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Article Citation - WoS: 9Citation - Scopus: 9Elektrik Direnç Punta Kaynağı ile Birleştirilen %15 Deforme Edilmiş Twıp Çeliğinde Kaynak Akımının Mikroyapı ve Mekanik Özellikler Üzerindeki Etkisi(Gazi Univ, Fac Engineering Architecture, 2020) Aydın, Hakan; Tutar, Mümin; Davut, Kemal; Bayram, AliÇalışmada, %15 deforme edilmiş TWIP saclarının elektrik direnç punta kaynağıyla birleştirmelerindekaynak akımının mikroyapı ve mekanik özellikler üzerindeki etkisi incelenmiştir. Mikroyapıkarakterizasyonunda optik mikroskop, taramalı elektron mikroskobu (SEM), SEM/Enerji dağılımlı X-ışınıSpektroskopisi (SEM-EDS) ve SEM/Elektron Geri Saçılım Kırınımı (SEM-EBSD) teknikleri kullanılmıştır.Mekanik özelliklerin belirlenmesinde, mikrosertlik ölçümleri ve çekme testleri yapılmıştır. Kaynak akımıartışı ile erime bölgesindeki kaynak boşlukları azalırken, çekirdek çapı, çökme miktarı ve ısı tesiri altındakibölge (ITAB) genişliği yaklaşık lineer bir şekilde artmıştır. Kaynak bölgesinde deformasyon ikizleri ortadankalkarken, ITAB’da iri tavlama ikizleri ortaya çıkmıştır. Ayrıca, kaynak akımı artışıyla ITAB’daki taneirileşmesi ve ikiz kalınlıkları artmıştır. Ancak, kaynak işlemi kaynak bölgesinde herhangi bir fazdönüşümüne neden olmamıştır. Bu sebeple, kaynak bölgesi sertlik değerleri temel malzemenin sertlikdeğerlerinden oldukça düşük kalmıştır. Genel itibariyle, en düşük sertlikler ITAB’da gözlenmiştir. Kaynakakımı ile kaynak bölgesi sertlik değerleri arasında herhangi bir korelasyon elde edilmemiştir. Kopma yükükaynak akımı ile artmıştır: En yüksek kopma yükü 10 kA kaynak akımında elde edilmiştir. Düşük kaynakakımlarında aryüzey tipi kırılma meydana gelirken yüksek kaynak akımlarında buton çekirdek tipi kırılmalarortaya çıkmıştır. Kırılma karakteristikleri genel itibariyle gevrek-sünek karışımıdır. Daha yüksekmukavemete sahip numunelerde gevrek-sünek kırılma bölgesinde sünek kırılma, gevrek kırılma bölgesindeise trans-granular kırılma karakteristikleri artış göstermiştir.Research Project Magnezyum Alaģımı-benzeri Bileģiklerin Sinir Kılavuz Kanalı Uygulaması Ġçin Elektroeğirme Ġle Üretimi(2019) Türkoğlu, Hilal Şaşmazel; Bişkin, Erhan; Davut, KemalPeriferik sinir yaralanmalarında, 5 mm'den büyük hasarlarda cerrahi müdahale yetersiz kaldıgı ve sinir dokusunun iyilesme hızı göreceli olarak yavas oldugu için olusan bosluk inflamasyon hücrelerinden olusan yara dokusuyla kapanarak sinir hattında fonksiyon kaybına sebep olmaktadır. Dolayısıyla büyük hasarların tedavisi için bölgenin çevre dokudan izole edilmesi gerekmektedir. Bu amaçla, yaygın olarak çalısılan ve klinik uygulamalarda tercih edilmeye baslanan sinir kılavuz kanalı öne çıkmaktadır. Biyouyumlu ve biyobozunum hızı düsük olması gereken sinir kılavuz kanalları, oksijen/besin/atık alısverisine olanak saglayacak ancak inflamasyon hücreleri girisine izin vermeyecek gözenek yapısında olmalıdır. Dolayısıyla, seçilen malzemenin biyobozunma hızı ve bozunma ürünlerinin toksisite derecesi ile üretim için tercih edilen yöntemin gözeneklilik kontrolü saglaması oldukça önemlidir. Literatürde, polimerler kullanılarak düsük maliyetli ve kolay üretilebilen sinir kılavuz kanalları üzerine çalısmalar yapılmaktadır. Metaller ise mekanik dayanım ve iletkenliklerine ragmen geleneksel yöntemlerle mikron seviyesindeki gözeneklilikte üretilemediklerinden, tercih edilememektedir. Bu projede, yaygın olarak stent uygulamaları için çalısılan, ticari seviyede kullanılmak üzere FDA onayı almıs ve biyolojik yeterliligi stent uygulaması için kanıtlanmıs bir magnezyum alasımına benzer kompozisyonda bir bilesigin sinir kılavuz kanalı uygulamaları için kullanılmak üzere gelistirilmesi çalısılmıstır. Magnezyum düsük yogunluk ve toksisite ile yüksek özgül dayanım ve elektrik iletkenligine sahip, vücutta bol bulunan ve metabolizmayı besleyici bir elementtir. Böylece gelistirilen kanalın, fiziksel, kimyasal