Welding special metals requires skill and knowledge
by Brent Williams
Because of its exceptional corrosion resistance, titanium is frequently chosen by companies to help increase the service life of parts and reduce life-cycle costs.
This material also offers an excellent strength-to-weight ratio: it is 45 percent lighter, yet more than three times stronger than mild steel. It is also relatively maintenance free. These characteristics make titanium an ideal metal for aerospace, marine, military, chemical, power generation, medical devices manufacturing and oil and gas extraction applications. However, titanium demands a high cost and can be very challenging to weld.
Following are some recommended tips to help successfully GTA weld titanium (ASTM Grade 5 titanium: Ti-6A1-4V).
Cleanliness is key
It is critical to keep titanium clean prior to and during welding. Because it is a highly reactive metal, titanium responds quickly (and negatively) to contaminants such as oils from forming and drawing process; shop dust; paint; dirt; body oils (from hands); cutting fluids and more. Encountering any or all of these contaminants can easily lead to localized corrosion or cause weld embrittlement and failure. To prevent such issues, always keep the welding environment as clean as possible and minimize airflow to avoid disrupting the shielding gas coverage that protects the weld pool.
Prior to welding, it is critical to pre-clean both the base material and the filler rod. During this process, always wear nitrile gloves dedicated to the task and begin by degreasing both components. Remove surface contamination by wiping the material with methyl ethyl ketone (MEK), acetone or other non-chlorinated solvent soaked into a clean, lint-free cloth. After cleaning them, place the filler rods in an airtight container until ready for use, as it protects against re-contamination.
Due to its reactivity, titanium easily forms a very hard oxide layer on its surface (similar to aluminum). This layer provides titanium with its corrosion resistance, but it also melts at a higher temperature than pure titanium. For that reason, it must be removed from the area to be welded. Use a die grinder with a carbide deburring tool or carbide file – set to a low grinding speeds – to remove the layer of titanium oxide without overheating the base metal, which can also lead to embrittlement. After removing the oxide layer, once again wipe the area to be welded using MEK, acetone or other non-chlorinated solvent, and allow it to dry completely before welding.
Two important words of caution: one, be certain to use the grinder exclusively for the task so as to avoid introducing contaminants from other jobs. Two, never use steel wool or other abrasives to remove the titanium oxide layer as it can contaminate the metal and lead to weld defects.
Employ proper preparation techniques
Part fit-up when welding titanium is very important since gaps between parts can lead to contamination from the underside of the weld. Prior to striking an arc, use a clamp or tooling to hold parts together and prevent movement during GTA welding. Typically, it is not necessary to pre-heat titanium under 1/8-in. thick, but some applications may require a pre- and post-heat to ensure weld integrity. Always follow the prescribed welding procedures regarding these matters. Also, to reduce the chance of contamination, and to minimize the heat and the amount of filler rod needed to fill the weld, avoid beveling the edges of titanium parts.
Shield the weld pool
The American Welding Society (AWS) recommends measuring shielding gas purity to ensure quality when GTA welding titanium. Most titanium applications call for a 100 percent Argon shielding gas that is at least 99.995 percent pure. In addition, the shielding gas should have no more than 20 parts-per-million (ppm) of oxygen and a dew point greater than –50 to –76 degrees Fahrenheit. Some applications demand even higher purity levels (up to 99.999 percent purity), so always consult the welding procedure. When specifications allow, a 75 percent Argon/25 percent Helium mixed gas may be used to improve arc stability and increase weld penetration. The gas should be set at 20 cubic feet/hour (cfh) to obtain ideal weld protection. Use a plastic hose to transport the shielding gas, as a rubber hose can allow oxygen to mix with the shielding gas and contaminate the weld.
There are additional considerations for shielding gas when GTA welding titanium, including back purging and the use of a GTAW torch with a trailing shield.
Back purging the underside of the weld prevents oxygen from contaminating welds from below. It is important to allow enough shielding gas to replace the air environment ten times over in order ensure complete weld protection. Similarly, adding a trailing shield to the GTAW torch keeps the shielding gas over the weld longer, decreasing the potential for weld contamination while the material is above the 500 to 900-degree threshold. Below that threshold, oxygen can no longer react with the titanium. Trailing shields are available through welding distributors; however, some companies also fabricate their own trailing shield to match their specific application.
Also, for complex geometries or large weldments that cannot be adequately shielded, an argon enclosure may be used. This enclosure is filled with argon as the air is vented in order to provide an inert atmosphere. Special gloves and viewing ports give welders access to the parts in the box while the enclosure protects the titanium parts during the weld and cooling process.
