FBE- Malzeme Bilimi ve Mühendisliği Lisansüstü Programı - Yüksek Lisans
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ÖgeAl.Fe.Si. ve Al.Fe.Si.Mn. alaşımlı alüminyum folyoların mekanik performans özelliklerinin karşılaştırılması(İTÜ Fen Bilimleri Mühendisliği, 1998)In this study..mechanical performans of aluminium foils of Al.Fe.Si. and Al.Fe.Si.Mn alloys have been investigated. The ability to produce a variety of shapes from flat shapes from flat sheet of metal at high rates of production has been one of the real technological advances of the twentieth century. This transition from hand - forming operation to mass -production methods has been important factor in the great improvement in the standart of living which an occured during the period. Because of the complexity of sheet forming operations,simple mechanical property measurments made from the tension test are of limited value.Over the years a number of laboratory tests have been developed to evaluate the formability of sheet materials.The Swift flat -bottom cup test is a standardized test for deep drawing.The drawability is expressed in terms of the limiting draw ratio.In the Olsen test and Erichsen tests the sheet is clamped between two ring dies while a punch, usually a ball, is forced against the sheet untill it fractures.The depth of the bulge before the sheet fracture is measured.These tests subject the sheet primarily to streching, while the Swift test provides nearly pure deep drawing. Howewer,most practical sheet forming operations provide a combination of both biaxial stretching and deep drawing.The Fukui test, which produce a conical cup using a hemispherical punch, provide a combination of both streching and drawing. A useful technique for controlling failure in sheet -metal forming is the forming limit diagram.The surface of the sheet is covered by a grid of circles, produced by electrochemical marking.When the sheet is deformed, the circles distord into ellipses.The major and minor axes of an ellipse XIthese two directions is measured by the percentage change in the lenghts of the major and minor axes.These strains,at any point on the surface,are then compared with Keeler Goodwin diagram for the material. Strain states above the curve represent failure, those below do not cause failure. The failure curve tension-tension was determined by Keeler and tension -compression region was first determined by Goodwin. Another approach to predicting sheet formability is the stretch -draw shape analysis. The forming limit of the material is established with the Olsen test and the Swift cupping test. Then the part is broken down into simple shapes and the percentage of draw and strech are calculeted from the geometry. This places the part on the forming limit diagram ( FLD ) and the degree of severity of the part can be established. FLD can be predicted theoretically,but the best way is to obtain it experimentally by performing tension test,deep darwing test, Erichsen test and bulge tests. Represantation of fracture data as a plot of major versus minor strains at the fracture gives the limit curve. Fracture occurs when the strain state at the end of the process is above the limit curve. The direction and the magnitude of maximum strain are easily determined by a grid pattern of small diameter circles on the surface of the blank.The grid pattern can be produced by electro-chemical marking, serigraph techniques or fotochemical etching. During deformation, grid circles are deformed into ellipses.The major and minor axes of ellipses represent the two principal strain directions in the test specimen.The strain in these two directions are measured by the percentage change in the lengts of the major and minor axes.The tension -compression region of the FLD is determined by the strain values obtained from tension and deep drawing tests at the instant of fracture.For deep drawing stampings,the strain state is tension-compression,and the major axes of circles elongate and minor axes are shortened.In tension- tension (stretching ) region both major and minor XIIaxes are greater than the initial circle diameter. This region of the FLD is determined from fracture strain values obtained from Erichsen and Bulge tests. Trough a complete analysis of the FLD and the actual deformations, die tryout and modification can be simplified, selection of optimum materials and lubricants can be made easier, and breakage of stampings in the production can be minimized. Formability is a function of both materials properties ( strain hardening exponent "n", strain rate sensitivity exponent "m", plastic anisotropy "r", thickness,grain size ) and process variables ( die design lubrication,workpiece geometry ).FLD offers a graphical method for the representation of material influences in formability. Material effects are embodied in the limit curve, the shape of which depends on microstructural features and composition.FLD is independent of the process variables such as lubrication and die design but the process variables affect the formability because of their effects on the uniformity of strain distribution in the stamping operation The aim of this study is to compare the formability of two Al alloys (AlFeSi and AlFeSiMn alloys ) in the form of foil at 0.2 mm thickness. These two alloys produced Hunter continuous casting