If your design calls for a heat-treatable aluminum alloy that combines medium-high strength with genuine arc weldability — and you do not want to pay for post-weld solution treatment — 7020 (AlZn4.5Mg1) is the grade you should evaluate before defaulting to 6061 or attempting 7075. Unlike copper-bearing 7xxx alloys, 7020 is a Cu-free Al-Zn-Mg composition designed to re-age naturally in the heat-affected zone after welding. This single difference explains why it dominates European welded bicycle frames, military bridging structures, and armored vehicle hulls where riveted 7075 shells would be heavier, costlier, or outright impractical.
Grade Identity, Standards, and Where 7020 Fits in the 7xxx Family
7020 is registered under the Aluminum Association wrought alloy system and covered by EN 573-3 (chemical composition) and EN 485-2 (mechanical properties for rolled products). The EN designation is EN AW-7020 [Al Zn4.5Mg1]. It also appears in ISO 209.2 and the withdrawn British standard BS 3L80. There is a Chinese near-equivalent 7A04 (GB/T 3190), but 7A04 often tolerates slightly different Zn:Mg ratios and may include minor Cr or Zr additions — do not treat them as interchangeable without a full composition cross-check.
Within the 7xxx series, alloys fall into two broad camps:
- Al-Zn-Mg-Cu (7075, 7050, 7049, 7475): peak-aged strength above 500 MPa but poor fusion weldability. The Cu content promotes hot cracking and the heat-affected zone overages rather than re-ages.
- Al-Zn-Mg (7020, 7005, 7004, 7039): deliberately copper-free or low-copper. Weldable with 5xxx filler (e.g. 5356, 5183). Lower peak tensile strength than 7075-T6 but far better as-welded properties and natural aging response after welding.
7020 sits in the mid-upper strength tier within the Cu-free group. It is noticeably stronger than 6061-T6 but retains far better weldability than 7075-T6.
Chemical Composition — EN 573-3
The following table reflects the EN 573-3 (EN AW-7020) registration ranges. Values outside these ranges are out of specification.
| Element | Composition (wt%) | Role in 7020 |
|---|---|---|
| Zinc (Zn) | 4.0 – 5.0 | Primary precipitation-hardening element; forms MgZn₂ with magnesium. |
| Magnesium (Mg) | 1.0 – 1.4 | Co-hardener with Zn; controls aging kinetics and strength ceiling. |
| Manganese (Mn) | 0.05 – 0.50 | Grain structure control; improves hot-workability and recrystallization resistance. |
| Chromium (Cr) | 0.10 – 0.35 | Stress-corrosion resistance; also retards recrystallization. |
| Zirconium (Zr) | 0.08 – 0.20 | Inhibits recrystallization in the HAZ during welding — critical for weld performance. |
| Zirconium + Titanium (Zr+Ti) | 0.08 – 0.25 | Combined grain-refiner contribution. |
| Iron (Fe) | ≤ 0.40 | Impurity; excessive Fe forms coarse Al₃Fe intermetallics that reduce toughness. |
| Silicon (Si) | ≤ 0.35 | Impurity; can combine with Mg to form Mg₂Si, competing with MgZn₂ precipitation. |
| Copper (Cu) | ≤ 0.20 | Kept intentionally low to preserve weldability; high Cu would move the alloy into Al-Zn-Mg-Cu territory with hot-cracking risk. |
| Titanium (Ti) | ≤ 0.20 | Grain refiner as TiB₂ or TiC nuclei in the melt. |
| Zinc + Magnesium (Zn+Mg) | 5.0 – 6.4 | Total Zn+Mg controls total volume fraction of precipitates available. |
| Others (each) | ≤ 0.05 | Trace elements per EN 573-3. |
| Others (total) | ≤ 0.15 | Total impurity limit. |
| Aluminum (Al) | Remainder | Matrix. |
Note the deliberate absence of copper above 0.20%. This is the structural reason 7020 welds far better than 7075. Also note the Zr requirement — not every 7xxx specification includes Zr, but 7020 does, and it is not optional for meeting as-welded property expectations.
