How to Choose a Metal for a Machined Part

The material decides much of a machined part's price and how it behaves. Start with what the part has to do: carry a load, resist corrosion, stay light, look good, survive heat or wear. Then pick the least expensive material that does all of it. These are the metals we're asked about most, and the properties that separate them.

Strength is not stiffness

Strength is how much load a material takes before it bends permanently or breaks. Stiffness, measured as the elastic modulus, is how much it deflects under load before that point. Strength varies widely between alloys and heat treatments, but stiffness barely changes within a family: every aluminum alloy is about 69 GPa, every steel about 200 GPa, and titanium about 114 GPa. If a part bends too much, a stronger alloy of the same metal won't fix it. More material in the right place, or a stiffer metal, will.

Aluminum

Aluminum weighs about a third as much as steel, machines faster than any other common metal, resists corrosion well and anodizes in many colors. It's the default for brackets, housings, plates and prototypes.

6061 vs 7075

• 6061-T6 is the general-purpose choice: a yield strength of about 276 MPa, good corrosion resistance, easy to weld, and it anodizes evenly. It is widely stocked and the least expensive.
• 7075-T6 is about 80 percent stronger, with a yield strength of about 503 MPa, close to many steels at a third of the weight. It is used for highly stressed parts such as suspension arms, aerospace fittings and bike components. It costs more, resists corrosion less well, and isn't suited to fusion welding because it cracks.

Other common grades: 2024 for fatigue-loaded aircraft parts, 5052 for sheet that has to bend, and cast tooling plate for flat, stable fixture plates. Our control arm is machined from 7075 for its strength.

Carbon and alloy steel

Steel is about three times as stiff as aluminum and costs less per kilogram, but it's heavier and rusts unless it's finished.

• 1018 is a low-carbon mild steel: easy to machine and weld, and inexpensive, but not very strong, and it can't be through-hardened.
• 4140 is a chromium-molybdenum alloy steel that can be heat treated. Pre-hardened 4140, around 28 to 32 HRC, is common for shafts, gears and tooling components.
• Tool steels such as A2, D2, H13 and P20 are made for dies, punches and molds. See mold steels.

4130 chromoly

4130 is a lower-carbon cousin of 4140 that welds well, which is why it's the standard tube for roll cages, aircraft tube frames and bike frames. Normalized 4130 has a yield strength around 435 MPa and a tensile strength around 670 MPa, much higher than mild steel tube of the same weight. See welding titanium and chromoly.

Stainless steel

Stainless steels contain at least about 10.5 percent chromium, which forms a thin protective oxide film on the surface. They're slower to machine than carbon steel because they work-harden, and they should be passivated after machining. See passivation.

• 303 has added sulfur that makes it easy to machine, at some cost to corrosion resistance and weldability.
• 304 is the general-purpose grade for food equipment, enclosures and brackets.
• 316 adds molybdenum for better resistance to chlorides: salt water, road salt and many chemicals. It is the usual choice for marine and medical parts.

17-4 PH

17-4 PH is a precipitation-hardening stainless steel. After heat treatment it's several times stronger than 304 or 316, with a yield strength around 1,170 MPa in the H900 condition, while keeping good corrosion resistance. It's used for shafts, valve parts, aerospace fittings and other parts that need both.

Titanium

Titanium weighs a little over half as much as steel, at 4.43 g/cm³ for Grade 5, and the Grade 5 alloy (Ti-6Al-4V) has a yield strength around 880 MPa, as strong as many alloy steels. It doesn't corrode in seawater or in the body, which makes it the standard metal for implants, and it feels warmer to the touch than aluminum because it conducts heat slowly. Its stiffness is about half that of steel.

The drawback is cost: titanium is expensive to buy and slow to machine. Heat doesn't flow away from the cut, so it's machined at lower speeds with sharp tools, rigid setups and plenty of coolant. Grade 2, commercially pure titanium, is weaker but easier to form and weld, with the best corrosion resistance. Grade 23 is a higher-purity version of Grade 5 used for implants. Our V1 Titanium Laptop Stand and the hardware on Ski Pole 1 are machined titanium.

Brass and copper

Free-machining brass (C360) is the easiest metal to machine, so it's common for fittings, valve bodies, connectors and decorative parts. Copper (C110) has the highest electrical and thermal conductivity of the common metals and is used for busbars, electrodes and heat sinks, but it's soft and gummy to machine.

Engineering plastics

• Acetal (POM, often sold as Delrin) is stiff, low-friction and stable in size, and machines cleanly. It is a common choice for gears, bushings and fixtures.
• Nylon is tough and wear resistant but absorbs moisture, which changes its size.
• PEEK holds up to continuous use around 250°C and to most chemicals, and is used in medical and aerospace parts. It's expensive.
• UHMW polyethylene is very slippery and impact resistant, good for wear strips and guides, but hard to hold to tight tolerances.
• Polycarbonate and acrylic are the usual choices for clear parts.

Side by side

Typical values for the condition shown. Check the material certificate and a design reference before relying on them.

Quick picks

• General parts, brackets and prototypes: 6061 aluminum.
• Highly loaded parts where weight matters: 7075 aluminum, or titanium if the budget allows.
• Welded tube structures: 4130 chromoly, or 6061 aluminum.
• Outdoor, salt or chemical exposure: 316 stainless steel, titanium, or anodized 6061.
• Wear surfaces: hardened steel, or hard anodized aluminum.
• Parts that mustn't deflect: steel, or a deeper section in aluminum.

Tell us what the part does and we'll suggest a material when we quote. See CNC machining, and carbon fiber vs aluminum if you're also weighing composites.

Related reading

• CNC machining processes
• Surface finishes for metal parts
• Carbon fiber vs aluminum
• Carbon fiber vs steel, fiberglass and plastic
• Mold steels: P20, 718H, NAK80, S136 and H13

Terms are defined in the glossary, and every guide is listed in the knowledge base.