Heat-resistant superalloys (HRSAs) are nickel and cobalt-based alloys prized for applications that call for strength, resistance to corrosion and oxidation, and resistance to contact wear needed at extremely high temperatures.
“[We see] HRSAs as all nickel- and cobalt-based alloys that exploit the yield-strength anomaly," noted Alex Minich, applications engineer at toolmaker Greenleaf Corp., Saegertown, Pa. He is referring to when yield strength increases with temperature, contrary to most materials that get softer as they get hotter, or lower yield strength. It seems to be an anomaly—hence the name.
The same resistance to heat (and increasing yield strength with temperature) that makes HRSAs desirable for such applications is what makes them a challenge to machine. Here’s the latest on how cutting tool manufacturers are making the job easier.
Probably the most prominent application for HRSAs is their use in the aerospace and defense industry, in the form of components for turbine engines used in jets, rockets and missiles. However, the materials are also widely used in the oil and gas industry. “Oil, gas and their derivatives and anything else that is corrosive and abrasive that needs to be stored, processed or transported at high pressure and temperature tend to require the strength and resistance to corrosion at elevated temperatures that only Ni-based alloys can offer,” said Minich.
Some HRSAs are also used in medical device manufacturing, not necessarily for heat resistance but for bio-compatibility as well as strength, stiffness and corrosion-resistance properties.
Minich also noted that not all alloys called HRSAs actually fill the bill. “Some would consider Jethete M152 to be an HRSA, but in our eyes it’s just a low-carbon martensitic stainless steel,” he said. “Most would also consider many titanium-based alloys to be HRSAs, because many alpha-rich titanium-based alloys are designed for service at elevated temperatures.” True HRSAs are only those nickel- and cobalt-based alloys that take advantage of the yield-strength anomaly, he stated.
While there are different types of HRSAs, they all share a major “chip challenge.” In standard metal cutting, the material is removed in the form of chips that are efficiently evacuated from the cutting zone, taking with them much of the heat generated by the cutting process, according to Bill Durow, global engineering project manager for aerospace at Sandvik Coromant, Mebane, N.C.
“When you’re cutting a piece of steel, for instance, it’s nice and shiny, but if you look at the chips afterwards, you’ll see they’ve turned a dark blue because of the heat they’ve absorbed from the metalcutting process,” he said. But with heat-resistant materials, that doesn’t happen. Instead of being absorbed by chips and evacuated with them, the friction-generated heat often stays within the process. “Typically, about 80 percent of the heat stays right in that cutting zone,” Durow said. “It goes back into the insert, which, if you think about it, is not a good situation for the insert.”
There’s another difference between chips from standard steels and those formed from HRSAs. In turning operations, the standard steel chips break away at a size and shape that allows them to be easily removed from the cutting zone. Not so when turning HRSAs. “When you’re turning nickel materials, it doesn’t like to break a chip,” said Durow. Instead, “you’ll get these long stringers. The chip can actually wrap around your tool. Worse, it can wrap around the workpiece and damage it.” That’s not a good situation when you’re making, say, critical engine parts.
One solution is to direct high-pressure coolant into the cutting zone, at pressures up to 100 bar (1,400 psi) to break the chip out of the way, according to Durow. “This is a lot more than just splashing water around the cutting zone,” he said. “We have nozzles that precisely direct the coolant at high pressure into the cutting zone, creating a hydraulic wedge that pushes the chip up over the insert, essentially bending it back in order to break it.”
HRSA materials are machined either with carbide tools or inserts, which can offer better finishing but at comparatively lower surface feet per minute (sfm) of cutting, or else with ceramic tools that enable much higher sfm. “Ceramics are usually geared strictly towards roughing, possibly semi-finishing, but not for finishing, where carbide has the advantage,” said William Fiorenza, product manager die & mold, Ingersoll Cutting Tool Co., Rockford, Illinois. “The lion’s share of cycle-time reduction is going to be found in the roughing process and not necessarily the finishing process. But when requirements include a pristine finish, then [use] solid carbide.”
The heat is a particularly vexing situation when using carbide tools or inserts, according to Randy Hudgins, national product manager of turning at Iscar USA, Arlington, Texas. “To form a chip, the process needs to plasticize the material, but the high nickel content in Inconel, Waspaloy and other HRSAs make them so resistant to heat that the temperatures needed to begin to plasticize them are enough be detrimental to your carbide. The binder for carbide is cobalt, and the melting point of cobalt is about 2,700°F; the temperature that it takes to plasticize these nickel-based alloys approaches over 3,000°F.” Hudgins said. “You’re in danger of melting your cobalt away.”
