Guide
6061 vs 6063 aluminum: which should a machined or extruded part use?
6061 vs 6063 aluminum is a choice between two heat-treatable aluminum-magnesium-silicon alloys that look alike on a parts list but are built for different jobs: 6061 is the stronger general-purpose alloy sold as bar, plate and structural extrusions, while 6063 is the leaner "architectural" alloy designed to be pushed through complex extrusion dies and to anodize with a bright, even surface. For a part machined from solid stock, 6061-T6 is usually the right call; for a part whose cross-section can come straight off an extrusion die and needs only a few machined features, 6063 is often the better route.
By Saola Manufacturing engineering team · Published · Updated
This guide is written for design engineers and buyers deciding which alloy to put in the material block, and for anyone who has received a quote that proposes one in place of the other. It covers strength, conductivity and anodize appearance, why the two alloys come in different stock forms, how each behaves under a cutter, a decision rule for extrusion-plus-machining versus cutting from bar, and how to write a material callout that does not leave "equivalent" open to interpretation.
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What is the difference between 6061 and 6063 aluminum?
6061 carries more magnesium and silicon plus deliberate copper and chromium additions, which make it stronger; 6063 is a leaner alloy with almost no copper or chromium, which makes it easier to extrude into thin, complex shapes and gives it a cleaner anodized finish.
Both alloys are registered by the Aluminum Association, which publishes the chemistry of every wrought alloy in its Teal Sheets. The composition table below shows where they part ways. The higher magnesium and silicon in 6061 form more of the Mg2Si precipitates that give 6xxx alloys their aged strength, and its copper adds a little more. The chromium in 6061 controls grain structure but also makes the alloy more quench-sensitive, so it needs a faster quench after solution heat treatment to reach full strength.
6063 goes the other way. Lower alloy content means lower flow stress in the extrusion press, so a mill can push it at higher speed through dies with thin walls, deep channels and sharp detail, and the low quench sensitivity lets the profile be quenched right at the press exit. That is why window frames, heat sinks, enclosure profiles, rails and tubing are so often 6063, and why a 6063 part almost always starts life as an extrusion.
| Element | 6061, % | 6063, % |
|---|---|---|
| Silicon (Si) | 0.40–0.8 | 0.20–0.6 |
| Iron (Fe) | 0.7 | 0.35 |
| Copper (Cu) | 0.15–0.40 | 0.10 |
| Manganese (Mn) | 0.15 | 0.10 |
| Magnesium (Mg) | 0.8–1.2 | 0.45–0.9 |
| Chromium (Cr) | 0.04–0.35 | 0.10 |
| Zinc (Zn) | 0.25 | 0.10 |
| Titanium (Ti) | 0.15 | 0.10 |
| Others, each / total | 0.05 / 0.15 | 0.05 / 0.15 |
Which is stronger, 6061-T6 or 6063-T5 and 6063-T6?
6061-T6 is clearly stronger: its specified minimum yield strength for extruded products is well above 6063-T6 and more than double 6063-T5.
Per ASTM B221, extruded 6061-T6 has a minimum tensile strength of 38 ksi and a minimum yield of 35 ksi, against 30 ksi and 25 ksi for 6063-T6. 6063-T5, the most common temper for architectural and enclosure profiles, is lower again, and its minimums drop slightly in thicker sections. The typical values in the table, which describe average production material rather than guaranteed minimums, keep the same ranking. Design to the minimums, never the typical values.
Stiffness is the exception. All three tempers share essentially the same elastic modulus, so if a 6063 profile deflects too much, switching to 6061 does not help; adding section depth does. The table also shows what 6063 gives back in return for its lower strength: higher electrical and thermal conductivity, which is one reason it is so common in extruded heat sinks and conductive profiles (dedicated bus-conductor work usually calls for 6101 instead), and a clearer anodized appearance.
