Machining HRSAs starts with a stable machine, rigid workholding, and a very stiff interface between spindle and toolholder
For Dale Mickelson, Yasda product manager at Methods Machine Tools Inc. (Sudbury, MA) and author of several books on hard milling, tackling heat-resistant superalloys (HRSAs) requires the perfect combination of machine, workholding, tooling, tool paths and coolant.
Mickelson is also the first to say you can’t cover everything in an article, or even a book, and he has been asked to cut his drafts in half to meet the practicalities of publishing. But let’s review some highlights.
Stiff is Good
Like titanium (see “New Ideas Give a Jolt to Titanium Machining,” ME, January 2018), machining HRSAs requires a very stable machine, rigid workholding and a very stiff interface between spindle and toolholder. And like our earlier discussion on titanium, “the jury is still out on the ideal implementations of these requirements.
Mickelson said his testing shows that BIG Plus chucks deliver twice the tool life of comparably sized HSK systems (e.g. BIG Plus 40 versus HSK 63A). However, Lee Johnston, applications engineer, Okuma America Inc. (Charlotte, NC), said similarly sized HSK systems are “slightly more rigid” and “in theory, you can push the tool a little harder.” He added that the bigger HSK 100 or 125 systems (for which there are no comparable BIG Plus spindles) would be even better because you have more pullback force in addition to the face contact.
The primary application for HRSAs is hot zone turbine components for both aircraft and power generation—parts like blades, blisks, brackets, valves, and manifolds, most of which are complex and contoured. That means using five-axis milling to achieve the required forms and tolerances in one clamping. And for smaller parts, solid-carbide round tools are needed, requiring particularly stiff toolholders.
For carbide tools, James O’Leary, process improvement specialist, LMT Onsrud (Waukegan, IL), recommended hydraulic or heat-shrink chucks that “can maintain a runout of ≤0.0002″ (0.005 mm) at a length of 4X the tool’s diameter.” But heat-shrink holders won’t work for ceramic tools, explained Mickelson, because they run at such high temperatures that the heat affects the hole in the holder. For ceramics, he recommended collet holders.
Both Mickelson and O’Leary pointed to dovetails as the preferred method to hold Inconels and other superalloys so as to minimize part movement and deflection. The idea is to cut a dovetail into the raw stock and then use dovetail vise jaws to hold the part. “It doesn’t require a lot of material, which is important with such high-cost alloys,” explained Mickelson, “but you can get by with a small amount of hold.” And as O’Leary explained, “holding the part with a dovetail provides free access to the side of the material in a horizontal or five-axis machining environment.”
Finishing the part by machining the sixth side you put the dovetail in is usually accomplished by clamping to one of the machined surfaces (e.g., a port face with four bolt holes). But if the part design doesn’t call for such a feature, Mickelson recommended building on a tab with a threaded feature that can be cut off later.
New Ceramic, Carbide Tools
There is wide agreement that ceramic tools are the best option for roughing HRSAs. And with ceramics, as Jan Andersson, global manager, TechTeam & marketing, Greenleaf Corp. (Saegertown, PA), explained, “the first step is setting the right speed to raise the temperature in the cutting zone to create a certain degree of plasticization of the material, making it easier to machine. Otherwise, you struggle with unpredictable tool life. After determining the appropriate speed, you use feed rate to manage thermal evacuation. The higher the feed rate, the more mass in the chip. The more mass in the chip, the more heat you transport away from the cutting zone. That controls the chemical wear component.”
The speeds involved might surprise you. Mickelson said he generally uses Greenleaf whisker-reinforced ceramics at 3000 sfm in Inconel 718, but with a very light chip load. For a 2″ (50.8 mm) diameter insert cutter, 3000 sfm works out to about 5700 rpm, which is in the high-torque range of a machine he’d commonly recommend, the Yasda PX-30i. “But ceramic inserts don’t require high torque because the chip load would be only two thou per flute in this example.” For finer contouring on something like a blade, you’d probably want to rough with a smaller solid-ceramic endmill, which means even faster speeds. For example, a ¾” (19-mm) end mill would have to run at over 15,000 rpm to achieve 3,000 sfm. So machines like the Okuma Blade T400 or the Yasda PX-30i have 18,000 and 20,000-rpm spindles, respectively.
Conversely, ceramics don’t deliver the required surface finish for most HRSA parts, so you’re forced to switch to carbide tools running at completely different feeds and speeds. Mickelson said you’d run carbide at 100 sfm in Inconel, with a 10 to 15 thou chip load. That means much more torque at a much lower speed. So as Johnston put it: “The machine configuration can be a compromise. It’s hard to get the best of both worlds.” Not to be left out, Andersson said that for “non-critical components, which are typically non-rotating, ceramics are suitable for finishing as well, especially our WG600 coated ceramic.”
