Aerospace Castings: Processes, Classes, Inspection and Callouts
An aerospace casting is defined by four choices on the drawing: the process and alloy specification, the AMS 2175 class that sets how much of each lot is inspected, the grade that sets how many imperfections are acceptable, and whether the casting is hot isostatically pressed. On certified aircraft, a casting factor from the airworthiness rules then sets how much extra strength margin the design carries.
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Choosing the casting process
The process follows from size, wall thickness, alloy and quantity. Investment casting covers most small and medium aerospace castings with thin walls and fine detail, sand casting covers large housings and low quantities, permanent mold suits repeat aluminum shapes, and vacuum melting and pouring is required for the nickel superalloys used in turbine hardware.
| Process | How it works | Where it fits in aerospace |
|---|---|---|
| Investment (lost wax) | Wax patterns, often injected into a metal die, are assembled on a sprue tree, dipped and stuccoed into a ceramic shell about 5 to 15 mm thick, dewaxed, fired and poured | Brackets, housings, valve bodies, impellers, turbine blades and vanes, thin wall structural castings |
| Sand | Metal poured into a mold of bonded sand around a pattern, with sand cores for internal passages | Large gearbox and pump housings, prototypes, low quantities where tooling cost must stay low |
| Permanent mold | Metal poured into a reusable metal mold, sometimes with sand cores | Repeat aluminum parts of moderate complexity where the mold cost is spread over quantity |
| Vacuum investment | Alloy melted and poured under vacuum into an investment shell | Nickel and cobalt superalloys; turbine blades cast equiaxed, directionally solidified or single crystal |
For a sense of investment casting capability, published general figures give typical tolerances of about 0.1 mm over the first 25 mm plus about 0.02 mm per additional centimeter, as-cast surface finish of 1.3 to 4 micrometers (50 to 125 microinch) RMS, and sections from about 0.6 mm to 75 mm. Foundries publish their own tolerance tables, and those govern your quote. Any datum, bore, sealing face or thread on a casting is normally machined, so the casting drawing should carry machining stock on those surfaces.
AMS 2175 classes and grades
Class answers how much inspection, grade answers how good the metal must be. The military document AMS-STD-2175 set this system; SAE lists it as cancelled in July 2003, the same month SAE issued AMS 2175, Castings, Classification and Inspection of, which carries the class and grade system forward. AMS 2175B, revised 25 April 2024, is the current revision, and its scope is the nondestructive testing methods, sampling frequency and acceptance criteria for metal castings. Many drawings still say AMS-STD-2175 or MIL-STD-2175, so confirm with the design authority that AMS 2175 at a stated revision is acceptable.
The class and grade definitions are in the paid document. The summary below is from a foundry's published class and grade cheat sheet (Milwaukee Precision Casting, 2016) that references the military standard's tables. Check it against the revision your program invokes.
| Class | Inspection, per the foundry summary |
|---|---|
| Class 1 | 100 percent visual, 100 percent penetrant or magnetic particle, 100 percent radiography |
| Class 2 | 100 percent visual, 100 percent penetrant or magnetic particle, radiography sampled per Table I |
| Class 3 | 100 percent visual, 100 percent penetrant or magnetic particle, radiography sampled per Table II |
| Class 4 | 100 percent visual, penetrant or magnetic particle sampled per Table I, no radiography unless specified |
| Grade | Intended use, per the foundry summary |
|---|---|
| Grade A | High stress, critical application |
| Grade B | Critical application, low margin of safety |
| Grade C | High quality grade, average margin of safety |
| Grade D | Low stress application |
An Advanced Technologies International paper for the defense metalcasting program (2018) adds a useful point: AMS 2175 grades correlate with the cyclic life of the casting's highly stressed surfaces, and the standard is silent on weld repair. It grades only the imperfections found by radiography, penetrant, magnetic particle and visual inspection after all finishing, including any foundry weld rework and final heat treatment. If a weld repair passes the specified grade, the standard treats the casting as acceptable.
Zoning the drawing
A casting rarely needs the same grade everywhere. Zone the drawing so the highly stressed fillets and bosses carry Grade A or B and the rest carries C or D. Uniform Grade A on a large housing drives rejection rate and cost without improving the part where it matters.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Radiographic inspection: E1742, E155 and E192
Radiography of castings uses two kinds of documents, and mixing them up is the most common inspection callout error. ASTM E1742 is the practice: it sets minimum requirements for radiographic examination and states that it controls the quality of the radiographic images, not the acceptance criteria for the part. Acceptance comes from reference radiographs that show what each discontinuity type looks like at increasing severity levels, plus the grade on your drawing. The nondestructive testing page covers method selection and personnel qualification.
| Document | Covers | Notes from the published scope |
|---|---|---|
| ASTM E1742/E1742M | Radiographic examination practice for metallic and nonmetallic materials | Controls image quality; acceptance criteria come from elsewhere |
| ASTM E155 | Reference radiographs for aluminum and magnesium castings | Illustrates castings 1/4 in and 3/4 in thick; 13 aluminum plates and 10 magnesium plates; film adjunct bought separately; E2422 (aluminum) and E2869 (magnesium) are the digital equivalents. E155-20 is the active edition |
| ASTM E192 | Reference radiographs of investment steel castings for aerospace | Thin wall steel castings to close tolerances, sections up to 1 in; six graded discontinuity types at eight severity levels plus twelve ungraded illustrations |
Three practical points. The reference set must match the alloy, thickness and radiographic technique, so a 2 in thick aluminum boss is outside what E155 illustrates unless the purchaser and foundry agree it applies. If the foundry uses digital radiography, the acceptance references change to the digital reference image standards. And the radiographic technique sheet, with film or detector, views and coverage, should be approved before production, because shooting angles that miss a critical fillet cannot be fixed by reading the film harder.
