Aluminum welding is by far the most documented process among non-ferrous metals, but it is titanium that is arguably the most impressive. Lighter than ferrous steels yet significantly stronger than aluminum, titanium provides the highest strength-to-weight ratio among metals commonly used in manufacturing and fabrication today. It is notably more expensive, but the expense is justified when taking into account the corrosion resistance, longer service life and lower maintenance/repair costs. Engineers who take the long view know that extending the life of a component more than pays for the added expense when taking into account labor and materials that would go into its repair/replacement. As such, titanium is used extensively in maritime, aviation, military, chemical, power generation, nuclear, desalination and medical applications.
Oxygen, nitrogen, hydrogen and foreign contaminants, however, are kryptonite to titanium. Contamination and defects created during the welding process wreck titanium’s strength and corrosion resistance and require this expensive material to be cut away entirely or scrapped. As such, preparation, cleanliness and protection during the weld process are critical. In this article we’ll look at key factors related to the welding of titanium and how you can ensure the optimal strength and corrosion resistance of your titanium tube and pipe1.
Handling and Prepping Titanium: Cleanliness Above All
If you walk away from this article with one message, it is that cleanliness is the key to success. So much so that it’s important not to touch the material with your bare hands. Body oils, along with other oils, grease and grime will contaminate the material. Wear nitrile gloves (or other lint-free gloves) during the prep process. Whenever possible, a dedicated workstation should be made available that minimizes the risk of cross contamination from other metals. This includes dust from aluminum, stainless steel and other common alloys.
Do not use cutting methods that leave a smeared surface – use a high-speed circular saws over a band saw when possible. A smeared surface may cause lack of fusion and should be filed to remove any smeared metal prior to welding.
The tools that you use to cut and grind your titanium should be dedicated to titanium. Stay away from soft grinding tools that may have materials embedded in them – typically use a carbide deburring tool or file. Don’t use grinding wheels or stainless steel brushes that you also use to prep other alloys. This can lead to cross contamination.
Recommended Cleaning Procedure:
1. Wearing nitrile gloves, apply an industrial cleaning agent such as acetone or methyl ethyl ketone (MEK) to a lint-free cloth and wipe the inside edges and outer surface of the pipe to remove any contaminants. Let this dissipate.
2. Like aluminum, titanium features oxides that must be removed prior to welding. Grind or file both the inner and outside surface of the pipe 1-inch back from the joint, as well as the actual edge that will butt against the other piece. Grind slowly to minimize heat input.
3. Do not use steel wool or abrasives in this task as these materials may contaminate the base metal.
4. Wipe the base metal once more with the acetone or MEK cloth and allow the moisture to dissipate before striking an arc. Do NOT use clorine-based cleaning agents.
5. Wipe down the filler metal with acetone or MEK to ensure no transfer of contaminants through the filler rod. If time passes between the cleaning of the filler rod and the beginning of the welding process, place that filler rod in an air-tight container. If the filler rod has been left exposed, clean again before welding.
6. Clip the end of the filler rod just before you begin welding to expose pure, clean titanium for the beginning of your weld.
An important safety note: dust created in the grinding and preparation of titanium can be volatile. Titanium powder is used regularly in pyrotechnics – as such, it is important to properly gather and dispose of dust created during preparation to both minimize chances for contamination and fire hazards in the workplace.
Joint Fit-Up Helps Minimize Heat Input & Exposure
As a general rule of thumb, we recommend that titanium tube or pipe thinner than 5 millimeters should be welded autogenously (square butt joint – no filler metal added). We typically don’t recommend adding filler metal until thicknesses exceed 5 millimeters, although AWS D10.6 recommends a v-groove on thicknesses exceeding 2.4 millimeters and a u-groove on thicknesses exceeding 9.5 millimeters. Ultimately, use your best judgment and follow your certified welding procedures. The advantage to welding autogenously is that you minimize heat put into the part (less time spent above the 500- to 800-degree Fahrenheit threshold where oxygen and titanium are known to react) and minimize the risk of contaminants entering the weld pool via the filler metal. Tight fit-up in all joint configurations is important to lower heat input and minimize surface area exposure to oxygen.
Shielding Gas and Back Purging Critical to Success
Titanium reacts the most with oxygen when it exceeds a certain temperature threshold. Conventional wisdom puts that threshold anywhere between 500- to 800-degrees Fahrenheit. That reaction leads to embrittlement and loss of corrosion resistance. As such, it is critical to protect the weld puddle with shielding gas until it drops below these temperatures (and why minimizing heat input is important). This includes mandatory back purging of the tube or pipe using any variety of commercially available dams and purges.
The majority of titanium pipe welding is done in open-air environments. A purged gas chamber offers extreme protection, but is expensive (in terms of cost and the shielding gas required to purge it), potentially cumbersome to weld in and does not always accommodate large work pieces. It is also more time-intensive.
