New materials for workpieces call for innovative cutting tools, tool coatings and machining strategies
New work materials are developed continually to improve the capabilities of finished parts, making them lighter and stronger, among other properties. When these materials catch on, cutting tools must adapt to their often challenging properties.
This feature focuses on cutting tools for three aluminum-containing materials: titanium aluminide, high-silicon aluminum and lithium-aluminum alloys. While none are brand new, keep in mind that it takes years and sometimes decades of application work to create demand for “new” materials. When metallurgical changes are made to enhance the performance of machined components, a drop in machinability is the typical result.
Titanium aluminide—particularly gamma TiAl—is being used to make jet engine turbine blades that make next-gen engines lighter and more fuel-efficient. There is also growing interest in using TiAl in automotive applications, such as turbofans and engine valves. Arcam AB, a Swedish 3D printing company, is researching using additive manufacturing to make TiAl turbochargers for the auto industry.
The main challenge of machining TiAl, which is roughly 50% titanium and 50% aluminum, is that it is both hard and abrasive. It is also sensitive to cutting conditions, making it prone to surface or subsurface cracking.
“Generally, when machining TiAl you want a tool with a sharp edge to minimize surface damage,” said Don Graham, manager, education and technical services for toolmaker Seco Tools LLC (Troy, MI). “There is also a conflicting problem. You may want light cuts to avoid compressing the surface and generating cracks, while at the same time you want a thicker chip to keep heat in the chip and away from the part.”
As a result, machinists strike a balance by using comparatively high speeds when endmilling TiAl, such as 100 sfm (which is slow compared to steel and cast iron) as well as using sharper cutting edges. “You need to go a bit lighter in inches per tooth, but not so light that the cutting edge heats up,” said Graham.
Given the high cost and difficulties of machining TiAl, why do engine makers want to use it? “First, it is lightweight, and second, its high-temperature properties are phenomenal,” said Graham. “Third, you may be able to replace heavy Inconel parts in the very hot back-end of the engine with this material, which is 50% lighter.” TiAl cannot burn in jet engines, making it a very desirable material in this application.
Bill Durow, global engineering project office manager for toolmaker Sandvik Coromant Inc. (Fair Lawn, NJ), agreed that TiAl is gaining traction in aerospace. “It is a replacement for nickel materials inside jet engines; it’s twice as dense as those materials, which helps with the thrust-to-weight ratio,” he said. “It’s being used on both low-pressure and high-pressure blades.”
Sandvik Coromant recently performed test cuts on a series of forged gamma TiAl parts. “The material was scaly and cratered, but the actual cutting wasn’t difficult,” said Mike Magro, senior machining application engineer for Sandvik Coromant. “We cleaned up the scale with button-style RCMT inserts and performed turning with GC4325 CNMG-style 80° inserts. In general, we had better success with turning. We created a generic shape for the feature, then performed cutoff with a standard CoroCut QD insert. We’re used to a lot of ugly materials in aerospace, and cutting TiAl wasn’t really a challenge.” Milling was performed with a CoroMill 300.
Magro noted the abrasive material produced very fine, dust-like chips that could clog coolant system filters. “Our test machine has a 30-μm prefilter and a 5-μm fine filter, and that seemed to take care of the issue,” Durow noted.
He also said that the TiAl ran a bit hotter than nickel during turning. “We ran the machine at 130 sfm, four thou per revolution; on the milling we ran 110 sfm and four thou per tooth; on cutoff, we ran 90 sfm at three thou per revolution,” said Magro.
Because it can withstand heat, other jet engine applications for TiAl include non-rotating elements, such as brackets traditionally made from 625 Inconel, he noted.
Materials such as TiAl may not always require new types of tools; in many cases, existing tools can be modified with new coatings. “We work with coating suppliers to identify and develop new coatings,” said Cory Cetkovic, product manager of the Sphinx line of cutting tools for BIG Kaiser Precision Tooling Inc. (Hoffman Estates, IL). “We send them a library of cutting tools to identify the best coating for specific materials,” he said. “With TiAl, we typically rely on multiple-layer coatings, such as a hard coating on the underlayer, which is used to generate strength, then a thin, smooth coating on top to resist BUE [built-up edge] and enhance chip evacuation.”
