The machinability of the commercially pure grades of titanium is similar to that of annealed austenitic stainless steel, while in the same respect titanium alloys are more comparable with the harder steels of similar strength level.
Thermal Effects on Machining
In general, machining difficulties with titanium originate from high cutting temperatures, chemical reactivity with tools and the relatively low modulus of elasticity of the material. Thus titanium produces a thin chip moving at high velocity over the tool face on a small contact area. A high contact pressure and low thermal conductivity result in an unusually high tool tip temperature. The high reactivity of titanium leads to galling and wear of the cutting tool. The relatively low elastic modulus of titanium can cause slender parts to deflect more than steel, giving rise to difficulty in maintaining tolerances and to problems caused by tool rubbing.
Lubrication During Machining
Flood lubrication with an appropriate cutting fluid brings down contact temperatures. For example, sulphurised or chlorinated additives reduce friction on a steel tool but should be carefully removed as soon as possible to eliminate the risk of stress corrosion, particularly during any subsequent heat treatment.
Machine tools should be in good condition and capable of maintaining a positive feed at a slow cutting rate. The cutting tip should be properly ground and sharp, and should never ride or dwell in the cut without removing metal. The tip should be retracted when returning across the workpiece.
In milling, the cutter is effective for only part of each revolution. During the cutting period, smearing of the titanium and galling of the teeth ultimately result in wear or chipping of the tool. During the rest of the revolution, the chip remains tightly welded to the tooth and is entrained with the next cutting stroke. This can to some extent be reduced by using a rigid set-up and climb milling which results in a thin exit chip more readily detached from the tooth. Water based coolants will reduce cutting temperatures and minimise galling.
Conventional high speed steel cutters can be used for low volume production of small parts and other light operations. Carbide tipped cutters are more useful for high production rates or extensive metal removal operations as in face milling or slab milling. When starting a new piece of work, it is always advisable to use a slow cutting speed and build up to higher speeds as experience indicates.
Turning and Boring
These are not particularly difficult operations when the correct cutting conditions are used. Chip breaking devices are desirable. Experience suggests that high speed steel may be best for form cutting, plunge cuts or interrupted cutting, but carbide tools are normally used for continuous cutting jobs, high production items or extensive metal removal operations. Non-ferrous cast alloy tools are suitable for severe plunge cuts, machining to dead centre and producing narrow grooves.
As with other machining operations, it is best always to use constant, positive feeds, to avoid dwelling in the cut and never to stop or slow down in the cut. Cutting fluids help cool the tool and aid in chip disposal. Dry cutting is not recommended except for avoiding chip contamination if the swarf is to be reclaimed. A full steady flow of water-based coolant is the most satisfactory type of fluid, a 5% solution of sodium nitrate in water gives good results or a 1 in 20 emulsion of soluble oil in water.
The galling tendency of titanium, accentuated by high cutting temperatures and pressures can result in rapid tool wear leading to out of round, tapered or smeared holes, and possible tap breakage in holes that are to be subsequently threaded. These difficulties can be minimised by using short, sharp drills, by supplying copious cutting fluid to the cutting zone, by using low speed and positive feeds, and by solidly supporting the workpiece, especially on the exit side where burrs would otherwise form.
A heavy duty stub-type screw machine drill is recommended for workpieces other than sheet. For drilling deep holes, oil feeding drills, or a series of short drills of various lengths, may be used in sequence. The drill should be kept cutting and never allowed to ride in the hole without cutting metal. Low and constant cutting speeds should be maintained throughout the course of drilling. Chips should be removed at regular intervals unless the coolant flow is sufficient to ensure this. Care is needed when drilling through holes, when it is often advisable to retract the drill just before breakthrough in order to flush the drill and hole and remove the chips. The final breakthrough is then performed smoothly under positive feed.
This operation is the one most likely to give problems with titanium. A limited chip flow and the severe galling tendency of titanium can result in poor threads, incorrect fits, tap seizures or broken taps. Titanium also tends to shrink onto the tap at the completion of a cut. Designers should not specify blind holes or through holes of excessive length, in both of which chips can become confined and thus cause rough threads or broken taps. Some relaxation in fit tolerance may be permissible, or difficulties can be minimised by reducing thread requirements to 55 to 65% of full thread and only tapping the fewest threads that the design will allow.
Tap design can sometimes be improved by using interrupted threads with alternate teeth missing, by grinding away the trailing edges of the tap, by grinding axial grooves in the thread crests along the length of the lands, and by employing either eccentric or concentric thread relief. Other suggestions include a spiral angle large enough to allow chip flow out of the hole ahead of the tap, and a relief angle large enough to prevent seizure but not so large that jamming occurs when backing out. Surface treatment of the tap by nitriding, oxide coating or chromium plating has proved successful in reducing galling and abrasion.
