Thurman Dwayne Mccay

Melt pool dynamics during laser welding

The dynamics of the melt pool and keyhole was investigated during CO2 laser welding using high-speed video photography and the laser reflectometer technique. A low-power argon laser beam, focused on the weld pool, provided illumination to obtain a direct image of the weld pool surface. The near-surface plasma emission background was decreased by using a narrow-bandwidth interference filter centred at the argon laser wavelength (514 nm). A variation in the shape of the keyhole opening with a characteristic frequency higher than 1 kHz was observed both during spot welding and during welding with a moving beam. For the case of spot welding with a 20 ms laser pulse, long-wavelength (about 1 mm) oscillations of the weld pool were observed with a frequency during the laser pulse and the first 5 ms after the laser pulse in the range 200-500 Hz. In the time interval starting at 25 ms and ending at approximately 40 ms from the beginning of the laser pulse, the long-wave oscillation frequency increased up to 1.3 kHz. The solidification time was determined to be approximately equal to the pulse duration for the spot welding. Surface deformation during cooling was also observed. This information is used to develop a model illustrating the dynamics of the post-pulse weld pool.

Melt instabilities during laser surface alloying

The depth of the melt layer during laser surface alloying plays a significant role in the shape of the melt pool. There appear to be critical depths at which the molten layer becomes unstable and alters its shape in response to convective flow patterns. The Marangoni number provides an understanding of the influence of the material properties on convection and subsequently the melt shape, being inversely proportional to depth squared, viscosity, and thermal diffusivity, and directly proportional to the surface tension gradient. The laser processing parameters affect the melt shape through their influence on depth. The current research sought to influence the Marangoni flow and control the shape of the melt pool using a rectangular beam.

Interferometric measurements of a dendritic growth front solutal diffusion layer

An experimental study was undertaken to measure solutal distributions in the diffusion layer produced during the vertical directional solidification (VDS) of an ammonium chloride - water (NH4Cl-H2O) solution. Interferometry was used to obtain concentration measurements in the 1-2 millimeter region defining the diffusion layer. These measurements were fitted to an exponential form to extract the characteristic diffusion parameter for various times after the start of solidification. The diffusion parameters are within the limits predicted by steady state theory and suggest that the effective solutal diffusivity is increasing as solidification progresses.

Thermal and solutal conditions at the tips of a directional dendritic growth front

The line-of-sight averaged, time-dependent dendrite tip concentrations for the diffusion dominated vertical directional solidification of a metal model (ammonium chloride and water) were obtained by extrapolating exponentially fit diffusion layer profiles measured using a laser interferometer. The tip concentrations were shown to increase linearly with time throughout the diffusion dominated growth process for an initially stagnant dendritic array. The process was terminated for the cases chosen by convective breakdown suffered when the conditionally stable diffusion layer exceeded the critical Rayleigh criteria. The transient tip concentrations were determined to significantly exceed the values predicted for steady state, thus producing much larger constitutional undercoolings. This has ramifications for growth speeds, arm spacings and the dendritic structure itself.

Plasma assisted laser surface alloying

The combination of plasma arc with a Nd:YAG laser during surface alloying produces a synergistic effect which significantly increases the depth of the melt pool beyond that expected by a summation of the individual depths. Incremental increases in laser power produce larger depth changes than incremental increases in torch amperage. Displacing the sources emphasizes the effect. This could be attributed to an increase in interaction time. The depth of the heat affected zone experiences a lesser effect, suggesting that the synergism produces more efficient energy transfer into the liquid, therefore reducing energy transfer into the solid or HAZ. This would be accomplished by changes in fluid flow distribution.