For that reason, heat-resistant coatings such as titanium aluminum nitride (TiAlN) or aluminum oxide (Al2O3) are applied over the substrate as part of the toolmaking process, he said.
The high temperatures cause trouble for carbide tools in another way. The cutting edge’s hot contact with the HRSA workpiece effectively workhardens the material, putting a scale on it. “Basically, it’s heat-treating it,” Hudgins said. “Let’s say that on every pass you’re taking an eighth of an inch depth of cut per side. What happens is, the cutting pass workhardens the material. Then you come back and take another eighth of an inch depth of cut. Well, that previous pass is now an eighth of an inch up the side of your carbide. So, now you’ve got that workhardened material in contact with your carbide and it starts to erode it. You get what we call a depth-of-cut notch. It starts notching your carbide.”
A way to address this is to vary the depth of cut, he said. “Let’s say you start out with a 150 thousandths depth of cut. You could then drop to 100, then to 75, then to 50. What that does is move that workhardened surface up and down the length of that carbide at different intervals. The workhardened material doesn’t have a chance to set up and start eroding away the carbide so quickly," Hudgins said.
Another challenge to factor into the cutting process is the complexity of the design of the part being cut, pointed out Ingersoll’s Fiorenza. And, he said, there’s more complexity than ever.
In milling applications in particular, “part and feature shapes have become more complex over the years,” Fiorenza said. “Because of advances in CAM and CAD software, part shapes are becoming more free-flowing. Where parts might have been more open in the past, designers are taking liberties with more detailed features in these different parts. Parts are being designed with smaller, tight-radius features, where cutters need to have a greater radial engagement. In situations like that, a greater amount of heat is generated due to that radial engagement. This can sometimes cause machining to be difficult. For example, the high temperatures can cause thin-walled part features to warp if proper machining techniques are not followed.”
They try to navigate these conditions with tight control over the process.
When milling with solid-carbide tooling, “keen edge preps need to be maintained and monitored during the process,” Fiorenza said. “Additionally, special insert designs can help optimize cutting performance—for example, specially designed rake face geometries, edge preparations and insert spotting in the cutter."
And while those software advances in CAD/CAM have made the parts—and therefore the cutting process—more complex, they are balanced by other software advances.
“The toolpath algorithms of today are nicely balanced and allow for high-speed machining techniques to be employed more readily. These more fluid toolpaths allow us to attack these high-temp materials in a more efficient manner, minimizing radial engagements,” Fiorenza said.
Part designs that are more challenging to machine represent only one area in which manufacturers’ expectations are evolving. There is also growing pressure on them—and subsequently machine builders and toolmakers involved in HRSA machining—to enable ever-shorter cycle times and reduced tool costs.
“The broad picture is that ceramic machining of HRSAs isn’t as much of a novelty today as it was in the mid-1980s, and current user objectives range from increasing throughput capacity—by increasing metal removal rates—to reducing overall cost while maintaining or improving process stability,” said Greenleaf’s Minich.
“The pre-pandemic years were the golden era of commercial aerospace—which we expect will return,” he continued. The [state-of-the-industry] graphs were all very green and upward-trending, and the main requirement for success was cycle time reduction.” Since the onset of the COVID-19 pandemic, however, he believes there has been a higher priority given to reducing cost.
Tool life affects both areas. Whether talking about milling or turning, carbide or ceramic, the tools used on HRSAs tend to, as the saying goes, “live fast and die young.” The life of these not-inexpensive tools is relatively short.
“There are a number of factors that are within our control that we’ve identified as high impact when it comes to the tool life of ceramics in machining of HRSAs,” said Minich. The factors include: tool selection; rigidity and stability; grade; shape (macrogeometry); edge preparation (microgeometry), toolpath/machining strategy; cutting conditions; speed; and chip thickness. “The more difficult of these variables to maximize is certainly tool life.”
The challenge differs depending on the cutting task, he said. “Milling and turning of HRSAs place rather different priorities on the material properties of a ceramic cutting tool. Tool life in milling benefits most from high transverse rupture strength—TRS—impact toughness, and resistance to crack growth as a result of thermal cycling. Turning requires a tool that retains chemical stability and hardness at higher temperatures, is more resistant to abrasive wear, but nonetheless has sufficiently high TRS to be able to handle the chip load and changes in the direction and magnitude of mechanical stress. Finally, any machining of HRSAs requires that the ceramic grade has appreciable resistance to crack growth.”
Greenleaf offers solutions for maximizing tool life in both milling and turning. “We meet the needs in milling, forging scale removal and heavily interrupted turning with XSYTIN-1—a unique silicon nitride-based grade. Turning, in the meantime, has been addressed with WG-600—a coated whisker-reinforced ceramic grade. At optimal cutting conditions, it’s capable of maintaining regular wear for over 20 minutes of cut time at a single point of contact in Inconel 718,” Minich said.
