Keyhole formation and power deposition in Nd:YAG laser spot welding

The purpose of this work is to understand keyhole formation observed during pulse Nd:YAG laser welding. An analysis of the evolution of the keyhole geometry during a laser pulse was carried out by radiography x-ray flash, for tantalum and a titanium alloy target. From the images obtained by x-ray radiography, we determined the evolution of the geometry of the keyhole (depth and width) during the laser pulse. The keyhole appears after a delay, and increases linearly. We compare the measured creation delay with a value obtained by an analytical thermal model. We present a calculation of the energy deposit law by Fresnel absorption; the aim is to calculate the ray trajectory in the keyhole to deduce the local absorption. The absorption increases strongly during the formation of the keyhole, and tends towards a limit value. Further, we compare the mean absorption during a pulse with a measurement using a differential microcalorimeter.