Dewey Wu General Manager & senior mechanical engineer at EPOC CRAFTER, 15 years in design engineering, quality, and metallurgy. Hands-on across CNC machining, metalwork, sheet metal, and prototyping (subtractive + 3D printing).
6061 aluminum is a magnesium and silicon (Al-Mg-Si) wrought alloy that reaches 310 MPa (45 ksi) tensile and 276 MPa (40 ksi) yield in the T6 temper per ASTM B209/B209M-21a. That property set covers most CNC parts, but the alloy designation alone does not tell you what to order. A 6061-T6 plate and a 6061-T651 plate hold the same strength minimums, yet one moves 0.010 in to 0.030 in during heavy machining while the other stays under 0.002 in. Pick the wrong temper on a thin wall and you scrap the part. This guide covers the four decisions that turn 6061 into a buyable spec: alloy choice against 7075, 6063, and 5052; temper choice among T6, T651, and T6511; product form and supply thickness; and surface finish. Your part geometry, feature type, setup, required surface finish, quantity, and inspection method all shift which answer is correct.
1. What 6061 Aluminum Is and Why It Dominates CNC Parts
6061 aluminum is a heat-treatable Al-Mg-Si alloy (UNS A96061) that combines 310 MPa (45 ksi) tensile strength in T6 with a density of 2.70 g/cm3, which is why it appears on more CNC quotes than any other aluminum grade. Its strength comes from magnesium silicide (Mg2Si) precipitates formed during solution heat treatment and artificial aging, not from the base aluminum. You get a material that machines cleanly, welds with standard filler, and anodizes evenly, without the cost or corrosion problems that come with high-strength grades.
The composition is controlled by narrow limits. Magnesium and silicon do the strengthening work; copper adds strength but pulls down corrosion resistance; chromium refines grain structure. The values below are the sheet and plate limits set by ASTM B209/B209M-21a, Table 1.
Table 1. 6061 chemical composition limits (ASTM B209/B209M-21a, Table 1)
| Silicon (Si) | 0.40 | 0.8 | Forms Mg2Si; drives precipitation strengthening |
| Magnesium (Mg) | 0.8 | 1.2 | Forms Mg2Si; primary strengthener |
| Copper (Cu) | 0.15 | 0.40 | Raises strength, lowers corrosion resistance |
| Chromium (Cr) | 0.04 | 0.35 | Refines grain, resists stress corrosion |
| Iron (Fe) | – | 0.7 | Residual |
| Manganese (Mn) | – | 0.15 | Residual |
| Zinc (Zn) | – | 0.25 | Residual |
| Titanium (Ti) | – | 0.15 | Grain refinement |
| Others, each | – | 0.05 | Residual |
| Others, total | – | 0.15 | Residual |
| Aluminum (Al) | remainder | – | Base metal |
The copper window matters for procurement. At 0.15 to 0.40 percent Cu, 6061 keeps enough copper to age-harden but stays low enough to anodize without the yellow-brown cast you get on copper-rich grades. That single line in the spec is why 6061 anodizes to a clean gray and 2024 or 7075 do not.
Mechanical and physical properties depend on temper. The T6 numbers below are the design values from ASM Handbook Vol.2 measured on standard specimens. Yield strength, not tensile, is your working design limit for static loads.
Table 2. 6061-T6 properties at a glance (ASM Handbook Vol.2, pp.404-405; Brinell at 500 kg, 10 mm ball, 30 s)
| Tensile strength | 310 MPa (45 ksi) | T6 | Fracture limit; margin above yield |
| Yield strength | 276 MPa (40 ksi) | T6, 0.2% offset | Design limit for permanent deformation |
| Elongation | 12% (1.6 mm); 17% (13 mm bar) | T6 | Formability; higher in thicker sections |
| Shear strength | 207 MPa (30 ksi) | T6 | Sizing pins, bolts, keyed shafts |
| Brinell hardness | 95 HB | T6 | Wear and machinability reference |
| Fatigue strength | 97 MPa (14 ksi) | 5×10^8 cycles, R.R. Moore, fully reversed | Cyclic-load limit; no true endurance knee |
| Elastic modulus | 68.9 GPa (10.0×10^6 psi) | Tension | Stiffness; deflection under load |
| Density | 2.70 g/cm3 (0.098 lb/in3) | 20 C | Weight; one-third of steel |
| Thermal conductivity | 167 W/m-K | 25 C, T6 | Heat sinks, cooling plates |
| Electrical conductivity | 43% IACS | 20 C, T6 | Grounding, conductive enclosures |
One number on that table drives more failures than any other. The fatigue strength of 97 MPa is measured at 5×10^8 cycles, and aluminum has no true fatigue limit, so a 6061-T6 part under repeated load will eventually fail even below its yield strength if you size it to static numbers. In a part that sees cyclic loading, designing to the 276 MPa yield instead of the 97 MPa fatigue strength is the mistake, because Mg2Si precipitates resist static deformation far better than crack initiation under reversed stress. The exception is a part that never sees reversed loading, where the yield value governs and fatigue does not apply.
