Theoretical calculation of keyhole aperture morphology in laser deep penetration welding

The keyhole, as the primary feature of laser deep penetration welding, represents the principal site for achieving light-thermal energy conversion in the welding process. To study the three-dimensional morphology of keyhole, it was assumed in this paper that the temperature of molten liquid at the edge of keyhole aperture is boiling point temperature, and the boiling temperature isotherm on melt pool surface was calculated based on two different treatment methods of heat source to estimate keyhole aperture profile. Then it was subsequently verified by using in-situ optical observation and the method about melt pool quick freezing to retain keyhole. The results show that keyhole aperture in laser deep penetration welding can be divided into the zone formed by laser direct heating and the zone formed by laser-induced vapor eruption from front keyhole wall. The former is closely related to the laser energy distribution on material surface rather than the laser energy distribution in the direction of keyhole depth, while the latter is more susceptible to the influence of erupted laser-induced vapor related to the energy distribution on front keyhole wall. When the welding speed is low, the tilt angle of front keyhole wall is larger, the zone formed by laser-induced vapor eruption from the front keyhole wall is not obvious, and the zone formed by laser direct heating is approximately circular in shape. The profile of boiling temperature on the laser-induced melting pool surface calculated by the direct calculation method of laser heat source acted on material surface is consistent with the zone formed by laser direct heating. When the inclination angle of front keyhole wall is reduced by changing welding speed or material, the impact of laser-induced vapor emitted from front wall on rear wall is increased, resulting in an elliptical or "gourd" morphology of the keyhole aperture.