Catching ’22’: Beat the challenges of titanium

Applications for machining titanium alloys are on the rise, especially in the aerospace industry. This growth results from more carbon fiber-reinforced polymer being put into aircraft for weight reduction purposes, according to Scott Walker, chairman of Mitsui Seiki (U.S.A.) Inc., Franklin Lakes, New Jersey. He explained that the composite material increases a plane’s susceptibility to electrolysis erosion when introduced into structures that traditionally incorporated a significant amount of aluminum. “A battery is basically carbon fiber, aluminum and water. That is why titanium has migrated. When you had all aluminum, you did not have that issue.”

Most titanium machining applications involve Ti6Al4V (6% aluminum and 4% vanadium) while others typically need Ti-5553 (5% aluminum, 5% molybdenum, 5% vanadium and 3% chromium) and 10-2-3 titanium (10% vanadium, 2% iron and 3% aluminum).

A Mitsui Seiki 5-axis trunnion horizontal machining center machines a titanium aerospace component. Image courtesy of Mitsui Seiki

“A lot of aircraft guys have tried to steer away from 5553 and 10-2-3 because of the difficulty and cost of machining it,” Walker said. “However, those materials are still required on the high-stress areas on the aircraft, like engine mounts, side body cords and landing gear components.”

To help jet engines run hotter and lighter than current designs, titanium producers are developing and testing new grades that might be even more challenging and costly to cut, he noted.

In the Cut
Effectively machining titanium, an element with the atomic number 22, begins by having the proper cutting tools. One toolmaker with offerings targeted at titanium is Allied Machine & Engineering Corp., Dover, Ohio. Other than Allied’s 4TEX indexable carbide drill, the best option for drilling titanium, according to Field Sales Engineer Nate Craine, is Allied’s APX drill with a special replaceable insert.

Although some end users consider titanium a heat-resistant superalloy, that’s not the case. “A lot of people like to lump titanium into the same category as high-temp alloys, but it’s its own breed of material that needs to be treated differently,” Craine said.

He emphasized it’s critical to understand that friction and inadequate chip evacuation cause titanium to workharden while being drilled. As a result, a titanium workpiece with a hardness of 32 HRC might workharden to 50 HRC inside the hole, which makes every process that follows drilling more complicated. “You could potentially destroy a boring tool,” Craine said.

During tool life testing at a customer’s facility, this 2.5″-dia. drill produced six holes per T-A insert for a total length of 90″ and two to three holes/edge with a –PWHR insert or four to six holes/edge with a –PW insert when drilling Timetal 17 (Ti5Al2Sn4Mo2Zr4Cr) with a hardness of 34 HRC at a cutting speed of 82 sfm and a feed rate of 0.0035 ipr. The high-strength, deep-hardenable forging alloy is primarily used for jet engines. According to Allied Machine, chip formation was much better when drilling at 131 sfm and 0.0055 ipr, but tool life can drop if the machining parameters are excessive. Image courtesy of Allied Machine & Engineering

One way to help avoid that problem is by selecting a tool with the proper relief angle so it doesn’t rub the workpiece material. “In titanium, you have the propensity for the material to pinch the side of the drill, so we developed a geometry designed for greater success,” Craine said.

Although that application-specific design requires the drill to run at a lighter feed rate, most titanium aerospace applications, for example, don’t have a need for speed, Craine added. “The material costs are more of a concern. If you can get that job done without damaging the part, then that’s a success.”

In addition to the relief angle, a drill must have the correct rake angle and hone to effectively cut titanium, noted Kevin Vanderbeck, East Coast field engineer for Allied Machine.

However, producing a cutting edge with the correct hone is a balancing act, according to Craine. Too much of a hone and the tool performs like it is dull, while an edge that is too sharp is prone to chipping.

To further enhance its T-A drills, Vanderbeck said Allied Machine performs a proprietary process “affectionately called the ultragrind” to create the ideal cutting edge for increased tool life and chip control. “That process minimizes fracture points along the cutting edge,” he added.

Without proper chip formation, titanium chips can become like ribbons and hard to evacuate. “If you are drilling anything more than three times diameter, that long ribbon will bind up and seize your tool,” Vanderbeck said. “It goes bad real quick, and a lot of times you don’t have time to hit that e-stop. By the time you see it, it is too late.”

Keep It Cool
Tool coatings and coolant play important roles when combating the high level of heat generated from cutting titanium. For coatings, Vanderbeck recommends TiAlN or Allied Machine’s proprietary AM200 or AM300. The advantage of AM200, he noted, is that the coating achieves the same level of lubricity to overcome built-up edge as TiAlN but at a lower temperature.

Craine added that Allied Machine offers polished inserts with AM300 coating to minimize the chance of titanium sticking to the coating.