Weldability of Titanium

welding titanium

Are you welding Titanium?

If so, look no further! But before you try welding titanium, there are a few things you need to know.

First, titanium is one of the most fascinating metals. It has strength similar to steel, but it is 45% lighter. Also, it maintains its mechanical properties at a wide range of temperatures. Titanium will work in below freezing temperatures without losing its toughness. But it also resists creep and oxidation at temperatures up to 600 C.

Titanium is reactive metal that forms a thin layer of titanium dioxide on its surface. This oxide layer offers excellent corrosion resistance and will survive indefinitely to acidic, chloride, and saltwater environments. While expensive initially, the lifetime cost titanium is actually quite low due to its extensive service life and reduced (or even non-existent) maintenance and repair costs.

TYPES OF TITANIUM

ASTM International recognizes 31 grades of titanium based on various combinations of mechanical properties, corrosion resistance, formability, ease of fabrication and weldability. These grades are divided up into four classes: commercially pure (CP, or unalloyed), alpha, alpha-beta, and beta.

The alloying elements determine the crystal structure of the material. Oxygen, nitrogen, and aluminum encourage an alpha structure, whereas vanadium, molybdenum and silicon act as beta stabilizers. The addition of other elements to the alloy can precisely control the crystal structure, and therefore the alloy’s properties and weldability.

Consequently, the first step to successful titanium welding is to familiarize yourself with the various alloys, their properties, and the considerations in choosing filler metal for each.

COMMERCIALLY PURE

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

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.

ALPHA-BETA ALLOYS

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.

BETA ALLOYS

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.

PROCESSES

welding titanium
Titanium and its allows are welded in a few different ways.

Gas-tungsten arc welding is the most widely used process for joining titanium and titanium alloys, with the exception of 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.

Common Defects in Welding Titanium

Titanium and its alloys are readily welded, given the adequate 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 extreme care is not taken.

POROSITY

welding titanium
The most common defect in titanium and its alloys welds is porosity. Porosity is caused when hydrogen gets trapped in cooling weld pools.

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

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.

CONTAMINATION CRACKING

welding titanium
Two common mistakes that lead to cracking in titanium welds are:

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.

LEARN MORE ABOUT WELDING

April is National Welding Month! All month long we are exploring the weldability of various metals. Learn tips and tricks for welding aluminum, copper, brass, bronze, and stainless steel.

WELDABILITY OF ALUMINUM

WELDABILTIY OF ALUMINUM HEADER

WELDABILITY OF COPPER, BRASS, AND BRONZE

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WELDABILITY OF STAINLESS STEEL

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