TIG Welding Aluminum TIG Welding Titanium TIG Welding Magnesium TIG welding of aluminum uses similar techniques and filler  materials as oxy-fuel welding. Consult with the particular  welding machine manufacturer for recommendations on  tungsten type and size, as well as basic machine settings  for a particular weldment because this varies with specific  machine types. Typically, the machine is set to an AC output  waveform because it causes a cleaning action that breaks  up surface oxides. Argon or helium shielding gas may be  used, but argon is preferred because it uses less by volume  than helium. Argon is a heavier gas than helium, providing  better cover, and it provides a better cleaning action when  welding aluminum.  Filler metal selection is the same as used with the oxy-fuel  process; however, the use of a flux is not needed as the  shielding gas prevents the formation of aluminum oxide on  the surface of the weld pool, and the AC waveform breaks  up any oxides already on the material. Cleaning of the base  metal and filler follows the same guidelines as for oxy-fuel  welding. When welding tanks of any kind, it is a good practice  to back-purge the inside of the tank with a shielding gas. This  promotes a sound weld with a smooth inner bead profile that  can help lessen pinhole leaks and future fatigue failures. Welding is done with similar torch and filler metal angles as  in oxy-fuel welding. The tip on the tungsten is held a short  distance ( 1 ⁄ 16  – 1 ⁄ 8 -inch) from the surface of the material, taking  care not to ever let the molten pool contact the tungsten and  contaminate it. Contamination of the tungsten must be dealt  with by removal of the aluminum from the tungsten and re- grinding the tip to the factory recommended profile.  Magnesium alloys can be welded successfully using the  same type joints and preparation that are used for steel or  aluminum. However, because of its high thermal conductivity  and coefficient of thermal expansion, which combine to cause  severe stresses, distortion, and cracking, extra precautions  must be taken. Parts must be clamped in a fixture or jig.  Smaller welding beads, faster welding speed, and the use  of a lower melting point and lower shrinkage filler rods are  recommended.  DC, both straight or reverse polarity, and AC, with  superimposed high frequency for arc stabilization, are  commonly used for welding magnesium. DC reverse polarity  provides better cleaning action of the metal and is preferred  for manual welding operations.  AC power sources should be equipped with a primary  contactor operated by a control switch on the torch or a foot  control for starting or stopping the arc. Otherwise, the arcing  that occurs while the electrode approaches or draws away  from the work piece may result in burned spots on the work.  Argon is the most common used shielding gas for manual  welding operations. Helium is the preferred gas for automated  welding because it produces a more stable arc than argon and  permits the use of slightly longer arc lengths. Zirconiated,  thoriated, and pure tungsten electrodes are used for TIG  welding magnesium alloys. The welding technique for magnesium is similar to that used  for other non-ferrous metals. The arc should be maintained  at about  5 ⁄ 16 -inch. Tack welds should be used to maintain fit  and prevent distortion. To prevent weld cracking, weld from  the middle of a joint towards the end, and use starting and  run off plates to start and end the weld. Minimize the number  of stops during welding. After a stop, the weld should be  restarted about ½-inch from the end of the previous weld.  When possible, make the weld in one uninterrupted pass.  The techniques for welding titanium are similar to those  required for nickel-based alloys and stainless steels. To  produce a satisfactory weld, emphasis is placed on the  surface cleanliness and the use of inert gas to shield the  weld area. A clean environment is one of the requirements  to weld titanium. TIG welding of titanium is performed using DC straight  polarity. A water-cooled torch, equipped with a ¾-inch  ceramic cup and a gas lens, is recommended. The gas lens  provides a uniform, nonturbulent inert gas flow. Thoriated  tungsten electrodes are recommended for TIG welding of  titanium. The smallest diameter electrode that can carry  the required current should be used. A remote contactor  controlled by the operator should be employed to allow the  arc to be broken without removing the torch from the cooling  weld metal, allowing the shielding gas to cover the weld until  the temperature drops. Most titanium welding is performed in an open fabrication  shop. Chamber welding is still in use on a limited basis, but  field welding is common. A separate area should be set aside  and isolated from any dirt producing operations, such as  grinding or painting. Additionally, the welding area should  be free of air drafts and the humidity should be controlled.  Molten titanium weld metal must be totally shielded from  contamination by air. Molten titanium reacts readily with  oxygen, nitrogen, and hydrogen; exposure to these elements  in air or in surface contaminants during welding can adversely  affect titanium weld properties and cause weld embrittlement.  Argon is preferred for manual welding because of better  5-24