5 Welding, Brazing and Bonding Methods.pdf

Welding Methods Introduction to Welding Fusion welding is achieved by melting the edges of the two pieces of metal to be joined (the base metal) and allowing the molten material to flow together so the two pieces will become one. This makes fusion welding distinct from brazing, becaus...

Welding Methods Introduction to Welding Fusion welding is achieved by melting the edges of the two pieces of metal to be joined (the base metal) and allowing the molten material to flow together so the two pieces will become one. This makes fusion welding distinct from brazing, because in a brazing process, the base metal is not heated enough to melt. There are three general types of fusion welding: gas, electric arc and electric resistance. Each type has several variations, some of which are used in aircraft construction. Additionally, some new welding processes have been developed in recent years that are highlighted for the purpose of information. 2023-11-16 B1-07d Maintenance Practices Page 190 of 335 CASA Part Part 66 - Training Materials Only  Gas Welding Gas welding is accomplished by heating the ends or edges of metal parts to a molten state with a high-temperature flame. The oxyacetylene flame, is produced with a torch burning acetylene and mixing it with pure oxygen. Hydrogen may be used in place of acetylene for aluminium welding. Nearly all gas welding in aircraft fabrication is performed with oxyacetylene welding equipment consisting of: Two cylinders, acetylene and oxygen Acetylene and oxygen pressure regulators and cylinder pressure gauges Two lengths of coloured hose (red for acetylene and green for oxygen) with adapter connections for the regulators and torch A welding torch with an internal mixing head, various sizes of tips and hose connections Welding goggles fitted with appropriately coloured lenses A flint or spark lighter A special wrench for the acetylene tank valve, if needed An appropriately rated fire extinguisher. The equipment may be permanently installed in a shop, but most welding outfits are of the portable type. Portable oxy rig 2023-11-16 B1-07d Maintenance Practices Page 191 of 335 CASA Part Part 66 - Training Materials Only  Electric Arc Welding Electric arc welding is used extensively by the aircraft industry in both the manufacture and repair of aircraft. It can be used satisfactorily to join all weldable metals, provided that the proper processes and materials are used. There are four types of electric arc welding: Manual metal arc welding (MMAW) Gas metal arc welding (GMAW or MIG) Gas tungsten arc welding (GTAW or TIG) Manual Metal Arc Welding Manual metal arc welding (MMAW) is the most common type of welding and is usually referred to as ‘stick’ welding. The equipment consists of a metal wire rod coated with a welding flux, clamped in an electrode holder connected by a heavy electrical cable to a low voltage and high current. The welding circuit consists of a welding machine, two leads, an electrode holder, an electrode and the work to be welded. Representative arc welding circuit 2023-11-16 B1-07d Maintenance Practices Page 192 of 335 CASA Part Part 66 - Training Materials Only  Stick welder - manual metal arc welder (MMAW) Gas Metal Arc Welding Gas metal arc welding (GMAW) was formerly called metal inert gas (MIG) welding. It is an improvement over stick welding because an uncoated wire electrode is fed into and through the torch, and an inert gas such as argon, helium or carbon dioxide flows out around the wire to protect the puddle from oxygen. The power supply is connected to the torch and the work, and the arc produces the intense heat needed to melt the work and the electrode. GMAW welding process 2023-11-16 B1-07d Maintenance Practices Page 193 of 335 CASA Part Part 66 - Training Materials Only  GMAW welding system MIG Welding Equipment This method of welding can be used for large-volume manufacturing and production work; it is not well suited to repair work because weld quality cannot be easily determined without destructive testing. MIG welder - Gas metal arc welder (GMAW) 2023-11-16 B1-07d Maintenance Practices Page 194 of 335 CASA Part Part 66 - Training Materials Only  Gas Tungsten Arc Welding Gas tungsten arc welding (GTAW) is a method of electric arc welding that fills most of the needs in aircraft maintenance and repair when proper procedures and materials are used. It is the preferred method to use on stainless steel, magnesium and most forms of thick aluminium. The first two methods of electric arc welding that were addressed used a consumable electrode that produced the filler for the weld. In TIG welding, the electrode is a tungsten rod that forms the path for the high-amperage arc between it and the work to melt the metal. The electrode is not consumed and used as filler, so a filler rod is manually fed into the molten puddle in almost the same manner as when using an oxyacetylene torch. A stream of inert gas, such as argon or helium, flows out around the electrode and envelopes the arc, thereby preventing the formation of oxides in the molten puddle. Tungsten Inert Gas (TIG) welding process Typical setup for TIG welding 2023-11-16 B1-07d Maintenance Practices Page 195 of 335 CASA Part Part 66 - Training Materials Only  TIG welder 2023-11-16 B1-07d Maintenance Practices Page 196 