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Fusion-Welding Processes Solid-State Welding Processes Brazing, Soldering, Adhesive-Bonding and Mechanical-Fastening Processes. Chapter Objectives. The principles of joining processes where no external heat is applied.
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Fusion-Welding Processes • Solid-State Welding Processes • Brazing, Soldering, Adhesive-Bonding and Mechanical-Fastening Processes
Chapter Objectives • The principles of joining processes where no external heat is applied. • Welding processes where heat is involved but is generated internally or else is insufficient to cause a phase change in the workpiece, and where no filler is used. • Advantages, limitations, and applications of these processes. • Unique applications of diffusion bonding combined with superplastic forming. • Economic considerations in the selection of welding processes.
Chapter Outline • Introduction • Cold Welding and Roll Bonding • Ultrasonic Welding • Friction • Resistance Welding • Explosion Welding • Diffusion Bonding • Economics of Welding Operations
31.1 Introduction • Solid-state welding processes in which joining takes place without fusion at the interface of the two parts to be welded. • To form a strong bond, it is essential that the interface be free of oxide films, residues, metalworking fluids, other contaminants, and even adsorbed layers of gas.
31.1 Introduction • Solid-state bonding involves one or more of the following phenomena: • Diffusion: The transfer of atoms across an interface; thus, applying external heat improves the strength of the bond between the two surfaces being joined, as occurs in diffusion bonding. • Pressure: The higher the pressure, the stronger is the interface (as in roll bonding and explosion welding), where plastic deformation also occurs at the interface.
31.1 Introduction • Relative interfacial movements: When movements of the contacting surfaces (faying surfaces) occur (as in ultrasonic welding), even very small amplitudes will disturb the mating surfaces, break up any oxide films, and generate new, clean surfaces—thus improving the strength of the bond.
31.2 Cold Welding and Roll Bonding • In cold welding (CW), pressure is applied to the workpieces through dies or rolls. • Because of the plastic deformation involved, it is necessary that at least one (but preferably both) of the mating parts be ductile. • During the joining of two dissimilar metals that are mutually soluble, brittle intermetallic compounds may form; these will produce a weak and brittle joint. • The best bond strength is obtained with two similar materials.
31.2 Cold Welding and Roll Bonding • Fig 31.1 shows the schematic illustration of the roll bonding or cladding process.
31.2 Cold Welding and Roll Bonding • Typical examples are the cladding of (a) pure aluminum over precipitation-hardened aluminum-alloy sheet (Alclad) and (b) stainless steel over mild steel (for corrosion resistance).
Example 31.1 Roll bonding of the U.S. quarter The technique used for manufacturing composite U.S. quarters is the roll bonding of two outer layers of 75% Cu-25% Ni (cupronickel), where each is 1.2 mm thick with an inner layer of pure copper 5.1 mm thick. To obtain good bond strength, the faying surfaces are cleaned chemically and wire-brushed. First, the strips are rolled to a thickness of 2.29 mm; a second rolling operation reduces the thickness to 1.36 mm. The strips thus undergo a total reduction in thickness of 82%.
Example 31.1 Roll bonding of the U.S. quarter Because volume constancy is maintained in plastic deformation, there is a major increase in the surface area between the layers, and it causes the generation of clean interfacial surfaces. This extension in surface area under the high pressure of the rolls combined with the solid solubility of nickel in copper (see Section 4.2.1) produces a strong bond.
31.3 Ultrasonic Welding • In ultrasonic welding (USW), the faying surfaces of the two components are subjected to a static normal force and oscillating shearing (tangential) stresses. • The shearing stresses are applied by the tip of a transducer which is similar to that used for ultrasonic machining.
31.3 Ultrasonic Welding • Fig 31.2(a) shows the components of an ultrasonic-welding machine for making lap welds. The lateral vibrations of the tool tip cause plastic deformation and bonding at the interface of the workpieces. (b) Ultrasonic seam welding using a roller as the sonotrode. • The shearing stresses cause plastic deformation at the interface of the two components, breaking up oxide films and contaminants, thus allowing good contact and producing a strong solid-state bond.