For applications where you machine 6061 into functional parts, the alloy earns its place through balance, not through a peak number, which is what the four gates below help you exploit. If your parts run in electronics enclosures or heat sinks, the 167 W/m-K conductivity and 43 percent IACS make 6061 a default for consumer electronics housings, and our CNC machining process holds these grades in stock.
2. The Four-Gate 6061 Selection Framework
Four-Gate 6061 Selection is a decision sequence that turns the generic label 6061 into a complete purchase spec by resolving four questions in order: alloy, temper, product form, and surface finish. Each gate has a wrong answer that costs you money, and each depends on the gate before it. You cannot pick a temper before you confirm 6061 is the right alloy, and you cannot set an anodizing callout before you know the product form.
Properties and applications alone leave you with data but no decision. The four gates below map the exact order a mechanical engineer or buyer moves through when a 6061 part goes from CAD to a released drawing.
Table 3. The Four-Gate 6061 Selection Framework
| Gate 1. Alloy | Is 6061 right, or does 7075, 6063, or 5052 fit better | Over-spec to 7075 at higher cost, or under-spec and fail under load | ASM Handbook Vol.2 |
| Gate 2. Temper | T6, T651, or T6511 | Warp on thin walls, or pay for stress relief you do not need | ASTM B209, AMS 4027P |
| Gate 3. Product form | Sheet, plate, bar, or extrusion, and at what thickness | Order a temper unavailable in your thickness | ASTM B209, B221, B211 |
| Gate 4. Surface finish | Bare, Type II, or Type III anodize | Blow tolerances on tight fits, or fail wear service | MIL-A-8625F |
According to Dewey Wu, “The parts we scrap almost never fail at Gate 1. They fail at Gate 2, because someone specced T6 plate for a 3 mm wall, and the part rolled 0.08 mm off flat after roughing. The alloy was right. The temper was the problem.”
The four gates carry through the rest of this guide. Gates 1 through 4 each get their own section, and the last section shows the framework applied to a real 6061-T6 job.
3. Gate 1. Alloy Choice: When 6061 Beats 7075, 6063, and 5052
Gate 1 confirms 6061 against three alternatives before you commit: 7075 for strength, 6063 for extruded profiles, and 5052 for formed sheet. 6061 wins when you need a balance of machinability, weldability, and corrosion resistance in a machined part, and it loses when one property dominates the design.
The core trade is 6061 against 7075. 7075 delivers nearly double the strength, but you pay for it in cost, weldability, and corrosion behavior. The values below come from ASM Handbook Vol.2, both grades in the T6 temper, measured on standard specimens.
Table 4. 6061-T6 vs 7075-T6 (ASM Handbook Vol.2; 6061 pp.404-405, 7075 pp.452-462)
| Tensile strength | 310 MPa (45 ksi) | 572 MPa (83 ksi) | 7075 for peak strength-to-weight |
| Yield strength | 276 MPa (40 ksi) | 503 MPa (73 ksi) | 7075 carries higher static load per section |
| Elongation | 12% to 17% | 11% | Similar ductility; 6061 slightly higher in bar |
| Fatigue strength | 97 MPa (14 ksi) | 159 MPa (23 ksi) | 7075 for cyclic-load parts |
| Density | 2.70 g/cm3 | 2.80 g/cm3 | Nearly equal; 7075 wins on strength per weight |
| Thermal conductivity | 167 W/m-K | 130 W/m-K | 6061 for heat sinks and cooling plates |
| Electrical conductivity | 43% IACS | 33% IACS | 6061 for grounding and conductive parts |
Strength is not the whole decision. 7075 contains 1.2 to 2.0 percent copper and 5.1 to 6.1 percent zinc per ASM Handbook Vol.2, which is what raises its strength and also what makes it hard to fusion weld and more prone to stress corrosion cracking. 6061 keeps copper at 0.15 to 0.40 percent, so it welds with standard 4043 or 5356 filler and resists corrosion without a protective coating in most environments.