The temper names matter as much as the alloy. Per ANSI H35.1, T5 means cooled from the elevated-temperature shaping process and then artificially aged, which is what a press-quenched 6063 extrusion receives. T6 means solution heat-treated and then artificially aged. 6063-T6 is a 6063 extrusion given a proper solution treatment or an effective press quench, so it reaches higher strength than T5 from the same die.
| Property | 6061-T6 | 6063-T5 | 6063-T6 |
|---|---|---|---|
| Tensile strength, minimum, ksi (MPa) | 38 (262) | 22 (152) up to 0.500 in; 21 (145) over 0.500 to 1.000 in | 30 (207) |
| Yield strength, minimum, ksi (MPa) | 35 (241) | 16 (110) up to 0.500 in; 15 (103) over 0.500 to 1.000 in | 25 (172) |
| Tensile strength, typical, ksi (MPa) | 45 (310) | 27 (186) | 35 (241) |
| Yield strength, typical, ksi (MPa) | 40 (276) | 21 (145) | 31 (214) |
| Elongation in 2 in, typical, % | 12 | 12 | 12 |
| Brinell hardness, typical, HB | 95 | 60 | 73 |
| Electrical conductivity, typical, % IACS | 43 | 55 | 53 |
| Thermal conductivity, typical, W/m·K | 167 | 209 | 200 |
| Elastic modulus, typical, Msi (GPa) | 10.0 (68.9) | 10.0 (68.9) | 10.0 (68.9) |
| Clear Type II anodize appearance | Good; can show a faint gray or yellow cast and more visible grain | Bright, clear and uniform; the usual architectural choice | Bright, clear and uniform |
Why is 6063 an extrusion alloy while 6061 is sold as bar and plate?
6063 is formulated to extrude fast through complex dies and to be quenched at the press, so mills offer it mainly as profiles and tube; 6061 is harder to extrude and quench-sensitive, but its higher strength makes it the default alloy for rolled plate, cold-finished bar and structural extrusions.
In practice this decides what you can buy. A distributor will usually stock 6061 as plate to ASTM B209, rolled or cold-finished bar to ASTM B211, and extruded bar, shapes and tube to ASTM B221, often in the stress-relieved T651 and T6511 tempers that hold their shape during heavy machining. 6063 is mostly sold as extruded profiles, tube and pipe in T5 or T6, with some extruded round and rectangular bar; thick 6063 plate is rarely offered.
The consequence for a machined part is simple. If you draw a block, a manifold or a bracket that has to be cut from solid, 6061 is the alloy a shop can buy in the size you need, with a mill certificate and a stress-relieved temper. If you specify 6063 for that kind of part, you are either paying for odd stock or inviting a substitution request. 6063 earns its place when the part's cross-section is the shape that comes out of the die, so most of the geometry is already there before any cutter touches it.
How do 6061 and 6063 machine, and how do they anodize?
6061-T6 machines cleanly with controllable chips and modest burrs, while 6063, especially in T5, is soft and gummy: it builds up on the cutting edge, makes long stringy chips and leaves heavier burrs.
Machining. The lower hardness of 6063 is the root cause. Softer material deforms ahead of the tool instead of shearing cleanly, so it smears onto the cutting edge, pushes burrs over the exit side of holes and slots, and tears on thin walls. Sharp, polished, high-rake carbide tools, generous coolant and climb milling help; so do deburring operations planned into the routing rather than left to the end. 6063-T6 is noticeably better behaved than T5 because it is harder. 6061-T6 is not a free-machining alloy either and still makes stringy chips on a lathe, but it holds an edge and a thread form much better, which is why it is the usual choice for tight-tolerance milled and turned parts. Neither alloy should be machined in the soft O or T4 temper if it can be avoided. For finishing burrs on either alloy, see deburring methods for machined parts.
Workholding on profiles. Thin-wall 6063 profiles distort under vise pressure and spring back after unclamping, so dimensions measured in the fixture may not match the free state. Fixtures that support the walls, soft jaws shaped to the profile, and light finishing passes keep the part true. Say on the drawing whether dimensions apply in the free or the restrained state.
