Welding is almost as old as the processing of metals by humans. For most of history, it has been regarded as an obscure art or a crude construction
technique. New discoveries and the availability of electric energy in the nineteenth century pushed the development of modern welding with an
ever- accelerating rate.
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.
Friction welding (FRW)
In this process, metals are joined through mechanical deformation. Because the metals are not melted, defects associated with melting-solidification
phenomena do not develop, and unions as strong as the base material can be made. This process can join components having relatively simple cross sections,
especially circular. It is preferred for the joining of turbine shaft and case components, and is occasionally selected for the joining of aluminum
landing gear components.
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.
Friction stir welding (FSW)
It is a solid-state process in which metals are joined through mechanical deformation. A cylindrical, shouldered tool with a profiled probe is rotated
and slowly plunged into the joint line between two pieces of sheet or plate material, which are butted together. This process can weld aluminum alloys
such as the 2xxx and 7xxx series, which were previously considered to be unweldable in aircraft structures.
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.
Flash welding (FW)
FW is a melting and joining process in which a butt joint is welded by the flashing action of a short arc and by the application of pressure. It is
capable of producing welds as strong as the base material.
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.
Resistance spot welding (RSW)
In this process, sheets of metal are joined by the heat generated by resistance to the flow of current from electrodes that also press the metal sheets
at the welding spot. It is the most widely preferred welding technique in the automotive industry, because of its low cost and ease of automation.
It is seldom applied in the aeronautic industry due to its occasional lack of reliability and its limitations for the joining of aluminum alloys.
Gas metal arc welding (GMAW)
This process is one of the most widely applied processes in the world because of its flexibility and low cost, but not in the aeronautics industry.
The reason is that the large size of the heat source (compared with processes such as EBW, LBW, and PAW) causes the welds to have poor mechanical
properties. However, this process was the main welding process for the construction of the fuel and oxidizer tanks for the Saturn V rocket
(2219 aluminum alloy for the first stage).
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.
Gas tungsten arc welding (GTAW)
This process has a more intense heat source than GMAW; therefore, it can produce welds with less distortion at a similar cost. For most structure-critical
applications, GTAW cannot compete with other welding methods such as electron beam welding, laser beam welding, or plasma arc welding. However,
GTAW and GMAW were chosen to weld the 2014 and 2219 aluminum alloys in the fuel and oxidizer tanks in the Saturn V rocket. Messerschmitt Bolkow
Blohm in Germany currently uses GTAW for the nozzle extensions of Inconel 600 in the Ariane launch vehicles.
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.
Plasma arc welding (PAW)
PAW features a constricted arc between a non-consumable electrode and the weld pool or between the electrode and the constricting nozzle (nontransferred arc).
If the heat intensity of the plasma is high enough, this process can operate in a keyhole mode, similar to that of laser or electron beam welding, although
with smaller maximum penetration. PAW was selected for of the Advanced Solid Rocket Motor (ASRM) for the Space Shuttle.
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 (LBW)
This process, together with electron beam welding, can deliver the most concentrated heat sources for welding, with the advantages of higher accuracy and
weld quality and smaller distortions. This process is ideal for welding and drilling of jet engine components made of heat-resistant alloys such as Hastelloy X.
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.
Electron beam welding (EBW)
As mentioned above, the high intensity of the electron beam generates welds with small HAZ and little distortion. This process presents an advantage over LBW,
in that it has no problems with beam reflection on the molten metal; however, it must operate in a vacuum. This characteristic makes this process especially
suitable for the welding of titanium alloys that cannot be welded in an open atmosphere.
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.
Diffusion welding (DFW)
This is a solid-state welding process that produces a weld by the application of pressure at elevated temperature, with no macroscopic deformation or relative
motion of the pieces. This process has proven particularly useful when combined with superplastic forming (SPF) of titanium alloys.
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.