So wählen Sie Werkstoffe für die CNC-Bearbeitung aus: Ein praktischer Leitfaden

Material selection for a CNC machined part is a constrained optimisation problem: find the material that meets every functional requirement — mechanical, environmental, aesthetic — at the lowest total cost, where total cost includes both material purchase price and the machining cost driven by how difficult the material is to cut. Over-specifying a premium alloy wastes money without improving the part. Under-specifying risks failure in service. The optimal choice is the most economical material that meets all requirements with adequate margin.

In practice, the material shortlist for most CNC machined parts is short. Aluminium alloys cover the majority of applications where weight, corrosion resistance, and machining economy are priorities. Stainless steels and carbon steels serve where strength, hardness, and durability dominate. Titanium earns its high cost only when strength-to-weight ratio or biocompatibility is a hard requirement. Brass and copper handle electrical, thermal, and fluid-fitting applications. Engineering plastics cover insulation, chemical resistance, and low-friction applications.

This guide walks through the five selection criteria and each material family in the depth needed to make a confident specification decision. Material choice interacts with surface finishing — not every alloy anodizes or plates the same way — so for finishing decisions, refer to our guide on anodizing Type II vs Type III. Both material and finish selection are part of every quote through our CNC-Bearbeitungsdienstleistungen.

The Five Selection Criteria

1. Mechanical Properties

Start with what the part must do structurally: carry a load, resist impact, maintain stiffness under thermal cycling, survive fatigue over a specified number of cycles. Key mechanical properties to evaluate are tensile and yield strength, hardness, stiffness (elastic modulus), fatigue limit, and toughness. A structural bracket on an aerospace assembly and a cosmetic enclosure cover have fundamentally different mechanical demands — the same analysis applies to every part, just with different threshold values.

Yield strength determines whether the part deforms permanently under the maximum expected load; tensile strength determines when it fractures; modulus determines how much it deflects elastically. For parts subject to repeated cyclic loading, fatigue data is critical — a material may have adequate static strength but fail prematurely under cyclic stress at a fraction of its yield strength.

2. Weight

Where mass is constrained — aerospace, automotive, handheld devices, robotics, portable instruments — the relevant metric is specific strength (yield strength divided by density) or specific stiffness (modulus divided by density). Aluminium alloys and titanium lead on specific strength among structural metals. Carbon fibre composite and advanced polymers exceed both, but are not CNC machined in the conventional sense. For most weight-sensitive applications, aluminium 6061 or 7075 is the starting point, with titanium reserved for the minority of applications where aluminium’s strength is insufficient.

3. Environmental Resistance

The operating environment narrows the material field quickly. Corrosive environments — salt spray, humid tropics, acidic process streams, marine — rule out unprotected carbon steel and many aluminium alloys without surface treatment. Stainless steel 316, anodised aluminium, and many engineering plastics resist these environments without additional coating. High temperatures narrow the field to stainless steels, nickel alloys, and engineering ceramics. Cryogenic applications favour austenitic stainless steels and aluminium alloys, which retain toughness at low temperatures.

UV exposure is relevant for exposed plastic parts. Standard ABS and polycarbonate degrade under prolonged UV; UV-stabilised grades or surface coatings are required for outdoor applications. Chemical resistance must be evaluated for each specific chemical — PEEK resists most organic solvents and many acids; nylon absorbs moisture and swells; POM (Delrin) is attacked by strong acids and oxidising agents.

4. Machinability

Machinability is the ease with which a material can be cut to dimension and finish. It directly determines machining cost: higher machinability means faster cutting speeds, less tool wear, and lower cycle times. The machinability difference between a free-machining aluminium alloy and a nickel superalloy is a factor of 20 to 50 times in cutting speed — the cost differential is substantial even before material price.

Machinability ranking (approximate, higher is better): free-machining brass and aluminium are at the top; plain aluminium alloys, followed by mild steel and 303 stainless, are in the mid-range; 304/316 stainless, titanium, and hardened steels are difficult; nickel superalloys, hardened tool steels, and some ceramics are extremely difficult. If two eliminate materials meet all functional requirements, the more machinable one will almost always deliver lower total cost.

