Methods for Improving Overall Corrosion Resistance and Strengthening Modification of Magnesium Castings

1. Remove Impurities to Improve Corrosion Resistance and Strengthen the Improvement

(1) Improving the Purity of Magnesium Alloy in Magnesium Castings

Generally, high purity magnesium alloy has better corrosion resistance than low purity magnesium alloy. Improving the purity of magnesium alloy is a common method to improve the corrosion resistance of magnesium alloy. When the impurity content does not exceed their allowable limit, the corrosion rate of magnesium alloy will be very low. For example, the corrosion resistance of AZ91 and AM60 under salt spray condition is better than that of pressure cast A1380 (Al-4.5Cu-2.5Si) and cold rolled steel. Improving the purity of magnesium alloy is mainly carried out by metallurgical methods, such as refining magnesium alloy, but the general cost is relatively high.

(2) Impurities Detoxification

It is technically difficult and costly to reduce the content of harmful elements in magnesium alloys to a lower level. In fact, it is not necessary to remove impurities from magnesium alloy to remove the harmfulness of impurities. If impurities can be converted into harmless substances, even if these impurities remain in the alloy, it will not cause any serious corrosion.

(3) Alloying

Alloying is an important means to change the chemical composition, phase composition and microstructure of magnesium alloy. It is an important way to improve the corrosion resistance of magnesium alloy.

From the point of view of improving corrosion resistance, the purpose of alloying is to promote the corrosion resistance of base phase and the formation and rational distribution of corrosion resistance barrier phase, so as to improve the corrosion resistance of magnesium alloy.

2. Adopt Special Casting Technology for Magnesium Castings

(1) Adopting the Pressure Casting Process

Pressure casting is the most commonly used production method of magnesium alloy components, especially suitable for mass production. When magnesium alloy is die-cast in cold cavity, the cooling rate of casting surface is high, which can be considered as a rapid solidification process. The cooling rate inside the casting is low, which is close to that of ordinary casting.

(2) Strong Plastic Forming Process

Rolling, forging and extrusion are also important methods to produce magnesium castings. Theoretically, these manufacturing processes will change the microstructure of magnesium alloy to a great extent, which will also lead to the change of corrosion resistance of magnesium alloy. For example, when the deformation is large, rolling, forging and extrusion will change the material from as-cast structure to processed structure, eliminate coarse columnar crystal, press pores, pinholes and looseness, and eliminate dendrites to make the grain of the material fine, dense and uniform, which will obviously improve the corrosion resistance, mechanical properties and comprehensive properties of the material. Especially, when the alloy composition, deformation temperature and deformation speed are suitable, new processes such as superplastic forming, isothermal forging of magnesium alloy and isothermal extrusion can strengthen and modify the structure and properties of the material to a greater extent. The corrosion resistance will also be improved.