From Struggle to Speed: How Ceramics Tools Dominate?Superalloys

The ever-increasing use of difficult-to-machine materials, such as iron-, nickel-, cobalt- and titanium-based superalloys, creates several challenges when machining these materials, as their outstanding properties lead to very low tool life and reduced productivity. Namely, nickel-based superalloys, especially Inconel 718, present poor machinability due to the high shear strength, occurrence of intensive work-hardened layer, presence of hard and abrasive particles in the microstructure, generation of extremely high temperatures at the tool-chip interface, low thermal conductivity impairing proper heat dissipation from the cutting zone, and high weldability of the workpiece material to the cutting tool, and thus extensive formation of built-up edge.

For these reasons, ceramic cutting tools have been emerging as an alternative to cemented carbide tools for roughing machining heat-resistant superalloys (HRSAs), due to their excellent heat and wear resistance, high hardness, and chemical stability. These characteristics allow them to be used in machining operations at very high cutting speeds, thus leading to huge material removal rates and enhanced productivity. In some cases, like milling operations can be used without coolant, which contributes to a more sustainable process.

However, with such high cutting speeds some recommendations must be followed. Besides the high cutting speed, a continuous contact between the cutting tool and the workpiece surface should be maintained, as these materials present lower fracture toughness than cemented carbide tools. Also, built-up edge should not be removed, as this may cause damage to the cutting edge. Moreover, a finishing allowance of over 0.3 mm should be taken in consideration when using these tools.

Ceramic tools can be based in alumina (Al2O3) or silicon nitride (Si3N4) depending on the intended application. For heat-resistant superalloys, alumina reinforced with SiC whiskers and SiAlON are the ideal solution. In the first, the incorporation of SiC whiskers into the alumina ceramic matrix results in increased strength, fracture toughness, thermal conductivity, thermal shock resistance and high temperature creep resistance, while SiAlONs are silicon nitride-based compositions, in which the addition of alumina to the ceramic matrix combines the strength of silicon nitride with the excellent wear and heat resistance of alumina, improving its high-temperature properties.

In the case of Inconel 718 machining, one important factor to consider is that the cutting speed should be high enough to induce a material softening effect. Higher cutting temperatures are noted at larger milling parameters due to the severer frictional and cutting deformation work. This phenomenon happens at a critical temperature that’s generated during cutting, where the hard Nb-rich carbide particles are dissolved and partial melting of grain boundaries occurs, reducing cutting force and chipping, thus consequently improving tool life. For example, during the milling process with SiAlON ceramic tools, a cutting speed larger than 600 m/min is generally recommended.

The potential of ceramics tools is tremendous, being undoubtedly a game changer for the manufacturing industry, since their utilization can considerably enhance productivity and reduce machining costs.

Bruno Guimar?es | Product Development - R&D Engineer, PhD

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