The density of magnesium alloy is less than 2g/cm3, and it is the lightest metal structural material at present. Its specific strength is higher than that of aluminum alloy and steel, but slightly lower than that of fiber reinforced plastics; its specific stiffness is comparable to that of aluminum alloy and steel, but much higher than that of fiber-reinforced plastics; Its corrosion resistance is much better than that of low carbon steel, which has exceeded that of the die cast aluminum alloy A380; its vibration reduction and magnetic shielding properties are much better than those of aluminum alloy. Since the dynamic viscosity of magnesium alloy is low, the filling speed of magnesium alloy under the same fluid state (equal Reynolds index) is much higher than that of aluminum alloy. In addition, the melting point, specific heat capacity and transformation latent heat of magnesium alloy are lower than those of aluminum alloy, so the melting energy consumption of magnesium alloy is less, the solidification speed is faster, and the actual die casting period of magnesium alloy is 50% shorter than that of aluminum alloy. What is more, magnesium alloys have a low affinity with iron and low ability to solve iron in solid solution, so it is not easy to adhere to the surface of the die, and the life of the die is 2 ~ 3 times higher than that of aluminum alloy.
The most commonly used die-cast magnesium alloys are American grades AZ91, AM60, AM50, AM20, AS41 and AE42, which belong to the four series of Mg-Al-Zn, Mg-Al-Mn, Mg-Al-Si and Mg-Al-Re respectively. The following aspects of die-cast magnesium alloys are mainly studied at present:
At present, AZ and AM series of magnesium alloy castings account for 90% of the automotive magnesium alloy castings. The strength of these magnesium alloys decreases obviously when the temperature is above 150℃. The die-casting magnesium alloys with creep resistance above 150℃ have been developed, such AS AS41A alloy (Mg43% Al1% Si0.35%Mn). Its creep strength at 175℃ is better than that of AZ91D and AM60B, and it has higher elongation, yield strength and tensile strength. The Beetle crankcases of VOLKSWAGEN used to adopt AS41 and AS42, but a modified alloy AE42 that has better creep properties at high temperatures has recently been introduced. Some trace elements, such as rare earth elements Y, Nd and Sr, have obvious grain refinement effect on the die-casting magnesium alloy, which can improve the strength and creep resistance of the magnesium alloy casting. For example, the recently developed AE42 has better creep resistance than the traditional McAlSi alloy, and can be used at 200 ~ 250℃ for a long time. However, the improvement of the high temperature performance of AS and AE alloys is still limited. The casting properties of AS and AE alloys are worse than those of AZ and AE alloys. In addition, the high cost of rare earth elements limits the production and application of AS and AE alloys.
At present, magnesium die castings are growing rapidly in applications that require safety and high fracture toughness. In order to improve the ability of energy absorption under working conditions, the fracture toughness of the material should be improved. This can be realized by reducing the amount of aluminium in the alloy. AM60 and AM50 are widely used in safety components such as dashboard brackets, steering wheel shafts and seats, and AM20 is currently used in the back frame of seats. In addition, the relationship between fracture elongation and temperature is quite close, especially when the temperature is above 50℃, the fracture elongation increases with the increase of temperature.
Corrosion resistance was also an obstacle to the wider application of magnesium alloys. Magnesium has high chemical activity, and magnesium based alloys and composites are prone to micro-cell corrosion. Generally, low purity die-cast magnesium alloys have poor corrosion resistance. The corrosion resistance of high purity die-cast magnesium alloys (such as AZ91D) with strictly specified impurity elements such as Fe, Ni and Cu as well as AE42 containing rare earth has exceeded that of die-cast aluminum alloy A380 in salt spray test and is much better than that of low carbon steel. The corrosion resistance can be improved by adjusting the chemical composition, treating the surface and controlling the microstructure. Although there are many methods to improve the corrosion resistance of magnesium alloy parts, poor corrosion resistance is always a technical obstacle to realize wide application of magnesium alloy parts unless the problem is solved from the material itself.
The addition of AL (2.5%), BE (0.0005% ~ 0.03% of Be) or CA alloy to the magnesium alloy can also effectively prevent the oxidation of the liquid magnesium alloy. At present, some researchers are working on the flame retardant properties of magnesium alloys. If the research is successful, magnesium alloys will be smelted and cast just like aluminum alloys, which will have a wider application prospect.
Magnesium alloy matrix composites that are reinforced with silicon carbide particles have been studied and developed for many years. At present, although the development has not reached the stage of commercial application in the field of die casting, castings such as impellers, bicycle cranks and automobile cylinder liners have been made by sand casting and precision casting. Besides, there is a development trend to combine this composite material with semi-solid casting and apply it in the field of die casting and squeeze casting.