With high tensile strength and extreme heat resistance up to around 750°C, nickel-chromium alloys are used wherever very high temperatures are present, such as in gas turbines, rocket engines, spaceship components, nuclear power reactors and pumps. But even outside the aviation and aerospace industry, markets are shifting to favour heat-resistant parts: Machines used for power generation need to become more efficient in all sectors and make do with the least amount of cooling possible.

The high temperature resistance of nickel-based alloys often presents process managers with challenges during machining, not least because there is still relatively little experience to draw on compared to other materials. If heat-resistant super alloys are milled with carbide indexable inserts, the cutting speed is only around 45 metres/min. Milling cutters with ceramic cutting tool materials achieve cutting values that are approximately 20 times higher during roughing: with cutting speeds of up to 1,000 metres/min, they achieve a much higher metal removal rate.

The higher feed rate easily compensates for the depth of cut, which is generally lower for ceramic cutting edges than for carbide edges: In specific applications, the metal removal rate of Inconel 718 with ceramic cutting edges is more than 10 times higher during roughing. Machining times that are around 90% lower are an important factor in production planning, especially in sectors like the aviation industry, where order books are full and machine capacity is limited.

Milling with ceramic cutting edges imposes demanding requirements on the machine, cutting tool material and machining process. The biggest difference between ceramic and carbide cutting edges is the extremely high temperature in the cutting zone: spindle speeds of over 10,000rpm generate the high frictional heat required, which melts the material and carries it out of the cutting zone. At temperatures of around 1,200°C, the energy between the tool and the component is so high that bright orange sparks can be seen – not unlike those created during grinding.

Application: Roughing engine parts

Walter has optimised existing face milling cutters with ceramic indexable inserts, as well as the corresponding milling processes themselves, for roughing operations on a new component made from Inconel 718 for an engine component manufacturer. Roughing the facets was identified as a critical machining stage on the conical turned part, which has a diameter of over 1m. The total machining depth on 12 areas was around 25mm. Using traditional tools, this stage would take around 50 minutes.

“Engine manufacturers face high workloads and the high hourly rates of modern machinery,” says Marlon Ries, Application Development Engineer at Walter. “Creating additional machine capacities was therefore the most important factor for the customer. Lower machining times also reduce the total machining costs significantly. We adapted an existing tool body with five indexable inserts and eight cutting edges per insert for this application and optimised the entire machining process over the course of several test series.”

Heat and speed – risk factors for process reliability

Milling with ceramic cutting edges requires high-speed milling machines that can accelerate the spindle to more than 10,000rpm when required. These speeds represent a great challenge for the tools used. Over the course of the process, the insert seat and geometry of the milling cutter were optimised to create a higher level of operational smoothness.

Another challenge was that the heat created in the machining zone causes metallic fumes, which condense on the tool and clog its moving parts. To counter this, Walter covered the wedge clamp assembly parts with PVD coating, which has a higher melting point than the base material. This reduces deposits on the clamping system to a minimum.

There is one thing that makes Inconel special: when milling components, the material can rebound during machining operations or expand subsequently. Tests have shown that a smaller camber angle positively influences the tool life. The inserts are rotated at a certain angle in two levels to create a soft cut. This improved tool geometry results in less noise and vibrations.

Keeping thermal loads on the tool and workpiece low

The frictional heat requires special adaptations to the milling process, in that the ideal run-in path, speed, feed and depth of cut need to be selected.

“Ceramic cutting tool materials originated in turning, where thermal loads are relatively stable,” explains Ries. “During milling, by contrast, the temperature on the cutting edge varies because the cutting process is interrupted. The abrupt change between frictional heat and cooling puts a strain on the cutting edge. We planned the tool paths to ensure the cutting process is as uninterrupted as possible.”

To prevent thermal shock caused by the tool cooling down, cooling lubricants are not used when milling with ceramic cutting edges. The process temperature is preferably discharged with the chips, rather than introducing high temperatures into the component. Walter drilled three holes in the milling body’s wedge, through which pressurised air is introduced into the machining area when machining cavities. This makes for reliable chip evacuation.

“The customer was initially concerned that the very high temperature of nearly 1,200°C could cause warping on the component or increase surface hardness, as can be the case during grinding,” says Ries. “To prevent this, the outside of the turbine housing is not milled in one step. After each section, the component is rotated to the opposite side before machining continues. The tolerances of a tenth of a millimetre were met without problems. We ran many tests and even we were surprised to find that the parts were only lukewarm directly after milling.”

Lower costs per component despite higher tool costs

Most of the heat created during the machining operation is absorbed by the cutting edge and the chips. These extreme heat and speed loads quickly leave their mark on the comparatively hard ceramic cutting edges.

“Signs of wear caused by temperature-related chemical wear or adhesion appear relatively quickly,” says Ries. “Customers are often surprised that even tools that look quite worn still achieve very good results. The wear mark width during roughing was 2mm. When milling with ceramic cutting edges, the indexable inserts might be on the machine for as little as 10 minutes before becoming worn. But if work is carried out 20 times faster, the higher tool costs quickly pay off.”

Using ceramic indexable inserts and the Walter milling body optimised for this application, machining times are drastically reduced in this case. Compared with a face milling cutter with seven cutting edges (diameter 80mm), tool life fell from 29 minutes to seven minutes per assembly, while the machining time decreased by 92%. The costs per part for these roughing operations are now only 7% of the previous figure.

Suitable for different applications

While Walter created the face milling cutter with ceramic inserts for a very specific process, the company’s machining experts are already thinking ahead. After all, components made from nickel-based alloys are becoming increasingly common.

“The pilot process with the engine component manufacturer was highly successful,” adds Ries. “There are already plans to adopt the milling body for other production processes as well.”