HIP for castings
Hot isostatic pressing closes internal shrinkage and gas porosity in a casting by holding it at high temperature under inert gas pressure. Published summaries describe argon as the usual gas, pressures from about 7,350 to 45,000 psi with 15,000 psi (100 MPa) the most common, and soak temperatures from about 900 F for aluminum castings to about 2,400 F for nickel superalloys. The process eliminates internal voids and microporosity through plastic deformation, creep and diffusion bonding, and improves fatigue resistance.
ASTM A1080 is the practice for hot isostatic pressing of steel, stainless steel and related alloy castings. For aluminum and superalloy castings, HIP requirements usually come from the material specification, the prime's process specification or the drawing. Points to settle on the PO:
- Whether HIP is required, and the specification and cycle or the supplier's qualified cycle
- The sequence: HIP normally comes before the final solution and age, because the HIP thermal cycle disturbs the heat treated condition
- Radiography after HIP if acceptance is to be judged on the HIPed casting
- Surface oxidation removal, since titanium and nickel castings can pick up oxidation in the HIP vessel that must be machined off
- A HIP certification with vessel, load, temperature, pressure and time
HIP does not replace good foundry practice. It is a way to raise fatigue performance and reduce rejection on castings that are already sound at the surface.
Common aerospace casting alloys and their specs
The alloys below appear on many aerospace casting drawings: aluminum-silicon-magnesium alloys for housings and structure, 17-4 precipitation hardening stainless for strong corrosion resistant castings, and 718 type nickel alloy for hot section and high strength castings.
| Alloy | Specification | What the published scope says |
|---|---|---|
| A356 | AMS 4218 | Aluminum alloy castings, premium grade, 7Si-0.3Mg (A356); issued 1961, revision A reaffirmed October 2015 |
| A357 | AMS 4219 | 7.0Si-0.55Mg-0.12Ti-0.06Be (A357.0), solution and precipitation heat treated; applies to sand, permanent mold, composite mold and investment castings for high strength parts such as hydraulic pump and structural components |
| High strength aluminum castings | AMS-A-21180 | Allows the purchaser to call out no weld zones or require written permission for foundry weld rework |
| 17-4 PH | AMS 5344 | Corrosion resistant steel investment castings, homogenized, solution and precipitation heat treated to 180 ksi (1241 MPa) tensile strength |
| 718 type nickel alloy | AMS 5383 | Investment castings, 52.5Ni-19Cr-3.0Mo-5.1Cb-0.90Ti-0.60Al-18Fe, vacuum melted, homogenized and solution heat treated; for parts needing strength up to 1300 F and oxidation resistance up to 1800 F; revision F |
NASA MSFC-STD-3029 rates 356.0 and A356.0 castings highly resistant to stress corrosion cracking in all tempers, which makes these alloys a reasonable choice for hardware exposed to salt air; the aluminum alloys page has the rating context. When properties are verified on test bars, state whether the bars are separately cast, integrally cast or cut from the casting, and where. For 17-4 and 718, the heat treat condition after any welding or HIP must be named; see stainless and specialty steels and nickel superalloys.
Casting factors in design
A casting factor is an extra multiplier on the design loads for cast structure, and the inspection plan earns a lower factor. For transport category aircraft, 14 CFR 25.621 sets the rules, and they show directly how inspection trades against weight.
| Casting type and factor | Inspection required (each casting) | Static test |
|---|---|---|
| Critical, 1.25 or greater | 100 percent visual and penetrant or equivalent on surfaces, radiographic or equivalent on internal areas | Three castings, ultimate load at a 1.25 factor, deformation at 1.15 times limit load |
| Critical, 1.50 or greater | Same as above | One casting, ultimate load at a 1.50 factor, deformation at 1.15 times limit load |
| Critical, 1.0 or greater | Same as above, plus process qualification, proof of product and process monitoring showing property variation equivalent to wrought products | One casting to the strength and deformation requirements of 25.305 |
| Non critical, 1.25 or greater | 100 percent visual and penetrant on surfaces, radiographic internal inspection | None listed |
| Non critical, 1.5 or greater | 100 percent visual and penetrant on surfaces | None listed |
| Non critical, 2.0 or greater | 100 percent visual only | None listed |
A critical casting under the rule is one whose failure could preclude continued safe flight and landing or cause serious occupant injury. Casting factors need not exceed 1.25 for bearing stresses. Other aircraft categories, launch vehicles and spacecraft have their own structural design standards, so the stress analyst should state the factor used and the inspection it assumes. The point for purchasing is that the class and grade on the drawing are part of the structural justification; relaxing them to save cost changes the analysis.
