This Time It's Personal
Welding training at Delta's TechOps Center raises the bar
By Mike Sammons
When it comes to welding, Delta Air Lines Technical Operations Center (TOC) at Atlanta's Hartsfield International Airport performs plenty, with more than 500 welding operators located there. It maintains and repairs a full range of aircraft, engines, and components including landing gear, accessories, and ground support equipment. TOC's welding training and certification program actually exceeds industry standards (AMS 1595 for aircraft welds, soon to be replaced by AWS D17.1, and AWS D1.1 for structural welds).
"We don't disregard defects on the ends of the joint even though AMS 1595 allows us to discard the last half inch. The reason is that some of our parts have ends," says Jody Collier, SCWI (Senior Certified Welding Inspector) and Delta's program manager, welding training. "Another criterion where we are stricter than the AMS standard is that we require at least one tie-in in the middle area of the joint that will be X-rayed. Stops and starts are more likely to have defects like crater cracking. Because repair welding of aircraft parts often necessitates sequence welding and stopping often in order to control heat input, we require our welding operators to demonstrate the ability to stop and start without creating defects."
TIG welding at TOC is far more difficult than almost any other maintenance application. Typical parts include a magnesium transfer case that costs $47,000, a 3-inch diameter, .020-inch wall titanium air duct that costs $800 per foot, and the .012-inch 157 stainless-steel skin of an exhaust sleeve on a Boeing 767 engine.
When welding on material this sensitive, operators should not be limited by the capabilities of their equipment. Unfortunately, some TOC machines were 30 to 35 years old; one machine had a serial number of 73 and another was built in 1962. Basic TIG power sources inherently last a long time, and Delta's clean operating environment further extended their life. However, just because a machine can still strike an arc doesn't mean that it should remain in use.
"We had a TIG machine from the 1980s. It had a terrible bottom end on it, about 20 or 30 amp, which doesn't lend itself to aircraft welding very well," says Robert Trudelle, TOC welding training instructor. "For example, the .012-inch exhaust sleeve is welded at 15 to 18 amps. We need something that can weld thin metals."
In addition to limitations on thin ferrous metals, older machines do not perform as well on nonferrous metals because they do not feature modern squarewave technology or incorporate advanced arc starting features (see sidebar story on page 24). Thus, at the same time TOC began intensifying its training program, it recognized the need to modernize its fleet of AC/DC TIG power sources.
TIG welding has always been an art, but the TOC training program adds science.
Collier says that, "The structure and consistency of our training program improve welding results. Before we had too many performance highs and lows. Delta's continuous improvement team assessed the operator's skill level and our training program. As a result, they determined we needed to fund and support a world-class training program that would produce operators whose skills surpass any industry standard."
Prepared by the Hobart Institute of Welding Technology, Troy, Ohio, the TOC training program for GTAW (TIG welding) covers all the materials encountered in aircraft maintenance and repair: carbon steel, nickel-based steels, cobalt-based alloys, titanium, aluminum, and magnesium, plus the relevant alloys of these metals. A hefty training manual covers every aspect of the TIG process, TIG equipment, and welding terms and symbols. Each metal type has a chapter devoted to it, enabling the operator to learn about that alloy's welding characteristics (including an alloy data sheet), pre-weld preparations, and welding instructions. Practical exercises, such as a square groove butt joint on .020-inch titanium (Group 7: 6AL4V) or a fillet weld T-joint on .063-inch 6061-T6 aluminum, test for real-world skills.
"The training class usually consists of an eight-week course, and it combines classroom and lab experience. The classroom portion especially provides the structure we feel promotes good skill growth," notes Trudelle. The program is not easy, and it does not allow for skills to atrophy. Every operator must take and pass a re-certification test every two years.
"Because of this structure, I get a more efficient personnel draw," says Mark Nolan, lead welder, department 400 (which is responsible for engine-related repairs). "Engine manufacturers like Pratt & Whitney and General Electric specify welding requirements, and of course we have to satisfy the FAA. With operators trained on welding seven metals and tested in both the flat and vertical positions, we have a large talent pool for any job required."
Welders at TOC respect the training program not just for its results, but for the quality of its instructors. Collier and Trudelle have a combined 34 years of welding experience, much of it related to aircraft.
"Because they know aircraft components and the metallurgical properties of aircraft metals, Collier and Trudelle can apply that knowledge in training," says Charles Pierce, a welder in department 400. "They bring out our skills in ways that help us prepare for real-world challenges. That's a big difference between Delta TOC training and training you'll get anyplace else."