machine as strip, then cold rolling operation begins to get desired thickness and annealed to get desired mechanical properties. Since rather high solidification rates are encountered in the twin -roll casting of wrought aluminium alloys, their as-cast microstructures are often not in eguilibrium. The aluminium solid solution is super satureted and the intermetallic phases are often metastable and aluminium -rich. Moreover, in spite of a very fine dendritic structure, both micro and macro segregation patterns are observed in as-cast sheet.Hence, homogenization is an essential step in the production process of such alloys particularly when they are submitted to forming operations in the post production applications. XIIIIn the experimental studies, firstly the chemical compositions of these two aluminium alloys are determined by spectral analysis ( Table -1 ), then tension test, Erichsen test and Swift test have been used to evaluate the formability of these alloys in the form of foil at 0.2 mm thickness. The result of these mechanical test are given in Table -2 Table-1 : The chemical compositions of the alloys studied ( as Weight % ) Table -2 The result of mechanical test of the Al alloys studied When the chemical compositions of these two alloys are compared, the most noticeable difference is Fe contents of these alloys. The other XIVimportant observation in the chemical composition of AlFeSiMn alloy is decreasing Si content while increasing Mn content compared to the chemical composition of AlFeSi alloy. Fe / Si ratio is 7.81 for AlFeSiMn alloy and 1.14 for AlFeSi alloy. During production of these alloys, the Fe/Si ratio is expected to be max 2.5,but the high Fe / Si ratio in AlFeSiMn alloy influences the process of the foil production in a negative way. However the high Fe/Si ratio is also desirable for use of scraps in high amount during Al foil production. Tensile test result show that foils of AlFeSi andAlFeSiMn alloys have similar yield and tensile strength values. Any problem which may occur during forming would be related to the ductility of these materials. The % elongation value which is a measure for ductility in tension, is the most basic data in comparison of forming performance.When the % elongation values of these two alloys given in Table -2 are compared,it has been observed that the AlFeSiMn alloy foil has a better ductility than the AlFeSi alloy foil. Strain hardening exponent "n" and normal plastic anisotropy "r" values for AlFeSi and AlFeSiMn alloy foils have been determined in three dimensions ( 0°,45° and 90° ) from tensile tests. The strain hardening exponent (n) determined by tension test as a measure of strech forming capacity and high value of n is desirable. Increasing n value reduces peak strains and increases the limit strain of the FLD. When the strain hardening exponent values of these two Al alloys given in Table-2 are compared, it has been observed that AlFeSiMn alloy foil has higher "n" value than the AlFeSi alloy foil.The high "n" value means that this material has high uniform plastic deformation,hence better formability. The average plastic anisotropy value, r, is a measure of normal anisotropy which characterizes the resistance to thinning.The r value must xvMajor Strain (%) s 1 Minor Strain (%) s 2 Figure 1. The forming limit diagrams of AlFeSi and AlFeSiMn alloys in the form of foil at 0.2 mm thickness.8 XVIIThe average plastic anisotropy value, r, is a measure of normal anisotropy which characterizes the resistance to thinning.The r value must be larger than one.Deep drawability of the materials increases with the increase of r value. The average plastic anisotropy value r is 0.54 for AlFeSi alloy and 0.76 for AlFeSiMn alloy. Therefore AlFeSiMn alloy has higher resistance to thinning and better deep drawability than AlFeSi alloy. The variation of normal anisotropy r with the plane of sheet is called planar anisotropy, Ar, and is responsible for earing in deep draw cup. The ideal value of Ar is zero.When the planar anisotropy (Ar) values of both alloys given in Table -2 are compared, the AlFeSiMn alloy which has Ar value smaller than the Ar value of AlFeSiMn alloy is indicates less earing during deep drawing,hence better performance in forming of AlFeSiMn alloy is expected. When the results of Swift and Erichsen tests given in Table -2 are compared, Swift and Erichsen values of AlFeSiMn alloy are little higher than the values of AlFeSi alloy. This result also indicates that AlFeSiMn alloy foil has better formability than AlFeSi alloy foil. Tensile, Erichsen and Swift test are mechanical tests which give indirect information about formability.These tests can compare materials in their forming performances, but can not give information what type of sheet has to be used or what has to be done for a succesfull forming operation. To overcome these,the forming limit diagrams of these two alloys are obtained given in Figure 1. The forming limit of AlFeSiMn alloy is above the forming limit of AlFeSi alloy as shown is Figure 1. This means that AlFeSiMn alloy has better formability than AlFeSi alloy. When one takes into account the result of tensile, Erichsen and Swift tests, beside forming limit diagrams of these two alloys, it is concluded that AlFeSiMn alloy foil will show better performance in forming than AlFeSi alloy foil.
