If your design currently uses a 7075-T6 forging because you need 600+ MPa tensile in the short-transverse direction but you cannot afford the forging cost, 7055-T7751 plate is worth a hard look. 7055 delivers roughly 10–15% higher strength than 7075-T6 at equivalent temper, with fracture toughness that often beats 7050-T7451 in thin sections. The trade-off: it is harder to machine, more quench-sensitive, and not widely stocked outside aerospace mill sources.
This article covers what 7055 actually is, how T77 temper differs from the more familiar T6 and T73, the composition-to-property logic, CNC machining behavior, and the specific information a shop needs on your drawing before quoting.
What 7055 Is — And What It Isn’t
7055 is an Al-Zn-Mg-Cu-Zr wrought alloy in the 7xxx series (ISO designation: AlZn8MgCu). It sits at the top of the strength curve among commercially available aluminum alloys, just below 7068. The critical alloying difference from 7075 is higher zinc (nominally 8.0% vs. 5.6%) and a deliberate zirconium addition (0.08–0.15%) that controls recrystallization during solution treatment. 7055 does not use chromium as a grain refiner — it uses zirconium instead — which affects both quench sensitivity and corrosion performance.
7055 is almost always specified in the T7751 temper (stress-relieved by controlled stretching). The T77 designation is a three-step artificial aging process: solution heat treat, quench, controlled stretching (TX51), then a multi-stage precipitation aging that produces a finer, denser distribution of η’ (MgZn₂) precipitates than the single-stage aging used in T6. This gives higher strength without the full over-aging sacrifice of T73, while retaining better corrosion resistance than T6.
Key specification documents for 7055:
- AMS 4206 — 7055-T7751 plate, the primary aerospace procurement spec
- AMS 4325 — 7055-T77511 extrusion
- ASTM B209 — aluminum and aluminum-alloy sheet and plate (general)
- MMPDS-17 (formerly MIL-HDBK-5) — statistically derived design allowables for aerospace
- EN 485-2 — European standard for Al-Zn-Mg-Cu wrought products (EN AW-7055 is not fully harmonized; check with your mill)
7055 is not a general-purpose alloy. Typical availability is limited to plate gauges (mostly 6–50 mm) and select aerospace extrusions. If your RFQ asks for 7055 round bar in 5 mm diameter, a legitimate supplier will tell you that does not exist — 7055 is not produced as small-diameter bar stock.
Chemical Composition: Why It Matters for Machining
The composition of 7055 per AMS 4206 / ASTM B209 drives everything from chip formation to tool wear. Below are the registered limits; note that zinc and magnesium are the primary strengtheners, copper adds both strength and stress-corrosion resistance, and zirconium suppresses recrystallization.
| Element | Weight % (min) | Weight % (max) | Role |
|---|---|---|---|
| Zinc (Zn) | 7.6 | 8.4 | Primary strengthener; forms MgZn₂ precipitates |
| Magnesium (Mg) | 1.8 | 2.3 | Co-strengthener with Zn; controls aging response |
| Copper (Cu) | 2.0 | 2.6 | Strength and SCC resistance |
| Zirconium (Zr) | 0.08 | 0.15 | Recrystallization control; replaces Cr used in 7075 |
| Iron (Fe) | — | 0.15 | Impurity; excessive Fe forms brittle Al₇Cu₂Fe particles |
| Silicon (Si) | — | 0.10 | Impurity; kept very low to avoid Mg₂Si depleting Mg |
| Manganese (Mn) | — | 0.05 | Impurity control |
| Chromium (Cr) | — | 0.04 | Not intentionally added; Zr replaces Cr |
| Titanium (Ti) | — | 0.06 | Grain refinement during casting |
| Others (each) | — | 0.05 | |
| Others (total) | — | 0.15 | |
| Aluminum (Al) | Remainder | Balance | |
The tight Fe/Si limits (0.15 and 0.10 max) are not trivial — they directly affect fracture toughness. High-purity ingot practice is required. This high Zn content also means the alloy is electrochemically active; galvanic corrosion with steel fasteners or tooling is a real concern that must be addressed in design and assembly.
