Characterization of ultrafine grained AA 6082 Aluminum alloys modified with Zr-Sc and processed by Equal Channel Angular Pressing

Günümüz dünyasındaki global yarış, malzeme mühendisliği alanında da yeni teknolojileri ve üretim yöntemlerini beraberinde getirmektedir. Bu kapsamda geliştirilen yöntemlerden bir tanesi Aşırı Plastik Deformasyon(APD) tekniğidir. APD tekniklerinden en popüler olanı Eş Kanallı Açısal Ekstrüzyon/Pres-EKAE/P (Equal Channel Angular Extrusion/Pres) yöntemidir. Alüminyum alaşımları ise APD uygulamalarında en çok kullanılan alaşımdır. Alüminyum alaşımlarının üzerinde yoğunlaşmanın en önemli nedenlerinden biri, EKAP sonrası yapılacak yaşlandırma ısıl işlemleri ile mekanik özelliklerinin en üst seviyelere taşınabilmesidir. Ancak yüksek deformasyon sonrasında yapı, deformasyon enerjisinden dolayı kararsız hale geçmektedir ve özellikle aşırı plastik deformasyonlardan sonra yapılacak yaşlandırma ya da yüksek sıcaklıkta şekillendirme (süperplastisite) işlemlerinde hızlı bir şekilde yeniden kristalleşmeye uğramaktadır. Bundan dolayı alüminyum alaşımlarında yeniden kristalleşmeyi önlemek ve yüksek sıcaklık proses şartlarından etkilenmemesini sağlamak amacıyla Zr ve Sc gibi yeniden kristalleşmeyi geciktiren alaşım elementleri ilavesi yapılması gerekmektedir. Bu çalışmada EKAP tekniği kullanılarak AA 6082 ve AA 6082-Zr-Sc alaşımları aşırı plastik deformasyona tabi tutulmuş ve daha sonra EKAP işlemi görmüş alaşımlara farklı sıcaklıklarda yaşlandırma ısıl işlemleri yapılmıştır. Sonuç malzemelere mikroyapı karakterizasyonu yapılarak EKAP ve yaşlandırma ısıl işleminin etkileri incelenmiştir. EKAP sonunda her iki malzemenin ulaştığı sertlik değerleri aynı olmuştur. Zr ve Sc içeren alaşımın yüksek sıcaklıklarda dahi EKAP sonrası ulaştığı yapıyı ve sertlik değerlerini koruduğu, Zr ve Sc içermeyen AA 6082 alaşımının ise düşük sıcaklıklarda yeniden kristalleşmeye uğradığı tespit edilmiştir. Zr ve Sc ilavesinin EKAP öncesi ve sonrasında mikroyapının gelişiminde pozitif yönde etkili olduğu belirlenmiştir. Anahtar Kelimeler: Aşırı plastik deformasyon, eş kanallı açısal pres, alüminyum alaşımları, AA 6082 alüminyum alaşımı, Zr ve Sc ilavesi.Today’s global competitions bring together new technologies and production methods in materials engineering. In the recent years, in parallel to development of nano technologies, intensive studies are made on nano structured materials and their production methods. In these studies, new production methods are developed for producing nanomaterials, which exhibits numerous superior properties. One of these new production methods is severe plastic deformation. Severe plastic deformation (SPD) has emerged as a promising method to produce ultrafine-grained materials with attractive properties and today the SPD techniques are rapidly developing and are moving from lab-scale research into commercial production. Equal Channel Angular Pres (ECAP) is an effective method of enhancing the strength of metallic alloys through (sub) grain refinement to the submicrometer level by introducing intensive plastic deformation into materials through repetitive pressing. In the past several decades, the aluminum alloys of 6xxx group have been studied extensively because of their technological importance and their exceptional increase in strength obtained by precipitation hardening. AA 6082 alloy is one of the most popular alloys that are widely used for medium strength structural and automotive industry applications. In this study, due to its wide range of application, AA 6082 was used and ECAP was chosen as a severe plastic deformation technique. The measured mechanical properties were compared with the commercially produced AA 6082 alloy. Furthermore, the effect of Zr and Sc additions to the AA 6082 alloy were investigated. The aim of this study is to produce ultrafine-grained AA 6082 aluminum alloys that will be an alternative to commercially produced 6XXX alloys. The aluminum samples were subjected to ECAP process at room temperatures using