The ‘in’ metal for manufacturers

(Photo: Courtesy of Makino)

Known by the symbol ‘Ti’, titanium is an increasingly popular ‘go-to’ metal for manufacturers.

“Titanium is both strong and lightweight—as strong as steel but weighing only 56 per cent as much as steel. That gives it the highest strength-to-weight ratio of any of today’s structural metals. To produce structures of the same strength, far less titanium is required than other metals,” reads information from the International Titanium Association (ITA), a non-profit trade group based in Northglenn, Colorado.

Heat and corrosion resistant, titanium is highly regarded by people who make airplanes and airplane parts.

“The commercial aerospace industry is the single largest market for titanium products primarily due to the exceptional strength-to-weight ratio, elevated temperature performance and corrosion resistance,” states the ITA. “Titanium applications are most significant in jet engine and airframe components that are subject to temperatures up to 1100ºF (593ºC) and for other critical structural parts…as new titanium products, alloys and manufacturing methods are employed by the aircraft industry, the use of titanium will expand.”

Indeed, titanium is also finding its way into the medical parts sector and world of oil and gas.

For all its advantages, however, titanium has a reputation for being difficult to cut. “It doesn’t like to be bullied,” is how Brian Hamil, product engineering manager, SGS Tool Company, in Munroe Falls, Ohio puts it.

With that in mind, what’s the best way to cut this metal?

We asked some experts, and here’s what they had to say:

What’s the biggest challenge in cutting titanium?

(Photo: Courtesy of Makino)

“Probably the biggest problem people report is heat build-up at the cutting edge, because titanium is a poor conductor of heat,” says Don Graham, manager of education and technical services, Seco Tools, in Troy, Michigan.

“Titanium is regarded as a heat-resistant super alloy (HRSA). One of the biggest challenges [in cutting it] is chip control,” adds Brian Sedesky, applications/sales engineer, Horn USA, in Franklin, Tennessee. “Chip control is difficult because the titanium alloys will retain their strength at high temperatures. Titanium does not soften and flow into the chip form of the cutting tool like steels do. The requirements of the cutting tools are also demanding when machining titanium. The cutting tools must have a sharp enough edge to shear the material in the cutting zone but be strong enough to withstand the repeated cutting forces of the material.”

What’s the best way to deal with heat build-up on the cutting-edge?

“Typically the heat build-up from machining is controlled with coolant and by using an appropriate cutting tool that minimizes the heat generated during cutting,” says Kurt Ludeking, product manager for turning, Walter USA, Waukesha, Wisconsin. “A sharp cutting edge and appropriate chipform geometry can help quite a bit in controlling the heat.”

According to Brian MacNeil, milling products and application specialist at Sandvik Canada in Mississauga, Ontario the parameters that affect heat are, in order of biggest impact:

1) Vc cutting speed, m/min (ft/min)

2) ae cutting width, mm (inch)

3) fz feed per tooth, mm/tooth (inch/tooth)

4) ap cutting depth, mm (inch)

(Photo: Courtesy of Makino)

After setting appropriate parameters, “the next thing we can do is introduce high precision coolant at the highest available pressure,” says MacNeil. “Grades and geometries can also play a part in reducing heat by using sharp ground geometries and thin PVD coatings that prevent edge rounding of the geometry itself. Programming techniques can also help to reduce heat.”

An article on the Sandvik Coromant website reads, “If steel were stiff modeling clay, titanium would be frozen Silly Putty.” It can be ‘gummy’ in other words. What challenges does this pose for cutting and how do you overcome them?

“Titanium is a very high strength material but has a low modulus of elasticity. This causes the material to want to move away from the cutting tool. Or causes the tool to deflect,” says MacNeil. “You want to use very positive tools with sharp edges to overcome this challenge.”

As a rule of thumb, is it a good idea to use lots of coolant when cutting titanium? Is it possible to cut titanium with minimal coolant?

“I do know that people are researching Cryogenic machining and Minimal Quantity Lubricant (MQL) machining. Sandvik is heavily involved globally in this type of research as well with a number of universities here in Canada. These [applications] are not mainstream or readily available yet. Until then the best and most productive results will always be with precise delivery of coolant under the highest possible pressure,” says MacNeil.

Have more of your customers been asking about titanium?

