Welding of Aeronautical Components

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Introduction to Aerospace Welding Technology

Welding has evolved from an ancient metal-working technique to a sophisticated joining method critical to modern aerospace manufacturing. Throughout history, welding was often viewed as an obscure art or crude construction technique. However, scientific discoveries and the widespread availability of electrical energy in the nineteenth century accelerated the development of modern welding technologies at an unprecedented rate.

Today, welds are increasingly replacing rivets across various components in both military and commercial aircraft. This transition serves dual purposes: improving structural integrity while simultaneously reducing manufacturing costs. In commercial aircraft, diffusion, laser, and electron-beam welding have become preferred joining methods. Meanwhile, electron-beam welding continues gaining prominence for joining titanium alloys in military aircraft applications. Large commercial airplanes are now implementing laser-beam welds to replace rivets in substantial sections of the fuselage. Additionally, several innovative welding processes initially developed for the space industry show promising applications in aeronautical manufacturing.

This article examines nine key welding processes relevant to aeronautical component manufacturing: 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.

Solid-State Welding Processes in Aerospace

Friction Welding (FRW)

Friction welding joins metals through mechanical deformation rather than melting. This solid-state process avoids defects typically associated with melting-solidification phenomena, enabling unions as strong as the base material. FRW is particularly effective for joining components with relatively simple cross-sections, especially circular ones. The aerospace industry prefers this method for joining turbine shaft and case components, and occasionally for aluminum landing gear components.

Linear friction (fretting) welding has been investigated by major engine manufacturers including General Electric and Pratt & Whitney as an alternative for manufacturing and repairing high-temperature alloy blisks (bladed disks) for jet engines. While detailed information about this technology remains limited, FRW is believed to be successfully implemented in engines for next-generation fighter aircraft.

Friction Stir Welding (FSW)

Friction stir welding represents another solid-state process where metals join through mechanical deformation. The process utilizes a cylindrical, shouldered tool with a profiled probe that rotates and slowly plunges into the joint line between two butted sheet or plate materials. FSW can successfully weld aluminum alloys in the 2xxx and 7xxx series, which were previously considered unweldable for aircraft structures.

Welds produced through FSW demonstrate 30% to 50% greater strength compared to arc welding, with fatigue life comparable to riveted panels. While the absence of holes improves structural integrity, this advantage is partially offset by the presence of a small heat-affected zone (HAZ), residual stresses, and microstructural modifications in the welding zone.

Flash Welding (FW)

Flash welding combines melting and joining processes wherein a butt joint is welded through the flashing action of a short arc followed by pressure application. This technique produces welds with strength equivalent to the base material.

FW can effectively weld aluminum and temperature-resistant alloys without requiring special surface preparation or shielding gas. Its ability to join sections with complex cross-sections makes it valuable in the aeronautical industry, where it joins rings for jet engines manufactured from temperature-resistant alloys and extruded aluminum components for landing gear systems.

Arc-Based Welding Technologies for Aircraft Components

Resistance Spot Welding (RSW)

In resistance spot welding, metal sheets join through heat generated by electrical resistance as current flows between electrodes that simultaneously compress the metal sheets at the welding spot. Despite being the most widely used welding technique in automotive manufacturing due to its low cost and automation potential, RSW finds limited application in aeronautical manufacturing because of occasional reliability concerns and limitations when joining aluminum alloys.

Gas Metal Arc Welding (GMAW)

GMAW ranks among the world's most widely applied welding processes due to its flexibility and cost-effectiveness, yet it sees limited use in aeronautical manufacturing. This limitation stems from the relatively large heat source size compared to more focused processes like electron beam welding (EBW), laser beam welding (LBW), and plasma arc welding (PAW), resulting in welds with inferior mechanical properties.

Historically, GMAW served as the primary welding process for constructing fuel and oxidizer tanks for the Saturn V rocket using 2219 aluminum alloy for the first stage. Currently, one notable GMAW application is the automated welding of Patriot missile vanes, which feature an investment-cast frame of 17-4 PH stainless steel with sheet metal of identical composition welded over it. This application leverages GMAW's cost advantages in a context where extreme reliability requirements are somewhat less stringent than in manned aircraft.

