Parts destined for aerospace applications must have good tensile and fatigue strength, yet be lightweight and corrosion resistant. These requirements drive part design and material choices, which go hand-in-hand with manufacturing process selection.

For many aerospace parts, investment casting provides the best combination of capabilities and total manufacturing costs. In this blog we’ll explain why.

Aerospace Requirements

The need to reduce mass pushes engineering teams to chose high specific strength alloys like titanium and 7000 series aluminum. Mass is further minimized by designing complex parts that avoid the need for welds or fasteners. Thin walls and webs are typically used for strength and stiffness and tolerances are tight.

Corrosion-resistance is important in some applications too. This adds stainless steel and superalloys like Inconel to the list of candidate materials.

Manufacturing Challenges of Aerospace Materials

Many of the high strength alloys preferred for aerospace applications have poor machinability. In addition, bad chip formation characteristics tend to wear cutting inserts rapidly.

Features like deep pockets with tight internal radii need long thin cutting tools, which tend to deflect under load. At the same time, cutting forces can deform thin walls, making it hard to maintain tight tolerances. Other features, like cooling channels and galleries, are impossible to machine and push designers towards assemblies, which add weight and cost.

What’s needed is a process capable of producing complex shapes that are very close to the final dimensions required. This is where investment casting comes in.

Overview of the Investment Casting Process

Casting involves pouring molten metal into a cavity the shape of the part being made. The metal solidifies, the part is removed, and then usually goes for machining.

In investment casting the mold is made by coating a wax pattern with layers of ceramic slurry. The pattern is then melted out from this “shell”, which overcomes one of the biggest limitations of other casting processes: the need to remove the pattern. Molten metal is then poured in and allowed to cool. Once the metal is “frozen”, the ceramic shell is broken open to release the part.

Hollow internal regions are formed by placing cores in the wax pattern. These are made from soluble materials so they can be washed out from the cast part.

Reasons for Preferring Investment Casting for Aerospace

Investment casting offers numerous advantages for producing metal aerospace parts.

• Can produce complex geometries with thin walls, which minimizes secondary machining, and enables part consolidation
• Capable of holding tight tolerances
• Reproduces fine detail
• No draft angles needed (the sloping vertical walls other casting processes need so the pattern can be lifted out)
• Can produce re-entrant features
• Smooth surface finishes
• Cooling rates, and hence grain structure, strength and hardness, are controlled through variations in shell thickness

Limitations of Investment Casting

Investment casting is an expendable mold process, and mold production involves a series of steps. (Injection molding the wax pattern, pattern assembly, pattern coating, and burn-out (melting) of the wax.) However, for parts with complex geometries, especially those cast in hard-to-machine aerospace alloys, the benefits outweigh these costs.

A second limitation is the long lead time for sample and prototype part production. This can be compressed by using simulation to model metal flow and solidification and 3D printing the initial patterns.

Aerospace Applications of Investment Casting

Any metal part can be investment cast, up to a maximum weight of around 100 pounds (depending on alloy and design), but the process is usually preferred for smaller, more intricate forms. Examples of aerospace parts produced by investment casting include:

• Nozzle assemblies (often cast in superalloys)
• Valve mounts
• Manifolds
• Mounting brackets
• Flanges

Your Source For Investment Casting Services

Aerospace parts are usually designed with complex geometries that minimize mass and often produced in hard-to-machine alloys. Together, these give engineering teams no option but to choose the investment casting process. With its ability to reproduce complex shapes and thin walls, investment casting minimizes machining while also avoiding the need to join smaller pieces together.

As a leader in investment casting, Impro produces many different parts for customers in the aerospace industry. If you need a source for complex, high precision, near-net shape metal components, we can help. Contact us for a quote or to discuss your application.