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This work looks specifically at benefits and enhancement mechanisms associated with Plasma Augmented Laser Welding-PALW, on low carbon structural steel (4 mm and 6 mm plate). ATaguchi experimental methodology was used to assess PALW systems based on a 2 kW CO2 laser and a 2 kW Nd: YAG laser at Coventry University. This enabled a quantification of the significance of welding variables. allowing the most important process factors to be highlighted such as to maximise weld penetration. At constant maximum operating laser power, the most significant factors were found to be: welding speed, focal position and source configuration. The plasma leading configuration was shown to be superior to laser leading, this being attributed to arc pre-heat lowering the required laser power density for keyhole initiation resulting in m~re of the incident laser power being available to enlarge the weld keyhole, so propagating penetration depth. Factorial PALW work was also performed on both 6 mm and 4 mm plate using a 5 kW CO2 I~ser at Corus' Rotherham facility. Enhancements in the full penetration speed limit of up to 200 % were observed in 4 mm plate (2 kW laser power). The greatest penetration enhancements were noted at speeds of between 200300% of the laser alone full penetration speed limit, a consequence of keyhole stabilisation at elevated speeds. The synergistic potential of augmentation was fully manifested in the material at speeds ~ 100% of the laser alone full penetration speed limit; the extended keyhole stability enabled by PALW allo~ing enhancements in welding speed and penetration. The stabilisation of the keyhole at high speeds has been related to suppression of the transition of the dominant absorption mechanism in the keyhole from inverse Bremsstrahlung ~o Fresnel. It is proposed that keyhole plasma, further energised via inverse Bremsstrahlung photon absorption, acts to support the keyhole walls via its increasedlapour pressure, so resisting keyhole shrinkage and collapse at elevated speeds. Laser: arc power ratios were assessed on the 5 kW CO2 laser at Corus (Rotherham) using predominantly 2 kW and 4 kW laser powers. In each case a'laser: arc power ratio of -1.4: 1 was shown to be .J optimum for maximising penetration. The use of an energy utilisation parameter, fusion-mm3/J, aided the assessment of a complex hybrid process. Only when the keyhole root closes at the full penetration speed limit can the full benefits of augmentation be manifested in creating workpiece fusion. At this condition PALW showed equivalence with the laser alone in terms of fusion-mm3/J, the higher overall output power from PALW generating more fused material than the laser but at the same fusion-mm3/J 'efficiency rate'. An energy flow diagram for PALW was produced based on the experience and understanding ofPALW gained in this work.

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