Materials Matter: Choosing the Right Alloy for Investment Casting
Investment casting works well for complex parts with fine features. Being near-net shape, it minimizes secondary machining, which can lower costs and reduce lead time. It’s not ideal for all metal alloys though. This blog discusses what’s involved in choosing the right alloy for a part to be made by investment casting.
Consider the Application First
Requirements for strength, creep-resistance, corrosion-resistance, and appearance are usually set by how the part being cast will be used. For example, many gas turbine applications prioritize creep-resistance and high-temperature strength. Medical devices for implantation need bio-compatibility, and parts going into marine applications need excellent corrosion-resistance. Preferred alloys for these applications are, in order, nickel superalloys, titanium and stainless steel.
Only when a shortlist of alloys is available should casting be considered.
Suitability for Investment Casting
Reasons for choosing investment casting over other methods are:
- Ability to produce thin sections
- Reproduction of fine detail
- Excellent surface finish
- Maintains tight tolerances
- No draft angles or parting lines
These characteristics result from how the mold is made. The process begins with a wax pattern that replicates the part. After attaching wax runners, gates, and in the case of smaller parts, additional patterns, the assembly is coated with a silica sol ceramic slurry. This dries to form a hard shell, after which the wax is melted out to leave the mold cavity.
Metal is poured in and allowed to solidify, then the ceramic shell is broken apart and individual parts are separated from the metal feeding system.
This multistep process reduces the finishing operations needed, which, depending on part design, can more than offset the additional costs.
A few alloys can react when they come into contact with the ceramic shell. This occurs mostly when the silicon content is high. The aluminum casting alloy, A390, which contains 17% silicon, is an example of a metal where a discussion with the foundry is recommended before finalizing the material choice.
Other alloys that need careful consideration before deciding to investment cast are zirconium and those with high levels of molybdenum. Investment casting is usually possible, but the foundry may wish to modify the process or apply some different controls.
Some alloys, particularly superalloys, become very reactive when heated above their melting point. This can result in oxides forming in the cast part. To avoid this, foundries cast them under vacuum.
“Castability” is a term foundry expert use to describe how easy or difficult a metal is to cast. It relates primarily to fluidity or viscosity, which determines how well it flows into the mold. Cooling and shrinkage behaviors are also part of castability.
Castability is especially important when parts have thin walls and fine detail. Alloys known for good fluidity, and therefore good for investment casting such parts are:
- Aluminum (some high silicon aluminum alloys have excellent castability, but compatibility with the ceramic shell must be considered)
- Low carbon steels
- Nickel-based alloys
Alloys that are more difficult to cast include stainless steel, some tool steels, and some grades of titanium. It is important to stress that these can still be investment cast, but the foundry will need to maintain tight control over the process, possibly including casting under vacuum.
Partner with the Casting Experts at Impro
Investment casting is preferred when parts have fine detail and intricate features, and to minimize secondary machining. Almost all alloys can be investment cast, but a small number pose challenges for the foundry.
When choosing the alloy to make a part from, application requirements should come first. However, once a shortlist of candidate materials is available, a discussion with the intended casting partner or foundry is recommended.
Impro has extensive investment casting expertise and works with many alloys, including those used in aerospace, automotive, energy, and the medical industry. Using the investment casting process, we produce parts in stainless steel, alloy steel, superalloys, aluminum, and copper. Please contact us to discuss your investment casting needs.