Saving weight without losing strength – aluminium alloys for turned parts compared.

Aluminium is rarer in the turned parts market than often assumed. Most industrial applications are more economical in stainless steel or free-cutting steel. But where weight matters, aluminium is unrivaled. This article looks at the most important alloys.

Aluminium has about one third the density of steel with similar specific strength (strength per weight) in the high-strength alloys. That makes aluminium the material of choice for lightweight, mechanically loaded components. Typical application fields: aerospace, high-end mechanical engineering, robotics, sensor technology with weight requirements.

The most important alloy for CNC turned parts is AlCuMgPb (material no. 3.1645, formerly 2007). It contains copper, magnesium and lead – the latter as a chip breaker. Very good machinability, high strength (Rm ≈ 380 MPa), good anodizing properties. The standard choice for medium to highly loaded aluminium turned parts.

For maximum strength, AlZnMgCu1,5 (3.4365, formerly 7075) is used. Tensile strength above 500 MPa – approaching steel, at one third of the weight. Poorer machinability than 2007, but in demand where strength is critical. The standard in aerospace applications.

AlMgSi1 (3.2315, formerly 6082) is a forging alloy with good corrosion resistance – the typical choice for components used outdoors or in humid environments without surface protection. Poorer machinability than 2007 or 7075, but the right choice for corrosive applications.

Machining aluminium is demanding because aluminium tends to weld onto the tool. On our Swiss-type machines we use special tool geometries and sharp cutting edges for aluminium machining. The coolant strategy also differs from steel – usually with higher pressures and finer nozzles.

An economic note: aluminium is similar to stainless steel in material price, but faster to machine. However: without surface protection (anodizing, coating), aluminium is corrosion-sensitive in industrial environments. The finishing costs often bring the unit price back in line with stainless steel. Use aluminium only when weight really matters functionally.

We supply age-hardenable aluminium alloys in the appropriate heat treatment condition, the so-called temper. T4 denotes solution-annealed and naturally aged material, T6 additionally artificially aged for maximum strength. T3 stands for strain-hardened material after solution annealing; T651 supplements T6 with controlled stretching to relieve internal stresses. For tight tolerances, T651 in particular matters, because stress-relieved material barely distorts after machining. Higher strength tends to worsen chip formation, so we match tooling and cutting parameters to the respective condition.

Anodizing creates an oxide layer that grows into the material. Typical coating thicknesses are 5 to 25 µm and provide corrosion and wear protection, electrical insulation and a defined appearance including coloring. Hard anodizing produces considerably thicker and harder layers for highly loaded parts. Important for design: about half of the layer grows outward, so the diameter increases measurably. We account for this dimensional growth when defining the finished dimensions before coating.

During turning, aluminium tends toward burr formation and built-up edge, where material accumulates on the cutting edge and tears out edges. At cross holes, grooves and thread run-outs, this creates burrs that interfere with function, assembly and coating. That is why we plan deburring firmly into the process – depending on the geometry, manually, by vibratory finishing, or through a targeted machining strategy. This rework adds time and unit cost, which we state openly in the quote rather than hiding it in the price.

Aluminium expands at around 23 µm per meter and kelvin, roughly twice as much as steel. On a 100 mm long part, a temperature change of 10 kelvin already means a good 23 µm of dimensional change, which can blow IT6 or finer. We therefore inspect tight tolerances at a controlled 20 degrees Celsius and let parts acclimatize before measurement. For pairings of aluminium and steel, we also account for the differing thermal expansion so that fits work reliably across the entire operating temperature range.

A special design case is threads in aluminium. Because of its low strength compared to steel, directly cut internal threads wear quickly under repeated assembly and tend to strip. For highly loaded or frequently disassembled connections, we therefore recommend thread inserts (such as wire thread or self-tapping inserts) or a more generous thread length. Considering this early in the design avoids expensive rework. For aluminium turned parts with load-bearing threads, we flag this point early and agree on the most sensible solution with you.

We check feasibility, suggest optimizations and get back to you within 48 hours with a first assessment.