“Yes, we see it more and more. It used to be just the big aircraft companies or their first-tier suppliers [that used titanium], but now it’s getting into some of the smaller shops too. A lot of them don’t have much experience with titanium or some of the high temp alloys…if you’re a small shop, you want a tooling supplier that has experience with titanium, since [small shops] generally don’t have in-house [titanium experts],” says Ludeking.

Some experts recommend the following cutting speeds for titanium: 60 m/min for roughing and three to four times that when finishing. Would you agree? 

“Yes, I would agree if we are cutting Ti6Al4V which is an Alpha/Beta titanium. We tend to use it as the benchmark for speed and factor speed for other material from it. For example Ti-5553 which is a Beta Ti, we would use half the speed as we would for Ti6Al4V,” says MacNeil.

Titanium is very popular in the aerospace industry. Do you expect the use of titanium to increase for civilian and military aircraft?

“Absolutely. Titanium’s combination of properties makes it highly desirable for the demanding applications of aircraft. Use will almost certainly increase in the future…[titanium] applications are expanding. Its corrosion resistance brings it into some other areas that are not typical aerospace,” says Ludeking.

These other areas include automotive, medical parts and oil and gas drilling.

Some experts say Trochoidal milling (in which the cutter is programmed to move in a circular pattern) is the best milling solution for titanium. What’s your opinion?

“This is a very secure and efficient method of milling in HRSA. The low engagement (usually less than 10 per cent of the tool diameter) allows us to increase our feed and the short contact time in cut allows us to increase the speed,” says MacNeil.

“Trochoidal milling is one of the machining techniques [that] can be advantageous if it is applied correctly and uses the right cutting tool,” says Tom Hagan, milling product manager at Iscar Tools in Oakville, Ontario. “In many cases this technique does indeed significantly improve the situation. Material is machined by producing thin chips with high sfm. The tool must have the necessary cutting geometry and coatings but also must have the correct helix angle for smooth and stable cutting for titanium material.”

In your opinion, are carbide tools the best choice for cutting titanium? 

“With regards to finish and rough milling, in my opinion, no doubt, yes,” says Hagan. “Finish machining allowance (material to be removed per pass) is small, chips are thin and the heat does not affect the tool dramatically. In rough milling, low heat conductivity of titanium is a key factor and heat resistance of a tool material is very important. Tungsten carbide has higher heat resistance compared with high speed steel (HSS), even with rich cobalt (Co) content and coatings. However, the thermal shock resistance of HSS is greater than carbide…as a result, HSS and HSCo (high speed cobalt) milling tools continue to be widespread in industry.”

Do you have any tips for optimal machine tool set-up when cutting titanium? Should you only use rigid equipment?

“Overall rigidity is key, as well as part fixturing,” says Matthieu Guillon, global segment manager, aerospace and defence, Kennametal, in Latrobe, Pennsylvania.

“That is a Catch-22 question. Most of the parts in the aerospace industry are very complicated shapes that require large clearance in overhangs of the tooling. As for rigidity, rigid equipment is always preferable. However, in many cases we will machine non-rigid work pieces or have to deal with insufficient stiffness (poor work and/or tool holding, high overhang), or the machine tool is not in the best condition,” says Hagan.

What are the most common types of tool wear experienced when cutting titanium?

“Thermal cracks, edge breaking and chipping of the cutting edge,” says Hagan.

“Build-up and crater wear along with chipping are usually the primary wear mechanisms in titanium machining,” notes Ludeking. “To combat these and get the best tool life, you need a cutting edge and chipform geometry suited to titanium…to get the sharpest edge and avoid build-up, many manufacturers use uncoated carbide for titanium applications.”

Do you have any tips for tapping titanium alloys?

“Tapping titanium alloys such as Ti6Al4V is more difficult than tapping most alloyed materials, but certainly doable with the appropriate taps and techniques…tapping speed is critical for cutting threads in titanium alloys and will result in tap failure and/or shortened tap life if not followed. We recommend a tapping speed of 10-13 sfm both turning into the tapped hole and exiting out of the tapped hole,” says Mark Hatch, product director at Emuge Corporation in West Boylston, Massachusetts.

Any final thoughts about titanium?

“Best machining practices are critical to use when cutting titanium or any HRSA,” says MacNeil. “Simple habits such as rolling into cut, keeping tools in cut and rolling around corners are often overlooked and can give you a benefit in terms of tool life.”

And finally: “Don’t be afraid of it. Understand it. It’s not a black hole. We tool guys understand titanium and know how to deal with it,” says Hamil.