Anodizing. Both alloys take Type II and Type III coatings under MIL-A-8625. 6063 is the preferred cosmetic alloy: its low iron, copper and chromium give a bright, clear, uniform clear coat and even dye uptake. 6061 anodizes well for protection, but its higher alloy content can give a faint gray or yellow cast, a less saturated dyed color and more visible grain or die lines. If a 6063 extruded cover and a 6061 machined bracket must match after anodize, expect a shade difference; keep color-matched parts on one alloy or approve a color range, as covered in anodize color matching.
When does an extruded 6063 profile plus CNC operations beat cutting the part from 6061 bar?
Choose an extruded 6063 profile with machined features when the part has a constant cross-section, its loads fit inside 6063-T6 minimums with your design margin, and the volume is high enough to pay for a custom die; otherwise machine it from 6061 bar or plate.
A custom extrusion die is a one-time cost, but every part made from it starts near net shape: fins, channels, slots and wall thicknesses come from the die, and the machining is reduced to cutting to length, facing, drilling, tapping and a few pockets. Cutting the same part from solid 6061 removes most of the stock as chips, which costs spindle time and material on every part. The table turns that trade into a rule a buyer can apply at the RFQ stage.
Two cautions keep the rule honest. First, standard extrusion dimensional tolerances per ANSI H35.2 are much looser than machined tolerances, and they vary with the profile's size and wall thickness, so any surface that locates, seals or carries a bearing should get a machining allowance and a machined callout. Second, if the part needs 6061-level strength but you still want the extrusion route, 6061-T6 and 6061-T6511 profiles exist; they cost more per foot and cannot match 6063 for thin-wall detail, but they keep the strength.
| Check | Favors 6063 profile + CNC | Favors machining from 6061 bar or plate |
|---|---|---|
| Cross-section | Constant along one axis (rail, heat sink, channel, enclosure body, tube) | Varies in all three axes (blocks, manifolds, brackets with bosses) |
| Stress at worst load | Below 6063-T6 minimum yield of 25 ksi (172 MPa) with your safety factor, or below 6063-T5 minimums for T5 | Needs the 35 ksi (241 MPa) minimum yield of 6061-T6 |
| Material removed | Mostly cut-to-length, holes, threads and shallow pockets | Pockets and contours remove most of the stock either way |
| Volume | Repeat production that amortizes a custom die | Prototypes and low volume, or a design still changing |
| Cosmetic finish | Visible anodized surfaces, color-matched families | Functional anodize or no finish |
| Heat or current path | Heat sinks, conductive rails and profiles | Conductivity not a requirement |
| Thread and feature quality | Few threads; inserts acceptable in thin walls | Many fine threads, tight bores, sharp edges |
How should a drawing call out 6061 or 6063, including an "or equivalent" note?
Name the alloy, the temper and the product specification together, and replace "or equivalent" with an explicit list of approved alternates so the supplier never has to judge equivalence.
"6061 or equivalent" is the callout that causes the most trouble, because the supplier decides what equivalent means and the drawing no longer controls strength, finish or conductivity. These examples leave no room for interpretation:
- Machined from solid:
MATERIAL: ALUMINUM ALLOY 6061-T651 PLATE PER ASTM B209 OR 6061-T6/T6511 BAR PER ASTM B211 OR B221. NO SUBSTITUTION WITHOUT WRITTEN APPROVAL. - Extruded profile with machined features:
MATERIAL: ALUMINUM ALLOY 6063-T5 EXTRUSION PER ASTM B221, DIE NO. [xxx]. ALTERNATE: 6063-T6 OR 6061-T6 PER ASTM B221. MATERIAL CERTIFICATE REQUIRED. - Cosmetic anodized family:
ALL PARTS IN THIS ASSEMBLY SAME ALLOY AND TEMPER. DO NOT MIX ALLOYS WITHIN ONE LOT.
Note the direction of each alternate. 6061-T6 can usually replace 6063 on strength, but it changes anodize appearance and conductivity, and it is harder to extrude in the same thin-wall shape. 6063 replacing 6061 lowers the strength minimums, so any stress check done on 6061 figures must be rerun before approval. A bare alloy number with no temper is the other common gap: it allows soft O or T4 stock that machines poorly and fails the strength assumption.
