MACHINING NEW MATERIALS
smooth coating on top to resist BUE [built-up edge] and enhance chip evacuation.” When milling TiAl, Seco Tools recommends fi ne-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 fi rmly, 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.”
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 fi nish 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 hard- ness increased from 320 to 400 Brinell with only minimal increases to the cutting speed. “A good ballpark cutting speed for eco- nomical 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 fl ank wear.” Because of its aluminum content, an-
A Seco Tools Double Octomill, with 16 cutting edges, which is used to cut high- silicon aluminum in automotive applications.
With the challenge of line notching, more shops are ex- perimenting 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 fl ood 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,
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AdvancedManufacturing.org | January 2017
other TiAl issue is that some tool coatings have a chemical affi nity for aluminum, which creates machining problems. Tonne recom- mends titanium boride coatings, which retain the tool’s sharp edge and do not have an affi nity 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 Ken- nametal 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 auto- motive and aerospace applications. Auto manufacturers are
Photo courtesy Seco Tools
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