The most recent product Greenleaf has created specifically with cost savings in HRSAs in mind is XSYTIN-360. “As a solid end mill made from the XSYTIN-1 material, it offers the productivity of ceramic milling at diameters previously reserved for carbide, with significantly higher tool life—as measured by the volume of material removed per tool—than best-in-class tungsten carbide solid round tools,” he said. “Because of the transverse rupture strength and impact toughness of XSYTIN-1, XSYTIN-360 is also a more accessible tool in that it can be applied at lower speeds, reducing spindle requirements. And XSYTIN-360 can also be reground, offering further cost savings,” he concluded.
At Sandvik Coromant, recent innovations include new turning grades. “Our latest development is a brand-new turning grade that we developed for last-stage machining applications with aerospace engine components in the area of HRSA turning. It’s called S205,” said Sandvik Coromant’s Durow. “Because of coatings and new substrates, it resists heat much better than the previous grades and therefore can handle 30-50 percent higher cutting speeds. There are some new post-process treatments on the inserts as well. This CVD coated grade S205 is available in almost all of our standard insert portfolios.”
The company has also optimized its CBN—cubic boron nitride—portfolio, Durow said. “CB7014 is a high-speed CBN turning solution for nickel-based alloys.” The 7014 grade has been around for a while, but the company has recently optimized some of the geometries to better support HRSA work.
“CBN has typically been used in hard part machining. Very hard steels for gears and things of that nature,” he added. “But we found out this CBN material also works very well in aerospace materials. The problem was the edge performance when used on those. Whereas typically you would want a different edge prep for machining those hard steels, HRSA materials or materials like to be sheared. We needed to make a sharper edge line. So, we actually tweaked some of the geometries on those different inserts to work very well with those HRSA materials.”
These details are very important to aerospace companies, Durow noted. “They like that process security. They want to push a button and walk away knowing that the tool is going to last for a specific amount of time. They could do lights-out production. They don’t want to have to worry about something failing during the operation because the parts are extremely expensive, and the regulations they need to abide by are quite extensive.”
At Iscar, new carbide and ceramic grades have been developed for Inconel and other HRSAs, according to Randy Hudgins, national product manager of turning at Iscar USA. “Our IC806 carbide grade was developed specifically for Inconel 718 and it’s use spread to other heat-resistant alloys,” Hudgins said. “It was so successful that our engineers developed a grade with an even harder substrate for finishing and running at higher surface speeds—grade IC804.
“When turning Inconel, [it used to be that] if you got to 100 sfm, you were doing pretty good,” he continued. “With this IC806, we’re approaching 200 sfm and getting decent tool life. Then they developed IC804, with a harder substrate, and with that we’re at over 250 sfm.
Along with these grades, Iscar now has SiAlON grades—silicon aluminum oxygen nitrite. SiAlON is basically a ceramic—namely IS35 and IS25. “In our nomenclature, the bigger the number, the tougher the grade; the smaller the number, the harder or more wear-resistant the grade,” said Hudgins. “So, the IS35 is the tougher of the two. I usually start with IS35 because it works very well for cutting through the work-hardened scale that develops on these alloys. And with these grades, instead of 200, 250 sfm, now we’re at 600 to 800 sfm.”
Recent innovations at Ingersoll Cutting Tools center on a new ceramic line that offers two unique insert designs, according to Fiorenza. “These designs are new to the industry and the market,” he said. “Released in late 2020, this new line has been achieving a very high rate of success in many demanding HRSA rough milling applications.”
CERASFEED Hi-Feed indexable ceramic milling cutters use 9 mm and 12 mm indexable inserts with high-feed insert geometries. The system’s strong insert clamping allows for “blistering” feed rates, according to Fiorenza. “The density of the inserts is higher for increased productivity,” he said.
On the ceramic side, the company’s new SiAlON grade IN76N enables better throughput on demanding milling processes, he said. According to company literature, its sfm rate is up to 33 times greater than solid carbide (3,000 sfm, contrasted with 60–90 sfm for carbide).
Fiorenza said he is continually surprised by customers who minimize the importance of the total cost of production in HRSA machining. “In aerospace, the most expensive item in the machining process can be the workpiece,” he pointed out. “Yes, the machining center is the most expensive thing in some cases, but landing gear, as an example, can run to over $1 million apiece. And the least expensive thing is typically the cutting tool or the insert that goes into the cutting tool. So, you would think that the greatest amount of attention would be applied to the total cost of successfully driving those inserts. That doesn’t always happen, but it should.”
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