gate is very thin, slurry will splash when passing through inner gate, causing air entrapment; when injection speed is moderate and inner gate is thick, it will be easier for slurry to form a laminar filling method when passing through inner gate. When semi-solid die casting, try to use thicker gates while avoiding shrinkage cavities, shrinkage porosity and pores.
During semi-solid die-casting, inner gate should be 2 to 2.5 times thicker than conventional die-casting inner gate of same size, but should not exceed wall thickness of casting. Thickness of inner gate of mold used in test is 2.5mm, and wall thickness of casting is 16mm. Therefore, filling and solidification process of semi-solid squeeze casting was analyzed when gate thickness in mold was 5 mm. From above simulation analysis, it can be concluded that casting performance is better when pouring temperature is 600℃ and injection speed is 0.5m/s. Simulation was carried out when pouring temperature was 600℃, injection speed was 0.5m/s, and inner gate thickness was 5mm.
When inner gate is thickened, splashing of slurry when passing through inner gate is greatly suppressed. Filling method is close to laminar flow, which allows gas in mold to be discharged sequentially to avoid air entrapment, and also reduces probability of shrinkage cavities, shrinkage porosity and pores.

 

Figure 5 Effect of mold optimization on filling and solidification of A356 alloy castings