When we review a drawing for an RFQ, we look first at the material block: alloy, temper, product specification, whether alternates are listed, and whether the anodize callout needs one alloy across the assembly. For turned parts from aluminum bar, our CNC turning page lists 6061-T6 among the bar materials, and 6063 is not on that list. Our CNC milling covers secondary operations on near-net parts: datum faces, bores, holes and threads. If the starting stock is a profile you furnish or one bought to your die drawing, say who supplies it in the RFQ. Send the drawing, the material note and the finish callout together through the quote form, and see how to prepare a machining RFQ for the rest of the package.
How can you tell 6061 from 6063 on a received part?
Chemistry is the only conclusive test: the mill certificate or a spectrometer reading shows copper and chromium, which are well above 6063's limits in 6061.
Per the Aluminum Association registration, 6061 requires at least 0.15% copper and 0.04% chromium, while 6063 allows at most 0.10% of each, so a certificate's copper line usually settles the question. A handheld XRF can measure copper, but light elements such as magnesium are hard for it to quantify reliably, so treat it as a screen; optical emission spectroscopy is more complete. See PMI and XRF material verification for what each method can and cannot confirm.
Hardness and eddy-current conductivity are useful supporting checks. Typical Brinell hardness is about 95 HB for 6061-T6 against about 60 HB for 6063-T5, and typical conductivity is about 43% IACS for 6061-T6 against about 55% IACS for 6063-T5, so a large gap points to the alloy and temper. They cannot separate 6063-T6 from an under-aged 6061 with confidence, which is why they back up chemistry rather than replace it. The method for hardness and conductivity checks on finished parts is in how to verify aluminum temper and hardness.
Frequently asked
Is 6063 aluminum good for CNC machining?
It can be machined well, but it is softer and gummier than 6061-T6, so it needs sharp, polished tools, good coolant and planned deburring. It makes sense when the part starts as a 6063 extrusion; for parts cut from solid, 6061-T6 is the easier and more widely stocked choice.
Is 6063-T5 the same as 6063-T6?
Same alloy, different heat treatment. T5 is cooled from the extrusion process and then aged; T6 is solution heat-treated, or effectively press-quenched, and then aged, which gives it higher specified minimum strength. Name the temper on the drawing, because the two are not interchangeable on strength.
Can 6061 be extruded into custom profiles?
Yes. 6061-T6 and T6511 profiles are common for structural shapes. The alloy extrudes more slowly than 6063 and cannot reach the same thin-wall detail or surface finish, so a profile designed around 6063 may need thicker walls or simpler features in 6061.
Which alloy is better for a heat sink?
For extruded heat sinks, 6063 is the usual choice because it conducts heat better than 6061 and extrudes into tall, thin fins. Machined heat sinks from plate are often 6061 simply because that is the plate stock available.
Is 6063 more corrosion resistant than 6061?
Its lower copper content tends to give slightly better general corrosion resistance, and it is widely used outdoors in architecture. Once either alloy is anodized or coated, the finish matters far more than the choice between them.
Sources
- International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys (Teal Sheets), The Aluminum Association, August 2018 edition (with addenda through 2025)
- ASTM B221-21, Standard Specification for Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes, ASTM International, 2021
- ASTM B211/B211M-23, Standard Specification for Aluminum and Aluminum-Alloy Rolled or Cold Finished Bar, Rod, and Wire, ASTM International, 2023
- ASTM B209/B209M-21a, Standard Specification for Aluminum and Aluminum-Alloy Sheet and Plate, ASTM International, 2021
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, US Department of Defense, 1993, with Amendment 1 (2003)
- Aluminum Design Manual, The Aluminum Association, 2020 edition (minimum mechanical properties of extruded products)
- ANSI H35.1/H35.1M, Alloy and Temper Designation Systems for Aluminum, and ANSI H35.2, Dimensional Tolerances for Aluminum Mill Products, The Aluminum Association (editions current in 2026)
- ASM Handbook, Volume 2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, ASM International, 1990 (typical properties)