Fusion Welding Types and Equipment

• Fusion welding joins metals by melting their edges, allowing them to flow together, differing from brazing where the base metal does not melt.

Types of Fusion Welding

• Gas, electric arc, and electric resistance welding are the three main fusion welding types, each with variations used in aircraft construction.
• New welding processes have also been developed.

Gas Welding

• Gas welding heats metal edges to a molten state using a high-temperature flame, typically oxyacetylene, which burns acetylene mixed with pure oxygen.
• Hydrogen can replace acetylene when welding aluminum.

Gas Welding Equipment

• Gas welding equipment includes acetylene and oxygen cylinders, pressure regulators, colored hoses (red for acetylene, green for oxygen), a welding torch with mixing head and tips, welding goggles, a flint lighter, a special wrench for the acetylene tank valve, and a fire extinguisher.
• Gas welding equipment can be portable or permanently installed.

Electric Arc Welding

• Electric arc welding is common in aircraft manufacture and repair, suitable for all weldable metals when using proper processes and materials.
• There are four types of electric arc welding.

Manual Metal Arc Welding (MMAW)

• Manual metal arc welding (MMAW), or ‘stick’ welding, uses a flux-coated metal wire rod held by an electrode holder connected to a low voltage and high current power supply.
• The welding circuit consists of a welding machine, leads, electrode holder, electrode, and the workpiece.

Gas Metal Arc Welding (GMAW or MIG)

• Gas metal arc welding (GMAW), formerly MIG welding, feeds an uncoated wire electrode through the torch, using an inert gas (argon, helium, or carbon dioxide) to shield the weld from oxygen.
• The arc between the torch and the work melts the work and the electrode.
• MIG welding is suitable for high-volume manufacturing but less so for repair work due to difficulty in quality determination without destructive testing.

Gas Tungsten Arc Welding (GTAW or TIG)

• Gas tungsten arc welding (GTAW), or TIG welding, is ideal for aircraft maintenance and repair, especially for stainless steel, magnesium, and thick aluminum.
• TIG welding uses a non-consumable tungsten electrode to create an arc, melting the metal; filler rod is fed manually.
• An inert gas (argon or helium) protects the molten puddle from oxidation.

Electric Resistance Welding

• Electric resistance welding, including spot and seam welding, joins thin sheet metal components during manufacturing.

Spot Welding

• Spot welding clamps material between two copper electrodes and applies pressure while electrical current flows through the electrodes and material.
• The material's resistance generates heat, melting the metal, and the pressure unites the molten spots as they solidify.
• Current, pressure, and dwell time are carefully controlled based on material type and thickness.

Seam Welding

• Seam welding uses copper wheels as electrodes to create overlapping molten spots, forming a continuous seam.

Characteristics of a Completed Weld

• A completed weld should have a smooth seam with evenly spaced ripples of uniform thickness, minimal splatter, good penetration (approximately 1.6mm or 1/16th of an inch), a slightly convex build-up, and a smooth taper into the base metal.
• There should be no oxide formation, blowholes, porosity, or projecting globules, and the base metal should show no signs of burns, pits, cracks, or distortion.
• Filing welds is discouraged as it weakens the joint, and filling welds with solder or brazing material is not recommended.
• When re-welding a joint, all old weld material must be removed.

Welding Parameters

• Changing the position or type of welded joint may require adjusting the current value, electrode, polarity, arc length, or welding technique.
• Current values depend on electrode size and welding position; electrode size is determined by metal thickness and joint preparation; electrode type is determined by welding position.
• Manufacturers specify the polarity for each electrode.

Types of Welds

• Bead welds fuse two steel plates at a butt joint, applied in one or multiple passes; tack-welding is recommended to ensure alignment and reduce warping.
• Groove welds are used on butt or outside corner joints, especially for metals 6.3 mm (1/4 in.) or more in thickness, with single or double grooves depending on plate thickness.
• Fillet welds join T- and lap joints, with the electrode held at a 45° angle and tilted 15° in the welding direction, using weaving motions for thicker plates.
• Butt joints place two material edges together without overlap, using a filler rod for strength, suitable for metals 1.6–3.2 mm (1/16–1/8 in.) thick.
• Flanged butt joints weld thin sheets (1.6 mm or 1/16 in. thick or less) with flanged edges, typically without a filler rod.
• Tee joints connect the edge of one piece to the surface of another, common in tubular structures, often requiring beveling for penetration.
• Edge joints fasten two pieces of sheet metal together where load stresses are minimal, often bending the edges and welding along the seam.
• Corner joints form a box or enclosure corner, with or without a filler rod, depending on load stress.
• Lap joints are less common in aircraft structures with oxyacetylene welding but are commonly used with spot welding.

Brazing (Non Fusion)

• Brazing joins metals using a non-ferrous filler metal or alloy with a melting point higher than the metals being joined, best suited for large surface area contact configurations.
• Brazing requires a flux to remove oxides and ensure a clean metal surface for bonding, and acetylene or hydrogen can be used as fuel gas.

Silver Solder

• Silver solder is used in aircraft work for high-pressure oxygen lines and parts needing to withstand vibration and high temperatures.
• Silver solder joins copper, nickel, silver, and thin steel parts, producing stronger joints than other brazing processes.
• All silver solder joints must be physically and chemically clean, using flux to remove oxides and aid solder flow.
• Recommended joint types for silver soldering are lap, flanged, and edge joints, providing a seam wider than the base metal thickness for strength.

Adhesives vs Mechanical Fasteners

• Adhesives distribute stress evenly, whereas mechanical fasteners create stress concentration points, and adhesives improve aesthetics by eliminating protruding bolt heads.

Structural Adhesives vs Welding, Brazing, and Other Thermal Joint Methods

• Adhesives distribute stress evenly across the bond line, while welding, brazing, and other thermal joint methods create stress concentration points that lead to premature failure.
• Adhesives bond various materials, distribute stress evenly, resist flex and vibration, seal and protect the joint from corrosion, join irregular surfaces, add negligible weight, and bond dissimilar and heat-sensitive materials.

Adhesive Joint Types

• Lap/Overlap Joint formed by partially overlapping substrates.
• Offset Joint similar to the lap joint.
• Butt Joint formed by bonding two objects end to end.
• Scarf Joint an angular butt joint that increases surface area.
• Strap Joint a combination overlap joint with a butt joint, available in single or double configurations.
• Cylindrical Joint uses a butt joint to join two cylindrical objects.

Types of Stresses in Adhesive Bonds

• Shear stress results from surfaces sliding over each other.
• Peel stress occurs when a flexible substrate is lifted from another, concentrating stress at one end.
• Cleavage stress occurs when rigid substrates are opened at one end, concentrating stress.
• Tensile stress evenly distributes stress across the bond, causing elongation.
• Compressive stress evenly distributes stress across the bond, causing compression.

Adhesive Bonding

• Surface preparation is critical for adhesive bonding; bond strength depends on adhesion between the substrate and adhesive.
• Surface oxidation or rust impedes adhesion and must be removed via cleaning.
• Joint failure typically arises from poor design, inadequate surface cleaning/preparation, or improper adhesive selection.
• Assemblies should be thoroughly tested during the design phase.

Testing Bonded Seams

• Bonded seams can be tested by the tap test, ultrasonic equipment, or visually.
• Ultrasonic inspection is the most effective method for detecting flaws, using reflected waves to indicate discontinuities.
• Cracks in enamel covering bonded seams may indicate delamination; a feeler gauge can probe for separation.