Q. Explain briefly the Friction welding techniques.
Discuss any two solid state welding methods.
Ans. Solid State Welding: solid State welding refers to joining processes in which coalescence results from application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process is below the melting point of the metals being welded. In other words, fusion of the parts would not occur using only the heat that is externally applied in these processes. No filler metal is utilize in solid-state process.
The essential ingredients for a successful solid state weld are that the two surfaces must,
(a) Be very clean.
(b) Be brought into very close physical contact with each other to permit atomic bonding.
The solid-state welding group includes :
(i) Explosion welding (ii) Friction welding
(iii) Ultrasonic welding (iv) Roll welding
(i) Explosion welding
(ii) Friction welding : Friction welding (FRW) is a solid-state welding process in which coalescence is achieved by frictional heat combined with pressure. The friction is induced by mechanical rubbing between the surface, usually by rotation of one part relative to other. The parts are driven towards each other with sufficient force to form a metallurgical bond. The typical application of welding two cylindrical parts is shown in Figure.
(i) Friction/inertia welding is finding varied application for joining steel, superalloys, non-ferrous metals and combinations of metals.
(ii) It frequently replaces brazing and (metallic) arc, electron beam, pressure, flash or resistance butt welding.
(iii) Among its varied applications are :
(a) Production of steering shafts and warm gears, control shafts, axle shafts, engine valves, transmission shafts etc, for automobile industry.
(b) Production of bimetallic shafts, joining of superalloy turbine wheels to steels shafts, joining of then-walled containers to bases etc.
(c) Production of cutting tools like drills, tapes, reamers and some of the shanked milling cutters where HSS cutting body is welded to carbon steel shanks.
(d) Manufacture of aluminium steel transition joints, copper-to-aluminium transition pieces hole electrical and refrigeration industry (dissimilar metal welding).
(e) Welding together relatively simple forgings to make complex shapes.
(f) Fabrication of axle cases where a forgings or axle spindle is welded to tubular banjo or beam (transportation industry).
(i) Simplicity of operation.
(ii) Once the welding parameters for a job have been determined, the welding takes only a few seconds.
(iii) Low power requirements.
(iv) As compared to conventional flash or resistance butt welding, friction/inertia welding produces improved welds at higher speed and lower cost, less electric currents is required and costly copper fixtures to hold components are eliminated.
(v) Surface impurities and oxide films are broken up and thrown off during the friction heating process.
(vi) With inertia/friction welding there is less shortening of the component, as compared to that in flash or butt welding.
(vii) There is no flux, gas, filler metal or slag present to cause imperfections in welds. Also no smoke, fumes or spatter are produced.
(viii) Heat is localized at the weld and is quickly dissipated so that there is only a slight effect on the parts joined. The heat-affected zone adjacent to the weld is confined to a narrow band and therefore does not affect the temper of the surrounding area. The weld may not have to be heat treated.
(ix) Repeatability is reported as excellent and several jobs have been fully automated.
(i) The use of this process is restricted to flat and angular butt welds, where one part is normal to the other part.
(ii) Sometimes, quite a heavy flash is forced out in all inertia and friction welds.
(iii) So far the process has been applied only to the joining of small pieces in the form of bar stock.
(iv) Flash from medium and high carbon steels being hard, must either be removed while it is hot or annealed before it is machines.
(v) If tubing is welded, flash may have to be removed from inside the joint.
(vi) Thrust pressures in inertia welding will range from 700 to 2800 kg/cm2 which neeeds a heavy rigid machine.