Pretty colors are fine for titanium jewelry. However, the colors blue, violet, green, gray, and white indicate atmospheric contamination in a gas tungsten arc (GTA) welded titanium component. In critical applications, welds exhibiting such colors may suffer reduced strength and loss of ductility and could (or must) be rejected. Responsible fabricators owe it to their customers and themselves to produce welds that meet standards such as those outlined in AWS D1.9, Structural Welding Code — Titanium, as well as their own high standards. This article provides an introduction to titanium and the GTAW process, focuses on best practices, and outlines common pitfalls. It is especially written with smaller companies in mind, as they perform the bulk of GTA welding.
Titanium and its alloys offer excellent corrosion resistance to acids, chlorides, and salt; a wide continuous service temperature range, from –322°F (liquid nitrogen) to 1100°F; and the highest strengthto-weight ratio of any metal. For example, the most widely used grade of titanium alloy, ASTM Grade 5 (Ti-6Al-4V), has a yield strength of 120,000 lb/in.2 and a density of 282 lb/ft³. In comparison, ASTM A36 steel has a yield strength of 36,000 lb/in.2 and a density of 487 lb/ft³, while 6061-T6 aluminum has a yield strength of 39,900 lb/in.² and density of 169 lb/ft³. In short, titanium is about 45% lighter than steel, 60% heavier than aluminum, and more than three times stronger than either of them. While expensive initially, titanium lowers life cycle costs because of its long service life and reduced (or nonexistent) maintenance and repair costs. For example, the Navy replaced coppernickel with titanium for seawater piping systems on its LDP-17 San Antonio Class of ships because it expects titanium to last the entire 40 to 50 year life of the ship. In addition to military applications, other common uses for this light, strong, and corrosion-resistant metal include those for aerospace, marine, chemical plants, process plants, power generation, oil and gas extraction, medical, and sports.
Shielding Gas Is Critical
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 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 varying thicknesses of the oxide layer). 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° to 1000°F. Use 800°F as a reasonable median unless procedures, standards, or codes indicate otherwise).
One of the most common mistakes when welding titanium is not verifying the many variables that contribute to good shielding gas coverage prior to striking the first arc. Make it a practice to always weld on a test piece before beginning each “real” welding session. To ensure that gas purity meets your requirements, AWS recommends using analytical equipment to measure shielding gas purity prior to welding. Gas purity varies by application. Typical specifications require that the shielding gas (typically argon) be not less than 99.995% pure with not more than 5 to 20 ppm free oxygen and have a dew point better than –50° to –76°F.
Clean, Clean, Clean
Contamination from oil on your fingers, lubricants, cutting fluid, paint, dirt, and many other substances also causes embrittlement, and is a leading cause of weld failure. When working with titanium, follow the three Cs of welding: clean, clean, clean. Keep a clean work area, one free from dust, debris, and excess air movement that could interfere with the shielding gas. Clean the base metal and bag parts not immediately welded, clean the filler rod, and wear nitrile gloves when handling the filler rod and parts.
ASTM International recognizes 31 grades of titanium. Different grades address the need for various combinations of mechanical properties, corrosion resistance, formability, ease of fabrication, and weldability. While the various properties of these grades can be somewhat overwhelming (see the boxed item for a brief explanation), the welding of titanium is relatively similar to other alloy metals. The following images and advice demonstrate the basic best practices for welding titanium, expanding on the information given previously.
A standard GTA power source with high-frequency arc starting, remote amperage control capabilities, a postflow shielding gas timer, and an output of at least 250 A will work well for welding titanium. Set polarity to DCEN (straight polarity). Gas tungsten arc torches can be air or water cooled, depending on equipment preference, as most welds will be short and at lower output levels. Water-cooled torches are smaller, more maneuverable, and permit welding at higher amperages for extended periods, while air-cooled torches cost less. Notice the home-fabricated torch holder, which keeps the torch from falling on the floor.
For welding titanium, use a 2%-ceriated tungsten electrode sized to carry the required welding current: 1 ⁄16 in. or smaller for welding at <125 A; 1 ⁄16 to 3 ⁄32 in. for 125 to 200 A; and 3 ⁄32 or 1 ⁄8 in. for welding >200 A. Use a gas lens to evenly distribute the gas and create a smooth gas flow, and use a cup with a diameter of at least 3 ⁄4 to 1 in. A larger cup will enable you to make a longer weld.