What to call out on the drawing or PO
- Alloy, material specification and revision, and heat treat condition at delivery
- Casting process if it is constrained, or state that it is the supplier's choice
- AMS 2175 class and grade, zoned where grades differ, with the revision
- Radiographic practice and reference radiographs (for example ASTM E1742 with E155 or E192), and digital equivalents if allowed
- Penetrant or magnetic particle requirements and acceptance
- Test bar type, location and the properties to be reported
- HIP requirement, specification and sequence relative to heat treatment
- Weld repair: allowed, allowed with documentation, or prohibited in named zones
- Foundry first article: technique sheets, sectioning and dimensional layout approved before production, then frozen
- Marking, serialization and traceability to melt or heat lot
- DFARS 252.225-7009 specialty metals compliance for steel, nickel and cobalt castings on defense contracts
For machined castings, the first operation should pick up datum targets defined on the casting drawing, and the machined part receives its own AS9102 first article. Compare with forging when the part carries high cyclic loads, and with metal additive manufacturing for low quantity complex shapes.
Get castings sourced and machined
A finished casting usually involves a foundry, a HIP or heat treat source, a radiography lab and a machine shop. Send the casting and machined drawings with class, grade and alloy callouts, and we source the chain from qualified suppliers, flow down the inspection and weld repair requirements, check the radiographic reports and certs against the PO, and deliver with a certificate package on one PO. A person replies within one business day, and a mutual NDA comes before files. If the drawings are export controlled, follow the controlled programs process. Back to the processes hub.
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Questions
What is the difference between casting class and casting grade?
Class sets the inspection effort, meaning how much of the lot is radiographed and surface inspected, and is tied to how severe the service is. Grade sets the acceptance level, meaning how much porosity, shrinkage or inclusion is allowed in the inspected area. A Class 1, Grade B callout means every casting is radiographed and judged to a strict acceptance level. Both come from AMS 2175, issued when AMS-STD-2175 was cancelled.
Is AMS-STD-2175 still current?
No. SAE lists AMS-STD-2175 as cancelled in July 2003, the same month it issued AMS 2175, Castings, Classification and Inspection of, which carries the class and grade system. The current revision is AMS 2175B, revised in April 2024. Many legacy drawings still cite AMS-STD-2175 or MIL-STD-2175, so ask the design authority to confirm AMS 2175 at a stated revision before ordering, and record that on the PO.
Does ASTM E1742 tell the inspector what to reject?
No. ASTM E1742 controls how radiographs are made and the quality of the images. Its scope states it is not intended to establish acceptance criteria for parts. Acceptance comes from the grade on the drawing judged against reference radiographs such as ASTM E155 for aluminum and magnesium castings or ASTM E192 for thin wall steel investment castings, or from the customer's own acceptance standard.
Should aerospace castings be HIPed?
HIP is worth specifying on fatigue loaded castings and on alloys that are prone to internal shrinkage porosity, because it closes internal voids and improves fatigue resistance. It adds cost, a thermal cycle and an outside process step. Put the HIP before final heat treatment, decide whether radiography is judged after HIP, and require a cert with temperature, pressure and time. For low stress Class 3 or 4 castings it is often unnecessary.
Can a foundry weld repair an aerospace casting?
Often yes, unless the drawing or material spec forbids it. AMS 2175 does not address weld repair; it grades the finished casting after all finishing and heat treatment. AMS-A-21180 for high strength aluminum castings lets the purchaser define no weld zones or require written permission. If you restrict repairs, do it by zone on the critical surfaces only, because blanket bans raise cost and lead time.
What is a casting factor?
It is a multiplier applied to design loads for cast structure to cover the extra variability of castings. Under 14 CFR 25.621 for transport aircraft, factors range from 1.0 to 2.0 depending on whether the casting is critical and how thoroughly each casting is inspected and tested. More inspection earns a lower factor and a lighter part. Your stress analyst should state the factor and the inspection it relies on.
Related
Sources
- SAE Mobilus: AMS 2175B, castings, classification and inspection of
- SAE Mobilus: AMS-STD-2175 record
- Milwaukee Precision Casting: AMS-STD-2175 class and grade cheat sheet
- Advanced Technologies International: in-process weld rework and AMS 2175 (2018)
- ASTM E1742/E1742M radiographic examination
- ASTM E155-20 reference radiographs, aluminum and magnesium castings
- ASTM E192 reference radiographs, investment steel castings for aerospace
- ASTM A1080 hot isostatic pressing of steel and related alloy castings
- Hot isostatic pressing, process parameters
- Investment casting, process and typical tolerances
- SAE Mobilus: AMS 4218 premium grade A356 castings
- DIN Media: SAE AMS 4219F A357.0 castings
- ANSI Webstore: SAE AMS 5344H 17-4 investment castings
- SAE Mobilus: AMS 5383F nickel alloy investment castings
- eCFR via Cornell LII: 14 CFR 25.621 casting factors
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