ve biyolojik olarak üstün performans göstermesi öngörülmüstür. Söz konusu metalik alasım-benzeri bilesigin üretiminde, geleneksel yaklasımlardan farklı olarak, proses kolaylıgı ve üstün gözeneklilik kontrolü saglayan elektroegirme yöntemi seçilmis ve alasımın bilesenlerinin nitratlı bilesikleri ile PVA çözelti ham maddesi olarak kullanılması suretiyle elektroegirme yapılmıstır. Bu süreçte, ham madde/çözücü konsantrasyonları, sıcaklık, viskozite gibi çözelti parametreleri ile voltaj, uzaklık, besleme hızı gibi proses parametreleri gözle muayene ve SEM görüntüleriyle optimize edilmistir. Daha sonra, argon atmosferi altında kontrollü kalsinasyona tabi tutularak bilesenlerin istenilen alasım kompozisyonunda kristallesmesi ve alasım harici bilesenlerin uzaklastırılması saglanmıstır. Kalsinasyon sürecinde uygulanan, çoklu sıcaklık/süre kademesinden olusan ısıl islem profili, elektroegirilmis numunelere yapılan termal analizler sonucunda olası faz dönüsümlerinden kaynaklanan endotermik ve ekzotermik piklerin belirlenmesi ve bu piklere matematiksel reaksiyon kinetigi yaklasımının uygulanmasıyla tasarlanmıstır. Kalsinasyon süreci sonrası elde edilen nihai numunelerin kristalografik yapısı, elemental kompozisyonu, morfolojik özellikleri ve alasım harici bilesenlerin uzaklastırıldıgı/uzaklastırılamadıgı sırasıyla XRD, EDX, SEM ve XPS ile tayin edilmistir. Ayrıca elde edilen numunelerin absorpsiyon/sisme kapasitesi, ıslatılabilirligi, geçirgenligi ve bozunma hızı gibi fiziksel ve kimyasal özellikleri de tamamlanan projedeki karakterizasyon çalısmaları kapsamında gerçeklestirilmistir. Çalısmaların son basamagında, gelistirilen sinir kılavuz kanalı materyali adayının hücre-materyal etkilesimi, fare fibroblast hücre hattı ile MTT analizi, hemositometrik sayım ve çesitli boyama/görüntüleme teknikleri kullanılarak hücre canlılıgı, yapısma, yayılma ve üreme kabiliyetleri bakımından incelenmistir. Tamamlanan proje çalısmasından elde edilen fiziksel, kimyasal ve biyouyumluluk performans verileri, elektroegirme yöntemiyle magnezyum temelli alasım benzeri bilesikten olusan fibröz gözenekli yapıların sinir kılavuz kanalı uygulamalarında kullanılabilme potansiyeli olabilecegini göstermistir.Article Strain Hardening Behavior Characterization of Dual Phase Steels(2018) Davut, Kemal; Şimşir, Caner; Çetin, BarişThe requirements for higher passenger safety, improved fuel economy and weight reductionin automobile industry necessitates the usage of advanced high strength steel (AHSS)grades. Dual phase (DP) steels are the most widely used one among AHSS. DP steels becomeincreasingly popular, since they provide a combination of sufficient formability at room temperatureand tensile strength over 1000 MPa. The current standards for DP steels only specifiesyield and tensile strength. Steels from various producers have considerably different compositionand microstructure; however they still have the same grade name. Combined withthe inherited heterogeneous microstructure, those steels exhibit different strain hardeningbehavior. The aim of this study is to evaluate the strain hardening behavior of DP800 steels,obtained from different vendors and thus having different compositions and microstructures.The strain hardening behavior was characterized with tensile tests performed along rollingand transverse directions. The microstructure has been characterized with optical andscanning electron microscopes. The martensite fraction, grain size of ferrite and chemicalcomposition has been correlated to the strain hardening behavior. The results show thatthe steel with more micro-alloying addition has finer ferritic grain size, which cause higherinitial strain hardening rate. The steel with higher Mn and Cr has higher martensite fraction,which cause strain hardening rate to be higher at higher strain levels.Article Citation - WoS: 46Citation - Scopus: 49Recrystallization and