Buying Aerospace Special Processes the Right Way
A special process is one whose result cannot be fully checked by inspecting the finished part, so aerospace buyers control the process itself: the governing spec, an approved or Nadcap accredited processor, the right sequence, and a cert for every step. Aerospace Sourcing matches your parts to qualified processors, flows down the specs and source approvals, and checks each process cert before the parts ship.
What makes a process special
A process is special when the quality of its output cannot be verified by later measurement or inspection without destroying the part. You cannot look at an anodized bracket and know the coating was sealed correctly, or measure a bolt and know whether hydrogen is still trapped in it. So the process has to be proven and controlled instead.
AS9100D handles this in clause 8.5.1.2, validation and control of special processes. It requires the organization to validate these processes, revalidate them periodically, define how processes, documents and people are approved, set the conditions for keeping that approval, and keep records. IAQG guidance for auditors lists typical examples: heat treatment, chemical processing, composites, nonconventional machining, nondestructive testing, joining, coating and surface enhancement.
Typical buyer requests in this category include:
- Anodize on aluminum, including hard coat and dyed finishes
- Chemical conversion coating, often called chem film or by the trade name Alodine
- Electroplating and electroless plating, such as nickel, cadmium substitutes, silver and gold
- Passivation of stainless steel
- Heat treatment of aluminum, steel, titanium and nickel alloy parts
- Shot peening
- Nondestructive testing such as penetrant, magnetic particle, radiography and ultrasonic
- Precision and oxygen cleaning for fluid and propulsion hardware
For background on each process family, see our library pages on surface finishing, heat treatment, nondestructive testing and precision and oxygen cleaning.
Nadcap and prime approved process sources
Most aerospace drawings and purchase orders require two things from a processor: a quality system, and approval for the specific process, either through Nadcap accreditation or by the prime contractor directly. Check both before you place the order, not after the parts come back.
Nadcap is an industry managed accreditation program for critical processes, administered by the Performance Review Institute (PRI) under the Nadcap Management Council, whose members represent subscriber companies. It was founded in 1990 by aerospace manufacturers and now covers 24 critical process areas, including heat treating, nondestructive testing, welding and chemical processing. Accredited suppliers are listed in PRI's Qualified Manufacturers List through its online platform, formerly called eAuditNet.
Nadcap is not always enough. Many primes keep their own lists of approved processors for their specifications. Boeing's D1-4426, Approved Process Sources, is a well known example. Boeing's Q020 clause requires sellers and their subcontracted process sources to use processors approved in D1-4426 where it applies, to flow that requirement into subcontracts, and to keep a certificate for each D1-4426 process that names the processor and the processing dates. Other primes publish similar special process lists in their supplier quality requirements.
What this means for a buyer:
- Read your contract and the prime's quality clauses to find which processes need an approved source
- Check the processor's approval for the specific spec and process code, not just the company name
- Confirm approval is current on the date the parts are processed
- Put the approved source requirement on the purchase order so it flows to every tier
Process houses that want to be considered for this work can join our supplier network for free.
The governing specs and what they cover
Call out the spec, the type and class, and any options the spec leaves to the purchaser. A finish callout with only a spec number leaves the processor guessing, and a default the spec chooses for you may not be the one you want.
