In-situ study of keyhole behavior during a laser pulse applied to the dissimilar metal joint

In the present study, the method of frontal observation of the keyhole through the fused quartz window is applied to the dissimilar combinations between stainless steel 316L and different metals chosen to illustrate four typical cases of mismatch in physical properties: a much lower vaporization temperature (316L/magnesium alloy AZ31), a much higher vaporization temperature (316L/pure niobium), a more reflective and conductive metal (316L/aluminum alloy A5754) and an extremely reflective metal (316L/pure copper). A standalone Yb:YAG laser pulse was applied to the dissimilar couple/quartz and metal/quartz joints. Each of these cases was studied using image treatment of the obtained high-speed videos and post-mortem observation of the interaction zone. Basing on the analysis of the physical properties of the metals and their interdependencies, the first criterion of keyhole development in the dissimilar joint is proposed. It is concluded that in case of Yb:YAG laser welding of stainless steel with metals having thermal conductivity ≤200 W · m −1 · K −1 , the keyhole development is dominant in the metal having lower vaporization temperature, while for the 316L combinations with the metals having thermal conductivity >200 W · m −1 · K −1 , the keyhole development remains dominant on the 316L side, but its progression is slowed down by the neighboring metal.