Gas Tungsten Arc Welding (GTAW)

GTAW utilizes a more concentrated heat source than GMAW, enabling the production of welds with reduced distortion at comparable cost. For most structurally critical applications, GTAW cannot compete with more advanced methods like electron beam welding, laser beam welding, or plasma arc welding. Nevertheless, GTAW and GMAW were selected for welding 2014 and 2219 aluminum alloys in the Saturn V rocket's fuel and oxidizer tanks. Currently, Messerschmitt Bolkow Blohm in Germany employs GTAW for Inconel 600 nozzle extensions in Ariane launch vehicles.

Additionally, GTAW is extensively used for welding most ducting and tubing systems on commercial aircraft. The stainless steel and Inconel (nickel alloys) heat exchanger cores, louvers, and exhaust housings for both commercial and military jet engines are also typically welded using GTAW. The process is further utilized for plug welds in the stainless steel vanes of Patriot missiles.

Plasma Arc Welding (PAW)

PAW features a constricted arc between a non-consumable electrode and either the weld pool or the constricting nozzle (in non-transferred arc configuration). When the plasma's heat intensity reaches sufficient levels, the process can operate in keyhole mode, similar to laser or electron beam welding, though with more limited maximum penetration capability. PAW was selected for the Advanced Solid Rocket Motor (ASRM) development for the Space Shuttle program.

A recent innovation in this field is variable-polarity plasma arc welding (VPPA), commercialized by Hobart Brothers. This variation was specifically developed by the aerospace industry for welding thicker sections of aluminum alloys, particularly for the Space Shuttle's external fuel tank. In VPPA, the negative portion of the current cycle provides cathodic cleaning of the aluminum workpiece, while the positive portion facilitates penetration and molten metal flow during keyhole mode operation.

Advanced Beam Welding Processes for Aerospace Applications

Laser Beam Welding (LBW)

Laser beam welding, along with electron beam welding, delivers the most concentrated heat sources available for welding applications. This concentration offers advantages including superior accuracy, enhanced weld quality, and minimal distortion. LBW performs exceptionally well when welding and drilling jet engine components manufactured from heat-resistant alloys such as Hastelloy X.

LBW is poised to replace riveting for joining stringers to skin plates in Airbus 318 and 3XX aircraft series. This transition is expected to generate significant cost savings, considering that riveting operations typically consume approximately 40% of the total manufacturing labor hours allocated to aircraft structural assembly.

Electron Beam Welding (EBW)

The high-intensity electron beam generates welds characterized by minimal heat-affected zones and negligible distortion. Compared to laser beam welding, EBW offers the advantage of avoiding beam reflection issues on molten metal surfaces. However, EBW requires vacuum conditions for operation, making it particularly suitable for welding titanium alloys that cannot be welded in atmospheric conditions.

Titanium alloys are extensively used in military aircraft due to their lightweight properties, high strength, and excellent performance at elevated temperatures. EBW applications for titanium components in military aircraft have continuously expanded. Since the mid-1970s, McDonnell Douglas has employed EBW for pylon posts and wing components made from Ti-6Al-4V alloy for F-15 fighters. Similarly, the wing boxes supporting variable geometry wings in Tornado and F-14 "Tomcat" fighters also utilize electron beam welded Ti-6Al-4V components.

Diffusion Welding (DFW)

Diffusion welding represents a solid-state process that creates joints through the application of pressure at elevated temperatures without causing macroscopic deformation or relative movement between the components. This technique has proven particularly valuable when combined with superplastic forming (SPF) of titanium alloys.

The SPF/DFW combination enables the production of complex geometries in a single manufacturing step. Rolls-Royce applies SPF/DFW for manufacturing wide chord, hollow titanium fan blades for the front sections of commercial engines (specifically the RB211-535E4 and Trent 700 models). Pratt & Whitney is also exploring DFW applications for titanium alloy blade joining. In certain cases, the quality and cost-effectiveness of welded titanium joints allow them to replace conventional riveted aluminum components.

A notable example is an Airbus A310 and A320 wing access panel that transitioned from riveted aluminum to SPF/DFW titanium construction, achieving weight savings exceeding 40%. The success of titanium SPF/DFW has stimulated extensive research aimed at developing analogous processes for aluminum. The fundamental challenge in aluminum DFW stems from the metal's inability to dissolve its oxides, unlike titanium. Consequently, residual oxide at aluminum joint interfaces significantly reduces diffusion weld strength, preventing widespread adoption of aluminum SPF/DFW techniques.