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Materials Force Equipment Development


 

Machines and tooling must adapt to deal with the new generation of materials, such as titanium and composites

 

By Bruce Morey
Contributing Editor 

 

Driven by the need to reduce weight, industries from energy exploration to aerospace are adopting composites and titanium alloys in ever-greater volumes. New titanium alloys such as titanium 5553 are providing higher strength, up to 180 ksi (1241 MN/m2) yield, and good corrosion resistance and fatigue properties. Traditional epoxy-based composites are the material of choice in many new aircraft. Metal-matrix composites, if rare, are no longer laboratory specimens.

These new materials present different and unique machining challenges. Composites are abrasive on typical tools, and produce potentially damaging dust. Titanium, on the other hand, while not particularly harder than many other metals, retains its hardness at high temperatures. Cutting temperatures for titanium alloys can reach 1100–1200°C if not properly controlled. Material removal rates can be as little as one-fourth the rate achieved when machining more common metals. Titanium also conducts less heat than other materials—the chip does not transport heat away from the cutting tool. Machining this material has its own rules.

Coupled with titanium's tendency to chatter, these issues mean the ideal cutting machine is dynamically stiff with higher horsepower and torque at lower rpm, according to Dan Cooper, senior application specialist for Cincinnati Machine (MAG Cincinnati, Hebron, KY). "The typical spindle speed when machining titanium may be 200–300 rpm, compared to 10,000–15,000 rpm for aluminum," says Cooper.

To meet these special requirements, MAG Cincinnati last year introduced a new machine, the Ti-Profiler, that produces 0.33 hp (0.25 kW) per rpm, compared to typical aluminum cutting machines that might produce 0.03 hp (0.022 kW) per rpm, according to Cooper. He describes the Ti-Profiler as a vertical, five-axis multispindle (three, four, or five-spindle) profiler capable of delivering up to 340 hp (254 kW) in total. A variation on the design of Cincinnati Machine's Wide Range profiler, the Ti-Profiler runs at a maximum of 3500 rpm compared to the 7000 rpm of the Wide Range. This profiler can produce large parts. "Right now, we are quoting a machine whose rails are 490' [149-m] long," says Cooper, "and can produce up to five parts that are about 45" [1.1-m] wide."

Greg Hyatt, vice president and chief technical officer for the Machining Technology Laboratory, Mori Seiki USA (Rolling Meadow, IL), agrees that machining the harder titanium alloys, like Ti 5553, requires a machine that delivers high torque at low rpm. The typical parts are complex as well. "Almost all of the machines we provide for these applications are five-axis machines. To improve the damping for these applications, our NMV and NT machines use an octagonal ram. It provides the damping of the box-way design, but eliminates the thermal deformation of box ways. The spindle centerline is not displaced, even with heavy utilization of the high-speed positioning."

Hyatt says that the larger horizontal machining centers used on the tougher beta-alloys, like Ti 5553, deliver 1000 N•m of torque, in applications where cutting speeds are below 500 rpm. He reports cutting speeds for alloys like Ti 5553 at 150–250 fpm (45.7–76.2-m/min) with a carbide tool.

 

 

 

 

"We have observed a dramatic increase in the use of composites," notes Hyatt, led by aerospace applications. "Machining composites is as different from titanium as possible." Composites require much higher spindle speeds coupled with higher acceleration rates. Spindle speeds are approximately 10,000–20,000 rpm, and accelerations exceed 1 g.

Composites produce dust that may be a health hazard for both humans and machines. To control dust, Mori Seiki is developing a "zero chip" spindle and tool combination, in collaboration with Kennametal (Latrobe, PA). "As we cut composites, we suction the dust and chips through the tool and spindle, and capture it behind the machine," explains Hyatt. They are targeting a mid-2008 introduction of the tool and spindle.

Tooling engineering for hard alloys or composites presents its own problems.

Tools designed for machining titanium and other hard alloys must have specific features, according to Michael Standridge of Sandvik Coromant (Fair Lawn, NJ). Features include:

  • Positive cutting geometries with sharp edge lines,
  • Insert shapes that provide the proper lead angle to allow chip-thinning and resist notch wear,
  • Tools with coatings and grades of carbide that have the proper balance between heat resistance and toughness, and
  • High-pressure coolant.

