Magnesium alloy is a lightweight material. It is widely used in industries such as aerospace, automotive, and motorcycles because of its excellent specific strength and low density. Magnesium alloys have irreplaceable advantages in manufacturing critical load-bearing structural components. Forging is a metal processing method. Although it accounts for less than 1% of magnesium alloy processing, it plays an important role. Magnesium alloy forged parts are mainly used for components that bear large loads. These components are especially important in helicopters, automobiles, and motorcycles. Even though the usage is low, these parts remain key structural elements.
The forging process of magnesium alloys is similar to that of aluminum alloys. However, there are distinct differences in operation and process requirements. By analyzing the advantages, characteristics, common defects, and solutions in magnesium alloy forging, we can improve the quality and performance of forged parts.
Advantages of Magnesium Alloy Forging
Forging offers several advantages compared to magnesium alloy casting.
High Strength Performance: Forged parts show higher static and dynamic strength when the grain flow is aligned with the main load direction. During forging, the metal grains are compressed and aligned with the force direction, improving both strength and toughness.
Dense Microstructure: During forging, the microstructure of magnesium alloys becomes denser. This eliminates the porosity and looseness commonly found in casting. The dense structure enhances the mechanical properties, making forged parts suitable for applications that require high airtightness.
Excellent Airtightness: The dense and uniform structure of magnesium alloys after forging provides excellent airtightness. This is essential in applications that require high sealing capabilities.
Characteristics of Magnesium Alloy Forging
Magnesium alloys have unique processing characteristics during forging, which impose specific requirements on the process.
Strict Temperature Control: Magnesium alloy billets have high temperatures and a strong coefficient of friction. This makes it harder to fill deep vertical blind holes during forging compared to aluminum alloys. Therefore, the inner and outer radii and rib thicknesses of magnesium alloy forged parts should be larger than those of aluminum alloys. This typically requires an increase of 50%. The forging process design must consider the material's flowability.
Material Selection and Treatment: To ensure good mechanical properties, extrusion billets are typically used for magnesium alloy forging. The extrusion billets undergo homogenization treatment before extrusion to reduce material anisotropy during forging and improve the final product's mechanical properties. Additionally, treating the surface defects of cast ingots and homogenization is crucial.
Mold Temperature Control: Magnesium alloys are sensitive to deformation temperatures. If the billet contacts a mold that is too cold, surface cracking may occur. This can prevent normal forging. Therefore, both billet temperature control and preheating of the mold are essential for ensuring forging quality.
Sensitivity to Deformation Rate: Magnesium alloys are sensitive to deformation rates. If the deformation rate increases, the plasticity of magnesium alloys decreases, which can lead to cracking. For complex-shaped forged parts, multiple heating and forming steps are required. The temperature should be gradually reduced with each forging step to prevent grain growth.
Common Defects in Magnesium Alloy Forging and Preventive Measures
Magnesium alloy forging is a mature process, but common defects still need attention.
Coarse Grain Ring Fracture and Transgranular Cracking: These defects are common in magnesium alloy forging. They often occur when the forging temperature is too low or the deformation rate is too fast. Therefore, it is essential to control both temperature and deformation rate carefully during the forging process.
Surface Corrosion and Oxidation: Magnesium alloys have poor corrosion resistance. The surface of forged parts is prone to pitting corrosion, which appears as dark gray powdery spots. If not treated promptly, these areas may develop into small holes or depressions, leading to cracking. To prevent corrosion, suitable lubricants should be used, and salt-based lubricants should be avoided. The lubricant should be removed immediately after forging, and the surface should be treated by pickling and drying to eliminate the oxide layer.
Edge Cracking: Magnesium alloys are sensitive to tensile stresses during forging, and improper edge trimming can cause cracks. To avoid edge cracking, proper trimming processes should be followed, ensuring smooth surfaces and reducing stress concentration.
Process Optimization in Magnesium Alloy Forging
To improve the quality of magnesium alloy forged parts, the following optimizations are critical.
Temperature Control Management: The forging process of magnesium alloys requires strict control of both billet and mold temperatures to avoid cracking caused by temperature differences. Proper temperature control management improves process stability and reduces production defects.
Deformation Rate Optimization: Choosing the correct deformation rate is key to ensuring the quality of magnesium alloy forging. By controlling the deformation rate, the risk of cracking due to reduced plasticity can be minimized. For complex shapes, multiple heating and forming steps with gradual temperature reduction help optimize the forging quality.
Surface Treatment and Anti-corrosion Measures: To prevent surface corrosion of magnesium alloy forged parts, the surface should be cleaned, pickled, and dried promptly after forging. Using suitable lubricants and avoiding thick coatings are also vital measures to prevent pitting corrosion.
Conclusion
The magnesium alloy forging process has unique challenges and advantages. Although the forging characteristics of magnesium alloys differ from those of aluminum alloys, magnesium alloy forged parts offer high strength, excellent airtightness, and dense microstructures. These properties are vital in industries such as aerospace and automotive. By controlling factors such as forging temperature, deformation rate, mold temperature, and surface treatment, the quality and reliability of magnesium alloy forged parts can be significantly improved, ensuring their suitability for critical applications.
