Gregory A. Buck

Non‐Dimensional Characterization of the Friction Stir/Spot Welding Process Using a Simple Couette Flow Model Part I: Constant Property Bingham Plastic Solution

A simplified model for the material flow created during a friction stir/spot welding process has been developed using a boundary driven cylindrical Couette flow model with a specified heat flux at the inner cylinder for a Bingham plastic material. Non‐dimensionalization of the constant property governing equations identified three parameters that influence the velocity and temperature fields. Analytic solutions to these equations are presented and some representative results from a parametric study (parameters chosen and varied over ranges expected for the welding of a wide variety of metals) are discussed. The results also provide an expression for the critical radius (location of vanishing material velocity) as functions of the relevant non‐dimensional parameters. A final study was conducted in which values for the non‐dimensional heat flux parameter were chosen to produce peak dimensional temperatures on the order of 80% of the melting temperature for a typical 2000 series aluminum. Under these conditi...

A Simplified Approach for the Determination of Critical Velocity for Cold Spray Processes

A simple technique employing the first law of thermodynamics was used to predict the critical impact velocity for cold spray processes based on material properties of the particles and substrates. It has been shown that during its interaction with the substrate, a particle should reach around 70% of its melting temperature to obtain good mechanical bonding. To characterize the results in a general way, a non-dimensionalization of the relevant parameters was conducted and validated to determine the combination of cold spray process variables required for the particle to reach the critical impact velocity.

OBSERVATIONS OF TRAVELING DISTURBANCES FROM A POINT SOURCE IN ROTATING DISK FLOW

An experimental study of the stability of the flow due to a rotating disk was conducted using a point source of disturbances of fixed frequency. In order to isolate the traveling disturbance modes from the stationary modes, a novel technique employing a hot-wire anemometer fixed to the rotating disk was used to track the monochromatic disturbances as they appear in the boundary layer. The results show the development of a curved wedge shaped region downstream of the source, in which two distinctly different traveling disturbance modes arise, one mode dominating on the inboard side of the wedge, and the other on the outboard side. These results are consistent with theoretical predictions, which identify the origin of the former with the crossflow instability (C-F mode) and the latter with an instability due to the curvature of external streamlines (S-C mode), which is predicted to arise at lower critical Reynolds number, but with smaller amplification rate. Measured C-F and S-C mode propagation angles, phase velocities and C-F disturbance wavelengths exhibit acceptable agreement with recent numerical predictions.

Preliminary Design of a Calorimeter for Experimental Determination of Effective Absorptivity of Metal Substrates During Laser Powder Deposition

A thermal model of the laser powder deposition (LPD) process has been developed and tested. Results obtained from the model, however, are dependent upon the magnitude of the laser energy absorbed during the process. Although spectral absorptivities of metal surfaces are described in literature, during the LPD process, the powder increases the energy delivered to the substrate. There are no published data regarding this affect. Therefore, the SDSM&T Additive Manufacturing Laboratory (AML) is developing a calorimeter to experimentally investigate the affect of the powder on laser energy absorption at the metal substrate. The preliminary design is described in this paper with discussion on measures being taken to increase the accuracy of experimental data.Copyright

Thermal Control of Laser Powder Deposition: Heat Transfer Considerations

Laser based solid free-form fabrication is an emerging metallurgical forming process aimed at rapid production of high quality, near net shape products directly from starting powders. Laser powder deposition shares, with other free-form technologies, the common characteristic that part fabrication occurs directly from a 3-D computer aided design (CAD) model. The microstructure evolution and resulting material properties of the component part (strength, ductility, etc.) fabricated using laser deposition are dependent upon process operating parameters such as melt pool size, laser power, head (manipulator) speed, and powder flow rate. Presently, set points for these parameters are often determined through manual manipulation of the system control and trial and error. This paper discusses the development of a path-planning, feed-forward, process-driven control system algorithm that generates a component part thermal history within given constraints, thereby assuring optimal part quality and minimizing final residual stresses. A thermal model of the deposition process drives the control algorithm. The development of the thermal model is the subject of this paper. The model accounts for temperature-dependent properties and phase change processes. Model validation studies are presented including comparisons with known analytic solutions as well as comparisons with data from experiments conducted in the laser laboratory at SDSM&T.Copyright

Numerical Simulation of an Axisymmetric Ethanol Reforming Reactor for Hydrogen Fuel Cell Applications

Hydrogen fuel cell technology is currently capable of providing adequate power for a wide range of stationary and mobile applications. Nonetheless, the sustainability of this technology rests upon the production of hydrogen from renewable resources. Among the techniques under current study, the chemical reforming of alcohols and other bio-hydrocarbon fuels, appears to offer great promise. In the so called autothermal reforming process, a suitable combination of total and partial oxidation supports hydrogen production from ethanol with no external addition of energy required. Furthermore, the autothermal reforming process conducted in a well insulated reactor, produces temperatures that promote additional hydrogen production through the endothermic steam reforming and the water-gas shift reactions, which may be catalyzed or uncatalyzed, with the added benefit of lowered carbon monoxide concentrations. In this study, an adiabatic ethanol reforming reactor was simulated assuming the reactants to be air (21% O2 and 79% N2 ) and ethanol (C2 H5 OH) and the products to be H2 O, CO2 , CO and H2 , with all constituents taken to be in the gaseous state. The air was introduced uniformly through a ring around the side of the reactor and the gaseous ethanol was injected into the center of one end, with products withdrawn from the center of the opposite end, to create an axisymmetric flow field. The gas flows within the reactor were assumed to be turbulent, and the chemical kinetics of a simple four reaction system was assumed to be controlled by turbulent mixing processes. Air and fuel flow rates into the reactor were varied to obtain six different levels of oxidation (air-fuel ratios) while maintaining the same total gaseous mass flow out of the reactor. The numerical results for the reacting flow show that hydrogen production is maximized when the air-fuel ratio on a mass basis is held at approximately 2.8. These findings are in qualitative agreement with observations from previous experimental studies.Copyright

Laser Deposition Process Design Via Thermal Analysis‐Thermal Model Development

Research being conducted in the Laser Deposition Laboratory of the Advanced Material Processing center at the SDSM&T has as one objective the development of a path‐planning, feed‐forward, process‐driven control system algorithm that generates a component part thermal history within given constraints, thereby assuring optimal part quality and minimizing final residual stresses. These constraints are maximum temperature, temperature gradient, and cooling rate. A thermal model of the deposition process drives the control algorithm. The development of the thermal model is discussed herein. Results from the model are compared to known exact solutions and to experimental data. Results from these comparisons indicate that the model is appropriately accounting for underlying physical phenomena.