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Additively manufactured parts produced using selective laser melting (SLM) are prone to defects created during the build process due to part shrinkage while cooling. Currently defects are found only after the part is removed from the printer. To determine whether cracks can be detected before a print is completed, this project developed print parameters to print a test coupon with inherent defects – warpage and cracking. Data recorded during the build was then characterized to determine when the defects occurred. The test coupon was printed using two sets of print parameters developed to control the severity of warpage and cracking. The builds were monitored using an accelerometer recording at 12500 samples per second, an iphone recording audio at 48000 samples a second, and a camera taking a photo every build layer. Data was analyzed using image comparison, signal amplitude, Fourier Transform, and Wavelet Decomposition. The developed print parameters reduced warpage in the part by better distributing heat throughout the build envelope. Reducing warpage enabled the lower portion of the part to be printed intact, preserving it to experience cracking later in the build. From physical evidence on the part as well as time stamps from the machine script, several high energy impulse events in the accelerometer data were determined to be when cracking occurred in the build. This project’s preliminary investigation of accelerometers to detect defects in selective laser melting will be used in future work to create machine learning algorithms that would control the machine in real time and address defects as they arise.

Selective laser melting is a promising metallic additive manufacturing process with many potential applications in a variety of industries. Through a gracious donation made by Lawrence Livermore National Laboratory, California Polytechnic State University received and installed an SLM 125 HL selective laser melting machine in February 2017. As part of the initial setup effort, a preliminary machine verification study was conducted to evaluate the general print quality of the machine with default parameter settings. Coincidentally, the as-printed microstructure of SLM components was evaluated through nil strength fracture surface examination, an alternative to conventional polish-and-etch metallography. A diverse set of components were printed on the SLM 125 HL to determine the procedural best practices and inherent constraints. Additionally, the mode and mechanism of failure for a defective Lawrence Livermore National Laboratory component fabricated at their facility was investigated. From these studies, extensive documentation in the form of standard operating procedures, guidelines, templates, and summary reports was generated with the intent of facilitating future selective laser melting research at Cal Poly and strengthening the learning of students interfacing with the novel technology.

<p> The thaw rates of the active layer above the permafrost zone from a series of sites along the Hudson Bay coastline at lat. 56° have been examined with respect to temperature and moisture gradients, the characteristics of the surface layer and the bulk thermal properties for each profile. The thermal properties have been examined using firstly a Fourier approach with the parameter of degree days and using secondly a graphical approach employing thermal relationships obtained in the laboratory analyses by Kersten (1949).</p> <p> It was found that thaw rates are controlled by the interaction of a number of environmental factors of which vegetation appears to be the most important.</p> <p> The two approaches to the derivation of thermal properties give quite different results, such that the graphical approach is deemed to be unsuitable to field application.</p> / Thesis / Bachelor of Science (BSc)

<p> Levitation melting was used to study the oxidation kinetics of free falling iron droplets. Single droplets of Armco iron were deoxidized and allowed to fall through oxidizing columns of known heights and then quenched in Silicone Oil. The rate of oxygen pick up by a droplet was found to be dependent upon its initial temperature, its size, and the composition of the reacting gas. The proposed mechanism is presented with kinetic data.</p> / Thesis / Master of Engineering (MEngr)

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Yes / Titanium alloy Ti-6Al-4V is known for both its excellent mechanical properties and its low surface hardness. This study explores a two-step process for depositing a hard nanocrystalline coating onto the surface of the Ti-alloy, followed by surface melting, which embeds hard nanoparticles into a thin surface layer of the alloy. The treated surface layer was studied using X-ray diffraction, scanning electron microscopy, and Vicker's micro-hardness testing. The results of the study show that the surface of the Ti-6Al-4V alloy can be successfully hardened by embedding nanosized Al2O3 particles into the surface using gas tungsten arc welding to melt the surface of the material. Surface melting the Ti-6Al-4V alloy with a 50A welding current produced the maximum microhardness of 701 HV0.2kg. The micro-hardness of the treated surface layer decreased with the increasing size of the nanoparticles, while the roughness of the surface increased with the increasing welding current. The heat input into the surface during the surface melting process resulted in the formation of various intermetallic compounds capable of further increasing the hardness of the Ti-6Al-4V surface.

