Browsing by Author "Davut, Kemal"

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Citation Count: 2
A 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, Dermot; Metallurgical and Materials Engineering; Department of Metallurgical and Materials Engineering
This 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.
Citation Count: 2
Development and properties of austempered low alloyed white cast iron
(Walter de Gruyter GmbH, 2021) Erdogan,M.; Davut,K.; Kilicli,V.; Department of Metallurgical and Materials Engineering
This study examined the response of low-alloy white cast iron to austempering heat treatment. In addition, it investigated the microstructure and mechanical properties of austempered low-alloy white cast iron. The low-alloy white cast iron specimens were austenitized at 900 °C, followed by quick quenching into a salt bath at 375 °C, and held there for 15 to 120 minutes for austempering heat treatment. Microstructural features were studied by optical, scanning electron microscopes, and XRD analysis. The mechanical properties were determined by hardness and unnotched Charpy impact toughness tests. As a function of those austempering times, a microstructural map was constructed to show how the transformation products develop, quantitatively. The experimental results showed that the austempering heat treatment produced a microstructure consisting of eutectic carbides + ausferritic structure in low-alloy white cast iron. It can be concluded that the low-alloy white cast iron can be austempered, similar to ductile cast irons. Improved hardness and impact toughness values have been obtained in austempered low-alloy white cast iron. © 2021 Walter de Gruyter GmbH, Berlin/Boston, Germany.
Citation Count: 0
Effect 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, Dermot; Metallurgical and Materials Engineering; Department of Metallurgical and Materials Engineering
This 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.
Citation Count: 6
Effect of austenitizing temperatures on the microstructure and mechanical properties of AISI 9254 steel
(Walter de Gruyter Gmbh, 2021) Murathan, Omer Faruk; Davut, Kemal; Kilicli, Volkan; Department of Metallurgical and Materials Engineering
In this study, the effect of austenitizing temperatures and low-temperature isothermal heat treatment (below martensite start temperature) on the microstructure and mechanical properties of AISI 9254 high silicon spring steel has been investigated. Experimental studies show that ultra-fine carbide-free bainite, tempered martensite and carbon enriched retained austenite could be observed in isothermally heat-treated samples where the as-received sample consisted of fine pearlite. A high tensile strength of similar to 2060 MPa, a total elongation of similar to 8 %, and absorbed energy of 105 J were achieved in a commercial high-Si steel by austempering below the Ms temperature. A good combination of strength and ductility has been obtained in prolonged austempering below the martensite start temperature (225 degrees C) from an austenitizing temperature of 870 degrees C.
Citation Count: 0
Effect of batch and continuous annealing processes on crystallographic texture and formability of an if steel
(Association for Iron and Steel Technology, AISTECH, 2017) Aldikacti,G.; Baskaya,U.; Davut,K.; Department of Metallurgical and Materials Engineering
IF (interstit ial free) steels are used in many applicatio ns, specifically in automotive industry due to their excellent formability. This formability behaviour is directly related to the annealing process, which influences both the crystallographic texture and the related "r value". For annealing, two main routes, namely batch and continuous annealing processes, are used in the production of IF-steels. The present study aims at comparing the same grade of IF steels produced via batch and continuous annealing processes. For this purpose, the texture and microstructure analysis were performed by electron backscatter diffraction (EBSD) technique in a scanning electron microscope (SEM). The formability behaviour of the steel sheet was characterized by determining the hardening curve and yield surface. It has been found that all samples show a strong ND fiber; {111}//ND. On the other hand, the drawability and related "r value" of continuously annealed samples were better than batch annealed ones. This behaviour is explained by the ultra-low carbon content, lower yield strength and equiaxial grain structure. © Copyright 2017 MS&T17®.
