Guides / Alloys
Why an Extrusion Alloy Keeps Showing Up on Machined Parts
September 27, 2026 by The Alu CNC Parts editorial desk / how this is checked
6061 is the alloy most machining articles default to. 6063 keeps appearing anyway, usually on heat sinks, enclosure rails and anything with an extruded cross-section. That is not an accident and it is not the same situation as 6082 standing in for 6061. 6063 and 6061 are not regional equivalents. They are built for different jobs.
They are not the same job
| 6061-T6 | 6063-T5 / T6 | |
|---|---|---|
| Yield strength | ~276 MPa | ~145 MPa (T5) / ~214 MPa (T6) |
| Machinability | Very good, breaks into a clean chip | Fair, gummier, more built-up edge |
| Extrudability | Moderate | Excellent, the reason it exists |
| Anodizing | Excellent, the general purpose default | Excellent, often clearer and brighter |
| Thermal conductivity | ~167 W/(m·K) | ~200 W/(m·K) |
| Typical form on a quote | Plate, round bar, block | Extruded profile, tube, heat sink stock |
Published figures vary by source and by section size, so treat these as typical values, not a guarantee for your specific temper and lot.
6061 is a general purpose structural alloy sold as plate, bar and block, meant to be machined from solid. 6063 is an architectural and extrusion alloy: lower alloy content, softer, and built to flow cleanly through an extrusion die. Machining it well is a secondary property, not the reason it was chosen.
Why 6063 lands on a CNC quote at all
Almost nobody machines a part from solid 6063 billet by choice. What actually happens is that the part starts life as an extrusion, either a standard heat sink or rail profile or a custom die, and the CNC work is finishing the ends: facing to length, drilling and tapping mounting holes, cutting slots, milling a flat for a label.
That is a very different scope from machining a bracket out of solid 6061. The bulk of the shape already exists. The machining is light, localized, and the alloy was never a choice for the machinist, it was fixed by whatever the extrusion was drawn in.
Two reasons 6063 gets specified for that extrusion in the first place:
Thermal conductivity. On a heat sink, 6063’s higher conductivity is a real, measurable advantage over 6061. This is the single most common reason 6063 appears on a thermal management drawing.
Anodize appearance. Lower iron and copper content means 6063 anodizes with fewer visible impurities. Architectural and cosmetic extrusions are specified in 6063 partly because the clear anodized finish comes out brighter and more consistent than 6061 run through the same tank.
When to push back on 6063 for a machined-from-solid part
If a drawing calls for 6063 bar stock or block, and the feature is not riding on thermal or extrusion properties, ask why. Common cases worth a conversation with the designer:
A bracket or housing machined entirely from solid. 6061 will machine faster, hold a better finish on the same tool path, and run about twice the yield strength for the same wall thickness. There is rarely a reason to pay for a slower cut on a weaker part.
A part needing threads or fine features. 6063’s gumminess shows up worst in tapping and small-diameter drilling, where a smeared chip packs the flute instead of clearing it. 6061 is more forgiving here.
Anything qualified by test or by material certification. Switching alloys after a part has been validated is a paperwork and requalification problem, whichever direction it runs.
What this means for the RFQ
If the drawing shows an extruded profile with light end-machining, put “6063 extrusion, [spec] temper” on the request and let the shop confirm they can source that specific profile or die. If the drawing shows a part machined entirely from solid bar or plate, 6061 is very likely the right default unless something specific, thermal performance or a matched architectural finish, says otherwise. Getting this right before the RFQ goes out saves a round of “did you mean” emails and gets you a quote against the alloy the part actually needs.
Sources
Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.
Frequently asked questions
Is 6063 a bad alloy to machine?
Not bad, just gummier. It is softer than 6061 and tends to smear rather than break into a clean chip, especially at low cutting speed. A shop that runs sharp tooling and the right feed handles it fine. What it will not do is match 6061's crisp finish on a heavy roughing pass.
Why does a heat sink drawing specify 6063 instead of 6061?
Thermal conductivity. 6063 conducts heat somewhat better than 6061, and it is the alloy the extrusion die was cut for in the first place. The machining on that drawing is almost always finishing an extruded fin stock, not removing metal from solid bar.
Can I substitute 6061 for 6063 to get a better machined finish?
Only if the part is not riding on 6063's extrudability or its thermal or anodize properties. A structural bracket machined from solid, yes. A finned heat sink or an anodized architectural profile, no, because those shapes exist as 6063 extrusions and are not available in 6061 in the same form.
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