When the part carries a fatigue-critical load and joins by mechanical fasteners instead of welds, 7075-T6 is the better choice, because its 159 MPa fatigue strength at 5×10^8 cycles beats 6061-T6 at 97 MPa by more than 60 percent. The exception is any welded assembly or any part that sees salt or humidity without a coating, where 6061 wins on weldability and corrosion resistance regardless of the strength gap. For the full trade including cost and machining behavior, see 6061 vs 7075 aluminum comparison.
The other two alternatives resolve faster. 6063 shares the Al-Mg-Si system but runs lower strength, and it exists mainly for extruded profiles with thin walls and cosmetic surfaces. 5052 is a non-heat-treatable Al-Mg alloy that outperforms 6061 on formability and marine corrosion, which is why it goes into bent sheet-metal parts instead of machined blocks.
Table 5. When to leave 6061 for 6063 or 5052
| 6063 | Extruded profile, thin wall, cosmetic anodized surface | Better extrudability and surface finish than 6061 | Lower strength; not for high-load machined parts |
| 5052 | Bent or formed sheet, marine exposure | Higher formability and saltwater corrosion resistance | Non-heat-treatable; cannot reach T6 strength |
For a machined 6061 part, alloy substitution rarely helps once the design needs both strength and machinability, because 6061 is the only common aluminum that holds 276 MPa yield while still cutting at a 95 HB hardness that limits tool wear. The exception is a pure sheet-metal part with no machined features, where 5052 in our sheet metal fabrication process forms tighter bend radii than 6061 without cracking.
4. Gate 2. Temper Choice: T6 vs T651 vs T6511
Gate 2 sets the temper: T6 for general machined parts, T651 for flatness-critical plate work, and T6511 for machined extrusions. All three reach the same 290 MPa (42 ksi) tensile and 240 MPa (35 ksi) yield minimums per ASTM B209/B209M-21a, so temper is not a strength decision. Temper is a residual-stress decision, and it is where most 6061 parts go wrong.
The difference is stress relief. T6 is solution heat treated and artificially aged, which locks residual stress into the material from the quench. T651 adds a controlled stretch of 1.5 to 3 percent permanent set before aging, per AMS 4027P, which pulls that stress out. T6511 applies the same stretch to extruded bar and profile. When you machine a stressed billet, the stress redistributes as you remove material, and the part moves.
Table 6. 6061 temper selection for machined parts
| T6 | Solution treat + age | High (quench stress retained) | Symmetric parts, low stock removal, thin sheet under 0.25 in | ASTM B209 |
| T651 | Solution treat + stretch 1.5% to 3% + age | Low | Thin walls, large flat plates, tight flatness, heavy stock removal | AMS 4027P |
| T6511 | Same stretch, extruded form | Low | Machined extrusions, long structural profiles | ASTM B221 |
The stress difference shows up as measurable movement. A 6061-T6 part can move 0.010 in to 0.030 in after heavy stock removal, while a 6061-T651 part stays under 0.002 in under the same cut. That is a 5x to 15x difference in dimensional stability, driven entirely by the stretch step, not by any change in strength or composition.
When you machine a thin wall under 3 mm or a large flat plate that must hold flatness, T651 is the correct choice, because its stretched condition holds movement under 0.002 in where T6 would roll 0.010 in or more off flat after roughing. The exception is a thin sheet part under 0.25 in thick, where T651 plate is not stocked and T6 sheet is your only supply option anyway, which Gate 3 explains. For the full temper decision including cost and machining strategy, see 6061-T6 vs T651 temper selection.
