Elementel W ve C tozlarından mekanik alaşımlama süreçleri sonucu geliştirilen WC toz alaşımı ile sinter WC-Co alaşımının mikroyapısal karakterizasyonu

Abstract

Sunulan yüksek lisans tez çalışması elementel W ve C tozlarının atritör ve top değirmeninde uzun süreler öğütülüp mekanik alaşımlanarak (MA) katı hal reaksiyonları ile oda sıcaklığında WC sentezlemesi ve bunu takiben sentezlenen tozların bağlayıcı metal ilaveli ve ilavesiz sinterleme çalışmalarını içermektedir. Öğütücü cihaz ve mekanik alaşımlama süresi değişken olarak alınmıştır. Artan mekanik alaşımlama süresiyle birlikte her iki cihazın, W ve C tozlarını mekanik alaşımlamadaki etkisi ve mikroyapıda meydana getirdikleri değişiklikler incelenmiştir. Bu incelemelerde öncelikle mekanik alaşımlanmış tozlara, uzun öğütme işlemi neticesinde başlangıç tozlara göre partikül boyutundaki değişimleri tesbit etmek amacıyla Lazer Partikül Boyut Analizi yapılmıştır. Mekanik alaşımlama ile oluşabilecek fazların tesbiti X-ışınları ile yapılırken, tozların morfolojileri Taramalı Elektron Mikroskobu (SEM) çalışmaları ile belirlenmiştir. Bağlayıcısız sinterleme işlemi sonrası numuneler tekrar X-ışınları ve SEM çalışmaları ile karakterize edilmişlerdir. Karakterize edilen numunelerin tozları kobalt metali bağlayıcısı ile harmanlanıp pekiştirilerek tekrar sinterleme işlemine sokulmuşlardır. Elde edilen numuneler optik mikroskop ve SEM çalışmaları ile karakterize edilmiştir.

Powder metallurgy can be referred as a lost art. Unlike clay and other ceramic materials, the art of molding and firing practical or decorative metallic objects was only occasionally applied during the early stages of recorded history. Sintering of metals was entirely forgotten during the succeeding centuries, only to be revived in Europe at the end of the 18th century, when various methods of platinum powder production were recorded. Long before furnaces that could approach the melting point of metal were developed, P/M principles were used. About 3000 B.C., the Egyptians used a "sponge iron" for making tools. Powder metallurgy practices were used by the Incas and their predecessors in making platinum. The technique used was based on the cementing action of a lower melting binder, a technique similir to the present practice of making sintered carbides. The use of powder metallurgy to form intricately shaped parts by pressing and sintering was introduced in the 19th century. The first commercial application of powder metallurgy occurred when carbon and later osmium, zirconium vanadium, tantalum and tungsten, was used for incandescent lamp filaments. Tungsten was soon recognized as the best material for lamp filaments. It was known that tungsten granules combined readily with carbon at high temperatures to yield an extremely hard compound so that this material was used as the basis for a very hard, durable tool material known as cemented carbides. Due to low sintering temperatures and net design features P/M is a capital intensive technology offering economies in materials, labor and energy. In addition, the major advantages of powder metallurgy techniques with respect to ingot metallurgy are the elimination of segregations and ensuring of a fully homogeneous, fine-grained, pore- free and high alloy structure. Because of this advantages P/M products are widely used in the field of automobiles, washing machines, refrigerator and air conditioning compressors, bicycles and lawnmowers. XIII Farm machinery and industrial hydraulic equipment are large users. Data processing equipment, office copiers, postage meters and similar machines may actually have more than a hundred P/M parts designed into them. For the outdoorsman, the fishing reel, firearm, tape decks and phonograph turntables also utilize components made by powder metallurgy. P/M processing consists of three basic steps: i) powder preparation (blending), ii) compaction, iii) sintering. In the preparation step, raw powders with the desired characteristics such as size, shape, distribution, aggregates and surface area, are blended with lubricants (zinc stearate or a stearate derivative) and alloying additives. Lubricants act as an ejection facilitators for the compact from the tooling and prevents scoring of the punches, dies and core rods. Blending is performed long enough to obtain a homogeneous mixing in a special mixing device. The powder is then compacted under pressure into a rigid die of the desired shape and size. Compaction process consolidates and densifies the loose powder into a green compact of sufficient strength and density. In general, the compaction pressures range from 50 to 500 tons /cm2. The compression ratio-the ratio of the volume of loose powder to the volume of the compact made from it- vary considerably depending upon the characteristics of the raw materials used. As the sintering process, the green compact is then heated in a furnace at a '. relatively high temperature (from 1120 °C to 100-150 °C above) in a protective atmosphere, where true chemical bonding of the particles and recrystallization occur accros the particle interfaces. Protective atmospheres serve as to cleanse the compact and prevent any undesirable reaction. All these steps lead to achieve a higher performance compact. Characteristics of the elemental powders (particle size, distribution, aggregates, surface area) dominate densification and microstructure development. Ideal characteristics are small particle sizes, non- agglomeration, equiaxed particle shape, narrow size distribution and high purity or controlled impurities. Particles agglomerate because of the surface forces between constituent particles. The surface binding force per unit weight is inversily proportional to particle size. The voids between aggregates are often larger than those between constituent particles, requiring longer sintering times. Also, the densification of the individual agglomerates causes shrinkage from each other which widens the voids between them. The particle size distribution affects the compact's final density. Narrow particle size distribution is crucial for achieving high sintered density. Blending is carried out to obtain controlled particle size distribution, for better compaction and sintering. The variables are physical characteristics of powders, mixer sizes, powder volume in the mixer rotational speed and time of mixing. Environmental factors such as humudity and atmosphere also affect the final quality of the powder mixture. Most powder mixing is performed in rotating containers where a diffusion mix occurs by the motion particles into the powder lot. Optimal powder volumes are % 20 to % 40 of the mixer capacity. Rotating speed and time are important where a slow speed prolongs the XTV time necessary to obtain adequate mixing while rapid rotation interferes with the flow arising out of the centrifugal force imparting to the powder. Additionaly, powder should be prevented from extensive free falling to avoid preferential size settling. Prolonged mixing hardens particles and increases the probability of contamination. Die wall lubrication or lubrication of powder is necessary to minimize interparticle friction. Liquid-phase sintering is usually a three stage process, with no clear distinctions between them. In the initial stage the mixed powders are heated to a melting temperature of one or more compenents. During this stage rapid densification occurs while the wetting liquid exerts a capillary force on the solid particles. As the system minimizes its surface energy, the pores undergo elimination as an rearrangement during which compact behaves as a viscous solid. As a result, a reduction in the densification occurs and its degree depends on the amount of liquid phase, particle size and solubility of the solid in the liquid phase. Finer particles, lower green densities and regular particle shapes are favoured for a better rearrangement The second stage which is characterized with pore rounding, densification, and grain growth starts when densification tends to slow down, and diffusion and solubility effects begin to govern the process, where the liquid phase becomes a carrier for the solid phase atoms in a process called "solution-reprecipitation". The pores are smoother and the density between % 70 and % 92 that of the theoritical one. Diffusion of small particles into large particles occurs due to concentration gradient in the liquid. Grain growth occurs late in the stage, so the grain size is larger than the initial particle size. This stage contributes to both grain coarsening and densification. By the final stage of sintering, the pores are spherical and closed, and grain growth is evident. A homogeneous mixture of powder with additives gives improved sintered properties, therefore an effective milling is crucial. Example systems with liquid phase formation during sintering are WC-Co, W-Cu, Cu-Sn, W-Ni-Fe, Fe-P, TiC-Ni and Fe-Cu-C. The development of modern liquid phase sintering start with the production of cemented carbides. In the early 1900's in the sintering of carbides with metallic binder alloys the liquid phase was typically consisting of iron, nickel or cobalt. The liquid-phase sintering approach today enables the production of dense, pore-free cutting materials such as the cemented carbides which are an integral compenent of industrial operations including mining, machining, metal forming, grinding, drilling and cutting. Their widespread use is due to the composite properties such as high strength, high hardness, low thermal expension coefficient, and reasonable toughness. Mechanical Alloying (MA) is a solid state powder processing technique involving repeated welding, fracturing, and rewelding of powder particles in a dry, high-energy ball charge. The process was developed in the late 1960's to produce oxide-dispersion strengthened nickel- and iron-base alloys for applications in the aerospace and automobile industries. The intimate mixing of the constituent metals on a very fine scale can also result in the formation of a variety of stable and metastable phases. XV The process of mechanical alloying consists of loading the powder mix and a grinding medium (generally hardened steel balls) in a container under a protective argon atmosphere (to minimize /avoid oxidation and nitridation during the milling operation) and milling for the desired length of time. About 1-2 wt.