of 335 CASA Part Part 66 - Training Materials Only  Electric Resistance Welding Electric resistance welding, either spot welding or seam welding, is typically used to join thin sheet metal components during the manufacturing process. Spot Welding Two copper electrodes are held in the jaws of the spot-welding machine, and the material to be welded is clamped between them. Pressure is applied to hold the electrodes tightly together while electrical current flows through the electrodes and the material. The resistance of the material being welded is so much higher than that of the copper electrodes that enough heat is generated to melt the metal. The pressure on the electrodes forces the molten spots in the two pieces of metal to unite, and this pressure is held after the current stops flowing long enough for the metal to solidify. The amount of current, pressure and dwell time are all carefully controlled and matched to the type and thickness of the material to produce the correct spot welds. Spot welding 2023-11-16 B1-07d Maintenance Practices Page 197 of 335 CASA Part Part 66 - Training Materials Only  Seam Welding Rather than having to release the electrodes and move the material to form a series of spot welds, a seam-welding machine is used to manufacture fuel tanks and other components in which a continuous weld is needed. Two copper wheels replace the bar-shaped electrodes. The metal to be welded is moved between them, and electric pulses create spots of molten metal that overlap to form the continuous seam. Seam welder Example of a good weld 2023-11-16 B1-07d Maintenance Practices Page 198 of 335 CASA Part Part 66 - Training Materials Only  Characteristics of a Good Weld A completed weld should have the following characteristics: The seam should be smooth, with the bead ripples evenly spaced and of a uniform thickness. There should be little to no splatter on the surface of the plate. The bead should have a good penetration (approximately 1.6mm or 1/16th of an inch). The weld should be built up and slightly convex, providing extra thickness at the joint. The weld should taper off smoothly into the base metal. No oxide should be formed on the base metal close to the weld. The weld should show no signs of blowholes, porosity or projecting globules. The base metal should show no signs of burns, pits, cracks or distortion. Although a clean, smooth weld is desirable, cleanness and smoothness do not necessarily mean the weld is a good one; it may be dangerously weak inside. However, when a weld is rough, uneven and pitted, it is almost always unsatisfactory inside. Welds should never be filed to give them a better appearance since filing deprives the weld of part of its strength. Welds should never be filled with solder, brazing material or filler of any sort. When it is necessary to re-weld a joint, all old weld material must be removed before the operation begins. Remember that reheating the area may cause the base metal to lose some of its strength and become brittle. This should not be confused with a post-weld heat treatment that does not raise the metal to a high enough temperature to harm the base material. What is considered a good weld for different types of joint 2023-11-16 B1-07d Maintenance Practices Page 199 of 335 CASA Part Part 66 - Training Materials Only  Position Welding Techniques Introduction to Techniques of Position Welding Each time the position of a welded joint or the type of joint is changed, it may be necessary to change any one or a combination of the following: Current value Electrode Polarity Arc length Welding technique. Current values are determined by the electrode size as well as the welding position. Electrode size is governed by the thickness of the metal and the joint preparation. Electrode type is determined by the welding position. Manufacturers specify the polarity to be used with each electrode. Arc length is controlled by a combination of electrode size, welding position and welding current. Since it is impractical to cite every possible variation occasioned by different welding conditions, only the information necessary for the commonly used positions and welds is discussed here. Flat Position Welding Four types of welds are commonly used in flat position welding: bead, groove, fillet and lap joint. Each type is discussed separately in the following paragraphs. 2023-11-16 B1-07d Maintenance Practices Page 200 of 335 CASA Part Part 66 - Training Materials Only  Bead Welding The bead weld utilises the same technique that is used when depositing a bead on a flat metal surface. The only difference is that the deposited bead is at the butt joint of two steel plates, fusing them together. Square butt joints may be welded in one or multiple passes. If the thickness of the metal prevents complete fusion from being obtained by welding from one side, the joint must be welded from both sides. Most joints should first be tack-welded to ensure alignment and reduce warping. Proper bead weld 2023-11-16 B1-07d Maintenance Practices Page 201 of 335 CASA Part Part 66 - Training Materials Only  Groove Weld Groove welding may be performed on a butt joint or an outside corner joint. Groove welds are made on butt joints where the metal to be welded is 6.3 mm (1/4 in.) or more in thickness. The butt joint can be prepared using either a single or double