Solid State and Special Welding Questions and Answers

Answer: D

Heat comes from a chemical reaction in thermit welding.

Thermit welding uses a mixture of aluminium powder and iron oxide. Once ignited, the aluminium reduces the iron oxide in a strongly exothermic reaction, producing molten iron at well above 2,500 °C, which runs into a mould around the joint. It is widely used for joining railway rails.

The other processes get their heat physically, not chemically: resistance welding from the electrical resistance of the joint to a heavy current, friction welding from rubbing the parts together under pressure, and tungsten arc welding from an electric arc.

Answer: B

The untrue statement is that the process essentially requires vacuum.

A laser beam travels through air without difficulty, so laser beam welding is done in the open, usually with a shielding gas. It is electron beam welding that normally needs a vacuum, because electrons are scattered by air molecules.

The other two statements are true: a laser is amplified light energy (Light Amplification by Stimulated Emission of Radiation), and its power density is far higher than an arc's, which gives deep, narrow welds. Since only one statement is untrue, 'All of the above' is wrong.

Answer: D

The keyed answer is high frequency AC current.

In plasma arc welding the plasma gas is ionised in the torch. A high frequency unit in the circuit sends a high frequency AC current between the tungsten electrode and the constricting copper nozzle, which ionises the gas and lets a pilot arc form; the arc then heats and fully ionises the gas, and the plasma jet carries the main arc to the work. The paper names the high frequency current as what generates the plasma.

Low frequency AC would not break the gas down in this way, so A and B are wrong. Option C, high frequency voltage, describes the same equipment loosely and is the closest distractor; the key prefers the current, so choose D.

Answer: A

Ultrasonic welding is usually restricted to metal about 3 mm thick.

Ultrasonic welding vibrates the parts against each other at high frequency (typically 20 kHz or more) under light pressure. The rubbing breaks up surface films and forms a solid-state bond, but the vibration energy that can be delivered is limited, so it suits thin sheets, foils and wires rather than heavy sections.

Thicknesses of 9, 12 or 15 mm are far beyond what the vibrating tool can join; the process is used for thin, dissimilar or delicate parts such as battery tabs and electrical connections.

Answer: C

Electroslag welding is performed in the vertical position.

Electroslag welding joins thick plates in a single pass. The plates stand on edge with a gap between them, and water-cooled copper shoes close the gap on each side. A pool of molten slag, kept hot by the current passing through it, melts the electrode wire and the plate edges, and the weld builds upwards as the shoes move up the joint.

The molten slag and metal would run away in flat, horizontal or overhead positions, so the process only works vertically.

Answer: D

A collimated light beam is the heat source in laser welding.

A laser produces a narrow, parallel (collimated), coherent beam of light that can be focused to a very small spot with an extremely high power density. The beam melts the metal along the joint, giving deep, narrow welds with little heat-affected zone.

TIG and MIG welding use an electric arc as their heat source, and percussion welding is a resistance process that uses a sudden discharge of electrical energy, not light.

Answer: A

LASER stands for Light Amplification by Stimulated Emission of Radiation.

In a laser, atoms are excited to a higher energy state; when a photon passes, it stimulates them to emit identical photons in step with it. Repeated passes between mirrors amplify the light into an intense, coherent beam. 'Stimulated emission' is the physical process, and 'amplification' is what the device does to light.

'Augmentation' and 'Sustained Emission' are invented substitutes for the two key words, so the other three options are wrong.

Answer: A

Railway rails are widely joined by thermit welding.

Thermit welding needs no electricity: a mixture of aluminium powder and iron oxide is ignited in a crucible above the joint, and the reaction produces molten steel that runs into a mould round the rail ends. The equipment is portable, which suits welding rails on site to make long welded track without joints.

Ultrasonic welding is for thin sheets and foils, explosive welding for cladding plates, and percussion welding for small parts such as wires; none is used to join rails in the field.