When milling TiAl, Seco Tools recommends fine-grain solid-carbide tools, such as its line of Jabro end mills, which cost more than standard end mills but outperform them in this challenging application. When turning, if the setup is solid and the part is held firmly, high lead angles can help minimize DOC line notching in TiAl, which can create tool chipping and exaggerated tool wear.
“If you can increase the lead angle, you get more of the cutting edge into the cut and spread the width of that notch out over more of the cutting edge,” said Graham. “You might think ‘if I get 30% more of the cutting edge involved in the cut, I’ll get 30% more tool life,’ but it can actually be up to four times more.”
With the challenge of line notching, more shops are experimenting with diamond tools when machining TiAl. “When you use diamond, you have to reduce the cutting edge temperature to prevent tool dissolution,” said Graham. As a result, Seco Tools recommends high-pressure flood coolant with high heat capacity right at the cutting edge, such as its Jetstream Tooling system.
Aerospace suppliers have made major investments to machine TiAl. For example, Mitsui Seiki has supplied many of the machine tools being used to machine TiAl for jet engine turbine blades. The largest current user is AeroEdge, Japan, which is using 28 dedicated machines to produce TiAl blades. Scott Walker, president of Mori Seiki (USA) Inc. (Franklin Lakes, NJ), agreed that diamond tools perform very well in this application. Mitsui Seiki demonstrated CVD diamond-coated tools that were applied with a cutting speed of 100 m/min (328 sfm), rpm of 3,979, feed per tooth of 0.04mm (0.0015”), a feedrate of 637mm/min (25 ipm), a rough stepover of 0.5mm (0.02”) and finish stepover of 0.4mm (0.01”). The tools cost $475 each. However, due to cost, most of Mitsui Seiki’s customers machining TiAl are instead using very sharp carbide tools with a hard coating.
Edwin Tonne, senior engineer, product engineering for Kennametal Inc. (La Vergne, TN), noted that gamma TiAl has a very high tendency for work hardening, and that higher cutting speeds sharply increase hardness. In one test, workpiece hardness increased from 320 to 400 Brinell with only minimal increases to the cutting speed.
“A good ballpark cutting speed for economical tool life would be 30–40 mpm (roughly 100–230 sfm), which is about 30–40% slower than when cutting Ti 6-4,” said Tonne. “You need high-rake, high-clearance inserts, and if you go past 0.004 ipt, you’ll have a tendency to chip or even destroy the tool. If you keep it below 0.004 ipt, you should be operating in a safe condition with just normal flank wear.”
Because of its aluminum content, another TiAl issue is that some tool coatings have a chemical affinity for aluminum, which creates machining problems. Tonne recommends titanium boride coatings, which retain the tool’s sharp edge and do not have an affinity for aluminum.
Finally, keeping radial engagement (the arc of contact) low helps improve machinability. “If you’re at 10% of radial engagement (a small width of cut), you can go a bit higher in cutting speeds and feeds,” said Tonne. He noted that TiAl was successfully facemilled using the Kennametal 7745 VOD-441 insert and the 7713VR-701 button cutter, which were designed for aluminum. Likewise, solid-carbide XE and XER end mills were also effective.
New Aluminum Alloys
New materials are making inroads in other applications as well. High-strength aluminums are being used in more automotive and aerospace applications. Auto manufacturers are facemilling, drilling and boring cylinder heads and engine blocks made from high-silicon aluminum, which is challenging to machine.
Pure aluminum is lightweight and soft, but adding silicon carbide and other forms of silicon dramatically increases its strength, hardness, stiffness and longevity. However, it also makes it very abrasive. The higher the silicon content, the more abrasive the alloy. For example, Reynolds 390 aluminum is 17–19% silicon carbide and very abrasive.
As a result, very hard, micrograin solid-carbide tools or diamond cutting tools are preferred. However, diamond has the clear advantage in this application. Even very hard carbide grades have extremely short tool life, about 3–5 minutes, compared to 1 hour for diamond, according to Seco Tools’ Graham.
These automotive applications typically require large-diameter milling cutters. Seco Tools is applying its Quattromill, with four carbide cutting edges, and Double Octomill, with 16 cutting edges, in these applications. “However, it’s more likely that we will use milling cutters that take special diamond tools, including solid-diamond and diamond-tipped drills,” said Graham. “Some are standard tools, but a lot of the orders involve specials.”