Speaking of compromise, Greenleaf’s XSYTIN-1 is an entirely new type of “phase-toughened” ceramic that does not require full plasticization of the material in order to cut. As a result, you don’t have to run XSYTIN at the same speeds you’d run whisker- reinforced ceramics, though you can. This broadens the applications to larger land-based turbine components that can’t be spun as fast during machining. And the extreme flexural strength of the XSYTIN material also means the micro-geometry at the edge need not be specific to the application, making it easier to select the right tool. XSYTIN tools have just a small brushed A-hone, a simple feature that’s actually “incredibly important,” explained Andersson.
“You want the smallest possible contact area between the cutting tool material and the material you’re machining to concentrate the heat in this zone,” he said. “But traditional ceramics are brittle, requiring larger, more complex edge prep to withstand the forces generated by machining.”
The small XSYTIN hone concentrates the heat in a smaller zone, making the material easier to machine and also introducing less force on the material. Another big payoff: You can run it at the same speed as whisker-reinforced ceramics, while using feed rates that are 50% to 100% higher, increasing productivity by 50-100%.
Andersson added that other applications calling for XSYTIN are forgings and castings with skin and scales. “That kind of surface is very difficult to machine, but the flexural strength of XSYTIN allows you to do so at high levels of productivity and predictability.”
Finally, XSYTIN is now available in ballnose end-mill configurations. That’s not new for ceramics, but before XSYTIN users have been forced to tilt ceramic ballnose tools to keep the end out of the cut since sfm goes down to zero at the center of a ballnose. (Remember that ceramic requires a high sfm to cut.)
“Because of the strength of the XSYTIN material, you don’t have to do that, which makes it much easier to apply.” This issue may help explain an interesting point raised by Johnston: that when machining with ceramics, the amount of material left is less predictable with end milling than it is with face milling.
Cutting HSRAs at any speed generates temperatures too high for normal tungsten-carbide substrates, explained LMT Onsrud’s O’Leary, adding that “the running environment is also quite abrasive since there is a significant amount of carbide precipitates that are present to the alloy.” Even with finishing at 100 sfm, you can’t use standard carbide tools for these materials.
Fortunately, said O’Leary, “higher transverse rupture strength substrates have enabled us to improve the edge geometry of our tooling, allowing for better chip formation when cutting superalloys. This chip formation leads to greater heat removal and better part finish. Tooling geometry advancements such as variable pitch, variable helix angles, and tapered cores help to create a better cutting tool that minimizes tool vibrations, which are detrimental in machining superalloys.”
O’Leary added that LMT Onsrud’s EMC and MaxQ end mills are manufactured with newer CNC grinders, “which have given us the ability to maintain a higher degree of control during flute grinding on these tools and to achieve tighter tolerances in the flute’s geometric relationships.”
As you might expect, coating is critical as well. O’Leary pointed to Onsrud’s new ENDURASpeed in-house PVD coating, saying it both adds to the hardness of the tool and increases the tool’s ability to withstand the heat generated during machining. Mickelson referenced silicon type coatings and the carbide tools offered by Hitachi, Union Tool, and Fraisa.
Toolpaths, Other Considerations
Smart machining techniques like trochoidal milling are even more important when it comes to HRSAs.
“The key programming technique we look for in Inconel and other superalloys is keeping the tool in the cut as long as you can,” said Mickelson. “Because every time you enter and exit the cut you lose tool life. So we create pocketing routines that move down into the part helically. If you have a flat, you cut down to that flat helically and then finish the flat surface.” The goal with ceramic tools is to “keep the material melt,” as Mickelson put it. Losing the melt and then re-engaging reduces tool life.
This is also why Mickelson recommended using the option in today’s CAM software that forces a radius on every toolpath when roughing, even at a corner, “because otherwise, a tool cutting a sharp corner can lose enough surface footage to lose the melt. You don’t want any sharp tool paths.”
Mickelson added that when cutting with ceramics, it’s best to deliver air coolant (either through the spindle or externally) for both chip evacuation and cooling, while everyone agreed that high-pressure liquid coolant should be used with carbide tools. High-pressure liquid coolant is also needed for any drilling and tapping.
5ME (Cincinnati) has published data showing improvements in material removal rates, tool life and surface finish when cutting Inconel 625 with its liquid nitrogen cryogenic coolant system (see “Deep Freeze Helps Cuts Challenging Materials,” ME, Dec. 2017). But while Inconel 625 is commonly used in the oil and gas industry, there appears to be little published data with cryogenic cooling on the Inconels used in aerospace, according to Johnston.
Coincidentally, Mickelson is now testing a system that blasts dry ice (CO2) into the cutting zone. Called DIPS (Dry Ice Powder Cooling System), it promises to deliver greatly improved tool life but it’s too early to say if it meets expectations.
