Knowledge

Machining nickel-based alloys

Where there are very high thermal and mechanical stresses - as is the case with many components used in aerospace, the automotive industry or power generation - nickel-based alloy components are often used. 

What are nickel-based alloys?

Nickel-based alloys are materials where the main component, the heavy metal nickel, is created with at least one other chemical element (usually by means of a melting process). Nickel-copper, nickel-iron, nickel-iron-chrome, nickel-chrome, nickel-molybdenum-chrome, nickel-chrome-cobalt, low-alloy nickel alloys (with a nickel content of up to 99.9%) and other multi-component alloys are used. 

Wrought alloys and casting alloys

In general, a distinction is made between the following two groups of nickel-based alloys: wrought alloys and casting alloys. Wrought alloys are used in turbine construction for discs and rings and are ideal due to their properties for a temperature range up to 730 degrees Celsius. Casting alloys are mainly used for components with high thermomechanical loads and complex geometry. The components are cast to almost the final shape with a polycrystalline structure and are only machined slightly. 

Properties of nickel-based alloys

The common nickel-chrome alloys in particular are characterised by extreme heat resistance up to approximately 750 degrees Celsius, and can therefore withstand constant loads close to the melting point. They also demonstrate great expansibility and tensile strength, along with low heat conductivity coupled with good cold forming properties and high corrosion resistance. Low density, high chemical resistance and high wear resistance make the alloys especially interesting for such high-temperature applications where aluminium and steel is unstable.

It is these good application properties of the alloys which in turn make mechanical machining more difficult: where the tool life is short, only relatively low cutting speeds can be used. When machining aluminium with uncoated carbide tools, a tool life of several days is standard. For cast iron with spheroidal graphite, the tool life drops to around an hour, and for nickel-based alloys, it is between five and ten minutes. 

Cutting materials for nickel-based alloys

High-speed steel

"High-Speed Steel (HSS)" is used for machining nickel-based alloys due to the high degree of toughness for applications with an interrupted cut such as milling, thread cutting, broaching and striking. With nickel-based alloys, cutting speeds between 5 and 10 m/min can be used. The feed rates per tooth can be set relatively high at 0.1 to 0.16 mm due to the toughness of HSS.

Carbide

Carbides consist of metallic carbides, usually tungsten carbide, which are embedded in a soft metallic binding phase and therefore belong to the composites. Carbide tools with comparatively low cutting speeds of 20 to 40 m/min are generally used for nickel-based alloys. Higher cutting speeds lead to the rapid overloading of the cutting material and can therefore not be used in a process-secure manner in most cases. 

Boron nitride

After diamond, cubic boron nitride (cBN) is the second hardest known material. It is harder, more wear resistant and more expensive than ceramic. Due to the properties of cBN, high cutting speeds can be used during turning operations. cBN is not used for milling nickel-based alloys. It is however used for turning Inconel 718. Cutting speeds between 400 m/min and 600 m/min are recommended. Compared directly with TiAlN coated carbide tools, cBN has a 100 percent longer tool life at a cutting speed vc of 50 m/min. When it comes to finishing unstable structures, cBN is the first choice in industrial applications. 

Ceramics

Ceramics are sintered from ceramic powder without the addition of binders. DIN ISO 513 categorises the ceramics into five groups:

CA = Ceramic, main component aluminium oxide (Al2O3)

CM = Composite ceramic, main component aluminium oxide (Al2O3), as well as other oxide components

CN = Silicon nitride-ceramic, main component silicon nitride (Si3N4 )

CR = Whisker-reinforced ceramic, ceramic, main component aluminium oxide (Al2O3)

CC = Ceramic, all above-mentioned ones, but coated

Ceramic tools retain their hardness including at high temperatures, which arise when milling heat-resistant super alloys (HRSAs). Compared with solid carbide tools, a speed some 20 or 30 times higher can be achieved.

Ceramic cutting materials originally stem from turning operations. The thermal load remains relatively stable during turning. When milling, the temperature on the cutting edge varies however as the cut is interrupted. The abrupt change from frictional heat to cooling puts strain on the cutting edge. To prevent a thermal shock as the tool cools down, no coolants are used when milling with ceramic cutting edges. SiAlON ceramics (silicon-aluminium oxide-nitride) are usually less sensitive to temperature fluctuations than whisker-reinforced ceramics, which is why they are a better choice for milling operations. 

The prerequisite for milling with ceramic cutting edges is high-speed milling machines which are capable of accelerating the spindle to more than 10,000 rpm in some cases, which is an additional challenge for tools.

Whilst tool systems with ceramic indexable inserts are available in various forms on the market and are used in the industry sector, milling tools with a tool diameter of less than 16 mm are not yet as popular for the aforementioned reasons. For a long time, there was no alternative to tools made of high-speed steel and carbide.

In addition to the chemical wear caused by temperature, there is often a built-up edge formation with ceramic cutting materials: In the heat, which is generated in the machining zone, metal vapours are produced which merge with the surface of the cutting material - parts of the ceramic can flake when released.