Mechanical Properties: T7751 vs. T6 and T73
The table below gives minimum and typical values for 7055-T7751 plate (AMS 4206, 12.7–38.1 mm thickness range). Data in the L (longitudinal) and LT (long-transverse) directions are design allowables from MMPDS-17. Short-transverse (ST) values are significantly lower and often dictate part orientation in machining.
| Property | Direction | Minimum | Typical | Standard |
|---|---|---|---|---|
| Tensile Strength (UTS) | L | 593 MPa | 620–635 MPa | AMS 4206 / ASTM E8 |
| Tensile Strength (UTS) | LT | 586 MPa | 600–620 MPa | AMS 4206 / ASTM E8 |
| Tensile Strength (UTS) | ST | — | 550–580 MPa | MMPDS-17 S-basis |
| Yield Strength (0.2% offset) | L | 545 MPa | 565–585 MPa | AMS 4206 / ASTM E8 |
| Yield Strength (0.2% offset) | LT | 538 MPa | 550–570 MPa | AMS 4206 / ASTM E8 |
| Yield Strength (0.2% offset) | ST | — | 510–530 MPa | MMPDS-17 S-basis |
| Elongation (in 50 mm) | L | 7% | 9–11% | ASTM E8 |
| Elongation (in 50 mm) | LT | 6% | 8–10% | ASTM E8 |
| Compressive Yield Strength | L | 565 MPa | 580–600 MPa | ASTM E9 |
| Fracture Toughness KIC (L-T) | — | — | 28–33 MPa√m | ASTM E399 |
| Hardness | — | — | 190–200 HB | ASTM E10 (500 kgf, 10 mm ball) |
| Density | — | — | 2.86 g/cm³ | — |
| Modulus of Elasticity | — | — | ~71 GPa | ASTM E111 |
How to read this table for real-world decisions: A design engineer considering 7055 over 7075-T6 gains roughly 70 MPa in L-direction yield strength (545 vs. ~470 MPa minimum). But elongation drops from 7075-T6’s typical 11% to about 9%. If your part has a minimum elongation requirement of 10%, 7055-T7751 may not meet it — check the spec, don’t assume. Also note that fracture toughness KIC values for 7055-T7751 overlap with 7050-T7451 in the 25–35 mm thickness range, which is by design: the T77 aging was developed precisely to give T6-level strength with T73-level toughness.
Corrosion Behavior and Service Limits
7055-T7751 has significantly better exfoliation corrosion and stress-corrosion cracking (SCC) resistance than 7075-T6, particularly in the ST direction. Per AMS 4206, T7751 temper is rated for SCC threshold stress of at least 240 MPa in the ST direction (ASTM G47 alternate immersion test). This is comparable to 7050-T7451 and far better than 7075-T6, which typically fails below 50 MPa ST in the same test.
However, 7055 is still a high-zinc alloy. It is not suitable for sustained immersion in seawater without protective coating or cladding. Intergranular corrosion attack can initiate at exposed end-grain in machined pockets if not properly sealed. Chromic acid anodizing (MIL-A-8625 Type I) or sulfuric acid anodizing (Type II) with dichromate seal is standard. Hard anodizing (Type III) is used for wear surfaces but should not be applied to fatigue-critical areas — the coating thickness reduces fatigue life in 7xxx alloys by 10–30% depending on geometry and stress concentration.
CNC Machining 7055: What Changes from 7075
Machinability Overview
7055 machines like a stronger, slightly more abrasive version of 7075. Its chip is still Type C (segmented, short-breaking) in turning and milling — you will not get the long, stringy chips of 6061 — but tool wear accelerates by roughly 20–30% compared to 7075-T6 at the same cutting parameters. This is due to the higher volume fraction of hardening precipitates and the slight abrasive effect of Al₇Cu₂Fe constituent particles.
Work hardening in 7055 is moderate — less severe than austenitic stainless steels, more than 6061. This means climb milling (conventional cut direction) should always be used in CNC operations. A climb-milled surface on 7055-T7751 in a rigid setup can achieve Ra 0.4–0.8 μm with a sharp carbide tool; contrast this with conventional milling, which smears the surface and can produce Ra 1.6–3.2 μm with burnishing artifacts that mask subsurface micro-cracks.
Starting-Point Cutting Parameters
The parameters below are reference starting points only. They assume a rigid CNC machining center or lathe with flood coolant, sharp carbide tooling, and workpiece fixturing that minimizes overhang. Your actual parameters must be dialed in for your specific machine, toolholder, tool geometry, coolant pressure, and part rigidity. For thin-walled parts (wall thickness < 2 mm), reduce depth of cut by 50% and increase feed by 10% to avoid chatter-induced dimensional scatter.
