When designing the structure of aluminum and magnesium alloy castings, several key considerations should be taken into account to ensure optimal performance and manufacturability. First, due to their relatively low melting points, these alloys can be produced using a variety of casting methods. Therefore, the chosen casting process must be considered early in the design phase to ensure that the structure is suitable for the selected technique.
Aluminum and magnesium alloys benefit from thin-walled structures with uniform thickness, which helps prevent shrinkage and porosity while enhancing overall strength. However, if wall thickness varies, risers or thickened sections should be incorporated to promote proper feeding during solidification and maintain casting quality.
These alloys are also prone to oxidation and gas absorption due to their low density, which leads to lower metallostatic pressure and difficulty in gas removal. As a result, porosity and oxide inclusions are common, reducing mechanical properties and airtightness. To mitigate this, avoid large horizontal surfaces, ensure smooth transitions, and prevent turbulence in the molten metal flow.
For castings requiring good sealing, care must be taken to avoid core supports that do not fuse properly with the casting, as this can lead to leaks. Additionally, because aluminum and magnesium alloys exhibit higher shrinkage and lower elastic modulus, they are more susceptible to deformation and cracking. Therefore, structural design should focus on minimizing stress concentration and improving rigidity.
This includes using cross-sections like I-beams, slots, or boxes to increase moment of inertia, designing reinforcing ribs appropriately to avoid stress concentration, and avoiding sharp edges around holes by using flanges. In wheel castings, reinforcing ribs help connect the hub and web, while sloped connections between thick walls and cast parts reduce stress.
Thin-walled castings can be strengthened with ribs or stepped sections, and bolts should be used strategically—smaller diameter bolts spread force more evenly, preventing deformation. Large nuts and washers can also help reduce pressure on bosses.
For shell-shaped castings, avoid abrupt shapes to prevent stress concentrations. Since these alloys have limited elasticity, press-fit designs require careful clearance calculation, and threaded holes should be longer than those in cast iron or steel. For aluminum, L/d = 2, and for magnesium, L/d = 2.5.
Magnesium alloys have higher compressive strength than tensile strength, so asymmetrical cross-sections are often used to take advantage of this property. Also, magnesium castings are prone to corrosion, so surface treatments are essential. Avoid water-trapping pockets in parts like water pumps, and eliminate sharp corners that could damage protective coatings.
Finally, consider using bimetal or insert casting techniques to overcome structural challenges in aluminum and magnesium alloy designs. These strategies help improve both functionality and durability.
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