Material properties

A quick guide to the machinability of aluminium alloys

The machinability of aluminium alloys can be difficult to predict, even for machinists. There are so many variables. One is the condition of the material, not to mention its physical properties – alloying elements, microstructure, hardness, yield strength, tensile strength, elongation, work hardening.

You can view this issue in the same way as a restaurant chef does when preparing a meal: The raw material matters. With aluminium, having the right material will improve 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

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.