31.3 Ultrasonic Welding • The ultrasonic-welding process is versatile and reliable. It can be used with a wide variety of metallic and nonmetallic materials, including dissimilar metals (bimetallic strips).
31.4 Friction Welding • In the joining processes described thus far, the energy required for welding (typically chemical, electrical, or ultrasonic) is supplied from external sources. In friction welding (FRW), the heat required for welding is generated through (as the name implies) friction at the interface of the two components being joined.
31.4 Friction Welding • Fig 31.3 shows the sequence of operations in the friction-welding process: 1. Left-hand component is rotated at high speed. 2. Right-hand component is brought into contact under an axial force. 3. Axial force is increased; flash begins to form. 4. Left-hand component stops rotating; weld is completed. The flash subsequently can be removed by machining or grinding.
31.4 Friction Welding • The weld zone usually is confined to a narrow region; its size depends on the following parameters: • Amount of heat generated. • Thermal conductivity of the materials. • Mechanical properties of the materials at elevated temperature. • Fig 31.4 shows the shape of the fusion zones in friction welding as a function of the axial force applied and the rotational speed.
31.4 Friction Welding Inertia friction welding • This process is a modification of FRW, although the term has been used interchangeably with friction welding. • The energy required for frictional heating in inertia friction welding is supplied by the kinetic energy of a flywheel. • The flywheel is accelerated to the proper speed, the two members are brought into contact, and an axial force is applied.
31.4 Friction Welding Inertia friction welding • As friction at the interface slows the flywheel, the axial force is increased. • The weld is completed when the flywheel has come to a stop. • The timing of this sequence is important for good weld quality.
31.4 Friction Welding Linear friction welding • In a further development of friction welding, the interface of the two components to be joined is subjected to a linear reciprocating motion, as opposed to a rotary motion. • In linear friction welding, the components do not have to be circular or tubular in their cross-section. • The process is capable of welding square or rectangular components (as well as round parts) made of metals or plastics. In this process, one part is moved across the face of the other part using a balanced reciprocating mechanism.
31.4 Friction Welding Friction stir welding • In conventional friction welding, heating interfaces is achieved through friction by rubbing two contacting surfaces. In the friction stir welding (FSW) process, developed in 1991, a third body is rubbed against the two surfaces to be joined. • Fig 31.5 shows the principle of the friction stir-welding process. Aluminum alloy plates up to 75 mm thick have been welded by this process.
31.5 Resistance Welding • The category of resistance welding (RW) covers a number of processes in which the heat required for welding is produced by means of electrical resistance across the two components to be joined. • These processes have major advantages, such as not requiring consumable electrodes, shielding gases, or flux.
31.5 Resistance Welding • The heat generated in resistance welding is given by the general expression where H = Heat generated in joules (watt-seconds) I = Current (in amperes) R = Resistance (in ohms) t = Time of current flow (in seconds)
31.5 Resistance Welding • Equation (31.1) often is modified so that it represents the actual heat energy available in the weld by including a factor K, which represents the energy losses through conduction and radiation. • The equation then becomes where the value of K is less than unity.
31.5 Resistance Welding • Fig 31.6(a) shows the sequence of events in resistance spot welding. (b) Cross-section of a spot weld showing the weld nugget and the indentation of the electrode on the sheet surfaces. This is one of the most commonly used processes in sheet-metal fabrication and in automotive-body assembly.
31.5 Resistance Welding • The total resistance is the sum of the following properties: • Resistances of the electrodes • Electrode–workpiece contact resistance • Resistances of the individual parts to be welded • Workpiece–workpiece contact resistance (between the faying surfaces)
31.5.1 Resistance Spot Welding • In resistance spot welding (RSW), the tips of two opposing solid, cylindrical electrodes touch a lap joint of two sheet metals, and resistance heating produces a spot weld. • In order to obtain a strong bond in the weld nugget, pressure is applied until the current is turned off and the weld has solidified. • Fig 31.7 shows the schematic illustration of an air-operated rocker-arm spot-welding machine.
