Aerospace Forgings: Processes, Grain Flow, Specs and Callouts
Choose a forging when a part carries fatigue, impact or pressure loads that would cut across the grain of plate or bar: open die and rolled rings for large, simple shapes in low quantity, closed die for repeat parts, and isothermal or hot die forging for titanium and nickel parts that need near net shape. Control it with a material spec, a heat treat spec, an ultrasonic class and first article approval.
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Which forging process fits the part
The process is set by size, shape complexity and quantity, in that order. Open die work handles very large and simple shapes one at a time, closed die tooling pays off when the same shape repeats, and the heated die methods exist because titanium and nickel alloys do not flow well against cold steel dies.
| Process | How the metal moves | Typical aerospace output | Tooling and quantity |
|---|---|---|---|
| Open die | Worked between flat or simple dies that never fully confine the metal; swage dies, V dies, mandrels and loose tools shape the profile | Bars, blanks, hollows, shafts, preforms, very large billets for machining | Little or no custom tooling; one piece or a few hundred |
| Closed die (impression die) | Pressed or hammered into a shaped cavity in a die set | Fittings, brackets, links, landing gear and engine mount details, wheels | Custom die set with high cost and long lead time; pays back on repeat quantities |
| Rolled ring (one piece) | A preform is upset and pierced, then a ring mill thins the wall between a drive roll and an idler roll while the diameter grows | Engine cases, flanges, bearing races, tank and interstage rings | Ring mill plus simple tooling; custom sizes without impression dies |
| Hot die | Dies run hotter than conventional forging but cooler than the workpiece | Titanium and nickel alloy shapes closer to net than conventional closed die | Heated dies, longer die life than isothermal |
| Isothermal | Dies held at or near the workpiece temperature, slow strain rate | Thin web titanium and nickel superalloy parts, disks | Expensive die materials and atmosphere control |
Scot Forge, a large open die and ring producer, publishes rolled ring capacity up to 252 in outside diameter and 49 in face height, rolled ring weights to 60,000 lb and open die weights to 300,000 lb. Capacities differ widely between forge shops, so treat those as one supplier's published envelope, not an industry limit.
Grain flow: why a forging outperforms the same shape cut from plate
A forging is chosen for its grain flow. Hot working above the recrystallization temperature replaces the coarse cast grain of the ingot with finer grain, closes shrinkage and gas porosity left from casting, and lets the forger align the elongated grain with the contour of the part. A lug machined from plate cuts straight across the plate's grain at the radius; the same lug forged has grain that follows the radius.
That matters most for stress corrosion and fatigue. For high strength aluminum, the aluminum alloys page explains how sustained tension in the short transverse direction starts stress corrosion cracking. A closed die forging lets the designer keep the short transverse grain out of the loaded path. For rings, Scot Forge states that rings rolled and welded, or cut from plate, carry excess material and are more susceptible to fatigue than rolled rings with controlled grain flow. That is a producer claim, and it is the usual argument for specifying rolled rings on fatigue loaded rings and flanges.
Grain flow can also go wrong. Laps, folds and flow through, where metal folds over itself or the grain is forced across a thin section at the flash line, are forging defects that inspection must find. The grain pattern is commonly checked by sectioning and macroetching a first article forging, which makes forging first article approval a destructive event, not just a dimensional check.
What to put on the drawing about grain
- Show the grain direction requirement on any forging feature where load direction matters, typically lugs, clevis ears and flanges.
- Locate the parting line, or state where it is not permitted, if it would fall across a highly stressed radius.
- Require macroetch of the first article, and state who approves the grain flow pattern.
