In Flash and Resistance butt-welding processes modifications of the basic resistance welding process have allowed the welding of butt joints. An important distinction is that the conventional resistance spot welding process is a fusion welding process as metal is joined from the molten state. In flash butt welding the resistance caused between 2 surfaces form a molten edge, however the pressure employed will force this molten metal to the outside of the joint causing a flash to be produced leaving the material below this to be joined in the plastic condition, hence this process is considered to be of the solid state group. This process is also used in strip steels mills to join lengths of strip and also used to join smaller lengths of rail into lengths of up to 300m at the rolling mill.
B A
Solid materials to be welded
The faces are placed in close proximity and a high current and voltage is passed through the joint.
The current is switched off and an axial pressure is applied.
The materials are joined in the plastic condition and a flash is produced.
The joint faces are moved slightly apart causing small gaps to occur creating many brief arcs. Resistance heating between facets causes the heat required for welding.
Flash C
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3) Combustion of Gases
Oxygen & acetylene will combust to produce a flame temperature of 3,200 C. Other fuel gases may be used for oxy-fuel gas cutting, as this requires a lower temperature. The intensity of heat in a chemical flame is not as high as other heating methods and as such a longer time needs to be spent applying the heat to bring a metal to its melting point as heat is dissipated by conduction, convection and radiation
The gas welding process is not as widely used these days though it is a handy standby as there is not much that cannot be done with this process in the hands of a good craftsman.
4) Welding with Pressure
Friction (42)
A most useful Welding Process in this group is Friction Welding where heat is generated by moving the two parts to be welded together to generate heat, then applying pressure to weld components together. The joint is made while the material faces remain in the plastic condition and is thus a solid phase welding process.
Generally one of the components to be welded is rotated in a chuck and the other is held in the same axis in a stock. The 2 surfaces are brought into contact and friction is generated between the 2 faces. This caused heat to be produced which eventually brings the faces into their plastic condition. The rotation is arrested and an axial load is applied to the components forcing any liquid out of the joint to form a flash. The faces are now joined in the plastic condition. A variation of this process is Inertia Welding (44) where a flywheel is left in motion as the axial load is applied. As there is no liquid phase in the weld metal this process enables a great many materials to be joined together including aluminium to steels, ceramics to metals etc. There are a great many variations on the process with Friction Stir Welding at the cutting edge of this technology.
Diffusion Bonding (45) is also a solid phase process where parts to be welded are loaded in compression and heated to within 75% of their melting point where a high level of plastic movement takes place. A perfect surface is thus created between bonding faces, with the diffusion of atoms causing molecular bridges. This process can be used to create very complex fabrications that would be impossible to make by any other means.
4 WELDING WITH PRESSURE 41 Ultrasonic welding
42 Friction welding
44 Welding by high mechanical energy 45 Diffusion welding
47 Gas pressure welding 48 Cold pressure welding
3 GAS WELDING 31 Oxy-fuel gas welding 311 Oxy-acetylene welding
32 Air fuel gas welding
THE WELDING INSTITUTE
Welding Inspection of Steels WIS 5
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5) Beam Welding
High-energy beam processes are used in specialist applications where the high cost of the equipment is outweighed by the implications of failure in any component i.e. many aerospace applications. These processes utilises a focal spot of extreme high energy that vaporises the metal and forms a keyhole through the welded seam. This resultant vapour cloud surrounds the beam keeping the keyhole patent. The seam is generally traversed beneath the beam and solidification takes place behind the moving keyhole. Butt welds are always made with a square edge preparation and weld fit up is extremely critical.
In-Vacuum Electron Beam (511) has the highest penetrating power of these processes and can weld >100mm thick steel in a square edge butt. It is commonly used in the aerospace industry for the welding of titanium alloy components, where protection from oxidation is critical. It may also be used to weld high carbon and difficult to weld steels by practically removing the risk of hydrogen associated cracking. Out of vacuum EB (512) reduces operating costs, but looses the high degree of protection from oxidation and reduces the amount of penetration through divergence effects in the beam focal spot.
Laser (52) (Light Amplification through Stimulated Emissions of Radiation) light has been used for welding/cutting for many years, though the CO2lasers (522) initially used had a major drawback in that the beam required manipulation by a series of mirrors that restricted the use of this process. With the development of the Nd-YAG Laser (A crystal containing neodymium in ytterbium aluminium and garnet) (521) a frequency of laser light is produced that can be passed through a fibre optic making this system of welding extremely flexible. High-energy beam welding allows very fast welding speeds with a narrow HAZ and producing a very minimal amount of distortion.
5 BEAM WELDING 51 Electron beam welding
511 Electron beam welding in a vacuum 512 Electron beam welding out of vacuum
52 Laser welding
521 Solid state LASER welding 522 Gas LASER welding
The Keyhole effect Beam focal spot
Static
ultra-high energy beam Solidified weldSquare edge seam
Direction of travel of the joint Completed
Weld
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7) Other Welding Processes
In this category of welding processes all those processes that cannot be classified within the other groups are given here.