- May 29, 2014
Cutting super alloys can be a formidable machining challenge
Nickel and cobalt-based superalloys may not be Kryptonite, but drilling holes in Hastelloy X, Inconel 625, Waspaloy and Haynes 25 is about as tough as it gets in the world of metal removal. With machinability ratings in the low teens, these nightmare materials test the patience of even the most cool-headed machinist.
Unfortunately, modern aircraft won't fly without superalloys, and nuclear power plants wouldn't run. Their high strength, fatigue and corrosion resistance, and excellent mechanical properties make these materials the perfect choice for gas turbine blades, rocket motor components, oil and gas applications, and a host of other demanding environments. It also makes them a real bugger to cut.
Drilling in particular can be a super-challenge—chip evacuation and heat buildup are a concern in any holemaking operation, and doubly so in superalloys. Fortunately, four holemaking experts are here to share their insights on drilling these toughest of all materials, the superalloys.
Allies in drilling
Allied Machine & Engineering Corp., Dover, OH, manufactures replaceable-tip drilling systems and custom holemaking solutions. Product manager Robert Brown says there are several key factors to successful drilling of superalloys. "These materials have poor heat transfer properties, and can be quite abrasive. Because of that, we generally recommend indexable products running at greatly reduced speeds and feeds."
But not too low, warns Brown, as work hardening is also a concern—managing feed rates is a balancing act between drill breakage and premature failure. "You can expect lots of flank wear, with rounding of the drill corners. Tool life is normally about 25 per cent of a comparable alloy steel job."
If you're a newbie to super-drilling, Brown says you should double your expectations for tool usage. That's why drills with replaceable tips are a good idea—swapping out a $30 insert is far more cost-effective than replacing a $300 drill body when a superalloy gets the upper hand during holemaking.
Allied recommends C-2 or K-20 carbide together with its proprietary AM200 or AM300 coatings, which Brown says have the heat resistance critical to tool life in high-temp materials. As well, the company has several tool geometries designed to improve chip formation and clear the hole. Above all, you need plenty of coolant and a stiff, accurate setup. "The job is going to be hard enough without environmental problems like excessive runout or weak workholding."
The big picture
Susan Valenti, holemaking product manager at Ingersoll Cutting Tool, Rockford, IL, says there are many variables to consider when running high-temp or exotic materials. "At Ingersoll, we look at the whole picture: the machine, the holding method, the rigidity of the part set-up, the number of holes per part, the tolerance of the holes, long run versus short and coolant capability. These factors help determine the drill we recommend to help the customer achieve their goals."
That recommendation may include tipped, indexable or solid carbide drills. "Each has a place in a demanding shop environment. It's a matter of finding the most cost-effective solution." For example, Ingersoll offers its Gold-Twist drill, a two-piece replaceable tip drill with deep flutes that aid in chip evacuation. Aside from relatively low costs for tip replacement in the event of a bad day, this type of drill offers multiple geometries to suit the application.
"With exotics, heat is the enemy, as these materials have a high tendency to work harden," Valenti says. "Coolant with high-pressure and volume is critical, as is good chip control—classic C-shaped chips, and sixes and nines are preferable. These pass freely through the drill flutes and take the heat away from the drill point. Also, pecking and dwelling should be avoided when drilling exotic materials, as this creates additional heat."
For superalloys, Valenti recommends an "M" geometry, which has a lightly-honed cutting edge for freer cutting and reduced heat generation. "A good set-up with minimal run-out on this type of tool will give you +.002/-.000 hole tolerance," she says. For lower volume jobs, open tolerances, or roughing applications prior to semi-finishing or finishing, an indexable drill with a strong, through-hardened body and inserts designed for tough-to-break chips is suitable. "You need a workhorse when ploughing a hole in superalloys."
In deep hole applications over 5xD, a pilot hole 1/2 to 1 diameter deep should be used. This can be said for most any deep hole drilling, but is especially true when drilling superalloys, and is critical to straightness, concentricity, and hole size. Valenti says the pilot drill should have the same tip geometry and a point angle as large or larger than the follow-on drill. "Over 8xD, it's the same scenario, but you should drill the pilot hole at least 1.5 diameters deep."
Valenti summarizes like this: "No matter how you look at it, superalloys are extremely challenging to drill. You need the right tooling together with the right people to support your application. Look for a partner who can provide the best options to match your holemaking requirements for these difficult materials."
