7075 Aluminum Machining Guide: Optimizing Feeds, Speeds, and Tool Selection

What is 7075 Aluminum? Composition, Properties, and Common Tempers

At its core, 7075 aluminum is a cold-treated forging alloy belonging to the 7xxx series, where zinc is the primary alloying element. It’s often referred to as “aircraft aluminum” or by its old trade name, “Zicral,” and for good reason. Its composition is engineered for peak performance: typically containing 5.1-6.1% zinc, 2.1-2.9% magnesium, and 1.2-2.0% copper, with smaller amounts of chromium. This specific blend creates a tight, fine-grained structure where the formation of MgZn₂ precipitates during heat treatment leads to significant strengthening. The result is an alloy with a tensile strength that can reach 83,000 psi (≥560 MPa) and a yield strength of around 73,000 psi (≥450 MPa), putting it in the same league as many steels while being about one-third the weight.

The general properties of 7075 make it a standout choice for demanding roles. It offers an excellent strength-to-weight ratio, good fatigue resistance, and average machinability. However, it’s crucial to note its limitations: its corrosion resistance is lower than that of alloys like 6061 (though it can be improved via cladding or anodizing), and it is generally considered non-weldable by conventional means due to a high susceptibility to solidification cracking.

Understanding Tempers: T6, T651, T7351, and More

The “temper” designation is critical when ordering 7075, as it defines the alloy’s thermal-mechanical processing history and directly dictates its mechanical behavior on the machine. The most common temper encountered is T6 (solution heat-treated and artificially aged). This state provides the highest strength and is widely used for structural components. T651 is a subset of T6, indicating the material has been stress-relieved by stretching after heat treatment, which improves dimensional stability during machining—a vital characteristic for complex parts.

Another important temper is T7351. As highlighted in machinist forums, this is a key differentiator. The T7351 temper undergoes an over-aging process, which sacrifices a portion of the ultimate tensile strength (making it more than 10% softer than T651) to dramatically improve stress corrosion resistance. This over-aging can also affect chip formation. Users report that 7075-T7351 can sometimes feel “gummy” compared to the more brittle, free-chipping nature of 7075-T6. This underscores a vital lesson: 7075 is not a monolithic material. Always verify the specific temper, as it will influence your cutting parameters, tool selection, and even workholding strategy to manage the different material behaviors.

7075 vs. 6061: Key Differences Every Machinist Must Know

While both are aluminum alloys, 7075 and 6061 are as different as sprinters and marathon runners. Confusing them can lead to failed parts, broken tools, and inaccurate quotes. For a machinist, understanding these differences is non-negotiable.

• Strength and Hardness: This is the most dramatic difference. 7075-T6 has a tensile strength nearly double that of 6061-T6 (83,000 psi vs. 45,000 psi). This means 7075 can withstand much higher stresses, but it also means it’s harder on cutting tools. The increased hardness contributes to its superior wear resistance as a finished part.
• Machinability and Chip Formation: 6061 is famously easy to machine, producing small, easily evacuated chips. In its common T6 temper, 7075 also machines well, often producing a fine, slightly more abrasive chip. However, as noted with the T7351 temper, its behavior can vary. The higher strength of 7075 requires more horsepower to cut and generates higher cutting forces.
• Corrosion Resistance and Weldability: Here, 6061 holds the advantage. Its magnesium and silicon composition gives it very good corrosion resistance and excellent weldability. 7075 has poorer inherent corrosion resistance and is generally considered non-weldable for structural applications, often requiring mechanical fastening instead.
• Cost and Application Philosophy: 7075 is typically more expensive than 6061. The choice between them is fundamentally application-driven. Use 6061 for general-purpose parts, weldments, frames, and applications where excellent corrosion resistance and good machinability are priorities. Choose 7075 when the design is driven by maximum strength and minimum weight—think aircraft fittings, missile parts, high-performance bicycle components, and molds.

The forum advice that “If you can machine stainless, you won’t have any problems with 7075” is a useful, if rough, benchmark. It points to the need for rigid setups and appropriate tooling more akin to machining harder materials, even though 7075 is still an aluminum.

Core Challenges in 7075 Aluminum Machining: Gummy Chips, Deformation, and Tool Wear

Successfully machining 7075 aluminum requires anticipating and mitigating its specific challenges. Moving from theory to practice, these are the most common hurdles machinists face.

