Investment casting

Investment casting is used in preference to graphite mould casting when close tolerances, thinner sections, smaller draft angles and better surface finishes are required.

Rammed graphite mould (similar to sand) casting

The tooling for rammed graphite is less expensive than that for investment casting and the process is more suitable for large castings.

Cast components of up to 2700kg (5950lb), equal to 4500kg (9900lb) of steel, have been produced in commercially pure and alloyed grades. Larger structures are often manufactured by welding together two or more individual castings.

Titanium does not require special pattern equipment, and in many cases existing patterns for steel or other alloys can be used directly for production of titanium castings.

Cast mechanical properties are close to those of wrought products and the corrosion resistance of wrought and cast materials is identical.

For highly critical applications, consolidation of the cast component can be achieved by hot isostatically pressing (HIP) the component at just below the Beta transus temperature (882°C) at pressures around 100 MPa.

Hot working

Forging, extrusion, rolling and other elevated temperature processes, play an important role in providing shapes with desired properties and maximum metal utilization.

Forging

The aim of forging is to produce a shape, as close to possible to the final dimensions. This minimizes machining and optimises the use of the metal. The unique nature of the forging process is the ability to change the metallurgical structure and mechanical properties. This may be achieved in the whole forging or at specific locations within the forged part.

In applications where high integrity, consistency and controlled properties are required in a specific shape, forging is the optimum process route. Forging is the optimum process route. Forging allows full advantage to be taken of the low density and high strength of titanium.

All grades of titanium can be die forged. The only limitation are commercial; for small batch sizes, conventional open die forging or machining from a solid may be more economical.

Extrusion

All of the engineering grades of titanium can be extruded, and both gross and near net shape profiles are possible. Extrusion is particularly suited to production of long components, or items with complex cross sections involving re-entrant angles. The use of extruded high strength titanium alloy profiles for the production of aero engine rings by flash butt welding has been standard practice for many years.

Rolling

Hot rolling is used to produce sections and profiles. Hot rolled sheet and plate are extensively used for aircraft components.

Superplastic forming

Superplastic forming under high pressures and temperatures has recently found growing application. It is a hot metal operation capable of making intricate parts in a single operation. Diffusion bonding is often incorporated in the same process to create hollow or honeycomb structures.

Superplastic forming is used for ducting, aircraft wing access panels, nozzles and engine casings and blades, high pressure oil field sample bottles, and structural rectangular sections. The process involves placing titanium between the two halves of a die and heating to a pre-determined temperature. Hot argon gas is then pumped into the die at the appropriate pressure to force the titanium to deform superplastically into the shape of the lower die. Superplastic forming can be performed on fine grained (less than 10´m, ASTM grain size 10) titanium alpha-beta alloys such as Ti-6Al-4V. Superplastic forming gives the following benefits:

The best results will be obtained from machines with a built-in monitoring system and positive downfeed. Titanium can be sawn much faster with carbide tipped tooling.

Torchcutting

Torchcutting with oxyacetylene flame can be accomplished in sections up to 15cm (6in) thick. Care must be taken to remove all surface contamination before welding. Hard wheel abrasive grinding, using clean wheels with large grains can be used to clean cut edges without alpha phase contamination as excessive heat build-up is avoided.

Grinding

In grinding the difference between titanium and other metals is the activity of titanium at high temperatures. At the localized points of wheel contact titanium can react chemically to the wheel material. The important factors to 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 vapor-phase rust inhibitors of the nitrite amine type give good results with aluminum oxide wheels.

Correct wheel speeds

A good rule is: use a half to one third of conventional operating wheel speeds to get the best results with titanium.

