When heated in air above 650°C (1200″F), titanium tends to oxidize rapidly. At elevated temperatures it has the propensity for dissolving discreet amounts of its own oxide into solution. For these reasons, the welding of titanium requires protective shielding, such as an inert gas atmosphere, to prevent contamination and embrittlement from oxygen and nitrogen. The relatively low coefficients of thermal expansion and conductivity minimize the possibility of distortion due to welding.

Pure titanium is quite ductile (15 to 25% elongation) and has a relatively low ultimate tensile strength, approximately 207 MPa (30 ksi). Some limited amounts of oxygen and nitrogen in solid solution markedly strengthen titanium, but embrittle it if present in sufficient quantity. Carbon exerts a similar but less intensive effect. Hydrogen also promotes embrittlement when present above specified limits. These elements are usually unintentionally added by contamination when the metal is processed. Inten- tional additions of various alloying elements may result in tensile strengths exceeding 1380 MPa (200 ksi), but there is a resultant sacrifice of ductility. The combination of high strength, low density, and excellent corrosion resistance results in a very desirable strength-to-weight ratio, up to temperatures as high as 650°C (1200°F).

The weld metal tensile, impact and hardness properties for titanium and titanium alloy welding electrodes and rods are shown in Table T-7. These data were from multipass gas tungsten arc welded plate of 12.7 mm (0.5 in.) thickness, or greater, with plate and filler metal of identical composition.

Titanium alloys may be classified according to their ability to produce tough, ductile welds. One such rating is shown in Table T-8. All alloys rated A or B in Table T-8 are considered usable in the as-welded condition for most applications. Many alloys of limited weldability can be subjected to postweld annealing to improve ductility. All of the weldable titanium alloy grades in the annealed condition will produce joint efficiencies close to 100%.

Filler Metals

When welding titanium and titanium alloys, the filler metal should have the same nominal composition as the base metal. Filler metal is usually used in the form of bare rod or wire, depending on the welding process and the type of operation (manual, emiautomatic, or automatic). Refer to ANSIJAWS A5.16, Specification for Titanium and Titanium Alloy Welding Electrodes and Rods. The compositions of standard titanium welding electrodes and rods are shown in Table T-9.

When welding commercially pure titanium, an unalloyed filler metal can tolerate some contamination from the welding atmosphere without significant loss in ductility. The ERTi-1,-2, -3, and -4 filler metal classifications are designed for this purpose, as are those in AMS Specification 4951 (available from the Society of Automotive Engineers. Unalloyed filler metal may be used to weld titanium alloys when weld metal ductility is more important than joint strength. Joint efficiencies of less that 100% can be expected.

 

For cryogenic applications where base metals with extra-low interstitial impurities are specified, the filler metals should also be low in those impurities. To be effective, the welding must be done with equipment and procedures that prevent contamination of the weld metal with carbon, oxygen, nitrogen, or hydrogen. The quality and cleanliness of the filler metal are important considerations in the welding of titanium. Filler metal can be a source of serious contamination of the weld metal from inclusions, dirt, oil, and drawing compounds on the filler metal surfaces. The relatively large surface area-to-volume ratios of wire or rod used make cleanliness very important. Physical defects in wire, such as cracks, seams, or laps, can entrap surface contaminants, and make their removal difficult or impossible. The filler rod or wire should be carefully inspected for mechanical defects, thoroughly cleaned, suitably handled, packaged, and stored to prevent contamination.

Weld Stress Relief

Residual stresses in weldments are relieved during annealing or solution heat treatment. A stress-relieving heat treatment might be applicable when it is not necessary to heat-treat the weld to obtain the required mechanical properties. Weld stress relief can be beneficial in maintaining dimensions, reducing cracking tendencies, or avoiding stress-corrosion cracking in certain alloys. However, stress relieving treatments of heat-treatable titanium alloys may be detrimental. Thermal stress-relieving could alter the mechanical properties of the weld by an aging reaction with heat-treatable alloys. This response might reduce the beneficial effects expected from stress relieving because of reduced weld ductility resulting from aging.

