The properties of titanium make it an excellent choice of engineering material for a range of applications, from aerospace to chemical processing due to its high relative strength and corrosion resistance properties. These properties can be improved by alloying various elements to enhance specific properties making titanium a truly versatile engineering alloy.
The chemical characteristic that makes titanium so corrosion resistant is the oxide layer that forms on its surface. This occurs as titanium is a very reactive material, reacting readily with oxygen and nitrogen. This oxide layer however, protects it from corrosion in most situations, most notably chloride containing environments.
The mechanical properties of titanium, including excellent toughness even at low temperatures and good creep resistance at up to 600⁰C make it the material of choice for many aerospace welding applications.
Welding titanium has its own challenges, due mainly to the very same characteristics that make it so corrosion resistant. The reactiveness that promotes the resilient oxide layer that protects it also makes it difficult to weld. The selection of the correct titanium welding filler wire, but also the shielding gas, both play an important role in ensuring a good quality weldment.
The mechanical properties of the alloy are determined by the crystallographic structure of titanium. This allotropic alloy has two different microstructures depending on the temperature and chemical composition.
Titanium alloys are divided into four categories, commercially pure (CP), alpha alloys, alpha-beta alloys and beta alloys. The alloying elements determine the crystal structure of the material with oxygen, nitrogen, aluminium encouraging an alpha (CPH) structure and vanadium, molybdenum and silicon being beta stabilisers.
Unalloyed titanium (Grades 1-4, CP titanium) is alpha phase at room temperatures and changes at the alpha/beta transition temperature of about 880⁰C. The different grades from 1 to 4 have increasing levels of oxygen, nitrogen and carbon as impurities. The higher the level of impurities the less ductile and higher strength is the alloy. The more commonly used titanium welding filler wire is Grade 2 (VBC Alloy 0070) which is supplied to MSRR 9500/0070 or AMS 4951 specifications for aerospace welding. Grade 2 weld wire is used to weld other CP titanium grades but also for many of the other titanium grades where ductility is more important than strength.
The alpha and near alpha alloys are not heat treatable. Grade 6 titanium, Ti-5Al-2.5Sn is used in aerospace applications due to its good weldability and stability and strength at high temperatures.
The alpha-beta alloys, typified by the most commonly used titanium alloy Ti-6Al-4V, is an excellent all round alloy. It is heat treatable and has the combination of strength and corrosion resistance making it useful in applications as diverse as aerospace, marine applications and medical devices. It is readily welded with a matching weld filler metal.
Beta alloys such as Grade 19 Ti-3Al-8V-6Cr-4Mo-4Zr are heat treatable and for the most part weldable. They are more dense than Ti-6Al-4V but have high strength and good creep resistance for use in armour plating. They are typically welded in the annealed state with filler wire of matching composition.
As mentioned previously, the shielding gas plays a crucial role in achieving a good weld. This means the correct type of gas, preferably high purity argon, should be used when TIG welding. Poor shielding will become obvious when discoloration is seen on the weld, ranging from a light straw colour for mild contamination to dark blue to powdery white for worst-case contamination. The weld pool is protected by the usual gas shroud but there is also a need for a trailing shield to prevent the “just welded” area from contamination until it has cooled to below about 300⁰C. In addition, the underside of the weld should be protected similarly.
Also of huge importance is the cleanliness of the components being welded and the titanium filler wire. Any residual grease, dirt or moisture must be removed by degreasing before welding. These will result in hydrogen contamination, a common source of porosity and embrittlement in the weld. Likewise, entrapped air in the shielding gas can cause cracking due to excessive absorption of oxygen and nitrogen. These both increase the tensile strength and hardness but at the cost of a reduction in the ductility of the joint, so much so that cracking can occur. To protect against this high purity (BIP) argon should be used.
The desired properties of the welded joint in conjunction with the alloy being joined will determine the filler wire alloy selection. Most of titanium alloys are readily welded with the exception of high beta content alpha-beta alloys. Grade 2 titanium filler wire is frequently used when welding CP Titanium or for dissimilar titanium alloys to give the best combination of ductility and strength. Otherwise a filler wire with matching chemistry to the parent titanium alloy is often used.
The titanium alloy aerospace welding filler wires supplied by VBC Group are produced with high purity surface characteristics and comply with AMS, MSRR 9500 & AWS specification requirements and have full technical support.