Explanation: In “keyhole” welding in which there is sufficient energy/unit length to cause evaporation and hence a hole in the melt pool. This hole is stabilized by the pressure from the vapour being generated.

Explanation: The “keyhole” behaves like an optical black body in that the radiation enters the hole and is subject to multiple reflections before being able to escape. Nearly all the beam energy is absorbed once the keyhole is formed.

Laser beam welding – 15-25.

• Inverse Bremmstrahlung, leading to plasma re-radiation.

Explanation: Any hump on the surface will cause localized higher absorption and an explosion due to instantaneous evaporation. The keyhole walls are fluctuating with flow velocities up to 0.4 m/s.

Explanation: The keyhole contains considerable metal vapour, which is partially absorbing and hence capable of forming a plasma on further heating. This hot plasma vapour emerging from the keyhole may ionize the shroud gas. Ionized gas has free electrons and is thus capable of absorbing or even blocking the beam.

– Gap tolerance.

– Surface condition.

Explanation: The maximum welding speed for a given thickness rises with an increase in power. The fall off shown at the higher power levels of 2kW could be attributed to the poorer mode structure given by most lasers when working at their peak power.

– spot size x PRF x (1% overlap).