Select the right filler metal and tungsten electrode
Use a pointed, 2-percent ceriated or lanthanated tungsten electrode when GTA welding titanium. Follow this sizing for applications: when welding below 90 amps, use a 1/16-inch tungsten electrode; use a 3/32-inch tungsten electrode for applications between 90 and 200 amps; and use a 1/8-inch tungsten electrode for applications over 200 amps.
For most applications, match the filler rod strength exactly to the titanium base metal. Some applications may call for a different filler metal based on the desired mechanical properties of the weld or the service conditions. An example would be to use lower strength filler metal in order to improve ductility. Thin sections of titanium can be welded autogenously, or without filler. Regardless of the situation – welding consumables, techniques and quality requirements for titanium should be based on welding codes and procedures that apply to the industry or application.
Make the correct equipment selection
For the best results when GTA welding titanium, use an inverter-based power source that provides high frequency arc starting and at least a 250-amp output. Ideally, this power source should also provide remote amperage control capabilities and a post-flow timer.
Direct current electrode negative (DCEN) is commonly used to weld titanium. The process provides good arc control and when done properly can produce consistent results with good penetration and travel speeds. Alternating current (AC) can also be used if additional oxide cleaning is required. While considered a specialized variation, AC welding has become more common as GTAW inverters offer extended AC balance control – so the cleaning or electrode positive (EP) cycle can be set to minimal levels. The equipment will depend on the weld procedure.
Depending on the application, air-cooled and water-cooled torches may both be used to weld titanium. Air-cooled torches work well for portable applications, short welds and/or applications under 150 amps. Water-cooled torches are best for high-amperage applications. They are also preferred for production or continuous welds since they are lighter and typically easier to maneuver than air-cooled torches. Because they are typically smaller than air-cooled torches, water-cooled models also offer more accessibility to the weld joint. The decision to use a water-cooled torch should be considered when purchasing a power source. Packages are available with the matching torch, water circulator and other accessories that are optimized to work together.
The addition of a gas lens to either torch helps ensure smoother and more consistent shielding gas coverage of the weld, and it can help reduce instances of contamination. Gas lenses are commonly used for short welds on titanium, as they provide a less turbulent and more consistent flow of shielding gas. As welds increase in length, other methods, like a trailing shield or inert gas enclosure must be used.
Executing successful techniques
After completing all preparations properly and selecting the right equipment, begin welding with a freshly cut, ground and contaminant-free tungsten. Allow the shielding gas to surround the weld area for a few seconds before striking an arc using the inverter’s high-frequency start feature.
The titanium weld pool forms easily and tends to be sluggish, similar to that produced when GTA welding stainless steel. For that reason, using similar torch and filler metal angles, and torch speed is appropriate. Use a “dab” technique, keeping the filler metal within the shielding gas envelope. Because excessive heat can cause the weld to crack, it is important to minimize input as much as possible, too.
Upon completing the weld, allow the shielding gas to continue covering the weld for 25-50 seconds to prevent the atmosphere from contaminating the weld. Some codes require shielding gas post-flow times even longer depending on the welding amperage and material thickness. Post-flow of shielding gas is very important, because the titanium is high reactive until the weld cools below 500 degrees Fahrenheit. Always check the welding procedures for post-flow requirements.
Finally, know the proper color or a titanium weld. The color of a cooled weld indicates the thickness of the resulting oxide layer and whether the shielding gas has sufficiently protected the weld from contaminants. Figure 1 provides a guide for determining the quality of the weld based on its color. Typically, bright silver and sometimes a slight gold or straw color are ideal. Any further discoloration indicates excessive heat input or poor shielding that could result in weld embrittlement. Regardless of color, additional tests, including dye penetrant inspection, hardness testing, x-rays, ultrasonic and destructive tests, should also be used to fully evaluate the soundness of the weld or to qualify a procedure. In addition, refer to the appropriate welding code, procedure or standard for each specific application for weld quality and color requirements.
Remember, titanium may be challenging to weld, but the difficulty lies just as much in preparing and cleaning the material properly, and selecting the right welding equipment, as in the actual welding process. Always take the proper precautions before ever striking an arc. Doing so will save the cost of scrapping this expensive material, and can help ensure that companies gain the corrosion resistance and long-term service life desired from using titanium for their applications.
Brent Williams is marketing manager, TIG solutions, Miller Electric Mfg. Co. and technical contributor, Weldcraft