Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
Browse
7 results
Search Results
Master Thesis Investigating the effects of selective laser melting (SLM) process parameters on the microstructure, mechanical properties, and biocompatibility of 316l stainless steel implants(Biomedical materials, 2024) Türkpençesi, Büşra İrem; Yılmaz, Benay UzerEklemeli imalat, kemiğin anizotropik özelliklerini taklit eden metalik biyomalzemelerin üretilmesine imkan tanır. Seçici lazer ergitme (SLE) yöntemiyle üretilen 316L paslanmaz çeliğin üretim parametrelerinin malzeme özellikleri ve biyouyumluluk üzerindeki etkileri incelenmiştir. Çalışma kapsamında, üç farklı parametre seti (1B, 2B ve 3B) kullanılarak üretilen SLM numuneleri, geleneksel yöntemle üretilen 316L paslanmaz çelik ile karşılaştırılmıştır. Malzemelerin mikroyapısal özelliklerini ortaya çıkarmak için optik mikroskop, taramalı elektron mikroskopu (SEM), elektron geri saçılım kırınımı (EBSD) yöntemleri kullanılmıştır. Vickers mikrosertlik ölçümleri, yüzey ıslanabilirliği ve sitotoksisite özellikleri detaylı olarak değerlendirilmiştir. Mikroyapı sonuçlarına göre, geleneksel ile SLM numuneleri arasında çok farklı yapılar olduğu sonucuna varılmıştır. SLM numunelerinin kendi içindeki mikroyapılarının değişmesinin sebebi de üretim parametrelerinin değişkenliğidir. EBSD analizlerine göre, geleneksel çelikte yüksek oranda twin boundariler görülmüştür, bunun sebebi üretim esnasında yeniden kristallenme oluşmasıdır. SLM numunelerinde yüksek soğuma oranlarından kaynaklı alt-tane denilen yapılar yüksek oranda bulunmuştur. 1B numunesinde, %62.7 oranında <110>//BD tekstürü gözlemlenmiş ve bu numunenin anizotropik özellikler sergilediği belirlenmiştir. Mekanik testler, SLM numunelerinin sertlik değerlerinin geleneksel çeliğe kıyasla önemli ölçüde yüksek olduğunu ortaya koymuştur. 1B numunesi, 240.6 ± 19.3 HV ile en yüksek sertlik değerine ulaşmıştır. Ayrıca, yüzey ıslanabilirliği sonuçları, SLM ile üretilen numunelerin daha hidrofilik yüzeylere sahip olduğunu ve bunun biyouyumluluğu artırdığını göstermiştir. Hücre kültürü testlerinde, geleneksele göre SLM numuneleri canlılık açısından daha iyi sonuç vermiştir. Bu da hem anizotropik yapı elde edilip hem de biocompatible bir malzemenin SLM ile üretilebileceğini göstermiştir.Master Thesis Development of material flow stress and damage models for 304 stainless steel(01. Izmir Institute of Technology, 2024) Akdoğan, İbrahim Berk; Güden, MustafaPaslanmaz çelik 304 alaşımı üzerine yapılan önceki deneysel ve sayısal çalışmalar çoğunlukla martensitin hacim kesrine bağlı akış gerilimi davranışının belirlenmesine odaklanmıştır. Alaşımın hasar davranışı darbe ile ilgili uygulamalarda eşit derecede önemlidir. SS 304 alaşımının dinamik yükleme davranışını simüle etmek amacıyla hasar modellerinin belirlenmesi konusunda henüz sistematik bir çalışma yapılmamıştır. Bu tezde, Johnson ve Cook (JC) akış gerilimi ve JC hasar denklemlerinin parametreleri bir paslanmaz çelik 304 alaşımı için deneysel olarak belirlenmiştir. Belirlenen parametreler daha sonra bu parametreleri çıkarmak için kullanılan deneysel testlerin modellenmesiyle doğrulanmış ve kalibre edilmiştir. Sayısal modeller LS-Dyna'da uygulanmıştır. B4C kaplı ve kaplanmamış SS 304 plakalar üzerinde 800 m s-1'de gerçekleştirilen deneysel balistik testler, belirlenen model parametreleri kullanılarak Ls-Dyna'da simüle edilmiştir. Son olarak, mikroskobik çalışmalar test edilen alaşımdaki martensitik dönüşümün ve yüksek gerinim oranlarındaki adiabatik ısının martensit oluşumunun kapsamını azalttığını açıkça göstermiştir. Son olarak, martensit fraksiyonu literatürdeki denklemler kullanılarak farklı gerinim oranlarında analitik olarak tahmin edilmiştir.Master Thesis The Deformation Rate Sensitivities of Additively and Conventionally Fabricated 316l Alloys(01. Izmir Institute of Technology, 2021) Enser, Samed; Güden, MustafaThe compression stress-strain behavior of a Scanning Laser Melt 316L (SLM-316L) and an annealed and extruded commercial 316L (C-316L) were determined between 1x10-3 s-1 and 2500-3150 s-1. SLM-316L deformed by twinning and slip, while C-316L by martensitic transformation and slip with no fracture until about 0.51 strain. The higher yield strength of SLM-316L than C-316L was attributed to the higher dislocation density of SLM-316L. The higher work hardening rate of C-316L alloy was proved due to the higher resistance of martensite plate than twin boundary to the dislocation motion. As the strain rate increased, both alloys showed increased flow stresses. However, the rate sensitivities declined as the strain increased due to the adiabatic heating at high strain rates. The Johnson and Cook flow stress