Thesis (M.S.) -- Worcester Polytechnic Institute. / Keywords: Hot cracking; Porosity; Includes bibliographical references.

Automotive and aerospace industries consume a significant amount of Al alloys in structures and framing. There is, however, a significant challenge to join the alloy components by laser welding. A key problem is the presence of large amount of porosity in the welds. This research work aimed to understand factors affecting porosity formation in laser welding of AA6014 Al alloy and identification and verification of a suitable method for the porosity reduction and elimination. AC-170PX (AA6014) Al alloy was welded, for the first time, using a 5 kW disk laser in two different configurations: fillet edge and flange couch joints using a number of different filler wires. The experimental results showed that laser power (2-5 kW) and welding speed (20-50 mm/s) had a significant influence on porosity generation. Also, the introduction of a 0.2 mm gap between the sheets significantly reduced porosity for the fillet edge joint while it had a marginal effect for the flange couch joint. The effect of the chemical composition of the filler wire on the AA6014 weld quality was also evaluated for the first time by using different filler wires (AA3004, AA4043, AA4047 and AA5083) over a range of laser powers and welding speeds. The increase in Mg and Mn content in the filler wire's composition was found to reduce porosity in comparison with high silicon content filler wires. Nanosecond pulsed Nd:YAG laser cleaning was investigated as a surface preparation method for laser welding for AA6061, and its effect on porosity at various welding parameters was examined. The effect of laser cleaning on porosity reduction during laser welding using a filler wire has not been reported before. The surface characteristics before and after laser cleaning were analysed. The results showed that laser cleaning played an essential role in significantly reducing porosity in both the fillet edge and flange couch joints at different levels of power and laser welding speed. The developed surface preparation technology as a method for porosity reduction in laser welding has been successfully implemented in one of the largest UK/international car manufacturers. To study the laser cleaning process, a novel Smoothed particle hydrodynamics (SPH) meshless model has been implemented using a new 3-D multi-phase transient model. For the first time, a study was conducted to validate the temperature field distribution predicted in SPH method under nanosecond pulsed laser heating. The need for special surface treatment of the kernel truncation was also investigated. The proposed model accurately predicted the laser ablation depth and the crater shape and was validated using a significant number of experimental and numerical data reported in the literature. Moreover, a primitive laser welding model has been created to predict the material flow inside the welding pool. The research work has resulted in four publications in peer-reviewed journals. The research highlighted that future work should include the development of a more advanced SPH model for the prediction of porosity in laser welding and to fully describe the relationship between laser cleaning and porosity reduction in laser welding.

Made available in DSpace on 2014-06-11T19:34:58Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-09-11Bitstream added on 2014-06-13T20:06:01Z : No. of bitstreams: 1 cruz_tg_dr_guara.pdf: 6855273 bytes, checksum: c4d4c4e4cbb6489a37fda09a1a0d409c (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Este trabalho propõe uma metodologia, baseada no processamento digital de imagens, para o monitoramento do processo de conformação cerâmica com amidos comerciais. No estudo da formação dos canais porosos e o processo de gelatinização foi utilizada a microscopia óptica a quente e análise do comportamento dos amidos com o aumento da temperatura. Para a caracterização dos poros durante as etapas da sua formação, foi utilizada a reconstrução por extensão de foco. Como resultado complementar às técnicas utilizadas foi desenvolvida uma metodologia, com base em tratamento estatístico, para caracterização espacial da porosidade. Foi feito o mapeamento das concentrações de porosidade e visualização em 3-D dessas regiões. / A methodology is proposed to establish standards for consolidation casting ceramics process with commercial starches, based on digital image processing. Hot stage light microscopy has been used to study porous channels formation and gelling process, evaluating starches behavior with temperature rising. Depth-from-focus reconstruction and quantitative microscopy have been applied to characterize pores during their formation. As an supplementary result, a new method for statistical based spatial characterization of porous three-dimensional distribution has been developed. It provides 3-D maps for visualization of porosities concentration distribution.

La porosité d'une plaque composite carbone / époxy de type RTM est connue par tomographie X. Une méthode de détermination de cette porosité par mesure de l'atténuation des ondes longitudinales à travers l'épaisseur de cette plaque est proposée. Ces mesures sont effectuées sur des surfaces de dimensions variables (quelques cm2 à quelques mm2) et permettent l’obtention de cartographies. Une correspondance porosité (tomo X) – atténuation (onde US) est déduite et analysée en fonction de la structure du matériau composite. Dans chaque cas, on estime la qualité des relations obtenues et on en déduit les limites de validité de la correspondance porosité-atténuation. Des premiers résultats de tomographie acoustiques sont obtenus. / The porosity of a composite plate in carbon / epoxy of type RTM is known by used of tomography X. A method of determination of this porosity by measure of the mitigation of the longitudinal waves through the thickness of this kind of plate is proposed. These measures are made on surfaces of different sizes (from some cm2 to some mm2) and allow the obtaining of cartographies. A correspondence porosity (tomo X) - Mitigation (US wave) is deducted and analyzed according to the structure of the composite material. In every case, we estimate the quality of the obtained relations and we deduct the limits of validity of the correspondence between porosity and mitigation. First results of acoustic tomography are obtained.

