Titanium machining on a grand scale

Titanium has always placed particular demands on tools and machines during cutting. In recent years, titanium 3.7165 has become prevalent among lightweight materials for its outstanding properties of strength, corrosion resistance, and a capacity to withstand stresses. Containing 6% aluminium and 4% vanadium, the alloy – usually called Ti-6Al-4V – remains one of the most difficult materials to process in machining.

Specific applications call for specific alloys. For example, landing-gear manufacturers often use titanium 5553 from VSMPO-AVISMA Corp. because it is strong, tough, and less susceptible to structural changes from heating.

Ti-5553 is also one of the hardest materials on the market to machine. Problems – such as point heat due to poor conduction, associated chemical changes in the material such as embrittlement at higher temperatures, and the formation of built-up edges – occur to a greater extent with it than with other titanium alloys. That means matching cutting speed, feed rate, and penetration depth is important to avoid damaging Ti-5553. According to WFL experts, manufacturers should not exceed a cutting speed of 135ft/min. because severe shear stresses can develop at speeds as low as 180ft/min.

Using suitable cooling lubricants is just as important as the correct cooling strategy. A quick and continuous removal of swarf must be guaranteed; the heat dissipation occurs to a much greater extent via the tool. Removal of the forging skin – also called elephant skin – is an additional challenge with this material. The upstream forging process and resulting thermal and metallurgical influences give this skin very high surface hardness.

The low modulus of elasticity means that titanium tends to evade the pressure of the tool and to fuse with the cutting edge. Low cutting speeds, a relatively high and even feed rate, and the proper tools reduce the risk of fusion.

Vibration-free, fixed, sharp tools must in any case be ensured. High speed steels (HSS) with a high cobalt content, carbide, or stellite are used as cutting materials.
 

How it works

For roughing and rough turning, the front rake angle should be between -6° and 6° and between 0° and 15° for final turning. The angle of clearance should always be around 7°. For carbide, the angle of inclination should be -4° and for HSS steels it should be 0° to 5°.

As titanium tends to fuse with the tool, climb milling is preferred to conventional milling. The wedge-shaped swarf is thus separated at the thinnest point and damage to the milling cutter is reduced. For HSS, the front rake angle of the milling cutters should be 0° to 10°, and for carbide and stellite, it should be 0° with an angle of clearance of 12°.

The particular material properties of titanium become apparent during grinding. The relatively high-friction coefficients produce high temperatures during grinding that result in chemical reactions between the metal and the abrasive grain, leading to burning and smearing of the workpiece surface. Local overheating blunts abrasive grains fairly quickly, causing them to just slide over the surface. Even if the ground surface is not visibly burnt, surface tensions may be present, leading to grinding cracks that affect fatigue strength.

Water-based solutions, such as aqueous sodium nitride or water soluble oil, are normally used as cooling lubricants. Sulphurated or chlorinated oil can also be used at temperatures below 390°F, in which case the workpiece must be cleaned following machining.
 

Experience is the decisive factor

All the above demonstrates that experience is required in the selection and use of the tools and machining strategies.

A current WFL project involves the processing of landing gear parts for commercial aircraft. In such cases, it is customary to request a concept study from the supplier of the processing machines, that includes the essential processing steps. The blanks for the components are usually large, cost-intensive forged parts.

It is essential that the ability to cater for critical aspects of machining during the manufacture is shown early in the conceptual phase. For example, consider that different material thicknesses in the blank workpiece require modified machining strategies. Heat-affected zones must also be taken into consideration together with the cutting forces that occur. In the elephant skin, these are generally around 70% to 80% higher than those of hardened steel.

“Materials that are hard to cut like titanium have influenced the development of the WFL machines. We provide individual solutions for just these kind of demanding applications, which also include topics like cooling and production strategy as well as the actual machine,” explains Dieter Schatzl, marketing manager at WFL.

Reinhard Koll, WFL applications engineering manager, adds, “We are in a position to adapt the MILLTURNs precisely to the requirements of our customers – such as the emergency retraction, for example, which retracts the tool from the workpiece immediately if a power outage occurs, thus preventing damage to the workpiece.”
 

Harmonious overall concept

WFL pays particular attention to implementing customer requirements in such a way that they become an integral part of the overall concept. It is possible to respond to the particular requirements of customer projects through a special design for guides and spindles and to adapt the machine components to the respective loads.

Cooling is especially important where titanium machining is concerned. As titanium burns at temperatures above 1,600°F, appropriate fire and explosion protection must be ensured. The advantage of the individual spindle development is also visible in the coolant feed; the cooling lubricant can be fed directly to the cutting edge via the milling spindle with a pressure of up to 2,900psi. An additional factor is that it is also possible to measure the workpieces inside the WFL multitasking centers when work is in progress.

“The customer wants a machine where processing is secure. This also involves support in the choice of tooling and above all in the machining strategy, as just one reject part represents a huge economic loss with workpieces of this size,” Schatzl states.

WFL Millturn Technologies Inc.
www.wfl-usa.com

IMTS 2014 booth #S-8666