A slow speed is essential, particularly with the stronger alloys to reduce cutting torque. Some of the paste type cutting compounds give good results, while a flood of sulphurised mineral oil is also satisfactory. Soluble oils appear to be less useful in tapping operations.
Grinding of titanium can present difficulties since the metal has a tendency to load the wheel, which can lead to higher temperatures at the metal/wheel interface, high residual stresses in the ground surface and a generally unsatisfactory surface finish. These difficulties can be largely eliminated by the choice of suitable wheels, by the use of lower wheel speeds and feed rates, and by flooding the grinding area with an inhibitor or cutting fluid type of coolant. The best surface finish is obtained with silicon carbide wheels which can be used at superficial speeds of 700 to 1200 m.min-1.
A medium grit size (60 to 80), with a medium hardness, medium structure, vitrified bond, used with a highly chlorinated or sulphurised cutting oil normally gives good results. Aluminium oxide wheels must be used at lower speeds such as 550 to 600 m.min-1 and similar grit characteristics to those given for silicon carbide are suitable. Whenever coolant is used, good filtration is necessary to eliminate surface defects caused by circulation of titanium particles.
After grinding, pickling should be carried out in order to remove the surface layer of metal. Provided that the surface finish is good, subsequent inspection will show up any overheated areas as bright patches.
This can be a convenient and rapid method of cutting rod or bar when the cut surface finish is not particularly important. Rubber bonded silicon carbide wheels can be used, with superficial speeds of 2500 to 5000 m.min-1. A copious flow of a 10% solution of nitriteamine rust inhibitor or a soluble oil suspension is essential to keep temperatures down and avoid burning of the metal and rapid wheel wear.
Conventional electrochemical machining electrodes made from copper, brass, stainless steel or copper-tungsten alloys are applicable to the electrochemical machining of titanium. Electrolyte composition determines the effectiveness of the operation and, for titanium, formulations based on the use of sodium chloride have been found to be effective.
Metal removal is by anodic dissolution and so the workpiece is not subjected to hydrogen discharge. Thus there is no danger of hydrogen pick-up with consequent loss of ductility when titanium is machined by this method. It has little effect on mechanical properties in general although the fatigue strength may be slightly lower than that resulting from the use of mechanical finishing methods. This is because these often impart compressive stresses to the metal surface whereas electrochemical machining tends to remove stressed layers.
Chemical milling of titanium consists of four main operations:
1. Cleaning or surface preparation
3. Chemical etching
4. Rinsing and stripping
The cleaning of titanium surfaces serves two purposes, firstly the removal of oil, grease and other organic contaminants, and secondly the removal of oxides. Elimination of oil and grease is essential for uniform mask adhesion and is normally accomplished by vapour degreasing followed by alkaline cleaning. Removal of the surface oxide is required in order to provide a uniform surface for etching. Light oxide can be removed by pickling in a mixture of nitric and hydrofluoric acids, while with heavy oxide scales the pickling must be preceded by grit blasting or conditioning.
Masking entails application of an acid resistant coating to protect those parts where metal removal is not required. The mask is usually applied either by dip, spray or flow coating techniques, the particular method chosen depending upon the shape and size of the component to be machined. Vinyl polymers or Neoprene elastomers have been found to be the most successful for titanium. Patterns on the masked workpiece are generally applied by means of templates, followed by scribing or cutting of the mask with a special knife and then manual peeling to expose the area to be etched.
A good etching solution should be capable of removing metal at a uniform and pre-determined rate without adversely affecting dimensional tolerances or mechanical properties. Solutions are normally mixtures containing hydrofluoric acid. Typical production tolerances for chemical milling of titanium are 0.05-0.13 mm. After etching is complete, the components should be thoroughly washed and the masking compound removed either by hand or by use of a suitable solvent.
Electrodischarge machining of titanium requires operating gaps between the tool and the workpiece that generally range from 0.05 mm to about 0.4 mm. The smaller gaps are normally used for finishing work where a smooth surface is required while the larger gaps are for roughing work carried out at higher metal removal rates.
Selection of the electrode material is based on consideration of a number of factors. For titanium, the best metal removal rates and workpiece and tool wear rates have been obtained with copper and zinc electrodes. However, in practice either brass, tungsten, copper or graphite are normally used. The dielectric material recommended is one of the hydrocarbon fluids but where there is contamination of the workpiece surface by carbon, use of demineralised water or some other non-carbon containing dielectric should be considered.
There is the possibility of microcracks being formed when titanium is electrodischarge machined and so care must be taken to inspect the component after machining has been carried out.
As in most machining operations, surface speeds in the sawing of titanium must be kept low and continuous positive feed rates employed. Coarse pitched high speed steel blades having a pitch of 4.2 to 8.5 mm have been found to be the most satisfactory type.
Band sawing of titanium is possible on either horizontal or vertical machines. The thickness of the workpiece determines the blade pitch, the thicker the material, the greater the blade pitch. Positive feeds have to be maintained and a coolant is required.