Kendall J. Hollis

Weld pool flows during initial stages of keyhole formation in laser welding

Weld pool transport phenomena during the transition from conduction-mode laser spot welding to keyhole laser spot welding of titanium were studied by numerical simulation. A range of laser powers were simulated and temperature dependent evaporation recoil pressure and cooling were applied as boundary conditions on the weld pool surface. Simulation results predicted a complex time-varying flow pattern during weld pool development. The surface-normal flow at the weld pool centre oscillated between upwards and downwards during the simulation time due to interaction of competing effects of evaporation recoil and surface tension pressures and laser heating and evaporation cooling. The results show that the laser weld pool flow dynamics play a key role during the transition from conduction-mode laser welding to keyhole welding.

The effect of substrate temperature on the thermal diffusivity and bonding characteristics of plasma sprayed beryllium

Abstract Plasma spraying is under investigation as a method for in-situ repair of damaged beryllium and tungsten plasma facing surfaces for the International Thermonuclear Experimental Reactor (ITER), the next generation magnetic fusion energy device, and is also being considered as a potential fabrication method for beryllium and tungsten plasma-facing components for the first wall of ITER. Investigators at Los Alamos National Laboratorys Beryllium Atomization and Thermal Spray Facility have concentrated on investigating the structure-property relationship between the as-deposited microstructures of plasma sprayed beryllium coatings and the resulting thermal properties of the coatings. In this study, the effect of substrate temperature on the resulting thermal diffusivity of the beryllium coatings and the thermal diffusivity at the coating/beryllium substrate interface was investigated. Results have shown that increases in the beryllium substrate temperature can improve the thermal diffusivity of the beryllium coatings. Results also indicate that the thermal resistance at the interface between the beryllium coating and the beryllium substrate were minimal and showed little dependence on the substrate temperature. The effective bond strength and failure characteristics of plasma spray beryllium on beryllium surfaces were predominately dominated by mechanical interlocking at low substrate temperatures and increased metallurgical bonding at higher substrate temperatures.

Evaluation of plasma-sprayed tungsten for fusion reactors

Tungsten coatings are being considered for a variety of uses inside fusion reactors. Measurements of several properties of vacuum plasma- sprayed tungsten important for fusion energy applications are reported. Of vital concern is the thermal conductivity of the sprayed tungsten. Over the temperature range 25 to 1500 ‡, the thermal conductivity was approximately 60% of the value for high- purity tungsten. Of greater importance to reactor safety is the reactivity of tungsten with steam. It was found that the volatilization and reaction rates of plasma-sprayed tungsten from 800 to 1200 ‡ are similar to hot rolled tungsten. Several other useful properties have also been reported. The elastic constants of the sprayed tungsten were measured ultrasonically. The deposits were anisotropic; for example, the Young’s modulus measured in a direction parallel to the substrate was greater than that measured in the direction perpendicular to it, and their elastic moduli were approximately 30% lower than tabulated values for bulk tungsten. The average bulk density of the sprayed tungsten was approximately 17.4 g/cm , which is 90% of the theoretical density.

Particle temperature and flux measurement utilizing a nonthermal signal correction process

In-flight measurement of the surface temperature of plasma-sprayed particles is important for the correlation of particle characteristics to coating structure and properties. However, the use of optical pyrometry for particle surface temperature measurement has inherent uncertainties due to nonthermal emission signals in the plasma/particle plume. This nonthermal signal is especially pronounced near the torch exit and in regions of the plume where there are few particles. This work shows that both plasma and particle vapor signals in the form of line emission and continuum emission can be compensated for when calculating particle temperatures from the emission of the plasma/particle plume. The nonthermal signals have been estimated and removed from the raw spectral data, producing a more accurate calculated particle temperature. Using spectral shape fitting to determine the particle temperatures allows the radiant intensity to be used for particle flux estimation, thus providing more information on the state of the particle plume. Additionally, the spectral emittance of molybdenum particles sprayed in air was found to more closely resemble the emittance of pure molybdenum than the emittance of MoO3.