5. Finish and Appearance

The required surface finish and appearance affect both material selection and post-machining operations. Anodising — the most common finishing operation for aluminium — requires 6061, 6063, or 7075 for predictable, high-quality results. High-copper alloys like 2024 anodise poorly. Electroplating (nickel, chrome, zinc) is compatible with most metals but requires careful surface preparation. Powder coating adheres well to steel and aluminium.

If the part will be visible in service and colour or gloss is a requirement, surface finish matters from the first machining operation. As-machined tool marks photograph differently under different finishes; parts destined for high-visibility applications typically require polishing or blasting before coating.

The Material Families

Aluminium Alloys

Aluminium is the default material for a large share of CNC machined parts. It is lightweight (density approximately 2.7 g/cm3), naturally corrosion-resistant through a self-forming oxide layer, highly machinable, and available in a wide range of alloys with well-understood properties. The most important alloys for CNC machining are:

• 6061-T6: the all-purpose workhorse. Yield strength approximately 276 MPa, excellent machinability, weldable, anodises cleanly, readily available in bar, plate, and extrusion. The default choice for structural parts, enclosures, heat sinks, brackets, and any application where weight and machining cost matter.
• 7075-T6: a higher-strength alloy (yield strength approximately 503 MPa) developed for aerospace. Machines well, but is harder to weld and more expensive than 6061. Use 7075 where 6061 lacks the strength margin or where strength-to-weight ratio is critical. Note that 7075 is more sensitive to stress corrosion cracking in certain environments.
• 5052-H32: excellent corrosion resistance and formability, used primarily in sheet metal applications rather than machined billet. Good choice for marine environments.
• 2024-T3: high strength with good fatigue characteristics, used in aerospace where 7075 is not specified. High copper content reduces corrosion resistance and anodise quality; requires clad or coated protection in corrosive environments.

Steel and Stainless Steel

Steel is the choice when strength, hardness, and durability are the primary requirements and weight is secondary. The key grades for CNC machining:

• A36 and A1018 mild steel: low cost, good machinability, adequate strength for most structural applications, but requires coating or plating for corrosion protection. The default for structural plates, fixtures, and non-corrosive-environment parts.
• 4140 alloy steel: higher strength and hardness than mild steel; heat-treatable to a wide range of hardness levels. Used for shafts, gears, and tooling. Machinability is lower than mild steel, especially after heat treatment.
• 303 stainless: the free-machining grade of austenitic stainless. Good corrosion resistance, machines readily with the addition of sulphur as a free-machining additive. First choice when stainless is needed and machinability matters.
• 304 stainless: the most widely used stainless grade, with better corrosion resistance than 303 but lower machinability. Work-hardens rapidly during cutting; requires sharp tools and appropriate feeds. Used for food-contact, medical, and general corrosive-environment parts.
• 316 stainless: adds molybdenum for superior resistance to chloride corrosion. The choice for marine, pharmaceutical, and chemical-process applications. Machines similarly to 304 but is slightly more difficult.

Titanium

Titanium alloy Ti-6Al-4V (Grade 5) combines a yield strength of approximately 880 MPa with a density of only 4.43 g/cm3 — a specific strength exceeding most steels — with excellent corrosion resistance and full biocompatibility. These properties make it the dominant material for aerospace structural components and orthopaedic implants where its combination of properties is essential.

Titanium is expensive in both material and machining cost. Its low thermal conductivity concentrates heat at the cutting edge, accelerating tool wear. Cutting speeds are a fraction of those achievable in aluminium. Reserve titanium for applications where its specific combination of properties — particularly strength-to-weight, corrosion resistance, and biocompatibility — cannot be matched by a less expensive alternative. For parts that need only corrosion resistance, stainless steel is dramatically cheaper to machine. For parts that need only light weight, aluminium 7075 offers adequate strength at much lower machining cost.