| Spec | What it covers | What to specify |
|---|---|---|
| MIL-PRF-8625 | Anodic coatings on aluminum and aluminum alloys: six types and two classes | Type, class, color if dyed, seal, thickness if the drawing sets one |
| MIL-DTL-5541 | Chemical conversion coatings on aluminum | Type I or II, Class 1A or 3 |
| AMS 2700 | Passivation of corrosion resistant steels | Method and type if your engineering requires one, verification test |
| AMS 2770 | Heat treatment of wrought aluminum alloy parts | Alloy and final temper |
| AMS 2759 and slash sheets | Heat treatment of steel parts, with detail sheets for alloy families and processes | Alloy, required strength or hardness range |
| AMS 2774 | Heat treatment of nickel alloy and cobalt alloy parts | Alloy and condition |
| AMS 2750 | Pyrometry: furnace classes, instrumentation, calibration and surveys | Usually invoked through the heat treat spec |
| AMS 2430 and AMS 2432 | Shot peening, automatic and computer monitored | Media, Almen intensity, coverage, areas to peen and mask |
| ASTM E1417 | Liquid penetrant testing | Penetrant type, method and sensitivity, acceptance criteria |
| AMS 2759/9 and ASTM B850 | Hydrogen embrittlement relief baking after plating and some chemical processes | Part strength or hardness, so the processor can apply the right bake |
Anodize types and classes
MIL-PRF-8625 defines Type I chromic acid anodizing, Type IB low voltage chromic acid, Type IC non-chromic as a non-chromate alternative to Types I and IB, Type II sulfuric acid, Type IIB thin sulfuric as another non-chromate alternative to Types I and IB, and Type III hard anodic coatings. Class 1 is non-dyed and Class 2 is dyed. The current document is revision F with amendment 2, effective November 2020.
Chem film types and classes
MIL-DTL-5541 Type I coatings contain hexavalent chromium and Type II coatings do not. Class 1A is for maximum corrosion protection. Class 3 is for lower electrical resistance, where the part needs a conductive surface for grounding or bonding. The spec defaults to Type I if no type is given, which is a good reason to state the type on every drawing.
Passivation
AMS 2700 offers Method 1, nitric acid, with several types for different steels, and Method 2, citric acid. Either method can be used unless the purchaser specifies otherwise. Free machining and high carbon martensitic grades need the right type so the bath does not attack the part. Drawings that still call out QQ-P-35 are usually processed to AMS 2700, but confirm with engineering.
Heat treat and pyrometry
Heat treat is only as good as the furnace control behind it, which is why aerospace heat treat specs invoke AMS 2750 pyrometry. Ask your processor which AMS 2750 furnace class and instrumentation type its equipment is qualified to, and confirm it covers the temperature tolerance your spec requires.
AMS 2750 sorts furnaces into classes 1 through 6 by temperature uniformity, from the tightest at class 1 to the loosest at class 6, and defines instrumentation types A through E. It requires system accuracy tests, which check the control and recording system against an independent test thermocouple, and temperature uniformity surveys, which map temperature across the qualified work zone. Class and instrumentation type together set how often those tests are due.
For aluminum, AMS 2658 sets hardness and electrical conductivity acceptance criteria for wrought aluminum parts after heat treatment, and is commonly used to confirm alloy, temper and heat treat condition. If your parts are heat treated after machining, ask for those results on the cert.
Hydrogen embrittlement relief after plating
High strength steel parts that are plated or chemically processed can absorb hydrogen and crack later under load, sometimes days after installation. The control is a bake soon after processing, and it has to be on the purchase order because the plater cannot apply it correctly without knowing the part's strength.
AMS 2759/9 covers hydrogen embrittlement relief baking of heat treated steel parts to remove hydrogen picked up during plating and certain other chemical processes such as stripping, chemical milling, pickling and etching. Its applicability is tied to the strength or hardness of the steel, and the purchaser is expected to give the processor the final heat treat condition. ASTM B850 is a general guide to post-coating treatments of steel for reducing hydrogen embrittlement risk. It says the relief heat treatment is done after plating but before any secondary conversion coating, and that the treatment is chosen based on the actual tensile strength of the steel. It also notes that baking reduces risk but does not guarantee freedom from hydrogen damage.
On the order, give the material, the heat treat condition and the strength or hardness range, and require the bake to be recorded on the plating cert.
Shot peening and NDT
Shot peening and NDT are both controlled by procedure and by people, so specify the method details and require qualified personnel.
AMS 2430 covers automatic peening with metallic shot, glass beads or ceramic shot. AMS 2432 adds computer monitoring, recording process inputs in real time, and incorporates AMS 2430 by reference. Intensity is set and verified with Almen strips under SAE J443. The A strip covers the common range, with the N strip for lower intensities and the C strip for higher ones. The drawing should give the intensity, the coverage, and which surfaces are peened and which are masked.
