Titanium and its alloys can be readily hot worked at temperatures generally somewhat lower than those used for steels. To minimize surface contamination, titanium should be held at high temperatures for only a short time before forging. The rate of contamination, relatively low up to 700°C, increases rapidly with increase of temperature.
All forging furnace atmospheres contain free or combined oxygen, and some absorption of this element inevitably occurs. In addition to visible scaling, diffusion of oxygen results in hardening of a relatively shallow underlying layer. The effect of nitrogen is not usually significant at preheating temperatures. Subsequent operations such as machining will remove the hardened surface layer, and the final product will have hardness similar to forging stock.
Hydrogen, however, diffuses more rapidly than oxygen and may penetrate the full section of the work piece, which can have a serious effect on properties. Such material can only be recovered by prolonged vacuum annealing. Hydrogen is absorbed from both reducing and oxidizing gas- and oil-fired furnaces, but at a tolerably slow rate under strongly oxidizing conditions.
The order of preference of preheating atmospheres is therefore dried air (electric heating), undried air (electric heating), oxidizing oil- or gas-fired furnaces. Direct flame impingement must be avoided.
Forging. Techniques for press and hammer forging of titanium are essentially the same as for low-alloy steels. Good handling methods and plant layout will reduce the number of reheats necessary, minimizing contamination during forging. Because of the rapid cooling and the fairly narrow hot working range, the chilling effect of tools should be reduced to a minimum by keeping contact time as short as possible. Preheating the tools also helps. Repeated light blows, or attempts to continue forging at too low a temperature, may promote internal cracking and should be avoided. Moreover, a large number of reheats with only a small amount of deformation between heats is also detrimental, because it leads to a coarsening of the microstructure and consequently poor mechanical properties.
In drop forging, die contours should have larger radii and fillets than those used for steel; the lower thermal contraction of titanium requires a smaller shrinkage allowance. Trimming should be carried out hot; furnace, drop hammer and trimming press should be as close together as possible to minimize preheating and avoid wasting time and heat. A final stress-relief anneal is recommended.
Annealed and solution treated sheet can be pressed, stretch-formed, spun and dimpled, but maximum deformation depends upon the load being applied slowly. Good results are achieved with hydraulic presses, the rubber-pad method being useful for forming light-gauge parts. Drop hammer forming, with heated blanks, is widely used for sheet metal parts of complex contour. Punch presses, which should be slowed down to half or one-third their normal speed, can also be used.
Blanks may be prepared for forming by shearing, sawing, nibbling or blanking, using slow cutting speeds. Edge condition is important, and edge cracking may be minimized by keeping the guillotine blade sharp and close fitting or by heating metal before shearing. All burrs must be removed and, for more difficult forming operations, cut edges may need filing or polishing.
Simple shapes can be formed at room temperature, deformation being limited by the strength and springiness of the material. Solid lubricants such as soap, molybdenum disulphide or graphite are preferred to mineral oils and greases. ICI "Trilac" coating and polythene sheeting have been found to effect considerable improvement in difficult pressing operations.
For more complicated designs, the work piece and, where possible, the dies should be heated to facilitate forming. The use of heat in forming increases ductility, which is reflected in lower minimum bend radii and reduces both the load required to effect deformation and subsequent spring-back, thus ensuring greater accuracy.
Furthermore, at elevated temperatures, the spread between yield and ultimate strengths is increased, which also aids forming. The temperature to select for hot forming depends upon the alloy and the severity of the shape to be produced. Good results can be expected using temperatures of about 200-300oC for commercially pure titanium and IMI Titanium 230, and 550-650oC for IMI Titanium 317 and 318.
With heating in conventional furnaces there is always some surface contamination and a risk of hydrogen absorption. Vacuum treatment, though ideal, is rarely practicable, so it is customary to use ordinary electric furnaces; hydrogen pick-up is not usually excessive. Fuel-fired furnaces should be avoided if at all possible; titanium rapidly absorbs free or combined hydrogen from the surrounding atmosphere, and this can be serious, particularly with thin sections.
Superficial hardening by oxygen diffusion is almost inevitable at the higher annealing and preheating temperatures suggested for some titanium alloys. The hardening effect is insignificant at low annealing temperatures but above 600°C may lead to surface embrittlement. Both the oxide film and the underlying oxygen-rich layer should therefore be removed by one of the methods of surface treatment; this is particularly important for high-strength alloys.
Machining and Grinding
Titanium and its alloys can be machined successfully on conventional machine tools provided that certain requirements are satisfied. In all machining operations rigidity of both work piece and cutting tool is desirable. Best results will be obtained if the cutting tools have a good surface finish. If the cutting tools are in good condition, it is no more difficult to machine titanium than an alloy steel of equivalent strength.
Titanium has a tendency to gall or smear on to other metals. Sliding contact between the work piece and its support should be avoided, and the use of roller steadies and running centres is recommended.
Turning. In general, cutting speeds should be low and feeds as coarse as practicable. A good surface finish can be obtained with very coarse feeds by using suitably shaped tools with a large nose radius. This will, however, be limited by work piece rigidity as a large nose radius causes increased tool loads and work piece deflection. Due to the lower elastic modulus of titanium, these deflections are greater than would occur on steel workplaces.
