Welding titanium is still considered an uncommon process because titanium applications are only used for specialized niches where its unique properties make it the preferred selection when considering the lifetime cost of the structure. But with a proper understanding of the requirements and adaptation of equipment and techniques, welding-titanium becomes feasible and rewarding.
Special precautions must be taken when welding titanium and titanium alloys because they are susceptible to contamination from atmosphere gases. But welding titanium is possible if these alloys are shielded from air in a vacuum or an enclosure containing inert gas.
High specific strength at intermediate temperatures
Some basic properties that make titanium alloys the natural choice for a specific range of applications are low density (about 60% relative to steel), appreciable strength at intermediate temperatures, and corrosion resistance in marine environment and in chemical process operations. Commercial applications are numerous due to their corrosion resistance to a variety of agents present in chemical plants and seawater for marine applications.
Titanium for corrosion resistance
Besides unalloyed or commercial titanium, different classes of titanium alloys are described by reference to metallurgical types called alpha, alpha-beta and beta, which indicate the main phases present in the microstructure, as seen under an optical metallographic microscope on the surface of suitably prepared, ground, polished and etched specimens.
Different properties from different structures:
Different alloys have different compositions. They are grouped according to their prevailing microstructure. But before welding titanium one must also check the condition, or the last heat treatment that was performed.
Welding titanium and titanium alloys can be performed by friction welding to themselves and to other materials, with notable success - most or all of the contaminated metal is expelled in the flash material ring subsequently removed.
Resistance Welding is used for welding titanium and titanium alloys, in sheet metal or extruded thin sections, much as it is for other materials. The fact that the material has high electrical resistance and low thermal conductivity helps in obtaining good welds while limiting the electric current to normal values. The schedules for resistance welding titanium necessitates must take these particular characteristics into account.
Although titanium is a highly reactive material, which should be protected from air contamination when hot, there is no need to employ inert gas when spot or seam welding, although the metal should always be clean and the electrodes must be thoroughly water cooled.
Fusion Welding Options
Oxyacetylene welding is not applicable to welding titanium or titanium alloys. Of the fusion welding from the arc welding processes, those used for welding titanium alloys, considered one of the easiest materials to weld, are mostly GTAW (Gas Tungsten Arc Welding or TIG), Direct Current Straight Polarity, Electrode Negative, and PAW (Plasma Arc Welding), with argon or mixtures of argon and helium. GMAW (Gas Metal Arc Welding or MIG) of titanium, although being performed successfully, requires special attention, like Direct Current Reverse Polarity, Electrode Positive, and is considered for joints at least 13 mm (0.5 in) thick.
Titanium alloys are considered to have good weldability, but due to their extreme reactivity, precautions must be taken to shield the molten pool and the hot metal from contact with air. Clean the metal with utmost care, and chemically pickle or at least stainless steel wire brush. Remember that Chlorinated Fluoro Carbon (CFC) solvents are forbidden for cleaning titanium and titanium alloys because they produce embrittlement. Use Acetone or Methyl Ethyl Ketone (MEK) instead.
In the past TIG welding titanium could be performed only in chambers, equipped with glove ports and viewing ports, filled with argon. But with accumulating experience, especially of large structure welding, this is no longer an absolute need, provided sufficient "trailing" inert gas is continuously provided.
Welding titanium is done with straight polarity direct current (tungsten electrode connected to negative pole). Power supply should have high frequency facility for arc initiation without contact to avoid contaminating the weld with tungsten bits.
PAW with keyhole technique permits high productivity welding of thick plates at higher depths and at higher speeds (although slower than those used in high energy processes), with less porosity problems when compared to GTAW. One version called micro-plasma, employing low currents, is used for welding light gages.
Welding-titanium should be followed by stress relieving in order to prevent cracks, and also to avoid stress corrosion cracking in service.
High energy welding-titanium processes are widely used, both as Laser Welding, which needs proper means of inert gas protection for the molten pool and the hot joint, and as Electron Beam Welding, where the contamination problems are taken care of in a vacuum chamber. A thin titanium foil is sometimes interposed between surfaces of difficult to Electron Beam weld materials, like alloy steels, to contribute to weld ductility.
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