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ÖgeIsıl püskürtme süreci için çalışma parametrelerine bağlı basitleştirilmiş model(İTÜ Fen Bilimleri Enstitüsü, 2010)Günümüzde yüksek kapasiteli üretim hızı ile kalın kaplamaların sahada ve otomatize üretim hatlarında uygulanabilmesine olanak sağlayan kaplama yöntemleri arasında ?ısıl püskürtme? yöntemleri dikkat çekmektedir. Isıl püskürtme yöntemleri arasında süreç kontrolü bakımından ?plazma? püskürtme yöntemi oldukça avantajlıdır.Yıllardır teknik olarak mevcut olmasına rağmen ilk günden beri deneysel ve sezgisel yöntemler ile işletilmekte olan kaplama prosesleri, insanların zorlayıcı çevre şartları altında yüksek özellikli alaşım kaplama ihtiyaçlarının artması sebebi ile genişleyen bir uygulama alanı bulmaktadır. Bu ihtiyaçlara yönelik daha hızlı ve daha kontrollü kaplamalar artık bilgisayar modelleme araçları kullanılarak üretilmeye çalışılmaktadır.Plazma püskürtme prosesinde yapılan modelleme çalışmalarında tabanca'nın nozül tasarımı, plazma arkının karakterini belirleyen katod ? anot yerleşimleri, toz enjektörü ve tabanza ağzına takılan ek aparatlar ele alınmıştır.Bu çalışmada ilk aşamada plazma püskürtme yöntemi ile itriya stabilize zirkonya tozları kullanılarak kaplamalar gerçekleştirilmiş ve kaplama esnasında tozların hız ve sıcaklıkları kaydedilmiştir. İkinci aşamada plazma püskürtme tekniğini baz alan bilgisayar modeli oluşturulmuş ve elede edilen verilerle kıyaslanmıştır.
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ÖgeThe effect of cleaning chemicals on spatter phenomena at laser welding of stainless steels(İTÜ Fen Bilimleri Enstitüsü, 2010)Benzinli enjektörler paslanmaz çelik olan parçalardan oluşmaktadır. Lazer kaynak ise yüksek hız ve otomasyon özelliğinden dolayı benzinli enjektörlerin parçalarının birleştirilmesinde kullanılmaktadır. Lakin, lazer kaynak esnasında görülen sıçrama gibi kalite hataları enjektörlerin performansını etkilemektedir. Yıkama prosesi ise kaynak bölgelerinin yüzey özelliklerini değiştirdiği için bu tür hataların oluşumunda önemli rol oynamaktadır.Bu çalışmada, yıkama prosesinin etkisini görmek için farklı tipte paslanmaz çelik iki enjektör parçası iki çeşit yıkama kimyasalıyla yıkandıktan sonra lazer kaynaklanmıştır. Deneysel çalışmalar üç aşamada yapılmıştır. İlk olarak örnekler hazırlanmış denemeler yapılmıştır. Karakterizasyon aşaması ise son kademedir. Yıkama kimyasalları olarak yağ alma kimyasalı ve korozyon inhibitörü kullanılmıştır. Lazer kaynak işleminde ise Nd:YAG lazer tipi kullanılmıştır. Kaynaklı parçaların karakterizasyonunda yıkama kimyasalları ile sıçrama hatası arasındaki ilişkiyi anlamak için kimyasalların buharlaşma hızları ölçülmüştür ve XPS (X- ışını fotoelektronik spektrometresi) ile GDOES ( Akkor boşalımı optik emisyon spektrometresi ) analizleri yapılmıştır. Örnekların hazırlaması ile denemeler fabrika koşullarında, RBTR (Robet Bosch Türkiye) benzinli enjektörler fabrikasında yapılmıştır. Karakterizasyon çalışmaları İTÜ MMM (İstanbul Teknik Üniversitesi Metalurji ve Malzeme Mühendisliği) departmanında yapılmıştır.