Cleaning

Prior to welding, brazing or heat treating, titanium components must be washed clean of surface contaminants and dried. Oil, fingerprints, grease, paint, and other foreign matter should be removed using a suitable solvent cleaning method. Ordinary tap water should not be used to rinse titanium parts. Chlorides and other cleaning residues left on titanium can lead to stress-corrosion cracking when the components are heated above about 290°C (550°F) during welding, brazing and heat-treating. Hydrocarbon residues can result in contamination and embrittlement of the titanium. Parts to be welded or brazed usually have a light oxide coating in the vicinity of the joint. The coating can be removed by pickling in an aqueous solution of 2 to 4% hydrofluoric acid (used with proper precau- tions) and 30 to 40% nitric acid, followed by appropriate water rinsing and drying. Hydrogen absorption by titanium alloys is generally not a problem at temperatures up to 60°C (140°F). The part should be handled, after pickling and rinsing, with lint-free gloves during assembly in the welding or brazing fixture. The fixturing itself should be thoroughly cleaned and degreased prior to loading the workpieces.

Oxide scale formed at temperatures above 595°C (1 100°F) is difficult to remove chemically. Mechanical methods, such as vapor blasting and grit blasting, should be used for scale removal. Mechanical operations are usually followed by a pickling operation to ensure complete removal of surface contamination.

To control porosity in welding operations, the surfaces to be joined are often given special treatments, including draw filing, wire brushing, or abrading the joint and adjacent surfaces prior to fitup and final cleaning. Sheared joint edges frequently require special treatments to remove entrapped dirt, metal slivers, and small cracks because these edge discontinuities promote weld porosity.

Preweld cleaning operations should be accomplished immediately prior to welding. If this is not practical, the parts should be stored with a desiccant in sealed bags or in a humidity-controlled storage room. Alternatively, thorough degreasing and light pickling of parts just prior to welding or brazing is strongly recommended. Mechanical abrasion of the faying surfaces followed by washing with a suitable solvent, may be used in lieu of pickling treatment.

Protection During Joining. Because of the sensitivity of titanium to embrittlement by oxygen, nitrogen, and hydrogen, the entire component or that portion to be heated above about 260°C (500°F) must be protected from atmospheric contamination. Protection or shielding is commonly provided’ by a high-purity inert gas cover in the open or in a chamber, or by a vacuum of 0.013 Pa (104 torr) or lower.

During arc welding, titanium must be protected from the atmosphere until it has cooled below about 425°C (500oF). Adequate protection can be provided by an auxiliary inert gas shielding device when welding in the open. For critical applications, welding should be done in a gas-tight chamber that is thoroughly purged of air prior to filling with high-purity argon, helium, or mixtures of the two.

The purity of the shielding gas influences the mechanical properties of the welded joint. Both air and water vapor are particularly detrimental. The purity of commercial inert gases used for welding is normally satisfactory, but care must be taken to ensure that moisture and air are not entrained into the gas delivery system. The dew point of the gas should be measured at the welding location or as it is purged from a welding chamber. A dew point of 4 0 ° C (40°F) at the point of weld is the approximate maximum moisture limit. Shielding gases have a dew point of     -51°C (-60°F) or lower.

The inert gas at the cylinder or other source must be sufficiently dry to allow a margin for some moisture pickup in the delivery system. One method of checking gas purity is to weld a sample piece of titanium, prior to welding the workpiece itself, then to bend it. The surface appearance and the degree of bending are a good indication of the gas purity. A second sample should be welded and bent after the workpiece is completed to assure that the shielding was satisfactory during welding.

The color of a weld bead on titanium is often used as a measure of the level of contamination or the shielding gas purity. A light bronze color indicates a small amount of surface contamination; a shiny blue color indicates a greater amount of surface contamination. Neither of these levels of surface contamination is desirable, but may be acceptable on a single or final weld pass, provided the surface layer is removed before the weldment is placed in service. A white, flaky layer on the weld bead indicates excessive contamination, which is not an acceptable condition. In multipass groove welds, no surface contamination is acceptable and must be removed before depositing additional passes. If a white or gray flaky oxide is present, the gas shielding system should be inspected, and the cause of contamination corrected. The contaminated weld metal should be removed because it is likely to be brittle.

When brazing or diffusion welding titanium parts, they must be protected by high-purity inert gas or processed in a vacuum. The time at temperature should be as short as practical because hot titanium (a “getter”) absorbs oxygen, nitrogen, and hydrogen by diffusion when available in even minute amounts.

Gas Shielded Arc Welding

The three processes normally used for joining titanium are gas tungsten arc, gas metal arc, and plasma arc welding.

Welding with all three processes can be done with manual, semiautomatic, or automatic equipment. Manual and automatic welding can be done in the open or in a chamber filled with inert gas. Semiautomatic welding is usually done in the open, but could conceivably be performed in a chamber.