the Bc route, which means rotation of the samples, is 90o after each pass for a maximum 4 passes. MoS2 was used as lubricant to reduce friction between the samples and die. The two intersecting channels of the ECAP die had an angle of Φ: 90o and the outer arc of curvature of ψ:20o. This configuration induces an equivalent strain of approximately 1 after each pass. ECAP processed and post-ECAP aged samples was examined by transmission electron microscope (TEM). Samples were prepared by longitudinally cutting 1 mm thick disks from the billets, manually grinding and polishing. Twin jet electrolytic thinning was then carried out at –35°C with a 30% HNO3 solution in methanol at 18 V. Differential scanning calorimetry (DSC) was employed to identify the precipitation reactions. The cell was equilibrated at 25ºC and then heated to 600ºC at 10?C/min in a dynamic argon atmosphere (1 l/h). Pre-ECAP solid-solution treatment combined with post-ECAP aging treatment has been found to be effective in enhancing the strength of AA 6082 aluminum alloys. An increase of 33% in Vickers hardness was obtained in the post-ECAP aged material in 100oC-8h (140 HV) compared to the T6 treated commercial AA 6082 Aluminum alloy (105 HV). At higher aging temperature (more than 100oC for AA 6082 and more than 180oC for AA 6082-Zr-Sc after 1st pass), the both alloys showed an increasing softening with time due to recovery or/and grain coarsening. Deformation introduced during ECAP process was shown to have a big impact on the DSC analysis of both alloys. Deformation accelerated the precipitation of Mg2Si particles. The precipitation of the β” and β’ phases occurred at significantly lower temperatures with increasing ECAP strain. The presence of Sc did not affect significantly the precipitation cycle occurred in the AlMgSi alloys. While AA 6082 alloys showed the recyrstallization (RX) peak in DSC runs, the RX peak did not occurred in the AA 6082-Zr-Sc alloys due to the effect of the recrystallization retarding elements. Zr and Sc containing AA 6082 alloy showed a greater structure stability compared to commercial AA 6082 alloys.  Keywords: Severe plastic deformation, ECAP, aluminum alloys, AA 6082, Zr and Sc additions

Development of anisotropy coefficient (r-value) for the cold rolled if steels

Otomotiv endüstrisinde iç ve dış panellerin üretiminde kullanılan, çok az miktarlarda karbon ve azot içermeleri nedeniyle pres altında şekillendirmeye uygun çeliklerdir. Arayer atomsuz çeliklerde, şekillendirilebilme özelliğini olumsuz yönde etkileyen arayer atomlarının (C ve N), çelik üretim prosesi esnasında bileşimden kontrollü bir biçimde uzaklaştırılması Titanyum (Ti) ve/veya Niobyum (Nb) atomlarının C ve N atomları ile reaksiyona girerek çökelti fazları oluşturması ile olmaktadır. Sıcak haddeleme prosesinde slab fırınında belli sıcaklığa kadar ısıtılarak çökeltilerin çözeltiye alınması sağlanır. Bu işlem nihai sıcak band dokusunu ve tane büyüklüğünü belirleyici ilk prosestir. İkmal ve sarılma sıcaklıkları da arayer atomsuz çelik üretiminde önemli olan proses parametrelerdir. Her iki proses parametresi de çökelti ve tane büyüklüğünü önemli ölçüde etkileyerek çeliğin nihai mekanik özellikleri üzerinde büyük oranda etkili olmaktadır. Şekillendirilebilme özelliğinin önemli bir göstergesi olan r değerinin arttırılmasında en önemli parametre soğuk ezme oranıdır. Artan tavlama sıcaklığı tane büyüklüğünün artmasında etkili olmaktadır. Çelikhane prosesinden itibaren sıcak haddeleme ve soğuk haddeleme prosesleri arayer atomsuz çeliğin mekanik özellikleri üzerinde etkili olmaktadır. Endüstriyel ölçekte gerçekleştirilen bu çalışmada, çelik kimyasal bileşiminde bulunan Ti ve Nb elementleri ile soğuk haddeleme ve sürekli tavlama parametrelerinin optimizasyonu sonucunda mekanik özelliklerde oluşan değişimler irdelenmiştir. Yapılan çalışmaların sonucunda Ti+Nb elementleri içeren kompozisyonlarda, yüksek ezme oranları (%80) ve tavlama sıcaklıklarında (870°C) yüksek r değerleri elde edilmiştir. Otomotiv