Machining titanium and titanium alloys

The metal cutting characteristics of titanium and titanium alloys are similar to those of austenitic steels. Machining by any of the various metal cutting methods poses no problems if the following points are observed:

1. the cutting edge of the tool is subject to high thermal loads due to the relatively low specific heat, thermal conductivity and density of titanium,
2. due to its low modulus of elasticity, titanium yields under the pressure of the cutting tool, and
3. titanium tends to weld to the tool.

Titanium must therefore be machined at low speeds, with relatively high and uniform feeds and generous use of cutting fluid; a sharp tool should be used which is clamped in the holder with a minimum of vibration. In addition it should be noted that any hard surface layers will result in increased tool wear and should be removed prior to machining e.g. by shot blasting and/or pickling.

Please refer to the attached table for guideline values on the machining of titanium and titanium alloys.

1. Turning and milling

Suitable tool materials for turning include high-cobalt high-speed steels, hardmetals and stellites. Tungsten carbide grades with cobalt binders have proved to be the most successful of the hardmetals, with the K20 group of materials displaying the longest edge lives.

Stellites allow higher metal removal rates than the high-speed steels and compared with hardmetals result in improved chip formation and lower reworking times. Hardmetal tools should have a rake angle between -6° and +6° for roughing and between 0° and 15° for finish turning. When using high-speed steels, rake angles of between 5° and 15° should be selected depending on loading. A clearance angle of around 7° is always advisable. The inclination angle on hardmetal cutting edges should be -4° and with high-speed steel cutting edges 0 - 5°.

Due to titanium 's tendency to weld to the tool during milling, climb milling is preferable to conventional milling. In this way, the comma-shaped chip is separated at the thinnest point and damage to the cutter from built-up edges and chip adhesion is reduced to a minimum.

The rake angle of the cutter should be 0 - 10° for high-speed steels and 0° for hardmetals and stellites, with a clearance angle of 12°.

2. Drilling

Titanium should be drilled at low speeds and high feeds on a rigid, vibration-free drilling machine using plenty of coolant. Chlorinated cutting oils are recommended for deep holes to reduce friction. Manual drilling should be avoided where possible.

High-speed steels and hardmetals are the materials used for drills. It is essential that the drills are ground to a point with a chisel edge no greater than 1.5 - 2 mm. The point included angle should be approx. 140° when drilling through holes so as to reduce the breakthrough distance. The clearance angle should be between 10 and 15°.

If chip choking is encountered, the rake angle must be increased by grinding a rounded edge, as choked chips can frequently result in welding between drill and workpiece.

3. Thread cutting

Cutting threads requires a rigid machine design and good tool holding. Good cutting conditions are achieved at peripheral speeds of 5 - 15 m/min (unalloyed titanium), 3 - 10 m/min (titanium alloys, annealed) and 1 - 5 m/min (titanium alloys, age-hardened).

Manual thread cutting should be avoided where possible, as uneven pressure will shorten the edge life of the tool. External threads should be cut on a lathe, as thread-cutting dies may seize and result in sub-standard threads. The thread depth should be increased gradually (0.25 - 0.5 mm per pass).

The tendency of titanium to smear and seize is particularly pronounced in tapping. For this reason it is very important to use a chemically active lubricant such as sulfurous cutting oil or compounds containing molybdenum sulfide or graphite. The tap should have a thick core and a shortened cutting edge. It should be strongly back tapered and flank relieved. The rake angle should be 5°.

If a tap breaks during cutting it can be removed using nitric acid, which dissolves steel but not titanium.

An alternative method of producing small-diameter internal threads is circular milling. The problems encountered when using taps, in particular with blind holes, such as chips impeding the cutting process, are eliminated with this method, in which the chips are removed during milling. Other advantages of thread milling are short machining times and reliable, high-quality production.

Good results when using circular milling to cut M8 internal threads with a depth of 2 x diameter in components made of TiAl6V4 titanium alloy have been achieved with a TiCN-coated thread milling cutter (flute pitch = 7°). A cutting speed of 100 m/min and a feed rate of 670 mm/min have proven successful. Manufacturing a thread of the above-mentioned dimensions took approx. 10 s.