O’Leary said it is beneficial to understand the material’s ultimate tensile strength (UTS) and heat-treating condition ahead of time in order to determine the best operating parameters. “If you’re altering tool operating parameters for each material batch, it is probable the new material has a different heat treat and UTS. A good practice is to chart the UTS of each material you bring into the shop. This will assist in attaining solid operating parameters that will be successful at each UTS/Hardness range.”
He added that although UTS is not an industry standard, it is “available on every material certification and is used for traceability. UTS is also used by SolidCAM in their iMachining algorithms to develop the cutting tool data recommendations for machining materials.”
Finally, after a roughing process that’s literally melting the surface of the superalloy, it’s important to let the parts cool before finishing. Mickelson’s preferred approach is to rough the whole part, without pulling it out of the fixture, then rough additional parts in sequence before switching to finishing.
On a machine like the Yasda PX-30i, with 32 pallets in a stacker, the first part is well cooled by the time it comes back for finishing, all without human intervention. Mickelson also recommends zero point fixtures on the pallets to speed the process of re-clamping parts for additional operations or adding new parts.
The Ultrasonic Effect
DMG Mori USA (Hoffman Estates, IL) has introduced an even stranger innovation for machining superalloys than cryogenic cooling: rotary ultrasonic machining. The device is an ultrasonic actuator added to a machine tool’s standard HSK spindle that oscillates the tool axially as it’s spinning. The degree of oscillation can vary between 0 and 10 μm, depending on the application, and the system automatically adjusts the ideal resonant frequency of the oscillation based on real-time feedback. Because the technology is scalable across platforms, the tool can spin as high as 50,000 rpm during ultrasonic machining on the ULTRASONIC 20 linear model.
Luke Ivaska, advanced technologies product specialist for DMG Mori, said that by reducing process forces, the system delivers an eight-fold increase in tool life and doubles or triples the surface finish quality (to under 0.1 μm Ra). “We can also improve productivity by two to three times, depending on the specific alloy.”
The cutting tools used can be carbide, ceramic or other off-the-shelf tooling. And because the ultrasonic head can be changed out automatically, tools of different types can be combined on the same part. In one case study, DMG Mori reported that the complete machining of an Inconel 718 turbine blade from a raw block, using ceramic and carbide end mills, took 23 minutes versus 32 minutes for conventional machining.
While there are standard alloys for aerospace, new ones are gaining traction. “For most of the last 15 years, aerospace engines have used pretty uniform materials. Inconel 718 at 46 to 48 Rc is the bread and butter of the industry,” said Greenleaf’s Andersson. And there continues to be huge demand for legacy engine parts, using these same materials. But the latest aircraft engines are designed to work at even higher temperatures and pressures in order to achieve higher efficiencies, and Andersson goes so far as to assert that all these new engines have new superalloys that are much more difficult to machine. “The strategies used for Inconel 718 or 40 Waspalloy are no longer applicable,” he said.
Sometimes, nearly everything has to change: feeds, speeds and tool geometries. “It’s not uncommon to have to go from a negative rake to a positive rake insert. In some materials it’s the opposite,” said Andersson.
Take a new material called Inconel 718 Plus. “It sounds very similar to 718 but it machines completely differently. I would say Inconel 718 Plus differs from Inconel 718 more than Inconel 718 differs from Inconel 625. The OEMs and the Tier 1s are very much aware of this. But when you get farther down the Tier scale into contract manufacturers and people bid on a job for 718 Plus as if it were 718, they soon find out they’re about to lose their shirts.”
IN100 and ME16 are two more new nickel-based superalloys that require new tool parameters. ME16 also exemplifies another new challenge: the introduction of powdered metal (in addition to new wrought or cast alloys). According to Andersson, this “significantly changes how it interacts with the machining process.”
Similarly, Mickelson said that cutting 3D-printed Inconel is much different than cutting standard Inconel. “It seems more like a CPM material or a hardened steel than an Inconel. It seems harder and more abrasive. We’d use ceramic cutters but with slightly lower surface speeds and a light chip load.”
DMG Mori’s Ivaska said “Beta Phase titaniums offer a challenge due to the high molybdenum content. However, with ULTRASONIC we have realized over eight times the tool life at double the productivity.” Finally, Andersson also reported that there are not many new cobalt-based superalloys, though there is a higher degree of cobalt in some new components.
In any case, it’s wise to pay attention to the details and seek out experience. As Andersson related: “We work closely with the OEM research facilities and other R&D centers, cutting materials that are not yet introduced. It’s very common for us to get phone calls from people claiming to have materials no one has ever machined before and we can give them exact cutting data because we have machined it in our R&D facility.”