| Operation | Tool Type | Speed (SFM / m/min) | Feed (IPT / mm/tooth) | DOC (in / mm) | WOC (in / mm) |
|---|---|---|---|---|---|
| Rough Milling | 3-flute carbide, 45° helix | 800–1200 SFM / 245–365 m/min | 0.006–0.010 IPT / 0.15–0.25 mm/tooth | 0.080–0.200″ / 2–5 mm | 40–70% tool dia. |
| Finish Milling | 3-flute carbide, 45° helix, sharp edge | 1000–1400 SFM / 300–425 m/min | 0.003–0.006 IPT / 0.08–0.15 mm/tooth | 0.010–0.040″ / 0.25–1.0 mm | 5–10% tool dia. |
| Rough Turning | Carbide insert, CCMT/CNMG, positive rake | 600–1000 SFM / 180–300 m/min | 0.008–0.015 IPR / 0.20–0.38 mm/rev | 0.060–0.160″ / 1.5–4 mm | — |
| Finish Turning | Carbide insert, sharp edge, polished flank | 800–1200 SFM / 240–365 m/min | 0.003–0.006 IPR / 0.08–0.15 mm/rev | 0.010–0.040″ / 0.25–1.0 mm | — |
| Drilling | Carbide drill, 140° point, through-coolant | 300–500 SFM / 90–150 m/min | 0.004–0.010 IPR / 0.10–0.25 mm/rev | Full diameter | — |
| Tapping | Spiral-flute HSS-Co tap, TiN coated | 30–60 SFM / 9–18 m/min | Per pitch | — | — |
Tooling, Coolant, and Chip Control
Tool material and coating: Uncoated micrograin carbide with a sharp, polished cutting edge (edge radius < 5 μm) works best for finishing because it eliminates the built-up edge (BUE) that forms on coated tools at aluminum cutting temperatures. For roughing, TiB₂ (titanium diboride) or diamond-like carbon (DLC) coatings reduce aluminum adhesion but require careful break-in. TiAlN and AlTiN — coatings optimized for steel — offer no advantage on 7055 and can worsen BUE because aluminum has a chemical affinity for aluminum in the coating.
Coolant: Flood coolant at 8–12% concentration (semi-synthetic or soluble oil) with minimum 20 bar (290 psi) pressure. Through-spindle coolant is strongly preferred for drilling and deep pocket milling to evacuate chips. 7055 chips are short but hot; recutting a chip can instantly smear it onto the tool flank. If you see aluminum transfer on the tool (a shiny silver deposit), increase coolant pressure or reduce speed by 10–15%.
Chip breaking: 7055 typically produces acceptable C-shaped chips in turning. In milling, very light radial engagement (< 10% tool diameter) can produce thin foil-like chips that pack into flutes — increase feed per tooth or radial engagement slightly to thicken the chip.
Residual Stress and Distortion: The Real Problem
This is where 7055 machining projects go wrong. 7055-T7751 plate carries significant residual stress from the quench, partially relieved by controlled stretching. When you machine away 60% or more of the material from one side of the plate (typical for structural pockets in aerospace brackets), the part will distort — bow, twist, or both.
Mitigation strategies in order of effectiveness:
- Rough + stress-relieve + finish sequence: Rough machine leaving 1–2 mm stock, then thermally stress-relieve (175–200°C for 2–4 hours; this is below the aging temperature and does not significantly degrade T77 properties), then finish machine.
- Symmetric stock removal: Machine equal amounts from both faces in alternating passes if geometry allows.
- Pre-stretched plate selection: Specify TX51 temper (stress-relieved by controlled stretching), which is standard for 7055-T7751, but verify with the mill cert that stretching was performed per AMS 4206 requirements (1.5–3% permanent set).
- Accept that some parts will not work as machined from plate: If your part is a deep, asymmetrical pocket with thin walls, 7055 may simply be the wrong product form — consider a forging or a redesign that balances material removal.
Surface Treatments and Post-Processing
7055 responds to the standard aluminum surface treatments, with some caveats:
- Anodizing (Type II, sulfuric acid): Produces a uniform 5–25 μm coating. The high copper content (2.0–2.6%) gives the anodic layer a slightly yellowish tint vs. the clear or light-gray finish on 6061. Not a defect — just different chemistry.
- Hard anodizing (Type III): Achieves 25–75 μm coating, Rockwell C 60+ surface hardness. But the coating is brittle — do not hard-anodize sharp internal corners or threads without radius relief. Fatigue debit of 10–25% is well-documented in 7xxx alloys; factor this into your design margin.
- Chemical conversion coating (Alodine / Iridite, MIL-DTL-5541 Type I or II): Standard pre-paint treatment for 7055. Type I (hexavalent chromium) provides better corrosion resistance but faces tightening environmental restrictions. Type II (trivalent chromium) is the forward path but may require requalification for aerospace parts.
- Shot peening: Effective for improving fatigue life on machined 7055 surfaces. Almen intensity 0.008–0.012A with S110 or S170 cast steel shot, 100–200% coverage per AMS 2430, can increase fatigue limit by 15–25% compared to as-machined surface.
