Lap Joint Fillet Weld Procedure

Welded Joints

• A butt joint is made by placing two pieces of material edge to edge, without overlap, and then welding.
• A plain butt joint is used for metals 1.6–3.2 mm ((1/16–1/8 in.) in thickness.
• A filler rod is used when making a butt joint to obtain a strong weld.
• The flanged butt joint can be used in welding thin sheets 1.6 mm (1/16 in.) thick or less.
• The edges are prepared for welding by turning up a flange equal to the thickness of the metal.
• If the metal is thicker than 3.2 mm (1/8 in.), it may be necessary to bevel the edges so that the heat from the torch can completely penetrate the metal.
• Bevels may be either single or double-bevel type or single or double-V type.

Tee Joints

• A tee joint is formed when the edge or end of one piece is welded to the surface of another.
• These joints are quite common in aircraft construction, particularly in tubular structures.
• The plain tee joint is suitable for most thicknesses of metal used in aircraft.
• Heavier thicknesses require the vertical member to be either single or double bevelled to permit the heat to penetrate deeply enough.
• It is good practice to leave a gap between the parts, about equal to the metal thickness, to aid full penetration of the weld.

Lap Joints

• A lap joint is made by overlapping two pieces of metal and welding them together.
• The lap joint is seldom used in aircraft structures when welding with oxyacetylene, but is commonly used and joined by spot welding.
• The single lap joint offers very little resistance to bending and cannot withstand the shearing stress to which the weld may be subjected under tension or compression loads.
• The double lap joint offers more strength, but requires twice the amount of welding required on the simpler, more efficient butt weld.

Edge Joints

• An edge joint is used when two pieces of sheet metal must be fastened together and load stresses are not important.
• Edge joints are usually made by bending the edges of one or both parts upwards, placing the two ends parallel to each other, and welding along the outside of the seam formed by the two joined edges.
• The thin stock edge joint requires no filler rod since the edges can be melted down to fill the seam.
• The joint shown in illustration B, being thicker material, must be bevelled for heat penetration; filler rod is added for reinforcement.

Corner Joints

• A corner joint is made when two pieces of metal are brought together so that their edges form a corner of a box or enclosure.
• The corner joint shown in the illustration requires no filler rod since the edges fuse to make the weld.
• It is used where the load stress is not important.
• If higher stress is to be placed on the corner, the inside is reinforced with another weld bead, as in the next illustration.

Brazing and Soldering Methods

• Torch brazing is a method of joining two pieces of metal by brazing (non-fusion) using brass or bronze as the filler metal.
• The three recommended types of joint for silver soldering are lap, flanged, and edge.
• With these, the metal is formed to furnish a seam wider than the base metal thickness and provide a joint that holds up under all types of loads.

Bonding and Bond Inspection Methods

• Adhesives distribute stress evenly across the bond line, while mechanical fasteners create stress concentration points which lead to premature failure.
• Adhesives improve the aesthetics of the final assembly since they leave no bolt heads sticking out.
• Adhesives minimize or eliminate secondary operations, like punching holes, required with many fastener applications.
• Adhesives distribute stress load evenly over a broad area, reducing stress on the joint.
• Adhesives resist flex and vibration stresses and form a seal as well as a bond, which can protect the joint from corrosion.

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 which lead to premature failure.
• Whether bonding metal to metal, plastic, glass, rubber, ceramic, or another substrate material, adhesives distribute stress load evenly over a broad area, reducing stress on the joint.

Types of Stress

• Tensile stress: joint stress distribution is illustrated as a straight line; stress is evenly distributed across the entire bond and the object tends to elongate.
• Compressive stress: joint stress distribution is illustrated as a straight line; stress is evenly distributed across the entire bond.
• Cleavage stress: occurs when rigid substrates are being opened at one end; note: the stress is concentrated at one end.
• Peel stress: occurs when the stress is concentrated at one end, causing the bond to peel away from the substrate.