Resistance Welding Processes Questions and Answers

Answer: A

The heat generated is \(H = I^{2}Rt\).

Resistance welding relies on Joule heating: a heavy current \(I\) passed for time \(t\) through the resistance \(R\) of the joint releases heat \(I^{2}Rt\). Because the heat rises with the square of the current, resistance welding machines supply very large currents at low voltage for a fraction of a second.

The other forms misplace the square. \(IR^{2}t\) and \(IRt^{2}\) are not Joule's law, and 2IRT has no physical basis; only the current is squared.

Answer: D

The voltage does not depend on the length of weld.

In resistance welding the voltage only has to drive the welding current through the resistance at the joint. That resistance is set by the composition of the metal (its resistivity), the area of contact the current passes through, and the thickness of the parts being joined.

The overall length of a weld is made up by making more spots or running the seam further; each spot or section is welded under the same conditions, so the length does not change the voltage required. Composition, area and thickness all do, which is why they are wrong answers here.

Answer: C

The one that is not a type of resistance welding is pressure welding.

Pressure welding is the broad family of processes that join metal by applying pressure, with or without heat. Resistance welding is one member of that family, not the other way round, so pressure welding is not a resistance welding process.

Spot welding, butt welding and percussion welding are all genuine resistance processes: in each, the heat comes from passing a heavy current through the parts at the joint, with pressure applied to complete the weld.

Answer: A

Projection welding is a multi-spot welding process.

Small projections are pressed into one of the parts beforehand. When the electrodes clamp the parts and the current flows, it concentrates at the projections, so several spot welds form at once, in a single operation.

Cross-wire welding of mesh is one use of the same principle, with the wires themselves acting as projections, but it is an application, not a description of the process. Continuous spot welding describes seam welding, and making cantilevers is not what the process is for.

Answer: B

Seam welding is a continuous spot welding process.

Instead of stick-shaped electrodes, seam welding uses two rotating copper wheels. The sheets pass between them while current is applied in pulses, producing a row of overlapping spot welds that forms a continuous, leak-tight seam. It is used for fuel tanks, drums and similar containers.

Making several spots at once is projection welding, and welding mesh is done by cross-wire (projection) welding. Cylindrical objects can be seam welded, but that is a use, not what seam welding is.

Answer: D

Flash butt welding is resistance welding.

The two ends to be joined are clamped and brought lightly together with current flowing. Contact at high points causes flashing (arcing and expulsion of molten metal) until the ends are heated through; they are then forged together under heavy pressure. The heat comes from the current through the joint's resistance, so it belongs with resistance welding, like spot and seam welding.

It uses no gas flame and is not an arc welding process with either polarity.

Answer: C

Spot welding is used on a lap joint.

In spot welding two copper electrodes squeeze the overlapping sheets and pass a heavy current through them; the weld nugget forms where the sheets touch, between the electrodes. The sheets must overlap so that the current passes through both, which is a lap joint, as in car body panels.

In butt, T and corner joints the parts do not overlap face to face, so there is nowhere for the electrodes to clamp both sheets together.