Isothermal and hot die forging for titanium and superalloys
Isothermal and hot die forging solve one problem: conventional dies chill the workpiece surface, so the skin flows less than the core and thin sections cannot be filled. The ASM Handbook article on isothermal and hot die forging (Volume 14A, 2005) describes the difference as die temperature.
| Method | Die temperature, per ASM Handbook Vol. 14A |
|---|---|
| Conventional hot forging | Dies typically warmed to about 95 to 425 C (200 to 800 F), tool steel dies in hammers or presses |
| Hot die forging | Die about 110 to 220 C (200 to 400 F) below the workpiece temperature |
| Isothermal, titanium alloys | Die at about 760 to 980 C (1400 to 1800 F) |
| Isothermal, nickel base alloys | Die at about 980 to 1200 C (1800 to 2200 F) |
At nickel superalloy temperatures the die itself becomes a refractory metal problem. Published patents on superalloy disk forging describe isothermal forging at roughly 1900 to 2000 F with TZM molybdenum dies, a combination that requires vacuum or inert gas because TZM oxidizes readily when hot. The cobalt and refractory alloys page covers TZM itself. The practical effect for a buyer is that isothermal capacity is concentrated in a small number of forge shops, die sets are expensive, and the press runs slowly, so these forgings are justified by material savings on expensive alloys and by thin sections that cannot be made any other way.
For the alloys themselves, see titanium alloys for alpha case after forging temperatures and nickel superalloys for 718 aging behavior.
The specifications that govern a forging
A forging is controlled by four layers: the alloy material spec for the forging product form, the heat treat process spec, an ultrasonic inspection requirement, and first article and production controls. The drawing or PO has to name each one, because none of them pulls in all of the others.
| Document | What it covers | Status as verified |
|---|---|---|
| SAE AMS 2375 | Control of forgings requiring first article approval: how a forging gets preproduction approval and the controls on later production forgings | Revision F, revised September 2022, listed as current by SAE; E is superseded |
| SAE AMS-F-7190 | Steel forgings for aircraft and aerospace equipment, special ordnance and related accessories | AMS-F-7190B, published 8 August 2016, listed as current by BSI; the military original MIL-F-7190 is cancelled |
| SAE AMS-STD-2154 | Process for ultrasonic inspection of wrought metals, which includes forgings | Issued March 1998, reaffirmed November 2010; SAE also lists lettered revisions, so state the revision on the PO |
| ASTM E2375 | Practice for ultrasonic testing of wrought products 0.250 in (6.35 mm) and thicker, with five acceptance classes in its Table 1 | Active; ASTM lists E2375-22, adopted to replace MIL-STD-2154 |
| AMS 2770, AMS 2774, AMS 2759 family, AMS 2801 | Heat treatment of wrought aluminum, nickel and cobalt, steel, and titanium parts | Covered on the heat treatment page |
Two points trip buyers. First, the material spec and the heat treat spec are separate. A forging may arrive in a solution treated condition from the forge and be aged by your machine shop's processor, or it may arrive fully heat treated; the PO should say which, and the heat treater certifies to AMS 2770, AMS 2774, the AMS 2759 slash sheets or AMS 2801 accordingly. Second, a cancelled military document on an old drawing, such as MIL-STD-2154 or MIL-F-7190, needs a decision on the SAE or ASTM replacement before the order is placed, or the forge will quote to whatever it assumes.
Ultrasonic acceptance classes
Most aerospace forgings are ultrasonically inspected, and the class sets how small an indication is rejectable. The class letters are easiest to compare in a published table. NASA Johnson Space Center process specification PRC-6504, revision G (January 2020), lists the following alarm levels as flat bottom hole diameters in inches.
| Class | Single discontinuity | Multiple discontinuities | Linear discontinuity | Loss of back reflection |
|---|---|---|---|---|
| AAA | 1/64, or 25 percent of 3/64 | 10 percent of 3/64 | 1/8 in long and 10 percent of 3/64 | 50 percent |
| AA | 3/64 | 2/64 | 1/2 in, 2/64 | 50 percent |
| A | 5/64 | 3/64 | 1 in, 3/64 | 50 percent |
| B | 8/64 | 5/64 | 1 in, 5/64 | 50 percent |
| C | 8/64 | Not applicable | Not applicable | 50 percent |
The same document requires at least Class A for flight hardware, states that linear discontinuities are not acceptable on flight hardware regardless of length, and requires the drawing to be zoned when different areas of a part carry different classes. In 2014 the JSC document replaced its references to AMS-STD-2154 with ASTM E2375, which shows how program documents migrate between the two. Use the document and class your own program invokes; the table is here so you can see how much tighter AAA is than A before you specify it.
