If you’ve spent any time machining 6082-T6 extrusions with thin walls or intricate cross-sections, you’ve probably hit a wall: the extrusion die wears fast, section complexity is limited, and the part cost climbs. EN AW-6351-T6 was developed to solve exactly this problem — it extrudes more easily than 6082 while retaining enough strength for structural applications. For a CNC machinist, that means you’re more likely to receive near-net-shape blanks and spend less time roughing. This article covers what 6351 is, where it outperforms 6082, and what matters when you machine it.
Grade Identity and Standards
6351 is a medium-strength Al-Mg-Si wrought alloy in the 6xxx series, standardized under EN 573-3 as EN AW-6351 and under the former British designation BS HE30. In China, it aligns closest with GB/T 3190 6A02 (formerly LD2), though the silicon and manganese windows differ slightly — they are near-equivalents, not direct substitutes.
The alloy’s defining formulation is approximately AlSi1Mg0.5Mn per EN 573-3. The magnesium content (0.40–0.80%) and silicon content (0.70–1.30%) place it squarely in the heat-treatable 6xxx group, but the manganese addition (0.40–0.80%) is what distinguishes it from 6060 and pushes its extrudability past 6082 in complex profiles.
| Standard | Designation | Notes |
|---|---|---|
| EN 573-3 | EN AW-6351 | European standard, wrought alloy |
| EN 755-2 | EN AW-6351 [Al Si1Mg0.5Mn] | Extruded products specification |
| BS (legacy) | HE30 | British standard, superseded but still referenced |
| GB/T 3190 | 6A02 | Chinese near-equivalent; verify Mn/Si window |
| ISO 209 | AW-6351 | International designation |
Chemical Composition (EN 573-3)
The composition is balanced around three objectives: adequate solution-ageing response from Mg₂Si precipitation, good extrudability from moderate silicon and controlled manganese, and acceptable corrosion resistance for general structural use.
| Element | Content (wt%) | Role in 6351 |
|---|---|---|
| Si | 0.70–1.30 | Forms Mg₂Si precipitates; excess Si aids age hardening |
| Fe | ≤ 0.50 | Impurity; forms AlFeSi intermetallics, reduces ductility |
| Cu | ≤ 0.10 | Kept low for corrosion resistance; trace hardener |
| Mn | 0.40–0.80 | Controls recrystallization during extrusion; improves toughness |
| Mg | 0.40–0.80 | Primary age-hardening element with Si |
| Cr | ≤ 0.10 | Grain refiner; minimal in 6351 |
| Zn | ≤ 0.20 | Impurity; no structural role |
| Ti | ≤ 0.20 | Grain refiner; typical for Al alloys |
| Others each | ≤ 0.05 | Total others ≤ 0.15 |
| Al | Remainder |
Mechanical Properties: T6 Temper
The data below comes from EN 755-2 for extruded profiles in T6 condition. These are minimum guaranteed values, not typical production numbers — real mill output often exceeds the floor by 5–15%, especially in thin-walled sections that quench faster.
| Property | Value | Condition | Standard |
|---|---|---|---|
| Tensile strength, Rm | ≥ 290 MPa | T6, extruded profile, thickness ≤ 25 mm | EN 755-2 |
| Yield strength, Rp0.2 | ≥ 240 MPa | T6, extruded profile, thickness ≤ 25 mm | EN 755-2 |
| Elongation, A | ≥ 8% | T6, gauge 5.65√S₀ | EN 755-2 |
| Elongation, A50mm | ≥ 6% | T6, alternative gauge | EN 755-2 |
| Hardness | ≈ 90–100 HBW | T6, typical (not specified minimum) | EN ISO 6506-1 |
| Shear strength | ≈ 180–200 MPa | T6, typical | — |
| Fatigue strength (5×10⁸ cycles) | ≈ 95–110 MPa | T6, R = −1, smooth specimen | — |
Key takeaway: 6351-T6 sits between 6063-T6 (~215 MPa yield) and 6082-T6 (~260 MPa yield) in the 6xxx family. It’s stronger than 6063 but sacrifices about 20 MPa of yield versus 6082-T6 in exchange for significantly better extrudability. For a CNC shop quoting a machined-from-extrusion part, this trade-off matters: the extruder can hold tighter as-supplied tolerances on complex profiles, which means you machine less off the blank.
Strength Comparison: 6351-T6 vs. Peer Alloys
All values are T6 temper, ≤ 25 mm section thickness, from EN 755-2 minimums.
| Alloy | Rp0.2 min (MPa) | Rm min (MPa) | A min (%) | Extrudability (relative) |
|---|---|---|---|---|
| 6060-T6 | 160 | 215 | 10 | Very high |
| 6063-T6 | 190 | 245 | 8 | High |
| 6351-T6 | 240 | 290 | 8 | Moderate-high |
| 6005A-T6 | 240 | 285 | 8 | Moderate |
| 6082-T6 | 260 | 310 | 8 | Moderate |
| 6061-T6 | 260 | 310 | 10 | Moderate |
Note: 6351-T6 and 6005A-T6 have near-identical published minimums, but 6351’s higher manganese gives it a practical edge in profile complexity. If your drawing calls for thin ribs or deep channels on a long extrusion, 6351 is the stronger choice.