The main concern with welding in the open is adequate inert gas shielding of (1) the molten weld pool and adjacent base metal (primary shielding), (2) the hot, solidified weld metal and heat-affected zone (secondary shielding, and (3) the back side of the weld joint (backing).

Primary Gas Shielding. Primary gas shielding is provided by the arc welding torch or arc welding gun nozzle. The nozzle size usually ranges from 12.7 to 19 mm (0.5 to 0.75 in.). In general, the largest nozzle consistent with accessibility and visibility should be used. Nozzles that provide laminar flow of the shielding gas are desirable because they lessen the possibility of turbulent gas flow where air mixes into the gas stream at its periphery. Proper shielding of the molten weld pool is critical.

Secondary Gas Shielding. The primary gas shielding advances with the arc welding gun and a secondary inert shielding gas is necessary to protect the solidified, cooling weld bead and the heat-affected zone. The hot weld zone must be shielded from the atmosphere until it has cooled to a temperature where oxidation is not a problem. The low thermal conductivity, and consequent slow cooling, of titanium requires that a considerable length of the hot weld be shielded; more than is usually provided by gas flow from an arc welding gun.

The common form of secondary shielding is a trailing shield; a typical design is shown in Figure T-15. It consists of a metal chamber fitted to the torch nozzle and held by a clamp. The inert gas flows through a porous metal diffuser screen over the weld area. The shield must be wide enough to cover the heat-affected zone on each side of the weld bead.

A trailing shield is used for machine or automatic welding where travel speeds are higher. In one important application, the trailing shield, used in welding pipe in the horizontal-rolled position, is curved to conform to the pipe surface.

For manual welding, a large gas nozzle or an auxiliary annular gas nozzle can be used with slow welding speeds. Trailing gas shielding can interfere with the visibility of the weld pool and manipulation of the manual arc welding torch.

Secondary shielding can be incorporated into the fixturing, as shown in Figure T-16. Inert gas passages are provided in the hold-down bars on both sides of the weld seam. Shielding gas flows from the arc welding torch and hold-down bars into the channel formed by the bars, displacing the air from above the weld.

Backing Gas Shielding. Inert gas shielding is required to protect the weld root and adjacent base metal from atmospheric contamination during welding. This is accomplished using gas passages in a backing (bar or ring) as shown in Figure T-17. The backing is incorporated into the fixturing (see Figure T-16), and contains a clearance groove under the joint that is filled with inert gas prior to welding. The inert gas pressure in the groove must be kept low to avoid forming a concave root surface. The backing should be tightly fitted along the entire length to ensure uniform weld quality.

The backing, which is often made of water-cooled copper, can serve to remove heat from the weld and accelerate cooling. Stainless steel backing may be used when lower cooling rates are acceptable. The root opening of the joint must be near zero to prevent the arc from impinging on and fusing the titanium weld to the backing bar. Contamination of the titanium weld metal may embrittle it, resulting in a cracked weld.

When welding pipe or tubing, the interior of the pipe must be purged of air with inert gas. Usually a volume of inert gas that is at least six times the volume of the pipe is required to displace the air. In large systems, internal dams can be placed on both sides of the joint to confine the backing gas to the vicinity of the weld joint. Internal dams must have an inlet for the inert gas, an outlet for the displaced air and inert gas to escape, and internal gas pressure must be low, 50.8 or 76.2 mm (2 or 3 in.) of mercury. Suitable dams are available commercially.

Welding in a Chamber. Many titanium weldment designs are not adaptable to welding in the open air; adequate inert gas shielding of weld joints would be difficult to achieve. An acceptable procedure is to weld such an assembly in an enclosed chamber filled with inert gas.

Two types of welding chambers are used: flow- purged and vacuum-purged. The welding atmosphere in a flow-purged chamber is obtained by flowing inert gas through the chamber to flush out the air. The volume of inert gas needed to obtain a welding atmosphere of sufficient purity in the chamber is about six times the chamber volume.

The appropriate inert gas flow rate and air displacement time for a specific chamber should be established by welding tests. During welding, inert gas flows through the arc welding torch to ensure adequate shielding of the molten weld pool. A low, positive gas pressure is always maintained in the chamber to prevent air from entering. The welding atmosphere should be monitored during actual welding operations by running weld beads on test coupons prior to, during, and after welding the actual assemblies. The test coupons should be evaluated visually and mechanically to verify that the chamber atmosphere was satisfactory during the welding operation.

