Orbital Welds Take Flight

Orbital Welding Produces Top-Grade Welds on Titanium Hydraulic Lines

 

The Citation X from Cessna Aircraft Co. is the world’s fastest business jet, certified for cruising at Mach 0.92 or approximately 600 miles per hour. The program was launched in 1990 and the aircraft entered service with the first retail delivery in August 1996. The jet’s high-speed cruise capability means it can save up to one hour’s flight time on transcontinental U.S. flights, flying from Los Angeles to New York with normal wind conditions in 4 h 10 min. Because of its ability to cruise at high speed at high altitudes, Cessna says the Citation X consumes less fuel than other jets on a transcontinental flight. The Citation X can climb to 43,000 ft in just 30 min at its maximum takeoff weight, quickly rising to transcontinental and transatlantic crossing altitudes. The plane is certified to fly as high as 51,000 ft – where fuel efficiency is optimized and airline traffic is nearly nonexistent. Cessna recently announced major improvements to the aircraft, including increasing its maximum takeoff weight from 35,700 to 36,100 lb, making it possible to carry up to seven passengers with full fuel in a typically equipped airplane.


Key Weight-Saving Feature
The use of titanium hydraulic and fuel lines is a key weight-saving feature. The Citation X is the first Cessna plane, and one of the first business jets ever, to use this advanced design alternative. The hydraulic and fuel lines on this plane comprise about 700 assemblies between 1/4 and 5/8 in. in diameter, with about 60% receiving a weld on both ends and the rest receiving a weld on one end. The use of titanium lines has created some interesting manufacturing challenges.

In the past, hydraulic and brake lines were flared and fastened with fittings, a process unsuitable for titanium, so Cessna engineers turned to orbital welding, a mechanized version of the gas tungsten arc welding (GTAW) process. In manual GTAW, the welder moves the welding torch and controls the welding current. In orbital GTAW, the tungsten is installed in a weld head that clamps on the tube or pipe. The tube remains in place while the weld head rotor revolves around the weld joint circumference to complete the weld. The welding is done in an inert atmosphere to protect the metal from oxidation as it is heated to melting temperature. The arc welding current is regulated with a control system, automating the entire process and providing a more precise and reliable method than manual welding. An advantage of orbital welding on this application is that once a weld program has been established, an orbital welding system can repeatedly perform the same weld in exactly the same manner, eliminating the normal variability, inconsistencies, and errors of manual welding. As a result, orbital welding doesn't require as much operator training. Productivity of orbital welding systems is also generally superior to manual welding, because it can produce high-quality, consistent welds at a speed close to the maximum welding speed.


Need to Meet High Quality Standards


To save weight, Cessna uses titanium hydraulic and fuel lines in the jet's construction. The company now uses the orbital welding equipment shown here to weld these lines.

"When we made the decision to go with orbital welding, our number one concern was to meet the high quality standards we require on all of our planes," said Bill Starnes, production superintendent. Cessna selected a Model 9AF-750 orbital welding head and a Model 207 welding power supply from Arc Machines, Inc., Pacoima, Calif. Standard Model 9AF-750 heads incorporate a two-sided clamping mechanism. An independent set of clamps on each side of the weld head holds the tubes or fittings in line, butted together and ready to be welded. These clamps eliminated the need for pretacking. The clamps can be quickly and easily replaced with a different size, so each weld head can weld a wide range of tube or pipe sizes. Clamps are available in all standard inch and metric tube and pipe sizes.
The model 207 welding power supply controls weld parameters such as welding current, primary and background amperes, travel speed, weld bead overlap, delay of rotation at the start of the weld, and current downslope at the end of the weld. The enclosed weld heads used for small-diameter tubing provide a chamber filled with inert gas that encloses the entire joint during the weld. A timed prepurge and postpurge are usually used to time the flow of inert gas into the weld head before arc initiation and to continue the purge for a timed period after the arc has been extinguished. This allows the weld tool to cool sufficiently to prevent oxidation before the weld head is opened to remove the welded tube.
 

Getting Up and Running
None of the shop personnel at Cessna had experience with orbital welding but were eager to learn. Arc Machines offered a training program that helped provide all of the operators with a clear understanding of what was needed to produce quality welds. The setups for orbital welding are straightforward and include setting the speed, rotation, dwell, and penetration parameters, which are different for each size of tubing. The operator is also responsible for selecting the right fitting, which is usually either a sleeve and a nut or a union.
 

Material preparation is the key to producing good welds.
"This is a very easy machine to operate," said Richard Stump, Cessna production foreman. "We have developed programs for all of the sizes we need to weld and the operator can easily change to the appropriate program and be pretty certain of producing parts that meet our demanding quality specifications. We use nondestructive testing on 100% of the welds. We have discovered that material preparation is the key to producing good welds. During the cutoff operation, we are careful to avoid any contamination or burring in order to provide the clean and smooth surface required for high-quality welding. Both material preparation and welding are performed in a special controlled environment to avoid possible airborne contaminants."

Cessna learned the importance of material preparation when the orbital welding equipment was first set up and tested. Initially, preparation and welding were performed on the factory floor. However, problems caused by airborne contaminants were seen in a small number of cases, so Cessna built a special air-conditioned room apart from the factory floor. This solved the problem. While the room is air-conditioned, it is not a special environment such as the clean rooms used in the semiconductor industry.

The orbital welding equipment has proven to be reliable. More than 150,000 welds have been produced during the past five years with only a few brief downtime incidents for the equipment.

"Cessna has been successful because we have maintained the highest standards and ideals over the years," said Lyman Ratcliffe, manufacturing engineer.

 

Reprinted from the Welding Journal, June 2002.