MACHINING NEW MATERIALS


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.”


In a test cut of TiAl, Sandvik Coromant used GC4325 CNMG inserts for turning.


As a result, machinists strike a balance by using com- paratively 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 Inco- nel parts in the very hot back-end of the engine with this ma-


50 AdvancedManufacturing.org | January 2017


terial, 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 replace- ment 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,


Photo courtesy Sandvik Coromant


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