Choosing the right material is half the engineering challenge in precision CNC machining. The alloy you specify determines everything downstream: achievable tolerances, surface finish, tool life, cycle time, and ultimately your per-part cost. Pick wrong, and you're paying for capability you don't need—or discovering at first article that the material won't hold the tolerance your design requires.
This guide covers the materials we machine most frequently for aerospace, defense, and high-performance industrial applications—from common aerospace aluminums to the exotic superalloys that separate general-purpose shops from shops built to handle difficult materials.
Material Selection at a Glance
| Material | Machinability | Achievable Tolerance | Relative Cost |
|---|---|---|---|
| Aluminum 6061-T6 | Excellent | ±0.0005" | $ |
| Aluminum 7075-T6 | Very Good | ±0.0005" | $$ |
| Stainless 316L | Moderate | ±0.001" | $$ |
| 17-4 PH Stainless | Moderate (condition dependent) | ±0.001" | $$ |
| Titanium Ti-6Al-4V | Difficult | ±0.001" | $$$ |
| Inconel 718 | Very Difficult | ±0.001" | $$$$ |
| Hastelloy C-276 | Very Difficult | ±0.0015" | $$$$ |
The tolerance values above are practical production tolerances—what a capable shop can hold consistently across a production run, not best-case numbers from a single prototype. Tighter tolerances are achievable but require slower feeds, additional inspection, and higher per-part cost.
Aerospace Aluminum: The Baseline
The most commonly machined aluminum alloy in aerospace. Good strength, excellent machinability, readily available, and cost-effective. The default choice for structural brackets, housings, and non-flight-critical components where weight matters but ultimate strength isn't the primary driver.
The high-strength aerospace aluminum. Nearly twice the tensile strength of 6061, making it the go-to for structural applications where stress analysis demands more than standard aluminum can deliver. Slightly harder to machine than 6061 but still in the "easy" category compared to steels and superalloys.
For most non-flight-critical parts, 6061-T6 is the right call. It's cheaper, machines faster, and anodizes better. Specify 7075 when your stress analysis requires it—not as a default upgrade.
Titanium: The High-Performance Compromise
The workhorse titanium alloy accounting for over 50% of all titanium used in aerospace. Combines high strength-to-weight ratio with corrosion resistance and biocompatibility. The material of choice for airframe structures, engine components, and medical implants where weight savings justify the higher material and machining cost.
Machining titanium requires specific knowledge. It's not that titanium is impossibly hard to machine—it's that it punishes bad practice instantly. The material has low thermal conductivity, meaning heat concentrates at the cutting edge instead of dissipating into the chip. Combined with its tendency to work-harden and gall on the tool face, titanium demands:
- Rigid setups with minimal overhang. Vibration is the enemy.
- Sharp, coated carbide tools with positive rake angles. Worn tools generate heat and work-harden the surface.
- High-pressure coolant (1,000+ PSI through-spindle) to break chips and manage heat at the cutting zone.
- Conservative speeds, aggressive feeds. Keep the tool moving. Dwelling in the cut generates heat and kills tool life.
- 5-axis machining where possible, to maintain optimal tool engagement angles and reduce setup count.
Ti-6Al-4V is typically machined in the annealed condition (AMS 4928 bar, AMS 4911 sheet). If your application requires solution-treated and aged (STA) condition for higher hardness, machine close to final dimensions before heat treatment, then finish-machine critical features afterward. Machining STA titanium is significantly harder and more expensive.
Inconel: When Temperature Kills Everything Else
The most widely used nickel superalloy in aerospace. Maintains strength up to 1300°F—where titanium softens and steels fail. Found in turbine discs, combustion liners, exhaust components, and any application where thermal cycling and high stress coexist. Extremely difficult to machine, with tool life measured in minutes rather than hours.
Inconel 718 is where many machine shops draw the line. The material work-hardens aggressively—if a tool skips or rubs instead of cutting, the surface becomes harder than the original material, making subsequent passes even more difficult. It also generates extreme heat at the cutting edge, welding chips to the tool face and causing rapid crater wear.
Successful Inconel machining requires:
- Ceramic or coated carbide inserts rated for nickel superalloys. Standard carbide won't survive.
- Through-spindle coolant at maximum pressure (1,000 PSI minimum). Flood coolant alone isn't enough.
- Positive, consistent engagement. The tool must always be cutting, never rubbing. Light passes are the enemy.
- Dedicated toolholders with balanced pullstud assemblies for vibration-free operation at the recommended speeds.