Machining is machining
David Vetrecin says successful drilling of superalloys is a matter of picking the proper carbide grade and tweaking the cutting parameters to suit. "Drilling, milling, turning, it doesn't matter. Surface speeds and feedrates will certainly be different on superalloys, but it all comes down to the basic principles of machining."
Vetrecin is the product manager for holemaking at Iscar Tools Canada, Oakville, ON. He says the biggest difference between holemaking and other machining operations—in any material—is chip evacuation and heat. "Drilling in general is more difficult. With turning and milling, the chips are free to go wherever they need to go, but drilling is a closed operation. There is only one way out for that chip. So it doesn't matter if you're drilling mild steel or Inconel, the key is clearing the chips out of the hole and keeping the tool cool."
Once you've accomplished this, the rest is...well, not gravy, but at least predictable: get the feeds and speeds dialed in and resign yourself to higher tooling costs. Like his competitors, Vetrecin says replaceable tip drills are the most cost-effective way to go for small holes, in Iscar's case these are available between 6 to 33 mm in diameter and up to 12x deep. "Compared to solid carbide, these tools are about 1/10 the price, and are suitable for all but the most demanding hole tolerance or straightness requirements."
One other thing Vetrecin recommends is the right toolholder. Hydraulic chucks offer runout accuracy to better than 0.003 mm (0.00012 in.) and clamping force up to 43 kg/mm. This is especially important when drilling superalloys, due to the high cutting torque involved. "Compared to collets, hydraulic have substantially more gripping force. I also like hydraulic because there's an oil bladder inside the holder, so you get some vibration dampening. This is really helpful with long drills, and where cutting forces are high."
Vetrecin says when drilling superalloys, you have to take every advantage possible. "There are a lot of things working against you. Being successful means effective chip management, proper coolant flow and the right toolholder. After that it's just a matter of applying good machining practices."
Asking the right questions
"In my job, I see a lot of new and difficult materials," says Jim Vavruska, product application specialist for tooling supplier Walter USA, LLC, Waukesha, WI. "One of the most common problems I hear about is the machining of high-temp alloys jobs, especially drilling: 718 Inconel, 600K or 400 Monel and even Hastelloy X: what's the best way to approach these materials?"
Vavruska says it's important to ask the right questions, starting with what type of coolant is being used. "I like a semi-synthetic water soluble, mixed to about 7-10 per cent. This creates a good balance of lubricity and cooling. Thru-the-tool coolant is best practice for these types of materials, with 250-650 psi optimal on drills up to about 16 mm (5/8 in.). Larger drills need greater volume because of the tool's larger coolant holes."
It's important to keep the heat in the chip with superalloys, Vavruska explains. If not, the material tends to swell around the drill, causing that awful squealing noise with which so many superalloy machinists are familiar. This leads to drill marks in the hole, as well as degraded tool life and increased flank wear.
Vavruska agrees that hydraulic holders or even shrinkfit holders are preferred, as these keep tool run out to a minimum and help with accuracy and tool wear. "Due to their higher cost, however, these holders are not an option for many shops. In this case, a good collet system can also be used, provided you have high-accuracy collets and keep your holders clean and free of debris and dents."
Above all, avoid starter holes. For holes shallower than 12 xD, coolant-fed carbide drills can often be deployed without one. "Anytime you drill, you introduce heat, and heat causes work-hardening in superalloys." For this same reason, pecking should be avoided. If you do need to peck, short retractions to clear the chips are best. "Never exit the hole until done drilling. It leads to whip and out of round holes, and causes premature wear to the drill margins."
Finally, the right point geometry is critical when drilling high-temp alloys. A very slight edge hone provides good shearing action, while a slight lip correction helps with chip control and tool life. Double margins and polished flutes, together with sub-micron carbide and top-notch coating tame even the nastiest of super materials. "Drilling technology has come a long way in recent years," says Vavruska. "Shops should avail themselves of the best cutting tools possible, especially when they're cutting difficult materials."
This last point is true no matter what material is in your machine. Quite simply, today's high performance drills make shops more competitive, and for superalloys, they're a must. Just remember to keep them cool, clear the chips, hold on tight, maintain straightness and rigidity, and budget accordingly. Happy drilling, Superman. SMT