Gummy Chip Formation and Evacuation

While 7075-T6 is typically known for producing manageable chips, issues with gumminess arise, particularly with certain tempers like T7351 or when using suboptimal cutting parameters. Gummy chips are problematic because they don’t break cleanly. Instead, they form long, stringy ribbons that can weld themselves to the workpiece (built-up edge), wrap around the tool, and scratch finished surfaces. This is often a symptom of the material being worked, not cut—excessive heat and pressure cause the aluminum to smear rather than shear. The solution lies in maintaining sharp, polished-flute tooling, using higher surface speeds to promote clean shearing, and employing high-pressure coolant to both cool the cut and blast chips out of the way.

Dimensional Distortion and Residual Stress

This is arguably the most insidious challenge in 7075 aluminum machining, especially for thin-walled or complex geometries. The high strength of 7075 is achieved through intense heat treatment, which can lock significant internal (residual) stresses into the raw stock. As material is removed during machining, these stresses rebalance, causing the part to warp or twist, sometimes dramatically. A user’s query about machining a 1000x70x25mm block that deformed after unclamping is a classic example. This isn’t a failure of technique in the moment; it’s a failure to manage the material’s inherent state. Strategies to combat this include using stress-relieved stock (like T651 temper), employing symmetrical machining sequences to balance stress release, and taking light, even finishing passes to minimize the introduction of new machining stresses.

Accelerated Tool Wear

Don’t let aluminum’s soft reputation fool you. The high strength and abrasive silicon particles present in 7075 can wear out cutting tools faster than when machining 6061. Tools can experience flank wear, edge chipping, and material adhesion. As one machinist noted, “I also found if the end mill is not almost perfectly sharp/new, the results are really bad, whereas in 6061 you wouldn’t even notice.” This demands a proactive tool management strategy. Using carbide tools with sharp, polished geometries designed for aluminum is essential. Coatings like ZrN (zirconium nitride) can help reduce aluminum adhesion. Furthermore, monitoring tool life and establishing replacement schedules before surface finish degrades is critical for maintaining consistency in production runs.

By recognizing these core challenges—gummy chips, part deformation, and tool wear—you can move from reactive problem-solving to a controlled, optimized machining process. The following sections will provide the specific parameters and techniques to build this optimized approach.

Optimizing Cutting Parameters: Feeds, Speeds, and Tool Selection for 7075

Successfully machining 7075 aluminum hinges on moving beyond generic parameters to a tailored strategy that leverages its properties. While some machinists treat it “exactly the same as 6061,” a nuanced approach yields superior tool life, finish, and efficiency. The goal is to generate clean, broken chips and manage heat effectively.

For cutting speeds (SFM), a robust starting point is 300–500 SFM for carbide end mills. You can often push towards the higher end of this range with sharp, polished tools and ample coolant. However, if you encounter gummy behavior—particularly with softer tempers like T7351—dropping to the 235–300 SFM range while increasing feed per tooth can help shear the material more cleanly rather than rubbing it. As one user experimenting with T7351 found, “running the tool slower (235 sfm) with faster feeds seems to work a little better.” The key is maintaining an aggressive chip load; a feed per tooth (IPT) between 0.004″ and 0.012″ is typical, with higher feeds within that range helping to promote chip evacuation and reduce heat buildup in the cut.

Tool selection is non-negotiable. High-quality, sharp carbide end mills with 2 or 3 flutes are standard. Tools specifically designed for aluminum feature high helix angles (around 38-45 degrees) and polished flutes to prevent chip welding. Coatings like zirconium nitride (ZrN) or uncoated, polished carbide are excellent choices to minimize aluminum adhesion. Drills should be carbide-tipped or solid carbide with a high point angle (130-140 degrees) and polished flutes. For all tools, rigidity is paramount; use the shortest flute length and largest diameter shank possible to combat deflection, which can mar surface finish and accelerate tool wear.

Coolant application is not just beneficial—it’s critical. Use a flood of water-soluble coolant to manage heat, wash away chips, and prevent re-cutting. For operations where flood coolant isn’t feasible, a powerful air blast with a mist lubricant can be an alternative, though flood is preferred for optimal tool life and surface finish in 7075 aluminum machining.

Best Practices for Fixturing and Workholding to Minimize Deformation

The high residual stresses locked within 7075-T6 and T651 plate can be your greatest adversary when holding tight tolerances. As material is removed, these stresses rebalance, causing the part to warp, twist, or bow—sometimes dramatically. The fixturing strategy must work to both restrain the part and manage stress relief.