Tool materials may be high-speed steel, cast alloy, or tungsten carbide. The "super" grades of high-speed steel are satisfactory, giving good results in turning where large feeds can be employed, and particularly where the surface is rough or the cut intermittent. Tungsten carbide may be necessary for heavy work on certain harder alloys or for intermittent cutting, but in general its use is confined to lighter, more continuous cuts. For economic use of carbide tools it is essential to regrind before wear becomes excessive, and mechanically clamped tips are an obvious advantage.
Threading. Single-point screw cutting is preferable to threading with a die. Conventional methods of screw-cutting can be used, but success can also be achieved when increments of cut of 0.25-0.50 mm are applied at right angles to the axis of the component. Cuts of less than 0.13 mm should be avoided. Machine tapping with cutting speeds up to 6 m/min is preferable to hand manipulation. Tapping of full threads should be avoided: a thread of 80% depth is much easier to tap and loses little strength.
Planing. Shaping and planing of titanium are not difficult, provided that the foregoing requirements of rigidity, speed and feed are satisfied. Tungsten carbide tools with a large radius, producing a broad and relatively thin chip, are most successful. As in all cutting operations, it is essential to use sharp tools and replace them before appreciable wear occurs. For planing, clamped circular buttons of tungsten carbide have obvious advantages.
Milling. In milling, the chief problem arises from chips welding on to the teeth, resulting in cutter chipping and breakage. This is minimized with climb milling, in which the tooth finishes its cutting stroke when moving parallel to the feed. Absolute rigidity is necessary to avoid chatter, but the chip is only attached to the tooth by a thin sliver which is easily broken off.
Drilling. Titanium may be drilled with short high-speed-steel drills; the holes should be as shallow as possible. A 140o point is best for sizes below 6-5 mm and a 90° or double-angle point for larger sizes. For holes of a depth greater than five diameters, it is helpful to retract the drill at intervals and clear the swarf. Flood lubrication with a heavily chlorinated cutting oil reduces frictional troubles.
Grinding. A reduction in wheel speed to a half or a third of the conventional speed, together with the use of a suitable coolant, will usually achieve an acceptable grinding ratio. Water-base soluble oils result in poor wheel life, but some chlorinated or sulphurised grinding oils, and solutions of vapour-phase rust inhibitors of the nitrite-amine type, are satisfactory.
Polishing. Titanium can be mechanically polished by techniques similar to those used for stainless steel; reductions in wheel or mop speeds are often beneficial. If a high polish is required, light pressures are necessary during the final operations. Good results have been obtained with a canvas wheel coated with 240E1 `Alundum` grit, which can be blended with stearic acid for a finer finish.
Descaling and Surface Treatment
When titanium and its alloys are heated in air, absorption of oxygen and, to a lesser extent, nitrogen, results in the formation of an outer layer of oxide and nitride and an underlying thin layer into which oxygen and nitrogen have diffused. Removal of this hardened metal layer is essential for optimum mechanical properties, and an integral part of any descaling process.
All types of scale can be removed in fused caustic soda, but use of an unmodified bath leads to hydrogen contamination and poor surface quality. The sodium hydride process results in good surfaces and efficient scale removal but, again, hydrogen contamination occurs. Consequently, neither process is suitable for thin sections.
Caustic soda with about 10% oxidizing additions can be used for slightly thicker material, descaling conditions being 20-30 minutes immersion (longer for very heavy scale) at 425°C. Reaction between titanium and any fused caustic soda bath may lead to a dangerous build-up of heat if a stack of thin sheet is descaled. Thin-gauge material should, therefore, be handled in small batches, at a temperature not exceeding 425°C.
Anti-galling Treatments. The tendency for titanium to gall when in sliding contact with itself or with other materials can be reduced by some form of surface treatment. This is particularly desirable for bearing surfaces and for threads of bolts. Both anodizing and `Sulfinuz` treatments reduce the galling tendency, while adherent nickel and chromium deposits provide good wear resistant surfaces. Cadmium plating or the use of anti-galling paints are effective in preventing seizure of bolt threads. Details of electro deposition and anodizing procedures are given in the following paragraphs.
Electrodeposition. Adherent metallic coatings can only be electrodeposited on to titanium if the surface is suitably prepared. A procedure which has been found successful for depositing nickel, chromium, zinc and cadmium on to some titanium alloys uses a pretreatment comprising: (1) Vapour degrease, (2) Hydrochloric acid etch, 5 min in concentrated HCl at 90-110°C, (3) Cold water rinse, (4) Nickel strike for 3 min, (5) Cold water rinse.
Anodizing. Surface properties of titanium and its alloys can be modified by anodic oxidation treatment, which covers the entire surface with a thin but compact oxide film. Almost any aqueous solution can be used, but immersion in a solution of 80% phosphoric acid, 10% sulphuric acid and 10% water gives a particularly coherent film. A potential increasing from 0 to 110 volts over ten minutes should be applied.
Anodized titanium has no affinity for dyestuffs, but the film itself shows interference colors, determined by the final anodizing potential.