sektörünün önemli oranda kullandığı arayer atomsuz çeliklerin mekanik özelliklerinde iyileştirmeler sağlanarak endüstrinin kullanımına sunulmuştur.Anahtar Kelimeler: IF Çelikleri, soğuk haddeleme parametreleri, r değeri, mekanik özellikler.Interstitial free steels (if steels) are very suitable material for automotive industry and they are used for inner and outer body panel applications due to their high formability properties. They have extremely high formability characteristics under forming presses. Interstitials damages formability characteristics of steel. Elimination of interstitials (C and N) is achieved through careful control of the steelmaking process by the addition of titanium and/or niobium to react with carbon and nitrogen to form precipitates. In the hot rolling process, firstly, slabs are heated upto some temperature and those precipitates are taken into solid solution. This process affects grain size and texture of hot rolled product. The other important process parameters in hot rolling are; finishing temperature and coiling temperature. Both parameters mainly affect grain size and size of the precipitates in hot rolled coil.and thus mechanical properties are affected. The most important parameter for formability is r value. And it is mainly affected by cold deformation.ratio. Also increasing annealing temperature increases grain size and results in higher formability. All the process parameters through steelmaking to the cold rolling process affects mechanical properties of interstitial free steels and thus affects r value of interstitial free steel. In this study, mechanical properties related with steel chemical composition by optimizing Titanium (Ti) and Niobium (Nb) contents and continuous cold rolling and annealing parameters were investigated in industrial scale. As a result, the highest r values were obtained at 80 % cold deformation ratio and annealing at 870 C for steel with Ti+Nb alloying addition. In this study, experiments were done in industrial scale. Firstly, interstitial free steels containing titanium and titanium+niobium were produced at steel shop. And then slabs produced at continuous casting machines were hot rolled After hot rolling process, those coils were sent to cold mill for cold rolling and annealing process. Also some coils were sent to galvanising line after cold deformation. Each process are controlled by high level process computers in Erdemir plants. Therefore process parameters for trial coils are taken from this computer results. Chemical analysis of heats were done at steel shop chemical laboratories and they were produced according to instructions given by metallurgical department. Mechanical test samples were taken after continuous annealing line and continuous galvanising line processes. Mechanical properties including yield strength, tensile strength, and elongation at fracture strain hardening exponent and anisotropy coefficient were determined by mechanical tensile tests. Tensile specimens were cut along rolling direction, transverse direction and 45° to the rolling direction. Samples were taken from the middle section of each coil. Tests were done on Zwick Z-100 testing machine. All tests were made at a strain ratio of 2 % Lo/min. The tensile test samples were processed according to DIN EN 10002-1 type 2 standards.The effect of process parameters on mechanical properties were investigated according to these results. Optical microscopy was used for the microstructural examinations and grain size measurements of the samples. Microstructural examinations were conducted on the cross sections perpendicular to the rolling direction. Microstructures of materials including ferrite grain size, carbide type and distributions were investigated with a leco 500 optical microscope. Also scanning electron microscopy was used for the determination of the microstructure and carbide distribution of the samples. The scanning electron microscope was a jeol 5600 jsm type machine. As a result of microstructure analysis