Welding knowledge in action
Working on the magnesium (Group 5: AZ31B) JT8 gearbox for a Boeing 727 clearly illustrates the difference between general TIG welding and TIG welding on an aircraft component.
The gearbox functions similarly to a car's transmission, except at $47,000 per half, it costs more than most cars. Like a transmission, it wears down. To salvage this expensive component, TOC welders cut out worn or damaged sections and replace them with new metal using a Syncrowave power source welding in the AC TIG mode. This buildup work, adding layer upon layer of weld metal, puts a lot of heat into the component. Unfortunately, excess heat warps the gearbox to the point where the two halves no longer match; the warped half then has to be scrapped.
"Today, we know the heat limits of this part, so we'll run a short bead then wait a few seconds for the part to cool before making the next pass," says TOC welder Vincent Brigante. "This is why it makes sense for the TOC training program to include a weld test that requires arc stops and starts."
Brigante did have problems, however, with the oil contaminating the magnesium. After spending a week adding new metal, he frequently encountered contaminated sections. Cutting out the contaminated section often meant cutting out all the new weld metal, too. Today, through proprietary welding repair techniques developed by Brigante, Trudelle, and others, Delta has eliminated this problem and extended the life of the gear box from 400 to 600 cycles to more than 12,000 cycles (a cycle is a takeoff and landing).
"Welding that gearbox got real personal," says Brigante. "We vowed never to scrap one, and we haven't scrapped one in more than 12 months."
Mike Sammons is a product manager for Miller Electric Mfg. Co.
Miller Electric Company
Senior Certified Welding Inspector (SCWI)
SCWI status is certainly special to the individual who has obtained the designation. It is AWS’ highest level of certification, says Wendy Sue Reeve, AWS. She explains that through successful examination, SCWI is awarded to an individual who has been a CWI for a minimum of six years, has a requisite 15 years of experience, and whose career has evolved into a supervisory or management position with responsibilities in the field of quality control and quality assurance. The person has an understanding of advanced nondestructive examination (NDE) topics, quality systems, procedure qualifications, and other identified SCWI knowledge areas.
Better Starts, Same Welding Arc
The new Syncrowave 250 DX, now being used by aircraft welding operators at Delta Air Lines, produces dramatically better arc starts through its selectable start condition feature. This three-function control ensures consistent and repeatable high-frequency AC and DC arc starts.
By controlling the initial volume of current, this machine offers "soft start," "standard start," or "hot start" arc starting conditions. Soft start helps operators working with small diameter tungstens or thin metals, while hot start improves starts with the larger diameter tungstens used for welding thicker material.
"It doesn't take much to blow through the .012-inch honeycomb skin of an exhaust sleeve," notes Scott Morris, TOC welder. "Before we had the new welding power source, we had problems welding at lower amperage. Now that we have the Syncrowave 250 DX, we have better control, better arc starts, and [we] don't fight problems with weld quality. Another thing I love about this machine is its Fan-On-Demand®. The cooling fan only runs when needed. This shop is quieter than most, and Fan-On-Demand lets us play our radio or talk. Before this we sometimes put in ear plugs."
Like all Syncrowave machines, the new machine's squarewave balance control permits tailoring the arc for more cleaning action to remove heavy oxide layers or more penetration on thicker aluminum and magnesium. Miller Electric introduced the Syncrowave welding machine with its squarewave AC output technology. Squarewave technology minimized the problems inherent with AC welding on aluminum and magnesium: arc stumbling, wandering, and outages.
Welding power sources with a sine wave output typically experience problems during the Electrode Negative (EN) to Electrode Positive (EP) transition of the AC sine wave. Sometimes, these older units 1) did not have enough "push" to drive the arc through the zero amp range and then re-establish the arc at the electrode or 2) they could not transition through the zero amp range quickly enough.
If five or six EN to EP cycles fail in a row, the welding output begins to resemble DC. If this occurs, a TIG machine reaches for open circuit voltage in an attempt to get enough voltage to re-establish AC welding output. Unfortunately, the excess voltage can cause current overshoots (voltage spikes). During the EP to EN half cycle this may produce tungsten migration, which degrades weld quality.
Squarewave technology shortened the switching time between EN and EP, so it created a more desirable arc. So desirable, in fact, that all higher-end AC TIG machines now feature squarewave technology (Miller's patent expired in 1994). Instead of a sloping sinusoidal wave, a squarewave makes a nearly vertical transition between EN and EP. This greatly increased arc stability, improved arc starts, and reduced arc wandering.