When contamination of the titanium during welding must be absolutely avoided, welding is performed in a vacuum-purged chamber. Air is removed from the chamber by a vacuum pumping system to a pressure of usually 0.013 Pa (lo4 torr) or lower. The chamber is then back-filled with inert gas having a dew point of -60°C (-76°F) or lower.

Accessibility to the work is through glove ports in the chamber. The gloves, welding torch or gun, fixturing, and other material installed or placed in the chamber must be impervious to air and water, and void of volatiles that can contaminate the titanium.

Joint Design. The weld joint designs used for welding titanium are similar to those used for steels. Actual joint design depends on several factors, including the welding process, type of operation (manual or machine), joint accessibility, and inspection requirements.

Edge preparation should be done by a machining process that does not contaminate the titanium or leave embedded particles on the surface. As mentioned pre-viously, root opening is important when welding with temporary backing. Fixturing or tack welds should be used to maintain uniform root opening during welding.

The design of a weldment, the types of joints, and joint locations can be limited by shielding requirements. When welding in a chamber, positioning for welding each joint must be considered during the design phase.

Preheat and lnterpass Temperature. Preheat and interpass temperatures must be kept low for welding in open air to avoid surface oxidation. Surface oxides dissolved in the molten weld metal can cause problems when the weld solidifies. A low preheat is generally employed to drive off adsorbed surface moisture prior to welding. Preheat and interpass temperatures should not exceed 120°C (250°F). Prolonged exposure to air at temperatures above 120°C (250°F) can cause an oxide film to form on the faying surfaces. This oxide film must be removed with a stainless steel wire brush or rotary carbide burrs prior to welding.

Gas Tungsten Arc Welding

Gas tungsten arc welding (GTAW) is commonly used to weld titanium and its alloys, particularly for sheet thicknesses up to 3 mm (0.125 in.). Welding in open air is best done in the flat position to maintain adequate inert gas shielding with the welding torch and secondary shielding devices. Specially designed secondary shielding devices may be required when welding in positions other than flat. Welding in positions other than flat may contribute to the amount of porosity in the weld metal.

Direct current electrode negative (DCEN) is normally used with Type EWTh-2 tungsten electrodes of proper size. Contamination of the weld with tungsten should be avoided because it embrittles the titanium. Electrode extension from the gas nozzle should be limited to the amount required for good visibility of the weld pool. Excessive extension is likely to result in weld metal contamination.

When welding in open air, welding should be terminated on a runoff tab or the welding torch should dwell over the weld with a postflow of shielding gas after shutting off the welding current. When a filler metal is added, the heated end of the welding rod must be held under the gas nozzle at all times to avoid contamination. If the tip of the rod becomes contaminated, it must be cut off before continuing the weld.

Welding conditions for a specific application depend on joint thickness, joint design, the weld tooling design, and method of welding (manual or machine). For any given section thickness and joint design, various combinations of amperage, voltage, welding speed, and filler wire feed rate can be used to produce satisfactory welds.

Typical welding conditions that can be used for machine gas tungsten arc welding of titanium are shown in Table T-10. The welding conditions generally do not have to be adjusted radically to accommodate the various titanium alloys, however, certain adjustments are often made to control weld porosity.

Gas Metal Arc Welding

Gas metal arc welding (GMAW) can be used for joining titanium. It is more economical than gas tungsten arc welding because of the deposition rates, particularly with thick sections. Selecting the correct welding conditions should produce a smoothly contoured weld that blends with the base metal.

With GMAW, the droplets of filler metal being transferred across the arc are exposed to much higher temperatures than the filler metal fed into a GTAW molten weld pool. The combination of high temperature and fine particle size makes the filler metal highly susceptible to contamination by impurities in the arc atmosphere. Consequently, the welding gun and auxiliary gas shielding must be carefully designed to prevent contamination of the inert gas welding atmosphere.

Equipment. Conventional GMAW power sources and control systems are satisfactory for welding titanium. Conventional GMAW guns are modified to provide the necessary auxiliary gas shielding needed for titanium.

Filler Metal Transfer. Titanium filler metal can be transferred by all three types of metal transfer: short-circuiting, globular, and spray. Globular transfer is not recommended for welding titanium because of excessive spatter and incomplete fusion in the weld. Short-circuiting transfer can be used for welding thin sections in all positions. When welding thick sections, in positions other than flat, incomplete fusion can be a problem because of the inherent low heat input.