effect of process parameters on microstructures analysed. In this study, texture analysis was conducted with a rigaku d-max ultima x-ray diffractometer with a pole figure attachment. Test samples were prepared from annealed samples, punched to a 4mm diameter and then metallographically prepared. The next step was data collection from an x-ray diffractometer. The xrd measurements of samples were carried out in the range of 20-90 2teta degrees with 0.02 steps. Then pole figure measurements were done with different alfa and beta angles in 110, 111 and 200 diffraction lines. As a result of that study, the best composition of interstitial free steel containing titanium and niobium were determined by optimization of titanium and niobium content. Also the best process parameters at the cold mill determined through analysis of results taken from mechancal tests and microstructure analysis in which the highest r value obtained. And interstitial free steel produced after that study put into use for the automotive market as a new quality of steel.Keywords: Interstitial free steel, cold mill process parameters, r-value, mechanical properties

The performance effects by surface treating for worn parts in the glass forming machines

Yüzey işlemleriyle malzemenin sertlik, süneklik ve yorulma gibi mekanik özellikleri yanında sürtünme ve aşınma, oksidasyon ve korozyona karşı dayanıklılık özellikleri geliştirilmektedir. Sürtünerek çalışan makina elemanlarında belirli bir süre sonra ortaya çıkan aşınma problemlerini azaltmak için birçok yüzey iyileştirme teknikleri uygulanmaktadır. Yaşanan malzeme ve ürün kayıplarını, cam şekillendirme makinalarında minimuma indirmek ve söz konusu bu nedenlerden kaynaklanan duruş ve üretim kayıplarını azaltmak amacı ile gerçekleştirilen bu çalışmada; cam üretim makinalarında kullanılan kalıp kollarının malzemelerine alternatif malzeme ve kaplama türleri denenmiştir. Cam şekillendirme makinalarında mevcut kalıp kolu malzemesi olan AISI 420 kalite çeliğe alternatif olarak seçilen yüzeyi kaplanmış (nitrokarbürleme, sert krom kaplama, akımsız nikel kaplama ve borlama) üç farklı kalitede çelik (AISI 1040, AISI 4140 ve AISI 5140 kalite çelikler) malzeme, kalıp kollarındaki gerçek aşınma şartlarını simule edebilecek şekilde tasarlanan laboratuvar deneyleri için standart numuneler olarak hazırlanmıştır. Hazırlanan numunelere, metal-metal deneyleri yapılarak aşınma davranışları incelenmiştir. Oda sıcaklığında yapılan deneylerde nem ve sıcaklık sabit tutulmuştur.  Metal-metal aşınma deneylerinde, pimlere M2 kalite takım çeliği üzerinde aşınma testi uygulanmıştır. Farklı kalitede çelikten imal edilmiş olan kaplanmış taban malzemelerine uygulanan disk üstünde pim deneyi sonucunda kullanılmakta olan sert krom kaplı ve kaplamasız AISI 420 kalite çeliğin bor kaplı AISI 4140 kalite çelik malzemeye göre aşınma oranının sırasıyla yaklaşık 18.67 ve 184 kat daha çok olduğu tespit edilmiştir. Laboratuvar deneylerinin sonucunda AISI 4140 kalite çelik malzeme üzerine borlama ön plana çıkmış ve kalıp kollarının çalışma koşullarında aşınmaya en dayanıklı kaplama ve taban malzemesi olarak borlanmış AISI 4140 kalite çelik kabul edilmiştir.  Anahtar Kelimeler: Borlama, cam şekillendirme makineleri, kalıp kolu.The materials science in our country has provided significant developments in recent years. Glass industry has also needed to develop its glass forming machines and their parts for manufacturing the high quality with low price in this competitive environment for being in line with the demands of customers. As a result of this change in the business environment, the selection of materials being used in glass forming machines and the surface treatments applied on these materials have developed in accordance with the technology. Friction, wear, oxidation and corrosion properties of materials can be improved in addition to an increase of the ductility, hardness and fatigue properties of materials by using surface treatement. At glass forming machines, the mechanism