Milling Titanium

The aerospace marketplace of today is very demanding as manufacturers continue to look for ways to produce more fuel-efficient aircraft.

Strength-to-weight ratios are more important than ever in the race to create the next big aerospace platform.

This means that new materials are constantly being developed to create parts for these platforms. Today’s aluminum, composite, INCONEL®, stainless steel, and titanium parts must not only be lighter, but stronger as well. The issue is that these materials can pose challenging problems during the machining process.

Titanium usage in aircraft production has increased recently thanks to the metal’s inherent strength and corrosion-resistant. In the Boeing 787 Dreamliner, for example, 14 percent of the aircraft is made from titanium parts, up from 7 percent in the company’s previous model, the 777-200.

Titanium 5.5.5.3

While the milling of titanium alloys is never an easy task, the problems are compounded when the material is a relative newcomer like today’s golden child, titanium 5.5.5.3 (Ti5Al5V5Mo3Cr).

During the chemical analysis of this relatively new titanium alloy, researchers noted that the oxygen content is lower than many other titanium alloys. They deduced that it was this property that most likely enhances the ductility and toughness of the alloy, making it more difficult to cut.

“Ti5.5.5.3 allows aircraft manufacturers to design much thinner wall sections due to its improved stability and strength over Ti6Al4V products,” explained Kevin Burton, product and application manager - milling and deep hole drilling for Sandvik Coromant.

What makes this titanium alloy different from its predecessors is the addition of other elements into the composition.

Titanium 6Al4V’s makeup, for example, is 6 percent aluminum and 4 percent vanadium. Ti5.5.5.3, however, is composed of 5 percent aluminum, 5 percent molybdenum, 5 percent vanadium, 3 percent chrome, among others.

“We can see that there are different elements, especially the chrome and vanadium, that make it harder to cut,” explained Louis-Jacques Boucher, Sandvik Coromant business development, aerospace segment, for Canada. “Harder to cut means there is more friction, which equals heat generation.”

Milling Titanium Diagram

Rolling into the cut keeps the chip thin on exit, reducing vibrations and giving the best tool life.

Because titanium is so heat-resistant, the heat from the cutting process is not dissipated by the chip, but is absorbed by the cutting tool.

“It’s a material that has to be machined at lower cutting speed (Vc) and feed rate than steel or aluminum,” said Boucher. “Also, titanium is not hard but it is abrasive. Therefore, the cutting parameters, when compared to steel and aluminum, are drastically reduced. If the proper machining techniques are not applied, poor tool life will result.”

Even when comparing the various titaniums on the market, there are marked differences in the machining recommendations. This is because some titanium alloys have had crystals added to add strength to their basic composition.

“If we take the Ti6Al4V as our 100 reference, the new series of titanium alloys such as Ti5.5.5.3 and Ti10.2.3 will have to run at -50 percent of the recommendations of the Ti6Al4V,” said Boucher. “The Vc selection will have the most important impact for tool life and productivity. Our starting value will be between 40 and 60 m/min. for roughing based on Ti6Al4V and can go up to 150 to 200 m/min. for finishing. Again, these cutting parameters have to be reduced when we are cutting the new generation of titanium.”

Also, a trial-and-error methodology does not tend to work well when cutting this material because of its high cost.

Problem-Solving

“Cutting speed is affected by the arc of engagement (ae) of the cutter,” explained Burton. “The larger the engagement, the lower the speed has to go. This is all due to heat generation at the cutting edge. In long edge milling, for example, we prefer an ae of around 15 percent maximum.”

To overcome the unique challenges of this material, choosing the right machining techniques and tool path can be as important as the geometry and coating of the tool.

“For example, rolling onto the surface in face milling can give up to three times the tool life when compared to moving straight onto the surface,” explained Burton.

Understanding the machining process as a whole is very important when selecting the proper tool for machining titanium.

Geometry, Edge Prep, Coolant

When milling titanium a positive geometry combined with a good edge preparation to protect the edge is necessary. However, depending on the type of part and cutting conditions, the edge preparation will vary.

To maximize stability, the machine tool should be set up as rigidly as possible, with the tool as close as possible to the spindle. And, of course, coolant must be used.

Because of the intense heat that is created at the cutting edge, using coolant will help avoid edge buildup, hammering at the cutting edge, plastic deformation, and microthermal cracks at the surface of the insert.

Tools with through-spindle coolant can deliver coolant directly at the cutting edge, where it is needed the most. Also, using a high-pressure coolant pump (for example, 70 bars and higher) is essential when cutting a deep pocket. It will help stop the chips from welding to the cutting edge and ensure that they are not recut.

“Because the cost of the material is so high, manufacturers cannot afford to create scrap,” explained David Vetrecin, rotating tools product manager for Iscar Canada. “There are some aerospace parts that spend many hours being machined, and the last thing you want to do is have an insert fail, causing a scrap part to be produced.”

As the cost of material rises, and as the machining process becomes more complex, an examination of the cutting process as a whole is necessary to determine the correct tool and tool path.

“When cutting titanium you must examine the fixturing, machine tool setup, spindle condition, and coolant usage as well as tooling to be used,” said Vetrecin.

Generally speaking, people use a machine tool that can produce high torque and low RPMs and is as rigid as possible. This can help eliminate unstable cutting conditions that can produce chipping and failure of the insert.

“If there is an increase in spindle load or if chip formation is not normal, chances are your inserts are failing,” said Vetrecin. “This means that you may need to re-examine not just your insert choice, but your machining setup as well.”

Titanium 5.5.5.3 is a complex material. Lead-times for getting this alloy are longer than for more common materials, but manufacturers, especially those in the aerospace and power generation supply chains, will be seeing more of it coming through their shops soon.

“I would guess that in the next five to eight years titanium 5.5.5.3 and materials like it will start replacing older versions of titanium,” said Vetrecin. “There is just too much upside to ignore.”

For more information, visit www.coromant.sandvik.com and www.iscar.ca.