By Ed Belitsky

Thirty years ago aluminum welding was not yet at the stage where it might be used with a high degree of confidence to replace rivets in airframe construction. This is no longer the case, but aircraft manufacturers in the world's most litigation-happy jurisdictions -- Canada and the United States -- are slow to make moves to replace the labour-intensive riveting technology that has worked so well for so long.

Military aviation markets, which are less sensitive to such considerations, have moved ahead with evolving technology. Airframe components in many modern military jets are joined using the best welding procedures available. It is only a matter of time before these proven techniques migrate into civilian aviation.

THE CHALLENGE

The challenge in aviation arises from the necessity to control weight through the use of lightweight metals. Welding of aluminum, magnesium and lightweight titanium alloys, which tend to flare brightly and vanish in a puff of smoke at welding temperatures, had to wait until flawless procedures were developed to exclude oxygen from the weld. Advanced techniques evolved with the application of gas plasma, laser and electron beams and the development of robotics driven by microprocessors using specialized software. The evolution of scanning and monitoring technology enhanced the application of robotics.

State-of-the-art robotics include monitoring and adjusting in real time a whole range of weld functions. Human welders would need more than three arms and unprecedented hand-eye coordination in order to compete. Robotic monitoring capabilities include optical, X-ray and laser vision. The monitoring devices control the flow of gas, current, rate of weld, direction of weld, rate of wire feed and the attitude of the welding head, which can adjust its attitude through a multiplicity of axis.

Probably the best example of where aluminum welding technology is going in the field of aeronautics is what Boeing is doing with its Delta IV orbital transfer vehicles at the company's giant new purpose-built plant in Decatur, Alabama. Here, large sheets of 3/8-in. aluminum are rolled into 16-ft dia. barrel sections, which are then joined at the butt ends into tubes with the use of stir-welding robots from ESAB of Laxa, Sweden. These barrel segments are then joined circumferentially into propellant tanks with spun aluminum end domes fabricated elsewhere with the use of plasma arc automatic welders from Liburdi-Pulsweld of Dundas, Ont. All the welding robotics are driven by software in PC consoles developed at Liburdi.

Pressure plates on mating adapters fabricated at Boeing using Liburdi technology are currently orbiting on the International Space Station.

Companies like Liburdi Engineering, Nu-Tech of Arnprior Ont. and Aerospace Welding Inc. of Blainville, Que., are regularly involved with welding rare alloys of aluminum, magnesium, and titanium to fabricate structural members, control and power components in the aerospace industry. This includes tubing for jet turbines, seam welding, exhaust systems, fuel tanks, engine mounts and landing gear.

REFURBISHING

Problems with jet engines include turbine blade erosion from sand and grit sucked into the intake and mechanical damage arising from ingestion of birds during takeoff and landings. State-of-the-art welding procedures are required to refurbish the compressor and power turbine blades in everything from jet interceptors to turbines that spin helicopter blades in machines such as the Apache. The problem first came to the military's attention when turbine blade erosion rose to a crisis point during Desert Storm when helicopters had to take off and land in a sandy environment. The resulting sand-blasting of the turbine blades reduced their efficiency to the point where they could no longer be operated safely within design parameters.

NEW TECHNIQUES

Techniques had to be developed to bring blades back to correct design specifications by replacing the lost blade materials with the right titanium alloys, according to engineer Chris Pilcher at Liburdi. They took the procedure a step further when they found that applying a thin coating of titanium nitride, a ceramic with a hardness index higher than that of sand, significantly reduced the rate of wear during Desert Storm-like operating conditions. New blades are also coated at Liburdi to make them more wear-resistant before they are installed in helicopter turbines.

Titanium nitride is the material that gives all those yellow-colored steel twist drills their superior performance and resistance to wear.

Nu-Tech Precision Metals Inc. employs the electron beam welding process (EBW) to patch-weld new material to the leading edge of titanium alloy jet engine blades which have been damaged during service. Michael J. Pates, project manager of Nu-Tech's Joining Technology Division, says the use of this high-vacuum technique reduces the risk of weld zone contamination from oxygen and nitrogen to zero. Minimum distortion and shrinkage, narrow heat affected zones and minimal thermal damage to the surrounding materials are but a few of the advantages of this fusion welding process.

Nu-Tech also uses conventional welding processes to build-to-design, repair and modify various aircraft components in aluminum, magnesium, stainless steels and nickel alloys and is approved to do so by Transport Canada.

Pates points out these welding processes are supported by state-of-the-art inverter power sources giving a full range of pulsing capability.

Dominic Zaccardelli, general manager at Aerospace Welding, Inc, says his company employs electron beams for the joining procedures of various lightweight alloys for aeronautical use. The company's fabrication and repair procedures include tubing, airframe structural members and engine components, engine inlet and nose cowl repairs, and damaged compressor or hot-section blades. Worn components are brought back to specs with the use of Plasma (or HVOF) coating techniques which can deposit a layer of new metal on old surfaces. The process is both manual and robotic, may be pre-programmed and is capable of depositing more than 300 different alloys or combinations of metals as required. Inspection and monitoring procedures at Aerospace Welding include NDT, X-ray, Strain and hydrostatic testing, according to Zaccardelli.

Larry Holt, welding automation engineer at ESAB Welding and Cutting Products of Florence, S.C., supplier of friction stir welding machines, sees this relatively new procedure as the answer to joining technology in aerospace in the future. While friction stir is currently being used to join extrusions, butt joints and lap joints in low-melt temperature alloys, there is a whole army of technicians at work expanding the application. It is now possible to join copper, silver, titanium and steel alloys. Holt says they have succeeded in joining copper to aluminum and dissimilar alloys using the technique.

The advantage in stir welding is that the adjoining metal is stirred into a plastic state using a rapidly rotating steel pin while adjoining sections are firmly forced together for a joint that is fully integrated without actual melting. The precise temperature control procedure avoids oxidation or generation of gases with resulting porosity. Holt says there is no need to prepare metal in any way nor use fluxes, inert gases or any of the rest of the paraphernalia required to shield the workpiece from oxygen and the operator's vision and epidermis from the deadly rays of an arc. Since actual heating is only to the state of plasticity, heat distortion resulting in welding inaccuracies is minimized, according to Holt.

He says current kinks still to be worked out of the process include how to safely apply pressure to work pieces consisting of compound curves and multi-dimensional shapes. Since current stir welding technology is at its best in joining long, straight pieces, it's a fair guess that joining aircraft skins and linear components will be among its first widespread applications in civil and commercial aviation.

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Refurbishment time cut by 75%

A new process can help to reduce the time required to refurbish aircraft engine turbine blades by up to 75%, according to Franklin, Wis.-based Hermle Machine Co., which developed the technique.

The process involves adding metal to the worn edge of the blade by means of a welded bead. The piece is then machined on a Hermle 600U 5-axis machining centre equipped with software designed by TTL Expert Systems, a British company.

Traditionally, refurbishing a blade required hand polishing. Machining and polishing efforts often resulted in a high rejection rate and sometimes meant scrapping the entire part.

The compressor blades, which formerly required a cycle time of 12 minutes, were machined to specification in a three-minute cycle using the new process, according to the company. The finished blades required only minimal hand polishing and the scrap rate was reduced.