Aluminum, magnesium, and other highly reactive metals are among the most active elements in the periodic table. Aluminum is the third most abundant element in the Earth's crust, making it a widely used material in industry. Both magnesium and aluminum are essential raw materials for manufacturing due to their light weight and high strength-to-weight ratio. However, because of their high chemical reactivity, softness, and low melting points, they are typically used in the form of alloys in industrial applications.
When designing the structure of aluminum and magnesium alloy castings, several key factors should be taken into account:
1. Due to their relatively low melting points, both aluminum and magnesium alloys can be cast using various methods. Therefore, during the design phase, it’s important to consider the specific casting process that will be used, as this directly affects the feasibility and quality of the final product.
2. It is recommended to use thin-walled structures with uniform thickness to minimize shrinkage porosity and improve the mechanical strength of the casting. For areas with uneven wall thickness, feeding systems such as risers or thickened sections should be incorporated to ensure proper solidification and avoid defects.
3. These metals are prone to oxidation and gas absorption, and their low density (about 2/3 to 3/4 less than cast iron or steel) results in lower metal static pressure, which makes it harder to remove gases from the melt. This can lead to porosity and slag inclusion, reducing mechanical properties and airtightness. To prevent this, large horizontal surfaces should be avoided, and smooth transitions should be designed to prevent turbulence during pouring.
4. Because of their low melting point, when designing castings with airtight requirements, care must be taken to avoid using core supports that do not fuse properly with the casting body, as this may cause oil leakage.
5. Aluminum and magnesium alloys have a higher tendency to shrink and exhibit lower elastic modulus, making them more sensitive to stress concentration. As a result, deformation and cracking can occur during casting or use. Proper structural design is crucial:
a) Use I-beam, slotted, or box-shaped cross-sections to increase stiffness and reduce deformation.
b) Reinforcing ribs should be designed carefully to avoid stress concentration at their edges. They should also not be too thick, as this can lead to shrinkage cavities at the junction with the wall.
c) Edges around holes should be reinforced, such as by adding flanges.
d) In wheel castings, reinforcing ribs are often used to strengthen the connection between the hub and the web. A thicker, sloped section is usually used between the heat zone and the casting wall.
e) Thin-walled castings can be strengthened using ribs or stepped sections. Long bolts should be avoided if possible, as they may cause localized stress and shadowing. Instead, use many small-diameter bolts with washers to distribute forces evenly and prevent deformation.
f) Forces should be transmitted along the axis of the casting to avoid bending or torsional stresses. Thin-walled shell-like parts should not have sharp or abrupt shapes.
g) Due to their low elasticity, aluminum and magnesium alloys are more prone to deformation. When designing press-fit components, clearance after deformation and repeated assembly should be considered. Threaded holes in these alloys should be longer than those in cast iron or steel—typically L/d = 2 for aluminum and L/d = 2.5 for magnesium.
6. Magnesium alloys have higher compressive strength than tensile strength, so asymmetric cross-sections that promote compressive stress are preferred in their design.
7. Magnesium alloys are susceptible to corrosion and require surface treatments to enhance durability. When designing castings that come into contact with water, such as Water Pump housings, avoid features that trap moisture. Also, sharp corners should be avoided to protect the protective coating.
8. Bimetallic casting and insert casting techniques can be useful in solving structural challenges in aluminum and magnesium alloy castings.
By considering these design principles, manufacturers can produce high-quality, durable, and functional aluminum and magnesium alloy castings suitable for a wide range of industrial applications.
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