An introduction to GTAW repair at Delta Air Lines
July 13, 2004
Pilots refer to flying experience as "seat time." For the 300-plus certified welders in Delta Air Lines' TechOps division, the term has a similar meaning. Certification is just the first step for them. Qualified GTAW welders here log plenty of seat time.
Whether they maintain ground support equipment like deicing trucks and luggage carts or critical aircraft components like engine gearboxes or cylinder sleeves, these welders know that lives depend on their expertise.
To train and certify the welders, Delta instituted a Welding Training Department at the airline's maintenance headquarters at Atlanta's Hartsfield International Airport. Although the majority of welders work at the TechOps division, the welding training instructors—program manager Jody Collier and instructor Robert Trudelle— also are responsible for training and certifying welders at the airline's smaller maintenance facilities, two of which are in Dallas and Tampa, Fla. All of Delta's maintenance facilities operate under the TechOps division.
The task is large. Delta's welders hold more than 1,000 different certifications. Each certification is differentiated by material type, process, and whether the part is nonflight hardware or an aircraft component.
The training and certification process for aircraft welders runs six to eight weeks, depending on the candidate's expertise. Each welder must be recertified every two years.
Welding training covers every welding process on every type of metal, from 1-inch thick carbon steel to 0.001-in. INCONEL® alloy or stainless steel. For all nonflight hardware, training focuses on three processes: gas metal arc welding (GMAW), shielded metal arc welding (SMAW), and gas tungsten arc welding (GTAW).
Manual welding for aircraft component maintenance and repair is all GTAW. The materials are thin, and the welds must be precise. Because filler metal doesn't cross the arc, spatter isn't a problem. The welding area is clearly visible, and postweld cleaning is minimal.
The welders' GTAW skill is relatively high, especially for aircraft component welders. Every test weld either is X-rayed or metallographically tested to spot various possible defects such as crater cracking or underfill, so preventing heat distortion is critical. All welders are trained to weld short runs—1¼2 to 2 in. depending on the part—to minimize heat input. Starting and stopping without creating defects is key to welding aircraft components.
Training and certification are conducted on eight different metal categories. The categories are taken from industry standards AWS D17.1 for aircraft welds and AWS D1.1 for structural welds. The categories are:
Training progresses through the categories, and practical exercises conclude each step. Each practical exercise must pass a visual inspection and either X-ray or metallographic testing before the welder advances.
After the eight-week training and certification process, welders are assigned to individual departments that require on-site welding or to the TOC's central welding shop, which has a pool of 30 welders. Once in the assignment, a newly certified welder undergoes a period of on-the-job training with a qualified welder before he or she is considered qualified.
Metal preparation is critical to GTAW repair. Weld training emphasizes the three C's: clean, clean, clean. Titanium and niobium, for example, require stringent cleaning procedures using solvents and wire brushing to clean the joints. For other metals, such as stainless steel, wire brushing alone is preferred to using abrasive cleaners.
It's standard practice to spend more time cleaning and preparing a part than welding it. In many instances, the weld process itself may take 30 seconds; the repair of a 1-in. crack in a niobium heat barrier shield that shrouds a jet engine is one example. Critical to that weld, however, is the preparation: cleaning the surface properly, fixturing the part, and ensuring the back side of the weld is shielded. For most aircraft repairs, the shielding gas is 100 percent pure argon. The exceptions are an 80/20 mix of helium and argon for certain aluminum applications in which distortion control is critical; and a 50/50 argon and helium mix for certain magnesium applications.
To shield the back side of the weld locally without having to wait to purge an entire vessel or cavity, Delta's welders use backup boxes. For example, to repair a 3-in. crack in a 6-in.-dia. INCONEL alloy exhaust duct, one alternative is to enclose the ends, slide an argon tube into the duct, and purge the entire inside.
A faster, more focused method is the backup box. Made of perforated material—typically copper—the backup box is contoured to fit on the back side of the exhaust duct. Once the box is clamped in place and the argon gas turned on, the back side is shielded immediately. The welder doesn't have to wait for a purge or employ an oxygen-monitoring system to ensure the shield is adequate. Backup boxes can facilitate quick and complete repairs, and given that aircraft components susceptible to wear will do so again and again, TOC welders' toolboxes contain 20 or more handcrafted backup boxes.
Delta welders also use a unique type of GTAW nozzle that provides a larger gas shield to any aircraft part. Whereas a standard-size nozzle provides about a 7¼16-in. shielding gas envelope, the larger GTAW cup provides up to a 1-in. envelope. The benefit of this nozzle comes into play on all of the stainless steel and nickel-based alloys.
