Many lightweight materials have come onto the market in recent years, but one continues to be a popular classic: aluminium. What makes aluminium so popular? What advantageous properties does it possess and what are the pitfalls of machining aluminium, particularly when it comes to milling? Find out in our overview!
Aluminium is indispensable in industrial production. Not least thanks to its special properties:
Aluminium is a non-ferrous metal and a chemical element with the symbol Al. It is used as a structural material, in electrical engineering, in the packaging industry as well as in the construction industry. In particular, its lower weight in comparison to other metals, high electrical conductivity and thermal behaviour are all critical factors in its application.
Compared to other metals, aluminium is soft and light, yet also tough. At 660.4°C, its melting point is significantly lower than that for other metals and it has a density of 2.70 g/cm³ – a third of the density of steel. The tensile strength of aluminium varies depending on its purity, from 45 N/mm² for absolutely pure aluminium, up to 90 N/mm² for commercially pure aluminium. Its thermal conductivity is 237 W/(m·K). This is higher than other base metals and is only exceeded by silver, copper and gold. Thanks to its high thermal conductivity, aluminium is also favoured for use in electrical engineering. Its physical properties make aluminium a good electrical conductor, and it is often used in energy technology for large conductor cross sections as an alternative to copper wires.
Aluminium quickly forms an oxide layer when exposed to the air. This gives the metal a silver-grey, somewhat dull appearance when unmachined. This oxide layer, which is approximately 0.05 µm thick, means that aluminium is highly corrosion-resistant and it also provides protection against further oxidation.
Aluminium is either used in its pure form or as an alloy depending on the application. The melt, with other metals, brings out or suppresses specific properties of this lightweight metal. Such alloys are produced in the form of cast aluminium alloys or wrought aluminium alloys. Metals such as manganese, magnesium, copper, silicon, nickel, zinc and beryllium are primarily used for aluminium alloys. The base material is normally Al99.5 (EN AW-1050A).
A distinction is made between hardenable and non-hardenable alloys, as well as between wrought or cast materials. Self-hardening aluminium alloys such as AlMg as well as pure and ultra-pure aluminium belong to the wrought materials as well as hardenable aluminium alloys such as AlMgSi, AlCuMg or AlCuSiMn. These aluminium alloys are particularly suitable for producing semi-finished products such as strips, sheets, wires or pipes.
Cast materials include aluminium alloys such as AlMgSi, AlSiCu AlCuTiMg or AlCuTi.
When only small quantities of the elements copper, nickel, zinc, silicon, manganese and magnesium are added, wrought alloys are produced. These increase the hardness and tensile strength of the aluminium. In return, the electrical conductivity is reduced. However, the material can still be formed, making it ideal for use in ships, aeroplanes or transport containers.
The most important cast aluminium alloy is the eutectic alloy of aluminium and silicon, which contains approximately 12 percent silicon. This has very low viscosity, and so retains its exceptional casting properties while keeping its high tensile strength. As a result, it has a long history of use in gearbox and motor housing in vehicle and aircraft construction.
Unfortunately, not all aluminium alloy groups and their associated materials are equally as machinable: For example, pure aluminium materials are very soft and quite difficult to machine due to their low tensile strength. These materials have a significant impact on the chip shape, wear, surface and cutting force. For this reason, it has proven worthwhile to divide aluminium wrought alloys into three classes.
Aluminium materials with low tensile strength belong to class 1. This includes non-hardenable alloys in unsolidified or partially solidified state, for example the 1000 series, 5005A and 5454. This class also includes hardenable alloys in a non-hardened state such as EN AW-6063, 6060 and 6082. Due to the low tensile strength, machining often results in adhesion and "smearing effects" of the chips. The result is stronger built-up edge formation. Suitable cutting fluids are recommended to minimise this.
Class 2 contains aluminium materials with increased tensile strength. This class includes non-hardenable materials in a work-hardened state, such as alloys in the 5000 series. Hardenable materials in a hardened state, e.g. alloys in the 6000 and 7000 series, also belong to this class. Their tensile strength lies between 300 and 600 N/mm2 and has no hard constituents, so they have a very low wear effect. Thanks to the higher tensile strength, there is also less built-up edge formation.
Class 3 contains free-machining materials, such as hardenable wrought materials with chip breaking additives, such as lead or bismuth. Materials such as EN AW-2011, 2007 or 6012 fall into this category. Due to the chip breaking additives, the materials in this class form short breaking chips and exhibit only a low tendency for built-up edge formation.
Although aluminium is generally regarded as easy to machine, this metal also has its challenges: Adhesion of chips and built-up edges are the dreaded issues when milling aluminium. One successful approach is high-speed machining with the right cutting fluid strategy – and of course the right tools. For example, the cutting forces correspond to around a third of the value for steel.
Therefore the top priority when milling aluminium is to quickly transport the chips away from the machining zone. A milling cutter with extremely smooth and very low-friction surfaces helps to divert the adhesive aluminium chips away. In contrast to milling cutters for steel, milling cutters for aluminium have a relatively low number of teeth, which also helps with chip transport. Specially adapted coating solutions also improve chip discharge.
In aluminium milling, cutting fluids take on the role of lubrication in order to minimise wear and frictional heat as well as pure cooling. Cooling is particularly important when machining aluminium, as the material undergoes greater thermal expansion than steel, for example. If the heat is well dissipated, the workpiece also remains more dimensionally accurate. Cooling lubrication with emulsion minimum quantity lubrication of water and cutting oils has proven particularly successful.
Ethanol is also recommended for cooling during aluminium milling processes: It is highly effective at cooling the aluminium chips and prevents clumping because the cutting fluid components are free of oil. This also makes it easier to blow the chips off, collect them and return them to the material loop as a clean, reusable material.
If cutting fluids are not desirable or are even off-limits, a DLC-(Diamond-like-Carbon) coating is an excellent alternative: This high-performance coating even permits dry machining, where everything runs as if it were lubricated. The properties of aluminium also help here, as it dissipates the process heat more readily than steel, for example.
To mill aluminium successfully, there's an easy rule of thumb: the higher the cutting speed, the smoother the surface of the aluminium. This does mean that wear on the milling cutter also increases at the same time though. However, special HSC milling cutters, which are used for high speed cutting (HSC), permit significantly higher RPMs compared to conventional milling cutters.
Hard aluminium alloys are the best for milling. However the cutting speeds are 100 to 500 m/min – depending on the cutting edge diameter and the resulting feed. If a cutting edge diameter of 2-4 mm is used for example, a feed of 0.02 – 0.03 is required. If the cutting edge diameter is 5 to 8 mm, the feed increases to 0.05, while a cutting edge diameter of 9-12 mm increases it to 0.10. Hard aluminium is machined with cutting speeds of 100 to 200 m/min and the same feed as for soft aluminium.
Key parameters when milling aluminium at a glance:
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