For penetrant inspection, ASTM E1417 sets minimum requirements for examining nonporous metal and nonmetal parts for surface-open discontinuities such as cracks, laps and porosity, and it replaced MIL-STD-6866. Penetrants are classified by type, fluorescent or visible dye, and fluorescent penetrants by sensitivity level, so state what your engineering requires. NDT personnel on aerospace work are usually qualified and certified to NAS 410, which sets the education, experience and examination requirements for each level.
Our inspection and metrology library page covers dimensional inspection, which is a different control from NDT.
Sequencing processes
Get the order of operations right before the first part is processed, because many sequencing errors cannot be undone. The drawing or a process routing should set the sequence, and when it does not, ask engineering rather than letting each processor decide.
Common sequencing logic:
- Machining and forming come first, along with any dimensional work that would cut through a coating.
- Heat treatment usually comes before finishing, since heat would damage most coatings. Stress relief may be scheduled between roughing and finishing cuts.
- Shot peening normally follows heat treat, because later heat can relax the compressive stress peening adds.
- Penetrant inspection is usually done before coatings or plating, since a coating can bridge and hide a surface crack.
- Plating, then hydrogen embrittlement relief on susceptible steel, before any secondary conversion coating.
- Passivation of stainless parts after machining and cleaning.
- Anodize or chem film on aluminum, then paint or primer if required.
- Final inspection and marking, with marking methods that do not damage the finish.
Specs and drawings can override any of these. Treat the list as a set of questions to ask, not as rules.
Masking
Masking tells the processor which surfaces must stay bare, and it is one of the most common causes of rework. If a surface must not be coated, the drawing has to say so.
- Identify masked areas on the drawing, with a view or note, not only in an email
- Call out threads, bores, bearing fits and electrical bonding surfaces, since coatings add thickness and change resistance
- Say whether a chem film touch-up is acceptable on masked edges or contact points
- For shot peening, show the peened areas and the areas that must be shielded
- Ask the processor to confirm the masking plan on complex parts before processing
Hard anodize and plating build thickness, so tight fits may need to be machined undersize before coating or masked. That is an engineering decision, and it belongs on the drawing.
Certs to expect for each process
Every special process step should produce its own certificate from the processor that did the work. Ask for one cert per process, per lot, that you can tie to the parts.
A complete process cert normally shows:
- Processor name and location
- Specification, revision, type, class and method as applicable
- Part number, quantity and lot or job number
- Date of processing
- Results where the spec requires them: thickness, hardness, conductivity, Almen intensity, bake time and temperature, inspection results
- A statement of conformance signed by an authorized person
These certs also feed your first article. AS9102 Form 2 lists the special processes on a part along with the specs and the processors, so the certs need to match what is recorded there. Our first article inspection guide covers how those records fit together.
What drives special process lead time
Transit and queue time usually matter more than the process itself. Each outside process adds a trip to the processor, time in its queue, the process, inspection and a trip back.
- Number of separate processes and processors in the routing
- Approved source requirements that limit you to one or two processors
- Batch processes that run on a schedule, such as some heat treat cycles and tank lines
- Bake requirements after plating
- Masking work on complex parts
- Rework when a callout was missing or wrong
We do not promise lead times. A person reviews each request and replies within one business day, and rush requests are prioritized. Our aerospace machining cost guide shows how outside processes add to the cost of a machined part.
What to put on the drawing and the purchase order
Put the technical requirements on the drawing and the commercial and source requirements on the purchase order. Between them, the processor should not have to guess anything.
On the drawing
- Spec, revision if it matters, type, class and method for each process
- Color for dyed anodize, and seal requirements
- Coating thickness where the drawing controls it
- Masked areas and peened areas
- Heat treat condition or final temper, and hardness or strength range
- NDT method, sensitivity and acceptance criteria
- Sequence notes where order matters
On the purchase order
- Nadcap accreditation or prime approved source requirements, with the clause or list that applies
- Material and heat treat condition, so plating and bake can be done correctly
- Required certs for each process and the data they must report
- Retention of process records
- Quantity, lot identification and need date
- Export control status of the drawing
Our aerospace RFQ checklist covers the rest of a complete request. If the drawing is ITAR or CUI controlled, follow our confidentiality page to send it instead of attaching it to a form.
