A quick guide to the machinability of aluminium alloys

You can view this issue in the same way as a restaurant chef does when preparing a meal: The raw material matters. With aluminium, choosing the right alloy and temper improves machinability, and thereby the final product.

In general, machining aluminium profiles is relatively inexpensive. I’m talking about processes such as sawing, turning, drilling, milling, threading and punching. One reason is that tool costs tend to be competitive due to low tool wear as a result of low specific cutting force. The cutting force in Al is only about a 1/3 of that with steel. In addition, the cutting speeds attainable with aluminium are much higher than for steel, thus leading to less-expensive machining compared to steel.

Still, you need to keep an eye on some basic points to achieve your machining cost and quality targets.

Chip breakage is critical aspect of aluminium machinability

Let me start with the two principal classifications of aluminium alloys: cast alloys and wrought alloys. Each classification is further divided into the categories heat treatable and non-heat-treatable. About 85% of aluminium is used for wrought products, such as rolled plate and foils, and extrusions. Here you can find large variations, with some alloys forming very small nice chips whereas others struggle with chip breakage.

For the category of cast alloys, machining is more about tool wear since these alloys possess excellent chip breakage.

For wrought aluminium, chip breakage is the most critical aspect of machinability. Good chip breakage prevents:

• Chip entanglement which can lead to operations stops and tool breakage
• Chip scratching of machined surfaces and deterioration of the surface finish

For 6xxx alloys, the general trend is that chip breakage increases with strength, both with harder alloys and tempers. However, this cannot always be translated to other alloy systems. For instance, it can be very difficult with some 7xxx alloys, despite their high strength, to get good chip breakage.

You can assess chip breakage with chip maps or chip counting. The first gives you a good overview of chip breakage at different cutting parameters, while the latter provides you with a more accurate measurement.

In addition:

1. Machinability should always be assessed on conditions as similar as possible to production
2. A single machine operation can incorporate cutting parameters over a wide range
3. To easily find the right cutting parameters and have a stable cutting operation, a broad interval of acceptable chip breakage is preferred

How machinability varies by alloy

Extrusion alloys are commonly benchmarked for cutting machinability on a simple 0–3 scale, where 3 is the best score achievable and 0 is the weakest. None of the standard extrusion alloys below score a full 3 for cutting, which reflects how the industry generally frames aluminium machining: differences between alloys are usually a matter of degree, not a night-and-day contrast.

1050A (0): Pure aluminium is soft and gummy. It resists clean chip formation, so it's the weakest performer on this list despite being easy to form.
6060 (1): Workable, but the lowest-scoring alloy in the 6000 series here. Chosen for surface finish and anodizing quality, not for machining ease.
6063 (2): A solid middle-of-the-road performer. Widely specified for architectural profiles, and its machinability is adequate rather than a selling point.
6005 / 6005A (2): Comparable to 6063. Chosen more for the strength-to-formability balance than for machining ease.
6082 (2): The highest-strength standard alloy in the range, and its machinability score matches the mid-strength alloys, so strength isn't traded away for machinability here.
6161 / 6463 (2): In line with the other mid-range 6000-series alloys.
HHS 360 / HHS 400 (2): The high-strength alloys in this range hold the same cutting score as the standard-strength alloys, which is notable given their higher tensile properties.

So where does 6061 fit in?
6061 isn't part of the table above, since it's classified under a different alloy-designation system than the alloys listed there. But still, it earns its own paragraph. 6061 is commonly described as an alloy that machines cleanly and handles secondary operations, holes, cuts, threading, well after extrusion. Compared with 6063, the trade-off is fairly direct: 6063 gives a better surface finish and anodizing result, while 6061 gives more strength and better machinability. For profiles that need heavy fabrication, welding and machining together, 6061 is generally the better-suited choice of the two.

Machining aluminium extrusions in practice

Sawing is usually the first cut after extrusion, and it can leave a finish clean enough that no further work on the cut surface is needed, provided the blade is suited to aluminium. Typical setups run blade diameters of 300–650 mm, blade widths of 2–4.2 mm, and rotation speeds of 1,500–2,800 rpm. Radial saws can handle profiles up to 500 mm wide.

Turning works best with alloys that produce short chips, and as a rule of thumb, an alloy should be machined at its highest practical temper, choosing a hardenable alloy where possible. Soft material tends to build up on the cutting insert, produce long chips, and cause burr formation, so cutting speed and feed need to be set correctly to keep chips falling away cleanly from the cut.

Drilling bits suited to extruded aluminium typically use a tip angle of around 130° and a spiral angle of approximately 40°, with ample clearance for chip evacuation. Bits are usually high-speed steel with a tungsten carbide or diamond tip.

Milling, or high-speed machining, generally defined as cutting speeds of 2,500 m/min and upward, reduces cutting forces as speed increases, which allows higher feed speeds and shorter machining time. It does require machine tools with better dynamics and more sophisticated controls to exploit those speeds properly. 

If you're already machining aluminium and running into problems, I'd encourage you to check out our guide to fixing aluminium machining problems. 

Special aluminium alloys 6061M and 6082M

One final point: Aluminium producers have traditionally added the low melting-point alloying elements lead (Pb) and bismuth (Bi) to 6xxx-series alloys to achieve good machinability. Examples are the 6042 and Hydro 6262 alloys.

That said, lead and bismuth are banned as alloying elements in some regions and applications. Where this is the case, good machinability alloys are 6061M and 6082M. These are special versions that are based on 6061 and 6082 with both tailored temper and composition for machinability.

Let me conclude simply by stating that excellent machinability of aluminium alloys is achieved by careful control of the alloy’s chemical composition and the parameters in all process steps.