- Frequent insert changes based on time-in-cut rather than visual wear. By the time you see wear, the surface is compromised.
Hastelloy and Other Nickel Superalloys
The go-to alloy for extreme corrosion resistance. Where Inconel is chosen for high-temperature strength, Hastelloy is chosen for chemical resistance—withstanding hydrochloric acid, sulfuric acid, and chloride-induced stress corrosion that would destroy stainless steels. Used in chemical processing, pollution control, and specialized aerospace applications.
Machining parameters for Hastelloy are similar to Inconel, with the added challenge of even higher gumminess and chip adhesion. Surface speeds must be kept low (40–60 SFM for carbide), and tool changes should be scheduled proactively based on cut time rather than waiting for signs of wear.
Stainless Steels: The Middle Ground
Stainless steels sit between aluminum's easy machinability and the superalloys' difficulty. For most precision applications, they offer the best balance of strength, corrosion resistance, and reasonable machining cost.
- 303 Stainless: Free-machining grade. Best choice when corrosion resistance is needed but weldability isn't. Machines almost as easily as carbon steel.
- 304/316L Stainless: General-purpose austenitic grades. Weldable, good corrosion resistance, but work-harden during machining. Use sharp tools and avoid re-cutting chips.
- 17-4 PH Stainless: Precipitation-hardened for high strength (up to 190 ksi). Often machined in Condition A (solution-treated), then aged to final hardness. Specify the condition on your drawing—machining hardened 17-4 PH is significantly more expensive than machining Condition A.
How Material Choice Affects Your Quote
When you submit a drawing for quoting, the material specification drives three cost factors:
- Raw material cost. Titanium bar stock is 10–15x the cost of aluminum. Inconel is 20–30x. For large parts with high buy-to-fly ratios, material cost can dominate the total.
- Cycle time. Cutting speeds for Inconel are 10–15% of aluminum speeds. A pocket that takes 5 minutes in aluminum takes 30–45 minutes in Inconel. The machine-time cost scales directly.
- Tooling consumption. A carbide end mill might cut 100 aluminum parts before replacement. The same geometry in Inconel might consume a tool insert every 2–3 parts. Tool cost per part can reach $20–$50 on superalloy jobs.
Over-specifying material is the most common cost driver we see. If your part operates at room temperature with no corrosion exposure, aluminum or standard steel is the right call—not titanium. Save the exotic alloys for applications that genuinely require their properties. If you're unsure, include the operating environment on your RFQ and we'll recommend the most cost-effective material that meets your requirements.
Heat Treatment Considerations
Material and heat treatment are inseparable in precision machining. The condition of the material when it arrives at the machine determines your machining strategy and final part properties.
- Aluminum: Usually machined in final temper (T6). No post-machining heat treatment required for most applications. Anodizing is a surface treatment, not a heat treatment.
- Titanium Ti-6Al-4V: Typically machined annealed. If STA (solution-treated and aged) is required, rough-machine before treatment, finish-machine after. Warn your shop—the aged condition is harder to machine.
- Inconel 718: Can be machined solution-treated (softer) or in the aged condition. Whenever possible, rough in solution-treated condition, age, then finish to final dimensions. This preserves tool life and reduces cost.
- 17-4 PH Stainless: Machine in Condition A (annealed), then age to H900/H1025/H1150 per your specification. Post-machining aging causes minimal dimensional change if stress-relieved properly.
Always specify the material condition on your drawing (annealed, T6, STA, H900, etc.). A drawing that says "Inconel 718" without specifying condition forces the shop to guess—and they'll quote the harder, more expensive condition to cover risk.
The Bottom Line
Material selection isn't just an engineering decision—it's a cost and manufacturing decision. The right material is the one that meets your performance requirements at the lowest total machining cost. That means:
- Don't over-specify. Titanium where aluminum works costs 5–10x more to machine.
- Specify the condition. Heat treatment state determines machinability and cost.
- Consider the buy-to-fly ratio. Large parts from expensive material stock can have material costs exceeding machining costs.
- Work with a shop that actually machines these materials. A shop that runs Inconel twice a year will quote defensively. A shop that runs it weekly knows the optimal parameters and tools—and that shows up in your price.
We machine titanium, Inconel, Hastelloy, and the full range of aerospace aluminums and stainless steels daily. If you need help selecting the right material for your application, send us the drawing with your operating environment and we'll recommend the most cost-effective option. View our complete materials and equipment list.