100 percent argon is recommended as both a shielding and backing gas in most titanium tube and pipe welding applications. Pay close attention to your weld procedures as they may dictate the purity levels and dew point of that argon. For instance, it may call for a shielding gas with no more than 20 parts-per-million (ppm) of oxygen and/or a dew point greater than -50 to -76 degrees Fahrenheit. Some applications may even require 99.999 percent purity.
Weld procedures may dictate the occasional use of a 75/25 or 70/30 argon/helium mix as a shielding gas, but this is not common. Helium, however, may be allowed as a backing gas as it provides the same general protection as argon. Argon is recommended as the primary ingredient in the shielding gas as it provides greater arc stability, greater density, is less expensive and more readily available.
Two components that you may not generally use in other TIG welding applications that are critical to the shielding process are a gas lens and a trailing shield. A gas lens replaces the standard collet body and improves the flow and coverage of the shielding gas around the tungsten, the arc and the weld pool. Trailing shields can either be purchased or fabricated to match specific joint configurations and provide a continuous secondary shielding gas source to ensure the weld puddle and heat affected zone stay protected until each drops below the 500- to 800-degree window. Use a clean, non-porous plastic hose to transport all shielding gasses as rubber absorbs oxygen that could contaminate the weld. Some welders will also use oversized cups to achieve additional coverage surrounding the weld, but this is only necessary within practical means.
Equipment Selection
Welding titanium tube and pipe is relatively straightforward as it is recommended to weld in a Direct Current Electrode Negative (DCEN) setting. As such, a transformer- or inverter-based welding power source with DC capabilities will suffice. AC welding capabilities are not necessary (unless you are regularly using the power source to weld other alloys as well). A few key considerations:
- High frequency arc starting capabilities are critical as the tungsten should never touch the base material.
- Pulsing capabilities are extremely helpful in reducing heat input, improving arc stability, and increase penetration. As such, inverters with higher pulsing frequencies may provide an advantage here.
- Select a welder with low amperage capabilities – a power source with a range of three to 200 amps provides an excellent range for most titanium tube/pipe welding applications.
Either an air- or water-cooled torch will suffice in this application, dependent on factors such as accessibility to the joint and welding amperage. Water-cooled torches are smaller and offer greater comfort and joint accessibility, but come in at a higher price point along with the need to purchase or add a cooling device. Air-cooled torches are a bit larger, but cost less and are likely suitable for the majority of titanium welding applications.
Consumables and Other Components
According to AWS D10.6, thoriated and lanthanated tungsten electrodes are preferred, although we’ve also seen customers use two-percent ceriated tungsten. The tungsten should be ground to a point and sized as follows: 1/16-inch or small for <90 amps; 3/32-inch for 90-200 amps; and a 1/8-inch tungsten for welding above 200 amps.
Filler metal is typically matched directly to the base metal, although there are some instances where a variation is used to achieve desired mechanical properties, such as using a filler metal with lower strength to improve ductility. Filler metal selection should always be dictated by the weld process.
Welding
While your success will depend largely on your preparation, there are a few important pointers to keep in mind when welding titanium tube and pipe:
- Remember to clean the filler metal and clip off the end to expose pure titanium before welding.
- Check shielding gas apparatus for leaks or flaws that may allow for oxygen or moisture to be introduced into the weld puddle.
- Allow 2-5 seconds of pre-flow to ensure proper protection of the joint.
- Use high frequency arc starting.
- Tack welds should be made under the same conditions as the final weld – no exceptions.
- Always keep the filler metal within the envelope of the shielding gas. If you stop welding or the filler metal becomes contaminated, clip it and start new.
- Let shielding gas flow over the weld for 20 to 25 seconds upon completion to protect the seam as it cools below the contamination threshold.
- Pre- and post-weld heat treatment is typically not required in these applications, but follow instruction as detailed in your weld procedures.
Upon completion, titanium will tell you if it is an acceptable weld or not by its color. Colors ranging from silver to straw to brown are typically acceptable. As you get into blues, greens, grays and eventually to white, these welds are unacceptable. Step back and examine each step of your process to determine where contamination is entering the weld. Eliminating that variable should help you get the colors you are looking for.
If contamination is present, the weld joint will have to be completely cut away and started anew – there is no quick fix for a contaminated titanium weld.
With these tips and resources, you should be on your way to making a successful weld on titanium tube and pipe. Always consult your weld procedures to guide you and, when in doubt – clean.
Consulted Resources:
American Welding Society. AWS D10.6/D10.6M:2000 Recommended Practices for Gas Tungsten Arc Welding of Titanium Piping and Tubing Miami: American Welding Society, 2000
http://www.weldcraft.com/2008/10/tig-welding-titanium-tubing/; Jack Fulcer, 2008
1 This article discusses general best practices – always follow certified weld procedures.