Aerospace Sourcing
Process comparison

Casting vs Forging for Aerospace Parts

Choose a casting when the part has internal passages, complex contours or thin walls that forging cannot fill, and the design can carry a casting factor and radiographic grading. Choose a forging when fatigue, impact or pressure loads run along the contour. For one to a few parts, a part hogged from plate or bar often beats both.

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Casting and forging side by side

A forging trades geometric freedom for wrought properties and grain that follows the part; a casting trades some property margin and more inspection for near net shape. The process attributes below come from the cited standards and published sources; our casting and forging pages cover each process in depth.

AttributeCastingForging
Grain structureSolidified structure with no worked grain direction; properties roughly the same in every directionWorked structure; grain elongated along the direction of working, strongest longitudinally and weaker transversely
Typical internal defectsShrinkage and gas porosity, inclusionsLaps, folds, flow through at the flash line; ingot porosity is worked closed rather than created
Inspection frameworkAMS 2175 class (how much is inspected) and grade (how much imperfection is accepted), radiography to reference radiographsUltrasonic class (for example ASTM E2375 or AMS-STD-2154), macroetch of grain flow, AMS 2375 first article control
Design penalty on certified aircraftCasting factor of 1.0 to 2.0 under 14 CFR 25.621, set by criticality and inspectionNo casting factor; wrought allowables in the loaded grain direction
Porosity closure optionHot isostatic pressing (for example ASTM A1080 for steel castings)Not normally needed
Shape freedomCored internal passages, undercuts, variable thin wallsShapes that can be worked in a die or between open tools; needs draft, radii and machining stock
ToolingPattern or wax injection die; sand patterns are cheap, investment dies cost moreOpen die: little custom tooling. Closed die: a machined die set with long lead time
First articleFoundry technique and sectioning approval, then the machined part AS9102AMS 2375 forging approval, often with a sacrificial forging for macroetch, then the machined part AS9102

Property comparison with the source for every number

For the same alloy, wrought forging specifications carry higher minimum strength and ductility than casting specifications, and published test data show the fatigue gap is larger than the tensile gap. Every value below is a specification minimum or a reported test result as labeled. Design to the minimums in the specification revision your program invokes, or to approved allowables, not to this table.

AlloyCast specification and stated minimumsWrought or forged specification and stated minimums
Ti-6Al-4VAMS 4985, investment castings, hot isostatically pressed, anneal optional: 130 ksi tensile, 120 ksi yield, 6 percent elongation, minimum, per the SAE title (revision E, 2018, is the latest SAE lists)AMS 4928, bars, wire, forgings and rings, annealed: 120,000 psi yield per the SAE title; tensile and elongation minimums vary by section size in the specification table
Alloy 718AMS 5383, investment castings, vacuum melted, homogenized and solution treated, for parts needing strength up to 1300°F: minimums are in the paid specification; confirm the current revision on ASSIST or SAEAMS 5663, bars, forgings and rings, solution and precipitation heat treated: 185 ksi tensile, 150 ksi yield, 12 percent elongation, 15 percent reduction of area, minimum, as published by Premium Alloys
17-4 PHAMS 5344, investment castings, heat treated to 180 ksi tensile, per the specification titleWrought 17-4 bar and forging specifications set minimums by H condition; see our 17-4 vs 15-5 PH comparison

What test data add

A US Bureau of Mines study (Report of Investigations 8897, 1984) cast Ti-6Al-4V in zircon sand molds and compared it with the AMS 4928H wrought requirements. Cast and annealed material exceeded the wrought tensile and yield minimums, yet none of the specimens met the 25 percent reduction of area minimum. HIP healed the internal porosity in the test specimens, but fatigue strength was not significantly improved, and both cast and cast plus HIP fatigue properties were significantly lower than those of wrought products. Charpy impact results compared favorably with the 17 ft-lbf cited for wrought and 19.2 ft-lbf cited for investment cast material.

The lesson for a buyer is that matching a tensile number does not make a casting equivalent to a forging. Ductility and fatigue are where castings give ground, and those are the properties that govern most rotating, cyclically loaded and pressure hardware.