According to Dewey Wu, “We keep 6061-T651 plate in stock specifically for thin-wall work. On a 3 mm wall, T6 will move enough to blow a 0.05 mm flatness callout after you rough one side. T651 holds it. The material choice at Gate 2 saves a stress-relief anneal you would otherwise pay for downstream.”
Engineering note. Temper substitution follows a one-way rule. An order calling for 6061-T6 can be filled from T6511 extrusion, but not the reverse, because the tempers are dual-certified in one direction only. Confirm the certification direction on the mill test report before you accept a substitution.
5. Gate 3. Product Form and Supply Thickness
Gate 3 matches your temper to a product form and a stocked thickness: sheet and plate for flat parts, bar and rod for turned parts, and extrusion for profiles. The trap at this gate is calling out a temper that does not exist in your thickness. 6061-T6 and 6061-T651 are not interchangeable across thickness because the standards split them by section size.
The split is set by ASTM B209/B209M-21a for sheet and plate. 6061-T6 sheet runs 0.006 in to 0.249 in thick, and 6061-T651 plate runs 0.250 in to 6.000 in thick. There is no overlap. Below 0.25 in you order T6 sheet; at 0.25 in and above you order T651 plate. A 6061-T651 callout on a 2 mm part is unfillable and stalls your order, because T651 plate starts at 0.250 in.
Table 7. 6061 product form and thickness by standard
| Sheet | 6061-T6 | 0.006 in to 0.249 in (0.15 mm to 6.30 mm) | ASTM B209/B209M-21a |
| Plate | 6061-T651 | 0.250 in to 6.000 in (6.30 mm to 150 mm) | ASTM B209/B209M-21a |
| Extruded bar, rod, profile | 6061-T6, T6510, T6511 | Section 0.250 in and over: 38 ksi tensile, 35 ksi yield min | ASTM B221-02 |
| Cold-finished bar, rod | 6061-T6, T651 | 0.125 in to 8.000 in dia; bar max 50 in2 cross-section | ASTM B211-03 |
Product form also changes the strength minimums you can hold. Extruded 6061-T6 per ASTM B221-02 carries a 38 ksi (262 MPa) tensile minimum, lower than the 42 ksi (290 MPa) minimum for sheet and plate per ASTM B209. Cold-finished bar per ASTM B211-03 returns to the 42 ksi tensile and 35 ksi yield minimums. If your design leans on the full 290 MPa, specifying an extruded form quietly drops your guaranteed floor to 262 MPa.
When your part is a long structural profile with a constant cross-section, an extruded 6061-T6511 form is the right call, because extrusion produces the shape near net and the T6511 stretch holds it straight for machining. The exception is any part needing the full 290 MPa tensile floor, where you specify plate or cold-finished bar to ASTM B209 or B211, not extrusion, since extruded B221 stock only guarantees 262 MPa. For long-run profile parts, our low volume production route sources extruded stock to the correct temper certification.
For aerospace and defense work, the supply spec tightens further. 6061-T6 sheet and T651 plate to aerospace requirements fall under AMS 4027P, which holds the same 42 ksi (290 MPa) tensile and 35 ksi (241 MPa) yield minimums but adds heat-treat traceability to AMS2772 and covers plate to 6.000 in. Reference this callout when the part needs a certified aerospace pedigree, and pair it with the tolerance controls in our DFM design guidelines.
Engineering note. Ordering thickness sets your temper before you finish the drawing. Fix the part thickness first, read the temper that ships in that thickness from Table 7, then write the callout. Writing the temper first and discovering the thickness conflict later is the common ordering error at this gate.
6. Gate 4. Surface Finish and Anodizing 6061
Gate 4 sets the surface finish: bare for internal parts, Type II anodize for cosmetic and corrosion-protected parts, and Type III hardcoat for wear surfaces. 6061 anodizes more evenly than most machinable aluminum grades because its copper stays at 0.15 to 0.40 percent per ASTM B209, low enough to avoid the dark, blotchy cast that copper-rich alloys produce. The gate matters because anodizing grows the part and changes your tolerances.
The two finishes that cover most CNC work are Type II and Type III per MIL-A-8625F. Type II is a conventional sulfuric anodize for appearance and corrosion protection. Type III is a hardcoat for wear resistance. The thickness ranges below come from MIL-A-8625F, and thickness is what drives the dimensional change you have to plan for.