Where 7055 Is Used — and Where It Should Not Be
Primary applications (all in the T7751 condition):
- Upper wing skin and stringer panels on commercial aircraft (Boeing 777 and later models use 7055 extensively in compression-dominated structure)
- Bulkhead webs and spar caps where compression yield strength is the limiting factor
- High-cycle fatigue brackets in aerospace where weight savings over 7075 justify the material premium
- High-performance sporting goods (some premium bicycle frames, but these are typically 7068 or 7075 — 7055 is uncommon outside aerospace)
Where 7055 is a poor choice:
- Parts requiring welding — 7055 is considered non-weldable by fusion processes due to hot cracking sensitivity; friction stir welding is possible but only in specialized applications
- Parts exposed to sustained seawater immersion without coating or cathodic protection
- Parts where minimum elongation exceeds 8% — 7055 has limited ductility, and notched sections can fail with minimal plastic deformation
- Small-diameter turned parts from bar — 7055 is not commercially available as bar stock under 50 mm diameter; verifying availability before designing is essential
- Cost-sensitive projects where 7075-T6 or 7050-T7451 is “good enough” — 7055 typically carries a 30–50% price premium over 7075 due to high-purity ingot requirements and limited mill sources
How 7055 Compares to Common Alternatives
| Alloy & Temper | UTS (L, MPa, typ.) | YS (L, MPa, typ.) | Elong. (L, typ.) | KIC (L-T, MPa√m) | Relative Cost | Best For |
|---|---|---|---|---|---|---|
| 7055-T7751 | 620–635 | 565–585 | 9% | 28–33 | 1.4–1.6× | Max compression strength + toughness |
| 7075-T6 | 570–580 | 500–510 | 11% | 26–30 | 1.0× (baseline) | General high-strength, good availability |
| 7075-T73 | 500–510 | 430–440 | 10% | 30–36 | 1.1× | Corrosion resistance over max strength |
| 7050-T7451 | 520–540 | 460–480 | 10% | 33–38 | 1.2–1.3× | Thick-section toughness + SCC resistance |
| 7068-T6511 | 680–710 | 640–670 | 8% | — | 1.5–2.0× | Absolute maximum strength, small parts |
Critical nuance on “equivalent” grades: 7055 has no direct European EN AW equivalent. EN AW-7049A (AlZn8MgCu) shares the generic AlZn8MgCu tag but has different Zr/Ti limits and is typically delivered in T6 or T73 tempers. It is not interchangeable with 7055 for aerospace applications without requalification. Similarly, the Chinese grade 7A55 (GB/T 3190) is conceptually similar to 7055 but the composition windows differ — do not assume material certified to GB/T 3190 meets AMS 4206 without side-by-side testing.
Information a CNC Shop Needs to Quote 7055 Parts
7055 is not a generic aluminum. When sending an RFQ, include the following — missing any one of these will either delay the quote or result in assumptions that may not work for your part:
- Material specification: “7055-T7751 per AMS 4206” (not just “7055 aluminum”). If you will accept 7050-T7451 or 7075-T73 as alternatives, state it explicitly — the shop cannot guess.
- Grain direction requirements: Which axis is L, which is LT, which is ST. For plate parts, call out the rolling direction on the drawing. A part machined with L in the short dimension instead of the long dimension can lose 10–15% of its design strength.
- Minimum thickness after machining: Verify that your finished part sits within the thickness range covered by the design allowables you are using. MMPDS-17 S-basis values for 7055-T7751 are only valid for 12.7–38.1 mm thickness; thinner or thicker sections require separate allowables or testing.
- Surface treatment specification: “Anodize per MIL-A-8625 Type II, Class 2 (sulfuric acid, dyed black), seal in nickel acetate” — not “make it black.” Include whether fatigue-critical surfaces are included in the treatment area and whether masking is required.
- Inspection requirements: Are you requiring mill certs (EN 10204 3.1)? Tensile coupons from the actual plate? Conductivity testing (typical for 7055-T7751: 38–42% IACS per ASTM E1004)? First-article inspection (AS9102)? NDT (penetrant inspection per ASTM E1417)?
- Quantity, delivery date, and tolerance envelope: Standard stuff — but on 7055, be realistic about lead times. Plate may need 8–16 weeks from the mill if not in distributor stock.
Other CNC Machining Articles in This Series
- 7075 Aluminum CNC Machining: Properties, Tooling, and Distortion Control
- 7050 vs. 7075 Aluminum: Which Alloy for Thick Aerospace Parts?
- Aluminum Temper Designations: T6, T73, T7451, and T77 Explained
- Residual Stress in CNC-Machined Aluminum Plate: Causes and Fixes
Data sources: AMS 4206D, MMPDS-17 Chapter 3, ASTM B209-14, ASM Handbook Volume 2 (Aluminum and Aluminum Alloys). Typical values are for 12.7–38.1 mm plate in T7751 temper. Always verify against the actual mill cert for your specific heat and lot.
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