Welding of Titanium Alloys

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Introduction to Titanium Alloy Welding

Commercially pure titanium and most titanium alloys can be welded using procedures and equipment similar to those employed in welding austenitic stainless steel and aluminum. However, the high reactivity of titanium and its alloys at temperatures above 550°C requires additional precautions to shield the weldment from air contact. Furthermore, both titanium base metal and filler metal must be thoroughly cleaned to prevent contamination during the welding process.

Weldability of Different Titanium Alloy Types

Alpha Titanium Alloys

Unalloyed titanium and all alpha titanium alloys demonstrate excellent weldability characteristics. The alpha-beta alloy Ti-6Al-4V and other weakly beta-stabilized alloys are also readily weldable. However, strongly beta-stabilized alpha-beta alloys experience embrittlement during welding, which limits their application in welded structures.

Most beta alloys can be welded successfully, though caution must be exercised when considering heat treatment for strength enhancement. Aged welds in beta alloys often exhibit brittleness, making post-weld age hardening treatments potentially problematic.

Unalloyed Titanium Grades

Unalloyed titanium is available in several commercial grades with purity levels ranging from 98.5 to 99.5% titanium. These grades achieve their strength through controlled variations in oxygen, nitrogen, carbon, and iron content. While strengthening through cold working is possible, it is rarely employed in practice. All grades are typically welded in the annealed condition, as welding of cold-worked alloys results in annealing of the heat-affected zone (HAZ), eliminating the strength benefits of cold working.

Alpha Alloy Welding Conditions

Alpha alloys including Ti-5Al-2.5Sn, Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-5Sn-2Zr-2Mo, Ti-6Al-2Nb-1Ta-1Mo, and Ti-8Al-1Mo-1V are consistently welded in the annealed condition to ensure optimal results.

Alpha-Beta Alloy Considerations

Alpha-beta alloys such as Ti-6Al-4V can be welded in either the annealed condition or in the solution-treated and partially aged condition, with aging completed during post-weld stress relieving. Unlike unalloyed titanium and alpha alloys, which can only be strengthened through cold work, alpha-beta and beta alloys offer the advantage of heat treatment strengthening.

The reduced weld ductility commonly observed in alpha-beta alloys results from phase transformations occurring in either the weld zone or the HAZ. These alloys can be welded autogenously or with various filler metals. Lower alloyed materials are commonly welded with matching filler metals, while filler metal of equivalent grade or one grade lower ensures optimal weld strength and ductility.

For Ti-6Al-4V alloy welding, filler metal of matching composition is typically used. The extra low-interstitial (ELI) grade significantly improves both ductility and toughness characteristics. However, using filler metals that enhance ductility may not completely prevent HAZ embrittlement in susceptible alloys. Additionally, low-alloy welds remain vulnerable to embrittlement through hydride precipitation, though proper joint preparation, filler-metal storage, and adequate shielding can prevent this issue.

Beta Alloy Welding Procedures

Metastable beta alloys including Ti-3Al-13V-11Cr, Ti-11.5Mo-6Zr-4.5Sn, Ti-8Mo-8V-2Fe-3Al, Ti-15V-3Cr-3Al-3Sn, and Ti-3Al-8V-6Cr-4Zr-4Mo are weldable in either annealed or solution heat-treated conditions. In the as-welded condition, these welds exhibit low strength but maintain good ductility. Beta alloy weldments are sometimes utilized in the as-welded condition, though welds in Ti-3Al-13V-11Cr alloy experience more severe embrittlement when age hardened.

To achieve full strength in metastable beta alloys, the recommended procedure involves welding in the annealed condition, followed by cold working through planishing, and concluding with solution treatment and aging. This comprehensive procedure also ensures adequate ductility in the final weld.

Titanium Welding Processes and Applications

Available Fusion Welding Methods

Several fusion welding processes are successfully employed for joining titanium and titanium alloys:

Gas-tungsten arc welding (GTAW) represents the most widely used process for titanium alloy joining, except for thick-section applications. Square-groove butt joints can be welded without filler metal in base metals up to 2.5 mm thick. Thicker base metals require joint grooving and filler metal addition. Critical to success is proper shielding of heated weld metal from atmospheric contamination by oxygen, nitrogen, and carbon, which severely degrades weldment ductility.