Grain Growth Kinetics of In718 Manufactured by Laser Powder Bed Fusion(Elsevier, 2022) Dogu, Merve Nur; Davut, Kemal; Obeidi, Muhannad Ahmed; Yalcin, Mustafa Alp; Gu, Hengfeng; Low, Thaddeus Song En; Brabazon, DermotThe recrystallization and grain growth behaviour of IN718 alloy additively manufactured by laser powder bed fusion (L-PBF) is presented herein. The effects of three different temperatures (1050, 1150 and 1250 degrees C) and holding times (15, 45 and 90 min) were investigated. The texture evolution of the samples was recorded via electron backscatter diffraction (EBSD). The as-built sample is composed of bowl-shaped melt pools, a chessboard-like grain pattern and has a cube texture {100}<001>. Recrystallized grains were observed in the samples treated at 1150 degrees C for 15 min, as well as the samples treated for longer periods and at higher temperatures. Recrystallization was observed to start from high dislocation density regions, including the overlapping melt pools and the borders of the chessboard-like pattern. The initial cube texture transforms into a first-generation cube-twin texture {122}<212> via a twinning-assisted recrystallization mechanism. Then, those recrystallization nuclei sweep through the high defect density matrix; during which almost no new twins are formed. The samples treated at 1250 degrees C are almost completely recrystallized, which forms a weaker cube texture and a stronger P-orientation {011}<112>. However, the growth of recrystallized grains is very limited due to the presence of non-coherent precipitates. (C) 2022 The Author(s). Published by Elsevier B.V.Article Citation - WoS: 22Citation - Scopus: 25Effect of Solution Heat Treatment on the Microstructure and Crystallographic Texture of In939 Fabricated by Powder Bed Fusion-Laser Beam(Elsevier, 2023) Dogu, Merve Nur; Ozer, Seren; Yalcin, Mustafa Alp; Davut, Kemal; Bilgin, Guney Mert; Obeidi, Muhannad Ahmed; Brabazon, DermotThe effect of various solution heat treatment temperatures (i.e., 1120, 1160, 1200 and 1240 & DEG;C) on the microstructure, grain morphology and crystallographic texture of IN939 fabricated by powder bed fusion-laser beam (PBF-LB) was investigated. Microstructural analyses showed that the high-temperature gradient and rapid solidification of the PBF-LB processing caused different resulting microstructures compared to conventionally pro-duced counterparts. The melt pool morphologies and laser scanning paths were examined in the as-fabricated samples in the XZ-and XY-planes, respectively. After the application of solution heat treatment at 1120 & DEG;C, the as-fabricated PBF-LB initial microstructure was still apparent. For solution heat treatments of 1200 & DEG;C and above, the melt pool and scanning path morphologies disappeared and converted into a mixture of columnar grains in the XZ-plane and equiaxed grains in the XY-plane. On the other hand, large equiaxed grains were observed when the samples were solutionized at 1240 & DEG;C. Additionally, g' phase precipitated within the matrix after all solution heat treatment conditions, which led to increase in the microhardness values. According to electron backscatter diffraction (EBSD) analyses, both as-fabricated and solution heat-treated samples had intense texture with {001} plane normal parallel to the building direction. The first recrystallized grains began to appear when the samples were subjected to the solution heat treatment at 1160 & DEG;C and the fraction of the recrystallized grains increased with increasing temperature, as supported by kernel average misorientation (KAM) and grain spread orientation (GOS) analyses.