Citation Count: 15
Effect of post fabrication aging treatment on the microstructure, crystallographic texture and elevated temperature mechanical properties of IN718 alloy fabricated by selective laser melting
(Elsevier Science Sa, 2022) Ozer, Seren; Bilgin, Guney Mert; Davut, Kemal; Esen, Ziya; Dericioglu, Arcan F.; Metallurgical and Materials Engineering; Department of Metallurgical and Materials Engineering
The effect of building direction and post fabrication aging treatment on the microstructure, crystallographic texture and high temperature mechanical properties of Inconel 718 (IN718) alloy fabricated by selective laser melting (SLM) method was investigated. After aging, arc-shaped structures seen in as-fabricated samples dis-appeared and converted into a mixture of columnar and equiaxed grains. Nano-sized gamma '' and/or gamma' precipitates were formed upon aging; however, MC type carbides and Laves phase encountered in as-fabricated samples were not dissolved completely after aging. Moreover, aging did not alter the texture ((001)//building direction (BD)) of as-fabricated samples. Mechanical properties of the alloys under tension were influenced by the build direction, aging time and test temperature. As-fabricated samples produced in vertical direction exhibited higher room temperature strengths with lower ductility due to orientation of overlapped prior melt pools. Room temperature tensile test results revealed that peak aging caused a significant improvement in ultimate tensile strength (UTS), from 1066.5 MPa and 998.4 MPa to 1408.5 MPa and 1330.4 MPa whereas elongation values decreased from 27.5% and 32.2% to 19.6% and 23.7% in vertically and horizontally built samples, respectively. Peak-aged samples (aged at 700 degrees C for 8 h) tested at 600 degrees C displayed serrated regions in their stress-strain curves due to dynamic strain aging (DSA). Although strength values of the samples displayed an expected decrease by temperature, ductility of the samples reduced to minimum at temperatures around 700-800 degrees C, which was attributed to intermediate temperature embrittlement.
Citation Count: 5
Effect 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, Dermot; Metallurgical and Materials Engineering; Department of Metallurgical and Materials Engineering
The 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/).
Citation Count: 4
Elektrik direnç punta kaynağı ile birleştirilen %15 deforme edilmiş TWIP çeliğinde kaynak akımının mikroyapı ve mekanik özellikler üzerindeki etkisi
(Gazi Univ, Fac Engineering Architecture, 2020) Aydın, Hakan; Tutar, Mümin; Davut, Kemal; Bayram, Ali; Department of Metallurgical and Materials Engineering; Industrial Engineering
Çalışmada, %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.
Citation Count: 24
Friction stir processing of dual phase steel: Microstructural evolution and mechanical properties
(Elsevier Science inc, 2019) Aktarer, S. M.; Kucukomeroglu, T.; Davut, K.; Department of Metallurgical and Materials Engineering
The influence of friction stir processing (FSP) on the microstructure and mechanical properties of a DP 600 steel has been studied. The microstructure evolution during the FSP has been characterized using electron back scatter diffraction (EBSD) technique and scanning and transmission electron microscopes. Standard tension and hardness tests were used to characterize the mechanical properties. The results show that the FSP produced a refined microstructure composed of ferrite, bainite, martensite, and tempered martensite which in turn increased the hardness and strength magnitudes by a factor of 1.5. The initially 2.83 mu m average grain size of ferrite has decreased to 0.79 mu m in the pin effected zone of (PE-SZ-I) of the processed region. Both EBSD and TEM observations showed regions with high dislocation density and sub-structures region in the processed zone. The grain size became coarser, the density of both dislocations and low-angle grain boundaries decrease, away from the processed zone. Moreover, phase fractions and hardness values were predicted using CALPHAD thermodynamic based software based on commercial material properties. Although the prediction does not take into consideration the influence of severe plastic deformation, the results were within 10% uncertainties of the experimental findings. The present study demonstrates that an ultra-fine grained structure can be obtained through the thickness of a 1.5 mm thick D P600 steel sheet via FSP. FSP can produce a range of different hardness and strength values; which can also be predicted successfully by inputting the composition and local temperatures reached during the FSP.