"Titanium and heat-resistant superalloys combine relatively low density (compared to steel) with high strength, and need to be sheared. A positive cutting geometry does that," says Bruce Carter, product manager for rotating tools for Sandvik Coromant. Optimal cutting angles vary depending on the application, but are typically around 11°. "We also recommend using round inserts and inserts with radius corners on them, which spread the chip out over a greater distance on the cutting edge," he explains. This geometry contributes to chip thinning and better cutting action.

Positive cutting geometries with sharper cutting edges are inherently weak. To overcome this problem, Sandvik Coromant has developed tools with multilayer PVD TiAlN coatings over specially developed carbide substrates. Standridge reports a 25–35% increase in tool life with these new generation grades, when compared to older inserts. Newer Sandvik Coromant products using this technology include GC1030 milling tool inserts, GC1105 turning tool inserts, and the CoroDrill 880 indexable-insert drill.

High pressure of 1000 psi (6.9 MPa) or more cools the cutting zone more effectively than normal flood coolant, aiding the cutting action and extending tool life, according to Standridge. "Just as important, highpressure coolant is only effective when applied directly to the cutting zone. We've developed tools with programmable ultra-high-pressure coolant for turning applications, called Jet Break. The tools maintain ultra-high-pressure coolant to the cutting edge through the entire toolpath."

Eliminating chatter through tool engineering means developing a way to reduce contact between the cutting tool and the workpiece, according to Cincinnati Machine's Cooper. MAG Maintenance Technologies, a sister company to MAG Cincinnati, recently collaborated with Technicut (Sheffield, UK) to offer that company's solid-carbide Raptor tool to customers. "The traditional way of creating brazed carbide tools limited the helix angle to 15°. The Raptor tool we are now offering can produce a much higher helix angle on larger tools. For example, we produce a helix angle of 60° on a 3" (76-mm) diam carbide cutter," explains Cooper.

A higher helix angle reduces cutting contact area with the part, decreasing chances for chatter and increasing material removal rates by thinning the chip and reducing the chip load. This allows increasing feed rates by 300–400%, according to Cooper.

"Our new tooling [for hard alloys like titanium] has a high flute density," says Cooper, "we are able to provide, for example, 1" [25 mm] diam solid carbide cutters with up to 20 flutes. With increased flute density coupled with the high helix angles, this tool can run at feed rates of up to 100 ipm [2500 mm/min] in finishing operations. This also produces a finer surface finish, important for titanium since a rough surface can build stress raisers. Typically, our 2" (51-mm) Raptor mill runs at 760 rpm and feed rates of 40–60 ipm [1.0–1.5 m/min].

Tom Hoffman global product manager for ATI Stellram (La Vergne, Tennessee) believes that positive cutting angles and approach angles of the cutter equal to 45° or less thins the chips. Specially designed substrates are also vital for tool design.

Taking positive cutting angles a step farther, ATI Stellram has developed a two-angle approach to providing strength through geometry. With titanium exhibiting a springback effect when cut, an additional clearance is needed to eliminate rubbing on the flank of the insert, explains Hoffman. "We use two angles to reinforce the cutting edge—a small primary angle that gives us the necessary strength and a higher secondary angle that gives us clearance. It's the best geometry to resist pressure and extend tool life." He also emphasizes a peripheral ground edge. "You don't want to use a molded insert or just ground facets on the ends, depending on the style," he explains. "This approach gives a sharp cutting edge and a controlled cutting edge." Finishing with a tight-tolerance hone protects the edge from premature chipping. "Too sharp an edge will chip the tool, reducing tool life," he explains.

The substrate for the company's tools uses ATI Stellram's patented X-Grade technology, available in three grades, that employs a ruthenium and cobalt alloy rather than standard carbide. The combination results in superior thermal cracking and propagation resistance, according to ATI Stellram, achieving higher metal removal rates. Their X500, and X700 grades were specifically designed for titanium and nickel-based alloys, while the X400 was designed for hardened steel.

When cutting titanium, Hoffman suggests reducing the arc of engagement between the tool and the work surface. This reduces heat while enabling higher spindle speeds. "For proper tool life, reducing the arc of engagement anywhere from 2 to 15%, compared to 50–100% for common steels, actually produces higher material-removal rates without a loss of tool life," he says. 

