Aluminum Quality & Inspection Services

Karl Gillihan

🔩 ALUMINUM

Dimensional and Material Verification for Aluminum Parts

Aluminum looks forgiving on a print, but it punishes loose inspection practice harder than steel because it moves. A 6061-T6 bracket relaxes after heavy material removal, anodizing adds 0.0002 to 0.001 in per surface, and conductivity drifts if the temper was cooked. Buyers searching for aluminum quality and inspection on ManufacturingBase are usually chasing one of three failures: features that walked off-tolerance after stress relief, a temper that never met spec, or a finish callout nobody measured.

ISO 9001AS9100NADCAP

Why aluminum dimensions move between machining and final inspection

Aluminum has roughly three times the thermal expansion of steel (about 13 microinch/in/degF for 6061), so a part measured at 78 degrees on the floor and again at 68 degrees in a climate-controlled lab can differ by half a thou over a 4-inch span. Any supplier inspecting tight aerospace tolerances on aluminum without a 68 +/- 2 degF soak is reporting noise. Ask whether their CMM room is temperature-controlled and whether parts are soaked before measurement. Residual stress is the second mover. Thick 7075 and 2024 plate carries locked-in stress from quenching, and when a shop hogs a pocket out of one side the part bows, sometimes 0.005 to 0.020 in across a 12-inch length. Good shops rough, stress-relieve or let it sit, then finish. Inspection has to happen after that settles, not on a part fresh off the spindle. A flatness number taken at the machine is meaningless if the part keeps relaxing overnight. Finish operations move dimensions too. Type II anodize grows roughly 50 percent of its thickness outward, so a 0.0005 in coating adds about 0.00025 in per surface, closing a slip-fit bore by 0.0005 in on diameter. Hard coat (Type III) is worse, often 0.002 in per surface. Competent inspection plans state explicitly whether dimensions are checked before or after finish, and on critical fits the answer should be after.

01

Verifying temper and alloy: it is not just a chip test

The most common aluminum quality escape is a part machined from the wrong temper or an under-aged lot. You cannot see the difference between 6061-T6 and -T4 across a bench. Hardness testing on the Rockwell B or E scale catches gross temper errors fast, and a properly heat-treated 6061-T6 lands around 60 HRB / 95 HRE. For 7075-T73, the buyer often wants T73 specifically for stress-corrosion resistance, and an as-supplied T6 lot will pass a dimensional check while failing the application. Electrical conductivity testing (eddy current, reported in %IACS) is the aerospace gold standard because it correlates with both temper and over-aging. 7075-T73 should fall in a defined %IACS band; a reading outside it flags improper heat treat even when hardness looks fine. NADCAP-accredited shops doing aerospace aluminum will have conductivity meters and the relevant AMS conductivity tables. If a supplier offers no conductivity capability and you are buying flight hardware in 7075 or 2024, that is a gap. For full traceability, material certs (mill cert per AMS or ASTM B209) tie the lot to chemistry and mechanicals. On controlled programs, suppliers should retain certs and ideally do an incoming verification, even a spark-OES or XRF check, to confirm the alloy is what the cert claims. XRF will not separate tempers, but it confirms you did not get 2024 when you ordered 6061.

02

Surface finish and cosmetic inspection on a soft, smearable metal

Aluminum machines to a beautiful finish but smears, and that smearing hides and creates inspection problems. A dull or chattered insert leaves a built-up edge that reads as a rougher Ra than the geometry warrants, and burrs roll over rather than break clean. On 5052 and other soft non-heat-treatable grades this is worse because the material gums. Inspection should distinguish a genuine surface defect from a smear artifact, often by lightly deburring and re-checking. Cosmetic grading matters most when the part anodizes. Anodize is transparent and ruthless: every tool mark, scratch, and handling ding shows after finishing, and dye color shifts over machining marks. A real inspection plan for cosmetic anodized aluminum grades surfaces against a written acceptance standard (often a customer cosmetic spec or AMS 2470/2471/2469 for the anodize itself) before the part goes to the finisher, because anodizing a scratched part just makes the scratch permanent and obvious. Profilometer Ra readings are the quantitative backstop. Typical milled aluminum lands 32 to 63 microinch Ra, fine finish passes hit 16, and a bead-blast or anodized cosmetic surface is graded visually plus Ra. State the measurement direction; Ra across the lay reads higher than along it, and a part can pass one way and fail the other.