material models of both alloys were further determined for the adiabatic and isothermal conditions. The martensite formation in C-316L specimens and twinning formation in SLM-316L alloys decreased at high strain rates compared to quasi-static strain rates. The XRD spectra of C-316L also confirmed the reduced martensite formation at high strain rates. The reduced twin and martensite formation at high strain rates were attributed to the increased stacking fault energy due to the adiabatic heating of the test specimens. The increase of stacking fault energy at high strain rates promoted a higher fraction of the deformation by slip. Lastly, the reloading tests revealed a strain-rate history effect in SLM-316L and no strain-rate history effect in C-316L.Master Thesis Investigation of the Fatigue Behaviour of Metallic Components Used in Plate Heat Exchangers Under Variable Dynamic Loads(Izmir Institute of Technology, 2020) Hayta, Yiğit; Kandemir, Sinan; Kandemir, SinanPlate heat exchanger (PHE) is a component that provides heat to be transferred from hot water to domestic cold water without mixing of them with a high efficiency. Over the lifetime of the PHE, cyclic pressures act on the brazing points and the plates, and this may lead to fatigue failure. The fatigue behaviours of the PHEs which are designed by using copper brazed 316L and 304L stainless steels, were investigated in this thesis by performing strain based fatigue tests to also seek the feasibility of the use of 304L stainless steel in PHE production to reduce the cost. Besides, the microstructural investigation of the brazed regions was conducted and, the tensile tests for both non-brazed and brazed steel specimens were performed in order to determine the mechanical properties of the samples. The fatigue tests were carried out with twelve specimens for each sample groups at four different load levels as displacement (strain) controlled with a stress ratio of R=0 and 5 Hz frequency. Finite Element Analysis (FEA) was performed to determine the strain distribution on the plates of PHEs during their operation to estimate the lifetime of PHEs by using the generated lifetime curves based on the fatigue tests. Consequently, it was obtained that the ultimate tensile strength and fracture strain of non-brazed steel specimens are higher than those of the brazed specimens. The Scanning Electron Microscopy (SEM) analysis shows that; copper can diffuse into 316L easier than 304L and the use of copper foil with 50 µm thickness results in more defect at brazing regions compared to 100 µm thickness. Hereunder the fatigue test results, Weibull Analysis was performed and the fatigue life curves were generated. It was found that 316L brazed joint has approximately 33 times greater fatigue life than 304L brazed joint and filler metal thickness is more likely to have a linear relationship with fatigue life. Finally, fatigue lives of each sample group were calculated based on the loads determined by FEA. The results suggest that either 316L or 304L stainless steels can be used as PHE material as both materials satisfy the lifetime requirement of 15 years which was preliminarily defined by Bosch Thermotechnology (TT).Master Thesis Structural and Nanohardness Behavior of Low Energy, High Flux Nitrogen Implanted Austenitic Stainless Steel(Izmir Institute of Technology, 2018) Dal, Refika; Öztürk, Orhan316 austenitic stainless steels (SSs) are one of the most commercial and technological alloys and extensively used in the field of defence, nuclear and biomedical applications due to its excellent corrosion resistance in abrasive and erosive environment. However, this type of steel is rather soft, and these results in poor durability, in particular when this material (316 SS) is in contact with other surfaces. In addition, 316 SS is nonmagnetic at room temperature. In order to make the surface of 316 SS harder, nitrogen ion beam implantation and wear resistant method is applied. Earlier studies of high dose nitrogen ion implantation into the surface of austenitic SSs around 400 °C substrate temperature showed that an expanded austenite phase (The Nitrogen phase in the FCC lattice of 316 SS) gives excellent wear resistance with high hardness value. In this study, type 316 stainless steel (SS) was implanted with low energy (700 eV), high flux (2.9 mA/cm2) nitrogen ions at 400 °C substrate temperature in order to harden its surface. Microhardness and nanohardness measurements were carried out on the nitrogen implanted surface and on the nitrogen implanted cross-section under the applied loads ranging from 6 mN to 30 mN. Both microhardness and nanohardness data suggest that the hardness of the N implanted 