Citation Count: 5
Influence of Cu and Ni Alloying on the Microstructure and Mechanical Properties of Austempered Ductile Iron Castings
(Polish Acad Sciences inst Physics, 2019) Yalcin, M. A.; Cetin, B.; Davut, K.; Department of Metallurgical and Materials Engineering
Austempered ductile cast iron (ADI) offers a good combination of high tensile and fatigue strength, good ductility, toughness, wear resistance and damping characteristics, lower density in an economical way. This excellent combination of properties is due to the specific microstructure of ADI; which is composed of spheroidal graphite particles on an ausferritic matrix. The ausferrite consists of acicular ferrite and high carbon retained austenite; which is produced via austempering heat treatment after casting. The alloying additions of Cu or Cu + Ni increases austemperability, which means completely ausferritic structures can be produced on larger cross-sections. In the present study the effect of the alloying additions of Cu and Cu + Ni on mechanical properties and microstructure of ADI was studied. For that purpose, Y-block specimens having a lean composition, 0.8% Cu and 0.8% Cu + 0.4% Ni alloying additions were cast. After austempering treatment, mechanical tests, fractographic and metallographic examinations were performed. The results show that the Cu + Ni alloyed specimen has higher strength and elongation. The lean alloy on the other hand, has the highest nodularity and matrix hardness but the lowest strength and ductility. Those differences in mechanical properties were attributed to the fraction and morphology of the retained austenite regions of the matrix.
Citation Count: 0
Investigating the Effect of Auto-Focus and Auto-Threshold Algorithms in Advanced Nodularity Analysis of Austempered Ductile Iron Castings
(Polish Acad Sciences inst Physics, 2019) Cetin, B.; Kurtuldu, H.; Durkaya, G.; Davut, K.; Department of Metallurgical and Materials Engineering
Austempered ductile iron castings have a wide range of application fields including defense and heavy-duty industries due to their promising mechanical properties. Austempered ductile iron castings could provide excellent combination of high strength, toughness, and wear resistance at the same time. Unlike other spheroidal ductile cast irons, austempered ductile iron castings need to be processed by a special austempering heat treatment which enables the verification of the conformity of the nodularity level after the casting operations. Previous studies have shown that wide area mappings for determining nodularity are crucial to eliminate the representativeness problems. In this sense, some precautions should be taken for the accuracy of results in large scale nodularity examination. In this study, the effect of auto-thresholding and auto-focusing algorithms on the advanced nodularity analysis is investigated. The results suggest that auto-thresholding and auto-focusing methods have significant contributions for determining average nodularity levels and average graphite sizes.
Citation Count: 9
Machinability 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, Kemal; Department of Metallurgical and Materials Engineering; Manufacturing Engineering
In 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/).
Citation Count: 4
A Material Perspective on Consequence of Deformation Heating During Stamping of DP Steels
(Iop Publishing Ltd, 2017) Simsir, C.; Cetin, B.; Efe, M.; Davut, K.; Bayramin, B.; Department of Metallurgical and Materials Engineering; Manufacturing Engineering
Recent studies showed that, during stamping of high strength steels at industrially relevant production rates, local temperature in the blank may rise up to 200 degrees C - 300 degrees C due to deformation heating. Moreover, die temperature may also rise up to 100 degrees C - 150 degrees C for progressive stamping dies. Based on the common assumption that the blank softens as the temperature increases, thermal softening creates a margin in Forming Limit Diagram (FLD) and therefore the FLD determined at room temperature can safely be used for those cases. In this article, the validity of this assumption on DP590 steel is questioned by high temperature tensile tests (RT - 300 degrees C) at various strain rates (10(-3) s(-1) - 1 s(-1)). The results indicated a decrease both in uniform and total elongation in 200 degrees C - 300 degrees C range together with several other symptoms of Dynamic Strain Aging (DSA) at all strain rates. Concurrent with the DSA, the simulated FLD confirms the lower formability at high temperature and strain rates. Thus, it is concluded FLD determined at RT may not be valid for the investigated steels.
Citation Count: 0
Microstructural Analysis of Austempered Ductile Iron Castings
(2016) Çetin, Barış; Meco, Halim; Davut, Kemal; Arslan, Ebru; Uzun?, Mustafa Can; Department of Metallurgical and Materials Engineering
Austempered 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.
