Titanium and its alloys are remarkably resistant to the cracking problems experienced by many of the other alloy systems. Solidification and liquation cracking are virtually unknown and what could perhaps be called cold cracking, occurs generally only because of embrittlement arising from contamination, as discussed in
Part 1.
Porosity is the commonest problem, particularly when close square butt joints are used. It is generally attributed to hydrogen and cleanliness is therefore crucial in eliminating porosity. The porosity may be of one or a mixture of two types: firstly micro-porosity formed within the arms of the dendrites during solidification and secondly, larger pores that often align themselves along the weld centre line.
As discussed in Part 1, cleanliness is the key to defect free welds and this means that not only must the component be thoroughly degreased but so should the filler wires; weld preparation edges must be deburred and the highest purity shielding gas must be used. Ideally the gas should have a dew point of less than -50°C (39ppm H2O) and to maintain this low level the gas supply system should be free of leaks. Regular and frequent maintenance of the system is therefore essential, checking the joints for leaks and for damaged hoses. Ideally the gas supply should befrom a bulk gas tank, not cylinders, and delivered to the work stations via welded or brazed steel or copper tubing. Plastic hoses should be kept as short as possible; most plastics used are porous and will allow moisture to permeate through the hose wall; neoprene and PVC are the worst, Teflon one of the least porous. It is worth remembering that moisture can collect in the hose over a period of time so a porosity problem, say after a weekend shut down, may be an indication that this is occurring.
TIG filler wires should be cleaned with a lint free cloth and an efficient degreasant immediately before use. Following cleaning, the wire should not be handled with bare hands but whilst wearing clean, grease-free gloves. MIG wire presents more of a problem but devices to clean the wire as it passes through the wire feeder are available. For the best results wire that has been shaved to remove any embedded contaminants can be obtained.
A further potential source of contamination that is frequently overlooked is the use of air powered tools for wire brushing or dressing weld preparations and welds. Most compressed air contains moisture and oil so that, even when oil and moisture traps are fitted, it is possible to leave a thin film of moisture and/or oil on the surface to be welded. It is recommended that electrically powered tools are used at all times once the item has been degreased prior to welding.
Although regarded as a very minor problem, ductility dip cracking (where alloys experience a severe loss of ductility at a temperature below the solidification temperature) has been noted in some of the titanium alloys; the alpha-beta alloys containing niobium being the most susceptible with Ti-6Al-2Nb-1Ta-0.8Mo the most sensitive. The temperature range in which this loss of ductility occurs is between750°C and 850°C.
The cracking is intergranular and is thought to be partly the result of volume changes during the beta to alpha phase change coupled with the reduction in ductility.
A significant amount of welding of titanium alloys is carried out without the use of filler metals. When filler wire is used, generally a composition matching that of the parent metal is selected. There are, however, some exceptions. The welding of high strength but low ductility commercial purity titanium is generally performed with a low strength filler metal in order to achieve the desired weld quality. Similarly, unalloyed filler metal is sometimes used to weld alloys such as Ti-6Al-4V, thereby improving weld metal ductility by lowering the amount of beta phase that is formed. Extra low interstitial (ELI) filler metals are also available and may be used to improve weld metal ductility and toughness.
Most of the titanium alloys can be successfully fusion welded using the gas shielded welding processes and power beams; all can be welded using solid phase processes, friction and resistance welding. Welding parameters and weld preparations are similar to those that would be used to weld a carbon steel. From the welder's point of view, titanium iseasier to weld than steel, having good fluidity and high surface tension, easing the task of depositing sound full penetration root beads.
TIG welding is probably the most commonly used process in both manual and mechanised applications. The current is DC-ve, generally with high purity argon as the shielding gas, although helium or Ar/He mixtures may be used to improve penetration. Torch nozzles should be fitted with gas lenses to improve gas shielding and the ceramic shroud should be as large a diameter as possible. A 1.5mm diameter tungsten, for example, should be used with a 16mm diameter ceramic. Arc lengths need to be as short as possible to reduce the risk of contamination; 1 to 1.5 times the electrode diameter is regarded as a good rule of thumb. Arc initiation should be achieved by the use of HF current or Lift Arc to prevent tungsten contamination. The equipment must also be capable of continuing the shield gas flow after the arc is extinguished so that the weld can cool within the protective gas shield. It is also advisable to keep the tip of the filler wire within the gas shield until such times as it has cooled to a sufficiently low temperature.
A supplementary trailing gas shield will also need to be attached to the torch to provide protection to the cooling weld metal as the welder moves along the joint line. This makes manipulation of the welding torch more difficult. Most welders manufacture their own supplementary shields, shaped to closely fit the component; several shields would therefore be required to weld a range of pipe diameters. A backing gas is also necessary and back purging should be maintained for at least the first three or four passes in a weld. Backing gas purity should be better than 20ppm maximum oxygen.
MIG welding using argon or argon/helium mixtures may be used but this process will not provide the same high quality weld metal as the TIG process and it can be difficult to achieve the stringent quality levels required by aerospace applications. Dip transfer can lead to lack of fusion defects and spray transfer requires both leading and trailing supplementary gas shields, the leading gas shield to prevent oxidation of any spatter that may be remelted into the weld pool. The improvements in pulsed MIG power sources by the use of inverter technology and micro-processor control have obviated some of these problems and substantially narrowed the gap between MIG and TIG. MIG is, however, still difficult for the manual welder because of the difficulty of manipulating the MIG torch with a supplementary gas shroud. Because of these difficulties MIG welding is often mechanised or automated.
Plasma-TIG may be used for welding titanium, being capable of keyholing a weld up to 12.5mm thick. The same requirements for gas purity and weld pool protection required for TIG are also needed for plasma-TIG. Plasma-TIG is rarely used in a manual application and never in the keyhole mode.
Atmospheric contamination is best avoided by the use of a welding chamber or glove box that can be filled with argon. Purpose built glove boxes can be purchased but it is a simple matter to fabricate a chamber of an appropriate size using slotted angle eg DexionTM angle, to form the frame and covering this with a clear plastic or acetate sheet. The size of the component that can be welded within a glove box is necessarily restricted.
Electron beam, laser, friction, resistance spot and seam and flash welding are all used to weld titanium and its alloys. Electron beam welding, being carried out in a vacuum, needs no protective gas shield. Conventional friction welding may also be carried out without a protective shield although a gas shield should be used when friction stir welding. Similarly, no gas shield is required when resistance welding, although for the most critical applications a gas shield is recommended. Laser and flash welding both require gas shielding for the best results and least atmospheric contamination.
This article was written by Gene Mathers.