What is Semi-Solid Die Casting?

Semi-solid die-casting technology refers to the process of solidification of aluminium alloy from liquid to the solid state; when the temperature drops between the liquid and solid phase lines (solid-liquid two-phase zone), the microstructure of the aluminium alloy is made to be a liquid phase surrounded by a spherical solid phase by applying certain external influence. In this temperature range, the aluminium alloy can have a fixed shape like a solid but also can flow easily after being squeezed by a slight external force.

Principle of semi-solid die-casting

 Semi-solid metal forming technology breaks the traditional dendritic solidification mode. It has unique advantages in terms of composition and tissue uniformity, improving the formed parts’ comprehensive mechanical properties and reducing the formed parts’ internal defects. The processing temperature is lower than that of the liquid state. The solidification shrinkage is smaller, thus less (or no) casting looseness, and the filling is smooth, thus less (or no) porosity defects and oxide inclusions. The solution + ageing (T6) heat treatment can be carried out, and the casting quality of traditional casting methods can be improved. The lower forming temperature of semi-solid makes the mold less thermal shock and prolongs the mold life. Relative to forging and other heat deformation forming processes, semi-solid forming deformation resistance is small, so a large amount of deformation can be processed at one time to form complex shapes and high precision and performance requirements of the parts to overcome the traditional forging method in the high cost, can not form a copy of the shortcomings, with net forming, high quality, high performance, low energy consumption, low cost and many other unique advantages, by domestic and foreign research scholars as the 21st century the most promising Metal material processing technology is one of the most promising in the 21st century.

Advantages of the semi-solid die-casting process

1) Fine grain structure can be obtained without adding any grain refining agent, which eliminates the columnar crystal and coarse dendrite in traditional casting.

2) Low forming temperature (such as aluminium alloy can be reduced by more than 1200 ℃) can save energy.

3) Longer mold life. Solid lower temperature semi-solid slurry forms shear stress than the traditional dendrite slurry three orders of magnitude smaller, so the filling is smooth, the thermal load is small, and the thermal fatigue strength decreased.

4) Reduce pollution and unsafe factors because the operation is free from high-temperature liquid metal.

5) Less resistance to deformation, homogeneous processing can be achieved using less force, and it is easy to shape difficult-to-process materials.

6) Faster solidification speed, higher productivity and shorter process cycle time.

7) It is suitable for computer-aided design and manufacturing, which improves the degree of automation of production.

Design and development of semi-solid die-casting forming technology

Semi-solid die-casting mold design and development should follow some basic principles, such as in the design process should avoid or reduce the convergence, the need to increase the oil or electric heating system, consider the influence of temperature gradient, etc.; to ensure the smooth implementation of high-pressure shrinkage, after filling the casting to leave the shrinkage channel, the cross-sectional area of the sprue along the filling direction should be reduced by 10-20%; the inner gate should be set in the thickest position of the part, and the inner gate Because the semi-solid slurry contains a certain mass fraction of solid phase, the viscosity of the slurry is higher than that of traditional casting, and the die casting speed of the semi-solid slurry is faster than that of the liquid metal die casting in the low-speed die casting stage, and slower than that of the liquid metal die casting in the high-speed die-casting stage.

Semi-solid die-casting product characteristics

The different ratio of the liquid phase and solid phase in the semi-solid slurry is divided into high solid phase semi-solid, low solid phase semi-solid two kinds of semi-solid solid phase mode. According to the characteristics of the semi-solid manufacturing process, semi-solid die-casting products have the following characteristics.

1.Semi-solid castings are suitable for thin-walled parts forming, high dimensional accuracy, and near-net forming. They can be free from machining art processing when the requirements are not high. Semi-solid products have better microstructure, low processing temperature, small solidification shrinkage, can reduce thermal cracking and segregation, are easy to form thin-walled products, and is conducive to light product weight. Product flatness can reach 0.25mm~0.3mm and surface roughness below 3.2um. The cleaning workload is 30%~40% less than conventional liquid die casting.

2.Semi-solid casting has fine internal organization, good air tightness, few casting defects and good thermal conductivity. Take high solid phase semi-solid slurry, for example, with only about 50% liquid metal, spherical microstructure, and high viscosity fluid, through smooth sequence filling, promote aluminium solidification forming process with almost no shrinkage, porosity and thermal change.

3.Long mold life. The semi-solid slurry filled into the mold has released part of the latent heat of crystallization, which is more than 120 ℃ lower than the conventional aluminium alloy liquid temperature. The temperature difference with the mold is smaller, and the filling speed is low; the filling shape is smooth, which reduces the thermal fatigue and erosion of the mold and prolongs the mold life.

Advantages of semi-solid die-casting molding technology compared with traditional die-casting process

1.the semi-solid die-casting molding process can effectively control the content of internal pore defects in the casting. In the flow process, the semi-solid metal presents an orderly and controlled macro flow state. Using this characteristic, it can make the metal fluid fill the metal mold cavity in order according to the design sequence, push the cavity air to the end of the cavity, and finally discharge through the exhaust slot, avoiding the involvement of the cavity gas, reducing the porosity defects and improving the denseness of the casting.

2.semi-solid die-casting molding can also effectively control the content of internal shrinkage defects in the casting. Metal from liquid solidification to solid needs to go through liquid shrinkage and solidification shrinkage two volume shrinkage process; from semi-solid solidification to solid only need to go through solidification shrinkage a volume shrinkage process. Compared with liquid metals, the volume shrinkage of semi-solid metals during solidification is smaller. Under the condition of natural solidification, the same volume of semi-solid metal produces less shrinkage volume than liquid metal.

3.semi-solid die-casting molding can also effectively reduce the thermal cracking tendency of the casting. The root cause of thermal cracking is the internal stress generated by the alloy in the solidification process; semi-solid die-casting molding technology eliminates the internal stress generated by the dendrite lap while reducing the amount of alloy shrinkage, thereby reducing the tendency of thermal cracking.

4, High-quality semi-solid die-casting parts, due to the low internal porosity, castings can be high-temperature heat treatment, which can further enhance the capacity, and castings have good weldability. Semi-solid die-casting moulding technology also follows the high production efficiency of conventional die-casting moulding, which is ideal for preparing high-volume parts. Compared to conventional liquid-cast parts, semi-solid die-cast parts typically have better mechanical properties.

 

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