which provides the mould to adapt into the glass machine, is called as the arm mould. Although mould and arm mould materials show similar properties, the responsibilities they undertake are totally different from each other. If the expectation from mould material can be described as resistivity to heat and surface roughness at metal glass interface, then the expectation from arm mould can be limited as resistivity to wearing and general suitability to the machine's material. The aim of this study to find an alternative material to the arm mould material as coated or uncoated conditions which are used at the glass forming machines in order to decrease the loss of discarded materials and product at glass machines. As alternatives to the existing arm mould material AISI 420, various kinds of steels (AISI 1040, AISI 4140, AISI 5140) were coated with different kinds of coatings as nitrocarburizing, hard chrome coating, boronizing and electroless nickel coating. To simulate the real working condition of arm moulds at the laboratory conditions, standard specimens are prepared for pin on disc tests. Specimens were at standard of ASTM G99 with a diameter of 4 mm. Specimens and disc are always cleaned at each test and dry friction conditions are provided. During laboratory tests, humidity and room temperature is controlled at %50±5 and 20±2oC respectively. At metal-metal tests, specimens as a pin are fixed on a steel disc with a fixed load where the disc rotates at a constant speed. Tests are applied on M2 steel disc with 0.52 m/s sliding speed, under 5 kg load and at a total sliding distance of  6000 m. Before and after each wear test, the differences at specimens in terms of height (H) and weight (G) are measured with a sensitivity variance of 0.01 mm and 0.1 mg respectively at every 2000 m sliding distance. Steel specimens without any surface treatment have surface hardness values in the range of 170 and 210 HV. After the surface treatment (nitrocarburizing, hard chrome coating and electroless nickel coating) processes, these specimens surface hardness increased up to 800 HV. However, after the boronizing process, specimens surface hardness increased to the higher values up to 1110 - 1495 HV. The results of wear tests are analyzed according to the weight loss of the materials. The relative wear ratio (RWR) is defined as the ratio of the weight loss of a specimen to the weight loss of the reference material, which is defined as the most wear resistant specimen at the wear tests. The most wear resistant material was found as the boronized AISI 4140 steel specimen from the weight loss values. mFrom the results of the pin on disc tests, the specimens weight losses are calculated to identify the relative wear ratio values. Non-coated AISI 420 specimen had the weight loss of 0.0552 gr, hard chrome coated AISI 420 specimen had the weight loss  of 0.0056 gr, boronized AISI 4140 specimen had the weight loss  of 0.003 gr at 2.000 m sliding distance. From the comprarison of the RWR values of the studied steels, it is found that the boronized AISI 4140 steel is the highest wear resistance material. The RWR values show that the boronized AISI 4140 steel is 18.7 times better than hard chrome coated AISI 420 steel, and 184 times better than uncoated AISI 420 steel in terms of wear resistance. As the results of the wear tests, the boronized AISI 4140 steel is determined as the most wear resistant material in the studied steels and suitable to use as the arm mould material in the glass forming machines. Keywords: Boronizing, glass forming machines, arm mould

Effect of bias voltage modes on the mechanical properties of PVD ZrN coatings