A trailing shield extends the length of the weldment compared to a lesser length when welding with a cup alone. It is constructed similarly to the purge blocks (commercial shields are also available). Notice that the electrode is extended longer than the norm, which is only advisable when using trailing shields or oversized cups, as they provide extended gas coverage. Normally, the electrode should extend just far enough to permit visibility and access to the joint, or about 11 ⁄2 times the diameter of the electrode. To provide shielding gas coverage on the back and bottom sides of a joint, most facilities custom fabricate their own purge blocks from porous copper sheet and stainless steel.
The porous copper acts like a gas lens, evenly distributing the gas. To further smooth gas flow, the blocks are filled with stainless steel wool. Set the gas flow at 10 ft3/h for the purge blocks and trailing shield. Use 20 ft3/h for the torch.
When awkward joints preclude the use of standard purge blocks, welders fabricate shielding gas dams or chambers using stainless steel foil and fiberglass tape. To ensure purity, a rule of thumb is that the gas must flow long enough to exchange the gas inside the chamber ten times prior to welding.
For demanding applications and where complex parts need to be welded, consider a vacuum-assisted welding chamber. After loading parts, a vacuum pump quickly removes the air, and the chamber is then filled with inert gas for welding.
This gas manifold system distributes shielding gas to the torch and all purge blocks using separate gas lines; notice the use of surgical grade tubing for quality purposes. Because moisture content rises as cylinder pressure drops, consider switching cylinders when the pressure reaches about 25 bar.
First, select the appropriate filler rod to match the material grade (Table 1). Then, use a lint-free cloth and acetone or methyl ethyl ketone (MEK) to clean the filler rod just prior to welding. After cleaning, store the acetone in a safe place prior to welding. Also, read the manufacturer’s safety precautions. To prevent the body’s natural oils from contaminating the filler rod or base metal, always wear nitrile gloves when handling titanium.
To prevent contaminants from entering the weld pool via the filler rod (notice the discoloration on the end of the rod), clip off the end of the filler rod before every use. Store the filler rods in an airtight container when not in use.
To break down the oxide layer prior to welding, use a die grinder with a carbide deburring tool to prep the edges of the joint. Do not use the tool for anything else except titanium. Follow mechanical cleaning by cleaning with a lintfree cloth and acetone or MEK.
A carbide file — dedicated to titanium — may also be used to prepare the joint. Note the nitrile gloves, which are worn to prevent contamination. Simply wear welding gloves over the nitrile gloves to prevent accidentally handling clean titanium with bare hands.
To hold the purge blocks in place while welding, consider a fixture/clamp arrangement. The holes in the welding table allow weldments and purge blocks to be clamped in a wide variety of positions.
Notice the variety of stainless steel blocks and shims used to position and balance the purge blocks. The holes in the welding table make it much easier to position the purge blocks, as it permits access for the gas lines from the bottom side.
Use a stainless steel brush — dedicated for this one purpose — to remove any impurities (e.g., light oxide coating) that may develop before continuing to weld. If welds require visual inspection for QA/QC purposes, omit this step. Note that the bead length is just about 1 in. Short beads minimize heat input and ensure that the bead won’t “outrun” its shielding gas coverage.
After turning off the arc, hold the torch in position so that the postflow shielding gas continues to cool the weldment until its temperature drops below 800°F. Postflow duration will vary by the mass of the weldment, size of the weld, and total heat input (postflow was set at 20 s for the weld shown here).
To keep interpass temperatures below the critical 800°F threshold, use an infrared temperature gauge. Also, weld at the lowest amperage level that still produces complete fusion. Finally, do not travel too quickly, as that is a leading cause of porosity and weld failure. The front and bottom of the weld, which were properly shielded, show no evidence of contamination.
To demonstrate the importance of shielding all sides of a weldment, the purge block was intentionally removed from the backside of this fillet weld and two welds approximately 3 ⁄4 to 1 in. long were made.
The back of the weld shown in the photo to the left indicates a completely unacceptable weld. Note the progressive degree of contamination, with the “chalky dust” showing extreme contamination. The weld cracked internally with an audible “tink” after cooling for about 90 s. Welds with such contamination may not be repaired: scrap the entire part or cut out and completely remove the contaminated section.
When adding filler rod, be sure the rod end stays within the shielding gas envelope. Use a dab technique to lower overall heat input (as opposed to leaving the rod end in the weld pool, which increases the mass of metal and total heat necessary to melt it).
The color of a titanium weld indicates varying degrees of oxide thickness, or the degree to which the shielding gas failed to protect the weld from contaminants during the welding process — Fig. 21. Note that color is just one means of judging weld quality; dye penetrant inspection, hardness testing, X-ray, ultrasonic testing, and destructive tests may also be necessary to confirm acceptable quality. Table 5.3 in AWS D1.9 provides direction for judging weld quality.