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Grain structure, direction and why the loads matter

A forging is stronger along its grain and weaker across it; a casting has no strong direction but carries the risk of porosity anywhere. Which one is better depends on where the loads run.

A 2007 Gear Solutions article summarizing a Rexnord study makes the balanced case. Forging starts from a cast ingot whose porosity and segregation are worked out in the direction of working rather than removed, so properties are higher longitudinally and significantly lower transversely. In that study's comparison of a 5 inch cast low alloy steel plate with a 5 inch rolled 4340 plate, through thickness ductility was significantly higher in the casting, and notched fatigue specimens of cast steel outperformed the wrought plate in both directions. The article's conclusion is that cast steel is less notch sensitive than wrought steel, and that a poor product can come from either process.

That result is about heavy industrial steel sections, not thin aerospace forgings, but the principle carries over: a closed die forging is only better if the die and forging sequence put the grain along the load path. Lugs, clevis ears and flanges are where this pays off, and the forging drawing should say so with grain direction and parting line notes. A casting avoids short transverse weakness entirely, which is why a complex housing loaded in several directions can be a good casting application.

Casting factors earn back weight through inspection

On transport category aircraft, 14 CFR 25.621 applies a casting factor from 1.0 to 2.0 depending on whether the casting is critical and how each casting is inspected and tested; a factor of 1.0 on a critical casting requires process qualification and monitoring showing property variation equivalent to wrought products. A forging carries no such factor. The table of factors and their inspection requirements is on our casting page; the point here is that the casting's weight and its inspection plan are linked, and comparing a cast part to a forged part means comparing the factored casting design to the forging design.

Inspection: casting classes and grades vs forging ultrasonic classes

Castings are graded mostly by radiography of the finished shape; forgings are graded mostly by ultrasonic inspection at a simple intermediate shape plus macroetch of a first article. The paperwork and cost differ accordingly.

  • Castings. AMS 2175B (revised April 2024) sets class, the sampling of radiography and surface inspection, and grade, the acceptance level. Radiography follows ASTM E1742 for technique and reference radiographs such as ASTM E155 or E192 for acceptance. Class 1 means every casting is radiographed, and that cost scales with every piece.
  • Forgings. Ultrasonic classes from AAA (tightest) to C are defined in ASTM E2375 and AMS-STD-2154; NASA JSC PRC-6504 Rev. G, as one program example, requires at least Class A on flight hardware. AMS 2375F governs first article approval and freezes the forging process once approved.

Two consequences matter on a quote. A casting with Grade A zones everywhere drives rejection rate and cost, so zone the drawing. A forging's ultrasonic inspection must be planned at a stage where the beam can reach every zone, usually rectilinear or rough machined, which ties the forge, the inspection source and the machine shop together in the routing. Method details are on our nondestructive testing page.

Where HIP closes the gap and where it does not

HIP narrows the casting versus forging gap for internal porosity, not for everything. It closes internal voids by holding the casting at temperature under inert gas pressure, and the casting page covers cycle, sequence and certification.

Specify HIP on a casting when:

  • The casting replaces a forging or a hogged part in a fatigue loaded location and internal porosity is the limiting defect.
  • The alloy is prone to shrinkage porosity and radiographic rejection rates are eating the schedule.
  • The material specification already calls for it, as the AMS 4985 title does for Ti-6Al-4V investment castings.

Do not expect HIP to make a casting a forging. The 1984 Bureau of Mines study found HIP healed internal porosity in Ti-6Al-4V sand castings without significantly improving fatigue strength, which stayed below wrought levels. HIP also cannot fix surface defects, coarse cast grain or segregation; for cast 718, an ATI paper notes niobium segregation in the cast condition is not easily reduced by homogenization. When a stress analyst needs wrought fatigue allowables, the answer is usually a forging or a part machined from wrought stock, not a HIPed casting.

