Titanium 101: Best TIG (GTA) Welding Practices
Pretty colors are fine for titanium jewelry. However, blue, violet, green, grey and white colors indicate atmospheric contamination in a 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, the GTA welding 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 liquid nitrogen (-322°F) to 1100°F; and the highest strength-to-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 psi and a density of 282 lb/ft3. In comparison, ASTM A36 steel has a yield strength of 36,000 psi and a density of 487 lb/ft3, while 6061-T6 aluminum has a yield strength of 39,900 psi and density of 169 lb/ft3.
In short, titanium is about 45 percent lighter than steel, 60 percent 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 non-existent) maintenance and repair costs. For example, the Navy replaced copper-nickel 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 it 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 thickness 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°F 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 percent purity 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 it 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 side bar story for 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 advice and information given above.
|A standard GTA power source with high frequency arc starting, remote amperage control capabilities, a post-flow shielding gas timer and an output of at least 250 amps will work well for welding titanium. Set polarity to DCEN (straight polarity).|
|GTA 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 sized to carry the required welding current: 1/16-in. or smaller for welding at <125 amps; 1/16- to 3/32-in. for 125 to 200 amps; and 3/32- or 1/8-in. for welding >200 amps. 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 such as this one extends the length of the weldment compared to welding with a cup alone. It is constructed similarly to the purge blocks (See next caption. Commercial shields are also available). Notice that the electrode is extended longer than the normal, 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 1-1/2 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 cfh for the purge blocks and trailing shield. Use 20 cfh 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 10 times prior to welding.|
|For demanding applications and where complex parts need to be welded, consider a vacuum-assisted welding chamber. This model utilizes a steel riser with glove ports and features a hemispherical, Plexiglas dome for viewing. 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 25bar.|
First, select the appropriate filler rod to match the material grade (see Table 1 above). 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 lint-free cloth and acetone or MEK.
A carbide file—again 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 on 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 like the one shown here. 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 post-flow shielding gas continues to cool the weldment until its temperature drops below 800°F. Post-flow duration will vary by the mass of the weldment, size of the weld and total heat input (post-flow was set at 20 seconds 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 – 1 in. were made.|
|The back of the weld 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 seconds. 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 puddle, which increases the mass of metal and total heat necessary to metal it).|
a) Discoloration must be removed prior to additional welding.
b) On the weld and in the HAZ up to 0.03 in. beyond the weld.
c) Violet, blue and green discoloration is rejectable if additional welding is to be performed. Blue and green discoloration is acceptable on finished welds but must be removed prior to subsequent processing.
Note: Discoloration comes in various shades, hues and tones.
Common Grades of Titanium
Titanium is divided in four classes: commercially pure (CP, or unalloyed), alpha, alpha-beta and beta. Note that many companies and experts treat CP and alpha alloys as one group. The “alpha” and “beta” refer to phases of the metal’s crystalline structure at various temperatures. Adding oxygen, iron, aluminum, vanadium and other elements to the alloy can precisely control the crystal structure, and hence the alloy’s properties.
The most common CP grade are ASTM Grades 1, 2, 3 and 4. They differ by the varying degrees of oxygen and iron content; greater amounts of these elements increase tensile strength and lower ductility. Grade 2 is the most widely used, notably in corrosion resist applications. CP Grades have good ductility, good elevated temperature strengths to 572°F and excellent weldability. They cost less than alloyed grades, but have a relatively low tensile strength, such as 70,000 to 90,000 psi for Grade 2.
Grade 5 (Ti-6Al-4V), an alpha-beta, is the most widely used of any grade of titanium (50 to 70 percent of all uses, according various sources). The addition of aluminum and vanadium increases tensile strength to 120,000 psi and service temperature up to 752°F, but it also makes Grade 5 less formable and slightly harder to weld than Grade 2. It is used for a range of applications in the aerospace, marine, power generation and offshore industries.
Grade 23 is similar to Grade 5, but features reduced of oxygen content that improves ductility and fracture toughness with a just a slight loss of strength. Grade 9 strengths fall between Grade 4 and Grade 5, so it is sometimes referred to as a “half 6-4.” Grade 9 can be used at higher temperatures than Grade 4, offers 20 to 50 percent higher strength than commercially pure gradesS and is more formable and weldable than Grade 5.
The author would like to acknowledge the significant contributions to this article made by two people. Geoff Ekblaw has more than 40 years of experience (and the patience to pose for the photos in this article). He is the senior welder at Woods Hole Oceanographic Institution (WHOI, www.whoi.edu). Woods Hole Oceanographic Institution is a private, independent organization in Falmouth, Mass., dedicated to marine research, engineering, and higher education. Established in 1930 on a recommendation from the National Academy of Sciences, its primary mission is to understand the oceans and their interaction with the Earth as a whole, and to communicate a basic understanding of the ocean's role in the changing global environment. WHOI is world-renowned for its deep-sea submersible vehicle Alvin. Alvin makes extensive use of titanium and is most famous for its use in exploring the wreck of the Titanic.
Jody Collier is a senior certified welding inspector (SCWI) and Instructor/Developer, Welding Training/Certification with Delta Air Lines Technical Operations Center in Atlanta (Delta TechOps; 866-MRO-Delta or TechSales.Delta@delta.com). Delta TechOps is the largest airline MRO in North America, earning more than $312 million in revenue in 2006. In addition to providing maintenance and engineering support for Delta's fleet of 440 aircraft, Delta TechOps serves more than 100 aviation and airline customers from around the world, specializing in high-skill work like engines, components, base and line maintenance. Delta TechOps employs more than 6,500 maintenance professionals and is one of the most experienced MRO providers in the world with more than seven decades of aviation expertise.
American Welding Society (2007), Structural Welding Code, Titanium (AWS D1.9), https://www.awspubs.com/
Titanium Metals Corporation (1997), Titanium Design and Fabrication Handbook for Industrial Applications, http://www.timet.com/pdfs/ti-handbook.pdf
TWI (The World Centre for Materials Joining Technology) and The Titanium Information Group (1999), Welding Titanium, A Designers and Users Handbook, http://www.twi.co.uk/j32k/protected/pdfs/bpweldti.pdf [Visitors must register to download this file]
Donachie, Jr., Matthew (2000), Titanium, A Technical Guide, ASM International, http://asmcommunity.asminternational.org/portal/site/asm/