Zirkonyum Nitrür (ZrN) kaplamalar endüstriyel alanda yaygın uygulaması olan diğer fiziksel buhar biriktirme (FBB) kaplamalar ile kıyaslandığında, düşük yüzey sürtünme katsayısına, az sayıda metalik damlacık dağılımına ve düşük yüzey pürüzlülüğüne sahip olmasına rağmen, görece düşük sertliği ve kötü yapışma mukavemeti sebebiyle yaygın uygulama alanına sahip değildir. Genel kullanım alanı sadece demir dışı yumuşak metal ve alaşımların (alüminyum, bakır, pirinç vb.) talaşlı şekillendirilmesi prosesleriyle kısıtlıdır. Bazı özel üretim proseslerinin kullanımıyla, ZrN’in sürtünme katsayısı arttırılmaksızın, kaplama sertliği ve yapışma mukavemeti arttırılabilirse, ZrN kaplamalara yeni kullanım alanları oluşturmak mümkün olabilecektir. Özetle, eğer hem yüksek sertliğe ve yapışma mukavemetine, hem de düşük sürtünme katsayısına sahip bir kaplama üretilebilirse bu ideal bir kaplama olur ve endüstriyel uygulamada yaygın olarak kullanılabilir. Böyle bir kaplama geliştirmek amacıyla FBB kaplama proses parametrelerinin etkilerinin incelendiği bu çalışmada, öncelikle ZrN kaplamaların üretiminde azot kısmi basıncının optimizasyonu yapılmış, sonra belirlenen azot kısmi basıncı değerinde doğru akım bias, tek kutuplu değişken bias ve asimetrik çift kutuplu değişken bias voltajları uygulanarak elde edilen kaplamaların yapısı ve kalınlık, sertlik ve yapışma mukavemeti gibi mekanik özelliklerindeki değişimler incelenmiştir. Deneysel çalışmalar sonucunda, 8 mtorr azot kısmi basıncında, -200 volt asimetrik çift kutuplu değişken bias voltajıyla üretilen FBB ZrN kaplamanın en iyi mekanik özelliklere sahip olduğu tespit edilmiştir. Anahtar Kelimeler: FBB kaplamalar, ZrN, doğru akım bias, tek kutuplu değişken, asimetrik çift kutuplu.  When compared to the other Physical Vapor Deposition (PVD) coatings which are extensively used in industrial applications, Zirconium Nitride (ZrN) coating does not have a wide application in industry because of its relatively low hardness and low adhesion strength despite its low friction coefficient. Its common application field is restricted by machining soft and ductile non-ferrous metals and alloys like aluminium, copper, brass etc. under well-cooled machining conditions. Although surface friction coefficient of a material changes due to its surface roughness, ambient temperature and lubrication regime; when all the simulation conditions are fixed, it would be a physical material property that is variable characteristically for each material surface. The low friction coefficient of the polished surface of ZrN coating at room temperature and non-lubricated conditions enhances adhesive wear resistance of ZrN coated tools and considerably reduces the built-up edge formation and the material loss during machining and forming processes. Therefore ZrN coating has a significant effect to improve the wear resistance of the tools especially used in adhesive wear conditions. Furthermore, ZrN coating has lower surface roughness and less droplet (macroparticle) distribution than Titanium Nitride (TiN) and Chromium Nitride (CrN) coatings which are widely used in the industry; and this is also an important effect on reducing friction. However, ZrN coating has some unfavorable properties such as relatively low hardness and low adhesion strength. If the coating hardness and adhesion strength of ZrN coating could be improved without altering its friction properties by performing some special processes, it would be possible to extend its usage in the new applications. Briefly, if a coating which has both a high hardness and low friction coefficient and also a high adhesion strength can be produced; it would be an ideal coating which would have both adhesive and abrasive wear resistances and can be extensively used at all processes in industrial applications. ZrN might be an ideal coating for such a study because of its low friction coefficient and low droplet size and distribution. Furthermore, since zirconium metal has higher melting temperature and lower vapor pressure than the other cathode metals like titanium, aluminium and chromium; droplet transfer from the cathode to the coating would be at a minimum degree, so ZrN coatings contain much fewer and smaller droplets. In such a way, as a result of droplet decrease, the durability and homogenity of the coating increases and the surface roughness