03

Detecting porosity and internal defects in cast and plate aluminum

Wrought 6061 and 7075 plate is generally sound, but castings and thick plate can hide porosity, inclusions, and lamination. For die-cast or sand-cast aluminum structural parts, radiographic (X-ray) or CT inspection finds gas porosity and shrink that a dimensional check never sees. Aerospace castings are typically graded to ASTM E155 reference radiographs with a porosity acceptance class called out on the print. For machined plate parts where an internal void would be catastrophic, fluorescent penetrant inspection (FPI, per ASTM E1417) catches surface-breaking cracks opened up by machining, and ultrasonic testing catches subsurface laminations in thick plate. Penetrant is a NADCAP-controlled process in aerospace, so confirm the supplier or their subcontractor holds that accreditation rather than running an uncontrolled rattle-can process. The honest note: for most commercial 6061 and 5052 work, NDT is overkill and adds cost with no return. Reserve X-ray and FPI for flight-critical, pressure-containing, or fatigue-loaded parts where a hidden defect is a safety issue. A supplier who pushes full NDT on a non-critical 6061 bracket is padding the quote.

Frequently Asked Questions

Three layers. First, the mill cert per AMS-QQ-A-200/250 or ASTM B209 should state the temper and report mechanical properties; a true T6 6061 shows roughly 42 ksi yield and 45 ksi tensile. Second, a Rockwell hardness check is the fast floor verification: 6061-T6 reads about 60 HRB or 95 HRE, while T4 reads noticeably softer and annealed O temper is far lower. Hardness testing runs a few dollars per part and catches gross temper errors immediately. Third, for aerospace work, eddy-current electrical conductivity testing in %IACS confirms both temper and over-aging against AMS tables, which hardness alone cannot fully resolve. Expect a NADCAP-accredited aerospace supplier to offer all three. For commercial work, a mill cert plus a spot hardness check is usually enough, and adds maybe 1 to 3 percent to part cost on a typical run.

Both, but the critical-fit dimensions get measured after. Type II anodize grows about 50 percent of the coating thickness outward, so a 0.0005 in coating closes a bore by roughly 0.0005 in on diameter and grows a pin by the same. Type III hard coat grows more, often 0.001 to 0.002 in per surface. If a slip fit or bearing bore is dimensioned on the print, the supplier must either machine undersize to land in tolerance after anodize, or mask the feature and verify it post-finish. Cosmetic and surface inspection happens before anodizing, because anodize is transparent and turns every scratch and tool mark into a permanent, highly visible defect. A competent inspection plan states explicitly which dimensions are checked pre-finish and which are checked post-finish; if the print does not say and the fit matters, get it in writing before parts run. Re-anodizing a mis-sized part means stripping and re-coating, which adds days and cost.

At minimum, a CMM in a temperature-controlled room held at 68 +/- 2 degF, because aluminum's high thermal expansion makes shop-floor measurement of tight tolerances unreliable. Beyond the CMM, expect calibrated micrometers and bore gauges for in-process checks, a Rockwell hardness tester for temper verification, and a profilometer for Ra surface finish. For aerospace aluminum in 7075 and 2024, add an eddy-current conductivity meter for %IACS temper confirmation. For castings or flight-critical parts, X-ray or CT and fluorescent penetrant capability (in-house or NADCAP-accredited subcontractor) matter. Calibration traceability to NIST and a documented gauge R&R program separate a real quality system from a drawer of uncalibrated tools. On ManufacturingBase you can filter aluminum suppliers by ISO 9001, AS9100, and NADCAP accreditation, which is the fastest proxy for whether this equipment and the discipline behind it actually exist.

Almost always residual stress relaxing after machining. Thick 7075 and 2024 plate locks in stress from the quench, and when a shop machines material off one face the part bows to rebalance, often 0.005 to 0.020 in over a 12-inch length. If the supplier measured flatness right off the machine, the number was real at that moment and wrong by the time the part shipped or got clamped at your facility. The fix is process: rough machine, stress relieve or let the part rest, then finish machine and inspect after it has settled. For tight-flatness aluminum, specify stress-relieved plate (T651 instead of T6) and require the supplier to inspect after a documented dwell. Also confirm their inspection happened at controlled temperature, since a half-thou of thermal growth on a 4-inch part can masquerade as a flatness problem. Spell out the stress-relief and inspection-timing requirement on the print so it is contractual, not assumed.

It scales with how much you ask for. A first-article inspection (FAI) per AS9102 with a full balloon-and-report dimensional layout typically adds 1 to 3 business days and a few hundred dollars for the report, more for a complex part with 100-plus features. In-process and final dimensional checks on a production run add maybe 2 to 8 percent to per-part cost depending on tolerance count. NDT changes the math: fluorescent penetrant adds roughly 10 to 40 dollars per part at low volume, and X-ray or CT can add 50 to 200-plus dollars per part plus a day or two of turnaround. Conductivity and hardness testing are cheap, a few dollars each. The biggest hidden driver is tolerance: features held to +/- 0.0005 in or tighter force CMM time and possible re-machining, which can double inspection cost versus a +/- 0.005 in part. Specify only the inspection the application needs, and reserve NDT for critical parts.

Last updated: July 2026

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