Gas-metal arc welding (GMAW) serves as the preferred method for joining titanium and titanium alloys exceeding 3 mm thickness. This process utilizes either pulsed current or spray mode operation and offers cost advantages over GTAW, particularly when base metal thickness exceeds 13 mm.

Plasma arc welding (PAW) provides another viable option for titanium alloy joining. This process operates faster than GTAW and accommodates thicker sections, enabling single-pass welding of plates up to 13 mm thick using keyhole techniques.

Electron-beam welding (EBW) finds extensive application in aircraft and aerospace industries for producing high-quality welds in titanium and titanium alloy plates ranging from 6 mm to more than 76 mm thick. The high-vacuum welding environment achieves exceptionally low contamination levels in the weldment.

Laser-beam welding (LBW) is experiencing increased adoption for titanium and titanium alloy joining. Square-butt weld joint configurations are feasible, and the process eliminates vacuum chamber requirements, though gas shielding remains necessary. Base metal thickness limitations typically restrict applications to materials not exceeding 13 mm.

Friction welding (FRW) proves particularly useful for joining tubes, pipes, or rods where joint cleanliness can be achieved without atmospheric shielding requirements.

Resistance welding (RW) effectively joins titanium and titanium alloy sheet materials through either spot welds or continuous seam welds. This process also facilitates welding titanium sheet to dissimilar metals, particularly for cladding titanium to carbon or stainless steel plate applications.

Process Limitations and Considerations

Fluxes cannot be used with these titanium welding processes because they react with titanium to cause brittleness and may compromise corrosion resistance. Welding processes that typically employ fluxes, including electroslag welding, submerged arc welding, and flux-cored arc welding, have seen limited application. These processes are generally not considered economical due to their requirement for expensive fluoride-based fluxes.

Filler Materials and Electrode Selection

Filler Metal Composition Guidelines

Filler metal composition typically matches the grade of titanium being welded. For enhanced joint ductility when welding higher strength grades of unalloyed titanium, filler metal with yield strength lower than the base metal is occasionally employed. Welding dilution ensures the weld deposit achieves required strength levels. Unalloyed filler metal sometimes serves for welding Ti-5Al-2.5Sn and Ti-6Al-4V applications where improved joint ductility is prioritized.

Using unalloyed filler metals reduces the beta content of the weldment, thereby minimizing transformation effects and improving ductility. Engineering approval is recommended when employing pure filler metal to verify that welds meet specified strength requirements.

Alternative filler metal options include materials with lower interstitial content (oxygen, hydrogen, nitrogen, and carbon) or reduced alloying content compared to the base metal. While filler metals that improve ductility offer advantages, they do not eliminate the possibility of HAZ embrittlement in susceptible alloys. Additionally, low-alloy welds may increase susceptibility to hydrogen embrittlement.

Shielding Gas Requirements

Shielding gases for titanium and titanium alloy welding are limited to argon and helium, with occasional use of gas mixtures. Argon sees more widespread application due to its greater availability and lower cost compared to helium alternatives.

Electrode Specifications

Conventional thoriated tungsten electrodes (EWTh-1 or EWTh-2) are standard for GTAW of titanium materials. Electrode size selection is governed by the smallest diameter capable of carrying the required welding current. To enhance arc initiation and control arc spread characteristics, electrodes should be ground to a point. The electrode may extend one and a half times its diameter beyond the nozzle end for optimal performance.

Conclusion

Successful titanium alloy welding requires careful consideration of material type, welding process selection, and contamination prevention measures. While multiple welding processes are applicable, proper shielding, clean materials, and appropriate filler metal selection remain critical for achieving high-quality results. Understanding the specific characteristics and limitations of different titanium alloy types enables welders and engineers to select optimal procedures for their specific applications.