Pressure casting (referred to as die-casting) is to fill die-casting cavity (die-casting mold) with liquid or semi-solid metal at a high speed under high pressure, to form and solidify under pressure to obtain castings. Die-casting parts have high dimensional accuracy, generally equivalent to level 6~7, or even up to level 4; low surface roughness; high strength and hardness, 25%~30% higher than sand casting, stable dimensions, good interchangeability; can die-cast thin-walled complex castings, high production efficiency, long die-casting mold life, when die-casting aluminum alloys, can reach 80,000~200,000 times, so die-casting process is widely used in photovoltaics, 5G communications and automotive fields.
Aluminum and aluminum alloys have low density (close to 2.7g/cm3), which is about 1/3 of iron or copper; good electrical and thermal conductivity, second only to silver, copper and gold; good corrosion resistance: surface of aluminum is easy to naturally produce a dense and firm Al2O3 protective film, which can well protect substrate from corrosion. Good products can be obtained through passivation, powder spraying, coating, etc., so it is particularly suitable for die-casting production. This study collects domestic and foreign data, industry development status, expounds development trend of aluminum alloys and castings, aiming to provide a reference for their application.
Graphical results

 

Table 1 Chemical composition of American die-cast aluminum alloys (%)

 

Table 2 Chemical composition of EU die-cast aluminum alloys (%)

 

Table 3 Chemical composition of Chinese die-cast aluminum alloys (%)
It can be seen that composition of various standard aluminum alloy materials is similar, and alloy materials can be selected according to use requirements. Conventional die-cast aluminum alloys have no special requirements for elongation and thermal conductivity, are mainly used for automobile and motorcycle engine parts, such as engine covers, oil pans, cylinder blocks, transmission housings, etc.
With development of 5G technology, more and more die-cast aluminum alloys are used in communication base stations (see Figure 1), mainly for production of radiator shells. Since thermal conductivity of traditional ADC12 aluminum alloy is only 100W/(m·K), ENAC44300 is generally selected as die-casting material to improve thermal conductivity of parts; in addition, T5 heat treatment at 200~350℃ can be used. In addition to 5G communication field, photovoltaic industry also has an increasing demand for die-cast aluminum alloys, and representative component is inverter shell, as shown in Figure 2. "Photovoltaic + energy storage" has become standard configuration for photovoltaic development in many countries. After matching with energy storage, it will bring long-term and sustainable development momentum to photovoltaics. It is estimated that global photovoltaic installed capacity will increase by 370GW in 2025, and new demand for energy storage inverters will be about 74GW. Experts predict that by 2025, 2% of world's energy supply will come from photovoltaic power generation, and by 2055, photovoltaic power generation will deliver more energy, accounting for about 25% of the total energy supply, and by 2150 it will exceed 50%. In addition to need for thermal conductivity, this type of aluminum alloy also has certain requirements for elongation in order to prevent accidental explosions from producing fragments, usually requiring more than 5%. Main components in heat dissipation field are shown in Figure 3. They mainly include ADS housing, DC converter, base station radiator, photovoltaic inverter, charger, vehicle radiator and headlight radiator.

 

(a) Communication base station

 

(b) Base station heat dissipation housing
Figure 1 5G communication base station heat dissipation housing

 

Figure 2 Inverter heat dissipation housing

 

Figure 3 Representative parts of thermally conductive die-cast aluminum alloy
Applicable aluminum alloy materials in thermal conductivity field are shown in Table 4. Compared with ADC12 alloy, in order to ensure thermal conductivity, impurity alloy elements must be removed, especially elements that have a greater impact on thermal conductivity, such as Ti, Mn, Zr, Mg, etc. Effects of each element on thermal conductivity are shown in Figure 4. In addition, T5 heat treatment (200~350℃) can eliminate internal stress generated during die-casting process and improve thermal conductivity of material (see Figure 5). Figure 6 is a statistical analysis of high thermal conductivity die-cast aluminum alloy materials. It can be seen that the lower alloying element content, the higher thermal conductivity. In order to obtain a thermal conductivity of about 200W/(m·K), Si content is usually around 2%, or Ni is used as main alloying element, or a semi-solid process is used.

 

Table 4 Main elements of high thermal conductivity aluminum alloys (%)

 

Figure 4 Effect of alloying elements on thermal conductivity

 

Figure 5 Thermal conductivity of traditional die-cast alloys

 

Figure 6 Statistics of high thermal conductivity die-cast aluminum alloys
Transmission vehicle structural parts mainly include crossbeams, B-pillars, longitudinal beams, shock towers, etc., see Figure 7. According to requirements of vehicle body connection safety and connection, this type of product usually requires a higher elongation, generally more than 12%; corresponding die-casting aluminum alloy material is mainly Silafont-36 (AlSi10MnMg) developed by Rheinland Aluminum, and its performance under different states is shown in Table 5. Corresponding grade in EN1706 is ENAC 43500. This alloy uses Mn to replace Fe element to solve problem of die-casting sticking. At the same time, side effect of Mn element on elongation of alloy is not obvious. After T7 heat treatment, final elongation reaches more than 12%. However, since temperature of T7 heat treatment is 450~480℃, tiny pores inside die-casting expand and produce bubbles. At the same time, high temperature to low temperature process of solid solution process causes product deformation, which brings troubles to appearance and size of product, resulting in a significant increase in cost.

