Advice about endmilling nickel-based alloys.

Developments in aerospace technology have increased the use of nickel-based superalloys such as IN 100, INCO 718 and René 100. In fact, nickel-based materials account for 45 to 50 percent of the total material used in the manufacture of a gas turbine engine.

But due to their exceptional strength and corrosion resistance at high temperatures, these alloys are turning up in a widening array of applications, including rocket engines, steam powerplants, nuclear reactors, petrochemical plants and submarines. As a result, more machinists are having to contend with milling these hard-to-machine materials. Following are some basic techniques for endmilling nickel-based alloys that will lend predictability to the machining process.

A Question of Toughness

What makes a nickel-based alloy so difficult to mill? There are number of reasons, including:

• Nickel doesn't conduct heat very well compared to, say, steel or aluminum. This results in a higher temperature at the tool/workpiece interface, increasing the likelihood of tool tip breakage.
• Similarly, the high cutting zone temperatures that arise when machining nickel leads to premature tool wear.
• Though not as bad as aluminum, the formation of built-up edge when endmilling a nickel-based alloy is more noticeable than when cutting steel.
• The high machining forces created when milling nickel causes workhardening of the part.
• Nickel-based alloys contain refractory carbides such as chromium, titanium, tantalum, niobium, molybdenum and tungsten. These carbides appear near the grain boundaries in amounts ranging from 0.02 to 0.05 percent. This is enough to cause significant abrasive wear to the tool's cutting edge.

Perhaps the biggest material-related variable affecting the machinability of nickel-based materials is the percentage of nickel present in the workpiece. For example, Incoloy 901 is comprised of 42.5 percent nickel. Endmills can be run twice as fast in this material than in IN 100, an alloy containing 60 percent nickel.

Solid Cuts

When it comes to the equipment component of milling nickel-based alloys, a highly rigid machine tool equipped with a shrink-fit toolholder and a stub-length endmill is the ideal setup.

One issue frequently debated is milling tool geometry. For standard cylindrical endmills, the typical starting point for a rake angle is between 0° and 5°, with a clearance angle of 8° to 12° and a flute helix angle of 30°.

Theoretically, there is an optimal tool geometry for each type of material and each set of milling conditions. But I have found that a standard cutter with a rake angle of 5°, an eccentric clearance angle of 8° and a 30° helix angle provides excellent results for most nickel-based alloys.

In addition, high-quality cutters are essential. Interrupted cutting, which can fracture and chip the tools' cutting edges, are common failure modes when machining nickel-based materials. So to obtain consistent results, only use endmills made of the highest quality submicron-grade tungsten carbide.

In his widely read book Metal Cutting Principles, author Milton Shaw claims that "a high-cobalt-containing tungsten carbide containing relatively large amounts of TiC and tantalum carbide should be the most effective cemented carbide type for machining refractory nickel-based alloys." This combination of materials tends to block heat from entering the tool nose, shifting the heat zone up the rake face of the tool, increasing tool life.

Unfortunately, other than cobalt content, many tool manufacturers are unwilling to reveal the detailed composition of the carbide used in their endmills. But, as a rule of thumb, a tool that has a cobalt content of 10 to 12 percent will provide a good combination of hardness and transverse rupture strength, allowing it to resist fracturing and chipping.

All endmill cutters should have a maximum flute-to-flute runout of 0.0005". This ensures that each of the flutes produces chips of the same thickness. Uneven chip thickness from flute to flute will cause premature tool failure.

The flute corners should have a small radius (0.015" to 0.030") to reduce breakage. It is also essential to have a high-quality cutting edge that has neither serrations nor microcracks.

Well Heeled

Figure 1: An important development in the milling of nickel-based materials is the use of a helix feature on endmill form cutters, like this one from Accugrind Inc.

An important development in the milling of nickel-based materials is the use of a helix feature on endmill form cutters (Figure 1). The helix limits the heavy impact experienced by each flute as it enters the material. But most of today's form cutters lack this feature. Consequently, they are likelier to chip and fail prematurely.

The helix design distributes the tool entry forces over a broader area, reducing the impact at the edge of the flute. On form-style endmills, the helix angle must be small (10° to 15°) in order to ensure that the axial rake angle isn't highly negative. If the axial rake angle is too high, the resultant increase in cutting force will cause premature tool wear and failure.

One general point should be noted here: When selecting roughing and finishing endmills, keep in mind that the smaller the pitch, the longer the tool life and the higher the cutting force available. The larger the pitch, the lower the cutting force.

You should check with your cutting tool supplier for precise speed and feed information for your application. Typically, though, the speed for slot milling nickel-based alloys with a carbide endmill is about 10 m/sec.

Extending Tool Life

The use of tool coatings, such as TiN, to machine nickel-based alloys has been disappointing. My experience in the aerospace industry has been that the coating tends to wear off quickly.

There is hope, though. The new multilayer coatings, such as those that incorporate alternating layers of TiN and TiCN, can extend tool life by 30 to 50 percent, depending on the application.

Surprisingly, the use of a high-pressure coolant system results in a slight decrease in tool life compared to the use of simple flood coolant. Research I conducted showed that a high-pressure coolant stream reduces the chip length along the rake face. This causes more stress on the tool nose, leading to premature tool failure.

Figure 2: Using a CNC's circular interpolation feature, the helical-form milling cutter enters the part via a circular motion that pivots about point No. 1 until the full width of the cutter engages the material. This circular entry technique allows chip thickness on the cutting edge to build gradually as the tool enters the material. Tool life increases by 50 to 70 percent over directly feeding the mill cutter into the workpiece.

However, there is a procedure that can extend the cutting life of all types of endmills in all types of materials. Called circular entry, it takes advantage of the circular interpolation feature of a CNC control (Figure 2). Circular entry can reduce the damage created when an endmill enters the workpiece material.

Upon the approach to the part, the cutter stops just before contact (0.005" to 0.010"), then begins to pivot about a cutter tangent point at one corner of the slot to be milled. This technique gradually increases the chip thickness with a climb-milling action, without lowering the feed rate. This will increase cutter life by 50 to 70 percent.

However, the technique is not as effective on endmills that contain custom forms, because the circular entry process must then use the largest diameter of the form cutter to set the pivot point. Therefore, the smaller form features on the tool may not receive the full benefit of this circular entry technique.

Utilizing this technique—along with a stable machine tool, shrink-fit toolholding and a high-quality carbide endmill—will help the growing number of machinists contend with milling hard-to-machine nickel-based alloys.

About the Author
Frank Mullett recently retired from Pratt & Whitney, East Hartford, Conn., a division of United Technologies Corp. He has nearly 40 years of machining experience.