Ti-6Al-4V alloy has been used as biomedical implants for decades because of its superior mechanical properties, good biocompatibility, lack of allergic problems and good corrosion resistance. It is widely used as the tibial components in total knee arthroplastry and hip cup in total hip replacement. However, the mechanical properties of Ti-6Al-4V implant can be deteriorated due to corrosion pits. In the past decades, the rapid developments in additive manufacturing have broadened their applications in biomedical area due to the high geometrical freedom in fabricating patient-friendly implants. However, the high-localized thermal input and fast cooling rate during laser processing usually result in non-equilibrium phase with high residual stress. Therefore, it is necessary to apply proper post-treatments on the as-printed parts to ensure better properties. In this work, various post-treatments (e.g. post-heat treatments, hot isostatic pressing) were applied aim to improve the corrosion behavior of direct metal laser sintered Ti-6Al-4V parts. The effect of post-treatment temperature on the mechanical properties and corrosion behavior were examined experimentally. A discussion on factors influencing corrosion rate was presented, and the corrosion mechanism on the Ti-6Al-4V part in simulated body fluid was proposed. Based on the electrochemical measurement results, enhanced corrosion resistance was observed in the samples after high temperature HIPing at the annealing temperature (α+β region) of 799°C.

The accuracy issue of a rapid prototyping-direct laser sintering system is studied in this paper. The sources of errors are analyzed for their contribution to the final accuracy of built parts. The error sources are related to the hardware and software of the machine, the materials and the process. Special measures were exploited to improve the accuracy of the direct laser sintering system and process. For the errors caused by hardware like laser scanner, compensation by software was developed to correct the errors resulting from galvano-mirrors and F-θ lens. A compensation function mode was added to the slicing software to compensate the errors caused by material shrinkage and laser beam offset. Based on the analysis and improvement, a desired accuracy of 0.2mm has been achieved for the direct laser sintering system, which was verified by experiments. / Singapore-MIT Alliance (SMA)

Laser Sintering, a method of additive manufacturing, is used in the production of concept models, functional prototypes, and end-use production parts. As the technology has transitioned from a product development tool to an accepted production technique, functional qualities have become increasingly important. Tension properties reported for popular polyamide sintering materials are comparable to the molded properties with the exception of elongation. Reported strains for laser sintered polyamide are in the 15-30% range with 200-400% strains reported for molding. (CES Edupack n.d.) The primary contributors to poor mechanical properties in polyamide materials used during Selective Laser Sintering® are studied. Methods to quantify decreased mechanical properties are compared against each other and against mechanical properties of components fabricated using multiple process parameters. Of primary interest are Ultimate Tensile Strength (UTS) and Elongation at Break (EOB) of tensile specimens fabricated under conditions that produce varying degrees of ductile and brittle fracture. / text

In an effort to gain a more comprehensive and complete understanding of the thermal behaviors occurring during the selective laser sintering process, external temperature measurements were taken during the build process. To accomplish this, an infrared camera was aimed directly through a viewport on the front of the sinterstation. The temperature was monitored during the heating process which showed slightly non-uniform heating of the part bed surface. Temperatures were also recorded while the laser was sintering each layer and the subsequent cooling of the entire machine following the build. By directly capturing infrared images of the part bed’s surface, it is clearer how the temperature gradients behave and the impact such variables have on part build efficiency. / text

Direct Methanol Fuel Cells (DMFCs) are electrochemical energy conversion devices that convert chemical energy into electrical energy. The bipolar plate component of the DMFC is required to be fluid impermeable to prevent fuel leakage and electrically conductive to collect the electrons produced within the cell. Graphite possesses the properties of high electrical conductivity, low weight and resistance to corrosion that make it an attractive material for bipolar plates. However, the poor mechanical properties of graphite lead to prohibitive machining costs and increased production times. The objective of this research is to develop an indirect laser sintering (LS) process, involving the laser sintering of graphite powders mixed with a phenolic resin binder which offers the advantage of complex part production and testing of prototype bipolar plates in short times. Due to the nature of the indirect LS process, the as-produced (green part) plates are porous and possess low electrical conductivities (< 0.1 S.cm-1). This research describes a viable method to rapidly fabricate and test multiple graphite bipolar plate designs using indirect LS. This process involved identifying and selecting suitable graphite powder and binder systems based on their thermal and electrical properties and developing a post process heat treatment method for achieving electrical conductivity of 250 S/cm for LS graphite parts which exceeds the DOE target of 100 S/cm for bipolar plate materials. The post processing also covered a method of infiltration using cyanoacrylate which was capable of rendering porous brown parts fluid impermeable and suitable for use in DMFCs. The cyanoacrylate infiltrated LS graphite parts were characterized for flexural strength and electrical and thermal conductivities and bipolar plates were made and evaluated in a DMFC test stand. Various flow field designs including plates with varying channel and rib widths and triangular, elliptical and rectangular flow field cross sections were fabricated using indirect LS and their respective polarization curves were compared to commercially machined graphite plates. The fuel cell tests show the improvement in mass transport performance could be due to improved methanol distribution and water removal characteristics of triangular and elliptical cross sectional channels over rectangular channels of equivalent dimensions. / text