Brass and Copper

Brass alloys, principally C360 free-machining brass, machine faster than any other commonly used engineering metal — two to three times faster than mild steel at equivalent surface finish. Their machinability, combined with excellent corrosion resistance, makes them the standard choice for fluid fittings, valve bodies, connectors, instrument housings, and decorative hardware. The limitations are cost (brass is more expensive than mild steel) and weight (density approximately 8.5 g/cm3, higher than aluminium).

Copper is specified when electrical or thermal conductivity is the primary requirement. Oxygen-free copper (C101) achieves electrical conductivity approximately 60 times higher than aluminium and is used for bus bars, heat-sink interfaces, and electrical contacts. Copper machines adequately but generates built-up edge on cutting tools more readily than brass.

Engineering Plastics

Engineering plastics serve where metals are inappropriate — where electrical insulation, chemical resistance, low friction, or weight minimisation is the primary requirement. The most common grades in CNC machining:

• POM (Delrin/acetal): low friction, good dimensional stability, excellent machinability, FDA-compliant grades available. Standard for bushings, gears, rollers, and food-contact parts. Attacked by strong acids and oxidising agents.
• Nylon (PA6, PA66): good strength and toughness, low friction, absorbs moisture (up to 2-3% by weight) which can affect dimensional stability in tight-tolerance applications. Use moisture-stabilised grades for humid environments.
• PEEK: the highest-performance engineering plastic, with a continuous service temperature to 250 degrees Celsius, excellent chemical resistance, high strength, and low outgassing. Used in aerospace, medical, and semiconductor equipment where metal would be too heavy or electrically conductive. PEEK machines well but is expensive.
• Polycarbonate (PC): optically transparent, tough, and dimensionally stable. Used for sight windows, optical components, and prototyping. UV-stabilised grades for outdoor applications.
• ABS: low cost, good machinability, adequate mechanical properties for non-structural applications. Standard for prototypes, enclosures, and housings that do not require high strength or chemical resistance.

Quick-Reference Selection Table

A Practical Selection Process

Write down the part’s hard requirements first: the minimum yield strength under maximum load, the operating temperature range, the corrosive species it will contact, the weight budget if constrained, and any surface finish or compliance requirements (FDA, RoHS, biocompatibility). Eliminate every material that fails any hard requirement. Among those remaining, identify the most machinable option — it will usually deliver the lowest total cost. If two or three candidates are close, prototype in the cheapest one and switch to the premium option only if testing reveals a specific deficiency.

If you are specifying a new material for a new part, confirm your supplier stocks it in the form factor you need — bar, plate, tube, sheet — before committing to it in the design. Specialty materials ordered to spec add lead time and cost.

Häufig gestellte Fragen

What is the most common CNC machining material?

Aluminium 6061-T6 is the most common CNC machining material by volume in most job shops. It combines excellent machinability, good structural strength (yield strength approximately 276 MPa), corrosion resistance, light weight, and the ability to be anodised, at a reasonable material cost. It is the default starting point for a large share of machined parts.

How do I choose between aluminium and steel?

Choose aluminium when weight matters, when corrosion resistance in general environments is needed without additional coating, and when machining cost matters — aluminium machines three to four times faster than steel. Choose steel or stainless when higher strength or hardness is required, when the part is subjected to high wear or impact, or when the application temperature exceeds aluminium’s practical service range of around 150 degrees Celsius.

Which material is best for corrosion resistance?

Stainless 316 offers the best corrosion resistance among the commonly machined metals, particularly for marine and chloride-rich environments where 304 is vulnerable to pitting. Anodised aluminium 6061 performs well in atmospheric and mild chemical environments. For severe chemical service, PEEK and other engineering plastics often outperform metals entirely. The best choice always depends on the specific corrosive environment.

When is titanium worth the machining cost?

When the application genuinely requires its specific combination of properties: high strength-to-weight ratio and biocompatibility for orthopaedic and dental implants; strength-to-weight ratio and corrosion resistance for aerospace structural parts where aluminium lacks the strength; or corrosion resistance in aggressive environments where stainless steel is too heavy. For parts that need only corrosion resistance, 316 stainless is dramatically cheaper to machine. For parts that need only light weight, aluminium 7075 offers adequate strength for most applications at a fraction of the titanium machining cost.