1. Definition of Magnesium Alloy Die Casting
Pressure casting (also called die casting) is a metal casting process. The most significant difference between die casting and other metal casting processes is its high pressure and high speed. Die casting alloys vary, including zinc alloys, aluminum alloys, copper alloys, tin alloys, and magnesium alloys. Magnesium alloy die casting involves melting material in a furnace to 630℃-650℃, then using a die casting machine with high pressure and high speed. Magnesium alloy is continuously accelerated through gooseneck, feed pipe, mold runner, and ingate until cavity is completely filled. Molten magnesium alloy then cools and solidifies under high pressure to obtain casting.
2. Magnesium Alloy Die Casting Methods
(1) Hot Chamber Die Casting Machine
Hot chamber die casting machine involves immersing gooseneck, injection rod, hammer, and plunger ring into molten magnesium alloy. Gooseneck tube, injection tube, nozzle, mold runner, and product cavity are integrated into one unit (as shown in Figure 2-1). Probability of air entrapment during injection is low; molten metal temperature fluctuates little, allowing for production at lower temperatures. However, it can only be used for production of zinc and magnesium alloy die castings that are not severely corrosive to consumables, to avoid inconsistencies in alloy melt composition. Due to reduced ladling and pouring actions and component response time, hot chamber die casting machine has a shorter production cycle time, higher production efficiency, and larger output. A simplified diagram of various parts of hot chamber die casting machine is shown in Figure 2-2.

 

(2) Cold Chamber Die Casting Machine
Cold chamber die casting machine has a separate pressure chamber and a quantitative furnace. During operation, quantitative furnace transports molten magnesium alloy material from holding furnace through a feeding pipe to pressure chamber inlet. Molten alloy passes through pressure chamber, gating system, mold runner, and product cavity, filling under high pressure and high speed before cooling and solidifying to obtain casting. This is metal casting method of cold chamber die casting machine (as shown in Figure 2-3). Production cycle of cold chamber die casting machine is relatively long. During transportation, molten alloy is prone to oxidation and inclusions when exposed to air, and air can mix into casting, forming voids. This can be improved by adjusting process or using special processes (such as vacuum die casting). Cold chamber machines come in various models and specifications, with a wide range of adaptability to product size and alloy type. They offer excellent and stable injection performance, can be controlled in segments or multiple stages to meet needs of various die casting processes.
A simplified diagram of each part of cold chamber die casting machine is shown in Figure 2-4.

 

Figure 2-3 Schematic diagram of a cold chamber die casting machine

 

1-Injection punch 2-Pressure chamber 3-Mixed magnesium alloy 4-Fixed mold 5-Moving mold 6-Cavity 7-Gateway 8-Moving mold plate 9-Ejector plate 10-Material cake 11-Fixed mold plate
Figure 2-4 Schematic diagram of various parts of a cold chamber die casting machine

1. Application Scope of Magnesium Alloy Die Casting
Magnesium alloy die casting can produce complex structures, close to final dimensions of finished product, with less machining and high precision. It can be applied in various industries such as automotive gearbox housings, aerospace, 3C products, communication base stations, garden tools, and medical applications. Currently, hot chamber magnesium alloy die casting is the most widely used in 3C industry for mobile phone mid-plates, with AZ91D as a typical example. It is also subject of this paper.
2. Advantages and Disadvantages of Magnesium Alloy Die Casting
Advantages and disadvantages coexist, which is often a common characteristic of anything. Everything has its advantages and disadvantages, magnesium alloy die casting is no exception. Main consideration is whether advantages outweigh disadvantages, whether it can compensate for a certain characteristic to become stronger and better. This is basis for our selection. Advantages of hot chamber magnesium alloy die casting are:
1) It can produce complex structures, has high dimensional accuracy requirements, and excellent repeatability.
2) Short production cycle, with a minimum cycle time of 7 seconds, resulting in high efficiency and low manufacturing costs.
3) Good fluidity in die casting, with a large die casting thickness of 0.35mm and a dense surface structure.
4) Good machinability.
5) Relatively low probability of air entrapment during injection.
6) A molten metal temperature of 630℃ is sufficient for production, with minimal molten metal temperature loss.
Main disadvantages of hot-chamber magnesium alloy die casting:
1) Prone to combustion and explosion; magnesium dust has a low ignition point; molten magnesium reacts violently with water, potentially causing an explosion.
2) Poor sealing or insufficient purity of protective gas SF6 can lead to internal combustion in furnace, resulting in severe slag inclusions in product.
3) Severe beryllium loss from molten metal and increased iron content, making product susceptible to corrosion and oxidation.

Currently, smartphones utilize a magnesium alloy die-cast mid-plate combined with an injection-molded mid-frame shell, forming a unibody design after injection molding. Application of magnesium alloy die casting reduces CNC machining time, solves sand hole problem in fully die-cast products, and improves production yield. It is currently mainstream application solution for smartphones, and its characteristics are:
1) Magnesium alloy has a low density, is lighter than aluminum alloy, and cheaper than titanium alloy, which is beneficial for lightweighting and cost-effectiveness.
2) Magnesium alloy mid-plate combined with injection-molded mid-frame shell, including antenna, reduces problem of inaccurate antenna placement.
3) Lower cost, higher precision than stamped parts, capable of complex structures, and excellent repeatability.
4) Using a hot chamber die-casting machine, production cycle of one mold (1 out of 2) is as short as 7 seconds, resulting in high efficiency and low manufacturing costs.
5) Good airtightness of die-casting channel reduces porosity in product's rib area, increasing product strength.
6) Magnesium alloy dissipates heat quickly, resulting in rapid cooling after molding.
7) Magnesium alloy die casting has poor compatibility with mold steel, making it less prone to sticking to mold.