John A. Hopkins

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.

The influence of metals and carbides during laser surface modification of low alloy steel

The addition of both elements (Cr and Ni) and carbides (SiC and WC) during laser surface alloying under different processing speeds produced surfaces with both enhanced hardness wear resistance and corrosion properties compared to the base AISI 4340 steel material. These effects were due to the evolution of unique microstructures within the laser-processed region, which includes austenite, ferrite, martensite, Fe- and Si-based carbides and the retention of the original carbides (SiC and WC) in various combinations. The chromium and nickel stabilized the austenite and ferrite but reduced the formation of martensite that is useful to increase the hardness and prevent cracking. Also, the substantial dissociation of the original carbides (SIC and WC) into elemental silicon and tungsten supplemented the stabilization of ferrite and reduction in the hardness. The presence of the undissociated carbides and some martensite formation provided substantial increases in the microhardness. The improvement of both the mechanical properties and corrosion resistance might be self-exclusive due to the reduction of the carbides and the subsequent inability of the matrix to prevent cracking.

The nature and influence of convection on the directional dendritic solidification of a metal alloy analog, NH4Cl, and H2O

Using optical techniques, the onset and development of convection was characterized in the liquid ahead of a (transient) solidifying dendritic interface during Bridgman growth of NH4C1-H2O. Mushy zone growth rates were measured and correlated to three regimes: quiescent (dif-fusion dominated growth), cellular flow, and pluming flow. The growth rates were found to increase an order of magnitude as the system transitioned through the three regimes. Critical Rayleigh numbers, which are shown to be mushy zone height dependant, are calculated for the onset of the latter two regimes.

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.

Refractive index of NH 4 Cl–H 2 O as a function of wavelength: the effect of temperature and concentration

The refractive index of NH(4)Cl-H(2)O solutions has been measured over a wavelength range from 496.5 to 690 nm. The NH(4)Cl concentration was varied from 15 to 30 wt. % over a temperature range from 10 to 35 °C. We obtained mathematical equations relating the refractive index to wavelength, temperature, and concentration using the least-squares method. A knowledge of these properties is important for analyzing and modeling the dendritic growth of this system.

Convective flow effects on diffusion layers during NH4Cl−H2O dendritic solidification

Abstract Diffusion layer widths ahead of NH 4 Cl-71.5wt%H 2 O dendritic growth fronts are compared for microgravity and ground based experiments. It is argued from the data that both micro (dendritic stalk boundary layer) and macro (bulk liquid) convection significantly influence the size of the ground-based layer, enhancing the dendrites lateral growth but also limiting the tip growth. The use of the microgravity environment illustrates the significant aspects of convection free measurements in solidifying systems.

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.

Relationship between flow direction and dendritic growth rate in NH4Cl-H2O

evaluations of Barin et al. in 1973 t91 and 1977, u~ both of which were based on estimated values for the heat capacity of Mg3P208. Since experimentally determined lowand high-temperature heat capacities are used in the present assessment, the results obtained are more reliable. In summary, an error of 35 to 40 kJ . mol-~ exists in recent thermochemical compilations u,2] for the enthalpy and free energy of formation of Mg3P208. The error resulted from a change in standard state for phosphorus in the new compilations without appropriate corrections in the enthalpy of formation of the phosphate. These errors have been corrected in the new assessment presented in this communication.

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.