Crystallization and multiple melting behavior of bisphenol-A polycarbonate (PC) was investigated using differential scanning calorimetry (DSC) for the monitoring of thermal behavior and atomic force microscopy (AFM) for the morphology study. The exceedingly slow crystallization kinetics of PC and the feasibility of obtaining near monodisperse fractions provide distinct advantages for the elucidation of the effects of crystallization time, temperature, and molar mass on crystallization kinetics. The effects of molar mass on the glass transition temperature (Tg) and heat capacity change at Tg, and the amorphous density of PC were investigated. Similar to many semicrystalline polymers, PC exhibits a multiple melting behavior upon heating. While for each PC sample, the coexistence of low and high temperature endothermic regions in the DSC heating traces is explained by the melting of populations of crystals with different stabilities, melting-recrystallization-remelting effects are observed only for the lowest molar mass samples. The effects of crystallization temperature and molar mass distribution on overall crystallization kinetics were studied for some of the fractions, including the commercial PC-28K (Mw = 28,000 g.mol-1) sample. Regarding the kinetics of secondary crystallization, particular attention was placed on understanding the effects of molar mass, initial degree of crystallinity prior to the secondary crystallization, and secondary crystallization time and temperature. The secondary crystallization of PC follows the same laws discovered in previous studies of PEEK, PET, it-PS and ethylene copolymers, and the results are discussed in the context of a bundle-like secondary crystallization model. During isothermal annealing of semicrystalline PC-28K around the high melting endotherm, a significant increase of melting temperature along with peak broadening with time was observed. Independently, morphological studies using AFM showed that mean lamellar thickness increases with time during isothermal annealing. These results are discussed in light of isothermal thickening of lamellar crystals. Lastly, almost 200 DSC melting traces of varying molar mass PC samples thermally treated under various conditions were analyzed to calculate crystallinity (Xc), rigid fraction (RF), and rigid amorphous fraction (RAF). The correlation between RAF vs Xc, Tg, and Tg broadening are discussed. / Ph. D.

A slit die with a rectangular slot placed transverse to the flow direction has been used for both flow visualization and direct pressure measurements of the hole pressure (Ph). The results from these measurements have been used to evaluate the Higashitani-Pritchard-Baird (HPB) equation which has been proposed for determining the magnitude of the primary normal stress difference (N1) from the values of the hole pressure. The slit die was run at higher shear rates than those used by Pike. Both the tracer method and flow birefringence technique were applied to visualize the streamlines and stress field, respectively, in the slot region . Effects of the slot on the flow pattern and on the stress field were examined by changing the slot width and by rounding the corners of the slot. The validity of the HPB equation, which is derived from the Higashitani-Pritchard theory (H-P theory), was tested by comparing values of N1 predicted by the HPB equation and slit die data with that obtained from the cone-and-plate rheometer. The validity of the HPB equation was also tested by changing the slot dimensions. Flow visualization experiments were performed for polystyrene (Styron-678) and polycarbonate (Lexan) melts. It was found that both the flow and stress fields are asymmetrically distributed about the slot centerline and that secondary flows exist in the upper part of the slot. However, no visible vortices was found for LOPE even though σ_w was increased to 70 Kpa. The shear rate at which the vortices became visible is lower for a wider slot. Rounding both corners of the slot seems to have an effect on smoothing the streamlines across the slot. Polycarbonate, which exhibits lower fluid elasticity than polystyrene does, the streamlines and stress field are more symmetrically distributed about the slot centerline. Five polymer melts were used in measuring Ph . It was found that the values of N1 predicted from the HPB equation correlate well with those obtained by the C&P apparatus at low shear rates. The predicted values of N1 also agree well with 2G' even though the shear rate was increased to 70 sec⁻¹ for most of the polymers investigated. Changing the width of the slot did not have a significant effect on the magnitude of Ph , whereas the magnitude of Ph depended largely on the polymers investigated. The measured Ph was nearly zero for polycarbonate at σ_w = 40KPa and was about 70 KPa for polystyrene at σ_w = 8O KPa. This was attributed to the significantly lower fluid elasticity of polycarbonate relative to polystyrene. / M.S.