Why Material Selection Matters
Material selection is the most consequential decision in part design. The wrong material costs you twice - once when you build the prototype, and again when you redesign for production because the part failed in the field. This guide gives you the engineering framework to narrow hundreds of alloys down to the right one - backed by real property data, cost comparisons, and application-specific recommendations used by experienced mechanical engineers.
The Material Selection Framework
Before opening a material database, answer these five questions in order. Each one eliminates entire material families and narrows your search.
Material selection is the most consequential decision in part design. The wrong material costs you twice - once when you build the prototype, and again when you redesign for production because the part failed in the field. This guide provides a structured framework for choosing materials across CNC machining, 3D printing, sheet metal, and injection molding.
We'll cover property trade-offs with real numbers, cost comparisons at different volumes, and the selection methodology that experienced mechanical engineers use to narrow hundreds of alloys down to the right one.
What loads does the part see?
Static, cyclic/fatigue, impact, or creep. This determines your minimum yield strength and fatigue endurance limit.
What environment does it operate in?
Temperature range, chemical exposure, UV, humidity, sterilization (autoclave, EtO, gamma).
What are the dimensional requirements?
Tight tolerances favor metals (stable, machinable). Large, complex shapes favor plastics or composites.
What are the weight constraints?
Strength-to-weight ratio (specific strength) is what matters, not absolute strength. Aluminum and titanium win here; steel wins on absolute cost.
What is the target cost at volume?
Raw material cost per kg varies 10× between commodity steels and aerospace titanium.
Pro Tip
Answer these five questions before opening any material database. Each one eliminates entire material families and narrows your search from hundreds of alloys to 2–3 candidates.
Aluminum Alloys
Aluminum is the default CNC material for good reason: excellent machinability, good strength-to-weight, natural corrosion resistance, and moderate cost.
The two most common grades cover 80%+ of CNC aluminum work. Start with 6061-T6. Switch to 7075-T6 only when you need >40 ksi yield and can accept lower weldability and higher material cost. For cosmetic anodized parts, 6061 produces a cleaner finish.
| Property | 6061-T6 | 7075-T6 |
|---|---|---|
| Tensile strength | 310 MPa (45 ksi) | 572 MPa (83 ksi) |
| Yield strength | 276 MPa (40 ksi) | 503 MPa (73 ksi) |
| Elongation at break | 12–17% | 5–11% |
| Machinability | Excellent | Good (more tool wear) |
| Weldability | Good (loses T6 temper in HAZ) | Poor (stress-corrosion cracking risk) |
| Anodize quality | Excellent (clear, hard, color) | Good (slight yellow tint) |
| Cost (plate, $/lb) | $3–5 | $6–10 |
| Best for | Structural brackets, housings, fixtures | High-load aerospace, bicycle frames, drone arms |
Pro Tip
Selection rule: Start with 6061-T6. Switch to 7075-T6 only when you need >40 ksi yield and can accept lower weldability and higher material cost. For cosmetic anodized parts, 6061 produces a cleaner finish.
Stainless Steel
Stainless steel grades range from free-machining (303) to marine-grade corrosion resistance (316L) to heat-treatable high-strength (17-4 PH).
| Property | 303 | 304 | 316L | 17-4 PH (H900) |
|---|---|---|---|---|
| Tensile strength | 620 MPa | 515 MPa | 485 MPa | 1,310 MPa |
| Corrosion resistance | Good | Very good | Excellent (chloride environments) | Good |
| Machinability | Best (free-machining) | Moderate | Moderate | Moderate |
| Welding | Poor (hot cracking) | Excellent | Excellent | Good (age after welding) |
| FDA food contact | No | Yes | Yes | Yes |
| Best for | Fittings, shafts, bushings | Food/medical housings | Marine, chemical processing | High-strength structural |
Pro Tip
Use 303 when machinability is king and corrosion requirements are moderate. Use 316L for anything involving salt water, chlorides, or bodily fluids. Use 17-4 PH when you need stainless corrosion resistance and heat-treatable high strength (130+ ksi).
Carbon & Alloy Steels + Titanium
When cost matters more than corrosion resistance, carbon steel is the answer. When strength-to-weight is critical, titanium is unmatched.
Carbon and alloy steels dominate when cost matters more than corrosion resistance. Titanium (Ti-6Al-4V, Grade 5) delivers a strength-to-weight ratio that beats both aluminum and steel - 160 ksi tensile at 4.43 g/cm³ density (vs. 2.70 for aluminum, 7.85 for steel). But it's expensive ($25–$50/lb for bar stock) and difficult to machine (low thermal conductivity causes tool-tip heat buildup).