Temper Designations and Typical Mechanical Properties
7020 is most commonly supplied in the T6 (solution heat-treated and artificially aged) and T651 (stress-relieved by stretching after solution treatment) tempers. Less common tempers include T4 (solution treated and naturally aged) and O (annealed, for forming prior to re-heat-treatment).
The table below references EN 485-2:2016 for plate and sheet products in T651 condition. Values are minimum guaranteed, not “typical.” Actual mill certificates often exceed these by 5–15%.
| Property | Value (T651) | Test Condition & Standard |
|---|---|---|
| Tensile Strength Rm | ≥ 350 MPa | Room temperature, EN 485-2 / ISO 6892-1 (tensile axis parallel to rolling direction for plate ≤ 25 mm) |
| Yield Strength Rp0.2 | ≥ 290 MPa | Room temperature, same conditions as above |
| Elongation A50 | ≥ 8% | Room temperature, gauge length 50 mm, for thickness 6–12 mm plate |
| Hardness (typical) | ~115–135 HB | Brinell HBW 10/500, not a specification minimum — reference range from supplier data |
| Modulus of Elasticity | ~71 GPa | Typical for 7xxx aluminum; not specification-controlled |
For comparison, 7020-T651 sits between 6061-T6 (Rm ≈ 310 MPa minimum) and 7075-T651 (Rm ≈ 540 MPa minimum) in tensile strength. But the as-welded strength retention is where 7020 pulls ahead: with proper 5356 or 5183 filler, as-welded joint efficiency typically reaches 70–85% of parent T6 strength, with natural re-aging gradually increasing this over weeks at ambient temperature. A 7075-T6 weldment, by contrast, may drop to 40–50% of parent strength in the HAZ with very limited post-weld recovery — hence the widespread use of riveted or bolted joints for 7075 structures.
Corrosion Behavior and Stress-Corrosion Cracking
7020, like all 7xxx alloys, sits in the “susceptible” category for stress-corrosion cracking (SCC) in the short-transverse direction, particularly in the T6 temper applied to thicker sections. The Cr and Zr additions in 7020 partially mitigate this relative to earlier Cu-free 7xxx alloys (e.g. 7039), but the risk is real. Key engineering rules:
- Overaging (T73-type) improves SCC resistance at the cost of 10–15% strength, but 7020 is less commonly supplied in overaged tempers than 7075.
- Short-transverse grain orientation is the vulnerable axis — avoid through-thickness tensile stresses in thick plate designs.
- Protective coatings (anodizing to MIL-A-8625 Type II or III, or paint systems over chromate or chromate-free conversion coating per MIL-DTL-5541) are standard practice for aerospace and defense applications.
- Exfoliation corrosion in T6 can occur in aggressive marine/industrial atmospheres if the material has a heavily recrystallized surface layer; this is one reason Zr and Cr are specified — they pin grain boundaries and suppress recrystallization.
CNC Machining Behavior: What 7020 Brings That 6061 and 7075 Don’t
7020 machines like a slightly softer 7075 — which is generally good news. Chip formation is continuous and curling in the T6/T651 condition; the absence of copper means the alloy is less abrasive on cutting edges than 7075-T6, but the Zn+Mg content still demands sharp tools and avoids built-up edge. Key observations from shop-floor practice:
- Chip control: Chips are long and continuous like 6061-T6. Use chip-breaker insert geometries. Avoid low-feed finishing passes that create tangled “bird’s nest” chips around the tool.
- Burr formation: Comparable to 6061-T6. Deburring strategy matters especially on cross-drilled holes and slot exits. The material can exhibit a small but sharp exit burr in milling.
- Surface finish: Ra 0.8–1.6 μm is achievable on rigid setups with polished carbide inserts and proper coolant. Finer than Ra 0.4 μm generally demands a secondary process (vibratory finishing, electropolishing, or diamond turning).
- Workholding: Like all aluminum, 7020 is softer than steel jaws can mark. Use aluminum or nylon soft jaws for finished surfaces.
