No description available.

Selective laser melting of nitrogen gas atomized 17-4PH stainless results in up to 50% lower yield strength and 600% higher elongation compared to traditionally processed, wrought 17-4PH. This drastic difference in mechanical properties is commonly attributed to the presence of high volume fractions of retained austenite within the as-built microstructure. The factors leading to the increased level of retained austenite have not been clarified in the literature. Furthermore, the amount of retained austenite reported within published literature vary widely, even with the use of identical process parameters. Manufacturers of selective laser melting systems state that solution annealing and precipitation hardening will achieve traditional mechanical properties, thereby removing all retained austenite. Once again, it is not clear, how the recommended solution and precipitation treatments lead to the desired changes in microstructure. The research within this thesis establishes that there is up to 0.12wt% higher nitrogen content within additively manufactured 17-4PH, compared to traditionally manufactured 17-4PH, as a result of the powder atomization process. The increased nitrogen is able to stabilize the austenitic phase by reducing the Ms temperature below ambient temperatures. Fertiscope bulk phase analysis demonstrates that the processing atmosphere during selective laser melting cannot alter the fraction of retained austenite in the as-built material. The depression of the Ms temperature in the printed parts is confirmed by dilatometry. Due to the TRIP phenomenon, during sample preparation, it was found that the austenite would transform to 80% martensite at the surface. This transformation will greatly impact the phases detected when x-ray diffraction is used for analysis, leading to a wide variety of reported retained austenite values within literature. A mechanism based on the precipitation of nitrides during solution-treatment has been proposed to explain how heat-treatment of the printed parts can lead to a martensitic microstructure with comparable mechanical properties to those of wrought alloys. / Thesis / Master of Applied Science (MASc) / 17-4PH stainless steel is a martensitic alloy, that can be precipitation hardened when used in traditional manufacturing processes. Within a selective laser melting process, it will exhibit up to 50% lower yield strength and 600% higher elongation. This behaviour is caused by retained austenite, which is stabilized by the introduction of nitrogen during the powder atomization process. As a result, the alloy exhibits transformation induced plasticity. Existing literature states the alloy’s microstructure can be controlled by altering the selective laser melting process atmosphere or using heat treatment to achieve traditional mechanical properties. However, the production and preparation of samples generates a surface transformation which was misinterpreted as a complete bulk transformation. Therefore, the change in microstructure from altering the process atmosphere is only detectable through surface analytical techniques. It is proposed that the rapid cooling rates of SLM form a non-equilibrium state, keeping nitrogen in solution. Subsequent heat treatment allows the formation of nitrides resulting in the Ms being brought above room temperature.

No / Alpha phase sintering, sinter hardening, and mechanical properties of prealloyed Fe-1·5Mo base powder with and without additions of elemental Si, ferrosilicon, and carbon under various process conditions have been investigated. Liquid paraffin, as a new lubricating agent, was found to be useful in reducing segregation, interparticle and die wall frictions, as well as reducing ejection forces and die and tool wear. It was found that addition of Si to the base powder enhanced the sintering process by stabilisation of the alpha-phase and formation of two kinds of liquid phase at ~1045 and ~1180°C, corresponding to the solidus and liquidus temperatures, respectively. This addition increased the tensile strength of the as sintered Fe-1·5Mo from 174 to 445MPa owing to massive solid solution strengthening effect of Si. An optimum sinter hardenable alloy, of composition Fe-1·5Mo + 3Si + 1·2C, provided a high sintered density of 7·55g cm-3, tensile and bend strengths of7 64 and 1405MPa, respectively, with 2·5% elongation, after sintering at 1250°C for 1h under hydrogen or vacuum using moderate cooling rates of le20K min-1. Faster cooling rates caused brittleness and very low UTS for the high carbon steel. Full heat treatment improved the UTS by ~200MPa which was useful only for the high carbon steel with high cooling rates ge30K min-1. Depending on the cooling rate, the as sintered microstructures consisted of mainly fine or coarse pearlite, bainite, martensite, and some retained austenite with hardness in the range 250-720HV10. Some proeutectoid grain boundary cementites were also present in the as sintered high carbon steel. This work, therefore, has shown that high densities with acceptable microstructures and good mechanical properties are achievable with single stage compaction and single sintering operations by using the optimum process conditions and alloying composition without the need for a post-sintering heat treatment.

No / This paper reports the results of an investigation into the selective laser sintering of a prealloyed high-speed steel powder. The structured development of processing conditions for single lines, single layers and multiple layers of material is reported, as are the flexural modulus and strength of the single- and multiple layer components. Infiltration with bronze was used to improve the mechanical properties of the components and it is concluded that selective laser sintering of high-speed steel allied to bronze infiltration can produce material with the mechanical properties to allow for use in load-bearing applications, but that further work is required to improve the density, mechanical properties and build rate if selective laser sintering is to develop as a general manufacturing process for hard metals.

