To evaluate weld quality, it is important to inspect both the molten pools involved liquid/solid boundary and the heat affected zone (HAZ). In the present paper, we are focusing the molten pool observation; an in process monitoring, based on the non-contact laser ultrasonic technique, has been developed to visualise a molten pool configuration and identify weld defects and discontinuities.

When a laser pulse is irradiated to a surface of molten metals, an acoustic pulse is generated due to ablative photo-acoustic process. Photoacoustic signals are detected with laser interferometer as micro displacement on the opposite surface and then propagation time of ultrasonic waves which travel through the molten and solid metal are measured. Sound velocity depends on the temperature distribution along the path, therefore, the location of fusion point on the sound path is calculated from the propagation time and expected temperature distribution. Moreover, a configuration of the molten pool is visualised 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, energy: < 450 mJ/ pulse ) for generating ultrasonic waves and Michelson interferometer for detection . Molten pools to be observed are formed by static arc current using GTAW on type 304 stainless steel having 20 mm thickness.

Some configurations of molten pools, which are measured by applying the method, are presented. Comparison result with lateral cross section view is also discussed. Using the technique, it is possible to observe an invisible molten pool configuration during weld. Weld defects and discontinuities will be observable by the similar technique.