groove, depending on the thickness of the plate. The number of passes required to complete a weld is determined by the thickness of the metal being welded and the size of the electrode being used. Any groove weld made in more than one pass must have the slag, spatter and oxide carefully removed from all previous weld deposits before welding over them. Some of the common types of groove welds performed on butt joints in the flat position are shown. Groove welds on butt joints in the flat position 2023-11-16 B1-07d Maintenance Practices Page 202 of 335 CASA Part Part 66 - Training Materials Only  Fillet Joint Fillet Weld Fillet welds are used to make T- and lap joints. The electrode should be held at an angle of 45° to the plate surface and tilted at an angle of about 15° in the direction of welding. Thin plates should be welded with little or no weaving motion of the electrode and the weld should be made in one pass. Fillet welding of thicker plates may require two or more passes using a semicircular weaving motion of the electrode. Tee joint fillet weld 2023-11-16 B1-07d Maintenance Practices Page 203 of 335 CASA Part Part 66 - Training Materials Only  Lap Joint Fillet Weld The procedure for making a fillet weld in a lap joint is similar to that used in the tee joint. The electrode is held at about a 30° angle to the vertical and tilted to about 15° in the direction of welding when joining plates of the same thickness. Lap joint fillet weld 2023-11-16 B1-07d Maintenance Practices Page 204 of 335 CASA Part Part 66 - Training Materials Only  Welded Joints Basic Types of Joints Basic joints 2023-11-16 B1-07d Maintenance Practices Page 205 of 335 CASA Part Part 66 - Training Materials Only  Butt 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 this 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. This type of joint is usually made without the use of a filler rod. 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. These bevels may be either single or double-bevel type or single or double-V type. A filler rod is used to add strength and reinforcement to the weld. Types of butt joints 2023-11-16 B1-07d Maintenance Practices Page 206 of 335 CASA Part Part 66 - Training Materials Only  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, but heavier thicknesses require the vertical member to be either single or double bevelled to permit the heat to penetrate deeply enough. The dark areas in the illustration show the depth of heat penetration and fusion required. It is good practice to leave a gap between the parts, about equal to the metal thickness, to aid full penetration of the weld. This is common when welding from only one side with tubing clusters. Tight fitment of the parts prior to welding does not provide for a proper weldment unless full penetration is secured, and this is much more difficult with a gapless fitment. Types of tee joints showing filler penetration 2023-11-16 B1-07d Maintenance Practices Page 207 of 335 CASA Part Part 66 - Training Materials Only  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 shown in illustration A 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. Edge joints 2023-11-16 B1-07d Maintenance Practices Page 208 of 335 CASA Part Part 66 - Training Materials Only  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. The next type shown in the illustration is used on heavier metals, and filler rod is added for roundness and strength. If higher stress is to be placed on the corner, the inside is reinforced with another weld bead, as in the next illustration. Corner joints Lap Joints 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. Fluid tanks Single and double lap joints 2023-11-16 B1-07d Maintenance Practices Page 209 of 335 CASA Part Part 66 - Training Materials Only  Brazing and Soldering Methods Torch Brazing Joining two pieces of metal by brazing (Non Fusion) typically meant using brass or bronze as the filler metal. However, that definition has been expanded to include any metal joining process in which the bonding material is a non-ferrous metal or alloy with a melting point higher, but lower than that of the metals being joined. Brazing is best suited to joint configurations that have large surface areas in contact, such as the lap, or for fitting fuel tank bungs and fittings. Either acetylene or hydrogen may be used as fuel gas, both being used for production work for many years. Brazing example A brazing flux is necessary to obtain a good union between the base metal and the filler metal. It destroys the oxides and floats them to the surface, leaving a clean metal surface free from oxidation. A brazing rod can be purchased with a flux coating already applied, or any one of the numerous fluxes available on the market for specific application may be used. 2023-11-16 B1-07d Maintenance Practices Page 210 of 335 CASA Part Part 66 - Training Materials Only  Silver Soldering The principal use of silver solder in aircraft work is in the fabrication of high-pressure oxygen lines and other parts that must withstand vibration and high temperatures. Silver solder is used extensively to join copper and its alloys, nickel and silver, as well as various combinations of these metals and thin steel parts. Silver soldering produces joints of higher strength than those produced by other brazing processes. Flux must be used in all silver soldering operations to ensure the base metal is chemically clean. The flux removes the film of oxide from the base metal and allows the silver solder to adhere to it. Silver soldering All silver solder joints must be physically, as well as chemically, clean. They must be free of dirt, grease, oil and/or paint. After removing the contaminants, any oxide (rust and/or corrosion) should be removed by grinding or filing the piece until bright metal can be seen. During the soldering operation, the flux continues to keep the oxide away from the metal and aid in the flow of the solder. 