A point and line source analysis of the laser material interaction in hyperbaric keyhole laser welding

A continuous CO2 laser of 1.35 kW has been used to study the welding of 5 mm thick stainless steel for a range of external pressures (0.1 to 0.8 MPa) in shielding gas environments of pure helium, argon and nitrogen. It was found that keyhole laser welding could not be carried out for pressures significantly in excess of atmospheric pressure using argon and nitrogen shielding gases, but that the process was possible over the whole range of pressures using helium as a shielding gas. The data using helium is summarised in the form of weld pool profiles for a series of values of the external pressure in increments of 0.05 MPa over the entire range of pressures. The interaction of the laser light with the weld specimen is analysed using the concept of a point and line source representing the strength of the laser coupling to the material. The line source of heat represents the net effect of absorption processes in the keyhole and the point source represents the absorption of heat from the laser in the plume. The differential equation for steady state heat conduction is solved using these point and line sources and their strengths are adjusted until the calculated weldpool profile matches that provided by the results of experiment. It was found that as the external pressure was increased above atmospheric pressure, the strength of the line source decreased monotonically and that the point source increased. The increasing strength of the point source is due to enhanced inverse bremsstrahlung absorption in the plume with rising pressure. The effect attenuates the available power in the laser beam and is responsible for the declining strength of the line source with increasing external pressure. The analysis points to the crucial part played by the plume and that of the shielding gas employed in the hyperbaric welding processes.