Citation Count: 19
Microstructural and texture evolution during thermo-hydrogen processing of Ti6Al4V alloys produced by electron beam melting
(Elsevier Science inc, 2020) Dogu, Merve Nur; Esen, Ziya; Davut, Kemal; Tan, Evren; Gumus, Berkay; Dericioglu, Arcan F.; Department of Metallurgical and Materials Engineering
The present study was conducted to reveal the effects of building angles and post heat-treatments (2-step Thermo-Hydrogen Processing (THP) and conventional annealing treatment) on the density, microstructure and texture of Ti6Al4V alloy parts produced by Electron Beam Melting (EBM). The results showed that regardless of the building angle; the density, microstructure and crystallographic texture (defined with respect to building angle) of the as-produced samples were identical; having Widmanstatten a structure and columnar beta-grains which are parallel to building direction. The main texture component for the alpha phase was (10 (1) over bar0)//building direction, and for beta phase (001)//building or heat flow direction. The first step of THP, namely, the hydrogenation step, produced a needle-like microstructure and increased the local misorientations due to lattice distortion. On the other hand, after application of the second step of THP, dehydrogenation step, microstructure was refined, particularly alpha-grains that were larger than 10 mu m and located at grain boundaries. Moreover, THP randomized the crystallographic texture since it involves beta to alpha phase transformation, at which one beta-grain can produce 12 distinct alpha-variants. The grain boundary misorientation distributions also changed in accordance with the microstructural changes during the 2-step THP. On the other hand, annealing coarsened the grain boundary and Widmanstatten alpha phases; moreover, it changed the texture so that the basal planes (0001) rotated 30 degrees around the building direction.
Citation Count: 1
Nondestructive microstructural characterization of austempered ductile iron
(Walter de Gruyter Gmbh, 2023) Tuzun, Mert Yagiz; Yalcin, Mustafa Alp; Davut, Kemal; Kilicli, Volkan; Department of Metallurgical and Materials Engineering
Austempered ductile iron (ADI) has been preferred in a wide range of applications due its unique combination of high strength, good ductility, wear resistance and fracture toughness together with lower cost and lower density compared to steels. Magnetic Barkhausen noise (MBN) measurement offers a better alternative to traditional characterization techniques by being fast and non-destructive. A simple linear regression using only one single independent variable cannot correlate the MBN with the microstructure of ADI, since its microstructure is multi component. Multiple linear regression analysis (MLRA) was used to build a model that uses the characteristic features of microstructural constituents as input parameters to predict the MBN. For that purpose, Cu-Ni-Mo alloyed ductile iron samples austempered between 325 and 400 degrees C and for 45-180 min duration were used. The results show that MBN is most sensitive to the size and shape of acicular ferrite and retained austenite. Moreover, MBN is almost insensitive to the size, morphology and volume fraction of graphite particles. This indicates that retained austenite pins the domain walls more effectively than the graphite particles. Considering the results MLRA, MBN technique can be used to characterize the ausferritic microstructure of ADI.