How Aerospace Sourcing handles special process work
We are a sourcing service, not a process house. We do not run tanks, furnaces or peening machines. We match your parts to processors whose approvals fit your specs and your prime's requirements, and we manage the work through delivery.
- One quote, one lead time, one point of contact and one purchase order, even when several processors are in the routing
- Your specs, sequence, masking and approved source requirements flowed down to each supplier
- Each process cert checked against the drawing and the purchase order before parts ship
- A certificate of conformance with every order, plus the process certs your request requires
If the parts also need machining, our aerospace CNC machining page covers how machining and outside processing are combined into one job.
Request special processing
Send the drawing, the process callouts and the quantity through the special process request form. It is preset for special processes, so it asks for the specs, the approved source requirement and the material condition up front, which are the details that decide which processors can take the job. A person replies within one business day.
Questions
Is Nadcap accreditation required for every special process?
No. Nadcap is required when your contract, your prime or the drawing requires it. Many primes do require it for processes such as heat treat, chemical processing and NDT, and some also require approval on their own source list, such as Boeing D1-4426. Read your quality clauses, then check the processor's accreditation for the specific process in PRI's Qualified Manufacturers List.
What is the difference between Type II and Type III anodize?
Under MIL-PRF-8625, Type II is conventional sulfuric acid anodizing and Type III is hard anodic coating. Type II is common for corrosion protection and dyed colors. Type III builds a harder, thicker coating for wear resistance, so dimensional allowances and masking matter more. Class 1 is non-dyed and Class 2 is dyed, and both are called out alongside the type.
Should I call out Class 1A or Class 3 chem film?
It depends on what the surface has to do. Under MIL-DTL-5541, Class 1A is for maximum corrosion protection, and Class 3 is for lower electrical resistance where the part needs a conductive surface for grounding, bonding or shielding. Also state Type I or Type II, since Type I contains hexavalent chromium and the spec defaults to Type I when no type is given.
When is a hydrogen embrittlement relief bake required?
It applies to steel parts at higher strength or hardness levels that are plated or go through processes such as pickling, stripping or etching. AMS 2759/9 sets the bake requirements based on the steel's strength or hardness, and ASTM B850 says the bake comes after plating and before any secondary conversion coating. Give the processor the material and heat treat condition so the right bake is applied.
Why does process sequence matter so much?
Many steps cannot be reversed or repeated. Heat after peening can relax the compressive stress, heat after anodize can damage the coating, and a coating applied before penetrant inspection can hide a crack. Plating on susceptible steel needs its bake before any conversion coating. Set the sequence on the drawing or routing, and ask engineering when it is not defined.
What certs should come back from a special processor?
One cert per process, per lot, from the processor that did the work. It should name the processor, the spec, revision, type and class, the part number, quantity and lot, the processing date, the results the spec requires, and an authorized signature. Those certs also support the special process entries on an AS9102 first article report.
Related
Sources
- PRI: Nadcap program overview
- Elsmar: AS9100D clause 8.5.1.2 validation and control of special processes
- Boeing: Q020 D1-4426 Approved Process Source clause
- DLA ASSIST via ibiblio: MIL-A-8625F anodic coatings, types and classes
- Finishing and Coating: rollout of MIL-PRF-8625 revision F
- MIL-C-5541F chemical conversion coatings on aluminum
- ANSI: SAE AMS 2700 passivation of corrosion resistant steels
- Heat Treat Today: furnace classifications and AMS 2750
- SAE: AMS 2759/9 hydrogen embrittlement relief (baking) of steel parts
- BSI: ASTM B850 post-coating treatments of steel
- Shot Peener: specifications relating to shot peening
- ASTM: E1417/E1417M liquid penetrant testing
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