How to weld Titanium

Welding of Titanium & Filler wire selection guide

Titanium (Ti) is silver-colored metal.  It has a strong affinity for oxygen and forms an oxide layer on a clean surface. This leads to natural passivation and provides corrosion resistance to salt or oxidizing acid solution. Pure Titanium is very ductile and having low strength. A small amount of Aluminum, Oxygen, and Nitrogen in the alpha phase increases its strength. It has low thermal expansion and conductivity which improves its weldability.

How to weld Titanium

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Material availability:

Titanium is divided into four classes:

  1. commercially pure (CP, or unalloyed),
  2. Alpha,
  3. Alpha-beta and
  4. Beta

Note that many companies and experts treat CP and alpha alloys as one group. The “alpha” and “beta” refer to phases of the metal’s crystalline structure at various temperatures. Adding oxygen, iron, aluminum, vanadium and other elements to the alloy can precisely control the crystal structure, and hence the alloy’s properties.

The most common CP grade are ASTM Grades 1, 2, 3, and 4. They differ by the varying degrees of oxygen and iron content; greater amounts of these elements increase tensile strength and lower ductility.

Grade 2 is the most widely used, notably in corrosion resist applications. CP Grades have good ductility, well-elevated temperature strengths to 572°F, and excellent weldability. They cost less than alloyed grades but have a relatively low tensile strength, such as 70,000 to 90,000 psi for Grade 2.

Grade 5 (Ti-6Al-4V), an alpha-beta, is the most widely used of any grade of titanium. The addition of aluminum and vanadium increases tensile strength to 120,000 psi and service temperature up to 752°F, but it also makes Grade 5 less formable and slightly harder to weld than Grade 2. It is used for a range of applications in the aerospace, marine, power generation, and offshore industries.

Grade 23 is similar to Grade 5 but features reduced oxygen content that improves ductility and fracture toughness with just a slight loss of strength. Grade 9 strengths fall between Grade 4 and Grade 5, so it is sometimes referred to as a “half 6-4.” Grade 9 can be used at higher temperatures than Grade 4, offers 20 to 50 percent higher strength than commercially pure grades S, and is more formable and weldable than Grade 5.

Weldability concerns

  1. Embrittlement through contamination with air and carbonaceous (oil, grease, etc.) materials pose the biggest threat to the successful welding of titanium.
  2. Above 400 °C its oxidation resistance decreases rapidly. Therefore, it should be shielded by inert gas (Argon) to avoid contamination from OXYGEN and NITROGEN.
  3. Oxygen, Nitrogen, and Hydrogen in the form of impurities get picked up by titanium during welding if the weld pool is not protected from the atmosphere. Hydrogen solubility is increased with rising in metal temp. e.g. 8% at 300°C. Therefore, it is essential to give Argon shielding on the weld pool and also for trailing. Hydrogen and Oxygen inclusion in weld reduce the toughness.
  4. Hydrogen and Oxygen are picked up from moisture from the surrounding. Residual oil, a cleaning agent adhered to the surface causes Hydrogen and carbon pick up.
  5. Titanium can not be fusion welded directly to Stainless steel, Carbon steel, and Aluminum alloys due to the formation of brittle compounds with these alloys.
How to weld Titanium

Cleaning of Titanium before welding

  1. Prior to welding the component should be cleaned and dried thoroughly.
  2. Oil, Fingerprints, grease, paint /dye should be cleaned.
  3. Chloride and cleaning residues (Residues from Cleaning Agent) on titanium can lead to stress corrosion cracking when it is heated to above 300°C during welding. So, it is essential to clean thoroughly.
  4. Ordinary Tap water should not be used for rinsing the Titanium parts. De-mineralized (DM) water should be used.
  5. Before welding, light oxide coating on the weld edges should be removed by Pickling in an aqueous solution of 2-4 % Hydrofluoric acid and 30-40% Nitric acid Followed by DM water rinsing and Drying.
  6. After cleaning parts should be handled with Lint Free gloves even during welding.
  7. Any mechanical operation should be followed by pickling to ensure complete removal of scale or any contamination.
  8. To control porosity in welding, the edges should be scraped withdraw filing, wire brushing. This is required to remove entrapped dirt, small cracks.
  9. This cleaning is to be carried just before the start of welding. Wherever extended storage is required, it is necessary to store the parts in sealed bags containing silica gel or to be kept in a humidity-controlled room.
  10. Fixtures used for welding shall need the same cleaning process as Titanium parts.
  11. It is preferable to have a separate area to set aside for titanium fabrication.
  12. Water/moisture is a potential source of oxygen and hydrogen and all equipment, jigs, fixtures, etc. should be free from moisture.
  13. Flexible welding chambers or enclosures can be used for the welding of small components and project sites where possible.

Preparation of the joint for welding

Correct preparation of the weld joint is essential for arc welding of Ti alloys.

  • A square-cut edge can be used for all butt and corner welds for thickness upto1 mm.
  • Thicker sheets and tubes should be provided with a single V preparation with a 90° included angle and 0-0.5mm root face. This is essential to achieve consistent penetration during root pass welding.
  • Acid pickling of weld edges and weld coupons can be used to remove oxygen-contaminated metal from the surface of the titanium.
  • The surface of the weld preparation and adjoining metal is critical to the quality of the joint and should be scrupulously cleaned prior to welding.
  • The surface should be inspected to see whether a final polishing operation is necessary. The smoothness of weld edges is important for reduced porosity in arc welding.

