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Roll Bonding or Roll Welding
The pressure required for welding is applied through a pair of rollsCan be performed hot (Hot Roll Bonding)Surface preparation is important for interfacial bondingCladding examples:-Alclad: pure aluminum over precipitation hardened aluminum alloy sheet-Stainless steel over mild steel (for corrosion resistance)Roll bonding: bimetal strips for thermostats
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Ultrasonic Welding(a) Lap weldsThe faying surfaces of the two components are subjected to:
Static Normal force (applied through sonotrode)Oscillating shearing (tangential) stresses (applied by tip of transducer)
Efficient operation depends on proper coupling between transducer and sonotrodeShearing stresses cause plastic deformation at interface; creating clean surfaces for bondingTemperatures 1/3 – 1/2 melting point (therefore no melting or fusion)Different joining mechanism for plastics because plastics have lower melting point and there is some fusion
(b) Seam welding using roller as sonotrodeThe welding tip is replaced by rotating disks for the seam welding of structures in which one component is sheet, foil or polymer-woven material
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Friction Welding (FRW)
The heat required for the weld is generated through friction at the interface.One component remains stationeryThe other component is gripped in a chuck and rotated at high constant speedThe two members brought together with axial forceAfter sufficient contact, the rotating member is stopped while axial force is increasedOxides & other contaminants at the interface are removed by radial outward movement of hot metal at the interface
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Friction Welding (FW)
The shape of the welded joint depends on the rotational speed and axial pressure
Friction welding machines are fully automatedLow operator skill level required
FW can be used to join a wide variety of materials- Provided that one of the components has some rotational symmetry
Solid or tubular parts can be joined with good joint strength
Because of the combined heat and pressure, the interface develops a flash by plastic deformation of the heated zone.If desired, the flash can be removed by machining or grinding
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Friction Stir Welding (FSW)
The heat for joining is created by a third body rubbing against the two members to be joined.In this diagram this heat is generated by the probe between the two components
A relatively recent process (developed 1991)Successful with aluminum, copper, steel, titaniumCan use conventional vertical-spindle milling machineProcess relatively easy to implementWelds are: high quality, minimal pores, uniform material structureWelds produced with low heat input; therefore low distortionNo shielding gas or surface cleaning required
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Resistance Spot Welding
The simplest and most commonly used resistance-welding processThe tips of two opposing solid, cylindrical electrodes touch a lap joint of two sheet metalsResistance heating produces a spot weldA strong weld nugget is made by applying pressure until the current is turned off and the weld has solidified
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Resistance Spot Welding
A variety of electrode shapes are used to spot-weld areas that are difficult to reach
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Resistance Spot Welding
Widely used for fabricating sheet metal partsModern equipment is computer controlled for optimum timing of current and pressure; spot welding guns are manipulated by programmable robots
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Tests for weld nugget strength
(a) Tension shear tests are easy to perform and are inexpensive; therefore commonly used in fabricating facilities
(b) & (c) Both capable of revealing flaws, cracks, and porosity in the weld area(d) Peel test commonly used for thin sheets
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Resistance Seam Welding (RSEW)
A modification of spot welding: the electrodes are replaced by rotating wheels or rollersUsing a continuous AC power supply, the electrically conducting rollers produce a spot weld wherever the current reaches a sufficiently high level in the AC cycle.
(b) With a high enough frequency or slow enough traverse speed, these spot welds actually overlap into a continuous seam that is liquid and gas tight
Process used to make the longitudinal (side) seam of cans, mufflers, gasoline tanks, and other containers.
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High Frequency Resistance Welding (HFRW)
Similar to seam weldingException: employ high-frequency current (up to 450 kHz)
The heated edges are pressed together by passing the tube through a pair of squeeze rollsAny flash is then trimmed
Can fabricate structural sections (such as I beams) by welding the webs and flanges made from long flat pieces
(b) High Frequency Induction Welding (HFIW)Note the electrical coil
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Resistance Projection Welding (RPW)
High electrical resistance at the joint is developed by embossing one or more projections (dimples) on one of the surfaces to be welded.The projections may be round or oval for design or strength purposesHigh localized temperatures are generated at the projections (first contact)
Can weld nuts & bolts to sheets ((c) & (d))
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Flash Welding (FW)
Aka Flash Butt WeldingHeat generated very rapidly from the arc as the ends of two members begin to make contact and develop an electrical resistance at the jointAfter proper temperature is reached& interface begins to soften, an axial force is appliedappliedWeld is formed by plastic deformationGood weld quality because contaminants squeezed outMay machine later to improve appearanceMachinery usually automated; large
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Stud Welding (SW)
Aka Stud Arc WeldingSimilar to Flash WeldingThe stud (a small part; a threaded rod, hangar, handle) serves as one of the electrodes while being joined to another componentThe other component is usually a flat platePolarity for aluminum is usually DCEPPolarity for steel is DCEN
Applications in auto bodies, electric panels, shipbuilding
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Example 31.3 Resistance Welding vs. Laser-beam Welding in can-making industry
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Explosion Welding (EXW)
Pressure is applied by detonating a layer of explosive that has been placed over one of the components being joined, called the flyer plate.The contact pressures are extremely high; the kinetic energy of the plate striking the mating component causes a wavy interfaceThe impact mechanically interlocks the two surfaces, so that pressure welding by
l i d f i l k lplastic deformation also takes place
(b) The flyer plate is placed at an angleAny oxide films present at the interface are broken up and propelled out of the interfaceThe resultant bond strength is very high
The explosive may be a flexible plastic sheet or cord or in granulated or liquid form, which is cast or pressed onto the flyer plate
Process is particularly suitable for cladding a plate or slab with a dis-similar metal
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Note: Significant safety issues
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Diffusion Bonding
Aka Diffusion Welding (DFW)
Aka Hot-pressure welding (HPW)This arises from centuries old process of placing a weighted thin layer of gold foil over copper article and placing in an oven to create a product called filled gold
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Diffusion Bonding aka Diffusion Welding (DFW)
The strength of the joint results primarily from diffusion (movement of atoms across the interface) and secondarily from plastic deformation of the faying surfaces
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Diffusion Bonding-Superplastic Forming
Sheet metal structures can be fabricated by combining diffusion bonding with superplastic formingAfter diffusion bonding of selected locations on the flat sheets the unbonded (stop-off) regions are expanded in a mould by air or fluid pressure
These structures are thin and have high stiffness-to-weight ratios; particularly useful for aircraft and aerospace applications
DB-SF improves productivity by eliminating (i.e. reducing) the number of parts in a structure, mechanical fasteners, labour and cost
It produces parts with good dimensional accuracy and low residual stresses
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Case Study 31.1 Friction Welding of Pistons
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