Table 8. 6061 anodize types (MIL-A-8625F)
| Type II | Sulfuric, conventional | 0.00007 in to 0.0010 in | Cosmetic parts, corrosion protection, dyed color | Class 1 undyed, Class 2 dyed |
| Type III | Hardcoat, dense film | 0.0005 in to 0.0045 in (default 0.002 in) | Wear surfaces, valves, slides, pistons, gears | Class 1 undyed, Class 2 dyed |
Anodize thickness is not a coating on top of the part. The oxide film grows into the surface and out from it, roughly half each way, so a Type III hardcoat at the default 0.002 in per MIL-A-8625F adds about 0.001 in to each external dimension and shrinks each bore by the same amount. On a bearing bore or a threaded feature, that shift eats your tolerance if you machine to nominal and anodize afterward.
When a part has a bore or thread tighter than 0.001 in and also needs Type III hardcoat, machining to a pre-anodize dimension that accounts for growth is the correct approach, because the 0.002 in film per MIL-A-8625F changes both the bore and the outside diameter. The exception is a masked feature, where you specify no anodize on the critical surface and machine it to final size, since masking removes the growth entirely on that feature. Our surface finishing process maps these features before machining so the anodize allowance is built into the toolpath.
The drawing callout follows a fixed structure. MIL-A-8625F requires the type, class, color, and sealing to be specified, and leaving any of them blank lets the finisher choose for you.
Table 9. 6061 anodize drawing callout (MIL-A-8625F, Section 6.2)
| Standard | Spec and revision | MIL-A-8625F |
| Type | I, II, or III | TYPE II |
| Class | 1 undyed or 2 dyed | CLASS 2 |
| Color | Required for Class 2 | COLOR BLACK |
| Sealing | Special requirement if any | SEALING PER 3.8 |
A complete callout reads: ANODIZE PER MIL-A-8625F, TYPE II, CLASS 2, COLOR BLACK, SEALING PER 3.8. For the full anodizing decision including pre-anodize sizing, surface prep, and masking, see pre-anodize machining allowance for 6061.
Engineering note. Type III sealing is a trade, not a default. MIL-A-8625F states that Type III coatings intended for maximum abrasion resistance shall not be sealed, because sealing softens the surface. Specify sealing only when corrosion protection outranks wear on that part.
7. Machinability and What It Costs You
6061-T6 machines at a 95 HB hardness with short, breakable chips, which is why it holds a machinability rating near the top of the structural aluminum grades and runs faster than 7075 or stainless. 6061-T6 machinability is a cost lever: faster cutting, longer tool life, and cleaner surface finish all come from the same balanced hardness. The property that makes 6061 easy to cut is the same one that keeps its yield at 276 MPa.
The behavior to manage is built-up edge. 6061 welds to the cutting edge if chip thickness is too low or evacuation is poor, which tears the surface and shortens tool life. The fix is chip load and evacuation, not slower speed. Running the tool too slow makes it rub instead of cut, which builds the edge faster.
Table 10. 6061-T6 machining behavior and the cost it drives
| Hardness | 95 HB (ASM Handbook Vol.2) | Lower tool wear than 7075; longer tool life |
| Chip form | Short, breakable in T6 | Clean evacuation; fewer stoppages |
| Built-up edge | Forms at low chip load | Surface tearing; plan chip load, not lower RPM |
| Surface finish | Ra 0.8 um to 1.6 um from CNC | Meets most functional callouts without secondary ops |
| Thin-wall stability | Depends on temper (Gate 2) | T651 holds under 0.002 in; T6 moves 0.010 in or more |
Surface finish ties back to your finishing decision. A standard CNC finish on 6061-T6 lands at Ra 0.8 um to 1.6 um, which meets most functional requirements directly, but a cosmetic Type II anodize shows every tool mark, so a cosmetic part needs a finishing pass or bead blast before anodize. Speccing a cosmetic finish without budgeting that pass is the common surprise on the quote.
When a 6061-T6 part has a cosmetic anodized A-surface, adding a finishing pass to reach Ra 0.8 um before anodize is the correct approach, because the anodize film magnifies any tool mark left at Ra 1.6 um. The exception is a functional internal surface, where Ra 1.6 um straight off the tool is acceptable and the extra pass only adds machine time without value.