& COPY; 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).Article Citation - WoS: 11Citation - Scopus: 12A Comprehensive Characterization of the Effect of Spatter Powder on In939 Parts Fabricated by Laser Powder Bed Fusion(Elsevier Sci Ltd, 2023) Dogu, Merve Nur; Mussatto, Andre; Yalcin, Mustafa Alp; Ozer, Seren; Davut, Kemal; Obeidi, Muhannad Ahmed; Brabazon, DermotThis study is focused on a comprehensive characterization of virgin and spatter IN939 powders and the effects of a certain amount of spatter powder on the part quality of IN939 fabricated by the L-PBF process. A brown tint coloration formed Al2O3 oxide, pores, a 124.4% increase in the average particle size, a 10.2% decrease in the powder circularity, and a 7.5% decrease in the powder aspect ratio were observed in the spatter powder. Additionally, higher average grain size and lower nanohardness were obtained for the spatter powder. In order to understand the effect of a certain amount of spatter powder on the part quality, 10 wt% spatter powder was mixed with the virgin powder. This addition was found to decrease the flowability of the powder. Moreover, this addition decreased relative density by around 0.3% and increased surface roughness by around 80.8% in the fabricated samples (termed as V and SV). On the other hand, there was no considerable microstructural, texture, microhardness, and nanohardness difference between V and SV samples, although the spatter powder addition caused a 30.2% increase in the average grain size of SV. The overall texture for both V and SV samples exhibit (00 1)//BD.Article Citation - WoS: 13Citation - Scopus: 13Effect of Aging Treatment on the Microstructure, Cracking Type and Crystallographic Texture of In939 Fabricated by Powder Bed Fusion-Laser Beam(Elsevier, 2024) Ozer, Seren; Dogu, Merve Nur; Ozdemirel, Ceren; Bilgin, Guney Mert; Gunes, Mert; Davut, Kemal; Brabazon, DermotThis study aimed to provide a comprehensive understanding of how aging treatments (namely, HT1 and HT2) affect the microstructure, cracking behavior, and crystallographic texture of IN939 fabricated by powder bed fusion-laser beam (PBF-LB) method. Although both aged samples demonstrated similar grain structure and recrystallization behavior according to the electron backscatter diffraction (EBSD) analysis, as well as the precipitation of bimodal gamma ' phase and MC- and M23C6-type carbides, notable differences were observed in the size and morphology, particularly the gamma ' phase. The HT1 sample displayed coarsened primary gamma ' phase, with sizes reaching up to 2 mu m and exhibiting varied morphologies, including irregular and cuboidal shapes. Additionally, this treatment led to the formation of some gamma '-gamma eutectic regions and plate-like eta phase, along with the decomposition of MC-type carbides into M23C6-type carbides. In contrast, the HT2 sample displayed uniformly distributed spherical primary gamma ' phase with sizes ranging from 70 to 120 nm, accompanied by very fine secondary gamma ' phase. Furthermore, it was found that changes in both aged sample microstructures could result in the formation of strain-age cracks due to the gamma ' phase formation and liquation cracks due to the partial remelting of lower melting point phases. The findings also revealed that with the application of aging treatments, the hardness of the as-fabricated sample (339.8 +/- 3.4 HV) increased to 440.2 +/- 5.6 HV and 508.1 +/- 4.8 HV for the heat treatment of HT1 and HT2, respectively.Article Citation - WoS: 11Citation - Scopus: 17Machinability Evaluations of Austempered Ductile Iron and Cast Steel With Similar Mechanical Properties Under Eco-Friendly Milling Conditions(Elsevier, 2021) Eraslan, Dogancan; Balci, Ahmet; Cetin, Baris; Ucak, Necati; Cicek, Adem; Yilmaz, Okan Deniz; Davut, KemalIn engineering applications, to increase productivity and to decrease production costs, the selection of the proper engineering material is essential. At that point, machining operations directly affect the production costs. Therefore, determination of the material with the desired mechanical properties and easy-to-cut characteristics has a