Roll Bonding or Roll Welding

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5 Roll Bonding or Roll Welding The pressure required for welding is applied through a pair of rolls Can be performed hot (Hot Roll Bonding) Surface preparation is important for interfacial bonding Cladding examples: -Alclad: pure aluminum over precipitation hardened aluminum alloy sheet -Stainless steel over mild steel (for corrosion resistance) Roll bonding: bimetal strips for thermostats 5

6 Ultrasonic Welding (a) Lap welds The 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 sonotrode Shearing stresses cause plastic deformation at interface; creating clean surfaces for bonding Temperatures 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 sonotrode The welding tip is replaced by rotating disks for the seam welding of structures in which one component is sheet, foil or polymer-woven material 6

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8 Friction Welding (FRW) The heat required for the weld is generated through friction at the interface. One component remains stationery The other component is gripped in a chuck and rotated at high constant speed The two members brought together with axial force After sufficient contact, the rotating member is stopped while axial force is increased Oxides & other contaminants at the interface are removed by radial outward movement of hot metal at the interface 8

9 Friction Welding (FW) The shape of the welded joint depends on the rotational speed and axial pressure Friction welding machines are fully automated Low 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 9

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11 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, titanium Can use conventional vertical-spindle milling machine Process relatively easy to implement Welds are: high quality, minimal pores, uniform material structure Welds produced with low heat input; therefore low distortion No shielding gas or surface cleaning required 11

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13 Resistance Spot Welding The simplest and most commonly used resistance-welding process The tips of two opposing solid, cylindrical electrodes touch a lap joint of two sheet metals Resistance heating produces a spot weld A strong weld nugget is made by applying pressure until the current is turned off and the weld has solidified 13

14 Resistance Spot Welding A variety of electrode shapes are used to spot-weld areas that are difficult to reach 14

15 Resistance Spot Welding Widely used for fabricating sheet metal parts Modern equipment is computer controlled for optimum timing of current and pressure; spot welding guns are manipulated by programmable robots 15

16 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 16

17 Resistance Seam Welding (RSEW) A modification of spot welding: the electrodes are replaced by rotating wheels or rollers Using 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. 17

18 High Frequency Resistance Welding (HFRW) Similar to seam welding Exception: employ high-frequency current (up to 450 khz) The heated edges are pressed together by passing the tube through a pair of squeeze rolls Any 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 18

19 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 purposes High localized temperatures are generated at the projections (first contact) Can weld nuts & bolts to sheets ((c) & (d)) 19

20 Flash Welding (FW) Aka Flash Butt Welding Heat generated very rapidly from the arc as the ends of two members begin to make contact and develop an electrical resistance at the joint After proper temperature is reached& interface begins to soften, an axial force is applied Weld is formed by plastic deformation Good weld quality because contaminants squeezed out May machine later to improve appearance Machinery usually automated; large 20

21 Stud Welding (SW) Aka Stud Arc Welding Similar to Flash Welding The stud (a small part; a threaded rod, hangar, handle) serves as one of the electrodes while being joined to another component The other component is usually a flat plate Polarity for aluminum is usually DCEP Polarity for steel is DCEN Applications in auto bodies, electric panels, shipbuilding 21

22 Example 31.3 Resistance Welding vs. Laser-beam Welding in can-making industry 22

23 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 interface The impact mechanically interlocks the two surfaces, so that pressure welding by plastic deformation also takes place (b) The flyer plate is placed at an angle Any oxide films present at the interface are broken up and propelled out of the interface The 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 Note: Significant safety issues 23

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25 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 25

26 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 26

27 Diffusion Bonding-Superplastic Forming Sheet metal structures can be fabricated by combining diffusion bonding with superplastic forming After diffusion bonding of selected locations on the flat sheets the unbonded (stopoff) 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 27

28 Case Study 31.1 Friction Welding of Pistons 28