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Resistance Upset Butt Welding |
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Resistance Upset Butt Welding -
Upset butt welding is a resistance welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from the resistance to electric current through the area of contact of those surfaces.
Pressure is applied before heating is started and is maintained throughout the heating period. This pressure or force is later on increased to give a forging squeeze when the welding temperature* has been reached. When sufficient upset has been produced, the welding current is cut off and the force is removed.
Principle of Operation
The steps involved in making a resistance upset butt weld are given below:
(i) The two pieces to be butt welded are gripped firmly, one in each clamp and are correctly aligned so that when brought into contact one with the other by sliding the movable clamp to the fixed one, they fit together exactly.
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(ii) Force is applied so that the faces of two pieces touch together and remain under pressure.
(iii) A heavy current is then passed from one piece to another. The resistance to the electrical current flow heats the faces to fusion temperature.
(iv) Both pressure and current are applied throughout the weld cycle and when the faces (or ends) of the pieces become plastic, they are pressed together more firmly, upsetting the metal pieces to form a dense joint.
Upsetting takes place while the current is flowing and continues until after the current is shut off. The upsetting action mixes the two metals homogeneously and pushes out many of the impurities of the atmosphere. It also reduces the heat affected zone to a minimum.
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(v) The welding current is cut off.
(vi) Upsetting force is released as the welded joint has cooled to the desired temperature.
(vii) Workpieces are unclamped.
Good butt welding is obtained if
(a) Faces to be butt welded are dean, parallel and reasonably smooth.
(b) The two workpieces are equal in cross sectional area and of equal specific resistance. If the two pieces are of unequal specific resistance, the part having the lesser resistance should project farther from the clamping die than the other.
Similarly, if the two pieces have equal specific resistance, but unequal area of crosssection, one with the larger crosssectional area should project from the damping die farther than the other part.
(c) To facilitate heating at the abutting surfaces, the areas are sometimes restricted by bevelling the ends.
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Metals Welded
The following materials are butt welded in wire, bar (up to 30 mm diameter) strip or tube form.
(i) Copper alloys,
(ii) Low and high carbon steels
(iii) Stainless steels,
(iv) Aluminium,
(v) Nickel alloys,
(vi) Resistance alloys.
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