critical importance. This situation is currently gaining more importance in especially defense industry applications in which high strength engineering materials are heavily employed. In addition, tool performance and final product quality are directly influenced by the cooling and/or lubrication conditions in particularly interrupted cutting operations. In this study, machinability characteristics of G18NiMoCr3-6+QT1 cast steel (CS) and 1050-6 austempered ductile iron (ADI) with similar mechanical properties during milling operations were investigated. The tests were performed using TiAlN coated cemented carbide (WC-Co) end mills under dry, conventional cutting fluid (CCF), and minimum quantity lubrication (MQL) conditions. Under each condition, the variations of cutting forces, tool wear, average surface roughness (Ra), and subsurface microstructure and microhardness were analyzed for both materials and then compared to one another. Test results showed that 1050-6 ADI led to further tool wear in comparison to G18NiMoCr3-6+QT1 CS. According to obtained results, dry condition is more favorable than CCF and MQL conditions in terms of cutting forces, surface roughness, and tool wear for both types of material. In addition, examinations on subsurface microstructures showed that MQL conditions provided an effective cutting environment to maintain microstructural stability of workpiece materials. (c) 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Article Citation - WoS: 20Citation - Scopus: 22Understanding Corrosion Morphology of Duplex Stainless Steel Wire in Chloride Electrolyte(Mdpi, 2021) Ornek, Cem; Davut, Kemal; Kocabas, Mustafa; Bayatli, Aleyna; Urgen, MustafaThe corrosion morphology in grade 2205 duplex stainless steel wire was studied to understand the nature of pitting and the causes of the ferrite phase's selective corrosion in acidic (pH 3) NaCl solutions at 60 degrees C. It is shown that the corrosion mechanism is always pitting, which either manifests lacy cover perforation or densely arrayed selective cavities developing selectively on the ferrite phase. Pits with a lacy metal cover form in concentrated chloride solutions, whereas the ferrite phase's selective corrosion develops in diluted electrolytes, showing dependency on the chloride-ion concentration. The pit perforation is probabilistic and occurs on both austenite and ferrite grains. The lacy metal covers collapse in concentrated solutions but remain intact in diluted electrolytes. The collapse of the lacy metal cover happens due to hydrogen embrittlement. Pit evolution is deterministic and occurs selectively in the ferrite phase in light chloride solutions.Article Microstructural Analysis of Austempered Ductile Iron Castings(2016) Çetin, Barış; Meco, Halim; Davut, Kemal; Arslan, Ebru; Uzun?, Mustafa CanAustempered ductile iron (ADI) castings have a wide range of application areas in engineering designs due to their promising mechanical properties and lower cost. ADI has very good strength and toughness values at the same time its ductility is relatively high compared to most of the other cast irons. These promising mechanical properties originate from combination of specific graphite and matrix microstructure. The size, shape and fraction of graphite as well as the matrix microstructure influences the mechanical properties. In this paper the efforts regarding to a localization project of ADI is presented. In a more detailed manner, the first locally produced ADI which cannot satisfy the mechanical properties stated in ISO 17804 is compared with the original sample which is conform with the standard. The two pieces are inspected by mechanically and microstructurally by means of which necessary actions are detected for the local production. In other words the relation between the macro mechanical properties and the microstructural conditions are tried to be clarified.
