In-process identification of molten pool configuration: in-process monitoring of weld quality using laser ultrasonic technique (1)

摘要

A new method for in-process monitoring of welding based on non-contacting ultrasonic technique is proposed. When a laser pulse is irradiated onto a surface of base metal, an ultrasonic pulse is generated due to ablation of material from thc surface. The ultrasonic waves, which travel through both liquid and solid metal, are detected with a laser interferometer as micro displacement on the surface and then propagation time of the ultrasonic pulse is measured. Since sound velocity depends on the temperature distribution along the propagation path, it is capable to estimate the location of fusion point on the sound path from the propagation time with a temperature distribution model. Moreover, a configuration of the molten pool is reconstructed by scanning the laser beams on the surface which is being observed. Feasibility tests are performed with a pulsed Nd: YAG laser source (wavelength: 1064 nm, pulse energy: <450 mJ/pulse) for generating ultrasonic waves and Michelson interferometer for detecting. Molten pools are formed by static arc current using GTAW (welding current: 70-150 A) on type 304 stainless steel having 32 mm thickness. The estimated configurations molten pool are compared with lateral cross section views. Consequently, by applying the present method, it is found that ultrasonic signals are detected with good signal-to-noise ratio despite under welding operations and the molten pool configurations are estimated as a similar geometry within 1 mm or less accuracy. Accuracy of the estimated depth tends to be inferior as an increase of welding current; therefore advanced temperature distribution model and simultaneous detection system of the propagation time over the observing area are required in order to determine the more accurate configurations.