Parag Hegde, manager of the Global Machining Technology group at Kennametal, agrees that a titanium cutting tool needs a positive rake angle. Edge preparation is another matter. "For titanium alloys, the conventional wisdom is they should be machined with a sharp [or a smallhone] edge," says Hegde. "We did a lot of work to test this theory. We find that the edge preparation is very application-dependent. For example, we have found that a honed T-Land edge preparation works very well in some specific titanium applications."

Kennametal recommends their Grade KC5010 in the -UP geometry for finishing to medium-machining turning operations in heat-resistant materials and titanium. Using a PVD AlTiN coating with a microfinished edge over a fine-grain tungsten carbide substrate helps the substrate resist the high heat generated, while providing deformation resistance.

They are also trying out coolant delivery through nozzles built into the tool pocket, rather than flood coolant, to provide a jet of coolant where the edge is hottest. The coolant is directed through the tool and exits a nozzle directly in front of the insert. "We have just introduced this coolant technology into our new Z-Axis cutter," explains Hegde.

Composites present their own tool design challenges; whether they are the traditional epoxy-based carbon-reinforced variety, or the more advanced metal-matrix composites. Drilling is probably one of the more common—and demanding—machining operations on traditional composite materials, according to several sources. Drilling issues include delamination, fiber pullout, and burr formation. Kennametal, in partnership with Novator AB (Spanga, Sweden), offers an orbital drilling approach that Hegde says is particularly suited for composites, especially stacks of differing material often found in aerospace fuselages. He describes it as a helical interpolation process that allows making multiple features on the hole in the same operation, such as holes plus countersinks. They have also introduced a new diamond-coated solid-carbide drill that is specifically designed for epoxy-based composites, and report significantly reduced delamination and pullout in tests.

Layered composites in some MMCs present even more of a challenge, because the individual layers of composite, titanium, and or aluminum have different properties, and thus are machined in a different way. Approaching layered composites with a single tool is challenging, which in turn causes a bit of difficulty in creating a "single tool" solution to produce holes as a standard off-the-shelf product, says Sandvik's Standridge. "I think the whole tooling community is working on solutions to drill composites effectively," says Standridge. "This is true for metal-matrix as well as the traditional carbon fiber-epoxy composites used in aerospace."

Increasing demand is spurring new ways to create titanium alloys. Demanding applications are increasingly satisfied with metal-matrix composites. One company, Dynamet Technology Inc. (Burlington, MA), has advanced powder metallurgy to both meet demand for titanium alloys and titanium-based metal-matrix composites. They now offer powder titanium-alloy parts that are as fully dense and strong as parts made from traditional wrought material. In their CHIP process, Ti powder is blended, cold isostatically pressed, sintered in a vacuum furnace, and then finally hot isostatically pressed (HIP), according to Sue Abkowitz, vice president technology and operations at Dynamet Technology. 

Most exciting about this process is its ability to produce metal-matrix composites just as easily as titanium alloys. Using the same CHIP process, elemental titanium powder added with alloy powders and ceramic particles creates an MMC they call CermeTi, for Ceramic-Metal-Titanium.

Published data for a version using titanium-carbide reinforcement shows a 13% increase in yield strength and a 15% increase in elastic modulus over Ti6Al4V alone. "More importantly for many applications, increased wear resistance is the key property provided by the carbide particles in the matrix," explains Abkowitz. The improved wear resistance and higher modulus makes the properties more like steel, while retaining the much lighter weight and corrosion resistance of titanium. Abkowitz says applications for CermeTi include aerospace parts, sporting goods, medical implants, and industrial tooling, including shot sleeve liners for aluminum die casting, where the high-temperature erosion resistance is especially useful.

CHIP is a near-net shape process.finish machining is required to complete the part, whether composite or pure alloy. As with composites in general, machinability is a concern. "There are some issues machining this material, but the good news is that for anyone already familiar with machining titanium, the leap to CermeTi MMC is not great," explains Abkowitz. "It acts more like a metal than a composite material." This is because the reinforcement volume is not high and is a particulate rather than fiber.

 

This article was first published in the October 2007 edition of Manufacturing Engineering magazine.


Published Date : 10/1/2007

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