decreases. The aim of this study is to enhance the properties of ZrN coating through the optimization of PVD process parameters and to expand its market usage. In the first step of the experimental studies, the reactive gas (nitrogen) partial pressure used at the PVD ZrN coating was optimized. The basic mechanical properties such as coating hardness, coating thickness and adhesion strength of ZrN films deposited in various nitrogen partial pressures were determined and compared. Coating hardnesses were measured by using a Micro Vickers test device, coating thicknesses were measured by using Calotest ball cratering method and coating adhesion strengths were measured by using a scratch tester. Furthermore a standard Rockwell-C hardness measurement method was used for determining coating adhesions comparatively. As the result, the coating produced under 8 mtorr nitrogen partial pressure exhibited the best mechanical properties. In the next step of the experimental studies, 8 mtorr was kept constant as the nitrogen partial pressure and a bias voltage scan was performed by applying various d.c., unipolar pulse and asymmetric bipolar pulse bias voltages. The pulse frequency was kept constant as 50 kHz for all the coatings deposited by pulse bias voltages. Related to the ion energy improvement in asymmetric bipolar pulse bias mode, a grain orientation close to single crystal structure, dense film structures, enhancement in nitrogen content and improvement on the mechanical properties such as hardness, adhesion, thickness etc. compared to d.c. and unipolar pulse bias modes were achieved. As the result of experimental studies, the coating hardness equivalent to TiN coatings and friction and adhesion properties equivalent to CrN coatings were achieved in a single coating. The best results were achieved in ZrN coatings deposited by applying 8 mtorr nitrogen partial pressure and -200 volt asymmetric bipolar pulse bias voltage value.   Keywords: PVD coatings, ZrN, d.c. bias, unipolar pulse, asymmetric bipolar

The effect of RRA treatment on the mechanical properties of 7075 alloy

Bu çalışmada 0.85 mm kalınlığındaki 7075 kalite aluminyum alaşımı sacın, sertlik, mukavemet ve darbe direnci gibi mekanik özelliklerine, retrogresyon ve yeniden yaşlandırma ısıl işleminin etkisi incelenmiştir. Başlangıçta T6 temper durumunda olan alaşıma, 220°C’de 15 saniye – 60 dakika arasında retrogresyon uygulanmış, yeniden yaşlandırma 120°C’de 24 saat süreyle yapılmıştır. Retrogresyon ve yeniden yaşlandırma işlemi sonunda, sertlik önce artarak en yüksek değerine ulaşmış, daha uzun retrogresyon sürelerinde ise azalmıştır. Akma ve çekme mukavemeti ise retrogresyon süresine bağlı olarak azalırken, kopma uzaması ve darbe direncinde, retrogresyon süresine bağlı olarak sürekli bir artış görülmüştür. Sonuç olarak, retrogresyon ve yeniden yaşlandırma ısıl işlemiyle, belirli retrogresyon sürelerinde, T6 temper durumu seviyesine eşit ya da daha yüksek sertlik, mukavemet, süneklik ve tokluk elde edilmiştir.Anahtar Kelimeler: Aluminyum, retrogresyon ve yeniden yaşlandırma, çökelme sertleşmesi.In this study, the effect of retrogression and reageing heat treatment on mechanical properties such as hardness, strength and impact resistance of 0.85 mm thick 7075 quality aluminium alloy sheet has been investigated. The alloy, which was initially in T6 temper condition, was retrogressed at 220°C for various times between 15 seconds and 60 minutes, prior to reageing at 120°C for 24 hours. Hardness and electrical conductivity were monitored as an indicator of the microstructural changes during retrogression or reageing stages. Hardness was initially decreased giving a minimum as the retrogression time increases. Further retrogression times beyond increment in hardness resulted in a decrement of hardness again. Electrical conductivity was progressively increased as the retrogression time increased. Hardness of retrogressed and reaged samples initially increased as the retrogression time increased, exhibiting