Aluminium Alloys

Aluminium alloys are the light-metal family at the heart of modern machining — the wrought alloys of aluminium that make aircraft fittings, electronics housings, automotive parts, jigs and prototypes. Aluminium weighs only about a third as much as steel, machines faster than almost any structural metal, and can be alloyed and heat-treated across a wide range of strengths, and those facts together make it the default material of a large share of machined parts. To the machinist the family is best read the way this wiki reads every metal — by how each alloy behaves under the tool. Aluminium is never the material that destroys tooling, but it has its own enemies: gumminess, built-up edge, distortion and chatter. The grades a shop actually machines are dominated by the heat-treatable 6xxx and 7xxx alloys, above all 6061 and 7075, and this entry is about machining them.

How aluminium alloys are built

Pure aluminium is soft, light and corrosion-resistant but far too weak for structure, so engineering aluminium is always alloyed, and the alloys divide by how they gain strength. The non-heat-treatable alloys — 1xxx, the manganese 3xxx and the magnesium 5xxx — are strengthened by cold work, and they are the marine, chemical and sheet alloys, of which 5083 is the classic; they are formed and welded more than machined. The heat-treatable alloys — the copper 2xxx, the magnesium-silicon 6xxx and the zinc-magnesium-copper 7xxx — are strengthened by precipitation hardening, or age hardening: the metal is solution-treated, quenched and aged so that fine particles precipitate and lock the structure against slip. The machinist meets these in the fully hardened T6 temper and, for machining, the stress-relieved T651. Of them all, 6061 and 7075 dominate machined parts — 6061 the general-purpose structural alloy, 7075 the high-strength aerospace alloy — and the engineering numbers show the gap between the two.

The strength gap, and what it does not buy

The numbers explain when 7075 is worth its harder cut. In the T6 temper, 6061 develops a tensile strength of the order of 300 MPa while 7075 reaches roughly 500 MPa or more — about twice the yield strength for only a few percent more weight. The catch that matters most to the machinist is that stiffness does not follow strength: the elastic modulus of the two alloys is almost identical, near 70 GPa, so a part that flexes in 6061 flexes just the same in 7075. Upgrading the alloy buys strength and fatigue resistance, never rigidity — a deflection problem is solved with geometry or section, not by changing from 6061 to 7075.

Machining aluminium

For the machine, aluminium is the friendly extreme of the cutting spectrum. It is soft, so carbide tools cut it at surface speeds that would destroy them on steel, and it is a material where speed is an ally — several hundred metres per minute is ordinary, and the limits are chip evacuation and finish rather than tool wear. But that softness brings the classic aluminium problem: the metal is gummy, and a tool that is not sharp, or a cut that does not commit, makes the work smear, weld onto the edge and build up — the built-up edge that spoils finish and dimension. The remedies are the familiar ones turned the aluminium way: sharp tools with polished flutes and positive rake, feed high enough that each tooth cuts rather than rubs, coolant to clear the soft chips and carry heat, and climb milling. And because the metal is soft and light, thin walls and unsupported sections deflect and chatter under the cut, while its high thermal expansion means a large part measured warm is not the same size cold — all reasons the same feeds and speeds discipline, chip-formation awareness and carbide tooling apply here as everywhere, with the dials set the aluminium way.

6061 versus 7075 under the tool

The two alloys machine differently enough that the choice affects the whole job. 6061 is the easier, safer default. It is dimensionally stable, keeping its flatness even when most of the stock is machined away, it runs at the top of the speed range with a wide process window, and it is gentle on cutting edges. Its only enemy is its own gumminess — the stringy chips and built-up edge that are managed with sharp tooling and coolant rather than feared. 7075 is the harder cut. Its zinc-based hardening is harder on cutting edges, so tool life shortens, and — more important — 7075 plate carries residual rolling stress, so machining away deep pockets or thin walls can let the part distort as the stressed skin is released; the remedies are the stress-relieved T651 form, roughing staged to release stress progressively, and light finish passes on a stable part. In a quiet irony of the trade, the harder alloy often breaks its chips more cleanly than the gummy one, so 7075 is not difficult in the way 6061 is — it wears tools and moves under stress rather than smearing.

Choosing between them

Selection is a genuine trade, not a hierarchy. 6061 is chosen when the part is to be welded — 7075 is essentially unweldable, cracking as it cools — when it will serve in corrosive conditions, when it will be anodised to a controlled colour, when cost matters, or simply when nothing demands more; it is the default for housings, brackets, prototypes and the broad run of machined aluminium. 7075 earns its place when strength-to-weight, fatigue or wear genuinely govern — aircraft fittings, high-load links and competition parts machined from solid and mechanically joined. Both, in the end, are quick, economical material to machine, and the part that is truly demanding is the one that chooses the alloy for the properties it needs and machines each alloy on its own terms.

Aluminium in the job flow

Aluminium sits at the productive end of CNC machining, where its speed under the cutter rewards the shop with short cycles and good surface finish. Beside the stainless steels, with their work-hardening and heat, and the carbon and alloy steels, with their force and their chips, aluminium is the light metal that asks for sharpness and speed instead of power — and gives the machinist, in return, the fastest route from bar stock to finished part.