Elmaslı kesicilerde kullanılan matriks malzemelerinin serbest sinterlenmesi
Free sintering of matrix materials which are used for diamond cutting tools
Yazar: YUSUF KAHRAMAN
Danışman: PROF. DR. EYÜP SABRİ KAYALI
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Doğal taşların kesiminde çeşitli kesici takımlar kullanılmaktadır. Bunlardan biri de doğada bilinen en sert bilinen element olan elmastır. Yapay olarak üretilen elmaslar matriks malzemesi içerisine gömülerek karot, halat, testere, kesme teli vb. teknolojilerle doğal taş kesim sektöründe kullanılmaktadırlar.
Genelde bu kesici takımların üretimi toz metalurjisi yöntemiyle olur. Soğuk presleme işlemini takiben yapılan sinterleme işlemleriyle matriksin mukavemetinin arttırılması amaçlanır. Üretilen kompozit malzeme amaca uygun olarak testereye, karot ucuna lehimlenir veya halat üzerine monte edilir. Elmaslı kesicinin performansını etkileyen birçok faktör vardır. Burda ana unsur matriks dayanımını elmasla optimum çalışacak şekilde ayarlamaktır. Çünkü eğer matriks malzemesi yeterince dayanıklı olmazsa elmas ömrünü tüketmeden yapıdan uzaklaşır, aksi bir durumda ise elmaslar körelir ve de takım kesme işlevini yerine getiremez. Bunun dışında kesim parametreleri, çalışma koşulları vb. birçok faktör de kesim performansını etkilemektedir.
Kesilen taş cinsine göre matriksin kompozisyonunda ve elmas konsantrasyonunda değişikler yapılmaktadır. Bununla birlikte kesilecek taşın cinsine göre plastik ya da kauçuk kaplama yapılabilmektedir. Kobalt mekanik özelliklerinden ötürü çok sık kullanılan bir matriks malzemesidir. Bununla birlikte fiyatındaki senelere bağlı dalgalanmalardan ötürü ve de stratejik bir metal olmasından sektör kobaltın özelliklerini ikame edecek değişik alaşım türleri üzerine de arayışa girmiştir.
Tel ile kesim teknolojisinin doğal taş kesim işlemlerinde bir çok avantajı vardır. Zaman ve verimlilik bu avantajların en önemlileridir. Elmaslı tel ile kesim sırasında kesme işlemini yapan boncuk diye isimlendirilen yapay elmas parçacıkları içeren metal matriksli kompozit farklı üretim yöntemleriyle üretilebilmektedir. SPS ve de HIP bu yöntemlerden bazılarıdır. SPS ve HIP yönteminin performans olarak Serbest Sinterleme'den üstün olduğu yapılan çalışmalarla sabittir ancak Serbest Sinterleme yöntemi üreticileri üretim kapasitesi, ekonomik olması ve de bir takım dizayn kolaylıkları açısından cezbetmektedir. Bu çalışmada serbest sinterleme için hazırlanmış 4 farklı kompozisyondaki hazır alaşımlar toz metalurjisi ve de serbest sinterleme fırını ile üretilmiş farklı bileşenlere sahip matriks malzemelerin karakterizasyonu yapılmıştır. Piyasada MX4885, COBALITE, NEXT 400 ve de 61390 olarak isimlendirilen alaşımlar soğuk preslenip ardından serbest sinterleme yöntemiyle sinterlenerek mekanik deneylere tabi tutulmuş ardından da karakterizasyon çalışmaları yapılmıştır.
Bu çalışmada 4 farklı kompozisyona sahip serbest sinterlemede kullanılan hazır alaşım toz metalurjisi ile üretilmiştir. Üretilen silindirik matriks malzemeleri arasında sertlik, yoğunluk, basma dayanımı, XRD, SEM ve optik mikroskop verileri arasında kıyaslama yapılmıştır.
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Yapılan çalışmalar sonucunda Fe-Co-Cu içeriğine sahip MX 4885 numunesinin elmaslı kesici boncuklarda arzulanan temel nitelikler olan basma dayanımı ve sertlik özellikleri açısından en uygun malzeme olduğuna karar verilmiştir.

Abstract:
A diamond tool is a cutting tool with diamond particles distributed on the functional parts of the tool via a bonding material,electrolysis etc. Since diamond is very hard material, diamond tools have many advantages as compared with other kind of cutting tools such as borides and carbies. Diamond dressers consist of single-point or multipoint tools brazed to a steel body, and used for dressing process. There are different kind of tools including: chemically bonded types, electroplated types , disc type. Polycrystalline diamond (PCD) has a common usage in diamond cutting tools industry. Polycrystalline diamond (PCD) is producted at High Temperature-High Pressure (HT-HP) press conditions. Most wafers are polished to a mirror finish, then cut with an electrical discharge machining (EDM) tool into smaller, workable segments that are then brazed onto the sawblade, reamer, drill, or other tool. Often they are EDM machined and/or ground an additional time to expose the vein of diamond along the cutting edge. These tools are mostly used for the machining of nonmetallic and nonferrous materials. The grinding operation is combined with EDM for several reasons. For example, according to Modern Machine Shop,the combination allows a higher material removal rate and is therefore more cost effective. Also, the EDM process slightly affects the surface finish. Grinding is used on the affected area to provide a finer final surface. The Beijing Institute of Electro-Machiningattributes a finer shaping and surface geometry to the combination of the two processes into one. The process itself is accomplished by combining the two elements from each individual process into one grinding wheel. The diamond graphite wheel accomplishes the task of grinding, while the graphite ring around the existing wheel serves as the EDM portion. However, since diamond is not a conductive material, the bonding in the PCD work piece must be ample enough to provide the conductivity necessary for the EDG process to work. Polycrystalline diamond tools are used extensively in automotive and aerospace industries. They are ideal for speed machining (9000 surface feet per minute or higher) in tough and abrasive aluminum alloys, and high-abrasion processes such as carbon-fiber drilling and ceramics. The diamond cutting edges make them last for extended periods before replacement is needed. High volume processes, tight tolerances, and highly abrasive processes are ideal for diamond tooling.
