Welds are replacing rivets in a variety of components in both military and commercial airplanes, to improve both cost and structural integrity. Diffusion, laser, and electron-beam welding are preferred in commercial aircraft, while electron-beam welding is continually gaining ground for the joining of titanium alloys in military airplanes. In large commercial airplanes, laser-beam welds are poised to replace rivets in large parts of the fuselage. Some new processes developed for the space industry also show promise for the aeronautics industry.
Welds are replacing rivets in a variety of components in both military and commercial airplanes, to improve both cost and structural integrity. Diffusion, laser, and electron-beam welding are preferred in commercial aircraft, while electron-beam welding is continually gaining ground for the joining of titanium alloys in military airplanes. In large commercial airplanes, laser-beam welds are poised to replace rivets in large parts of the fuselage. Some new processes developed for the space industry also show promise for the aeronautics industry.
This article covers the following processes: friction welding, friction stir welding, flash welding, resistance spot welding, gas metal arc welding, gas tungsten arc welding, plasma arc welding, electron beam welding, and diffusion welding.
Linear friction (fretting) welding was considered by General Electric and Pratt & Whitney as an alternative for the manufacture and repair of high-temperature alloy blisks for jet engines. Although little has been disclosed about this technology, it is believed that FRW is being successfully implemented in engines for next generation fighter aircraft.
The strength of the weld is 30% to 50% greater than with arc welding, and fatigue life is comparable to that of riveted panels. The improvement derived from the absence of holes is compensated by the presence of a small HAZ, residual stresses, and microstructural modifications in the welding zone.
This process can weld aluminum and temperature- resistant alloys without special surface preparation or shielding gas. It can join sections with complex cross sections, and it is used in the aeronautical industry to join rings for jet engines made of temperature-resistant alloys and extruded aluminum components for landing gear.
One of the current applications of GMAW is in the automatic welding of the vanes of the Patriot missile. These vanes consist of an investment cast frame of 17-4 PH stainless steel over which sheet metal of the same composition is welded. This application benefits from the low cost of GMAW, while extreme reliability is not as important as in manned aircraft.
In addition, most of the ducting and tubing on commercial aircraft are welded by GTAW. The stainless steel and Inconel (Ni-Alloys) heat exchanger cores, louvers, and exhaust housings for jet engines, both commercial and military, are also welded by GTAW. Plug welds are also used in the stainless steel vanes of the Patriot missile.
One of the latest variations of this process is variable-polarity plasma arc welding (VPP A), commercialized by Hobart Brothers. This variation was developed by the aerospace industry for welding thicker sections of alloy aluminum, specifically for the external fuel tank of the Space Shuttle. In this process, the melting is in the keyhole mode. The negative part of the cycle provides a cathodic cleaning of the aluminum workpiece, while the positive portion provides penetration and molten metal flow.
Laser beam welding will soon replace riveting in the joining of stringers to the skin plate in the Airbus 318 and 3XX aircraft. Significant savings are expected to be made by replacing riveted joints by LBW. Riveting is estimated to consume 40% of the total manufacturing man-hours of the aircraft structure.
Titanium alloys are widely applied in military aircraft because of their light weight, high strength, and performance at elevated temperatures. The application of EBW to the welding of titanium components for military aircraft has been expanding constantly. Pylon posts and wing components in Ti-6Al-4V for the F15 fighter have been EB welded by McDonnell Douglas since the mid 70’s. The wing boxes that hold the variable geometry wings in the fighters Tornado, and F14 "Tomcat", are also Ti 6Al-4V EB welded.
In this case, complex geometries can be built in just one manufacturing step. SPF/DFW is being applied by Rolls-Royce for the manufacture of wide chord, hollow, titanium fan blades for the front of commercial engines (RB211-535E4 and Trent 700). Pratt & Whitney is also attempting to apply DFW for the joining of titanium alloy blades. In some cases, the quality and low cost enable welded titanium joints to replace riveted aluminum components.
A wing access panel for the Airbus A310 and A320 was switched from riveted aluminum to SPF/DFW titanium, thus achieving a weight saving in excess of 40%. The success of SPF/DFW with titanium stimulated much research with the goal of developing a similar process for aluminum. The fundamental difference between DFW of titanium and aluminum is that titanium can dissolve its oxides, and aluminum cannot. Therefore, the residual oxide at the interface of an aluminum joint dramatically reduces the strength of the diffusion weld. This problem has prevented the SPF/DFW of aluminum from being generally adopted.
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