Ceramic Cutting Tools and Heat Resistant Super Alloys

Sean Holt, aerospace industry specialist, Sandvik Coromant, outlines the requirements and techniques of ceramic tooling in this sector.

Machining of HRSA materials is difficult because of a number of factors. During the machining process, the cutting edge is exposed to strain and heat. This means that when cutting HRSA materials, the very high temperatures generated tend to cause rapid wear in conventional carbide inserts. The solution is to use ceramic inserts. Recent developments in ceramic cutting tools, materials and machining methods have greatly improved aerospace component manufacturing.

In this article, you will learn about a new ceramic grade that offers a 50% metal removal rate increase over traditional whisker ceramics. You will also read about new application techniques to increase productivity and learn how to reduce your manufacturing costs by a minimum of 30%.

Type of Ceramic Material

Sandvik Coromant has recently developed a number of ceramic grades for a range of tasks from turning to milling, as well as first stage machining and turn mill applications for engine casings. The company's grade CC670 (whisker reinforced ceramic) has proven itself to be an effective insert for heavy roughing applications.

Coromant has now introduced two additional grades, CC6060 and CC6065, which compliment grade CC670. These grades are a combination and mixture of silicon nitride and aluminium oxide commonly know as Sialon (silicon, aluminium, oxygen and nitrogen). CC6060 is optimized for long cutting lengths with effective programming techniques and for pocketing and profiling operations. CC6060 is also first choice for milling applications using ceramics.

This new grade provides excellent rates of metal removal, as much as 10 to 15 times that of carbide inserts, and is an economic alternative to whisker ceramics.

Turning Using Ceramics

There are two main wear criteria when machining using ceramic inserts: top slice – the layer of the top of the cutting edge is sliced off usually caused by excessive pressure; and notch wear – diffusion wear where the depth of cut sets the workpiece material line. Both of these destructive wear patterns can be reduced by applying the Sialon grades. For example, grade CC6060 has excellent notch resistance over whisker ceramics.

Careful programming techniques can reduce these wear patterns significantly but the correct selection of cutting speed and feed is also paramount. The speed should be balanced to create enough heat in the cutting zone to plasticize the chip but not too high to unbalance the ceramic. The feed should be selected to give a chip thickness that is high enough so as not to work harden the material but not so high that it causes edge frittering. These boundaries change depending upon the component material hardness and grain size.

Because of the increased notch resistance with Sialon grade CC6060, the cutting speed can be increased over whisker ceramics by around 30% up to 1,200sfm. A relatively high feedrate of about 0.008" (dependent on depth of cut) can also be achieved.

One of the most crucial factors that affect the cutting action in HRSA is the approach of the cutting edge in relation to the workpiece and the entry and exit of the insert.

Sandvik Coromant has developed many application techniques and tools for turning operations using ceramic inserts including insert shape selection, pre-chamfering the component, turning to a shoulder, optimized programming techniques to minimize notch on entry, and the ability to control the program and reduce the cutting arc of engagement into a corner.

Typically, applying these techniques can increase productivity and tool life by four times. The example to the right shows an effective way to machine against a shoulder to help reduce the arc of engagement on the insert.

To support the introduction of the Sialon grades, Sandvik Coromant will release a range of dedicated holders for the aerospace industry.

The manufacturing of engine discs today is a time consuming and costly process in HRSA materials. Many special tools are applied with different configurations to be able to machine the different features on an engine disc (closed pockets, hooked grooves, etc.).

However, in some respects, "a disc is a disc," regardless of size and position in the engine. The features found on a disc that measures 8" are in some respects similar to features found on a disc that measures 48".

Therefore, Sandvik Coromant has developed a standard range of cutting tools (including an antivibration solution for deep pockets) to hold these inserts to machine an aerospace engine disc, reducing the number of tools needed to turn a disc from the typical 20 to six.

Milling Using Ceramics

Because of the increase in aerospace manufacturing to meet the demands from aircraft manufacturers – 28,000 new and replacement planes by 2020 – cost reduction and productivity improvements programs must be implemented. The need to manufacture parts quicker and more reliably is very important.

As a metal removal solution and cost reduction technique, ceramic milling is an excellent option. Ceramic milling offers dramatic increases compared to carbide. Typical cutting speed for carbide would be about 120sfm, while ceramic milling can run at 3,500sfm: 30 times the speed of carbide.

The main application area for ceramic milling is machining of engine casings due to the high stock removal needed between bosses.

By applying the correct tool (low radial and axial run is needed), grade CC6060 Sialon grade and the correct application technique, great improvements can be made.

For example, unlike turning using ceramics, where coolant should always be applied, with milling, coolant should never be used as it has a negative effect on tool life. This is because the increase in thermal shock with cooling and heating of the cutting zone as the insert enters and exits the workpiece. This increases the chance of top slice on the insert. Above is an example of the affect of wear/tool life of ceramic with and without coolant.

Due to intermittent cutting, milling ceramics is a much cooler operation than turning. For this reason, the cutting speed needs to be kept high. The high cutting temperature generated when milling increases the temperature of the chip, making it highly sheared.

By applying CC6060 at the correct cutting data and feedrate, and by changing the direction of cut, increases of 50% in tool life can be achieved due to the reduced impact force on the insert.

A demonstration of the new ceramic face mill and CC6060 was carried out at a leading aerospace component manufacturer to reduce cost and increase productivity. This particular customer project was face milling Inconel 718 material, commonly used in engine casings. CC6060 reduced the cycle time on a single component feature by 97% and equated to an annual cost saving of $34,000.