Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
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Master Thesis Preparation and Characterization of Sintered Ti-6a1 Powder Compacts(Izmir Institute of Technology, 2004) Çelik, Emrah; Güden, MustafaSintered Ti6Al4V powder compacts were prepared using atomized spherical and angular powders in the porosity range of 29-63%. Cylindrical green powder compacts cold compacted at various compaction pressures and then sintered at 1200 C for 2 h. The final porosities and average pore sizes were determined as functions of the applied compaction pressure and powder type. The compression deformation behavior of Ti6Al4V powder compacts was also investigated at quasi-static (1.6x10-3-1.6x10-1s-1) and high strain rate (300 and 900 s-1) conditions using conventional mechanical testing and Split Hopkinson Pressure Bar techniques, respectively. The mean pore size of the compacts varied between 29 and 171 Um depending on the particle size range of the powders used and the compaction pressure applied. Microscopic studies of as-received powders and sintered powder compacts showed that sintering at high temperature (1200oC) and subsequent relatively slow-rate cooling in the furnace transformed the microstructure of spherical powder from the acicular alpha to the Widmanstatten microstructure and angular powder from bimodal to equiaxed+ Widmanstatten microstructure.In compression testing, at both quasi-static and high strain rate conditions, the compacts failed primarily by shear band formation along the diagonal axis 45 C to the loading direction. Increasing strain rate was found to increase both the flow stress and the compressive strength of spherical powder compacts while it did not affect the critical strain for shear localization. The mechanical properties of angular powder compacts were further shown to be a function of powder size; larger the particle size higher the percentage of equiaxed structure while in compacts of particles <100 um relatively large voids resulted in reduced strength and ductility. Microscopic analyses of deformed but not failed and failed spherical powder compact samples further showed that fracture occurred in a ductile (dimpled) mode consisting of void initiation and growth in alpha phase and/or at the alpha/beta interface and macrocraking by void coalescence in the interparticle bond region. In angular powder compacts, the failure was granular brittle type at the interparticle bond region while the compact samples of particles <100 um fractured transgranularly through the voids. The strength of the sintered compacts was further shown to satisfy the strength requirements for cancellous bone replacement. The strength of the compacts having porosity level of 40% and/or lower was comparable with that of human cortical bone.Compared to Ti powder compacts of previous studies, Ti6Al4V powder compacts provided higher strength and increased porosity level of the compacts suitable for cortical bone replacement.Master Thesis Designing and Manufacturing of Porous Spinal Cages Using Ti6a14v Foamed Metal(Izmir Institute of Technology, 2009) Dizlek, Mustafa Eren; Güden, MustafaOpen cell Ti6Al4V foams with varying porosities (50, 60 and 70%) were prepared at sintering temperatures between 1200 and 1350 °C using ammonium bicarbonate particles (315 - 500 .m) as space holder. Two different biomedical grade commercial, gas atomized spherical Ti6Al4V powders were used to prepare foams. Powder 1 was in size range of between 45 - 150 .m and Powder 2 in size range of between 30 - 90 .m. The foams were sintered under argon atmosphere in a tightly enclosed tube furnace. The resulting cellular structure of the foams showed bimodal pore size distribution, comprising macro pores (300 - 500 .m) and micro pores (1 - 30 .m). Compression tests of foam samples have shown that increasing sintering temperature or decreasing porosity increased the elastic modulus, yield and compressive strength and failure strain. The improvement in the mechanical properties of foams prepared using smaller size Ti6Al4V powder with bimodal particle distribution were attributed to the increased number of sintering necks and contact areas between the particles. The foam prepared with optimum porosity, pore size and mechanical properties for bone in-growth was further used to produce prototype porous spinal cages which are widely used in spinal surgery for vertebrae fixation. The geometries and size of the prototype spinal cages were determined through the measurements taken from human vertebrae. The foams for spinal cage preparation were first prepared in the form of plates and then core-drilled using water jet based on the design geometrical parameters determined for each vertebra segment.Master Thesis Determination of Material Constitutive Equation of a Biomedical Grade Ti6ai4v Alloy for Cross-Wedge Rolling(Izmir Institute of Technology, 2009) Kıranlı, Engin; Güden, MustafaIn the present work, the JC flow stress and damage parameters of a biomedical grade Ti6Al4V alloy that contained very low levels of interstitial elements were determined for the modeling its deformation in the CWR process. The JC models were determined through quasi-static (10-3-0.1 s-1) and high strain rates (300-1000 s-1) within the temperature range of 25-1150 oC. High strain rate tests were performed using both compression and tension SHPB testing devices. The damage model was determined using notched specimens of different stress triaxiality. The tested alloy flow stresses were found to increase with increasing strain rate for both compression and tension tests. This was proved that the alloy has a strain rate sensitive flow stress behavior. At increasing strain rates the failure strains in tension decreased. The reduced fracture strain was also confirmed by the microscopic observations. In statically tested samples the ductile fracture mode was composed of smaller but deeper dimples, while the dimples were observed to be shallow and larger in dynamically tested samples. The tensile fracture presumably started in a region and the b phase microscopically shown to deform plastically through the tensile axis. The compression failure mode of the alloy was found to be resulting from the shear band formation followed by the fracture of the shear band. High temperature test conducted at quasi-static strain rate showed that the stress values decreased greatly after about 800 oC due to a ->b transformation. Due to this two different JC material models valid between 25-600 oC and 800-1150 oC were developed. The determined JC parameters were found to be well agreed with the literature except the model obtained from the compression tests.