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Aerospace Aluminum Casting Tolerances

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Aerospace Aluminum Casting Tolerances

In pursuit of minimizing weight, aircraft components like manifolds, adapter ports and housings are brought closer to their design ideal, but also demands less variability in manufacturing. Casting is an important process for weight reduction, but a thorough understanding of casting tolerances is needed to ensure factors of safety are maintained.

Using Aluminum in Aerospace

Aluminum has a very high specific strength, meaning it can be made into parts that are light but strong. It also resists corrosion and fatigue. Some modern composites offer superior performance, but carry a significant cost penalty when compared to aluminum. It can also be difficult to verify the structural integrity of composites. For these reasons, modern aerospace structures still use aluminum extensively in both wrought and cast forms.

Aluminum is almost always used as an alloy. The main alloying elements used are silicon, copper, magnesium and zinc. These influence properties like corrosion and fatigue resistance and castability.

Aluminum alloys are defined in terms of series numbers that indicate the primary alloying element. A356.0 for example, an aluminum alloy used extensively in aerospace, is 92% aluminum and 7% silicon, with the balance comprising magnesium, iron, copper and other elements.

Casting Aluminum for Aerospace Applications

Aluminum is used extensively in modern aircraft. Brackets, housings, housing covers, pump covers, manifolds and port adapters are just a few of the many applications. Casting is a way of combining parts that would otherwise be welded or bolted together. Bolts can come loose and allow some movement. Welding some grades of aluminum is difficult and may necessitate the use of nondestructive evaluation techniques to verify joint integrity.

One way of reducing weight is to minimize wall thicknesses. This is achieved through precise placement of cores that form voids in the final part. For quality parts meeting tight tolerances over long production runs, core positioning is a critical aspect of the casting process.

Casting processes differ in their ability to provide geometric consistency. In addition, some need draft angles to let patterns release from molds increases wall thickness and can complicate machining.

Three casting processes used by Impro for aerospace parts are:

  • Investment casting – where a wax pattern is coated with ceramic slurry to produce a mold
  • Permanent mold casting – uses metal dies, similar to injection molding or die casting
  • Shell molding – forms a mold cavity in two halves by coating metal half patterns with sand.

Draft Angles

The investment casting process rarely needs draft angles added to the part being cast. This is mostly because the ceramic shell is broken away from the metal after solidification. In addition, draft angles are seldom needed in the wax patterns as these tend to shrink enough to release easily from the mold.

Permanent molds usually need 2° of draft when casting aluminum to ensure easy release. Shell molding requires 1° of draft for the pattern to come away from the sand shell.

Casting Tolerances

Tolerances are defined primarily as the maximum allowable or expected deviation in form per linear inch. On this basis, achievable tolerances for each casting process are:

  • Investment casting – +/- 0.010
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