On cost against 7075, 6061 runs faster and wears tooling less, so the machined-part cost gap is wider than the raw-stock gap alone. 7075 raw stock runs roughly 60 to 90 percent more than 6061 by industry pricing, and the higher tool wear and slower cutting on 7075 add to that on the machined price. For parts where 6061 meets the load requirement, the total cost case favors 6061 before you even reach the strength question, which is why we validate the alloy against your load case during rapid prototyping on the first article.
According to Dewey Wu, “The mistake we see on 6061 is people slowing the spindle to protect the tool. That builds a welded edge and tears the finish. On 6061-T6 you keep the chip load up and clear the chips, and the tool lasts longer at higher speed than it does crawling.”
8. A Real 6061-T6 Job
A semiconductor cooling cavity with a 3.2 mm wall came off the line at 70 percent first-pass yield, leaking at the weld seam after friction stir welding. The alloy was right. The temper was defensible. The warp came from residual stress stacking across the process route, and fixing it took a rebuild of the operation sequence, not a material swap.
The part. A closed-loop water-cooling cavity for a semiconductor inspection system, machined from 6061-T6, 425 x 218 x 72 mm, with a thinnest wall of 3.2 mm. Volume was 350 production pcs, preceded by five pilot rounds of 10 pcs each. The route combined 5-axis CNC roughing, segmented friction stir welding (FSW), low-temperature stress-relief aging, second-operation CNC finishing, and anodizing.
The problem. The 3.2 mm sealing face warped after friction stir welding, and the weld line leaked under water-pressure hold. In one 10-piece pilot batch, 3 pieces leaked, the sealing face warped up to 0.08 mm against a flatness limit of 0.04 mm or less, and first-pass yield sat at 70 percent. Root cause: machining residual stress and FSW thermal stress released together, and on a low-stiffness thin wall an unfavorable process sequence drove the sealing face into irreversible warp.
The fix. The route was rebuilt around stress control at every stage, not one material swap. The billet was given a 170 C low-temperature anneal to clear rolling stress. Roughing left 0.12 mm per side, followed by 48 h of natural aging to release cutting stress. FSW was run as segmented, symmetric, interrupted welds, then a 120 C stabilizing age locked the structure before the sealing face and locating holes were finish-machined to a flatness inside 0.035 mm.
The result. Overall cavity distortion dropped from a peak of 0.07 mm to within 0.03 mm. Water-pressure first-pass yield rose from 70 percent at pilot to 99.7 percent in production, with 1 microleak across 350 pieces, repaired and passed. Removing the leak rework and sealing-face regrind cut per-part rework cost by 62 percent and held delivery at 3 weeks against a 4-week plan.
Engineer quote. According to Dewey Wu, “Most shops only tune the friction stir weld parameters and miss how cutting residual stress and weld heat stress stack on top of each other. Finish-machine a thin wall first and then weld it, and you add the two distortions together, which leaves you almost no way to save the sealing face. Sequencing the operations and relieving stress in stages is the only way to control the warp and the leak at the source.”
9. FAQ
Is 6061 aluminum stronger than 7075?
No. 7075-T6 reaches 572 MPa (83 ksi) tensile and 503 MPa (73 ksi) yield per ASM Handbook Vol.2, nearly double 6061-T6 at 310 MPa tensile and 276 MPa yield. 7075 also holds a higher fatigue strength at 159 MPa versus 97 MPa at 5×10^8 cycles, which is why it goes into cyclic-load aerospace parts. 6061 wins on weldability, corrosion resistance, and cost, not on strength. Choose 7075 when strength-to-weight drives the design and the part joins by fasteners. Choose 6061 when the part welds, sees salt or humidity, or does not need the extra strength, since over-speccing to 7075 raises both material and machining cost with no benefit.
Do I need 6061-T651 or T6 for machining?