Selection of wheels

Silicon carbide can be used at 1200-1800 surface m/min (4000-6000 ft/min) to give optimum surface finish and minimum wheel wear but the high speeds essential with these wheels produce intense sparking which can cause a fire hazard unless the workpiece is flooded with coolant. However, 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 cu.cm (0.08 cu.in) 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, 3014ft²/min) will improve titanium surface grinding operations. Recommended abrasives include silicon carbide for cut off and portable grinding, and aluminum oxide wheels for cylindrical and surface grinding. To minimize residual ground surface stresses recommended down feeds are: 0.025mm (1 thou) per pass to 0.05mm (2 thou) then 0.013 mm, 0.010 mm, 0.007 mm, 0.005 mm, 0.002 mm per pass. For form grinding standard grinding oil is recommended and for other operations nitrite-amine based fluids have been used successfully.

Machining

Machining of titanium on conventional equipment is considered by experienced operators as being no more difficult than the machining of austenitic 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 aluminum 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 production rates. Where high speed steel tools must be used, such as on 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.

Disposable carbide tools should be used whenever possible for turning and boring since they increase production rates. Where high speed steel tools must be used, such as on 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. 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 feathered or discolored chips.

Milling

When facing, 'climb milling' should be used to lengthen the life of face milling cutters. 'Climb milling' produces a thin 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 and for cutters up to 25mm (1in) wish diameter there should be no more than three flutes.

Chemical milling

Very precise intricate milling of 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 (96—°F) metal is removed at a rate of 0.02 mm/min (0.8 thou/min). Areas which do not require material removal are masked with either a 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 except the clearance on the chamfer should be at least 10°. Reamers with the 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. Vapor blasting with the 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 results. 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 more chip clearance can be provided by sharply grinding away the trailing edge of 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.

Fabrication

Titanium has a low modulus of elasticity, this means it will deform considerably under load and then spring back. This characteristic must be allowed for when fabricating.

Springback is not an issue when hot creep, or superplastic forming is used. Because the material is more ductile at elevated temperatures, lower forming pressures may be used. Cold or warm pressing, stretch forming and spinning can also be used for forming titanium. Handling annealed commercially pure sheet is similar to working quarter hard stainless steel.

Bends can be made without undue difficulty with radii of 1 to 1.5t, in ductile grades of commercially pure titanium, and cold formable beta alloys such as Ti 15-3.

Welding

Titanium and most industrial titanium alloys are readily weldable. Properly made welds in the as-welded condition are ductile and, in most environments, are as corrosion-resistant as the base metal. Poor quality welds, on the other hand, might be severely embrittled and less corrosion-resistant than the base metal.

TIG welding(GTA)

The tungsten-arc inert-gas-shielded process is perhaps the most widely used method for welding titanium. It is used almost exclusively on sheet materials up to thickness' near 0.3cm (0.125in). It can also be employed successfully for heavier plate sizes although the MIG process is frequently used for these applications. The tungsten-arc process offers many advantages:

Friction welding

Friction welding has become a viable production method for parts having radial symmetry. The essence of the method is to convert rotational kinetic energy into heat to bring about controlled degrees of fusion and extrusion. Friction welding can be done in air.

Electron beam

Electron beam welding is quite attractive since it yields a low distortion weld where the fusion zone has a high depth-to-width ratio. All welding is done in a high vacuum chamber by mechanized equipment.

Electron beam welds have high integrity and the process is frequently used where the highest weld quality is desired.

all have their place in manufacturing. Techniques are available to weld titanium under a wide range of conditions both in the shop and in the field.

Welding requires adherence to rules of cleanliness and the exclusion of reactive gases including air.

Welding points

Joining

Titanium and its weldable alloys can be joined by fusion, resistance and other welding methods.
Titanium cannot be fusion welded to other metals but can be joined by normal mechanical fasteners,
brazing, friction welding and explosive bonding.

Cladding to other materials

A varied number of techniques have been developed to enabled titanium to be mechanically bonded to a lower cost backing material such as carbon steel, thereby lowering the overall cost of the final component.

Explosive bonding

Titanium sheet is placed at a closely controlled distance on top of the backing plate. Explosive spread uniformly on top of the titanium sheet, is detonated from a single point driving the titanium sheet down across the air gap to impact on the backing metal, producing an uncontaminated metallurgical bond of guaranteed shear strength. The continuity of the bond can be examined ultrasonically.