1018 (Low Carbon)
Easy to machine and weld. Case-hardenable. Best for pins, shafts, brackets where surface hardness + tough core is needed.
4140 (Alloy Steel)
Through-hardenable to HRC 28–32. Excellent fatigue resistance. Standard for gears, axles, and high-load shafts.
A2 / D2 Tool Steel
Air-hardening, wear-resistant. Use for fixtures, die components, and wear surfaces. HRC 58–62 achievable.
Ti-6Al-4V (Grade 5)
Use titanium when: weight savings directly impacts product performance (aerospace, medical implants, racing), the operating temperature exceeds 200°C (aluminum softens), or biocompatibility is required.
Pro Tip
Carbon steel needs surface treatment (plating, black oxide, paint) for corrosion resistance. Factor in finishing cost when comparing against stainless or aluminum.
Engineering Plastics
Plastics offer lower density, electrical insulation, and chemical resistance that metals can't match. The trade-off is lower stiffness, creep under sustained load, and tighter temperature limits.
| Material | Tensile Strength | Max Service Temp | Chemical Resistance | Cost ($/lb) | Best For |
|---|---|---|---|---|---|
| Delrin (POM) | 69 MPa (10 ksi) | 90°C (195°F) | Good (not strong acids) | $4–8 | Gears, bushings, snap-fits, low-friction guides |
| Nylon 6/6 | 82 MPa (12 ksi) | 100°C (212°F) | Good | $4–7 | Structural clips, cable ties, wear pads |
| PEEK | 100 MPa (14.5 ksi) | 260°C (500°F) | Excellent | $70–120 | Aerospace, medical implants, semiconductor |
| UHMWPE | 40 MPa (5.8 ksi) | 80°C (176°F) | Excellent | $6–10 | Wear strips, guides, food-contact surfaces |
| Polycarbonate (PC) | 63 MPa (9.1 ksi) | 120°C (248°F) | Moderate | $3–6 | Transparent housings, impact-resistant covers |
| ABS | 40 MPa (5.8 ksi) | 80°C (176°F) | Moderate | $2–4 | Consumer enclosures, injection-molded housings |
Pro Tip
Start with Delrin for precision mechanical parts (lowest moisture absorption, best dimensional stability). Use Nylon when you need higher strength and can tolerate ~2% moisture absorption. Use PEEK only when temperature or chemical environment demands it - it's 10–15× the cost of Delrin.
Composites: Carbon Fiber, Glass & Kevlar
Composites offer the highest specific stiffness and strength of any material class, but they bring manufacturing complexity.
Composites are not CNC-machined from billet - they're laid up, cured, and then trimmed. Only use composites when specific stiffness or specific strength requirements cannot be met by aluminum or titanium, and the program volume justifies tooling for lay-up molds. For one-off prototypes, machined aluminum is almost always faster and cheaper.
Carbon Fiber Reinforced Polymer (CFRP)
Specific stiffness 3–5× aluminum. Used in drone frames, satellite structures, sporting goods. Expensive ($50–$200/lb for prepreg).
Glass Fiber Reinforced Polymer (GFRP)
60% of CFRP stiffness at 20% of the cost. Excellent for enclosures, non-structural panels, and PCB substrates (FR-4 is GFRP).
Kevlar (Aramid)
Exceptional impact and abrasion resistance. Difficult to machine (fibers fuzz rather than shear cleanly). Best for protective housings and ballistic applications.
Pro Tip
When to use composites: Only when specific stiffness or specific strength requirements cannot be met by aluminum or titanium, and the program volume justifies tooling for lay-up molds.
Material Selection by Application
Use this application matrix to quickly find the right material for your specific use case - with alternatives and materials to avoid.
| Application | First Choice | Alternative | Avoid |
|---|---|---|---|
| Structural bracket (general) | Al 6061-T6 | Steel 1018 (higher load) | Plastics (creep under sustained load) |
| Precision bearing housing | Al 7075-T6 or SS 303 | Bronze (for self-lubricating bushings) | Carbon steel (corrosion without plating) |
| Medical instrument | SS 316L or Ti-6Al-4V | PEEK (non-metallic alternative) | Aluminum (anodize can flake in sterilization) |
| Consumer electronics enclosure | Al 6061-T6 (premium) or ABS (injection) | PC (impact resistance + transparency) | SS (too heavy for handheld) |
| Semiconductor wafer handling | Al 6061-T6 (hard anodized) or PEEK | Ceramic (alumina, silicon carbide) | Carbon steel (particle generation) |
| Outdoor/marine | SS 316L or Al 5052-H32 | Titanium (corrosion-free, weight matters) | Carbon steel, 7075 (pitting corrosion) |
| High-temp (>200°C) | Inconel 718 or Ti-6Al-4V | PEEK (to 260°C continuous) | Aluminum (softens above 150°C) |
Pro Tip
When your application doesn't fit a single row, prioritize the operating environment first (temperature, chemicals, moisture), then narrow by load requirements, and let cost break the tie.