% of a process control agent (PCA) (e.g. stearic acid, C18H36O2) is normally added to prevent excessive cold welding of the powder particles and to achieve a balance between welding and fracturing during the milling operation. The process variables include type of mill, intensity of milling, type and size of milling medium, milling atmosphere, ball-to-powder ratio, milling time, milling temperature, and nature and amount of the PÇA. The preparation of most metal carbides is usually performed by direct reaction of metals or metal hydrides with carbon at high temperatures. Most of the carbides must be heated at more than 2000 °C to suppress heterogeneities. The direct synthesis, however, can not be applied to synthesize some carbides, such as Fe7C3 (for which high pressure and high temperature, or crystallization from amorphous Fe-C alloys is employed). New methods for ceramic processing have recently been reviewed such as vapor phase chamical reaction and self propagating high-temperature synthesis, the latter being particularly capable of synthesizing metal carbides. A novel approach to synthesize carbides and silicides was recently proposed utilizing a process involving a comminution of the elemental powders at almost room temperature. In the last 20 years, increased attention was devoted to understanding the effects of comminution on chemical activity and also its ability to induce solid state reactions in high specific energy mills. The effect of grinding on reactivity of solids, for example in extractive metallurgy, has been previously studied. More recently, the possible mechanisms of mechano-chemical reactions were the subject of a number of studies and the roles played by the disordering of atomic structure and the generation of free redicals and deformed bonds at the generated surfaces were identified. It is generaly recognized that the internal energy of materials is enhanced during communition through plastic deformation, cold welding, and surface energy increases through fracturing. Phase transformations can take place in mills (such as ball mills) without an external heat source. The temperature rise of the mill, due to the fraction of impact energy converted to heat, usually does not exceed 100 °C. Cemented carbides are products of the powder metallurgy process; they consist primarily of minute particles of tungsten and carbon powders cemented together under heat by a metal of lower melting point, usually cobalt. Powdered metals such as tantalum, titanium, and niobium are also used in the manufacture of cemented carbides to provide cutting tools with various characteristics. In this study, mechanical alloying of elemental W and C powders to synthesize WC via solid state reactions at room temperatures was carried out under the ball mill and attritor milling conditions having same process parameters. The effect of milling type and milling time on WC production is investigated. Synthesized WC powders by mechanical XVI alloying was then sintered with and without adding metallic binder in order to show sintering behaviour of mechanically alloyed WC powders. Elemental powders of tungsten and graphite were used as starting materials. Mechanical alloying of W-C powders which are prepared in various milling time were performed in attritor and ball mill at a ball/powder ratio of 3:1, 1/4" WC balls and a speed of 144 rev./min. At the end of 5, 10 ve 30 hours milling times, a small amount of sample was removed for Particle Size Analysis, X-ray Difractometer and Scanning Electron Microscopy (S.E.M) studies. Following this step, MA powders were consolidated using uniaxial press. Green compacts were sintered in an industrial furnace with inert atmosphere at 1100 °C without adding any metallic binder. Sintered compacts were analysed by X-rays and S.E.M studies. Metallic binder, cobalt, added powders were consolidated again and then sintered in an industrial furnace with hidrogen atmosphere at 1450 °C. X-rays, optical microscopy and SEM studies for microstructure evoluation were carried out on sintered compacts. From the viewpoint of above studies it can be concluded that mechanical alloying of elemental W and C powders is very efficient in ball milling in comparison with attritor milling and the formation of WC increases with increasing milling times.