WELDING TITANIUM
Titanium and most titanium alloys are readily weldable, using several welding processes. Properly made welds in the as-welded condition are ductile and, in most environments, are as corrosion-resistant as base metal. Improper welds, on the other hand, might be embrittled and less corrosion-resistant compared to base metal.

The techniques and equipment used in welding titanium are similar to those required for other high-performance materials, such as stainless steels or nickel-base alloys. Titanium, however, demands greater attention to cleanliness and to the use of auxiliary inert gas shielding than these materials. Molten titanium weld metal must be totally protected from contamination by air. Also, hot heat-affected zones and root side of titanium welds must be shielded until temperatures drop below 800 degrees F.

Titanium reacts readily with air, moisture, grease, dirt, refractories, and most other metals to form brittle compounds. Reaction of titanium with gases and fluxes makes common welding processes such as gas welding, shielded metal arc, flux cored arc, and submerged arc welding unsuitable. Likewise, welding titanium to most dissimilar metals is not feasible, because titanium forms brittle compounds with most other metals; however, titanium can be welded to zirconium, tantalum and niobium.

In spite of the precautions which need to be taken, many fabricators are routinely and economically welding titanium, making sound, ductile welds at comparable rates to many other high performance materials. One of the important benefits of welding the commercially pure grades of titanium (i.e., TIMETAL 35A and 50A) is that they are over 99% pure titanium and there is no concern for segregation. The same is true of weld wire or rod in commercially pure grades.

WELDING ENVIRONMENT
Most titanium welding today is done in the open fabrication shop, although chamber welding is still practiced on a limited basis. Field welding is common. Wherever the welding is done, a clean environment is necessary in which to weld titanium. A separate area, specifically set aside for the welding of titanium, aids in making quality welds. This area should be kept clean and should be isolated from dirt-producing operations such as grinding, torch cutting and painting. In addition, the welding area should be free of air drafts and humidity should be controlled.

WELDING PROCESSES
Titanium and its alloys are most often welded with the gas tungsten-arc (GTA or TIG) and gas metal-arc (GMA or MIG) welding processes. Resistance, plasma arc, electron beam and friction welding are also used on titanium to a limited extent. All of these processes offer advantages for specific situations. However, the following discussion will be concerned primarily with GTA and GMA welding. Many of the principles discussed are applicable to all processes.

GAS TUNGSTEN-ARC (GTA) AND GAS METAL-ARC (GMA) WELDING
The GTA process can be used to make butt joints without filler metal in titanium base sheet of up to about 1/8-inch thickness. Heavier sections generally require the use of filler metal and grooved joints. Either the GTA or GMA welding process can be used, although GMA welding is more economical for sections heavier than about one-half inch. If the GTA process is used, care should be exercised to prevent contact of the tungsten electrode with the molten puddle, thereby preventing tungsten pickup.

POWER SUPPLY
A conventional power supply, connected d.c. straight polarity (DCSP), is used for GTA welding of titanium. Reverse polarity (DCRP) is used for GMA welding of titanium. A remote controlled contactor allows the arc to be broken without removal of the torch from the cooling weld metal, thereby maintaining inert gas shielding. Foot operated current and contactor control, high frequency arc starting and shielding gas timers are other desirable features.

WELDING TORCH
A water-cooled welding torch, equipped with a 3/4-inch ceramic cup and a gas lens, is recommended for GTA welding of titanium. A one-inch cup may be required for GMA welding.

Thoriated tungsten electrodes (usually 2% thoria) are recommended for GTA welding of titanium. Pointed electrodes (end blunted) help to control arc characteristics. The smallest diameter electrode which can carry the required current should be used.

INERT GAS SHIELDING
Protection needs to be provided to titanium weldments on cooling down to about 800 degrees F as well as to the molten weld puddle in order to prevent contamination by air. During GTA and GMA welding, argon or helium shielding gases of welding grade with dewpoint of -50 degrees F. or lower are used to provide the necessary protection. Separate gas supplies are needed for:

1. Primary shielding of the molten weld puddle.
2. Secondary shielding of cooling weld deposit and associated heat-affected zones.
3. Backup shielding of the backside of weld and associated heat- affected zones.

PRIMARY SHIELDING
Primary shielding of the molten weld puddle is provided by proper selection of the welding torch.

Standard water-cooled welding torches equipped with large (3/4 or 1-inch) ceramic cups and gas lenses, are suitable for titanium. The large cup is necessary to provide adequate shielding for the entire molten weld puddle. The gas lens provides uniform, nonturbulent inert gas flow.

Argon is generally used in preference to helium for primary shielding at the torch because of better arc stability characteristics. Argon-helium mixtures can be used if higher voltage, hotter arc and greater penetration are desired. Manufacturer's recommended gas flow rates to the torch should be used. Flow rates in the vicinity of 20 cfh have proven satisfactory in practice. Excess flow to the torch may cause turbulence and loss of shielding.

The effectiveness of primary shielding should be evaluated prior to production welding. An arc can be struck on a scrap piece of titanium with the torch held still and with shielding gas only on the torch. The shielding gas should be continued after a molten puddle forms and the arc is extinguished, until the weld cools. Uncontaminated, i.e., properly shielded, welds will be bright and silvery in appearance.

SECONDARY SHIELDING
Secondary shielding is most commonly provided by trailing shields. The function of the trailing shield is to protect the solidified titanium weld metal and associated heat-affected zones until temperature reaches 800 degrees F or lower.

Trailing shields are generally custom-made to fit a particular torch and a particular welding operation. A schematic of a trailing shield, useful for flat sheet or plate welding of titanium, is shown in Figure 12. Design of the trailing shield should be compact and allow for uniform distribution of inert gas within the device. The possible need for water-cooling should also be considered, particularly for large shields. Porous bronze diffusers have provided even and nonturbulent flow of inert gas from the shield to the weld.

BACKUP SHIELDING
The prime purpose of backup devices is to provide inert gas shielding to the root side of welds and their heat-affected zones. Such devices often look much like trailing shields and may be hand-held, or clamped or taped into position. Water- cooled copper backup bars (or massive metal bars) may also be used as heat sinks to chill the welds. These bars are grooved, with the groove located directly below (or above) the weld joint. About 10 cfh of inert gas flow per linear foot of groove is required for adequate shielding.

Makeshift shielding devices are often employed very effectively with titanium welds under shop or field conditions. These include use of plastic to completely enclose the workpiece and flood it with inert gas. Likewise, aluminum or stainless steel foil "tents", taped over welds and flooded with inert gas, are used as backup shields. When such techniques are used, it is important that all air, which will contaminate welds, be purged from the system. An inert gas purge equal to ten times the volume of the air removed is a good rule-of-thumb for irregular spaces. A moderate rate of inert gas should be maintained until the weld is completed.

Argon is generally selected in preference to helium for use in trailing shields and backup devices, primarily because of cost but also because it is more dense. Helium, with its lower density, is sometimes used for trailing or backup shielding when the weld is above the device.

It is important that separate flow controls are available for primary, secondary and backup shielding devices. Timer- controlled pre-purge and post-purge of torch shielding, and solenoid valves with manual switches interlocked with the welding current for secondary and backup shielding are also useful.