Proper welding techniques are critical with titanium and titanium alloys in order to maintain the weld’s strength matching that of the base metal and to maintain titanium’s natural corrosion-resistant properties.
There are several appropriate titanium welding techniques, and the welding equipment is the same or similar to other processes used for high-performance metals such as stainless steel or nickel-base alloys.
However, with titanium various contaminants such as dirt, grease, air, moisture, refractories and certain other metals are harmful to the titanium welding process and pose a risk of forming brittle compounds. Therefore, cleanliness is paramount to protect against such contamination leading to weld cracking or a lack of weld integrity.
Another requirement of welding with titanium is auxiliary inert gas shielding and protecting molten titanium weld metal from air contamination. Common welding techniques are not feasible, namely gas welding, shielded metal arc, flux corded arc, and submerged arc welding because titanium reacts with gases and fluxes. Metals that are suitable for use with titanium welding are zirconium, niobium and tantalum. Fabricators who take all necessary precautions in welding titanium routinely produce sound, ductile welds that compete economically with other high-performance metals.
A temperature and humidity controlled fabrication environment is ideal for titanium welding and a more common practice than chamber welding, still in limited use. Quality welds are highly dependent on cleanliness and an area set apart from grinding, torch cutting or other such operations is desirable.
Gas tungsten-arch (GTA or TIG) and Gas metal-arc (GMA or MIG) are the two most commonly used techniques in welding with titanium and its alloys with GTA being primary. Other methods used more rarely, but still viable, are resistance, plasma arc, electron beam and friction welding, also laser and brazing. Each method has its own benefits in particular situations. The following covers only GTA and GMA welding although most principles apply to all methods.
The GTA or GMA welding process can be used to make butt joints without filler metal in titanium base sheet up to approximately 1/8-inch thickness. Since heavier sections generally require the use of filler metal and grooved joints the GMA welding is more economical. In using the GTA method measures are necessary to prevent contact of the tungsten electrode with the molten material to prevent unwanted tungsten deposits.
All that is required for GTA welding is a conventional power supply, connected d.c. straight polarity (DCSP). GMA welding requires a reverse polarity (DCRP). To maintain inert gas shielding a foot operated remotely controlled current and contactor is used so that the arc can be broken without removing the torch from the cooling weld metal. Gas timers are also beneficial offering high frequency arc starting and shielding.
GTA welding of titanium is done with a water-cooled welding torch, a 3/4-inch ceramic cup, and a gas lens. GMA welding typically requires a one-inch cup. It is recommended that a pointed thoriated tungsten electrode (end blunted) with the smallest diameter that can carry the desired current be used for GTA welding of titanium.
Gas shielding of the titanium weldment is critical to the success of arc welding. Maintenance of an inert atmosphere until weldment cools below 426°C (800°F) is obtained by using three distinct gas streams, one to shield the molten weld puddle, one to protect the hardened weld metal and heat-affected areas, and one backup shield to protect the underside during welding and cooling.
Argon is the most commonly used shielding gas but occasionally argon helium gas mixtures are used when more heat and greater welding penetration are required.
Primary shielding of the molten weld puddle requires the standard water-cooled welding torches equipped with large (3/4 or 1-inch) ceramic cups to shield entire molten weld puddle and gas lenses which provide uniform, non-turbulent inert gas flow.
Argon is generally used over helium for primary shielding at the torch because of better arc stability properties. Argon-helium mixtures are used if higher voltage, hotter arc and greater penetration are desired. Recommended gas flow rates to the torch are around 20 cfh. Any excess flow to the torch may create turbulence and a loss of proper shielding.
A secondary trailing shield, usually hand-made to fit a particular torch, protects the solidified titanium weldment and the heat-affected zones until temperature reaches 800 degrees F or lower.
The backup shielding device provides inert gas shielding to the underside of weldments and the heat-affected zones. They can look similar to trailing shields and can be hand-held, clamped or in a taped position.
Hand-made shielding devices can be very effective with titanium welds including plastic that completely encloses the weld object, making sure it’s flooded with inert gas. Aluminum or stainless steel foil tents can also be effective as backup shields as long as air is effectively eliminated to prevent weld contamination. Inert gas elimination should be ten times the volume of air purged with some inert gas maintained until job is completed.
Argon gas is preferred to helium for trailing shields and for a backup because of its associated lower cost and density. Helium which has a lower density is commonly used for trailing or backup shielding if the welding process is to be done above the device.
Separate flow controls are used for all three shielding devices with 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.
Weld color is a good barometer of weld quality and any discoloration should cause cessation of weld operations for corrective measures. Bright silver indicates weld shielding is satisfactory and proper weld temperatures are being maintained.
Some weld discoloration such as a light straw coloring can be removed effectively with a stainless steel brush after which welding can continue. However, a dark blue oxide or white powdery oxide is a seriously deficient purge calling for cessation while the cause is determined. The discoloration matter should be completely removed and re-welded.
For the finished weld, an examination by a harmless liquid penetrant, radiography and/or ultrasound are typically utilized However, mechanical testing to evaluate weld integrity and quality is usually indicated. Tests such as weld bend testing and then tensile tests are common.