Magnesium alloy is a lightweight engineering metal material currently used in engineering applications. It has characteristics of light specific gravity, good thermal conductivity, strong electromagnetic shielding ability, and easy recycling. It is considered to be a "green material" with great development and application potential in 21st century. It has been used in automotive electronics, aerospace, communications and other industries.
Magnesium has a low density and is easy to burn, which is determined by its physical and chemical properties. Density of metallic magnesium at 20℃ is 1.738g/cm3, and density of liquid metallic magnesium is 1.58g/cm3; at standard atmospheric pressure, melting point of metallic magnesium is (650±1)℃ and boiling point is 1090℃. Melting point of magnesium alloy varies depending on alloy composition, usually between 400℃ and 630℃ (752°F to 1166°F), depending on alloying elements and their content. When heated in air, metallic magnesium begins to burn at 632℃ to 635℃. Therefore, preparation and alloy smelting process of magnesium are relatively complicated.
Magnesium and magnesium alloys have strong chemical properties, are easy to oxidize and burn in air, and generated oxide film is loose, while aluminum and aluminum alloys are not easy to burn because they have a layer of aluminum oxide on the outside to isolate air.
Main differences in process between magnesium alloy high pressure die casting and aluminum alloy high pressure die casting and their reasons are introduced.

1. Melting point difference: Melting point of magnesium alloy (about 430-630℃) is usually lower than that of aluminum alloy (about 580-670℃), but actual die casting temperature of magnesium alloy may be higher (about 620-680℃) to maintain fluidity, while aluminum alloy is 610-670℃.
Common magnesium alloy material grades have melting points: AZ91 (magnesium-aluminum-zinc alloy): about 470℃ (878°F); AM60 (magnesium-aluminum-manganese alloy): about 615℃ (1139°F); ZK60 (magnesium-zinc-zirconium alloy): about 520℃ (968°F).
2. Melting differences: Magnesium melt is very easy to oxidize or even burn. Magnesium alloy casting system needs to be equipped with a special gas protection furnace and protected with SF₆ mixed gas (such as SF₆+CO₂/N₂); aluminum alloys are oxidized to form a dense Al₂O₃ layer, and only conventional covering agents or nitrogen protection are required.
3. Reasons for difference: Magnesium has extremely high chemical activity and reacts violently with oxygen at high temperatures, so it must be isolated from oxidizing environment.

1. Fluidity: Fluidity of magnesium alloy is better than that of aluminum alloy, and heat transfer speed of magnesium alloy solution to mold is faster. In order to avoid premature solidification of magnesium liquid, flow rate of inner runner is generally fast. It allows faster injection speed (up to 100 m/s) and thinner wall thickness (up to 0.5 mm).
2. Injection pressure: Aluminum alloy requires higher pressure (usually 80-120 MPa), while magnesium alloy has good fluidity and lower pressure (40-100 MPa).
3. Reason for difference: Magnesium has low surface tension and low latent heat of solidification, and has stronger filling capacity, but it needs to be filled quickly to avoid premature solidification, requiring injection system to provide sufficient energy to meet requirements of fast filling. At the same time, casting performance of magnesium alloy is quite sensitive to mold temperature and pouring temperature, mold temperature and pouring temperature must be accurately controlled.

1. Type of die-casting machine: Magnesium alloy castings generally use hot chamber die-casting machines for castings less than 1Kg to ensure filling of thin-walled parts, cold chamber die-casting machines for large parts. Aluminum alloys are mostly made with cold chamber die casting machines (furnace separation), which are suitable for large pieces and high melting point alloys.
2. Reasons for difference: hot chamber die casting reduces risk of melt oxidation and is suitable for low melting point of magnesium; cold chamber die casting avoids aluminum from corroding iron parts.

 

Cold die casting machine

 

 

Hot die casting machine

1. Mold temperature: magnesium alloy molds need to be preheated to 200-300℃ to delay solidification, and aluminum alloy molds have lower temperatures (150-250℃). Wear of magnesium alloy molds is also lower than that of aluminum, mainly because of high viscosity of aluminum. Specific mold temperature range needs to be determined in combination with actual product structure.
2. Cooling system: magnesium alloys have a shorter cooling time (due to fast solidification), and mold requires an efficient cooling channel, which has thermal balance requirements for mold. Therefore, magnesium alloys are generally controlled by oil cooling + mold temperature controller.
3. Reason for difference: High thermal conductivity of magnesium leads to rapid solidification, filling and shrinkage need to be optimized by precisely controlling mold temperature.

1. Safety measures: Production of magnesium alloys requires strict explosion prevention (dust is flammable), fire prevention (melt explodes when it meets water) and waste gas treatment (SF₆ is a strong greenhouse gas). Aluminum alloys have lower risks, but ventilation and dust control are still required.
2. Reason for difference: Activity of magnesium brings higher danger. Although SF₆ is effective, it requires alternative technology (such as SO₂ or new environmentally friendly gases).

1. Corrosion resistance: Magnesium has high chemical activity and is easy to react with oxygen, water, etc. Oxide film is loose, has poor protection, and is easily damaged, especially in humid or salty environments. Magnesium alloy parts require coatings such as chemical conversion and micro-arc oxidation. Aluminum has low chemical activity and is easy to form a dense oxide film (Al₂O₃) on its surface. Oxide film is dense and stable, which can effectively isolate corrosive medium and usually does not require additional protection. It will still corrode in strong acids or strong alkalis. Aluminum alloys can improve corrosion resistance through anodizing, electroplating, etc. At present, aluminum alloys usually only need shot blasting.
2. Reasons for difference: Standard electrode potential of magnesium is low, and electrochemical corrosion is prone to occur, so surface treatment is more critical.

1. Cost: Previously, price of magnesium raw materials fluctuated greatly, but in the past two years, with national regulation, price of magnesium raw materials has tended to stabilize, which is no different from price of aluminum raw materials, and production efficiency of magnesium alloys is high (fast cooling and short cycle).
2. Application field: Magnesium alloys have good thermal conductivity and metal electromagnetic protection properties, are more suitable for electronic industry products than other alloys. Magnesium alloys also have good sound insulation and shock absorption. Magnesium alloys are mostly used in automotive parts (dashboards, wheels) and 3C products with high lightweight requirements; aluminum alloys are widely used in automobiles, aerospace and building structures.

Magnesium alloys and aluminum alloys each have their own advantages and applicable scenarios in die-casting. Magnesium alloy die-casting has advantages of lightweight, high fluidity and rapid production, but faces challenges in handling highly active materials and safety; aluminum alloys are known for mature processes, corrosion resistance and cost-effectiveness. Process differences are mainly driven by physical and chemical properties of materials, and equipment, process chains and safety measures need to be optimized in a targeted manner. When choosing a die-casting method, factors such as product requirements, production conditions, cost control and mold life should be comprehensively considered to select the most suitable casting process and materials.