Titanium is 45% lighter than steel, 60% heavier than aluminum, and more than three times stronger than either. While much more expensive than many other metals, it offers higher corrosion resistance, lower life cycle costs, longer service life, and lower maintenance and upkeep costs. Common applications include military, aerospace, marine, chemical, power generation, oil and gas extraction, and medical devices.
Titanium is a reactive metal that readily interacts with oxygen, is prone to oxidative contamination, and is difficult to weld. However, careful preparation, consistent technique, and extensive practice can accomplish a successful weld.
Welding of Titanium
Cleanliness is essential
Welding titanium welded tubes requires extremely clean base metal, filler metal and the welding environment must be perfect. Welders rubbing body lotions such as hand creams, grease from molding and stretching processes, shop dust, paint, dirt, cutting fluids and lubricants can all contribute to brittleness and weld failure.
Clean the work area, removing all kinds of debris, and make sure you choose a place with low airflow and try to keep the environment as wind-free as possible to avoid disturbing the shielding gas while welding. Next, clean the oil from the filler rod and the base metal:Wear nitrile gloves specifically designed for this purpose to prevent hand cream on the operator's hands or oil attached to other hands from rubbing on the titanium tube to be welded. Methyl Ethyl Ketone (MEK) is then applied to a clean lint-free cloth and the titanium is wiped to remove any remaining surface contaminants. At this point place the filler rod in a sealed container to prevent further contamination.
Remove the oxide layer formed by the reaction between titanium and oxygen on the pipe surface prior to welding. This oxide layer provides titanium with significant corrosion resistance. However, it must be removed prior to welding because it melts at temperatures higher than those of titanium and can enter the molten weld pool creating inclusions that weaken the interlayer.
Either a die grinder with a carbide deburring tool or a carbide file specifically for titanium will do a better job of removing the oxide layer from welded joints. It is recommended not to use velvets and abrasives. Because of contamination. Remember to use a low grinding speed to prevent overheating. After the oxide layer has been ground off, wipe the joint over again with a cloth dampened with MEK or acetone. Some solvents have a low flash point and the solvent must be completely evaporated before striking the arc.
Assembly is probably more important for titanium tubing than for any other metal tube, as it is critical to prevent oxygen from entering the weld. The joint should be square (no v-shaped gaps should be formed) to help reduce the amount of heat and weld metal required to fill the joint, which in turn reduces the potential for burns and contamination.
Clamp the part on a positioner or bench to ensure that the ends are butted together as tightly and accurately as possible.
Most thin-walled titanium tubing and pipe do not require preheating. However, if you plan to weld titanium tubing thicker than 1/3 inch, consult your welding equipment supplier, as some preheating and post-heating may be beneficial.
Importance of shielding gas coverage
Pure argon gas is recommended for welding titanium because of its purity and low water content. A 75/25 argon/helium mixture can be used to improve stability and increase penetration only when specified.
The American Welding Society (AWS) recommends measuring the purity of welding gases to ensure they meet the standards for each application. Shielding gases should be at least 99.995 percent pure, with no more than 20 parts per million (PPM) oxygen and a dew point above -76 degrees Fahrenheit. Other applications require 99.999% pure argon flow.
It's important to equip the torch with a tailpiece. Otherwise, the risk of oxygen contamination increases, and with it, the possibility of rupture. Some welders manufacture their own tailing shields, although there are many styles available for purchase. The trailing guard conforms to the shape of the tube and follows the GTAW torch along the tube. The shield provides additional argon protection for the weld after the torch and its argon gas flow. Setting the torch and trailing shield gas flow to 20 cubic feet per hour (CFH) provides better shield gas coverage.
Always use clean, non-porous plastic hoses to deliver shielding gas to the torch, trailing shield, and purge unit.
Filler Metal Options
Use filler metal when welding titanium tubing greater than 0.010 inch thick (see Figure 1). Generally, the filler metal should be the same grade as the titanium being welded-theoretically, it should be identical. Exceptions are allowed for certain applications, such as when the filler metal has a lower yield strength than the base material in order to improve ductility. However, any variation should be carefully tested and investigated to ensure that it meets the process requirements and specifications.
Picking the right flame and consumables
GTAW provides better control of the welder's heat input and molten pool than other welding techniques. A machine with high frequency arc start, remote current control capability, post-flow timer and a minimum 250 amp output gtaw inverter would be an excellent welding titanium.
Always set the machine polarity to DC electrode negative (DCEN). DCEN provides deeper penetration than DC electrode positive (DCEP).
Match the inverter to an air or water-cooled torch. Air-cooled cutting torches can provide good performance if your welding temperature is below 150 amps and their cost is lower than water-cooled cutting torches. On the other hand, water-cooled torches are small, easy to operate, and can weld at higher amperages for longer periods of time, with most welds on titanium being short, producing output levels below 150 amps.
Use a 2% tungsten metal electrode grounded in line with the welding current as follows
Up to 90 amps: 1/16 inch or less
90-200 amps: 3/32 of an inch
Over 200 amps: 1/8 inch
The gas lens distributes uniform, shielding gas and creates a smooth stream over the weld pool.
Strike an arc and start working
Start welding by first cutting off the end of the feed rod to show a pure, uncontaminated spot. Start the argon gas stream for a few seconds and then initiate the arc, making sure the weld area is completely covered.
Utilize the inverter's high-frequency arc start function. The torch angle, torch speed and wire fill angle are similar to those for welding stainless steel, providing better conditions for welding titanium tubing.
It is fairly easy to create a weld pool from titanium, but it may not be easy to move. Pushing the weld pool with the arc and filler rod usually gives good results, but the filler rod must be kept inside the shielding gas enclosure while welding. It is also important to minimize heat input because too much heat can break the weld. Use the dab technique with filler metal (at a steady travel speed).
After completing the weld, allow 20 to 25 seconds of subsequent flow to protect the joint as it cools to a threshold below 800 degrees Fahrenheit. Stop oxygen from reacting with titanium. Some welds may require temperatures below 500 degrees Fahrenheit.
Once the weld is complete, titanium shows its true colors. The color of the welded joint indicates the degree to which the shielding gas protects the weld from contamination and the thickness of the oxide layer (see Figure 2). The quality of titanium welds can be determined in addition to visual inspection, dye penetration, hardness examination, X-ray, ultrasonography, and destructive examination.