 

(a) Crossbeam

 

(b) B-pillar

 

(c) Longitudinal beam

 

(d) Shock tower
Figure 7 Traditional automotive structural parts

Table 5 Performance of Silafont-36 (AlSi10MnMg) in different states
According to Guohai Securities' research report, rear floor and front cabin of Tesla Model Y in 2022 are highly integrated, number of parts is reduced from 171 to 2 (Figure 8), and number of welding points can be reduced by more than 1,600. Due to large size of such products, they are prone to deformation during heat treatment and size cannot be well guaranteed. Therefore, performance of large one-piece die castings cannot be adjusted through heat treatment, and Silafont-36 (AlSi10MnMg) cannot be used in such products. In order to solve this problem, research and development of new heat-treatment-free materials is imminent; materials have become basis for development of one-piece die casting. At present, vehicle manufacturers such as Weilai and Xiaomi are laying out patents in materials. Parts die-casting companies such as Wencan and Hongtu, and raw material providers such as Shuaiyi Chi, Shenyuan Chuang, Chalco International, Nichikin Metal, and Suzhou Huijin have corresponding patent reserves. Universities such as Tsinghua University and Shanghai Jiaotong University are actively participating (see Figure 9).

 

Figure 8 Schematic diagram of Tesla's integrated die-cast front cabin and rear floor

 

Figure 9 Statistics of alloy grades of heat-free materials

 

Figure 10 Analysis of alloy elements of heat-free materials
Si content in heat-free alloy is between 6% and 9%. It can be seen from phase diagram that the lower Si content, the higher temperature of liquidus line, resulting in a decrease in fluidity of aluminum liquid. Secondly, as Si content decreases, temperature difference between liquidus temperature and solidus temperature increases, and probability of shrinkage cavities in die-cast product during solidification process also increases significantly, greatly increasing process difficulty; in addition, as Si content decreases, solidification shrinkage rate increases, resulting in increased demolding resistance, which brings difficulties to demolding and size control. In order to solve these problems, a three-stage vacuum pumping technology is adopted, as shown in Figure 11, which not only ensures forming of far end of product, but also ensures elongation. Principle of three-stage vacuum pumping is that when punch just passes aluminum liquid port, pressure chamber vacuum channel, hydraulic valve vacuum channel, exhaust plate vacuum channel are all opened and vacuum pumping begins. At this time, punch movement speed is in low-speed stage, speed is generally controlled between 0.15 and 0.3 m/s; when punch reaches pressure chamber vacuum pumping point, pressure chamber vacuum pumping channel is closed; punch continues to move forward, and when it is 100mm before high-speed speed position, see Figure 12. In order to avoid clogging of hydraulic vacuum valve, close hydraulic valve and enter high-speed filling stage. When injection is completed, exhaust plate vacuum channel is closed. In three-stage vacuum process, advantages of pressure chamber vacuum and hydraulic valve vacuum channels are large cross-sectional areas, which can reduce vacuum degree in mold in a short time. However, at beginning of injection, air will enter gap between molds (such as gap between push rods). At this time, vacuum channel of exhaust plate is always open, which effectively balances leakage of mold and keeps the entire injection process at a relatively high vacuum degree, usually within 5kPa.

 

(a) Schematic diagram of three-stage vacuum

 

(b) Schematic diagram of hydraulic valve

 

(c) Schematic diagram of exhaust plate vacuum
Figure 11 Schematic diagram of three-stage die casting vacuum pumping

 

Figure 12 Schematic diagram of die casting speed-displacement curve
Conclusion
Whether it is a new high thermal conductivity aluminum alloy or a heat treatment-free die casting aluminum alloy, to improve thermal conductivity and elongation, it is necessary to reduce content of alloy elements and keep types of alloy elements as small as possible to meet performance requirements. In addition to meeting product performance requirements, die-casting aluminum alloys should also have following properties to ensure die-casting process and product appearance quality: ① Good thermoplasticity near solidus temperature to achieve complex cavity filling and avoid shrinkage; ② Low shrinkage to avoid cracks and deformation during die-casting and improve dimensional accuracy; ③ Smaller solidification interval (temperature difference between liquidus and solidus) to reduce shrinkage; ④ Good high-temperature strength to avoid cracking during mold opening; ⑤ Good casting/mold interface performance to avoid reaction with mold and alleviate mold sticking; ⑥ Good physical and chemical properties, not easy to absorb air and oxidize in high-temperature molten state, meeting long-term insulation requirements.
Development trend of die-casting products is towards integration and large-scale development; with development of product characteristics and demand for personalized performance, die-casting aluminum alloys are developing towards low alloying; high vacuum die-casting has become an important means to solve process problems of new die-casting aluminum alloy materials.