|Nickel-based superalloys are used extensively in the hot end of jet engines.|
Shops, unfamiliar with the characteristics of highnickel materials, usually end up frustrated when they start working with these superalloys. Poor tool life, high costs and less-than-satisfactory surface finishes await the unwary.
Thanks to their high strength, corrosion resistance and ability to withstand extremely high temperatures, nickel-based superalloys are preferred for many aerospace and chemical-processing applications. Shops working with these materials must be aware of how the same metallurgical properties that provide these benefits also make it difficult to machine these alloys — but not impossible. Here are some tips and techniques for coping with the cutting idiosyncrasies of nickel alloys.
Work hardening: a major culprit
Most shops that machine steels or cast iron are used to running at high speeds, about 1,000 sfm. At those speeds, the heat generated softens the chip and pressure on the tool cutting edge will not be high. However, when cutting nickel-based superalloys the situation changes because these materials do not soften appreciably at elevated temperatures. This is the characteristic that makes these materials work well in high-temperature applications, such as in the hot end of jet engines. However, because these materials do not soften as a result of the high temperatures generated when a chip is formed, a lot of pressure can be put on the cutting edge. This can cause edge breakdown by chipping or deformation. High cutting temperatures also result because stronger materials generate more heat during chip formation, and have relatively low thermal conductivity
Don Graham, Manager of Turning Products at Seco Tools has some advice for successful machining of nickel-based alloys. “To lower temperatures and pressures when cutting superalloys, cutting speeds are usually 200 sfm to 250 sfm. Superalloys are expensive, a jet-engine ring or a shroud could be worth $100,000. Therefore, shops are very conservative with machining these materials, which is another reason for keeping cutting speeds low,” said Graham.
|The high-pressure coolant stream from Seco Tool’s Jetstream tooling cools and breaks chips into small, easy-to-manage pieces.|
Inconel 718, a high-temperature alloy with about 53% nickel, is the most common nickel-based material is use. More than half of the hot end of a jet engine is made from this material. Other, more exotic nickel-based materials, such as Waspalloy, Monel, Hastealloy and Rene 95 or Rene 88, are used in a variety of applications and are even more difficult to machine than Inconel 718.
These nickel-based materials readily work harden, or age harden, when machined. For example, after the first machining pass, an Inconel 718 workpiece with a starting hardness of 34 HRc might now have a hardness of 45 HRc in the first few thousandths of its surface. This hardened surface layer leads to what is called depth-of-cut-line notching on the cutting edge. Work hardening also makes it difficult to maintain precision tolerances, and the metallurgical integrity of the part surface. A damaged part surface can in turn compromise fatigue strength, which is one of the reasons that maintaining the edge condition of an insert is so important when machining superalloys.
An alternative to solid carbide
Niagara offers an alternative approach to milling Inconel and similar materials with its Excel line of end mills that combine a highgrade cobalt substrate with a wearresistant PVD coating. With these tools metal removal rates are said to be higher than carbide by taking heavy depths of cut with slow speeds and heavy chip loads.
As second-phase particles form due to age hardening, the alloy becomes both stronger and more abrasive, thus more difficult to machine. Therefore, it’s preferable to machine these alloys in their softer state. Typically, it is best to machine parts to near finish dimensions in the as-cast, solution-treated condition. After age hardening, only a final finishing operation is performed, providing the desired surface finish while minimizing the risk of distortion caused by heat generation.
Ceramic tools are usually used for rough turning because these tools can be run fast — up to 800 sfm. They also usually have a chamfered cutting edge for additional strength; this somewhat dull cutting edge work-hardens the material, limiting these tools to roughing applications. Because ceramics are somewhat brittle, it is wise to cut the feedrate in half when entering and exiting a cut to avoid damaging the tool.
However, ceramic tooling is ill suited to roughing applications with very coarse surfaces, such as out-of-round castings with ragged, rough surfaces that can chip ceramic tooling. In these cases, uncoated C2 carbide (6 percent cobalt, 94 percent tungsten carbide) is best, or C2 carbide with a titaniumaluminum- nitride (TiAlN) coating works well in some applications.
Because it is an intermittent cutting process, rough milling with ceramic tooling is best done dry; otherwise, coolants are recommended.
According to Sean Holt, Aerospace Industry Specialist at Sandvik Coromant, the choice of insert coating depends on the application — there are no rules about which is best. Some tooling manufacturers use a CVD coating on round inserts or inserts with a 45- degree approach angle because this is a thicker coating with better hightemperature capability than a PVD coating. However, PVD-coated inserts have greater resistance to notching.
For semi-finishing or finishing, carbide with a thin TiAlN coating applied by the physical vapor deposition (PVD) process is usually the tooling of preference. PVD coatings are thin and smooth, and they impart a slight residual stress to the carbide, which helps the tool resist chipping and notching. Compared with CVD coatings, PVD coatings more closely profile a tool’s sharp edge so the edges of PVD-coated inserts are inherently tougher than CVD coated edges.
Fine-grain carbide provides high hardness and wear-resistance while maintaining good toughness. Some manufacturers also apply a thin goldcolored TiN top coating over the bluish TiAlN to help operators identify which corner of an insert has been used.
Inserts with a positive rake are preferred for machining nickel alloys because this is a free-cutting geometry that efficiently shears the chip away from the workpiece to reduce heat generation and subsequent surface work hardening, and a positive rake minimizes edge build-up.
“We recommend using inserts with at least a 7- to 11-deg positive rake for semi-finishing and finishing cuts,” said Graham. “For aggressive, roughing cuts a neutral or 0-deg rake works well.”
A flaw-free surface finish is extremely important because a surface defect can be a crack-initiation site. Sharp tools are required because a dull tool can damage a workpiece surface. Also, it is a good idea not to index an insert while finish machining a part to avoid leaving a surface imperfection or “witness” mark where a new cutting edge was introduced.
|Seco Tool’s TS2000 and TS2500 grades have superior resistance to both heat and wear and have been developed for turning heat resistant superalloys and titanium. |
High-pressure, through-the-tool coolant delivery is a critical consideration when cutting nickel-based materials. Using a direct jet of coolant to the cutting edge results in longer tool life and/or higher cutting speeds. “You can run 20 percent faster with a 1,000–psi high-pressure coolant directed to the cutting edge and get the same tool life as with flood coolant delivery, or you can run at the same speed and get a 50-percent increase in tool life with a direct coolant jet,” said Holt.
Seco has developed its high-pressure Jetstream tooling technology that directs coolant directly to the cutting edge/ chip interface, allowing cutting of Inconel 718 up to 300 sfm. The coolant preserves the insert’s hardness and wear resistance. The Jetstream coolant stream helps to bend the chip away from the workpiece surface while also cooling the chip, making it easier to break. Also, new insert grades have been developed specifically for Jetstream cooling.
In addition to being able to run faster, the use of Jetstream cooling produces small, one-eighth- -inch chips. “We have developed sophisticated chip-breaker geometries that work well with other materials, but they do not work as well as using inserts with chip-groove geometries designed to be used with the Jetstream technology for managing nickel-based superalloy chips, said Graham. “This results in significant savings in chiphandling costs because we estimate that a chip hopper will contain four to ten times the amount of metal from compact Jetstream-produced chips as compared with conventional, long stringy chips that trap a large amount of air in the hopper.”
Beyond the Tools
Also critical to successful machining of superalloys is the rigidity and robustness of workholding and toolholding. If workholding fixtures are not perfectly rigid, workpiece movement, even slight, can make short work of the life of a cutter. Toolholder runout and a lack of concentricity can cause high tool wear destroy surface finish