Tackling Triple Nickel: General Industry Coverage

When it comes to milling aerospace parts from Ti5553, difficulty is in the eye of the beholder.

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• Sandvik Coromant
• ATI Stellram

Milling or otherwise machining the titanium alloy Ti5Al5V5Mo3Cr, or Ti5553, is something like having a bad first date, but a better second one. You get an unpleasant first impression of someone who later exhibits the opposite characteristics after you get to know them better. That’s a good thing, because the alloy has become an important workpiece material for critical aerospace structural components, such as landing gear.

As a result, new cutting tools and machining strategies are being developed for Ti5553. For example, John Palmer, U.K.-based global aerospace manager for ATI Stellram, LaVergne, Tenn., said the toolmaker developed its 5230VS chevron-style milling cutter when “looking at different ways to machine this challenging material.” However, by applying the correctly designed cutting tool with the proper milling strategy, toolpath and coolant supply in a rigid machine tool, “it becomes very easy to machine Ti5553,” he added. “You just have to think a bit more before you start to machine.”

Others interviewed for this article concurred. “Today, triple 5-3 has demonstrated to be very predictable with machining consistency similar Ti6Al4V,” said Michael Standridge, aerospace industry specialist for Sandvik Coromant Co., Fair Lawn, N.J. “The variances in the two materials create the need for different cutting data to be used to obtain similar tool life. Once you have your parameters set properly, triple 5-3 is relatively easy to machine.”

Courtesy of ATI Stellram

ATI Stellram offers a variety of milling cutters for machining titanium and other difficult-to-cut materials.

The main problem part manufacturers have when initially milling titanium is trying to transfer the knowledge gained from machining other metals, according to Gary Churchill, technical director for metal distributor Titanium Metal Supply Inc., Poway, Calif. “Then they sort of short circuit,” he said.

He recalled one shop that complained about hard and soft areas in large titanium forgings, which Churchill suspected was not the case. He asked if the customer had previously machined titanium. The response? “No, but we machine aluminum all the time.” After providing recommended speeds and feeds for titanium, the perceived problem of poor-quality material disappeared.

This article focuses on the properties of the workpiece material, design of the cutting tool system—including substrate, geometries and coatings—and appropriate coolant application to effect-ively mill Ti5553 to produce aerospace structural components. [Editor’s note: machine tool design for cutting titanium was covered in the September 2010 article “Ti Machines.”]

Made to be Difficult

Nicknamed “triple nickel,” Ti5553 is considered more difficult to machine than the more common Ti6Al4V because of its higher alloying elements of molybdenum, vanadium and chromium. “These higher content percentages make Ti5553 more challenging to machine, so your tool substrate, geometry and coating need to be right,” said Nick Trott, a technical sales manager for M.A. Ford Europe Ltd., Derby, U.K., a division of Davenport, Iowa-based M.A. Ford Manufacturing Co. Inc., which manufactures the tools. “Cutting parameters and milling strategy also play a big part in machining this alloy.”

Trott added that the alloying elements that make Ti5553 challenging to machine enhance the properties desired for high-load aerospace structural components, such as landing gear.

Palmer explained that heat treatments penetrate further into a Ti5553 part than one made of Ti64, increasing the linear load section strength in Ti5553 up to about 6 ” compared with about 2 ” for Ti64.

Aerospace part designers welcome the material’s ability to reduce part weight while not sacrificing strength, but chromium and molybdenum make the material “more aggressive [to machine],” Palmer said. “You must have a tool material capable of withstanding that type of cutting aggression.”

Balanced Cutting

Tools designed to cut other titanium alloys are also suitable for machining Ti5553, but not at the same speed. Some machinists reduce the cutting speed for Ti5553 to half of what is appropriate for Ti64, noted T.J. Long, engineering manager for indexable milling systems at Kennametal Inc., Latrobe, Pa.

Not all applications require cutting speeds that conservative, but triple nickel isn’t going to be cut faster. For example, when taking a 0.050 ” axial DOC with a high-feed mill, 140 to 160 sfm is appropriate for milling Ti64 but 100 to 120 sfm is the range for Ti5553, according to Terry Carrington, aerospace industry product manager for Iscar Metals Inc., Arlington, Texas.

Standridge concurred. “You must run slower to achieve a balance of tool life and tool security,” he said.

Balance is also the operative word when developing a substrate for tools designed to cut titanium. Such a substrate must have a balanced mixture of hard particles to resist heat and tough particles to absorb shock, Standridge emphasized.

That’s typically achieved, in part, by sintering submicron-grain carbide for hardness with 10 to 12 percent cobalt binder content for toughness, Trott noted.