Starting-Point Machining Parameters
The values below are starting references for 7020-T651 on a rigid CNC machining center (ISO 40 or HSK spindle) with coated carbide tooling and flood coolant. They are not universal recipes — actual optimal parameters depend on machine rigidity, tool holder runout, specific tool geometry, coating, workpiece clamping stiffness, and desired surface finish. Treat these as a baseline for your own trials.
| Operation | Cutting Speed Vc (m/min) | Feed per Tooth fz (mm) | Depth of Cut ap (mm) | Tool Notes |
|---|---|---|---|---|
| Rough Milling | 300–600 | 0.15–0.30 | 2–8 (radial ≤ 0.7×D) | 3-flute uncoated or DLC-coated carbide, high-helix (40–45°), corner radius ≥ 0.5 mm |
| Finish Milling | 500–800 | 0.05–0.12 | 0.3–1.0 | Polished uncoated carbide or PCD for long runs; use wiper insert geometry for floor finishes |
| Turning (Rough) | 350–600 | 0.15–0.35 (mm/rev) | 1–5 | Carbide insert, ISO K or N grade, positive rake 12–18° |
| Turning (Finish) | 500–900 | 0.05–0.15 (mm/rev) | 0.2–0.8 | Polished carbide or PCD; minimum nose radius to control surface finish vs. vibration tradeoff |
| Drilling | 120–250 | 0.10–0.30 (mm/rev) | — | Carbide with 130–140° point angle; peck cycle for holes deeper than 4×D; through-tool coolant if available |
| Tapping | 10–25 | — (pitch-dependent) | — | Spiral-flute or form tap (roll tap); form tapping produces stronger threads in 7020 because the Zn+Mg matrix work-hardens the thread flanks |
Coolant: Emulsion at 6–10% concentration, flood delivery. Minimum-quantity lubrication (MQL) is feasible for light finishing but not recommended for roughing due to the material’s tendency to heat up quickly under heavy cuts. 7020’s thermal conductivity (~140–155 W/m·K in T6) is lower than pure aluminum but typical for 7xxx — enough to pull heat into the chip, insufficient to skip coolant on roughing.
Tool wear pattern: With carbide tooling and correct parameters, primary wear is flank wear (VB). Built-up edge is rare in T651 condition if cutting speed stays above ~150 m/min. If you see aluminum smearing on the rake face, increase speed and check coolant concentration.
Fabrication: Welding, Forming, and What Changes After Heat
This is where 7020 earns its reputation. The alloy was explicitly developed for welded structures. Key facts:
- Filler metal: Use ER5356 (AlMg5) or ER5183 (AlMg4.5Mn) for GTAW/GMAW. Do not use ER4043 (AlSi5) — silicon can embrittle the Zn-Mg system.
- Post-weld aging: The heat-affected zone naturally re-ages at ambient temperature over 30–90 days, typically recovering to 70–85% of T6 parent tensile strength. Artificial post-weld aging at 90–100°C for 8–24 hours accelerates this recovery.
- Re-heat-treatment: Full solution treatment + re-aging after welding is possible but rarely performed — one of 7020’s selling points is that natural re-aging is sufficient for many structural applications.
- Formability: In the O or T4 temper, 7020 has good formability. In T6/T651, bend radii must be generous (≥ 3× thickness) to avoid cracking. Hot forming at 200–250°C extends formability but alters the T6 temper.
One caution: if you anodize after machining, the sulfuric acid anodizing bath (typically 15–20% H₂SO₄ at 18–22°C) can microscopically etch the surface more aggressively on Zn-bearing alloys than on 6xxx series. Chromic acid anodizing (Type I per MIL-A-8625) is gentler but less common. Specify your post-machining finishing route early, because anodizing thickness (5–25 μm for Type II, 25–50 μm for Type III hardcoat) must be accounted for in final dimensional tolerances.
Where 7020 Is Used — and Where It Isn’t
7020’s sweet spot is welded medium-high-strength structures where 6061-T6 is too weak and 7075 can’t be fusion-welded:
- Bicycle frames: A dominant material in European high-end aluminum frames from the 1990s through today. TIG-welded 7020 tubes with post-weld natural aging produce frames with strength-to-weight competitive with mid-range carbon fiber at lower cost.