31.5.1 Resistance Spot Welding • Fig 31.8 shows the two electrode designs for easy access to the components to be welded. • Fig 31.9 (a) and (b) shows the spot-welded cookware and muffler. (c) An automated spot-welding machine. The welding tip can move in three principal directions.
31.5.1 Resistance Spot Welding Testing spot welds • Spot-welded joints may be tested for weld-nugget strength using the following techniques: • Tension-shear • Cross-tension • Twist • Peel
31.5.1 Resistance Spot Welding Testing spot welds • Fig 31.10 shows the test methods for spot welds: (a) tension-shear test, (b) cross-tension test, (c) twist test, (d) peel test.
Example 31.2 Heat generated in spot welding Assume that two steel sheets 1-mm thick are being spot-welded at a current of 5000 A and over a current flow time of 0.1 second by means of electrodes 5 mm in diameter. Estimate the heat generated and its distribution in the weld zone if the effective resistance in the operation is 200 μΩ. Solution According to Eq. (31.1),
Example 31.2 Heat generated in spot welding From the information given, the weld-nugget volume can be estimated to be 30 mm3. Assume that the density for steel (Table 3.1) is 8000 kg/m3 (0.008 g/mm3); then the weld nugget has a mass of 0.24 g. The heat required to melt 1 g of steel is about 1400 J, so the heat required to melt the weld nugget is (1400)(1400)(0.24) = 336J. The remaining heat (164 J) is dissipated into the metal surrounding the nugget.
31.5.2 Resistance Seam Welding • Resistance seam welding (RSEW) is a modification of spot welding wherein the electrodes are replaced by rotating wheels or rollers. • Fig 31.11(a) shows the seam-welding process in which rotating rolls act as electrodes. (b) Overlapping spots in a seam weld. (c) Roll spot welds and (d) Mash seam welding.
31.5.2 Resistance Seam Welding • In roll spot welding, current to the rollers is applied only intermittently, resulting in a series of spot welds at specified intervals along the length of the seam. • In mash seam welding the overlapping welds are about one to two times the sheet thickness, and the welded seam thickness is only about 90% of the original sheet thickness.
31.5.3 High-frequency Resistance Welding • High-frequency resistance welding (HFRW) is similar to seam welding, except that high-frequency current (up to 450 kHz) is employed. • A typical application is the production of butt-welded tubing or pipe where the current is conducted through two sliding contacts. • Fig 31.12 shows the two methods of high-frequency continuous butt welding of tubes.
31.5.4 Resistance projection welding • In resistance projection welding (RPW), high electrical resistance at the joint is developed by embossing one or more projections on one of the surfaces to be welded. • Fig 31.13(a) shows the schematic illustration of resistance projection welding. (b) A welded bracket. (c) and (d) Projection welding of nuts or threaded bosses and studs. (e) Resistance-projection-welded grills.
31.5.5 Flash welding • In flash welding (FW), also called flash butt welding, heat is generated very rapidly from the arc as the ends of the two members begin to make contact and develop an electrical resistance at the joint. • Fig 31.4(a) shows the flash-welding process for end-to-end welding of solid rods or tubular parts. (b) and (c) Typical parts made by flash welding. (d) and (e) Some design guidelines for flash welding.
31.5.6 Stud welding • Stud welding (SW) also is called stud arc welding and is similar to flash welding. • The stud (which may be a small part or, more commonly, a threaded rod, hanger, or handle) serves as one of the electrodes while being joined to another component, which is usually a flat plate. • Fig 31.5 shows the sequence of operations in stud welding commonly used for welding bars, threaded rods, and various fasteners onto metal plates.