Sheet

Titanium sheet down to 2mm (0.1in) can be bonded explosively. Generally the backing plate should be at least twice the thickness of the clad material, with a minimum of 13mm (0.5in). Plates up to 3.5m (3.1ft) diameter or 15 square meters can be clad. For very large areas, smaller individual titanium sheets can be welded together, flattened and explosively bonded to large backing plates.

Resista-clad linings

This new, lower cost technique resistance bonds titanium of any grade in sheet or coil form to an inexpensive steel backing material. This is a patented process which produces a lining with 12.7mm (0.5in) longitudinal bond seams. The spacing of the seams can be adjusted to give optimal component strength. Prebonded sheets are available.

Cost

An analysis was conducted to calculate the effective annual cost of the various flue gas desulphurisation retrofit lining options. The application of Resista-Clad for lining or cladding of large- diameter pipes, ducts or vessels clearly offers significant cost savings over other methods of protection.

Roll-bond linings

Where large surface areas need only a thin layer of titanium to prevent corrosion attack then roll-bonding titanium onto lower cost materials is an attractive option. Titanium grades 1,2,7 and 11 can be bonded to a base plate by repeatedly passing through a powerful plate mill at temperature, causing the adjacent cleaned plate surfaces to bond in formation of an intermetallic layer.

Individual plates can be joined together by welding the base plates together and using strips to fillet weld and seal the titanium surface. Roll bonded plates are generally limited to low pressure service but again offers significant cost savings for corrosion protection of large diameter pipes, ducts or vessels.

Surface treatments

Titanium has a tendency to gall when in rubbing contact with itself and other surfaces. A number of techniques have been developed to overcome this problem.

Lubrication

Proprietary lubricants of the dry film type, based on molybdenum disulphide, graphite and the like have been developed specifically for titanium. They afford a measure of protection for a limited period and are reapplied as required. Application methods include spraying, dipping and brushing.

Dissimilar hardness

In many applications such as pumps, mixers and valves, titanium's tendency to gall causes gradual wearing of load bearing surfaces. One way to reduce this wear is to increase the differential hardness of the two materials in contact. For instance, a Grade 2 titanium pump case with a typical hardness of 150 HV may use Grade 5 internal components with a hardness of 360 HV.

Anodizing

Increasing the thickness of the titanium oxide surface layer can reduce galling. Where components are not subjected to continual wear, which will cause the layer to be removed, anodizing provides a simple cost-effective solution.

The deposition of low frictional coefficient polymers simultaneously with the growth of the anodic film provides both surface hardness and optimum corrosion resistance and lubricity. Several proprietary non-electrochemical processes are available which also provide low friction polymeric surfaces.

Electroplating

Hard chromium on titanium has long been the solution to wear in cases of reciprocating or limited angular movement. Modern methods also enable the co-deposition of polymers which similarly to those built into the anodic oxide film serve to reduce dramatically the surface coefficient of friction and component surface wear.

Ion implantation

Ion implantation is a cold, directional process where charged nitrogen ions are accelerated in a vacuum and impacted onto the titanium surface requiring treatment. The hardness of the titanium surface layer is increased to 1500 HV. Items such as gears and fasteners have been treated successfully by this technique.

Titanium nitride (tin) coating

In the TiN process, the surface hardness of titanium can be increased dramatically to 2600 HV. The components requiring treatment are placed in a vacuum chamber and heated to 400—°C (752—°F). A stream of titanium ions is reacted with the gaseous nitrogen to form TiN. As the components rotate in the vacuum, their surface layer is coated with TiN to a thickness of 2-6 ´m (0.08-0.23 thou) over a minimum period of three hours. The surface produced has a bright golden appearance. Very intricately shaped components up to 1 cu.m (35 cu.ft) can be treated in this way very quickly.

Currently, the automotive racing industry uses several TiN coated titanium components such as connecting rods and axle hubs, gaining at least a threefold increase in life.