How to Prepare and Weld Titanium and Titanium Alloys

To weld titanium and titanium alloys, first remove oxide, oil, moisture, and other contamination from the joint area, then protect all metal that remains hot enough to oxidize. Gas tungsten arc and gas metal arc welding are the methods discussed here, with shielding arrangements selected for the joint and workpiece thickness.

Preparing Titanium and Titanium Alloys for Welding

Before titanium or a titanium alloy is welded, the joint area and its surroundings must be cleaned carefully to remove oil, dirt, rust, and scale, and the surfaces must be kept dry.

1. Remove oxide scale. Oxide scale can be removed with a stainless-steel wire brush or file, by steam blasting or shot blasting, or by grinding with a silicon-carbide wheel. Acid pickling should follow immediately after scale removal to ensure that the surface is free of oxides and grease.
2. Degrease the surface. If scale is present, remove it before degreasing; if no scale is present, degreasing alone is sufficient. Suitable solvents may be used on surfaces contaminated by oil. One commonly used cleaning solution contains 3% hydrofluoric acid and 35% nitric acid by volume. The workpiece is immersed at room temperature for about 10 minutes, rinsed to remove the residual solution, and dried. A workpiece susceptible to stress corrosion should be rinsed after pickling with chloride-free water rather than tap water. Rubber gloves can leave plasticizer on the workpiece and may consequently cause weld porosity. Freshly sheared plate edges also require this pickling procedure because metal fragments and small cracks created during shearing can promote porosity during welding.

Cleaned parts should be welded immediately. If storage is necessary, place the parts in a container with desiccant or in a humidity-controlled room. Otherwise, perform a light pickling treatment again before welding.

Selecting a Welding Method

Titanium oxidizes readily in weld zones above 400°C. All areas above this temperature therefore require inert-gas protection during welding. Oxyacetylene welding, shielded metal arc welding, submerged arc welding, and electroslag welding are unsuitable for titanium and titanium alloys.

Gas Tungsten Arc Welding

Gas tungsten arc welding (GTAW or TIG) may be manual or automatic. By welding environment, it can be divided into open welding and chamber welding.

1. Open welding. A conventional arrangement uses a large-diameter torch nozzle, a trailing shield, and a backing-gas device. The trailing shield and backing device protect weld metal above 400°C with argon or an argon-nitrogen mixture so that it does not oxidize.
2. Chamber welding. Complex weldments can be difficult to shield effectively above 400°C, so they may be welded inside a rigid or flexible chamber.
   1. Rigid chamber. Before welding, evacuate the chamber to 1.3-13 Pa and then fill it with argon or an argon-helium mixture. The torch can be relatively simple, without a trailing shield, and no additional shielding-gas flow is required during welding.
   2. Flexible chamber. Air may be removed either by evacuating the chamber before welding or by repeatedly folding and purging it with argon. Because shielding-gas purity is lower in a flexible chamber, a conventional torch is still used and gas shielding must continue throughout welding.

Gas Metal Arc Welding

Gas metal arc welding (GMAW or MIG) is commonly used for medium-thickness and thick products. Short-circuit transfer is suitable for thin-wall workpieces, whereas spray transfer is suitable for medium-thickness and thick sections.

Compared with GTAW, GMAW has a higher heat input. Because more filler wire is deposited, a larger groove angle is normally required. Workpieces 15-25 mm thick commonly use a 90° single-V groove or an I-groove with a 1-2 mm gap and one weld pass on each side. GMAW can reduce the number of layers, reduce porosity, and lower production cost. A trailing torch shield is still required. Because the portion of the weld above 400°C is longer than in GTAW, the trailing shield must also be longer and water-cooled. The principal disadvantage is greater weld spatter.