Although the welding techniques employed for commercially pure titanium are also applicable to many titanium alloys, extra attention to ensure adequate shielding is necessary because these materials are less tolerant of contamination. As fabricators are largely concerned with welding sheet, the information given deals mainly with this form of material, but the basic principles described apply equally to the welding of thicker sections.
Plasma arc, inert-gas metal-arc and electron-beam processes may all be used for welding titanium in appropriate circumstances. Protection from atmospheric gases is essential; this is normally achieved by supplying argon to the heated surfaces or by operating in a totally enclosed argon-filled welding cabinet.
Materials to be joined must be scrupulously free from oxide or grease films. Titanium sheet, supplied descaled and pickled, has no significant amount of surface oxide. Before welding, however, the edges of sheets should be wire-brushed and degreased. Mild steel brushes should be avoided because of the risk of iron pick-up; stainless steel brushes give acceptable results but titanium wire brushes are preferable. Suitable degreasing agents include acetone and methyl ethyl ketone; chlorine-bearing organic liquids should not be used. Further handling of edges must be avoided. Tubes supplied with an anodized surface must be descaled at the ends before welding.
Time spent in preparing equipment and materials is amply justified, as cracked areas arising from severe contamination cannot be repaired without first removing the embrittled metal.
Edge preparation. Square edge preparations are generally satisfactory for butt welding sheet up to 2 mm (0.080 in) thick; with material thinner than about 0.75 mm, it is often practicable to flange or overlap sheet edges.
Porosity in titanium welds is partly related to edge condition. With commercially pure titanium this porosity is low and its effect on mechanical properties and formability slight. It is considerably reduced by ensuring that edges are smooth and by avoiding close square-butt preparations in manual welding. Sound welds can be produced in close square-butt joints with automatic or semi-automatic welding at speeds exceeding about 250 mm/min (10 in/min).
Open-air techniques. Manual welding produces excellent joints provided that speeds are limited to 75-100 mm/min (3-4 in/min) to avoid turbulence in the shielding gas. The torch should be held as upright as possible to ensure effective distribution of argon over the weld metal, and weaving must be avoided. When using added reinforcement, deviation from the vertical should not exceed 20°, and the filler wire should be kept within the gas shield and fed into the pool continuously. A dabbing technique should not be used, as it leads to turbulence and contamination of the filler wire. The diameter of filler wire should suit the welding current and is normally in the range 1.2-2.6 mm (0.05-0.1 in) diameter.
Automatic welding has distinct advantages, as conditions can be closely controlled and shielding devices used more conveniently. Also, higher welding speeds are possible, e.g. of the order of 500-750 mm/min (20-30 in/min) on 1.2 mm (0.05 in) thick sheet.
Where fully automatic techniques are inapplicable, semi-automatic equipment can combine the advantages of manual and automatic welding. In general, argon must be supplied to every heated surface; in certain instances, the underside of welds can be protected by mechanical means, but this procedure should be adopted only when no alternative is practicable.
Protection of upper surface of weld. The upper surface is protected by argon supplied via the welding torch. The flow required is governed by size and shape of nozzle, by the distance of nozzle from work piece and by the extent to which the electrode protrudes. Although the standard ceramic nozzles fitted to argon-arc welding torches can be used successfully, the extent of argon coverage provided at the weld area is not always adequate, and best results are obtained by fitting elongated shrouds, which are easily fabricated in any workshop, to the standard torch. Argon can be retained around the joint by baffles made from copper or aluminum foil formed into a trough and fixed alongside the seam by clamps or adhesive tape. In this way, the risk of contamination caused by entrainment of air in the argon flow from the torch is reduced. Use of such baffles is essential for corner and edge welds where geometry makes shielding difficult.
Protection of the under-surface of weld. Protection of the under-surface depends upon the geometry of the joint and the welding technique. In automatic welding of butt seams when substantial jigging fixtures are used, access of atmosphere to the weld root is restricted by clamping the sheet on a flat backing bar. Ordinarily, however, distortion during welding lifts the sheets, and severe atmospheric contamination may occur. It is therefore preferable to feed argon to the weld underside. For down hand butt seams, a backing bar incorporating a shallow channel for argon flow is effective.
