Laser–material interaction and process sensing in underwater Nd:yttrium–aluminum–garnet laser welding

Laser–material interaction and process sensing technology for local-dry underwater Nd:yttrium–aluminum–garnet laser beam welding were studied. The optical emissions induced by laser–water interaction were detected with an infrared (IR) optical sensor and observed with a charge coupled device camera. It was found that under laser irradiation, a kind of water–vapor plasma formed immediately above the water surface if the water depth was more than 3 mm. This plasma severely reduced the laser power reaching the workpiece. The water above the workpiece surface must be excluded by forming a local-dry cavity so as to perform underwater laser welding. In local-dry underwater laser welding, a coaxial gas-shielding nozzle was used to form a local-dry cavity. The IR optical signal and weld bead shape variations with various shielding conditions of the local-dry cavity were investigated. The experimental results showed that the IR signals could reflect well the status of the local-dry cavity and predict the weld quality. The optimal shielding condition could be determined by IR signal stability.