In recent years it is accepted that diamond tools are the most effective way to process natural stones. Increasing production rates and good surface finish characteristics of diamond tools make them indispensible in cutting tool industry.Since diamond has a great hardness value it can be used for different purposes such as sawing, drilling
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,cutting,polishing and grinding. As expected diamond tools such as wire saws have a common usage in cutting tool industry. Diamond is used as a reinforcing material in metal matrix.
Composition and the type of diamond can be altered due to the characteristics of natural stone. These diamond tools can be used to cut ceramics, granites,marbles,concrete etc. There are three ways to cut natural stones which are circular sawing, frame sawing and wire sawing. Sawing with circular saw consists in wearing away stone's constituents by diamond grits in metal matrix.
The diamond crystals in the metal matrix should stay for enough time so that until new diamond crystals appear diamond bead can work effectively. Aim of the manufacturer is to obtain simultaneous wear characteristics of diamond particles and metal matrix.
As it is mentioned composition, diamond type and characteristics of metal matrix should take into consideration for excellent cutting performance. Furthermore there are the other factors such as cooling efficiency, operator skills , manufacturing methods etc. which effect cutting performance directly.
Diamond bead dimensions can be changed due to the stone's characteristics but generally beads with small dimensions are desired for cutting efficiency. At present diamond beads which have diameter less than 6,3 mm are in service. There are also diamond beads with the diameter of 6.3, 7.3 , 8.5 and 11 mm.
Wire sawing has 4 major advantages over conventional solutions; flexibility, cutting speed , reduction of material waste and less polute wastes. In recent years there is an aim to work with multiwire cutting machines in cutting tools efficiency. During multiwire cutting process multiple wire ropes are set on the system that manufacturer increase cutting efficiency.
In wire sawing process, after machine is prepared for the production, vertical, horizontal and perpendicular holes are drilled on block surface. After preparation diamond wire saw passes through this holes and it is connected to the wire saw machine as a loop.
Wire is rotated with the wheel movement of the machine. Required tension, force and the other values can also be set on this machine. During cutting process water is used as a coolant at the same direction with the wire for removing particles.
During the cutting process metal matrix should be abraded in a such a way that diamonds that are in the matrix shouldn't pull out from their place before service life ends.
Matrix material and diamond should wear simultaneously. Co is commonly used for the matrix material for that purpose. Because Co has a high yield strength and good wear rate. Since cobalt is a strategic material and it's price is changable there is an aim to use some other materials in diamond tools cutting industry for that reason manufacturers are currently working on the other components which can be beneficial for natural stone cutting process.
There are also the other ways to improve cutting performance of diamond beads. Coating the diamond grits in metal matrix increases the tool life. Ti or other kind of proper coating materials can be used to improve servise life of diamond beads. Ti works for providing a good interface layer between diamond grits and matrix material. Thanks to good surface adherence tool life increases. By the way diamond size,
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concentration and type is an another thing that must be taken into consideration during the manufacturing process.
Aim of this study is to compare mechanical an microstructural properties of diamond tool materials which are prepared for free sintering process and currently in use. Most of these free sintering powders include Co as a main matrix material but in the deficiency of Co, W can be used to compansate these deficiency. Nickel is another constituent which increase mechanical properties of diamond tool.
There are 4 kinds of free sintering powders used in this study. These are named COBALITE, NEXT 400, 61390 and MX 4885. First of all 10 samples were prepared from each of these powders.
Totally 40 samples are cold pressed under the pressure of 50 kN. Samples were cylindrical and dimensiens were about 7,5 mm diameter and 15 mm height.
Samples were produced in this way just for simulating diamond bead shape which are montaged on steel wire ropes.
Before the cold pressing process granulation was an important process to implement. Granulation is not a must but it has some vital effects on material's properties and for that reason powders were mixed with a wax like material .
MX 4885's height was a little bit higher than the other materials ( one or two milimeters) it was most probably by the result of particle shape. Because powders with the flake shape are generally harder to consolidate. After the cold pressing process samples were sintered in a tunnel furnace under an inert gas atmosphere.
Sintered samples' dimensional and weight changes were noted. Hardness, SEM , Metallographic preperations, XRD and Optical Microscopy results were gathered to compare these 4 different free sintering alloys' characteristics.
As a result of all these characterisation tests it can be said that if a liquid phase sintering mechanism is active in a process, mixing, materials characteristics and processing parameters are important step to follow. After the sintering process liquid phase sintering was appeared in NEXT 400's structure.
Biggest change in material dimensions are gathered by the NEXT 400 sample. It was expected before the sintering process just because of Cu and Sn rates in the structures. By the result of these changes in dimensions highest rate of densification is gathered by the NEXT 400.
There were some local Cu island which can be clearly seen in the matrix. By the result of this effect barreling occured after compression tests. On the contrary 61390 was a succesful liquid phase sintering process which can be seen by the result of XRD,SEM,density measurements.
Samples prepared with MX 4885 powder gave best results for compressive stregth and hardness. Fe-Co solid solution was the main purpose of these results.
When we take into consideration all of the material properties included compressive stregth, shrinkages, stength tests, SEM and XRD analysis it can be seen that MX 4885 and 61390 gave competitive results.
Increasing Ni and Co rate in the structure has increased the mechanical properties of 61390 thanks to intermetalic phases formed during the sintering process.