4. Sawing

Titanium can be sawed without any difficulty using conventional hacksaws. Strong contact pressure and a generous cutting fluid supply are recommended. Best results are achieved with coarse (four tooth/inch) high-speed steel or hardmetal-tipped saw blades. A finer-toothed blade is recommended for thin-walled components. Sawing speeds around 25% lower than for steels can be used. Higher cutting speeds are achieved using circular saws. Suitable band saws can of course also be used.

It is particularly important when sawing that any hardened surfaces first be removed by shot blasting, grinding or turning, otherwise the saw blade will become blunt right at the beginning of the cutting process, which not only extends sawing time but also runs the risk of cutting off-line.

5. Grinding

The chemical and physical properties of titanium are most noticeable during grinding. The relatively high coefficient of friction produces high temperatures during grinding which lead to chemical reactions between the metal and the abrasives, resulting in burning and smearing of the surface of the workpiece. Localized overheating dulls the abrasives relatively quickly, which then merely slide over the surface. Even if there is no visible burning of the ground surface, there may be surface stresses which can lead to cracks and compromise durability.

Due to their good heat transfer and bond properties, vitrified-bond aluminum oxide and silicon carbide wheels of grade J, K or L have proved successful. While silicon carbide wheels generally provide a superior finish, the lower grinding speed of aluminum oxide wheels causes less residual stress in the workpiece.

A disadvantage of silicon carbide wheels is their higher sparking, necessitating a higher coolant delivery. Aluminum oxide wheels cause less sparking, but the abrasive grains on these disks dull more quickly and the wheels have to be dressed more frequently. Resin-bonded diamond wheels can also be used, preferably grinding wheels with tough rather than brittle abrasive grains.

The best results using aluminum oxide wheels are achieved at grinding speeds of 5 - 10 m/s. In contrast, silicon carbide wheels can be used at grinding speeds of 20 - 30 m/s.

Generous amounts of coolant should be used when grinding. Oil emulsions are recommended for fine finishes with low roughness heights. There is a risk of wheel adhesion for grains finer than 320.

It should be pointed out that from a specific particle size downwards, titanium reacts very easily with oxygen and burns to form titanium oxide. For this reason a certain amount of care is required when treating and handling titanium swarf.

6. Coolants

The most commonly used coolants for machining are water-based solutions (rust inhibitors or emulsion oils). The following coolants have proved successful for cutting titanium materials:

1. 5% aqueous solution of sodium nitride
2. 5 - 10% aqueous solution of water-soluble oil
3. Sulfurized or chlorinated oil - for use at low cutting speeds. Chlorinated coolants increase susceptibility to stress corrosion cracking and the formation of surface cracks when heated to above 200°C. For this reason the workpiece should be cleaned after machining.
4. Spirit - for use at low metal removal rates and cutting speeds.

7. Literature

Further details on the machining of titanium and titanium alloys can be found in the publications listed below:

1. W. Kreis, K.-H. SchröFder "Zerspanung der Titanwerkstoffe" Metall 29 (1975), H.1, p. 58/62
2. J.F. Kahles, M. Field, D. Eylon, F.H. Froes "Machining of Titanium Alloys" Journal of Metals (April 1985), p. 27/35
3. G. Calea, D. Drimer, C. Ionescu "Bearbeitbarkeit von Titan und Titanlegierungen" Zeitschrift für industrielle Fertigung 75 (1986), p. 616/20
4. N. Kumagai, K. Kamei, S. Inoue "Grinding of Titanium with Jet Infusion of Grinding Fluid" Société Francaise de Métallurgie, Vol. 4, p. 1841/46 Proceedings of the Sixth World Conference on Titanium 1988 Published by les éditions de physique
5. C.A. Brookes, R.D. James, F. Nabhani "Aspects of Ultra-hard Cutting Tool Wear in the Machining of Titanium Based Aerospace Alloys" Titanium & Aluminium, p. 183/94 Proceedings of the International Conference, Paris 27-28 February 1990 Published by IITT-International, France
6. B.K. Subhas "Some Experiment Studies on Chemical Reactivity in Machining of Titanium Alloys" Proceedings of the 1990 International Conference of Titanium Products and Applications, Vol. 1, p. 209/15 Published by Titanium Development Association, Dayton, Ohio.

Deutsche Titan, Nov. 2000

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