The densification and shear viscosity of borosilicate glass (BSG) + silica films on rigid substrates were studied. Optical measurement techniques were devised to determine the shrinkage profiles of the free and constrained films and in-plane stress in the constrained films. These films were prepared from slurries of the powder by tape casting. Sintering was carried out isothermally in a hot stage between 665°C and 775°C. The densification rates of both films were observed to be the same in the early stage of sintering but slowed down in the constrained film during the later stage resulting in a lower density. The activation energies for both free and constrained sintering were found to be 385 ± 10 kJ/mol. In-plane stress measurements in constrained films of the pure glass showed the stress to rapidly rise to a maximum level of 20 kPa during the initial stage of sintering and gradually decreased back to zero during the final stage. Densification rates can be given as a product of mobility and driving pressure. Activation energy determinations did not indicate a change in the densification mechanism such that a change in mobility can be ruled out as the reason for the reduced densification rate in constrained films. The stresses are substantially smaller than the estimated lower limit of the sintering pressure and had no observed effect on the densification of the constrained film during the early stages of sintering. However, the stresses could have prevented a few large pores from shrinking during the early stage of sintering leading to the lower density and larger pores observed in the constrained film after sintering. Shear viscosity determinations were also done using data obtained from the sintering of constrained films. The results showed that the density dependence of the shear viscosity is consistent with other work in sinter-forging experiments. However, the results also indicated that the shear viscosity is strongly dependent on sintering temperature. This can be attributed to the different microstructures that evolved when the films were sintered at different temperatures. / Ph. D.

At the present work the UO2 fuel production process was extensively studied and analyzed. The objectives of such investigation were to understand and analyze the influence of different additives and the variation of the production process steps on the microstructure and consequently in the mechanical strength of the nuclear fuel pellet. Moreover, an improvement of the qualitative characteristics of the ceramic fuel pellets was also aimed. For this purpose UO2 pellets without additives, the so-called standard pellets, pellets containing as additive for example AZB (Azodicarbonamid), black U3O8 (Oxidized uranium pellet scrap - OS), green U3O8 (Oxidized uranium powder - OP), keratin fibers (a non conventional additive) were produced. The introduction of these additives to the UO2 powder mixture prior or after the granulation production step and in different concentrations produced several microstructure configurations. As it would not be possible to analyze all of them here so during the investigation pre-tests some of them were separated to be studied in more detail. Pellets with AZB added after the granulation presented larger grains and larger pores than those with AZB added before granulation, also porosity free grains and a granulate structure instead of a homogeneous one. Pellets with OS present fine porosity distributed all over the pellet matrix with some porosity clusters whereas pellets containing OP show in its matrix porosity agglomerated in form of hooks. As for the grain size, a more uniform grain size distribution can be observed in pellets OS than in pellets with OP. The variations in the amount of keratin fibers added, sintering dwell time and green density resulted indeed in different microstructures. Nevertheless, some common characteristics among them were observed such as the presence of elongated pores, porosity clusters and larger grains located at the pellets borders while the smaller ones were concentrated more in the central part of the pellet. This distribution of grains was identified as bi-modal structure. The mentioned microstructure aspects certainly influence on the mechanical properties of the fuel pellet. However, the sintering parameters, the green and final pellet density and the pellet dimensions also have an influence on the mechanical characteristics of the pellets. For studying the influence of all these parameters on the pellet mechanical properties four testing procedures were utilized the so-called squirrel-cage where the mechanical resistance of the not sintered pellets against mechanical shocks was tested, the diametrical compression test (Brazilian Test) where the strength of sintered and not sintered pellets was studied, the Vickers indentation technique and the creep test where the pellet plasticity respectively at room and at elevated temperatures was analyzed. The squirrel-cage results showed that the pellets with keratin fibers were much more mechanically resistant than those pellets without it, which means that the keratin fibers acted, prior sintering, as a powder binder increasing the cohesion among the powder granules proportionating the green pellets higher mechanical resistance against impacts. The Brazilian test evaluated the influence of the pellet length to the pellet diameter (L/D ratio), the influence of different additives mixed to the UO2 powder and the different pellet production processes. The L/D influence analysis showed that if one fixes the pellet diameter and increase the pellet length the Weibull modulus (here a measure of the pellet lot reliability) will also increase. By comparing pellets with OS, OP and 0.3% keratin fibers it was observed that pellets with OS presented the highest volume of pores smaller than 10 mm while pellets with OP and keratin presented the highest volume of pores larger than 20 mm. It seems that this relevant characteristic favored to the highest Weibull strength value for pellets with OS. In the indentation test standard pellets, pellets with OS and pellets with keratin fibers were tested. The results showed that the calculated hardness for the standard pellets is slightly lower when compared to the values obtained by the pellets with keratin fibers. Also the pellets containing OS when compared to the keratin fibers pellets have in most of the cases a lower hardness. The calculated fracture toughness and fracture surface energy values show also a better mechanical behavior for the keratin fibre pellets than in the standard pellets. Standard pellets, pellets with 30%OP, which had the smallest grain size, pellets with keratin fibers, having the bi-modal structure and pellets with chromium oxide, which had the largest grain size, were tested in the creep furnace. The results showed that all pellets with additives presented a better creep behavior than the standard pellets. Among the pellets prepared with additives the comparison clearly showed that under lower stresses pellets with smaller grains have a better creep rate. By increasing the applied stresses we observe an improvement of the creep rate of the pellets with chromium oxide and keratin fibre even slightly overcoming the pellets with 30%OP at the highest applied stress. / Sacramento Santana, HH. (2014). Improving mechanical properties and microstructure development of fiber reinforced ceramic nuclear fuel [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/37199