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. 2023-11-16 B1-07d Maintenance Practices Page 211 of 335 CASA Part Part 66 - Training Materials Only  Silver solder joints 2023-11-16 B1-07d Maintenance Practices Page 212 of 335 CASA Part Part 66 - Training Materials Only  Bonding and Bond Inspection Methods Advantages of Bonding Metals Adhesives vs Mechanical Fasteners 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 minimise or eliminate secondary operations, like punching holes, required with many fastener applications. Bonds vs. bolts 2023-11-16 B1-07d Maintenance Practices Page 213 of 335 CASA Part Part 66 - Training Materials Only  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. They resist flex and vibration stresses and form a seal as well as a bond, which can protect the joint from corrosion. Adhesives easily join irregularly shaped surfaces, increase the weight of an assembly negligibly, create virtually no change in part dimensions or geometry, and quickly and easily bond dissimilar substrates and heat-sensitive materials. Adhesive Joint Design Lap/Overlap Joint Formed by placing one substrate partially over another substrate. Lap/overlap joint Offset Joint Very similar to the lap joint. Offset joint 2023-11-16 B1-07d Maintenance Practices Page 214 of 335 CASA Part Part 66 - Training Materials Only  Butt Joint Formed by bonding two objects end to end. Butt joint Scarf Joint An angular butt joint; cutting the joint at an angle increases the surface area. Scarf joint 2023-11-16 B1-07d Maintenance Practices Page 215 of 335 CASA Part Part 66 - Training Materials Only  Strap Joint Single or double; a combination overlap joint with a butt joint. Strap joint Cylindrical Joint Uses a butt joint to join two cylindrical objects. Cylindrical joint 2023-11-16 B1-07d Maintenance Practices Page 216 of 335 CASA Part Part 66 - Training Materials Only  Types of Stresses Several types of stresses are commonly found in adhesive bonds, including: shear peel tensile cleavage compressive. Shear Stress Results from two surfaces sliding over each other. Shear stress 2023-11-16 B1-07d Maintenance Practices Page 217 of 335 CASA Part Part 66 - Training Materials Only  Peel Stress Occurs when a flexible substrate is being lifted or peeled from the other substrate Note: The stress is concentrated at one end. Peel stress Cleavage Stress Occurs when rigid substrates are being opened at one end. Note: The stress is concentrated at one end. Cleavage stress 2023-11-16 B1-07d Maintenance Practices Page 218 of 335 CASA Part Part 66 - Training Materials Only  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. Tensile stress Compressive Stress Joint stress distribution is illustrated as a straight line; stress is evenly distributed across the entire bond. Compressive stress 2023-11-16 B1-07d Maintenance Practices Page 219 of 335 CASA Part Part 66 - Training Materials Only  Metal Bonding Application Requirements No matter what type of adhesive is used, surface preparation is critical to ensuring a lasting and stable adhesive bond. Bond strength is largely determined by the degree of adhesion between the substrate and the adhesive. The layer of surface oxidation or rust that is frequently present on metal substrates impedes adhesion and must be cleaned off in order to ensure an optimal bond. Some metals, such as steel, are treated with oils or other rust-preventative coatings that can also affect the bonding process. While certain adhesives can bond through surface contaminants, others may require a cleaning process before bonding. Joint failure rarely involves adhesive strength; rather it is due to poor design, inadequate surface cleaning and preparation, or improper adhesive selection for the substrates and the operating environment. Assemblies should always be thoroughly tested in the design phase to ensure that bonding will be successful during manufacturing and over the life of the device. Inspection of Bonding Seams Bonded seams may be tested by the tap test, by ultrasonic equipment or visually. Where equipment is available, ultrasonic inspection is the most effective method of detecting flaws in bonded seams. When ultrasonic sound waves strike a discontinuity, the reflected wave produces a distinctive signal showing where the problem is. Where bonded seams are finished with enamel, a crack in the enamel may indicate a delamination. In such cases, the seam should be examined carefully to determine whether the crack is limited to the enamel or there is actual separation of the seam. A thin feeler gauge can be used to probe the crack for evidence of delamination in the seam. 2023-11-16 B1-07d Maintenance Practices Page 220 of 335 CASA Part Part 66 - Training Materials Only