The machinability of silicon-aluminum is inversely proportional to the silicon content, according to Aaron-Michael Eller, product manager, advanced materials for Seco Tools. “There are many aluminum alloys, and the silicon content usually determines the machinability,” he said.
Since aluminum alloys are soft, they are susceptible to BUE—workpiece material that adheres to the tool cutting edge—leading to poor surface finish and shorter tool life. Build-up on the tool can also be deposited onto the workpiece, and when build-up is released from the cutting edge, it can take particles of the cutting tool with it, leading to premature tool failure.
Because of the potential for BUE, Seco Tools recommends solid-carbide tools with positive angles, sharp to light edge hone, and smooth, high-polish coatings for high-silicon aluminums. “The positive angles allow us to cut the soft material rather than pushing, or smearing it, which sometimes happens when using negative inserts,” said Eller. “The smooth, high-polish coatings add lubricity, allowing the cut material to flow away from the cutting edge. The edge hone gives us the added edge toughness needed to combat the abrasiveness of the material.”
Eller noted that polycrystalline diamond (PCD) tools have sharp edge preps and are polished uncoated in order to keep as sharp an edge as possible. Utilizing PCD grades with a submicron grain structure in these applications can also be affective against BUE.
BIG Kaiser’s Cetkovic noted that, as in machining TiAl, new coatings are enhancing the machinability of high-silicon aluminums. “We’re taking our standard Sphinx cutting tools for aluminum and applying new, innovative coatings, which lead to increases in both performance and tool life.”
Compared to uncoated tools, which can produce as few as five parts per tool, tools with new coatings can produce as many as 250 parts per tool. Again, a hard undercoating is followed by a smooth outer coating that enhances chip evacuation and helps prevents BUE.
“For aluminums, we have a series of straight-fluted micrograin carbide drills that enhance chip evacuation at extremely high operating speeds and feeds; they reduce the length the chip must travel to exit the tool,” he said. “We’re usually in the ball park of tens of thousands of parts per tool.” One high-silicon aluminum automotive application was as high as 72,000 parts using a drill with an aspect ratio of 40 to 1 (length-to-diameter ratio).
“While tools like these are expensive, shops using them understand they are not buying a drill, they are buying holes,” said Cetkovic. “Four hundred dollars for each tool may seem like a lot of money, but if it gives you 72,000 holes, it’s really not.”
In the global aerospace market, new aluminum-lithium alloys are gaining traction. The low-density alloy combines weight reduction with lower assembly and maintenance costs, and, with advanced welding practices such as friction welding as well as aerostructure redesign, offers up to 25% weight reduction. The lower alloy density and improved material properties allow optimized structural designs, and lighter aircraft produce fewer CO2 emissions.
One of the key global suppliers of aluminum-lithium is Constellium (Paris). Its Airware aluminum-lithium is the result of decades of R&D to support innovative aerospace programs, according to the company. Airware was launched in 2010 as a comprehensive offer of high-performance aluminum-lithium alloys that are well suited for all parts of the primary aerostructure. In addition to aluminum and lithium, Airware alloys also contain copper and silver as major additional elements.
“Airware guarantees higher durability due to its greater resistance to fatigue and corrosion, which enables airlines to target extended heavy maintenance intervals,” said Sylvain Henry, director, customer application engineering for Constellium. “Airware is endlessly recyclable without any loss of properties and contributes to the development of a sustainable aerospace industry.” He noted that Airware is already flying on major aerospace programs such as the Airbus A350 XWB as well as Bombardier’s C series. It has also been selected for space programs, including the Space X Falcon 9 Launcher and the Lockheed Martin Orion spacecraft.
Constellium has finalized the second phase of an investment program at its Airware cast house in Issoire, France. The cast house is now fully qualified by Constellium’s main customers.
As with other aluminum alloys, Airware requires standard machining operations such as contouring, skimming, pocketing and part finishing. These operations are performed on standard machine tools, such as vertical and horizontal machining centers, both at high speeds and standard speeds.
“Airware comprises a family of harder alloys, so machining can be different—but not significantly different—than other hard aluminum alloys, said Henry. “Some of our machining partners have developed special cutting tools to ensure similar efficiency and productivity levels.” Machining Airware generates large quantities of chips and offcuts. If these materials are effectively segregated, the material can be taken back into the manufacturing process.