Commercially Pure Titanium contains 98-99.5% titanium. The small additions of oxygen, nitrogen, carbon, and iron improve strength. CP alloys have the best weldability of titanium grades. This is due to their combination excellent corrosion resistance, good ductility and excellent weldability.
The most common CP grades are Grades 1, 2, 3 and 4. The difference between these is how much oxygen and iron are alloyed in them. Grade 1 is the most pure and also the weakest. Keep in mind that the mechanical properties increase with the grade number. Grades with more oxygen and iron have higher strength but lower ductility and weldability.
When welding CP Titanium, you should use a filler that is one or two PSI strength grades lower than the parent metal. The weld dilution with the base metal will increase in the strength of the weld metal.
Alpha alloys typically contain aluminum, tin, and trace amounts of oxygen, nitrogen, and carbon. Also, they have medium strength compared to other titanium alloys. Also, they have reasonably good ductility and excellent mechanical properties at cryogenic temperatures. Lastly, they are very weldable and are always welded in the annealed condition.
Alpha alloys do not respond to heat treatment. However, they can be strengthened by cold working. Alongside the CP Titanium grades, Alpha alloys possess the highest corrosion resistance of the Titanium groups.
As the name indicates, Alpha-Beta alloys of Titanium contain both crystal structures. They are formed by the addition less than 6% aluminium and varying amounts of the Beta forming elements. These include vanadium, chromium and molybdenum.
These alloys have medium to low strength compared to the other Titanium grades. Unlike CP and Alpha alloys, which can only be strengthened by cold work, Alpha-Beta alloys are heat treatable. Therefore, these grades can undergo machining while the material is still ductile. Then they can be heat treated to further strengthen the material.
Alpha-Beta alloys are generally weldable. However, their weldability is dependent on the amount of Beta present. The higher the Beta elements, the lower the weldability of titanium grades. Also, the higher the Beta elements, the more brittle the welds become. High-Beta grades are very strong and rarely welded.
Alpha-Beta alloys can be welded with various filler metals. It is common to use filler metal of an equivalent grade, especially for the lower alloyed materials. Another option is one grade lower to ensure good weld strength and ductility.
Beta alloys are the smallest group of titanium alloys. They are high strength, low weight, and highly corrosion resistant. Beta alloys are fully heat treatable, possess good hardenability, and are generally weldable.
Beta alloys are slightly denser than other titanium alloys. But, they have the highest strength and good creep resistance. These grades are welded in the annealed or solution heat treated condition. When welded, the joint has a low strength but is ductile. Next, they are cold-worked, then solution treated and aged. This increases strength but avoids embrittlement.
These alloys are welded using filler wire of matching composition. However, when welding higher strength titanium alloys, fillers of a lower strength are sometimes used to maintain weld metal ductility.
How To Weld Titanium
Because of the diverse weldability of titanium alloys, it is welded in a few different ways.
Gas-tungsten arc welding is the most widely used process for joining titanium. However, it should not be used when welding parts with thick sections. Joints can be welded without filler metal in base metals up to 2.5 mm thick. For thicker base metals, a filler metal is required and the joint should be grooved. It is imperative that weld pool is properly shielded from the atmosphere to prevent contamination with oxygen, nitrogen, and carbon, which will lead to embrittlement.
Laser-beam welding is becoming increasingly popular for joining titanium and titanium alloys. This process does not require the use of vacuum chambers, though gas shielding is still required. This process is more limited than some others, as the base metal thickness cannot usually exceed 13 mm.
Gas-metal arc welding is used to join titanium and titanium alloys thicker than 3 mm, using pulsed current or the spray mode. This process is less costly than GTAW, especially when a considerable thick base metal thickness (>13 mm).
Plasma arc welding is another viable process for joining titanium and titanium alloys. It is faster than GTAW and, similar to gas-metal arc welding, it can be used on thicker sections.
Friction welding is useful in joining titanium tubes, pipes, or rods, as joint cleanliness can be achieved without shielding.
Resistance welding is used to join titanium and titanium alloy sheet by either spot welds or continuous seam welds. Though it is not recommended, this process is also used for welding titanium sheet to dissimilar metals, such as carbon or stainless steel plate.
Can You Weld Titanium To Other Metals
You can weld titanium and other metals together, but there are specific steps you need to take when joining them. Welding titanium to steel together requires you to use 99.999% pure Argon gas with either the TIG or MIG welding procedure. Welding titanium to aluminum requires the temperature on the titanium side of the alloying melt boundary to remain below 2000° C. (Ref. Patent US4486647A)
Common Defects in Welding Titanium
Titanium is readily welded, given proper considerations and precautions have been taken. Titanium welds are not prone to solidification cracking or hydrogen cracking, unlike many other metals. However, they are prone to imperfections when welding if care is not taken.
Hydrogen arrives from moisture in the arc environment, or hydrocarbon contamination. Specifically, oils, greases, lubricants, and solvents contain hydrocarbons. While weld metal is in a molten state, it absorbs a high amount of hydrogen. Then, as it solidifies, it tries to expel the hydrogen.
However, if the weld is solidifying even moderately quickly, the hydrogen doesn’t have a chance to escape and instead forms small pockets in the weld. To reduce this risk, thorough cleaning of the welding surface and filler wire should be performed.
Degreasing: Remove any contaminants by using either steam, solvent, alkaline, or vapor degreasing.
Pickling: Use a surface treatment of HF-HNO3 solution to remove any surface oxide.
Mechanical cleaning: After pickling, use a clean stainless steel brush, to lightly grind away any remaining surface oxide on the metal surface.
Additionally, use a lint-free cloth to wipe away the impurities. Also take care not to touch the cleaned surface before welding.
Embrittlement, or lack of ductility in the metal, can be caused by weld metal contamination by either gas absorption or by dissolving contaminants on the surface of the metal. When titanium is heated, it becomes highly reactive and readily combines with oxygen, nitrogen, hydrogen and carbon to form oxides. The absorption of these oxides embrittles the weld. This potentially renders the weld and part useless.
Similar to countering porosity, thorough cleaning reduces the risk. Additionally, proper gas shielding of the weld puddle is critical.
Using the wrong filler metal: Attempting to weld titanium with any material other than titanium gives the weld a glass-like hardness that will result in cracking before it even cools.
Contamination: If iron particles or dust are present on the surface of the parent or filler metal, they will dissolve into the metal when welded. The resulting pockets of iron deposits in the weld metal frequently crack. These iron deposits also reduces the corrosion resistance quality of titanium. Therefore, when welding titanium it is best to do so in a reserved area clear of contaminants. Be particularly attentive to separate the area from steel fabrication operations, including your tools and gloves.