Effect of Dual Beam on Reduction of Porosity in CO₂ Laser Welding

In the recent past great study has been dedicated to porosity formation in laser welding. Using impressive apparatus for the experimentation Japanese authors have pointed out the main phenomenological causes of porosity produced by laser keyhole welding. Moreover, they have evidenced the effect of process parameters and bead configuration on porosity formation. Keyhole oscillation and consequently local vaporisation of the molten pool have been found to be present in all metals (stainless steel, carbon steel, aluminium and magnesium alloys) and in all process conditions (CO₂ or Nd:YAG systems, continuous wave or pulsed lasers). Since keyhole porosity can not be totally avoided, the laser welding process should be accurately selected and process parameters designed in such a way as to minimise pore formation. The paper is aimed at studying the effect of laser configuration, single beam or dual beam, on the keyhole porosity. A dual beam system, allowing the laser beam to be divided in two beams, both of them working in keyhole mode, has been studied and compared to the traditional single beam welding process. In order to compare the two laser configurations, quantitative features of pores have to be individuated. Since the pores in the bead are supposed to have a negative effect on fatigue strength of the welded component, the main characteristics of investigated porosity were the porosity percentage and the pore diameter. The experimental results show that the dual beam configuration is effective in pore area reduction, if the related process parameters (inter-beam distance and feed rate) are accurately selected.