Östemperlenmiş küresel grafitli dökme demirlerde alaşımlama ve ısıl işlem parametrelerinin optimizasyonu
(2019) Yalçın, Mustafa Alp; Davut, Kemal; Department of Metallurgical and Materials Engineering
Östemperlenmiş küresel grafitli dökme demir sunduğu yüksek mukavemet, tokluk ve aşınma direncine ek olarak düşük yoğunluk ve maliyet avantajı sayesinde mühendislik tasarımlarında yoğun olarak kullanılmaktadır. Bu malzemelerin güçlü mekanik özelliklerinin nedeni, ösferritik matris üzerinde küresel grafitlerden oluşan özel mikroyapısıdır. Östemperleme ısıl işlemi sonrasında oluşan ösferrit yapısı, iğnemsi ferrit ve karbonca zengin kararlı östenitten oluşmaktadır. Düşük küresellik değerleri ya da iğnemsi ferrit dönüşümünün eksik kalması nedeniyle östenit yapısının yeterince kararlı hale gelmemesi gibi sorunlar mikroyapıyı bozmakta ve mekanik özellikleri olumsuz etkilemektedir. Bu çalışmada Cu ve Cu + Mo + Ni alaşım elementlerinin, ısıl işlem parametrelerinin ve küresel grafit parçacıklarının boyutları ve dağılımlarının, östemperlenmiş küresel grafitli dökme demir malzemelerin mikroyapısına ve mekanik özelliklerine etkisi incelenmiştir. Bunun için, düşük alaşımlı ve düşük alaşıma Cu, Cu + yüksek Mo + düşük Ni ve Cu + düşük Mo + yüksek Ni elementleri eklenmiş Y-blok dökümler üretilmiştir. Y-blok parçalardan çıkarılmış numuneler östemperleme ısıl işleminden geçirilmiş; daha sonra mekanik testler yapılmış, kırılma yüzeyi ve metalografik incelemeler gerçekleştirilmiştir. Sonuçlar, Cu + düşük Mo + yüksek Ni ile alaşımlanmış numunenin daha yüksek mukavemet ve uzama değerlerine sahip olduğunu göstermektedir. Bu da, Cu ve Cu + Mo + Ni alaşımları eklenmiş parçaların karbonca zengin ve kararlı östenit yapısına sahip olduğunu; ve geniş kesit alana sahip parçalarda bile kalınlık boyunca homojen bir ösferritik yapı oluşturulabileceğini göstermektedir. Düşük alaşımlı numune ise, yetersiz alaşımlama ve östemperleme işlemi nedeni ile en iyi küresellik ve sertlik değerlerine sahip olmasına rağmen en düşük mukavemet ve uzama değerlerini vermektedir. Mekanik özelliklerdeki bu değişkenliğin, mikroyapıda bulunan kalıntı östenitin yapısı, miktarı ve dağılımı ile doğrudan bağlanıtılı olduğu gözükmektedir. Son olarak, Pearson Korelasyon Katsayısı yöntemi kullanılarak malzemenin mekanin özellikleri ve mikroyapısı arasındaki ilişki incelenmiştir. Sonuçlar, östenit tane boyutu ve akma dayancı, çekme dayancı ve süneklik arasında Hall-Petch benzeri bir ilişki göstermektedir. EBSD çalışmaları sonrasında iki farklı tip östenit yapısı görülmüştür: i) iğnemsi ferrit yapıları arasında kalmış ince östenit ve ii) östemperleme işlemi sırasında dönüşmemiş önceki östenit tanelerinden kalan blok tipi östenit. Çalışılan numuneler arasında, iğnemsi ferrit tane boyutu 0.7 m'den daha az değişkenlik göstermektedir. Bu nedenle iğnemsi ferrit tane boyutu ile mekanik özellikler arasında iyi bir korelasyon gözükmemektedir. Küresellik ve küresel grafitlerin boyutu arasında ise yüksek korelasyon gözükmektedir. Küresellik derecesindeki küçük farklar mekanik özellikleri etkilememekle beraber, küçük küresel grafit taneleri hem akma hem de çekme dayancı değerlerini iyileştirmektedir. Bu sonuçlar östemperlenmiş küresel grafitlli dökme demirlerin daha da geliştirilmesi ve iyileştirilmesine katkı sağlayabilir.
Citation Count: 3
A Potential Solution to Mystical Materials in Indentation Test
(Elsevier Science Bv, 2017) Billur, E.; Cetin, B.; Music, O.; Simsir, C.; Davut, K.; Department of Metallurgical and Materials Engineering; Automotive Engineering; Manufacturing Engineering
Various methods have been designed to determine the elasto-plastic properties of metals. Instrumented indentation test (IIT) is considered to be a good candidate to determine local properties after manufacturing operations. In order to acquire elastoplastic properties from IIT, either dimensional analysis or inverse analysis of the force-displacement curve is performed. However, the major drawback of those methods is the uniqueness of the solution. Some materials may exhibit almost identical force-depth curves, although they have different elastoplastic properties. Those materials are referred as "mystical materials". In this contribution, topological features of the indentation surfaces, i.e. indent size, pile-up and sink-in behaviour, are investigated to find a differentiating property. According to the results, indent size, pile-up and sink-in behaviour may help to find the unique solution to the inverse problem. (C) 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on the Technology of Plasticity.