For its chevron-style mill, Stellram selected its X500 carbide grade because it withstands crack propagation caused by heat or vibration through the workpiece and stays intact in the worst machining conditions, according to Palmer. He noted that the grade can rough Ti5553 at a “relatively high” surface speed of 80 to 150 sfm.

For finishing, Palmer added that ultrafine micrograin substrates offer the opportunity to produce sharp cutting edge profiles, precision ground or honed to provide a polished profile, rather than a standard ground surface found on tools for general milling. “If you have coarse-grain carbide and produce a sharp cutting edge, the edge is going to look sawtooth rather than razorlike,” he said.

Carbide isn’t the only tool material suitable for milling Ti5553. Cobalt and P/M HSS tools are often applied to cut titanium, particularly on older, less rigid machines. Still, Palmer feels the productivity gains carbide tools provide make HSS tools part of history. That’s because HSS traditionally run at 20 to 30 sfm and carbide tools can finish Ti5553 at up to 300 sfm, he noted.

Geometries at Work

Effectively milling titanium requires positive geometry to shear the material, reducing cutting forces, pressure and generated heat. “The positive cutting action is not just found in the edge line, but in the axial and radial rakes in the tool body tip seat,” Standridge said.

As previously noted, tools for milling titanium require sharp cutting edges. That’s because titanium exhibits a low modulus of elasticity, which leads to a “springiness” characteristic whereby titanium parts may move under the force of the cutting edge and then spring back, according to Iscar Metals.

Creating that sharp edge without negatively impacting the tool material and edge strength can be a challenge. Although an as-pressed insert has a stronger cutting edge than an insert with a ground edge, titanium typically needs to be cut with an edge that’s ground sharp, according to Carrington.

Courtesy of Kennametal

Kennametal Beyond Blast inserts channel coolant through the insert to the tool/workpiece interface to provide efficient coolant delivery, lubricity and heat transfer (see below).

Courtesy of Kennametal

An example of how Beyond Blast through-insert cooling works on a specific workpiece.

“Anytime you put a grinding wheel on a piece of carbide, you have reduced the integrity of that cutting edge—period,” he said. “You can’t put heat on a piece of carbide without changing its physical structure a little. It pulls binder out of that carbide.”

An edge that’s too sharp, however, can be easily damaged. “Typically, the primary failure mode in titanium machining is microchipping,” Long said, “which tends to progress to macrochipping.” To combat that, a light hone on the edge is suitable for stable machining environments and a T-land edge preparation may be required for less-stable conditions, he noted.

Sharpness is also a concern at the end of a tool. To protect that corner, or end edge, M.A. Ford offers a range of corner radii, Trott noted. “If you gash a tool with a sharp corner, it would not have the strength to tackle Ti5553 or any other titanium alloy for that matter,” he said.

Chip Shape

One common tool geometry isn’t always required when milling titanium. “Our titanium endmills don’t have chipformers or chipbreakers,” Trott said. “We don’t find them necessary.”

Because titanium “doesn’t particularly like to be machined, you have to make the cutting edges as sweet cutting as possible, but you also have to make them resilient to withstand wear and loading, which we believe we’ve achieved with our titanium endmills,” Trott said. “Once you have an endmill designed for the job, with the correct parameters and milling strategy, Ti5553 becomes pretty easy to machine.”

Once a chip is formed, it must be evacuated to avoid recutting it. To effectively export the chip from the cutting zone, ATI Stellram designed its titanium milling tools with a round flute to match the shape of the chip instead of the traditional V-type flute for standard cutters.

Palmer noted that the critical geometric feature is sufficient clearance on a tool’s cutting edge. Without it, built-up edge appears on the flank rake, which creates friction with the workpiece surface and additional heat. Eventually, a piece of the built-up metal deposits itself on the workpiece via friction welding, damaging the cutting edge and galling the workpiece.

“When machining steel, you talk about BUE on the front primary rake,” Palmer explained. “With titanium, yes, it does build up a slight deposit on the cutting edge, but the big danger is the buildup of material on the flank surface on the relief of the cutting edge.”

As with other materials, vibration must be minimized when milling titanium alloys. One way to achieve that is with variable, or differential, pitch, noted Long. Changing flute spacing can break up vibration-generating harmonics.

Long added that increasing the number of teeth on a cutter boosts productivity because the chip load and cutting speed are limited when milling titanium. But increasing the number of teeth reduces the ability to provide differential pitch and may cause chip packing. “You have to weigh the benefit of a higher number of teeth against having a coarser pitch tool with a differential pitch and/or a more open chip gash,” he said.

Although a rigid machine is extremely important when machining titanium, Long noted that plunge milling and high-feed cutters can boost productivity on less-rigid conventional machines.