First, understand the stock. Whenever possible, specify stress-relieved plate (often denoted as T651, which is solution heat-treated, stretched to relieve stress, and artificially aged). This provides a more stable starting point than non-stretched material. Even with stress-relieved stock, a strategic approach to machining sequence is vital. The core principle is to remove material symmetrically and in balanced stages. Avoid hogging out one entire side of a pocket or wall while leaving the opposite side thick; instead, take alternating, even passes from both sides to allow stresses to equalize incrementally.

Fixturing must provide uniform, distributed clamping force. Use a network of clamps or a vacuum chuck to spread pressure across the workpiece, avoiding isolated points of high stress that can induce distortion when released. For thin-walled or delicate features, machine them in the final finishing pass after the bulk of the material has been removed and the part has had a chance to “move.” In some cases, it may be necessary to rough the part, unclamp it to allow it to relax, then re-fixture and take a light finishing pass to hit final dimensions.

For long, slender parts (like the mentioned 1000x70x25mm stock), additional intermediate supports are essential. Use adjustable jack screws or custom soft jaws with raised supports along the entire length to prevent sagging from its own weight or clamping pressure. The goal is to fixture the part in a state as close to its free, unstressed condition as possible.

Advanced Techniques: Drilling, Tapping, and Achieving Superior Surface Finishes

Drilling and tapping 7075 require specific techniques to avoid tool seizure, poor thread quality, and broken tools. For drilling, pecking cycles are your friend, especially for depths beyond 3x diameter. This ensures chips are cleared and coolant reaches the cutting edge. A dwell at the bottom of each peck can help break the chip. For through-holes, reduce feed slightly as the drill exits to prevent breakout burrs and potential grabbing.

Tapping 7075 can be straightforward with the right preparation. Always use a spiral-point (gun) tap for through-holes or a spiral-flute tap for blind holes to efficiently evacuate chips. The tap must be sharp and designed for aluminum. A tapping fluid or high-lubricity coolant is recommended. For critical threads, consider thread milling. This CNC technique uses a rotating end mill to generate the thread profile, offering exceptional control, no axial tool pressure (reducing breakage risk), and the ability to use the same tool for multiple hole sizes. It is highly effective for 7075 aluminum machining where thread quality is paramount.

Achieving a superior surface finish often comes down to the final passes. For milled surfaces, use a light finishing pass (0.005″–0.015″ depth of cut) with a high spindle speed and a consistent, moderate feed rate. Climb milling is almost always preferred for aluminum as it produces a cleaner cut by shearing the chip away from the finished surface. Tool runout must be minimized; a wobbling tool will leave visible witness lines. For a near-mirror finish, consider using a single-purpose, sharp finishing end mill and potentially increasing the SFM while slightly reducing the feed per tooth for a finer cusp height.

Deburring is a final, critical step. The toughness of 7075 can create stubborn burrs. Manual removal with scrapers or files works, but for production, thermal energy methods (TEM) or cryogenic deburring can efficiently remove burrs from complex parts without damaging edges or affecting tolerances.

Heat Treatment and Post-Processing Considerations for 7075 Parts

7075 is a heat-treatable alloy, and understanding its tempers is crucial for both machining and final part performance. The most common temper is T6 (solution heat-treated and artificially aged to peak strength). However, as noted in the knowledge base, “The T7351 temper on 7075 is a more than 10% softer and weaker than the T651 temper.” This underscores a key point: not all 7075 is the same. T7351 (and T73) undergoes an over-aging process that sacrifices some ultimate tensile strength for dramatically improved stress-corrosion cracking resistance. It is also softer and can machine with a gummier feel.

When designing a part, select the temper based on the application’s needs: T6/T651 for maximum static strength, T73/T7351 for applications under sustained tension in corrosive environments (common in aerospace). It is generally not recommended to apply post-machining heat treatment to a finished 7075 part to change its temper, as the parts will warp significantly during the quenching process. Heat treatment should be done on the rough-machined blank, followed by finish machining to correct any distortion.

Post-processing for corrosion protection and aesthetics is common. 7075 has good anodizing characteristics. Type II (sulfuric acid) anodizing provides a decorative and moderately wear-resistant coating. For maximum corrosion protection and hardness, Type III (hardcoat) anodizing is used, though it adds significant dimensional growth (typically 0.0005″–0.002″ per surface), which must be accounted for in final part dimensions. Painting and powder coating are also viable after proper surface preparation.

Finally, for parts that have undergone significant machining, stress relieving might be considered. This involves heating the machined part to a specific temperature (below its aging temperature) for a set time to allow residual stresses from machining to relax without significantly altering the temper’s mechanical properties. This can be a valuable step for ensuring long-term dimensional stability in precision components.