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ÖgeÇeliğin yarı-katı halde şekillendirilmesinde kullanılacak kalıpların fiziksel buhar biriktirme (FBB) yöntemiyle kaplanması(İTÜ Fen bilimleri Enstitüsü, 2010)Metalleri yarı-katı halde şekillendirme prosesi, klasik dövme ve döküm işlemlerinin ayrı ayrı avantajlarını tek başına sunan yeni bir teknolojidir. Dendritik örgüsü özel yöntemlerle bozulmuş ve katılaştırılmış malzemenin katı ?sıvı aralığa ısıtılması ve metal enjeksiyon işlemine benzer şekilde basınçla şekillendirilmesi esasına dayanan bir yöntemdir. Önşekillendirme işleminden geçen sonra tekrar ısıtılan yapı, katı bir kütle gibi davranırken aynı zamanda sıvı gibi kolayca kalıp boşluklarını doldurmaktadır. Düşük kuvvet ve sıcaklıklarla, üstün mekanik özellikli aynı zamanda son şekle yakın parçaların üretilebiliyor olması bu teknolojiyi önemli kılmaktadır. Teknoloji; aluminyum, magnezyum alaşımları gibi ergime derecesi düşük alaşımlar için başarı ile endüstriyel boyuta taşınmıştır, aynı şekilde çeliğe uygulanması fikri ile çalışmalar bu yöne kaymıştır. 1980 sonlarında bu konuda ilk adımlar atılmış ve çalışma grupları oluşturulmuştur. Çelik ile çalışmanın en büyük zorluğu 1250 °C leri aşan proses sıcaklıklarına dayanacak nitelikli takım malzemelerinin olmayışıdır, prosesin endüstriyel boyuta gelmesi için bu sorunun çözülmesi gerekmektedir. Kullanılacak kalıp malzemesi yüksek sıcaklıklarda; mekanik, termal ve kimyasal yüklere dayanıklı olmalıdır. Klasik sıcak iş takım çeliği proses sıcklığında temper direncini tamamen kaybettiği için yetersiz gelir. FBB kaplamalar endüstride bir çok uygulama alanı bulmuştur ve bu uygulama içinde denemeler yapılmaktadır. Dövme kalıplarında yaygın kullanımı olan X32CrMoV3-3 sıcak iş takım çeliği; katodik ark fiziksel buhar biriktirme tekniği ile AlTiN ve AlTiON kaplanmıştır . Kaplama performansları, şekillendirme şartlarını simule edecek şekilde yapılan testler sonrası kıyaslanmıştır.
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ÖgeThe effect of surface roughness on mechanical behavior of commercially pure titanium implants produced by selective laser melting(Institute of Science and Technology, 2018-06-07)Implants produced by selective laser melting (SLM) have differentiating surface roughness caused by the process itself and applied surface treatments. Since surface quality and mechanical properties are critical parameters for implants and surface roughness is a known factor for stress concentration, it is aimed to investigate the effect of surface roughness on commercially pure titanium implants produced by SLM. Surface roughness is affected by several process parameters but this study focuses on the effect of position and orientation on surface roughness. First step is specified as determining the roughness measurement method ideal for this work. Therefore, 3 different roughness measurement methods as in confocal microscopy (CM), tactile profilometer and scanning electron microcopy (SEM) are compared by using the most common surface roughness parameter, Ra. Secondly, in order to determine the effect of position and orientation on surface roughness and determine roughness ranges across the build chamber, a build is designed with parts at 15 different positions with 6 different orientations that are specified considering the location of laser. Finally, for mechanical characterization, tensile bars are designed considering both the standards stated for tensile and fatigue test. Some parts are also post processed to see the post processing effect. For every position, 5 samples are subjected to tensile testing with 5 MPa pre-load until the part failure so that the average yield stress and UTS for every position are determined. Calculated average yield stresses are used to calculate fatigue test input. 5 stress levels are determined and 2 samples for low cycle, 3 samples for high cycle fatigue data are tested with load controlled, tension-tension fatigue test set-up with 60Hz frequency and R of 0.1. Surface finishing is differentiated by MPP and SB in addition to as-built form. Some samples are post processed with the standard post process of Materialise. Sand blasting and anodization are applied while for SB samples only sandblasting is applied. Then samples are tested to see pp effect on surface roughness and mechanical behavior. When the results are analyzed considering the roughness range and sample amount, the ideal roughness measurement method is determined as tactile profilometer because of its capability, repeatability, practical application and effectiveness if time and cost. The roughness range across the build plate is quantified for different positions and measured Ra values are in the range of 7 to 24 µm across the build plate. It is also concluded that at the right bottom side of the build chamber, and for the orientations perpendicular to the laser beam, surface roughness increases. Relation between Rz and thickness is specified and an equation is suggested to eliminate the effect of roughness on thickness. Since the thickness is effective on cross section calculations used for mechanical characterization, suggested equation is used to recalculate stress values measured with tensile and fatigue tests. Even though measured tensile and fatigue results indicate that increased surface roughness has a negative effect on tensile strength and fatigue life, recalculated tensile and fatigue results display no difference occurs with varying surface roughness. Therefore, it is shown that surface roughness has an effect on thickness hence has an effect on cross section that is affecting fatigue and tensile test results but it does not have a real significant effect on mechanical behavior of SLM printed cpTi parts.