- Military bridging and armored vehicle hulls: 7020-T651 plate provides ballistic protection comparable to 5083-H131 at lower weight, with the added benefit of weldable joints that maintain protection integrity.
- Motorcycle and motorsport components: Swingarms, subframes, and brackets where welding is required and 7075 is disqualified.
- Pressure vessels and cryogenic tanks: The alloy retains toughness at low temperatures, making it suitable for LNG and industrial gas storage where aluminum’s lack of ductile-to-brittle transition is an advantage.
- Railway structural components: EN 45545-compliant interior and exterior structures, particularly where MIG welding of extrusions is the joining method.
Where it is not the right call: elevated-temperature applications above ~120°C (overaging accelerates), marine immersion without protective coating (SCC risk in chloride environments), and applications requiring 7075-T6-level strength (540+ MPa tensile). For those, consider 7075-T6 with bolted or riveted assembly, or move to 7050 if thickness exceeds 75 mm.
7020 vs. 6061-T6 vs. 7075-T6: Selection Decision Table
| Criterion | 7020-T651 | 6061-T6 | 7075-T651 |
|---|---|---|---|
| Tensile Strength Rm (min) | ≥ 350 MPa | ≥ 310 MPa (EN 485-2, plate ≤ 25 mm) | ≥ 540 MPa |
| Yield Strength Rp0.2 (min) | ≥ 290 MPa | ≥ 260 MPa | ≥ 460 MPa |
| Fusion Weldability | Good (GTAW/GMAW with 5xxx filler) | Good (GTAW/GMAW with 4043 or 5356) | Poor — hot cracking; generally not fusion-welded |
| Post-Weld Strength Recovery | Natural re-aging: 70–85% of T6 over 30–90 days | HAZ drops to ~T4 level; post-weld heat treatment recovers to ~T6 | HAZ overages permanently; recovery is minimal without full re-heat-treat |
| Machinability | Good; less abrasive than 7075 | Good; forgiving chip formation | Very good chip break; more abrasive on tools |
| Corrosion Resistance | Moderate; SCC-sensitive in ST direction; needs protection in aggressive environments | Good general atmospheric corrosion resistance | Poor without cladding or coating; SCC-sensitive in T6 |
| Anodizing Quality | Acceptable; slightly duller than 6xxx | Excellent; clear or dyed anodizing yields bright, uniform finish | Fair; darker and less uniform than 6xxx |
Drawing and RFQ Preparation for 7020 Machined Parts
When you submit a drawing or RFQ for 7020 machined components, include the following to get an accurate quote and avoid post-order surprises:
- Full material callout: Not just “7020” — specify “EN AW-7020 [Al Zn4.5Mg1] T651” with the applicable standard (EN 485, EN 573, or equivalent mill-cert requirement).
- Grain direction: If the part carries significant tensile stress, state whether longitudinal (L), long-transverse (LT), or short-transverse (ST) grain orientation is required. For SCC-critical parts, avoid ST-direction tensile stress.
- Welding in scope: If the part will be welded into a larger assembly, tell the machine shop. Weld preparation bevels and post-weld machining allowance differ from standalone part strategy.
- Surface treatment: Anodizing type (chromic / sulfuric / hardcoat), thickness, color, sealing method, and whether the treatment is applied before or after final inspection.
- Tolerances and critical features: Standard ISO 2768-m or ISO 2768-f for non-critical dimensions; specify geometric tolerancing (flatness, perpendicularity, concentricity) for mating surfaces.
- Quantity and lead-time expectation: Single prototypes, low-volume production (10–100 pcs), or series runs all drive different fixturing and tooling strategies.
For more on aluminum alloy selection in machined components, see our articles on 6061-T6 machining considerations and 7075-T6 vs. 7050 trade-offs. For the weldability angle specifically, our guide to weldable aluminum alloys covers 5xxx and 6xxx alternatives.
Send us your drawing with material specification, temper, quantity, tolerances, and surface finish requirements. We machine 7020-T651 components on rigid CNC equipment with in-process verification — not by guessing parameters from a generic table.
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