Os objetivos deste trabalho foram a implantação e a otimização da técnica de sinterização por varredura a laser de filmes espessos, o estudo da cinética do processo e a avaliação das propriedades do composto BaTi0.85Zr0.15O3 (BTZ) sinterizado a laser, em comparação com os filmes sinterizados em forno. Pós nanocristalinos de BTZ foram sintetizados em baixas temperaturas com sucesso pela primeira vez (600ºC), por meio do método dos precursores poliméricos modificado. Foram obtidos pós nanométricos com tamanho de partículas primárias de ~20 nm e com aglomeração controlada, uma inovação para pós de BTZ preparados por rotas químicas. Para a deposição dos filmes, a estabilidade das suspensões de partida foi estudada e filmes espessos com excelente homogeneidade foram depositados utilizando a técnica de eletroforese (EDP). O desenvolvimento da técnica permitiu o controle da espessura do filme a partir dos parâmetros de deposição. A montagem experimental para a sinterização a laser foi otimizada de modo a permitir a sinterização de filmes com dimensões de até 70 mm de comprimento por 10 mm de largura e espessuras variáveis. Os tempos de patamares em cada etapa foram dependentes da velocidade e do número de varreduras. A temperatura máxima que se pode atingir no filme espesso, durante cada varredura e para uma potência nominal do laser fixa, foi correlacionada com a densidade relativa. Os processos térmicos envolvidos durante a varredura a laser atuaram de modo similar á sinterização em duas etapas (two step sintering). Com o intuito de melhorar a densificação dos filmes, passamos a adicionar o composto Bi4Ti3O12 (BIT) (1 e 2 mol %) ao BTZ durante a deposição. A utilização do sistema desenvolvido para a sinterização por varredura a laser em conjunto com o acréscimo do aditivo BIT resultou em uma diminuição no tamanho de grão dos filmes e uma importante diminuição da porosidade aparente. Para a aditivação com 2 mol% de BIT obtivemos filmes de ótima densidade (porosidade aparente de ~4%) e reduzido tamanho de grão (~200 nm), resultado inédito em se tratando de filmes espessos. A sinterização a laser resultou em filmes com maior permissividade dielétrica em relação ao filme sinterizado em forno elétrico. As reações que ocorrem entre o BTZ e o BIT foram exploradas usando conjuntamente as técnicas de espectroscopia de impedância, análise térmica e difratometria de raios-X. / The goals of this work were the implantation and optimization of the technique of sintering by laser scan of thick films, the kinetic study of the process and the evaluation of the physical properties of the laser sintered compound BaTi0.85Zr0.15O3 (BTZ), compared to thick films sintered in conventional furnace. Nanocrystalline powders of BTZ were for the first time, successfully synthesized at low temperatures (600ºC) using the modified polymeric method. Nanometric powders with primaries particles of ~20nm sizes and controlled agglomeration were obtained which was an innovation for BTZ powders prepared by chemical methods. For the films deposition, the suspensions stability was studied and thick films with excellent homogeneity were deposited using the electrophoresis technique (EDP). The developing of the technique allowed the thicknesses control using the deposition parameters. The characteristics of the experimental apparatus were optimized in such a way as to allow the sintering of thick films whose dimensions were up to 70mm in length, 10mm in width and variable thicknesses. The step times in each stage were dependent on the velocity and scan number. The maximum temperature that can be achieved in the thick film, during each scan, and for a fixed rated laser power was correlated with the relative density. The related thermal process during the continuous laser scan acted in a similar way as a two-step sintering. To improve the densification of the films, we started to add the compound Bi4Ti3O12 (BIT) (1 e 2 mol %) to BTZ during the deposition. The utilization of the system developed for the sintering by laser scan alongwith the adding of the BIT resulted in a grain size decrease and a significant decrease in apparent porosity. For the 2mol% additivation we obtained films with excellent density (apparent porosity of ~4%) and reduced grain size (~200nm), which is an unpublished result for thick films. The laser sintering resulted in films with a higher dielectric permittivity in relation to the conventionally sintered film. The reactions between BTZ and BIT were explored using the techniques of impedance spectroscopy, thermal analysis and X-ray diffraction.

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 / 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.

Laser sintering (LS) is an additive manufacturing process which uses laser surface heating to induce consolidation of powdered materials. This work investigates some of the process-structure-property relationships for LS of viscoelastic polymers. A one-dimensional closed-form analytical solution for heating of a semi-infinite body, with a convective boundary condition, by a moving surface heat flux was developed. This solution approximates the shape of the Gaussian energy distribution of the laser beam more accurately than previous solutions in the literature. A sintering model that combines the effects of viscoelastic deformation driven by attractive surface forces and viscous flow driven by curvature-based forces was developed. The powder-bed temperature was approximated using the thermal model developed herein. The effect of the enthalpy of melting for semi-crystalline polymers was accounted for using a temperature recovery approach. Time-temperature superposition was used to account for the temperature dependence of the tensile creep compliance. The results of the combined-mechanism sintering model will be compared to the classic Mackenzie-Shuttleworth sintering model. A lab-scale LS unit was constructed to fabricate test specimens for model validation and to test the applicability of materials to LS. In this work, sintering four materials, polycarbonate (PC) and three molecular weights of polyethylene-oxide (PEO) was predicted using the aforementioned thermal and sintering models. Samples were fabricated using the lab-scale LS unit and the sintered microstructures were investigated using scanning electron microscopy. The rheologic, thermal and physical properties of the materials were characterized using standard methods and the relevant properties were used in the models. The choice of an amorphous polymer, PC, and a semi-crystalline polymer, PEO, affords comparison of the effects of the two material forms on contact growth during LS. The three molecular weights of PEO exhibit significantly different tensile creep compliances, however, the thermal and physical properties are essentially the same, and therefore the effect of molecular weight and subsequently the rheologic characteristics on contact growth during LS will be investigated. The effects of particle size, laser power, and bed temperature were also investigated. / Ph. D.