Corrosion Resistance
6351 shares the general corrosion behavior of the 6xxx family: it forms a stable aluminium oxide passive layer in atmospheric and most aqueous environments. EN AW-6351-T6 has good resistance to rural, industrial, and mild marine atmospheres. It is not recommended for continuous seawater immersion without protection — for those conditions, consider anodized 6061 or 5083 marine-grade alloy.
6351-T6 is moderately resistant to stress corrosion cracking (SCC) in T6 temper. The copper content is deliberately kept ≤ 0.10% to avoid the SCC sensitivity seen in higher-copper 2xxx alloys. For maximum corrosion resistance in aggressive environments, architectural anodizing (Type II, Class I per MIL-A-8625) produces a durable finish on 6351.
CNC Machining 6351-T6: Practical Parameters
6351-T6 machines very similarly to 6082-T6 — both are in the “good machinability” bracket for aluminum. The slightly lower strength of 6351 versus 6082 means chip formation is marginally easier, but the difference is subtle enough that most shops use the same parameter set for both.
Starting parameters — these are reference points only. Actual values depend on your machine rigidity, tool holder type (hydraulic vs. shrink-fit vs. ER collet), coolant delivery, and workpiece fixturing.
Turning (CNC Lathe)
| Parameter | Range | Notes |
|---|---|---|
| Cutting speed (Vc) | 200–500 m/min | Uncoated carbide; use upper range for finishing |
| Feed (f) | 0.10–0.30 mm/rev | Roughing; finish at 0.05–0.12 mm/rev |
| Depth of cut (ap) | 0.5–5.0 mm | Roughing; finishing 0.2–0.5 mm |
| Insert geometry | Positive rake, polished chipbreaker | CCGT/DCGT with sharp edge, ~15° rake |
Milling
| Parameter | Range | Notes |
|---|---|---|
| Cutting speed (Vc) | 250–500 m/min | Carbide end mill; 2-3 flute for chip clearance |
| Feed per tooth (fz) | 0.08–0.25 mm/tooth | Slotting at lower end; profiling at upper end |
| Axial DOC (ap) | Up to 1.5×D | Reduce for long-reach or thin-walled work |
| Radial engagement (ae) | 10–60% D | Trochoidal preferred for deep slots |
Drilling
| Parameter | Range | Notes |
|---|---|---|
| Cutting speed (Vc) | 80–180 m/min | Carbide drill; HSS at 40–80 m/min |
| Feed (f) | 0.08–0.30 mm/rev | Diameter-dependent; peck for holes > 4×D |
These are starting references, not guarantees. Your machine’s spindle power curve, tool holder runout, coolant pressure, and workpiece rigidity will shift the optimal window. Always validate on a test piece. The numbers above assume a rigid CNC machining center or lathe with flood coolant and properly maintained tooling — a worn ER collet or weak fixture can halve your effective speed.
Tooling and Coolant Strategy
For 6351-T6:
- Tool material: Uncoated polished carbide works well. If you need coated tools, TiB₂ or DLC coatings prevent aluminum adhesion better than TiAlN or AlTiN, which can build up aluminum on the rake face.
- Chip control: 6351 produces continuous, ductile chips. Use positive-rake inserts with chipbreakers designed for aluminum (not steel-geometry inserts with chipbreakers tuned for short-chipping materials). Through-tool coolant improves chip evacuation in deep pockets.
- Coolant: Flood coolant (5–8% emulsion) is adequate. Minimum-quantity lubrication (MQL) works for finishing passes but may not clear chips fast enough in heavy roughing of pocketed extrusion profiles. If you’re machining dry, expect reduced tool life and possible built-up edge — 6xxx alloys are more forgiving than 5xxx in this regard, but not as forgiving as 2xxx.
- Workholding: Extruded 6351 profiles often have thin walls and unsupported spans. Use soft jaws, vacuum fixturing, or low-melt-point alloys for support. Clamp pressure should not distort the part — a common failure mode when machining thin-walled extrusions is spring-back after unclamping.
Surface Finishing Options
6351-T6 takes surface treatments well, comparable to 6063 and better than high-copper 2xxx alloys:
- Anodizing: Produces a uniform, clear-to-slightly-grey finish. Type II sulfuric anodizing (10–25 μm) is standard for architectural and industrial parts. Hard anodizing (Type III, 25–100 μm) is feasible with reduced growth on sharp edges.