Citation Count: 20
Recrystallization 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, Dermot; Department of Metallurgical and Materials Engineering
The 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.
Rulman çeliklerinin manyetik Barkhausen gürültüsü yöntemi ile tahribatsız karakterizasyonu
(2017) Arslan, Ebru; Davut, Kemal; Şimşir, Caner; Department of Metallurgical and Materials Engineering
100Cr6 çelikleri rulman yapımında yaygın olarak kullanılırlar. Isıl işlem uygulamaları ile değişen iç yapısal özelliklerine bağlı olarak; yüksek gerilimlere karşı dayanımı, yorulma ömrü, tokluğu, sertliği, aşınma direnci gibi özellikleri geliştirilebililir. Bu iç yapısal özelliklerin muayenesi, geleneksel metalografik ve XRD tekniklerinin bir arada kullanılmasıyla yapılabilir. Ancak bu yöntemler numune hazırlama gereksinimi bulunan yöntemlerdir. Bu sebepten; tahribatsız ve hızlı ölçüm alınmasını sağlayan Manyetik Barkhausen gürültüsü (MBN) tekniği, bu geleneksel karakterizasyon yöntemlerine alternatif olarak değerlendirilmiştir. Bu çalışmanın amacı 100Cr6 çeliklerinin MBN yöntemiyle tahribatsız muayene edilebilirliğinin araştırılmasıdır. Bu bağlamda, 100Cr6 çelik numuneler, içerisinde farklı içyapısal özelliklerin oluşturulabilmesi adına farklı sıcaklıklarda östenitlenip oda sıcaklığına ve -130°C'ye hızlı soğutulmuştur. Bu işlemlerin ardından numunelerin sertliği, karbür miktarları, dağılımları, kalıntı östenit miktarları, metalografik ve XRD ölçüm yöntemleriyle belirlenmiştir. Ayrıca, her bir numune MBN yöntemi ile de muayene edilmiştir. Son olarak, elde edilen içyapı sonuçlarıyla MBN sonuçları arasındaki bağlantılar incelenmiştir. MBN tekniğinin, kurulan lineer korelasyonlar kapsamında oda sıcaklığına hızlı soğutulan 100Cr6 çelik numunelerin iç yapısal özellikleriyle korele edilebilir olduğu gözlenmiştir; ancak tahmin aralıklarının oldukça geniş olması sebebiyle, bu numunelerin iç yapısında oluşan sertlik, kalıntı östenit ve karbür miktarıların doğrudan belirlenebilmesi için yeterince hassas olmadığı sonucuna varılmıştır. Birbirlerine bağlı olarak değişen bu özelliklerin MBN ile karakterize edilebilmesi; karbür çözünme kinetiğine bağlı olarak, tüm bu değişkenlerin hem kendi aralarında hem de MBN sinyalleri üzerindeki eş zamanlı etkileri arasında lineer olmayan bir ilişki kurulması ile mümkün olabileceği düşünülmektedir. Bunun haricinde, sıfır altı ısıl işlemine tabi tutulan numunelerde, kalıntı östenitin tamamen yok edilmesi, MBN sinyalleri üzerindeki kalıntı östenit etkisini tamamen ortadan kaldırılmıştır. Bu sebepten hem sertlik ve hem de kardür oranları için kurulan lineer korelasyonlarda belirgin bir iyileşme görülmüştür. Bu korelastonların R² değerleri kabul edilebilir düzeyde olmasına rağmen tüm iç yapısal değişkenlerin MBN sinyalleri üzerindeki eş zamanlı etkisi göz önünde bulundurularak lineer olmayan modeller kurulmasıyla iyileştirilebilinir. Bu doğrultuda, MBN tekniğininin, 100Cr6 çeliğinin iç yapısal karakterizasyonu konusunda, gelecekte yapılacak daha detaylı ve kapsamlı çalışmalar ile geliştirilebilir olduğu düşünülmektedir.