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What is Magnesium Alloy Materials?

Magnesium alloys are alloys based on magnesium and composed of other elements. Their characteristics include:

- Low density (1.8 g/cm³ for magnesium alloys)

- High strength

- Large elastic modulus

- Good heat dissipation

- Excellent shock absorption

- High impact load capacity, surpassing that of aluminum alloys

- Good corrosion resistance to organic substances and alkalis

The main alloying elements are aluminum, zinc, manganese, cerium, thorium, and small amounts of zirconium or cadmium. The most widely used magnesium alloy is the magnesium-aluminum alloy, followed by magnesium-manganese and magnesium-zinc-zirconium alloys. These alloys are primarily used in aviation, aerospace, transportation, chemical industries, and rocket manufacturing. Magnesium is the lightest practical metal, with a specific gravity of about 2/3 that of aluminum and 1/4 that of iron, offering high strength and rigidity.

Composition of Magnesium Alloy Materials for Die Casting

- Silver: When combined with rare earth elements, it enhances high-temperature tensile and creep properties but reduces corrosion resistance.

- Aluminum: Improves casting performance and acts as a solid solution strengthening element, providing precipitation strengthening at low temperatures, with minimal impact on corrosion resistance.

- Beryllium: At very low concentrations, it significantly reduces oxidation of the melt surface, though excessive amounts lead to coarse grains.

- Calcium: Provides grain refining effects, slightly inhibits melt oxidation, and improves creep resistance, but adversely affects corrosion resistance.

- Iron: Magnesium in low carbon steel crucibles does not react significantly, but the presence of iron in magnesium alloy negatively impacts corrosion resistance.

- Lithium: Increases evaporation and combustion risks, but improves corrosion resistance.

- Manganese: Refines grain, weakens iron and nickel's adverse effects, improves corrosion resistance and strength, and reduces hydrogen absorption.

- Rare Earth Elements: Increase heat resistance and strength.

- Strontium: Grain refining effect.

- Tin: Reduces melting point and improves fluidity.

- Yttrium: Significantly enhances high-temperature performance, such as oxidation resistance.

- Zinc: Has solid solution and aging strengthening effects, increasing alloy fluidity.

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Properties of Magnesium Alloy Materials for Die Casting Parts

1. High Specific Strength and Elastic Modulus: Magnesium alloys have a high strength-to-weight ratio and specific elastic modulus, allowing the production of thinner and lighter parts. They also exhibit high stability and dimensional precision in casting and processing.

2. Excellent Casting Performance: Magnesium alloy die-casting can achieve a minimum wall thickness of 0.5 mm, compared to 1.2–1.5 mm for aluminum alloys. Magnesium has a smaller crystallization latent heat than aluminum, meaning it solidifies quickly in the mold. Generally, the productivity of magnesium alloy die-casting is 30 to 50% higher than that of aluminum alloy.

3. Reduced Die Wear: Magnesium reacts slowly with iron, resulting in less wear on the die during casting. Compared to aluminum alloy, the service life of magnesium alloy die-casting molds is more than 1.5 times longer, often maintaining over 150,000 cycles.

4. Short Die Casting Cycle: Magnesium alloy die-casting has a short cycle time and can accommodate large casting areas, making it suitable for products requiring high surface quality.

5. Moderate Machine Cost: The cost of machines for magnesium alloy die-casting is moderate.

Both cold chamber and hot chamber die casting methods are used for magnesium alloys:

- Hot Chamber Die Casting: Generally used for thin-walled small to medium parts. This method is beneficial for maintaining the temperature of the alloy before it enters the mold cavity, enhancing fluidity and allowing quick mold filling for high-quality parts.

- Cold Chamber Die Casting: Typically used for thick-walled large to medium parts.

Types of Magnesium Alloys Used for Die Casting Parts

Magnesium alloys used for die casting are primarily categorized into deformed magnesium alloys and cast magnesium alloys. The main series of magnesium alloys for die casting include:

AZ (Mg-Al-Zn) Series:

- AZ91D: One of the most commonly used die-cast magnesium alloys, known for its excellent casting and mechanical properties. It contains approximately 9% aluminum and 1% zinc. Aluminum significantly enhances the alloy's strength and corrosion resistance, while zinc improves its fluidity.

- AZ61A: This alloy has a lower aluminum content (around 6%) but still exhibits good casting and mechanical properties. While its strength and corrosion resistance are slightly lower than those of AZ91D, it offers better resistance to thermal cracking.

AM (Mg-Al-Mn) Series:

- AM50A and AM60B: These alloys contain aluminum and manganese, offering excellent corrosion resistance and weldability. Although their strength is slightly lower compared to AZ series alloys, they are ideal for applications requiring high corrosion resistance.

AE (Mg-Al-RE) Series (RE stands for Rare Earth Elements):

- AE42: This alloy includes rare earth elements such as neodymium (Nd) or cerium (Ce) to enhance heat resistance and strength. AE42 alloys maintain good mechanical properties at high temperatures, making them suitable for high-temperature applications.

AS (Mg-Al-Si) Series:

- AS series alloys typically contain silicon to improve wear resistance and strength. However, in die casting applications, AS series alloys are less common than AZ, AM, and AE series alloys.

Other Magnesium Alloys:

- There are other magnesium alloys like Mg-Zn and Mg-Li that may be used in specific die casting applications. These alloys offer unique properties, such as low density (Mg-Li alloy) or high strength (Mg-Zn alloy).

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Surface Treatment for Magnesium Alloy Die Casting Parts

Chemical Conversion Coating Technology

- Phosphating: Uses phosphoric acid salt solution to form a phosphate film on the surface.

- Chromating: Forms a protective film of trivalent and hexavalent chromium compounds, improving corrosion resistance.

Organic Polymer Coating Technology

- Electrostatic Powder or Spray Paint, Electrophoretic Paint: Forms a protective organic polymer film on the surface, preventing corrosion.

Anodizing Treatment Technology

- Forms an oxide film through electrolytic reaction, improving corrosion and wear resistance. The oxide film consists of a dense barrier layer and a porous surface layer, which needs sealing.

Electroplating and Electroless Plating

- Electroless Nickel Plating: Effective in protecting magnesium alloys by improving corrosion and wear resistance, often involving zinc dipping for better adhesion.