Xin Sun

Ultrasonic spot welding (USW) represents one of the unique solid-state joining methods for lightweight materials such as magnesium alloy and aluminum alloy. However, the sonotrode vibration may have a detrimental impact on the sheet material and the existing welds, depending on the component geometry and vibration frequency. In this study, a modal analysis tool based on steady-state dynamics was developed for ultrasonic spot welding which features a cyclic load applied to the sheets during the joining process. Through predicting relative motion and shear stress at the faying surfaces, coupon geometry and weld spacing are identified as two major factors that affect the welding reliability and joint quality in USW. The model was validated via welding experiments on aluminum alloy and magnesium alloy and relevant characterization of temperature distribution, joint strength as well as fracture location.

Microstructural changes during thermal processing of a medium manganese steel containing (in wt%) 0.19C and 4.39 Mn were evaluated in situ with a high energy X-ray diffraction system (HEXRD). Samples with an initial fully martensitic microstructure were heated to intercritical annealing (IA) temperatures of 600 or 650°C, held for 30 min, and cooled to room temperature. Diffraction data were analyzed to determine the variations in austenite and ferrite phase fractions and phase lattice constants throughout the ICA cycles. On heating, the 2 vol. pct of austenite present in the starting microstructure decomposed, and cementite precipitation then occurred. During isothermal holding, the austenite fraction increased, up to 20% for the sample annealed at 650°C. The measured austenite fractions were less than those calculated by Thermo-Calc for equilibrium conditions, indicating that the 30-min hold time was insufficient to achieve near-equilibrium conditions. Observed changes in lattice parameters during isothermal holding were interpreted to reflect composition changes due to redistribution of the C and Mn between austenite and ferrite. The results are discussed in relation to the potential for controlling austenite stability during ambient temperature deformation.

Ultrasonic spot welding using different welding conditions was applied to join dissimilar metals of galvanized DP590 steel and AZ31B magnesium sheets. In situ high-speed imaging, digital image correlation, and infrared thermography were utilized to quantitatively study the interfacial relative motion, surface indentation, and heat generation across the joint faying interface and the sheet/sonotrode interfaces under the welding condition of moderate welding power and short welding time. For welds made with high power and long welding time, lap shear tensile tests as well as fatigue tests were carried out. Different fracture modes were observed after the lap shear tensile tests and fatigue tests performed under different peak loads. Post-weld cross-sectional analysis with scanning electron microscopy coupled with energy dispersive X-Ray spectroscopy revealed the variation of morphology and chemical composition at the joint interface for welds made with different welding conditions.

The use of 3D printed structured packing for the optimization of aqueous-amine based carbon capture in packed absorption columns is examined in this paper. An experimental testing system has been set up, and initial comparisons were made between metal, plastic, and 3D printed 16-inch packing elements and between three 8-inch 3D printed elements of different densities. Pressure drop measurements were obtained at various air flowrates under dry conditions. Measurements were also taken for a wet system by adding water at six different liquid flowrates. In each case, theoretical calculations for pressure drop were performed based on a model presented in the literature. It was found that, for the 16-inch dry column, the model slightly overpredicts the pressure drop. The model provides an accurate prediction for the dry 8-inch experimental data, especially for the two least dense packing elements. For the wet system, the model overpredicts the pressure drop, likely due to modeling deficiencies when the column reaches its loading limit. These results provide sufficient confidence to move forward with testing and process intensification of the CO₂ capture process.

We present a numerical study to investigate the effects of surface waves of falling films on the mass transfer from flowing gas phase into the liquid film. The model employs the volume of fluid method to explicitly track the gas-liquid phase interface and a one-fluid formulation to model the multiphase flow coupled with mass transfer between the two phases. The surface waves are generated by the time-varying injection rate of the falling film. The effects of surface wave frequencies and amplitudes on the overall mass transfer also are investigated for a counter-current gas-liquid flow of oxygen and water. Because of the solubility difference in the two phases, the model can capture the concentration discontinuity across the liquid-gas interface. Good agreement is obtained between the numerical results and experimental data reported in the literature. By systematically varying the frequencies and amplitudes of flow rate for the falling film, the surface waves generated on the film surface enhance mass transfer in general. The model prediction shows a proportional increase in mass transfer for small frequencies of surface waves but saturation at higher frequencies.