In this paper, it is clearly shown that the distribution of the initial porosity is a critical factor in the prediction of damage evolution and initiation of failure in a cast AM60B magnesium notch Bridgeman tensile specimen. Using X-ray computed tomography, the actual initial porosity distribution was obtained, and this distribution was input into a finite element code as an initial condition. The predicted damage evolution from this simulation was compared to the damage evolution of the experimental specimen as well as other simulated porosity distributions. This study shows that the simulation of the actual porosity distribution predicted well the damage evolution observed in the experiment. It is also shown that the initial distribution of porosity plays a vital role in the predicted elongation to failure of a notched specimen. The actual distribution was shown to fail at a significantly lower strain than random or uniformly distributed damage.

No description available.

No description available.

Background: New dental restorative material, “Cention Forte” and “Admira Fusion” have been available on the market for quite some time but are still new in comparison with traditional resin-based composite, “Ceram.x Spectra ST” and glass ionomer cement, “GC Fuji II LC”.  Aim: To compare the flexural strength, elemental composition, and porosity of new dental restorative materials (Cention Forte, Admira Fusion) with existing ones (Ceram.x Spectra ST, GC Fuji II LC).   Methods: Three-point bending test and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis was used to compare the flexural strength and composition among Cention Forte, Ceram.x Spectra ST, GC Fuji II LC and Admira Fusion. 10 test specimens were used for each material, 40 in total. The dimension of the test specimens were l (25 ± 2) mm x b (2,0 ± 0,1) mm x h (2,0 ± 0,1) mm. The statical analysis was performed with one-way ANOVA followed by Bonferroni’s post-hoc test.  Results: The three-point bending test showed that Cention Forte and Ceram.x Spectra ST had the highest mean flexural strength between the 4 materials tested, whereas GC Fuji II LC had the lowest. Admira Fusion had an intermediate value. SEM-EDS showed significant differences in material density and composition, it was seen that Ceram.x Spectra ST and Admira Fusion had the lowest porosity while GC Fuji II LC had the highest porosity.  Conclusion: It can be concluded that Cention Forte and Ceram.x Spectra ST are the strongest dental materials among the ones tested, with high flexural strength values. Therefore, they may be more suitable for use in high stress-bearing areas. GC Fuji II LC showed the lowest flexural strength values, making it less suitable for use in such areas. Admira Fusion showed intermediate values, making it a good option for applications with moderate stress-bearing requirements. SEM-EDS analysis showed that Ceram.x Spectra ST and Admira Fusion had the lowest amount of porosity, whereas GC Fuji II LC had the highest. The presence of these porosities weakens the material, a higher number of porosities implies lower flexural strength values.

No description available.

Experimental methods were developed and used to quantify the behavior of composite materials during heating to support development of heat and mass transfer pyrolysis models. Methods were developed to measure specific heat capacity, kinetic parameters, microstructure changes, porosity, and permeability. Specific heat and gravimetric data for kinetic parameters were measured with a simultaneous differential scanning calorimeter (DSC) / thermogravimetric analyzer (TGA). Experimental techniques were developed for quantitative specific heat measurement based on ASTM standards with modifications for accurate measurements of decomposing materials. An environmental scanning electron microscope (ESEM) was used in conjunction with a heating platform to record real-time video of microstructural changes of materials during decomposition and cooling following decomposition. A gas infusion technique was devised to measure porosity, in which nitrogen was infused into the pores of permeable material samples and used to determine the open-pore porosity of the material. Permeability was measured using a standard pressure differential gas flow technique with improvements over past sealing techniques and modifications to allow for potential high temperature use. Experimental techniques were used to measure the properties of composite construction materials commonly used in naval applications: E-glass vinyl ester laminates and end-grain balsa wood core. The simultaneous DSC/TGA was used to measure the apparent specific heat required to heat the decomposing sample. ESEM experiments captured microstructural changes during decomposition for both E-glass vinyl ester laminate and balsa wood samples. Permeability and porosity changes during decomposition appeared to depend on microstructural changes in addition to mass fraction. / Master of Science