Material Cost Comparison
Raw material cost varies significantly by alloy, form factor (plate, bar, tube), and market conditions. These ranges reflect typical 2024–2025 pricing for small-lot purchases.
| Material | Cost per lb (bar stock) | Relative to Al 6061 |
|---|---|---|
| Al 6061-T6 | $3–5 | 1.0× |
| Al 7075-T6 | $6–10 | 2.0× |
| SS 304 | $4–7 | 1.3× |
| SS 316L | $5–9 | 1.7× |
| 17-4 PH | $8–14 | 2.5× |
| Ti-6Al-4V | $25–50 | 8–12× |
| Inconel 718 | $30–60 | 10–15× |
| Delrin (POM) | $4–8 | 1.3× |
| PEEK | $70–120 | 20–30× |
Pro Tip
Raw material cost is only part of the equation. Titanium and Inconel are expensive to machine (slow speeds, high tool wear), so the total part cost is 3–5× the raw material premium. Factor in machining cost when comparing materials.
Common Material Selection Mistakes
Avoid these pitfalls that burn budget and delay schedules. Each mistake has real cost implications.
Defaulting to stainless steel "because it won't rust"
Anodized aluminum resists corrosion in most indoor environments at 1/3 the weight and 1/2 the cost. Reserve stainless for chloride exposure, food contact, or autoclave sterilization.
Specifying 7075 when 6061 works
7075 is stronger, but harder to machine, can't be welded reliably, and costs 2× more. Use it only when you've proven 6061 can't meet your strength requirement.
Ignoring thermal expansion
If your assembly uses mixed materials (aluminum housing + steel shaft), calculate the differential thermal expansion across your operating range. Al expands at 23.6 µm/m·°C vs. 12 µm/m·°C for steel - a 100°C delta on a 200 mm bore creates a 0.23 mm fit change.
Choosing PEEK for cost savings vs. metal
PEEK is $70–120/lb and machines slowly. It's justified for weight, chemical resistance, or radiolucency - not because "plastic is cheaper than metal."
Not specifying the temper/condition
"Aluminum 6061" without "-T6" means the shop may use -O (annealed), which has half the strength. Always callout the full material designation on your drawing.
Pro Tip
Create a "material requirements" checklist on your drawing title block: alloy + temper, surface treatment, hardness range, and special test/cert requirements. This prevents assumptions from creeping in.
Conclusion
Material selection is an engineering decision, not a materials science exercise. Start with the functional requirements (loads, environment, tolerances), narrow to 2–3 candidate materials using the tables above, then let cost and availability break the tie.
General CNC Parts
When in doubt, prototype in aluminum 6061-T6. It machines fast, costs little, and performs well in the vast majority of applications.
Corrosive / Medical / Food
For chloride exposure, food contact, sterilization, or biocompatibility requirements. Factor in 2–3× higher machining costs.
Weight-Critical / High-Temp
Upgrade to a higher-performance material only when test data proves 6061 isn't sufficient. Consider total program cost, not just material price.
When in doubt, prototype in aluminum 6061-T6. It machines fast, costs little, and performs well in the vast majority of applications. Upgrade to a higher-performance material only when test data proves 6061 isn't sufficient.
Further Reading
- Materials Library - searchable database of 100+ materials with properties and grades.
- Metals Catalog - aluminum, steel, stainless, titanium, and specialty alloys.
- Plastics Catalog - engineering plastics for CNC and injection molding.
- DFM Best Practices - 15 rules to reduce manufacturing cost.
- CNC Tolerances Guide - how material choice affects achievable tolerances.
Frequently Asked Questions
What is the best default material for CNC machined parts?
When should I use stainless steel instead of aluminum?
How much does titanium cost compared to aluminum?
What engineering plastic should I start with for CNC parts?
When are composites worth the cost and complexity?
What happens if I don't specify the temper on my material callout?
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