Choose T651 for thin walls, large flat plates, or any part with a tight flatness callout, because T651 holds movement under 0.002 in after heavy stock removal while T6 can move 0.010 in to 0.030 in. Both hold the same 290 MPa tensile and 240 MPa yield minimums per ASTM B209, so this is a stress decision, not a strength one. T6 is the right call for symmetric parts with low stock removal, or for sheet thinner than 0.25 in where T651 plate is not stocked. On an asymmetric part with heavy one-sided material removal, T651 prevents the warp that forces a second setup or a scrapped part.
Can you weld 6061 aluminum?
Yes. 6061 welds with standard 4043 or 5356 filler because its copper stays at 0.15 to 0.40 percent per ASTM B209, which keeps crack sensitivity low. Post-weld, the heat-affected zone loses some strength, and re-aging to T6 can partly restore it. For anodized parts, specify 5356 filler rather than 4043, because 4043 is high in silicon and anodizes to a dark line that shows against the parent metal, while 5356 stays close to the base color. 7075, by contrast, is considered unweldable by conventional fusion methods because it hot-cracks in the weld zone.
What temper should I order for a thin-wall part?
Order 6061-T651 for walls under 3 mm. The stretch of 1.5 to 3 percent per AMS 4027P relieves the quench stress that makes T6 walls roll off flat after roughing. On a 3 mm wall, T6 can move enough to blow a 0.05 mm flatness callout, while T651 holds under 0.002 in. The exception is a wall thinner than 0.25 in total part thickness, where only T6 sheet ships and you manage distortion through machining strategy instead, using symmetric stock removal and a stress-relief pass between roughing and finishing to keep the part within tolerance.
Does 6061 anodize well?
Yes. 6061 anodizes evenly because its copper content stays at 0.15 to 0.40 percent per ASTM B209, low enough to avoid the dark, blotchy finish that copper-rich grades like 2024 and 7075 produce. It accepts both Type II decorative and Type III hardcoat per MIL-A-8625F. Plan for oxide growth: a Type III film at the default 0.002 in adds roughly 0.001 in per external surface and shrinks bores by the same amount, so machine to a pre-anodize dimension on tight features. For a bore held tighter than 0.001 in, mask the feature or machine it after anodizing to hold the fit.
What surface finish does CNC machining hold on 6061?
A standard CNC finish on 6061-T6 lands at Ra 0.8 um to 1.6 um, which meets most functional requirements straight off the tool. For cosmetic anodized surfaces, a finishing pass or bead blast to Ra 0.8 um is needed first, because the anodize film magnifies any tool mark left at Ra 1.6 um. Built-up edge is the main finish risk on 6061, controlled by keeping chip load up and evacuation clear rather than slowing the spindle. Running the tool too slow makes it rub instead of cut, which welds aluminum to the edge and tears the surface.
What is the difference between 6061 and 6061-T6?
6061 names the alloy; T6 names the temper. 6061 without a temper suffix is the base composition and carries no guaranteed mechanical minimums. 6061-T6 has been solution heat treated and artificially aged to reach 310 MPa tensile and 276 MPa yield per ASTM B209. Always specify the temper on a drawing, because 6061 alone does not tell the supplier what strength or stress condition to ship. A part ordered as bare 6061 could arrive in the O temper at a fraction of T6 strength, which fails a design sized to 276 MPa yield.
Is 6061-T6 aluminum aircraft grade?
Yes, for non-primary structure. 6061-T6 meets aerospace sheet and plate requirements under AMS 4027P, which adds heat-treat traceability to AMS2772 on top of the ASTM B209 property minimums. It is used for secondary structure, brackets, and housings, not for primary fatigue-critical load paths like wing spars, where higher-fatigue grades such as 7075 or 2024 are specified. Reference AMS 4027P on the drawing when the part needs a certified aerospace pedigree, since a plain ASTM B209 callout does not carry the traceability an aerospace buyer requires.
Related Resources
Related Capability: CNC Machining– Confirms your 6061 temper and finish before cutting, so the part ships to spec.
Related Article: 6061 vs 7075 Aluminum – The full alloy trade including cost, machining, and welding behavior.
Related Article: 6061-T6 vs T651 vs T6511 – Which temper to specify to avoid post-machining warp.
Related Article: Machining 6061 for Anodize – Pre-anodize dimensions, surface finish, and masking decisions.
Related Standards Reference: Materials and Properties – ASTM B209, B221, B211, and AMS 4027P data behind these tables.
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