Keyhole formation study in pulse Nd:YAG laser welding

In this paper, we propose two experimental methods to understand keyhole formation observed during pulse Nd :YAC laser welding. The first one gives us the possibility to observe keyhole formation using flash X-ray radiography. The X flash is delay triggered from laser pulse beginning and so, it is possible to make X-ray radiographs at different times from the beginning of the interaction. The material tested is TA6V (Titanium alloy). The first results show that a keyhole appears less than one millisecond after the beginning of the interaction (laser pulse duration is 5 ms) and grows until the end of laser pulse. The keyhole has a conical shape. The second one allows us to determine laser - matter interaction efficiency. It is based on microcalorimetry and different materials behavior under laser irradiation are studied. With this method, we measure the incident laser beam energy and the part of energy absorbed by material. The results obtained on TA6V show that laser - matter efficiency vary from 32% to 80%. Laser - matter efficiency is lower in conduction mode (typically for peak power 0.5 kW) and grows with peak power, and therefore when the keyhole volume increases.