Protection during welding of Titanium alloys & welding consideration for Titanium Welding

  • Due to the sensitivity of Titanium to embrittlement by Oxygen, Hydrogen & Nitrogen, the entire weldment and its surrounding (Around 25 mm all round) should not be allowed to remain above 300°C and this area should be protected by Argon Trailing during welding till it cooled below 300°C as best practice.
  • A Trailing shield is required to use for welding Titanium grades. The shield from the gas cup may not be adequate, therefore a trailing shield is to be fitted to the welding torch to provide extra Argon which keeps the hot weld zone shielded with Argon longer than if it were just with a torch nozzle alone. This extra shielding gives time for the metal to cool below the temperature at which it oxidizes.
How to weld Titanium
  • High purity Argon gas must be used for welding Titanium. Recommended purity of Argon is 99.999%.
  • It is recommended to check the purity of Argon gas for each batch of cylinders. Therefore, it is recommended to weld a piece of the plate prior to production weld and check that piece for bend (Bend dia. -8 t) & Visually for Porosity. (Oxygen reduces the ductility).
  • Care to be taken to ensure that moisture and air are not leaked (through the faulty system –i.e. Regulators, Pipe connections, etc.).
  • Turning, Milling, Planning- These are the most popular methods used for edge preparation. Care should be taken that material is not overheated during machining.

Discoloration in Titanium

The color of the weld is often used as a measure of contamination level in Titanium Welding.

  1. A Silvery shiny color is indicative of correct shielding & it is desired to have this color. Satisfactory weld.
  2. Light & dark bronze/brown color indicates a very small amount of contamination.
  3. Light & Dark Blue color indicates the heavy amount of contamination. NOT Acceptable.
  4. Grey-blue, Grey, and White color indicate very heavy contamination. NOT Acceptable.
  5. When several passes are to be deposited in groove weld, no contamination in between passes/layers is acceptable.
  6. It should be noted that discoloration away from the weld bead does not necessarily indicate poor shielding. Indeed, dark brown ‘tramlines’ parallel to the weld bead is commonly encountered in fusion welding.
    1. Moisture comes from improper cleaning and drying of joint prior to welding
    2. Improper tack welding & wide root opening. Hardness testing can be used to provide supporting evidence for contamination as an alternative to color criteria since contaminated welds will exhibit higher hardness.
How to weld Titanium

Porosity in Ti weldment is caused due to the following:

  1. Moisture which comes from improper cleaning and drying of joint prior to welding
  2. Improper tack welding & wide root opening.
  3. Porosity increases with a decrease in welding speed.

TIG Welding techniques:

1.TIG welding power source should be equipped with a non-contact arc strike to prevent any tungsten contamination of the weld which occurs if a touch-down technique is used. Should the tungsten electrode touch the weld then both must be carefully examined before restarting. Any tungsten in the weld must be excavated.

2.The power source should also be capable of breaking the arc on completion of a weld run without stopping the inert gas flow.

3.Bigger size of the gas nozzle to be used to ensure the adequate shielding of the weld pool.

4.Pre-flow of 6-10 seconds and post-flow of 25-30 seconds is recommended.

5.A stringer bead technique is recommended for Ti welding.

6.Heat input to be kept to the minimum possible and never more than 1.0 KJ/mm.

7.No preheating is required for Ti welding.

Preheating and Interpass temperature for Titanium welding

1.Maximum Interpass temperature should be kept as low as possible and in no case can be more than 80°C

2.Staggered welding technique to be followed to avoid localized overheating of the weld zone.

Shielding & purging gases for Titanium welding

Titanium falls into a family of metals called reactive metals, which means that they have a strong affinity for oxygen. At room temperature, titanium reacts with oxygen to form titanium dioxide. This passive, impervious coating resists further interaction with the surrounding atmosphere, and it gives titanium its famous corrosion resistance. The oxide layer must be removed prior to welding because it melts at a much higher temperature than the base metal and because the oxide could enter the molten weld pool, create discontinuities and reduce weld integrity.

When heated, titanium becomes highly reactive and readily combines with oxygen, nitrogen, hydrogen, and carbon to form oxides (titanium’s famous colors actually come from the varying thicknesses of the oxide layer). The interstitial absorption of these oxides embrittles the weldment and may render the part useless. For these reasons, all parts of the heat-affected zone (HAZ) must be shielded from the atmosphere until the temperature drops below 800°F (note: experts disagree on the exact temperature, with recommendations ranging from 500°F to 1000°F. Use 800°F as a reasonable median unless procedures, standards or codes indicate otherwise). For welding:

  1. All gases used for welding and purging shall be delivered through clean, non-volatile residue (NVR) tubing with known low permeability (e.g. stainless steel, polyethylene).
  2. Nitrogen, oxygen, carbon dioxide, or hydrogen gas in any concentration, shall not be used for shielding or purging in any welding operation of Titanium.
  3. All gases used for shielding or purging shall have a dew point of -60°F (-51 °C) or better and oxygen content shall not exceed 50 ppm.
Titanium microstructure

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