Inspect at the stage where the shape is still simple. A forging is usually inspected in a rectilinear or rough machined condition, because the finished contour can leave zones the beam cannot reach. Agree the inspection stage with the forge and the machine shop, and record it. Method details, personnel qualification and accreditation are on the nondestructive testing page.
Forging drawings and machining stock
Most aerospace forgings are machined all over or nearly so, which means two drawings: the forging drawing, owned by the forge or the design authority, and the machined part drawing. The forging drawing defines the envelope, draft, parting line, datum targets for the first machining setup, grain direction requirements, and the material and heat treat condition at delivery.
- Machining stock. The forging envelope must leave enough stock to clean up scale, decarburization or alpha case, die wear and mismatch at the parting line on every machined surface. The forge proposes stock based on its process; the machine shop should confirm it can find the part inside the envelope.
- Datums. Give the forging drawing datum targets on surfaces that come off the die repeatably, not on flash lines. The first machining operation picks up those targets.
- Draft and radii. Closed die forgings need draft to release from the die, and generous radii fill better and carry grain around corners. Sharp internal corners on the forging envelope are a common source of laps.
- Test material. If mechanical properties are verified on prolongations or extra forgings, show the prolongation on the forging drawing and say which direction the test bars are cut.
- Identification. State where the forging number, heat code and serial number go, usually a raised or depressed pad, and make sure that pad survives machining or is transferred.
For complex machined parts cut from forgings, see 5-axis CNC machining for setup strategy on contoured shapes.
Die cost and lead time drivers
Forging lead time is mostly tooling and queue time, not press time. The drivers we see on quotes:
- Die design and sinking. A closed die set is a machined tool steel or superalloy block; a new die means design, machining and trial. Open die and rolled rings avoid this, which is why prototypes are often made open die or hogged from a hand forging before a closed die is committed.
- Material availability. Forge stock in aerospace alloys and in vacuum remelted grades is bought to a material spec with its own mill lead time. A forging cannot start before the billet arrives.
- Press and furnace scheduling. Large presses, ring mills and isothermal presses are few, and forge shops batch by alloy and temperature.
- Heat treatment and testing. Solution treatment, aging, test bar machining and mechanical testing add calendar time, especially when an outside lab does the testing.
- First article. AMS 2375 requires preproduction approval before production forgings follow, and when that approval includes sectioning for grain flow, plan for at least one sacrificial forging and the approval cycle.
Cost per piece falls quickly with quantity on closed die work because the die is amortized. For one to a handful of parts, an open die hand forging machined to shape, or a part machined from plate with a stress corrosion resistant temper, may be the economic answer if the stress analyst accepts the grain orientation. See lead times and delivery for how outside processing stacks up on a schedule.
First article for forgings
A forging first article has two parts and they are easy to confuse. The forging first article, controlled by AMS 2375 or a prime's equivalent, proves the forging process: die, stock, forging sequence, heat treatment, grain flow by macroetch, mechanical properties and ultrasonic quality. Once approved, the process is frozen, and changes to dies, forging sequence, source or heat treatment trigger a new approval. The machined part first article under AS9102 proves the finished part against the machined drawing, and it references the forging approval rather than repeating it.
What to require in the forging first article package:
- Forging drawing number and revision, die identification, forge source and forging sequence summary
- Material certification with heat number, chemistry and melt practice
- Heat treat certification with specification, revision and furnace identification
- Macroetch section photographs and the grain flow approval
- Mechanical test results with test bar location and direction
- Ultrasonic report with specification, class and inspection stage
- Dimensional layout of the forging envelope
For the machined part, the AS9102 first article buyer page explains forms 1 to 3, and material certs and traceability explains how to keep the heat number connected from billet to finished part.
