Investigation of the porosity formation mechanism in laser spot welding

The investigations were performed on a modeling of the porosity formation mechanism and its experimental verification in high power laser welding. Porosities formed in the weldment were classified into microporosity and macroporosity according to the porosity size. Moreover, the characteristics of porosities formed in laser welding of the keyhole mode were mainly investigated. Then, the validity of the proposed mechanism of porosity formation was also investigated by experiment. On the basis of the proposed mechanism of porosity formation, some ideas for suppressing porosity formation in high power laser welding were shown. The results were summarized as follows. 1) Spherical porosities were mainly observed in the thermal-conductive-mode laser welding, whereas keyhole-mode laser welding results in mixed spherical-irregular porosities. Moreover, it was clarified that the formation of irregular porosities was closely related to keyhole formation in keyhole-mode welding. The characteristics of porosity formation were determined by Δ Fhr(Resultant force of hydraulic pressure and reaction force(Frf) by pulsation of liquid surface) and θ (angle between Frf and ΔFhr). 2) The validity of the mechanism proposed for porosity formation became clear through the experiment. 3) The porosity formation was dependent upon the temporal fluctuation of the keyhole.