Commercially pure titanium contains 98-99.5% titanium. It consists of unalloyed titanium and titanium that is alloyed by small additions of oxygen, nitrogen, carbon and iron to improve strength. In industry, these grades are the most widely welded titanium alloys. This is due to their combination excellent corrosion resistance, good ductility and excellent weldability.
One of the great benefits of welding the commercially pure grades of titanium is that there is no concern for segregation.The most common CP grades are ASTM Grades 1, 2, 3 and 4. These differ by the varying degrees of oxygen and iron content. Grade 1 is the most pure, and the mechanical properties increase with the grade number. Grades with greater amounts of oxygen and iron have higher tensile strength but lower ductility and weldability. When welding CP alloys, 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 and tin, and trace amounts of oxygen, nitrogen, and carbon. They have medium strength compared to other titanium alloys. Also, have reasonably good ductility, excellent mechanical properties at cryogenic temperatures, and are generally very weldable. Additionally, they 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 stated in its name, alpha-beta alloys of titanium contain both phases. These alloys possess a characteristic two-phase microstructure. These are formed by the addition less than 6% aluminium and varying amounts of the beta forming constituents. These include vanadium, chromium and molybdenum.
These alloys have medium to low strength compared to the other titanium grades. Unlike the commercially pure 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 readily weldable. However, their weldability is dependent on the amount of beta present. Increased amounts of beta stabilizing elements reduces their weldability. Also, the most strongly beta-stabilized alloys become embrittled while welding. These very high strength, high beta content, alloys are therefore 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.
The Beta alloys of are the smallest group of titanium alloys. They are used when particularly high strengths are needed alongside light-weight and corrosion resistance. These 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. To obtain full strength and preserve ductility/avoid embrittlement, beta alloys are typically welded in the annealed condition. Next, they are cold-worked, then solution treated and aged.
Beta 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.
The weldability of titanium alloys is usually assessed by the toughness and ductility of the weld metal. Commercially pure grades are considered very easy to fabricate, whereas Titanium alloys show reduced weld metal ductility and toughness. The table below highlights the weldability of the common titanium and titanium alloy grades.