What to call out on the drawing or PO
- Alloy and material specification for the forging product form, including melt practice if required
- Forging process if it matters to you (closed die, hand forging, rolled ring), or state that it is the supplier's choice
- Delivery condition, and the heat treat specification and final condition after machining
- Grain direction requirements and parting line restrictions
- Ultrasonic specification, revision, class and zones, and the inspection stage
- First article requirement under AMS 2375 or the prime's forging control document, including who approves
- Test material location and direction
- Marking, serialization and traceability to heat and forging lot
- DFARS 252.225-7009 specialty metals compliance for defense contracts on steel, nickel, cobalt and titanium forgings; see the DFARS specialty metals page
Get forged and machined parts sourced
Forged parts usually involve a forge, a heat treater, an ultrasonic inspection source and a machine shop. Send the forging and machined drawings and we source the chain from qualified suppliers, flow down the forging control, ultrasonic class and heat treat requirements, check the first article package and certs, 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 process on controlled programs. Related: casting, technical cutting and the processes hub.
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Questions
When should a part be forged instead of machined from plate?
When the loads cross the grain of plate or bar at a critical feature, such as a lug, clevis or flange, or when stress corrosion in the short transverse direction is a concern. Forging lets the grain follow the contour. For one or two parts, machining from plate in a stress corrosion resistant temper may be cheaper if the stress analyst accepts the grain orientation. For repeat quantities of loaded fittings, a closed die forging usually wins on both properties and material use.
What does AMS 2375 control?
AMS 2375 covers control of forgings requiring first article approval. Its scope describes how a forging receives preproduction approval and the controls that apply to production forgings made after that approval. In practice the approved process is frozen, and changes to dies, forging sequence, source or heat treatment require review. Revision F, revised in September 2022, is the current revision listed by SAE.
Is MIL-STD-2154 still valid on an old drawing?
MIL-STD-2154 was cancelled. NASA JSC PRC-6504 records the cancellation date as November 15, 2004. SAE publishes AMS-STD-2154 as the industry version, and ASTM E2375 was adopted to replace the military standard. If an old drawing cites MIL-STD-2154, decide with the design authority which replacement and which revision applies before ordering, and put that decision on the PO so the inspection source certifies to it.
What ultrasonic class should a flight forging use?
Your program's documents decide. As one published example, NASA JSC PRC-6504 Rev. G requires at least Class A for flight hardware, with a 5/64 in flat bottom hole alarm level for a single discontinuity. AA and AAA are tighter and are used on rotating or fracture critical parts. Specifying AAA everywhere raises cost and rejection rate, so zone the drawing and apply the tight class only where it is needed.
Why is isothermal forging so expensive?
The dies must be held at roughly the forging temperature, about 980 to 1200 C for nickel alloys according to the ASM Handbook, so they are made from materials such as TZM molybdenum that must be protected by vacuum or inert gas. The press runs at slow strain rates, and few shops have the equipment. The payoff is near net shape on expensive alloys and thin sections that conventional dies cannot fill.
Does the forging first article replace AS9102 for the machined part?
No. The forging first article proves the forging process, including grain flow by macroetch, properties and ultrasonic quality. AS9102 proves the finished machined part against its own drawing. The machined part FAI should reference the approved forging, its drawing revision and source, so a change at the forge is visible as a change to the machined part's baseline.
Related
Sources
- SAE Mobilus: AMS 2375F, control of forgings requiring first article approval
- BSI Knowledge: SAE AMS F7190B:2016, steel forgings for aircraft and special ordnance
- SAE Mobilus: AMS-STD-2154, ultrasonic inspection of wrought metals
- ASTM E2375, ultrasonic testing of wrought products
- NASA JSC PRC-6504 Rev. G, ultrasonic inspection of wrought metals (Table 1 classes)
- ASM Handbook Vol. 14A: Isothermal and Hot-Die Forging
- Scot Forge: forged and rolled rings
- Gear Solutions: Forging 101
- US Patent 6932877: quasi-isothermal forging of a nickel-base superalloy
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