their maximum values, then finally decreased. Yield and tensile strength progressively decreased while elongation at fracture and impact resistance increased with increasing retrogression time. It is concluded that, equal or higher values of hardness, strength, ductility and toughness than those of T6 temper can be obtained with retrogression and reageing heat treatment. Improvement in hardness can be attributed to the increased number of precipitates in retrogressed and reaged microstructure. On the other hand, higher values of impact resistance are accompanied with overageing, which commences in longer retrogression time.Keywords: Aluminium, retrogression and reageing, precipitation hardening

Effect of Hf addition on reciprocating wear performance of ZrN coatings

Bu çalışmada, ZrN ve (Zr, %12 Hf)N kaplamaların aşınma davranışları incelenmiştir. Kaplamalar Zr ve %21Hf alaşımlı Zr katotların kombinasyonu ile üretilmiştir. Üretilen kaplamaların aşınma deneyleri, karşılıklı kazımalı aşınma deney cihazında, 1.5 N sabit yük altında ve 0.02 mm/s deney hızında 10 mm çapındaki Al2O3 bilyenin numune yüzeylerine sürtünmesi ile gerçekleştirilmiştir. Deney sonrası numune yüzeyinde oluşan aşınma izleri, yüzey profilometresi yardımı ile incelenmiş ve deney sonuçları aşınma iz alanları ölçülerek mm2 cinsinden alan kaybına göre değerlendirilmiştir. Deney sonuçları, (Zr, %12 Hf)N kaplamanın ZrN kaplamaya göre daha az aşındığını ve Hf ilavesinin aşınmaya karşı direnci artırdığını  göstermiştir.Anahtar Kelimler: Kaplama, FBB, ZrN, Hf, aşınma.In this study, a commercial ZrN and new (Zr, Hf)N coatings by alloying ZrN with Hf were deposited on hardened AISI D2 quality cold work tool steel plates by arc PVD technique. The coatings were prepared using combination of two cathodes under the equvalent PVD process conditions. ZrN and (Zr, %12 Hf)N coatings were obtained by utilising two Zr cathodes and one Zr and one Zr+21% Hf cothodes, respectively. The characterisation of the coatings were made by Scanning Electron Microscopic (SEM) examinations, X-ray diffraction analyses (XRD), hardness and adhesion tests as well as roughness and thickness measurements. Hardness measurements were carried out ultramicrohardness tester with a Vickers pyramid indenter. The indentation procedure consisted of 60 steps, with a waiting period between consecutive steps of 1 s. During the tests, load (P)-indentation depth (h)-time (t) data were recorded. Hardness measurements were performed under constant load of 25 mN. In order to investigate adhesion properties of the ZrN and the (Zr, 12% Hf)N coatings, scratch tests were used. The coatings were scratched up to 100 N normal load by using 10 mm/min scratching speed and 100 N/min loading speed. The dry sliding wear behaviour of ZrN and (Zr,12%Hf)N were examined by a reciprocating wear tester in ambient atmoshper. Alloying of ZrN coating with 12% Hf did not change the hardness significantly, but achieved an improvement on adhesion strength and wear resistance.Keywords: Coating, PVD, ZrN, Hf, wear

Effect of Parameters to the Coating Formation during Cold Spray Process

Abstract -In this study, the effects of parameters such as substrate hardness, substrate surface roughness and stand-off distance to the coatings formation and properties of the coatings during cold spraying process were examined. For this purpose, coatings were deposited onto the copper substrate with using of commercially available copper powder. Substrate hardness of 55 HV, 107 HV and 140 HV were obtained by heat treatment and deformation hardening methods. Different substrate surface roughness was obtained by using of SiC emery papers which have different grits and stand-off distance during cold spray process was manipulated as 5, 10, 20 and 30 mm. After the production of the coatings, characterisation procedures were performed by microstructural observations which are optical and scanning electron microscope surveys, porosity concentrations, coating thickness and hardness measurement