316 SS significantly increases compared to the hardness of the substrate material (by a factor of 3 to 4).The hardness increase is believed to be due to the high amount of nitrogen, the thick nitrogen implanted layer and macroscopic residual compressive stresses, the formation of which is verified by θ/2θ XRD scans as lattice expansions about 10 at. %. SIMS profiles suggest concentration-dependent diffusion behavior for the N implanted layers. Based on SIMS and SEM/EDX data, nitrogen implanted layers are 4-5 micron thick and constituting about 28 %.Master Thesis The Investigation of the Static and Dynamic Crushing Behavior of an Energy Absorbing Biomimetic Armor(Izmir Institute of Technology, 2017) Akbulut, Emine Fulya; Taşdemirci, Alper; Güden, MustafaIn this study, an innovative thin-walled energy absorbing structure was manufactured following by biomimicry rules and produced from AISI 304L stainless steel sheet material by deep drawing method. Manufacturing process was modelled in two stages to produce the numerical specimen containing residual stress/strain and thickness distribution using commercial software LS-DYNA. The balanus being a sea creature, consisting of an inner core structure and an outer shell structure, is the inspiration of this study. The balanus was compared to the other conventional geometries in terms of the energy absorption capacity and determined as highly advantageous configuration. Quasi-static crushing and drop weight experiments were conducted and modelled numerically. The observations indicated that the carried load by the balanus is greater than the arithmetic total of the carried load by the inner core and the outer shell separately due to the interaction effect. Besides, energy absorbing performance of the balanus improved under dynamic loading since the outer shell confines the inner core during the deformation and developed the energy absorption performance of it while the energy absorbing capacity of the other two decreased. After the end of the experimental studies, the energy absorption partitions between the components of the balanus were studied numerically and it was observed that the energy absorbing capacity of the balanus increases with increasing deformation velocity due to the strain rate sensitivity effect of the material and the differences of energy partition ratio between the two components decreases.Master Thesis Magnetic Characterization of Expanded Austenite Phase Formed on Nitrogen Ion Implanted 316 Stainless Steel Alloy(Izmir Institute of Technology, 2015) Karataş, Özgün; Öztürk, Orhan; Selamet, YusufAustenitic stainless steels (SSs) are technologically important alloys and highly resistant to corrosion in a variety of environments. Nevertheless, these materials have a few drawbacks; they are rather soft materials and susceptible to wear. Correspondingly, an improvement of the surface properties is often desirable. Ion beam techniques are widely used to enhance surface properties of these alloys. Surface modification of austenitic SSs by nitrogen ion beams at moderate substrate temperatures near 400 ºC, leads to the formation of a high N content phase. This phase, known as an expanded austenite phase, γN, creates a hard and wear resistant layer on the stainless steel. Additional property of this phase is related to its magnetic structure due to the large amount of nitrogen insertion and corresponding lattice expansion. In the current study, new data corresponding to structural and magnetic nature of the expanded austenite layers on austenitic 316 SS by low-energy, high-flux nitrogen ion implantation are presented. Phase and compositional analyses, surface topography and magnetic features of the nitrogen ion implanted layers were studied by a combination of experimental techniques involving XRD, SEM, AFM, MFM, VSM and MOKE. Nitrogen implantations were performed for 30, 90 and 240 minutes of processing time, at a fixed temperature near 400 °C. Relatively low-energy (0.7 keV) and high-flux (2 mA/cm2) ion beam conditions were carried out during the implantation. Combination of the aforementioned techniques provides strong evidence for the formation of the γN phase with mainly ferromagnetic characteristics. MFM imaging reveals stripe-like domain structures of the nitrogen ion implanted layers. Both VSM and MOKE analyses display hysteresis loops of the layers. Ferromagnetism in the γN layers are manifested by MFM, M and MOKE analyses. Ferromagnetic structure is linked to large lattice expansions 0 due to high nitrogen contents at. . s an interstitial impurity, nitrogen dilates fcc lattice of 316 SS i.e. Fe-Fe distance is increased, which strongly influences the magnetic interactions.