Tooling, lead time and quantity break-evens

Both processes front load cost into tooling and first article, so the break-even is about how many parts share that cost. We do not quote numbers here because they vary with size, alloy and supplier; the shape of the curve is consistent.

  • One to a few parts. Tooling and first article dominate. Sand castings with simple patterns, open die hand forgings machined to shape, or parts hogged from plate or bar usually win. Closed die forging and investment casting tooling rarely pay back.
  • Tens of parts per year. Investment casting dies and closed die sets start to pay back when the part is complex or the alloy is expensive, because each part saves material and machining time. Open die and ring rolled forgings stay competitive for simple shapes.
  • Hundreds and up. Closed die forging and investment casting both amortize tooling well; the choice returns to properties, inspection cost per piece and machining stock.

Lead time drivers differ. A new casting waits on pattern or die making, foundry trials, technique approval, radiography and possibly HIP. A new forging waits on die design and sinking, billet in the right melt practice, press scheduling, heat treatment and testing, and AMS 2375 approval with a sacrificial forging. Both then add machining and the machined part's AS9102 first article. Our lead times and delivery page shows how outside processing stacks up on a schedule.

One cost driver is easy to miss: specification and source changes. Once a forging process is approved, a change of forge, die or sequence reopens first article, and casting technique sheets are frozen the same way. Choose the supplier chain with the full production quantity in mind, not just the prototype.

Part geometry: which shapes suit which process

Geometry often decides before properties do. Ask what the finished part looks like and how much of a simple block would be cut away.

GeometryUsual fitWhy
Housings with cored internal passages, pump and valve bodiesCastingCores form passages that would need deep drilling and plugging in wrought stock
Lugs, clevis fittings, links, landing gear and mount detailsClosed die forgingGrain follows the load path through radii
Rings, flanges, bearing races, tank and interstage ringsRolled ring forgingCircumferential grain without impression dies
Thin wall, highly contoured nonstructural or lightly loaded partsInvestment castingNear net shape with fine detail
Large simple blocks and shaftsOpen die forging or plateLittle or no custom tooling
Prismatic brackets and fittings in small quantityHogged from plate or barNo tooling, shortest start

When a part hogged from billet beats both

A part machined from plate, bar or a hand forging block usually wins when quantity is small, the schedule is short and the stress analyst accepts the grain orientation of the stock. It skips pattern, die and foundry or forge first article entirely.

  • Prototypes and development hardware. Design changes are cheap when they are only a program change; a die or pattern change is not.
  • Small production lots where tooling would never pay back, especially in aluminum, where machining is fast and material is relatively cheap.
  • Parts whose loads align with the stock's rolling direction, or are low enough that the transverse properties of plate are acceptable.
  • Stress corrosion sensitive aluminum, where a resistant temper such as 7075-T7351 plate is accepted; see 6061 vs 7075 and aluminum alloys for short transverse limits.

The costs show up as material and spindle time. Most of an expensive titanium or nickel billet can end up as chips, thick plate has lower properties at mid thickness and in the short transverse direction, and deep pockets in thick stock move as residual stress is released. When quantity grows or the alloy is expensive, the near net shape of a casting or forging starts to repay its tooling. Our should cost and total cost guide walks through that trade on a buy.

Choose casting when, choose forging when

Choose a casting when

  • Internal passages, undercuts or complex contours would make a forging or billet part mostly chips or multiple welded pieces.
  • Loads run in several directions and through thickness properties matter more than peak longitudinal strength.
  • The design carries the applicable casting factor, and the program accepts AMS 2175 class and grade inspection with radiography.
  • Fatigue is not the governing failure mode, or HIP plus zoning gives enough margin as shown by test or allowables.

Choose a forging when

  • The part is fatigue, impact or pressure critical, and loads follow a contour the grain can follow.
  • The stress analysis needs wrought allowables without a casting factor.
  • The shape can be worked in a die or between open tools with sensible draft and machining stock.
  • Quantity justifies a closed die, or the part is a ring or simple shape that open die or ring rolling makes without one.

