Stainless Steel Investment Casting and Its Aerospace Applications
Stainless steel is preferred for many aircraft engine, structural, fuel and hydraulic system applications. Providing a combination of strength and corrosion-resistance, it supports weight reduction goals and facilitates factors of safety of 1.5.
Part mass is minimized by investment casting. This blog addresses what’s involved in investment casting stainless steel, discusses some typical aerospace applications of stainless, and reviews requirements for finishing after casting.
Investment Casting High Melting Point Alloys
Stainless steels like 17-4 PH (a precipitation hardening grade widely used in aerospace) melt at around 2,600 °F (1,427 °C). At such temperatures, it’s vital to avoid thermal shock and cracking the mold. In addition, cooling too quickly can have negative effects on the crystalline structure of the part being cast.
Investment casting uses a ceramic shell mold, produced by coating a wax pattern replicating the part to be made. Once the coating is dry the wax is melted out to leave the cavity that metal is poured into. To increase productivity when casting small parts, several are assembled on a central runner to form a “tree”.
To prevent cracking the mold is preheated, which reduces the rate of cooling and solidification.
An important consideration with stainless steel is the coefficient of thermal expansion. Although this varies between grades, in general stainless expands more than carbon steel. This must be taken into account during pattern design and tooling manufacture.
Benefits of Investment Casting for Aerospace Components
The investment casting process is more accurate than other expendable mold casting methods and is capable of forming thin sections and reproducing fine detail. Complex internal cavities and features are formed in the parts being cast by placing cores in the wax pattern and the ceramic shell mold.
Together, these enable casting of near net-shape parts. This reduces material consumption and the amount of metal removal needed after casting.
Many of the stainless steels used in aerospace have good machinability, unlike nickel-based superalloys. Consequently, the cost savings from reducing machining of stainless steel are smaller, although still appreciable. Of greater importance though is the ability to minimize part mass.
Finishing of Aerospace Components
Exacting tolerances and surface finish specifications mean investment cast aerospace components almost always require a degree of precision machining. Many will also need heat treatment to develop the required levels of strength, toughness and surface hardness.
Inspection is an additional requirement, with many aerospace components undergoing extensive non-destructive evaluation to check for cracks and internal defects.
Manufacturing Stainless Steel Aerospace Components at Impro
We cast a wide range of alloys. This includes the aerospace grades of stainless steel such as 17-4 PH as well as 304 and 306. Maximum cast part weight is 100 pounds and the largest parts that can be cast must fit within an envelope of Ø23.6 x 23.6 x 23.6”.
Precision machining capabilities include CNC turning, milling and grinding as well as 5-axis machining. Heat treatment and surface treatment services are also available for stainless steel parts produced by investment casting.
Inspection capabilities include X-ray analysis, fluorescent penetrant inspection and magnetic particle inspection. Quality certifications extend to AS9100 and NADCAP certification for special processes, welding, heat treatment, surface treatment, and materials testing.
Typical Stainless Steel Aerospace Components
Stainless steel is used extensively in aircraft structures, engines, fuel, and hydraulic systems. Parts produced by investment casting include:
- Valve mounts
- Swirl nozzles
- Engine rings
- Cooling tubes
- Mounting brackets and flanges
- Retainer seals
- Seat components
Your Partner for Aerospace and Stainless Steel Products
Slim safety factors, a need for corrosion resistance, and strict goals for mass minimization pose particular challenges for manufacturers of aerospace components. Impro Aerotek has significant expertise with the alloys used in aerospace, along with investment casting, precision machining, and inspection, and fully understands the quality requirements. If you need a source of high-quality stainless steel parts, we can help. Contact us for details.