- Powder coating: Excellent adhesion. Chromate conversion coating pretreatment (per EN 12487 or equivalent) is recommended before powder.
- Chemical brightening: Possible but cosmetic quality is below 6063 — 6351’s iron content (≤ 0.50%) and manganese produce a slightly less reflective surface.
- Alodine / chromate conversion: Standard yellow or clear chem-film per MIL-DTL-5541 Type I or II; good electrical conductivity retained.
Typical Applications
6351 was developed for structural extrusions in transportation and general engineering. In practice, if a part drawing calls for a complex extruded aluminum profile with moderate strength, 6351 is a candidate either explicitly or by equivalence to the now-withdrawn BS HE30 designation.
- Road transport: Truck chassis rails, trailer side-rails, bus body frame members, roof bows. These are long, thin-walled extrusions where 6351 saves extrusion die cost and reduces billet pressure versus 6082.
- Railway: Interior structural framing, window frames, seat tracks. Not for primary load-bearing structures (use 6005A or 6082).
- General engineering: Conveyor rails, machine guards, ladder sections, scaffold tubing, handrail profiles.
- Marine (non-immersed): Deck fittings, mast tracks, interior joinery in boats — anodized for protection.
- CNC-machined components: Mounting brackets, spacers, manifold blocks, sensor housings machined from extrusion cut-offs.
Machining Pitfalls with Extruded 6351-T6
Experienced CNC programmers know that extruded aluminum is not the same as rolled plate or drawn bar. 6351 extrusions bring specific challenges:
- Residual stress in the extrusion: Long extrusions carry quench-induced residual stress. If you machine asymmetrically — removing most material from one face while leaving the opposite face intact — the part will bow upon unclamping. Rough both sides alternately or stress-relieve (T6511 temper) when available.
- Profile distortion under clamping: Thin-walled extrusion cross-sections collapse under vise pressure. For hollow profiles, use expanding mandrels or internal support plugs during machining.
- Billet skin effect: The outer ~0.5 mm of an extrusion has a different grain structure from the core. For critical surfaces, remove at least 0.5 mm from the as-extruded surface to reach homogeneous material.
- Weld seams in hollow profiles: Porthole-die extrusions (hollow sections) contain longitudinal weld seams where metal flows reunite around the die bridge. These seams have slightly different grain structure and mechanical properties. Don’t put a critical machined feature — like a sealing surface or tapped hole — directly on a weld seam if you can avoid it.
When to Choose 6351 Over 6082 or 6061
This is the real engineering decision. Here’s the framework:
- Choose 6351-T6 if: The part starts as a complex extrusion (multi-cavity, thin ribs, deep channels) and you need moderate strength. The extrusion mill can hit tighter tolerances on complex profiles with 6351 than with 6082, reducing your rough-machining stock — and your cycle time.
- Choose 6082-T6 if: You need the maximum strength available from a 6xxx extrusion and the profile is relatively simple. The ~20 MPa yield advantage is real and matters for weight-critical structural design.
- Choose 6061-T6 if: North American supply chain is your priority. 6061 is a US-centric alloy with deep availability. 6351 is primarily a European and Asian extrusion alloy and may require longer lead times from North American mills.
- Choose 6063-T6 if: You need the best cosmetic surface after anodizing and strength is secondary. 6063 produces a brighter, more uniform anodized finish than 6351.
- Avoid 6351 if: You need high fatigue resistance (use 6082 or 2024), maximum corrosion resistance (use 6061 with chromate primer), or high-temperature strength above ~150°C (6xxx alloys lose significant strength).
RFQ Checklist for 6351-T6 CNC Machined Parts
When you send us a drawing for a 6351-T6 component, include the following to get an accurate quote the first time:
- Material specification: EN AW-6351-T6 per EN 755-2, or equivalent. Specify extrusion, not plate, if the part is cut from a profile.
- Temper: T6 (solution heat-treated + artificially aged). T6511 if stress-relieved by stretching is required.
- Extrusion profile: If the blank is a custom extrusion, provide the profile drawing or die number. For standard sections (angles, channels, tubes), specify the dimensional standard (e.g., EN 755-9).
- Surface finish: As-machined, anodized (Type II or III, color), powder coated, or chem-film. If anodizing, note whether dimensional growth must be accounted for (~50% of coating thickness per surface).
- Critical tolerances: Mark them on the drawing. Standard tolerances from ISO 2768-m or similar apply to unmarked dimensions.
- Quantity and delivery: Prototype batch, production volume, and required date.
- Inspection requirements: Dimensional report, material cert (EN 10204 Type 3.1), surface finish measurement, NDT requirements.
References: EN 573-3 (chemical composition), EN 755-2 (mechanical properties for extruded products), EN ISO 6506-1 (Brinell hardness). Machining parameters are engineering starting estimates and should be validated on actual equipment, tooling, and workpiece configuration.
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