Analysis of the nonthermal emission signal present in a molybdenum particle-laden plasma-spray plume

In-flight measurement of the surface temperature of plasma-sprayed particles is important for the correlation of particle characteristics to coating structure and properties. However, the use of optical pyrometry for particle surface temperature measurement has inherent uncertainties due to nonthermal emission signals in the plasma/particle plume. This nonthermal signal is especially bothersome near the torch exit and in regions of the plume where there are few particles. This work presents measurements of the nonthermal signals present when making temperature measurements of plasma-sprayed molybdenum particles. Changes in the nonthermal emission signals were found to be caused by particle vapor, the spectral plasma loading effect, and particle reflection of plasma light. Care must be taken to avoid particle temperature errors due to these effects.

The removal of co-deposited carbon/deuterium films from stainless steel and tungsten by transferred-arc cleaning

Abstract Deuterium and carbon were co-deposited onto tungsten and stainless steel samples using a deuterium plasma seeded with varying amounts of deuterated methane to simulate carbon-based deposited layers in magnetic-confinement fusion devices. Cathodic arc, or transferred-arc (TA) cleaning was employed to remove the deposits from the samples. The samples were characterized by ion beam analysis both before and after cleaning to determine deuterium and carbon concentrations present. The extent of TA cleaning was varied to determine the deuterium and carbon removal efficiency and sample erosion rate. The deuterium content was greatly reduced by the cleaning, thus demonstrating the possibility of using the TA cleaning technique for removing deuterium and/or tritium from components exposed to D–T fuels. The TA removal of carbon, tantalum and molybdenum contaminants from the same samples was also quantified. Removal of surface layers and significant reduction of subsurface concentrations of these contaminants were observed.

The removal of ion implanted deuterium from tungsten and stainless steel by transferred-arc cleaning

Tungsten and stainless steel samples have been ion implanted with deuterium in an accelerator to simulate hydrogen isotope ion implantation conditions in magnetic confinement fusion devices. The samples were characterized by ion beam analysis both before and after cleaning to determine deuterium concentrations present. The extent of transferred-arc (TA) cleaning was varied to determine the deuterium removal efficiency, surface roughening and sample erosion rate. The deuterium content was greatly reduced by the cleaning thus demonstrating the possibility of using the TA cleaning technique for removing deuterium from components exposed to D-T fuels.

Plasma transferred arc deposition of beryllium

The exceptional properties of beryllium (Be), including low density and high elastic modulus, make it the material of choice in many defense and aerospace applications. However, health hazards associated with Be material handling limit the applications that are suited for its use. Innovative solutions that enable continued use of Be in critical applications while addressing worker health concerns are highly desirable. Plasma transferred arc solid free-form fabrication is being evaluated as a Be fabrication technique for civilian and military space-based components. Initial experiments producing Be deposits are reported here. Deposit shape, microstructure, and mechanical properties are reported.

Negative Transferred Arc Cleaning: A Method for Roughening and Removing Surface Contamination from Beryllium and Other Metallic Surfaces

Abstract TA cleaning has been investigated for preparing the surface of beryllium plasma facing components (PFC’s) inside of the International Thermonuclear Experimental Reactor (ITER) prior to depositing beryllium by plasma spraying. Plasma spraying of beryllium was evaluated during the ITER Engineering Design Activity (EDA) for in-situ repair and initial fabrication of the beryllium first wall armor. Results have shown that surface roughening of beryllium, during the TA cleaning process, can result in bond strengths greater than 100 MPa between beryllium surfaces and plasma sprayed beryllium. In addition, the TA cleaning process was shown to be an effective method for removing contaminate layers of carbon and tungsten from the surface of beryllium. Investigations have been performed to characterize the different arc-types that occur during the TA cleaning process (Type I, II and III arcs) and the effectiveness of the TA cleaning process for potentially removing co-deposited layers of carbon and deuterium from the surface of beryllium, stainless steel and tungsten.

Bulge Testing and Interface Fracture Characterization of Plasma-Sprayed and HIP Bonded Zr Coatings on U-Mo

Bulge testing using a pressurized fluid to fracture the interface between bonded material layers along with three-dimensional digital image correlation to measure the sample distortion caused by pressurized fluid was applied to plasma-sprayed coatings. The initiation fracture toughness associated with the bonded materials was measured during the testing. The bulge testing of the uranium-molybdenum alloy plasma sprayed with zirconium and clad in aluminum is presented. The initiation fracture toughness was observed to increase with the increasing cathodic arc-cleaning current and the use of alternating polarity transferred arc current. This dependence was linked to the interface composition of oxide and mixed metal phases along with the interface temperature during spray deposition.