Porosity formation mechanism and its prevention in laser welding

During laser welding intense evaporation develops from the molten pool surface due to the high power density at the focusing point and the molten pool surface is deeply indented by the reaction force; the welding process proceeds with a deep hole, commonly called a keyhole. The formation of this type of keyhole makes feasible deep penetration welding with a narrow bead width and these are the special features of high energy density power source welding. The penetrated hole on the liquid surface is inherently unstable and its instability increases with increasing ratio of the hole diameter to the depth, in other words, the aspect ratio. In keyhole laser welding, the penetration depth increases with increasing laser output when the welding speed is constant; simultaneously, it is known that, during this process, there is a frequent occurrence of porosity (also called as cavity gas pocket) which is a characteristic of laser welding. However, the initiation mechanism of this has not been identified. Conventionally, the keyhole formed during CW laser welding is considered to be relatively stable and all the mathematical models concerning the keyhole welding phenomena have been constructed with the assumption that the evaporative reaction force, hydrostatic pressure and surface tension pressure (Laplace pressure) are in equilibrium under the quasi-steady state. However, it is extremely difficult to explain porosity formation using such an assumption. From the results of research concerning the occurrence of porosity in laser welding and its suppression, the authors have come to believe that the distinctive porosity formation during welding with pulsed and continuous lasers is caused by the unstable phenomena of the keyhole. Consequently, an attempt was made to observe directly unstable phenomena of keyhole. At present, there are numerous examples of investigation regarding the plasma behaviour and its characteristics during laser welding; however, there is no paper describing a comprehensive investigation into the relationship between the plasma behaviour and the keyhole dynamics from the viewpoint of solving the mechanism for the occurrence of defects and the correlation between the related emission spectrum and the acoustic spectrum. The following describes the results of observation of keyhole dynamics using equipment with a high time/ place resolving power and porosity formation mechanisms.