When welding thick sections in the flat and horizontal positions, spray transfer is preferred to take advantage of high heat input and high deposition rates. Pulsed spray welding provides spray transfer with lower heat inputs that is advantageous for welding thinner sections and in positions other than flat.

Plasma arc welding (PAW) is an extension of gas tungsten arc welding in that the arc plasma is forced through a constricting nozzle. Inert gas shielding of the weld is provided by a shielding gas nozzle and an auxiliary trailing shield similar to that used with GTAW and GMAW. Welding is accomplished with a transferred arc using direct current, electrode negative supplied by a constant current power source.

Argon, with a dew point of -60°C (-76°F) or lower, is generally used as the orifice gas and shielding gas, but helium-argon mixtures are sometimes used for shielding. Hydrogen must not be added to the inert gas because of its embrittling effects on titanium.

Plasma arc welding can be done using two techniques: melt-in and keyhole. The melt-in technique is similar to GTAW. The keyhole technique provides deep joint penetration for welding square-groove joints in one pass. The two techniques can be com- bined for welding groove joints in thick sections.

Square-groove joints in titanium alloys from about 1.6 to 12.7 mm (0.062 to 0.50 in) thick can be welded with one pass with the keyhole technique. Plasma arc welds tend to be undercut along the top edges and have convex faces unless filler metal is added during welding, or when a second pass is made as a cosmetic pass.

Electron Beam Welding

Electron beam welding (EBW) in high vacuum is well suited for joining titanium; oxygen and nitrogen contamination of the weld is held within acceptable levels. When electron beam welds are made in a vacuum or nonvacuum, inert-gas shielding requirements are the same as for arc welding.

The process variables are accelerating voltage, beam current, beam diameter, and travel speed. Beam dispersion increases with atmospheric density, pressure, or both.

Deep joint penetration in square-groove welds is obtained with high beam power density and a keyhole in the weld metal.

Laser Beam Welding

Laser beam welds can be produced in titanium by the conventional melt-in technique or by the keyhole technique. With the keyhole technique, as much as 90% of the laser beam energy can be absorbed, depending on the metal. Absorption efficiency is significantly lower with the melt-in technique. At an energy level of 15 kW, the maximum thickness of  Ti-6A1-4V alloy that can be welded in a single pass is about 15 mm (0.60 in.).

When welding with a high power density, ionization of metal vapor above the molten weld pool diffuses the laser beam and interferes with welding. This can be prevented by blowing the metal ions away from the weld pool with inert gas, preferably helium. Helium-argon mixtures can also be used. At the same time, a titanium weld must be shielded from the atmosphere to prevent contamination and embrittlement, as described previously for arc welding.

Other Processes

Titanium can also be welded using the diffusion, friction, resistance, and flash welding processes. Refer to American Welding Society Welding Handbook, Vol. 2, 8th Edition. American Welding Society, Miami, Florida. 1991.

Thermal Cutting

Titanium can be severed by oxyfuel gas cutting (OFC) at speeds approximately three times faster than an equivalent thickness of steel, however, the cuts result in a contaminated and hardened surface requiring some type of edge preparation before welding. The depth of hardening in titanium after OFC is less than 0.3 mm (0.010 in.), but the overall hardened zone can extend up to 1.6 mm (0.06 in.) deep. Titanium can also be cut using the plasma arc cutting (PAC) process. The cut face will be contaminated to some degree because of the exposure of the hot titanium to the atmosphere.

Safe Practices

The possibility of spontaneous ignition of titanium and titanium alloys is extremely remote. As in the case of magnesium and aluminum, the occurrence of fires is usually encountered where an accumulation of grinding dust or machining chips exists. Even in extremely high surface-to-volume ratios, accumulations of clean titanium particles do not ignite at any temperature below incipient fusion temperature of the air.

However, spontaneous ignition of fine grinding dust or lathe chips saturated with oil under hot, humid conditions has been reported. Water or water-based coolants should be used for all machining operations. Carbon dioxide is also a satisfactory coolant. Large accumulations of chips, turnings, or other metal powders, should be removed and stored in closed metal containers. Dry grinding should be done in a manner that will allow proper heat dissipation.

Dry compound extinguishing agents or dry sand are effective for titanium fires. Ordinary extiiliguishing agents such as water, carbon tetrachloride, arid carbon dioxide foam are ineffective and should not be used.

Violent oxidation reaction (explosion]) occurs between titanium and liquid oxygen or red-fuming nitric acid. Reference: American Welding Society, Welding Handbook, Volume 3, 8th Edition; American Welding Society, Miami, Florida.

 

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