If neither list fits cleanly, consider metal additive manufacturing for low quantity complex parts, or a welded or bolted assembly of simpler forged and machined details.

Callout examples for the drawing or PO

  • Cast Ti-6Al-4V housing: "Titanium alloy investment casting per AMS 4985, revision per PO, hot isostatically pressed. Castings classified and inspected per AMS 2175, Class 1, Grade B in Zone A, Grade C elsewhere. Radiography per ASTM E1742. Weld repair prohibited in Zone A. Foundry technique sheets approved before production."
  • Forged 718 fitting: "Nickel alloy forging per AMS 5663, revision per PO. Forging first article per AMS 2375 with macroetch of grain flow approved by engineering. Ultrasonic inspection per ASTM E2375, Class A minimum, Class AA in zone shown, at rough machined stage. Grain direction as shown at lugs."
  • Hogged aluminum bracket: "7075-T7351 plate per AMS 4078, grain direction as shown. Certified material test report with heat lot traceable to part."
  • All defense contract metal parts: DFARS 252.225-7009 specialty metals compliance where it applies; see DFARS specialty metals and material certs and traceability.

Confirm every specification revision with the design authority before release, and keep the casting or forging drawing and the machined part drawing linked so a change at the foundry or forge is visible in the machined part's baseline.

Get a casting, forging or billet option quoted

If you are deciding between processes, send the machined part drawing, material, quantity per year and any casting or forging drawing. We source the foundry or forge, heat treat, HIP, NDT and machining from qualified suppliers, flow down class, grade, ultrasonic and first article requirements, check the reports and certs against the PO, and deliver the machined part with its certificate package on one PO. A person replies within one business day, and we sign a mutual NDA before files. Export controlled drawings follow the controlled programs process. More process references are on the processes hub.

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Questions

Is a forging always stronger than a casting?

In its loaded grain direction, a forging in the same alloy usually has higher minimum strength and ductility, and published Ti-6Al-4V data show wrought fatigue strength well above cast. Across the grain the advantage shrinks, and a Rexnord study summarized by Gear Solutions found a cast steel plate had better through thickness ductility and notched fatigue than rolled plate. Compare the properties in the direction your loads actually run.

Does HIP make a casting as good as a forging?

No. HIP closes internal porosity, which improves consistency and often fatigue, but a 1984 Bureau of Mines study of Ti-6Al-4V sand castings found HIP healed porosity without significantly improving fatigue strength, which stayed below wrought levels. HIP does not change cast grain size or remove segregation or surface defects. Use it to raise casting quality, not to claim wrought allowables.

What is a casting factor and does a forging need one?

A casting factor is a multiplier on design loads for cast structure. Under 14 CFR 25.621 for transport aircraft it ranges from 1.0 to 2.0 depending on whether the casting is critical and how thoroughly each casting is inspected and tested. Forgings carry no casting factor. Comparing weights between a cast and a forged design is only fair once the casting factor and its inspection plan are included.

At what quantity does a forging die or casting tooling pay off?

There is no single number. It depends on part size, alloy price, how much machining the near net shape saves and the cost of first article approval. For one to a few parts, billet or an open die hand forging usually wins. Closed die and investment casting tooling starts to pay back with repeat quantities of complex or expensive alloy parts. Ask for quotes on both routes at your annual quantity.

When should I machine a part from plate instead?

When quantity is small, the schedule is short, the design may still change, and the stress analyst accepts the grain orientation and through thickness properties of plate or bar. It avoids tooling and foundry or forge first article. It costs more material and spindle time, and thick plate is weaker at mid thickness and in the short transverse direction, which matters for aluminum stress corrosion.

What inspection is typical for aerospace castings vs forgings?

Castings are usually inspected by visual and penetrant or magnetic particle methods plus radiography, at a sampling rate set by the AMS 2175 class and judged to the grade on the drawing. Forgings are usually ultrasonically inspected to a class such as A or AA under ASTM E2375 or AMS-STD-2154, with macroetch of a first article under AMS 2375. Both then get an AS9102 first article on the machined part.

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