What’s the difference of Material Magnesium Alloy And Aluminum Alloy?
What’ the Magnesium alloying elements?
The difference of Magnesium and Aluminum
Magnesium and aluminum have big differences in chemical and physical properties.
1: The mainly manifested in the chemical reaction activity.
2: The tightness of the oxides on the surface of the molten metal.
3: The thermal effect during the solidification process, and the ability to resist electrochemical corrosion.
4: Magnesium has stronger chemical reaction activity than aluminum. And the surface of molten metal cannot form a dense oxide protective film like aluminum alloy liquid. Or preventing the oxidation reaction from proceeding. So magnesium alloy is easy to burn or even explode.
5: Magnesium alloys are also easy to dissolve other metals. Including elements such as nickel and copper that severely reduce the electrochemical corrosion performance of magnesium alloys.
6: In addition, during the solidification of the same volume of aluminum alloy and magnesium alloy liquid. The heat released by the magnesium alloy is significantly lower than AL liquid.
Magnesium alloying elements
Why not using pure magnesium directly?
The mechanical properties of pure magnesium are very poor. It cannot meet the requirements for yield strength and tensile strength of industrial parts.
The addition of alloying elements can significantly improve the mechanical properties of magnesium. Making the strength-to-weight ratio of magnesium alloys second only to titanium alloys among various alloys, opening up a broad future for its industrial applications.
The second role of alloying elements is to improve the casting properties of magnesium alloys. Pure magnesium has a high melting point, poor fluidity, and large shrinkage.
Through alloying, the liquids’ temperature can be reduced. The fluidity can be increased. The casting performance of the magnesium alloy can be improved, and the shrinkage tendency can be reduced.
In view of the significant decrease in strength of magnesium alloys at temperatures above 150 degrees Celsius, the third effect of alloying is to enhance the creep resistance of magnesium alloys.
Aluminum and zinc are commonly alloying elements used in magnesium alloys
The alloying elements commonly used in magnesium alloys are aluminum and zinc. The alloying of aluminum can improve alloy strength and casting performance. Zinc can also improve the casting properties of the alloy.
In order to ensure the casting performance, the aluminum content of die-cast magnesium alloy is more than 3%, and the zinc content is less than 2%.
Otherwise, cracks are likely to occur. Alloys AZ91 (with zinc) and AM60B (without zinc) based on the Mg-An-Mn series are the main die-cast magnesium alloys used at room temperature. Trace amounts of rare earth elements Y, Nd, Sr have obvious refining effect on AZ91 alloy.
Sr-containing alloys have coarse Mg-Sr compounds and Mg17A112 phases at the grain boundaries, which are prone to intergranular fracture, while alloys containing Y and Nd are due to The rapid formation of slip bands and metamorphic crystals in the crystals mainly occur transcrystalline fractures. The toughness of the alloy is further improved. At present, the two series of alloys, AZ and AM, account for 90% of magnesium automotive structural parts, but their strength decreases significantly when the temperature is above 150°C
How to improve the creep resistance of alloy above 150℃
In order to improve the creep resistance of the alloy above 150°C, AS41A alloy (4.3% Al, 1% silicon, 0.35% manganese) has been developed, and the creep strength of this gold-containing alloy is in the range of 170°C. At the same time, it has good elongation, yield strength and ultimate tensile strength. Due to the low aluminum content, AS41A requires a higher casting temperature. AS series magnesium alloys are easy to form hard silicon dots in the structure due to the introduction of Si. Sodium and strontium can fundamentally improve the structure of the silicon phase and play a good metamorphic effect.
Application of rare earth elements in Mg-Al based alloys
Mg-Al-rare earth alloys have been developed using the beneficial effects of rare earth elements on the strength and creep resistance of Mg-Al based alloys. Die-casting AE42 alloy has more creep resistance than Mg-Al-Si alloy, and can be used for a long time at 200-250 degrees. Canada developed the AC series of magnesium alloys. The addition of Ca improves the phase structure of Mg2Si in the Mg-Si alloy and refines its grains, and its creep resistance is about 10 times that of AZ and AM series alloys. The tensile and tensile strengths are equivalent. And has good casting performance.
Die-cast magnesium alloys for industrial applications
At present, there are mainly 4 series of die-cast magnesium alloys that have been industrially used, namely AZ series (Mg-Al-Zn-Mn); AM series (Mg-Al-Mn); AS series (Mg-Al-Si); AE series ( Mg-Al-rare earth). Below, the table shows the nominal composition and properties of common alloys among them.