Reaction sintered silicon nitride (RSSN) was studied as a substitute coating material on the carbon-carbon material (RCC) presently used as a heat shield on the space shuttle, and on advanced carbon-carbon (ACC), a later development. On RCC, RSSN showed potential in a 538°C (1000°F) screening test in which silicon carbide coated material exhibits its highest oxidation rate; RSSN afforded less protection to ACC because of a larger thermal expansion mismatch. Organosilicon densification and metallic silicon sealing methods were studied as means of further increasing the oxidation resistance of the coating, and some improvement was noted when these methods were employed. / Master of Science

Yes / To interpret nitrogen sintering of chromium and manganese steels without the formation of deleterious oxides, but with manganese and carbon modifying the local microclimate, the role of the volatile Mn and carbothermic reactions were considered. Reduction of Cr2O3 by Mn vapour is always favourable. CO is an effective reducing agent, however, whereas at atmospheric pressure it will reduce FeO at ~730°C, temperatures some 500 and 700°C higher, i.e. above those for conventional sintering, are necessary for reducing Cr2O3 and MnO, respectively. Accordingly partial pressures must be considered and the sintering process is modelled at a conglomerate of several surface oxidised alloy particles surrounding a pore with graphite present and a tortuous access to the nitrogen-rich atmosphere containing some water vapour and oxygen. The relevant partial pressures were calculated and reduction reactions become thermodynamically favourable from ~200°C. Kinetics, however, dictates availability of CO and the relevant reactions are the water-gas, C + H2O = CO + H2 from ~500°C and the Boudouard, C + CO2 = 2CO, from ~700°C. Discussion of sintering mechanisms is extended to processing in semi-closed containers, also possessing specific microclimates.

Additive manufacturing introduces a new design paradigm that allows the fabrication of geometrically complex parts that cannot be produced by traditional manufacturing and assembly methods. Using a cellular heat exchanger as a motivational example, this thesis investigates the creation of a hybrid manufacturing approach that combines selective laser sintering with an electroforming process to produce complex, hollow, metal geometries. The developed process uses electroless nickel plating on laser sintered parts that then undergo a flash burnout procedure to remove the polymer, leaving a complex, hollow, metal part. The resulting geometries cannot be produced directly with other additive manufacturing systems. Copper electroplating and electroless nickel plating are investigated as metal coating methods. Several parametric parts are tested while developing a manufacturing process. Copper electroplating is determined to be too dependent on the geometry of the part, with large changes in plate thickness between the exterior and interior of the tested parts. Even in relatively basic cellular structures, electroplating does not plate the interior of the part. Two phases of electroless nickel plating combined with a flash burnout procedure produce the desired geometry. The tested part has a density of 3.16g/cm3 and withstands pressures up to 25MPa. The cellular part produced has a nickel plate thickness of 800µm and consists of 35% nickel and 65% air (empty space). Detailed procedures are included for the electroplating and electroless plating processes developed. / Master of Science

The densification behavior of porous gold films made from commercial circuit paste used in microelectronic packaging applications was studied. Constrained gold circuit films of 60-65μm thick were formed by multiple screen printing of the gold paste on rigid alumina substrates, while freestanding films were obtained by carefully peeling off gold films from the substrates after binder burn-out. Optical techniques were developed to determine the densification kinetics of the constrained and freestanding films at temperatures below 1000°C. The densification kinetics of gold films constrained on rigid substrates were observed to be significantly retarded relative to the free films, at all sintering temperatures between 650°C and 900°C studied. SEM studies revealed the microstructure of the constrained films to be much more porous than its freestanding film counterpart. Considerably higher sintering temperatures were required to obtain densities comparable to those of freestanding films. SEM studies also showed no significant difference in grain size between the sintered freestanding and constrained gold films. Inplane tensile stresses generated during constrained-film sintering, was determined to have a maximum value of 460 KPa at the sintering temperature of 750°C. The negligible difference in grain size between the sintered freestanding and constrained gold films, and the small magnitude of the measured tensile stresses, were both determined to be insufficient to account for the observed retardation in the densification kinetics of the constrained gold films. The activation energies for densification of the porous gold films during isothermal sintering, were found to be 21.54±1.03 Kcal/mole and 45.12​​±1.6 Kcal/mole for freestanding and constrained gold films respectively. These values corresponded very well with the activation energies for grain-boundary diffusion and lattice diffusion respectively, for gold as found in literature. Hence from our results of the activation energies for densification of the constrained and freestanding gold films, coupled with our studies on grain growth and stress, we suggest that the observed retardation in the densification kinetics of the constrained gold films are due to a change in the dominant diffusion mechanism during sintering of the porous gold films constrained on rigid substrates. / Master of Science