With titanium, the weld pool has little or no tendency to fall or burn through. Welds can, therefore, be made without backing bars provided that argon shielding at the weld root prevents contamination.
One method of achieving this is by means of a small-diameter thin-walled tube, suitably drilled and held in position with thin copper or aluminum foil; an argon reservoir is formed by attaching the foil to the component with adhesive tape.
In-chamber welding. Welding of complex titanium assemblies is most effectively carried out in a welding cabinet, either rigid or collapsible, filled with argon. A pressure vessel capable of evacuation to a pressure of 2μm Hg (0.0003 Pa), before filling with argon, is the most suitable, although double evacuation to higher pressures (0.1 mm Hg), with an intermediate and final back-fill with argon, will also give satisfactory results. A simple electrode holder may be used in place of the standard argon-arc torch.
Quality control. Close technical supervision, adequately trained operators and carefully studied welding procedures all contribute to satisfactory fusion welds in titanium. Correct shielding of upper and under surfaces, welding currents, speeds and gas flows must be established for each workshop; open-air welding plant must be so located that no external draught causes turbulence in the argon stream.
Titanium welds should have bright surfaces; straw-colored films indicate slight contamination, unlikely to affect mechanical properties; dark blue films (which are evidence of serious contamination) indicate that bend ductility maybe impaired, and suggest urgent revision of welding procedures.
The degree of weld contamination cannot, however, be accurately assessed by visual examination. The most satisfactory method is to check the difference in hardness between weld bead and heat-affected zone. Material adjacent to the weld is in the softest possible condition, as welding will have eliminated any effects of cold working or under-annealing. With good welding practice, the hardness increment, an extremely sensitive guide to the extent of contamination, should not exceed 20 HV for a single-pass weld.
Lining mild steel vessels with titanium. Titanium linings for mild steel vessels, a major advance in chemical engineering, call for special study by the welding technologist. Titanium cannot be directly joined to mild steel because brittle intermetallic constituents form during welding. Normally, titanium sheets are joined by fusion welding and the complete lining can then be attached to the steel vessel, by mechanical means if necessary.
Vessels can be lined on site but the weld under-surface must be adequately shielded and the soundness of the welds carefully checked.
Titanium, with an electrical resistance similar to that of stainless steel, is ideal for spot and seam welding. With sheet up to 1.5 mm (0.06 in), sound joints can be obtained with conventional equipment over a wide range of machine settings.
Joint quality depends largely on surface preparation. Titanium, normally supplied descaled and pickled, can generally be welded without further treatment, but it is useful to abrade the surfaces lightly to ensure uniform contact resistance.
Any scale, tarnish, dirt, oil or grease acquired during handling, storage or fabrication must be removed. Chemical treatments give the lowest contact resistances, but mechanical cleaning gives values almost as low and enables equally good joints to be made. After surface preparation, titanium can be stored for long periods without deterioration of welding characteristics provided that it is kept absolutely clean.
Shielding gas is not normally used in resistance welding because of the short welding cycle.
Spot welding electrodes should be made of heat-treated copper-chromium alloys. The working face should be domed, as the conventional truncated cone profile may give excessive indentation with titanium. A dome with a 75 mm (3 in) radius can be used for most gauges of sheet.
Seam welding. For seam welding, a heat-treated copper-chromium alloy is recommended for the electrode wheel, which should have a 75 mm (3 in) radius face. The welding load should be appreciably higher than for spot welding, because of the longer arc of contact between wheel and sheet. For first trials, a load two or three times that quoted for spot welding is suggested. Welding currents are best determined by practical test.
Initially, various welding currents should be tried with equal on-off times or with longer off-time, still keeping the total time close to the value shown. Metallographic tests are recommended to check the degree of overlap between successive spots, and care is necessary to distinguish between the columnar grains of the weld plug and the surrounding heat-affected zone of unfused metal. A high standard of metallographic preparation is required; examination under oblique light at low magnifications (x 10) is usually most informative.