Titanium and Titanium Alloys - Cutting, Grinding and Machining of Titanium Alloys

Cutting of Titanium Alloys

Titanium alloys can be cut cold by conventional power hacksaws, circular saws, band saws, shears, nibblers or water jet. Hot cutting using oxy-acetylene torch, plasma or laser will result in oxidation of the adjacent metal and for many applications this will require to be removed by grinding or machining before further processing is undertaken.

Alpha Case

Titanium components which have been hot worked, heat treated or otherwise exposed to air at elevated temperatures will have both a substantially thick oxide film and frequently a thin underlying oxygen enriched metallic layer known as the alpha case.

The ‘case’ is hard and brittle and must be removed before components are put into service. Normal methods of removal include shot blasting and pickling, hot salt bath descaling and machining, grinding and the like.

Grinding

In grinding, the difference between titanium and other metals is the activity of titanium at higher temperatures. At the localized point of the wheel contact titanium can react chemically to the wheel material. The important factors top consider in order to prevent this are:

Effective use of coolants – Water based soluble oils can be used, but in general result in poor wheel life. Solutions of vapour phase rust inhibitors of the nitrite-amine type give good results with aluminium oxide wheels. Coolant should be continuously filtered to remove fine solids. Coolant changes may need to be more frequent and regular than for grinding of steels.

Correct wheel speeds – A good general rule is, use a half to one third of conventional operating wheel speeds to get the best results with titanium

Grinding wheel selection – Silicon carbide can be used at 1200-1800 surface m/min (4000-6000 ft/min) to give optimum surface finish and minimum wheel wear. The high speeds essential with these wheels produce intense sparking which can cause a fire hazard. The work piece should be flooded with coolant to reduce sparking. Vitrified bond A60 wheels, hardness J-M have been successfully used at speeds of 450-600m/min (1500-2000 ft/min) while removing as much as 1.3 cubic centimeters (0.08in³) per minute.

Surface Grinding

Using a sharply dressed wheel, the largest wheel diameter and thickness available, harder wheels having ample power available and reduced wheel speeds (e.g. 280m²/min, 301ft²/min) will improve titanium surface grinding operations.

Recommended abrasives include silicon carbide for cut off and portable grinding and aluminium oxide wheels for cylindrical and surface grinding.

To minimize residual ground surface stresses recommended down feeds are: 0.025mm (0.001in) per pass to 0.05mm (0.002in) then progressively reducing to 0.013 mm, 0.010mm, 0.007mm, 0.005mm, 0.002mm per pass.

For form grinding standard grinding oil is recommended and for other operations nitrite-amine based fluids have been used successfully.

Machining of Titanium

Machining of titanium  on conventional equipment is considered by experienced operators as being no more difficult than the machining of austentic stainless steel.

Different grades of titanium varying from commercially pure to complex alloys do have different machining characteristics as do different grades of steel or different aluminium alloys. However, provided the following measures are given due consideration, little difficulty should be experienced.

Turning

Disposable carbide tools should be used whenever possible for turning and boring since they increase the production rates.

Where high speed steel tools must be used, such as drilling, super grades like T-15 are recommended.

Overhang should be kept to a minimum in all cases to avoid deflection and reduce the tendency for titanium to smear on the tool flank.

Drilling

High speed drills are satisfactory for the lower hardness commercially pure grades but super-high-speed steels or carbide should be used for the harder alloy grades and for deep holes. Sharp, clean drills just long enough for the hole being drilled and of sufficient length to allow a free flow of chips should be used. A dull drill impedes the flow of chips along the flutes and is indicated by the feathered or discoloured chips. Cutting and chip clearance can be improved by spiral point drills, rather than chisel edge. Spiral points reduce the large negative rake angle of the chisel edge drill, provide a proper clearance angle along the entire cutting edge and reduce thrust loading significantly.

Milling

When facing, ‘climb milling’ should be used to lengthen the life of face milling cutters. Climb milling produced a thing chip as the cutter teeth leave the work reducing the tendency of the chip to weld to the cutting edge.

Relief or clearance angles for face milling cutters should be greater than those used for steel. Sharp tools must be used.

End milling of titanium is best performed using short length cutters. Cutters should have sufficient flute space to prevent chip clogging. Cutters up to 25 mm (1in) diameter should have no more than 3 flutes.

Chemical Milling

Very precise intricate milling or titanium can be effected by using controlled  selective acid attack of the surface.

The titanium component is placed in a solution of 12-20% nitric acid and 4-5% hydrofluoric acid with a wetting agent. Solution temperature is maintained between 30-40°C (86-105°F). At 36°C (95°F) metal is removed at a rate of 0.02mm/min (0.08thou/min). Areas which do not require material removal are masked with either neoprene elastomer or isobutylene-isoprene co-polymer.

Reaming

Holes drilled or bored for the reaming of titanium should be 0.25-0.50mm (0.01-0.02in) undersize.

Standard high speed steel and carbide reamers are satisfactory. The clearance on the chamfer should be at least 10°.

Reamers with a minimum number of flutes for a given size should be selected to provide maximum tooth space for chip clearance.

Broaching

It is recommended that broach tools be wet ground to improve tool finish and give better tool performance.

Vapour blasting with coolant during broaching lengthens tool life and reduces the tendency to smear.

Broach and broach slots should be regularly inspected for signs of smearing and chip welding as these are indications of wear.

Tapping

Straight, clean holes must be drilled to ensure good tapping.

Reducing the tendency of titanium to smear to the lands of the tap by nitriding the taps, relieving the land, use of an interrupted tap or providing for a free flow of chips in the flutes will ensure sound threads are produced.

Chip clogging can be reduced by the use of spiral pointed taps, which push the chips ahead of the tool and give more chip clearance can be provided by sharply grinding away the training edges of the flutes.

For correct clearance , two fluted spiral point taps are recommended for diameters up to 8mm (0.3in) and three fluted taps for larger sizes.

Machining Do’s and Don’ts

Source: Titanium Information Group

For more information on this source please visit The Titanium Information Group.