Typically, aircraft parts made of those alloys contain a small amount of aluminum, and because of it, welders will get some mild oxidation while welding. A larger welding cup enables a cleaner weld that requires less amperage. If less amperage is needed to move the puddle because it's shielded perfectly and therefore is less sluggish, heat input and distortion are reduced. For example, if a welder is working on 0.0020-in. stainless steel at 13 amps, the larger cup can save 5 amps, which is considerable when heat distortion is a concern.
Most aircraft repairs are in locations that require unconventional welding techniques. The shielding afforded by the larger nozzle allows welders to extend their tungsten out farther to get into those locations. If the weld is inside an exhaust duct or radial drive sleeve, the tungsten can stick out as far as 11¼2 in. and the operator will still have adequate shielding (see introductory photo).
Given the range of metals used and their thinness, three fundamentals are critical in aircraft component GTAW repair: a clean part, clean shielding, and heat input. To illustrate this point, metals frequently encountered include aluminum, INCONEL alloys, niobium, and magnesium.
To repair the service wear on aluminum radial drive sleeves, the welder applies a buildup weld to cover the wear pattern, which allows the groove to be remachined. The goal is to create an edge no more than 0.0010 in. deep; any more risks distortion and will require more machining of the part. With an AC/DC GTAW inverter set to 70 amps AC, a 160-Hz frequency, and 70 percent electrode-negative balance, the operator uses 4043 aluminum filler wire with a tapered electrode1 instead of a blunt end. This allows him to pinpoint the heat instead of fanning it beyond the groove.
When repairing a jet engine air cycling machine of 5052TL aluminum, the welder sets his AC GTAW inverter to 75 amps. He uses 4043 aluminum filler wire and a 70/30 argon and helium shielding gas to counteract the thin material.
When repairing an INCONEL alloy exhaust duct, the goal is to control the heat buildup. To repair a deep gouge, the welder lays a bead of INCONEL 625 wire in the gouge to flatten it and then runs stringer beads along the cavity to build it back up. Running DC electrode negative at 40 amps, the welder's main concern is to control the heat to prevent warpage, which will distort the duct and make reassembly into the exhaust system impossible.
To rebuild a lug mount on the front cover of a magnesium engine gearbox, the welder spends three and a half hours building the mount with 1¼8-in. AZ101 magnesium filler metal. Approximately 2 in. deep by 11¼2 in. high, the mount is created by welding pass after pass and allowing the weld to cool down after every three passes. When the welder restarts, he changes direction to avoid creating any stress lines in the weld. Shielding gas is a 50/50 argon and helium mixture for more heat input and a hotter arc. This allows the welder to increase his travel speed because the metal puddles faster and he can get in and out of the weld quickly and cleanly. This is critical when considering the machining requirements for the lug mount.
Argon shielding and travel speed are critical in the repair of a niobium jet engine heat shield. When heated to welding temperature, niobium wire is sticky like bubblegum, so the welder must carefully slip it right into the center of the puddle. This is where the welder's skill is important, beyond just reading voltage and amperage. Also with niobium, if the metal isn't clean, the puddle doesn't flow as quickly, resulting in more heat input. This causes grain growth, which can show up as a defect in the part.
These are just some examples of situations in which the technical expertise of TOC welders is critical. The training and certification required to prepare the welders to handle such applications are equally crucial.
Jody Collier is program manager of welding training at the TechOps Division of Delta Air Lines Inc., P.O. Box 20706, Atlanta, GA 30320-6001, 404-714-3185, fax 404-714-3198, William.firstname.lastname@example.org, www.delta.com. Brent Williams is product manager of TIG Solutions at Miller Electric Mfg. Co., 1635 W. Spencer St., P.O. Box 1079, Appleton, WI 54912-1079, 920-734-9821, fax 920-954-3633, email@example.com, www.millerwelds.com.
1. For more information on welding with GTAW inverters, see "Selecting the right tungsten: How your choice affects AC GTAW" in the February 2004 issue of The FABRICATOR®, p. 28.
INCONEL is a registered trademark of Special Metals Corp.
Delta's TOC welders use a larger GTAW cup that provides a 1-inch shielding gas envelope for less heat input and distortion. The larger GTAW cup allows the welders to extend their tungsten out as far as 11¼2 in. to make welds in hard-to-reach locations.