Consultation
Verifying Aluminum Alloy Composition: OES vs Conductivity Meter
Two aluminum billets can have the same diameter, the same surface and the same weight, and one of them is the wrong alloy. If it reaches the furnace, heat treatment will not give the expected strength and the mistake surfaces at final inspection or in service. Whether it is caught at incoming inspection depends on the tool.
What a conductivity meter tells you
An eddy current conductivity meter is the common shop-floor tool for aluminum identification. It reads electrical conductivity as a percentage of IACS. Conductivity in aluminum depends on how much alloying element is dissolved in the matrix: elements in solid solution scatter electrons and lower conductivity, while elements tied up in precipitates have less effect. That is why conductivity also changes with temper. It is lowest right after quenching, when everything is in solution, rises through T4 and T6 as precipitates form, and is highest in the overaged T73 condition.
The problem for alloy identification is that the ranges overlap.
| Alloy | IACS, O condition | IACS, T6 condition |
|---|---|---|
| 6061 | 42 to 50 | 40 to 50 |
| 6063 | 57 to 65 | 50 to 60 |
| 6066 | 44 to 60 | 38 to 50 |
| 7050 | 44 to 50 | not shown |
| 7075 | 44 to 48 | 30.5 to 36 |
Source: SAE-AMS 2658B, Hardness and Conductivity Inspection of Wrought Aluminum Alloy Parts.
- In the annealed O condition, 6061 and 6066 can possibly be told apart. After T6, their ranges are almost identical and the meter cannot separate them.
- In the O condition, 6061 cannot be separated from 7050 or 7075. Only in T6 does the 6xxx versus 7xxx split appear, because the chromium in 7075 stays in solution and pulls its conductivity down.
So the meter answers "roughly which family, in which temper". AMS 2658B acceptance practice pairs conductivity with a hardness reading to confirm temper, because either alone can mislead. Neither confirms the alloy.
What OES tells you
Optical emission spectroscopy sparks the sample surface, splits the emitted light by wavelength and reads the intensity of each element's line. The result is a quantitative composition in weight percent, element by element, in a few minutes. Spark OES is the first-choice tool for incoming material checks and for confirming a mill certificate: fast, inexpensive, sensitive to parts per million.
A real batch of AA6061 forging stock measured against the ASTM specification:
| Element | Measured, wt% | ASTM spec for AA6061, wt% |
|---|---|---|
| Mg | 0.92 | 0.80 to 1.20 |
| Si | 0.66 | 0.40 to 0.80 |
| Cu | 0.22 | 0.15 to 0.40 |
| Cr | 0.09 | 0.04 to 0.35 |
| Fe | 0.12 | below 0.70 |
| Zn | 0.10 | below 0.25 |
| Ti | 0.012 | below 0.25 |
| Mn | 0.096 | below 0.15 |
Every element is inside specification and the alloy is confirmed beyond doubt. A conductivity meter would have given one number and a maybe.
Composition also matters beyond identification. Iron and silicon content in particular affect toughness in high-strength alloys, and a customer with a toughness requirement may specify tighter limits than the standard. OES is the only shop-floor tool that can check that.
When to ask for it
OES verification is available for any job. It is worth specifying when the material source changes, when the part carries load and the alloy must be certain, or when the customer's quality system requires composition records. When specified, the composition report is filed with the batch and stays attached through forging, heat treatment and final inspection.
Can a conductivity meter identify an aluminum alloy?
What does OES measure?
Is OES verification done on every order?
