Titanium weld strength relies on preparation, cleanness
March 1, 2013
Titanium is favored for its extreme strength and corrosion resistance, but improper weld preparation and the introduction of oxygen and other contaminants into the weld zone can render it useless.
Among nonferrous welding processes, aluminum is the most documented, but titanium is arguably the most impressive. Lighter than steel alloys 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, service life, and maintenance and repair costs. Engineers who take a 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 or replacement. For these reasons, titanium is used extensively in maritime, aviation, military, chemical, power generation, nuclear, desalination, and medical applications (see Figure 1).
Oxygen, nitrogen, hydrogen, and foreign contaminants affect titanium like kryptonite affects Superman. Contamination and defects created during the welding process ruin titanium’s strength and corrosion resistance and require this expensive material to be cut away entirely or scrapped. As such, preparation, cleanness, and protection during the weld process are critical.
Just as a strong foundation is critical to a building’s sturdiness, cleanness is critical to a titanium weld’s strength. Titanium is so susceptible to contamination that gloves are required for handling. Greases and oils—not only industrial products, but even the naturally occurring oils present in skin—contaminate the material. Nitrile gloves, or other lint-free gloves, are necessary during weld preparation.
If possible, do the preparation at a workstation dedicated to titanium products to minimize the risk of contamination from other metals. This includes dust from aluminum, stainless steel, and other common alloys. Likewise, use tools dedicated to titanium. Don’t use soft grinding tools that may have materials embedded in them. Instead, use a carbide deburring tool or file. Don’t use grinding wheels or stainless steel brushes that you also use to prep other alloys.
Prepare the workpieces and filler rod as follows:
An important safety note: Dust created in the grinding and preparation of titanium can be volatile (titanium powder is used for pyrotechnics). Because of this, it is important to gather and dispose of dust created during preparation to minimize the risk of a fire or explosion.
Also note that all of these steps should be done immediately before welding. If you clean the filler rod ahead of time, place it in an airtight container.
Do not use cutting methods that leave a smeared surface, which may cause lack of fusion. Use a high-speed circular saw rather than a band saw if possible. If the cutting process leaves a smeared surface, remedy the situation by filing the surface to remove any smeared metal before welding.
The advantage to welding autogenously is that you minimize heat input into the part (less time spent above the 500- to 800-degree-F 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.
As the welding temperature rises, so does the reaction between titanium and oxygen; the critical temperature range is 500 to 800 degrees F. That reaction leads to embrittlement and reduces the material’s corrosion resistance. Therefore, it is critical to minimize heat input when welding and to protect the weld puddle with shielding gas during and after welding until the temperature drops below the critical range. This includes mandatory backpurging of the tube or pipe using a commercially available dam.
The majority of titanium pipe welding is done in open-air environments. A purged gas chamber offers thorough protection, but it is expensive—factors include the cost of the chamber, the price of the shielding gas to fill the chamber, and the time it requires to set up and fill with gas.
Gas Selection. For most titanium welding applications, 100 percent argon is recommended as both a shielding and backing gas. Pay close attention to your weld procedures as they may dictate the purity levels and dew point of the argon. For instance, the procedure 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 F. 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 because 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 is more readily available.
Additional Hardware. Two components that you may not generally use in other gas tungsten arc welding applications that are critical to the shielding process are a gas lens and a trailing shield (see Figure 2). 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, nonporous plastic hose to transport all shielding gases; rubber absorbs oxygen that could contaminate the weld. Some welders also use oversized cups to achieve additional coverage surrounding the weld, but this is only necessary within practical means.
Welding titanium tube and pipe is relatively straightforward. It is recommended the weld be done with direct current, electrode negative (DCEN). A transformer- or inverter-based welding power source with DC capabilities will suffice. A few key considerations to keep in mind are:
Air- and water-cooled torches are suitable; the choice depends 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 at a higher price 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.
According to AWS D10.6, thoriated and lanthanated tungsten electrodes are preferred, although some welders use 2 percent ceriated tungsten. The tungsten should be ground to a point and sized as follows: 1⁄16 in. or smaller when the welding current is less than 90 amps; 3⁄32 in. for 90 to 200 amps; and 1⁄8 in. if the current exceeds 200 amps.
Filler metal typically matches the base metal, but on occasion a variation is used to change one of the mechanical properties, such as using a filler metal with lower strength to improve ductility. The weld process always dictates the filler metal choice.
While success in welding titanium depends mainly on cleanness and preparation, a few welding pointers help ensure a good weld.
When the weld is finished, titanium will indicate whether the weld is acceptable by its color. Silver, straw, and brown hues indicate acceptable welds. Shades of blue, green, and the continuum from gray to white mean the weld is unacceptable. If you see one of these colors, go back over the procedure to find the source of the contamination, cut out the joint, and start over.
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.Placeholder