Explanation: Due to the high absorptivity within the “keyhole” there is little operational difference when welding with long or short wavelengths. Shorter wavelengths do not get absorbed quickly by plasma therefore they are more useful in welding.

Explanation: The centre has a pressure which is a function of the surface tension and the curvature. This leads to pressure instability causing the “pinch” effect. The regions of high curvature flow to regions of lower curvature resulting in large humps.

Explanation: In butt joints, the gap must be small enough that the beam cannot pass straight through the joint. The gap should be smaller than half the beam diameter (<200 mm).

Explanation: For welds where there is a large gap the beam is sometimes rotated by rotating the lens off axis from the beam. Also, there is a chance of some drop out or an underfill in the weld.

Explanation: The welds which do not require filler material are called “autogenous” materials. If there are larger gaps, they can be corrected by adding filler material as a wire or powder.

Explanation: The gas shroud can affect the formation of plasma which may block the beam and thus the absorption of the beam into the workpiece. The formation of plasma is thought to occur through the reaction of the hot metal vapours from the keyhole with the shroud gas.

Explanation: The shroud gas enters the keyhole because of the fast emission of vapour from the keyhole. The plasma blocking effect will be less for those gases having a high ionization potential.

Explanation: Helium is favoured, in spite of its price, as the top shroud gas in laser welding. The shroud underneath the weld would be of a cheaper gas e.g. argon, N2 or CO2.

– poor absorption of the radiation.

Explanation: For welds of dissimilar metals there is the additional problem of the possible formation of brittle intermetallics. Crack sensitivity of the material refers to centreline cracking, hot cracking or liquation.

Explanation: It is due to the shrinkage stress building up before the weld is fully solidified and strong enough to take the stress. It usually happens in metal alloys having a wide temperature range over which solidification occurs, e.g. those with high C, S, and P contents.

Explanation: In arc augmented laser welding the arc from a TIG torch mounted close to the laser beam interaction point will automatically lock onto the laser generated hot spot. The temperature required for this phenomenon is around 300 °C above the surrounding temperature.

Explanation: Arc Augmented Laser Welding is used for either to stabilize an arc which is unstable due to its traverse speed or to reduce the resistance of an arc which is stable.

Explanation: The arc is on the same side of the workpiece as the laser which allows doubling of the welding speed for a modest increase in the capital cost. The locking only happens for arcs with a low current and therefore slow cathode jet.

Explanation: If two laser beams are used simultaneously then there is the possibility of controlling the weld pool geometry and the weld bead shape. It is known as twin beam laser welding.