Eb

Common Thickness Range (in.)

1/32-1/4

1/4-3+

1/8-3/8

Foil to 3

Ease of Welding

Good

Fair

Good

Excellent

Grooved Joint Required

Often

Always

No

No

Automatic or Manual

Both

Automatic

Both

Automatic

Mechanical Properties

Fair

Fair

Good

Excellent

Quality of Joint

Good

Fair-Good

Excellent

Excellent

Equipment Cost

Low

High

Moderate

Very High

Wire Required

Often

Always

Sometimes

No

Distortion

Very High

High

Moderate

Very Low

if the part is going to be subjected to fatigue loading, CP Grades 1, 2, and 3 do not require post-weld stress relief unless the part will be highly stressed in a reducing atmosphere.

Gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), plasma arc welding (PAW), and electron beam (EB) welding are the well-established fusion welding methods for titanium, and laser welding is a rapidly emerging technology. The relative merits of GTAW, GMAW, PAW, and EB welding are given in Table 4.4. Many common welding processes are unsuitable for titanium because titanium tends to react with the fluxes and gases used, including gas welding, shielded metal arc, flux core, and submerged arc welding. In addition, titanium cannot be welded to many dissimilar metals due to the formation of intermetallic compounds. However, successful welds can be made to zirconium, tantalum, and niobium.

Since fusion welding results in melting and resolidification, there will exist a microstructural gradient from the as-cast nugget through the heat affected zone (HAZ) to the base metal. Pre-heating the pieces to be welded helps to control residual stress formation on cooling. It is also a common practice to use weld start and run-off tabs to improve weld quality. Fusion welding usually increases the strength and hardness of the joint material while decreasing its ductility.

Attention to cleanliness and the use of inert gas shielding, or vacuum, are critical to obtaining good fusion welds. Molten titanium weld metal must be totally protected from contamination by air. Since oxygen and nitrogen from the atmosphere will embrittle the joint, all fusion welding must be conducted either using a protective atmosphere (i.e., argon or helium) or in a vacuum chamber. Argon, helium, or a mixture of the two are used for shielding. Helium gases operate at higher temperatures than argon, allowing greater weld penetrations and faster speeds, but the hotter helium shielded arc is less stable, requiring better joint fit-up and more operator skill. Therefore, argon is usually the preferred shielding gas. All shielding gases should be free of water vapor; a dew point of —50° F is recommended. The hot HAZs and root side of the welds must also be protected until the temperature drops below 800° F. If fusion welding is conducted in an open environment, local trailing and backing shields must be used to prevent weld contamination. The color of the welded joint is a fairly good way to assess atmospheric contamination. Welds that appear bright silver to straw indicate none to minimal contamination, while light blue to dark blue indicates unacceptable contamination.

Cleanliness is important because titanium readily reacts with moisture, grease, refractories, and most other metals to form brittle intermetallic compounds. Prior to welding, all grease and oil must be removed with a non-chlorinated solvent such as toluene or methyl ethyl ketone (MEK). Surface oxide layers can be removed by pickling or stainless steel wire brushing of the joint area. If the oxide layer is heavy, grit blasting or chemical descaling should be conducted prior to pickling. After pickling, the cleaned material should be wrapped in wax-free kraft paper and handled with clean white cotton gloves. Filler wire should be wrapped and stored in a clean dry location when not in use.

Gas tungsten arc welding is the most common fusion welding method for titanium. In GTAW, the welding heat is provided by an arc maintained between a non-consumable tungsten electrode and the workpiece. In GTAW, as shown in Fig. 4.35, the power supply is direct current with a negative electrode. The negative electrode is cooler than the positive weld joint, enabling a small electrode to carry a large current, resulting in a deep weld penetration with a narrow weld bead. The weld puddle and adjacent HAZ on the weld face are protected by the nozzle gas; trailing shields are used to protect the hot solidified metal and the HAZ behind the weld puddle; and back-up shielding protects the root of the weld and its adjacent HAZ. GTAW can be accomplished either manually or automatically in sheet up to about 0.125 in. in thickness without special joint preparation or filler wire. For thicker gages, grooved joints and filler wire additions are required. If high joint ductility is needed, unalloyed

Direct Current, Electrode Negative (DCEN)

Tungsten Electrode'

Workpiece

/Ions

Arc Gap

Direct Current, Electrode Negative (DCEN)

Arc Gap

Fig. 4.35. Gas Tungsten Arc Welding (GTAW) Schematic filler wire can be used at some sacrifice in joint strength. In manual welding, it is important that the operator make sure that the tungsten electrode does not make contact with the molten weld bead, as tungsten contamination can occur. Electrodes with ceria (2% cerium oxide) or lanthana (1-2% lanthanum oxide) are recommended because they produce better weld stability, superior arc starting characteristics, and operate cooler for a given current density than pure tungsten electrodes. Conventional GTAW equipment can be used but requires the addition of appropriate argon or helium shielding gases. Protection can be provided by either rigid chambers or collapsible plastic tents that have been thoroughly purged with argon. Other methods of local shielding have also successfully been used.

Plasma arc welding is similar to GTAW except that the plasma arc is constricted by a nozzle which increases the energy density and welding temperature. The higher energy density allows greater penetration than GTAW and faster welding speeds.

Gas metal arc welding uses a consumable electrode rather than a non-consumable electrode that is used in the GTAW process. In GMAW, as shown in Fig. 4.36, the power supply is direct current with a positive electrode. The positive electrode is hotter than the negative weld joint ensuring complete fusion of the wire in the weld joint. GMAW has the advantage of more weld metal deposit per unit time and unit of power consumption. For plates 0.5 in. and thicker, it is a more cost-effective process than GTAW. However, poor arc stability can cause appreciable spatter during welding which reduces its efficiency.

Electron beam (EB) welding uses a focused beam of high energy electrons resulting in a high depth of penetration and the ability to weld sections up to 3 in. thick. Other advantages of EB are a very narrow HAZ, low distortion,

Direct Current, Electrode Positive (DCEP)

Constant Voltage

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