Relationship between weld quality and optical emissions in underwater Nd: YAG laser welding

In underwater laser welding (LBW), the weld quality is very dependent on the shielding conditions of the local dry cavity when other welding parameters are fixed. Thus, diagnosing the stability of the local dry cavity is the key to monitoring of underwater LBW. In this work, a sensing system containing two optical sensors was set up to detect the infrared and ultraviolet waveband of the optical emissions induced in the welding. The effect of water on the laser welding was studied first by conducting direct underwater welding, which was performed without any method to exclude the water from the welding zone. A kind of plasma that has strong ultraviolet emission formed if the water depth was more than several millimeters, which considerably reduced the penetration depth. Then a gas-shielding nozzle was used to form a local dry cavity for underwater LBW. The relationship between the optical signals and the weld quality with various shielding conditions were investigated. The results show that the detected signal well reflects the shielding condition variations of the local dry cavity. The optimal shielding condition could be determined by signal stability.