Electric Resistance Welding
It is a type of pressure welding. It is used for joining pieces of sheet metal or wire. The welding heat is obtained at the location of the desired weld by the electrical resistance through the metal pieces to a relatively short duration, low voltage, high ampere electric current. The amount of current can be regulated by changing the primary turns of the transformer. When the area to be welded is sufficiently heated, the pressure varying from 25MPa to 55MPa is applied to the joining area by suitable electrodes until the weld is solid. The various types of electric resistance welding are as follows:
(1) Spot welding
It is used for welding lap joints, joining components made from plate material having 0.025 to 1.25 mm in thickness. The plate to be joined together are places between the two electrode tips of copper or copper alloy.
(2) Roll spot and seam welding
When the spot welds on two over lapping pieces of metal are spaced, the process of welding is known as roll spot welding. If the spot welds are sufficiently made close, then the process is called seam welding. This process is best for metal thickness ranging from 0.0.25 to 3 mm.
(3) Projection welding
It is similar to spot welding except that one of the metal pieces to be welded has projections on its surface at the points, Where the welds are to be made. In other words it is a multi spot welding process.
(4) Butt welding
The butt welding is of two type :
· Upset butt welding
· Flash butt welding
The upset butt welding is especially adopted to rods, pipes and many other components of uniform sections. The flash butt welding is extensively used in the manufacture of steel containers and in the welding of mild steel shanks to high speed drills and reamers
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Extrusion Processes:
Extrusion is the process of confining the metal in a close cavity and then allowing it to flow from only one opening , so that the metal will take the shape of the opening. The operation is identical to the sequeezing of toothpaste out of the toothpaste tube.
By the extrusion process, it is possible to make component which have a constant cross section over any length as can be formed by the rolling process. Some typical parts can be extruded are shown:
Compexity of parts that can be obtain by extrusion is more than that of rolling, because the die required being very simple and easier to make. Also extrusion is a single pass process,unlike rolling the amount of reduction that si possible in extrusion is large. Generally brittle materials can be very easily extruded. It is also possible to produce sharp corners and different angles. It is possible to gets shapes with internal cavaties in extrusion by the use of spider die. Large diameters, thin walled , tubler products with execellent concentricity and tolerance charasitic can be produced.
Types of extrusion:
1: Direct extrusion or forward extrusion
2: Indirect extrusion or backward extrusion
Indirect extrusion or backward extrusion
In order to completely overcome the problem, the backward hot extrusion as shown in figure, in this process the metal is confined fully by the cylinder, the ram which houses the die also compresses the metal against the container , forcing it to flow backward to the die in the hollow plunger or ram.
It is termed backward because of the opposite direction of the flow of the metal. Thus the billet in the container remains stationary and hence produce no friction. Also the extrusion pressure is not effected by the length. In the extrusion press since the friction is not loss. The figure of the backward extrusion is shown;
