To reduce weight and improve efficiency, the latest generation of passenger airliners, like the Boeing 787 and Airbus A350, are using more composite materials than aluminum. Aluminum is easier to work with though, and significantly cheaper, so it will remain an important aerospace material for the foreseeable future, especially in applications less weight and fatigue-resistance critical.

Two areas in particular that will be using aluminum for a long time are avionic housings and interior components. These are usually cast, followed by limited machining of mounting surfaces and holes. Here’s a closer look at the casting processes and their advantages.

Materials for Mass Reduction

Mass is the top issue in aerospace design and manufacture. The lighter the structure, the smaller the engines, the higher the efficiency, and the greater the payload. Mass is minimized by selecting materials with high strength-to-weight ratios, otherwise referred to a specific strength or strength per pound.

Carbon fiber-reinforced plastic, (CFRP), is both strong and light, and usage in aircraft structures is rising. However, it’s also expensive to produce and use, leaving the door open for other materials like aluminum.

Titanium is another material with high specific strength. Again though, it’s expensive and difficult to cast and machine.

Magnesium and aluminum have similar strength-to-weight ratios, but aluminum is more corrosion-resistant and doesn’t pose the fire risks of magnesium. It’s produced in a wide range of alloys, classified into seven series. The 7xxx series is often referred to as aerospace aluminum, although other alloys are also used, depending on specific application needs.

An additional beneficial characteristic of aluminum is that it can be anodized. This is a method of increasing levels of surface oxidation to raise hardness and impart an attractive appearance, without adding a coating.

Casting Methods for Aluminum Aerospace Components

Minimizing mass requires a casting process capable of producing thin sections and fine detail. This excludes sand casting from consideration, leaving:

• Investment casting
• Shell molding
• Permanent mold casting

Here’s a look at the relevant characteristics of each

Investment casting

An expendable mold process where a ceramic mold “shell” is formed over a wax pattern. Investment casting is for parts weighing up to 30 lbs and can produce complex shapes with thin walls and re-entrant features while maintaining tight tolerances and forming smooth surfaces. The complexity of the process means it comes into own when it’s important to minimize both machining and mass.

Shell molding

In this process a pattern is coated with a sand resin outer shell. The mold is usually made in two halves which are joined to form the mold cavity. Once the metal has solidified the mold is broken apart. Shell molding can produce larger parts than investment casting, but is limited in terms of the level of detail and complexity. Surface finish is not as good as that from investment casting.

Permanent mold casting

This uses metal dies to form the parts, with the liquid metal flowing in under gravity. Short cycle times are possible, and rapid cooling increases part strength, but the tooling is expensive. Together, these make permanent mold casting a high volume process. Furthermore, it’s not capable of the fine detail that investment casting can achieve.

Casting in Aluminum for Aerospace

Aluminum provides the optimal balance of mass, part complexity and cost for avionics housings and interior components like brackets and seat furnishings. Grades such as the 7xxx series offer high strength, along with good machinability and corrosion-resistance.

These parts, and others like hydraulic pump bodies, are readily produced by casting, which supports weight-reduction goals while minimizing machining. Investment casting achieves the highest accuracy and complexity but shell molding and permanent mold casting are alternatives, depending on part geometry and the quantities required.

If you need a source for quality cast metal aerospace parts, we have the resources and expertise you need. Contact us for a discussion or to request a quote.