Lightweight and strong, titanium has been a major boon for the aerospace industry, but the difficult-to-machine material can create major headaches. And, the headaches are not going away. Demand for titanium materials in the aerospace industry is rising with experts predicting that titanium usage will increase by an average of 20% throughout the next five years.
Aircraft makers use titanium for highly stressed structural components such door frames, landing-gear mounts, and wing attachments. These components typically have a high machining content, and the volume of swarf produced tends to exceed that of the finished component. This is primarily because a large number of pockets are cut from solid forgings – deep and shallow pockets, 5-axis pockets, triangular pockets – often with very thin walls or bottom faces.
When machining titanium, the material’s low thermal conductivity and its tendency to vibrate require a very rigid machine as well as optimum cutting edge preparation. These pose a huge challenge for milling – the most logical machining technology for these structural components – coupled with either solid carbide milling cutters or indexable cutters with the right inserts.
The main focus is on process reliability since these structural components are extremely expensive. At the same time, cost pressures demand high machining rates and high productivity performance.
These problems are very familiar to Dirk Masur, component manager for aerospace for Walter. They don’t lend themselves to piecemeal solutions he notes, but demand a comprehensive, system-wide approach.
“We focus on complete solutions. This means that we coordinate all aspects, such as the tool design, CAM program, and machining strategy – with the application in accordance with best practice consideration,” Masur says.
The result is modern tool concepts matched to the tasks in question, such as roughing, semi-finishing, or finishing.
Equally important in the development of productive and reliable titanium machining strategies, Masur says, is knowledge of the components being machined, and, more broadly, knowledge of manufacturing itself. Mazur says his team has built these up throughout years of experience.
“Our expertise is based on intensive partnerships with focus customers, universities, and machine tool manufacturers. Together, we are continually developing the machining operations for key components,” Masur says. “The machining operations are simulated and verified before the handover. This means that the user does not have any unpleasant surprises in terms of cost-effectiveness or process reliability.”
Testing in titanium
Walter demonstrated the performance of its current tool generations on a generic component made of a commercially available titanium variant, Ti6Al4V, the most frequently used type of titanium in the aerospace industry. However, Ti-5-5-5-3 and Ti-10-2-3 are also increasingly being used – for example, in landing gear components – and both materials have a higher thermal stability than Ti6Al4V and can only be machined with reduced cutting speeds. Walter designed test components with a range of typical pocket shapes which are the same as or similar to those found on components prevalent in the industry today.
The typical test includes tasks for a quartet of tools: The M3255 tangential helical milling cutter, the M4002 high-feed face milling cutter (roughing operations), the Ti40 solid carbide end milling cutter, and the modular ConeFit style with Ti50 changeable head (semi-finishing and finishing operations). This combination occurs on the plant floor, for example, when machining landing gear mounts.
The coatings, macro geometry, and micro geometry of the two solid carbide tools are designed for titanium machining. The M3255 and M4002 milling cutters were given indexable inserts made from the Walter WSM45X cutting tool material, with tough CVD coating suited to ISO S and ISO M applications.
High dynamic cutting
High dynamic cutting (HDC) – a machining strategy that uses constant tool engagement to lower vibration – improves cost-efficiency and process reliability. Machining conditions remain constant including the forces applied to tool cutting edges and the stable process temperature. Corresponding functions are provided by modern CAM software. HDC results in a higher tool life at a lower tool wear rate, enabling longer cutting periods, higher machining volumes, and lower energy consumption at the spindle. The Walter Prototyp Ti40 solid carbide milling cutter is suitable for this machining strategy.
What about aluminum?
Despite increasing use of titanium and composites, aluminum remains integral to modern aircraft. This is partly because of development of new alloys with improved properties, with the current trend toward aluminum-lithium (Al-Li) wrought alloys.
Al-Li alloys are lighter than other Al alloys and have a higher modulus of elasticity – welcome properties for the aerospace industry. Workpieces are often similar to those made from titanium. Both have a large number of pockets and high volumes of swarf. The main difference is that aluminum machining demands a high-speed cutting (HSC) process. Surface speeds higher than 10,000fpm when milling are common, and lower cutting speeds can result in the formation of build-up, lowering tool life. Working with aluminum also requires a great deal of expertise and component knowledge, especially when developing cost-effective, reliable processes.
Walter recently introduced a milling cutter tailored to aerospace requirements: the M2131 ramping milling cutter with 90° indexable inserts specializes in ramping and pocket milling. Milling bodies have high concentricity to provide indexable inserts with protection against centrifugal force. The milling cutter is pre-balanced to guarantee process reliability in HSC machining.
Walter’s WNN15 grade, manufactured using high-power impulse magnetron sputtering (HiPIMS), features an extremely resilient and smooth PVD coating that reduces friction and edge build-up. It also resists flank-face wear and has a high degree of cutting edge stability.
“Field tests have confirmed the technological advantages of the new indexable inserts in comparison to standard types,” explains Wolfgang Vötsch, product manager for milling at Walter. “Our application engineers have achieved increases in tool life of up to 200% with ease. In one case, we even succeeded in reaching an increase of almost 400%.”
“Our commitment to the aerospace industry demands that we use all the technical and intellectual tools at our disposal to develop intelligent machining strategies,” Masur sums up. “We don’t know exactly what tomorrow’s machining strategy will look like but we do know it will be even more effective than today’s.”
Walter USA LLC