Volfram ağır alaşımlarında başlangıç toz özelliklerinin sıvı faz sinterlemesi yoluyla yoğunlaşma süreçlerine olan etkileri

Abstract

Sunulan tez çalışmasında, başlangıç ve ön işlenmiş tozlardan yola çıkarak, toz metalürjisi (T/M) yöntemleriyle hazırlanan değişik bileşimlerdeki volfram ağır alaşımları nın yoğunlaşma süreçleri incelenmiştir. Bu amaçla, atritör ve top değirmeni gibi değişik ortamlar kullanılarak işlenen tozlar ve başlangıç tozların kombinasyonlarından 90U7Ni3Fe, 90U7l\li3Cu ve 9DW5Ni5Cu bileşimlerinde ağır alaşım toz karışımları hazırlanmış ve homojenize edilmiştir. Bu tozlar daha sonra, tek yönlü preslerle direngen kalıplarda ve soğuk izostatik presle esnek kalıplarda pekişti rilmişlerdir. Pekiştirilmiş numuneler, atmosfer kontrollü bir fırında sıvı fazda sinterlenmiş ve sonuçta yüksek yoğunlukta ağır alaşım numuneleri elde edilmiştir. Gerek ön işlemler sırasında gerekse sinterleme sonrasında numuneler, optik mikroskop, taramalı elektron mikroskobu (SEM) ve bu cihaza bağlı EDS destekli analizler ile, yoğunluk ölçümleri, tane boyutu ölçümleri ve görüntü çözücü cihaz kullanımı ile kalitatif ve kantitatif analizlerle mikro- yapısal olarak karakterize edilmiştir.

Powder Metallurgy (P/M) is an art of technology for producing variety of parts with unique properties. Con ventionally, parts are made by melting, casting and/or subsequent working (forging, extrusion, rolling, swaging etc. ) f ollowed by machining to get the required shapes. In the conventional processes, there is a substantial loss of material and more labor cost due to secondary operations. Powder metallurgy techniques are economical as compared to conventional processes by the way of reducing material wastes and labor costs. Moreover, there are certain exotic materials like tungsten, beryllium, and nickel-base alloys, which can not be processed by casting and conventional working route because of their inherent characteristics, besides metallic filtres required for gas or liquid separation and in oil-impregnated bearings, (their controlled porosity and self-lubricating property) can only be made by powder metallurgy techniques. In P/M processing technology, metals in the form of powders can be mixed to form alloys, moulded underpressure and heated to higher temperatures so as to sinter the particles together, which allows engineers to design and manufacture more complex shapes. One may to notice, but the P/M com ponents are used in almost all the equipments starting from the household goods such as television, radio, copy machines, typewriters, computers, washing machine, sewing machines to automobiles, trucks, tractors and aeroplanes and for many other military hardwares. The term powder metallurgy can be defined as the material processing technique used to consolidate parti culate matter, both metals and/or nonmetals, into discrete shapes. The process esentially consists of three steps: (a) Powder blending and mixing of powders, (b) compaction (c) sintering. First, powders that have the desired shape, size and other characteristics of importance are either blended by themselves or mixed with additives such VI / as binders or allay additions. The powder is then com pacted under pressure within a die at room temperature. Compaction is done in order to consolidate and densify the loose powder into a green compact that has a suf ficient strength for handling. The desired size and shape of the product are obtained in this step. After compaction, the green compact is heated in a furnace at a relatively high temperature in a protective atmos phere. This step is called sintering, which is usually a solid state process (no liquid or molten phase is Fnnmnrl^ T r, r, r-. T r, m - +. r, -~ ?! " 1 "., " 4. " U J J In any powder metallurgy product development, several parameters i.e. particle size, shape and its distribution modes of consolidation, addition of sintering aids and atmosphere affect sintered properties. The relationship between these parameters and sintered properties should be understood before embarking upon the manufacturing of the product. Characteristics of the starting powders, such as particle size, its distribution, aggregates and surface area have pronounced effect on densif ication and micro- structural development. An ideal powder mix should have small particle size, non-agglomeration, equiaxed particle shape, narrow size distribution and high purity or cont rolled impurities. Agglomeration of the particles takes place due to surface forces between constituent particles, This surface bonding force per unit weight is inversely proportional to particle size. The voids between aggre gates are often larger than those between the the consti tuent particles, which in turn requires longer sintering times. Furthermore, the densif ication of individual agglomerates leads to their shrinkage from each other thereby widening the voids between them. The particle size distribution influences the final achievable density in a compact. A narrow paticle size distribution is imperative for obtaining the high sintered density. Vll Blending is done to control particle size distribu tion so as to get better compaction and sintering. Powders are mixed to get the new compositions. The variables involved in blending and mixing are physical characteristics of powders, mixer size, powder volume in the mixer, speed of mixing and time of mixing. Environmental factors like humidity and atmosphere contribute to the quality of final mixture. The methods of powder mixing are diffusion, convection and shear carried out in rotating durms, screw mixer and blade mixer respectively. Most powder mixing is performed using rotating containers, where a diffusion mix occurs by the motion particles into the powder lot. A powder volume of 20 % to kO % of the mixer capacity i 3 usually considered optimal. A rotating speed and time also influence the mixing. A slow speed will prolong the time necessary to get adequate mixing while rapid rotation interferes with the flow arising out of the centrifugal force imparted to the powder. In addition, extensive free fall of powder in the mixer should be avoided tD prevent preferential siz£ settling. Prolong mixing work hardens the particles and increases the chance of contamination. To minimize the interparticle friction, die wall lubrication or lubrication of powder is done. The compaction of powders into the desired die shape involves both rearrangement and deformation of the particles causing the development of interparticle bonds. Because of die wall friction, there will be pressure gradient along the height of the compact. In double acting press, this pressure gradient is reduced. The pressure decay thus depends on compact height(h) to diameter (d) ratio. In general, when h/d 5 die compac tion is unsuccessful. For complex shapes involving undercuts or large h/d ratios, cold isDstatic pressing is only a viable approach. In this method, the compac tion is more efficient and higher densities with smaller pressure gradients can be obtained because of hydrostatic pressure. Densif ication during sintering is governed by the second law of thermodynamics, according to which every system tends to minimize its free energy. In powder metallurgy, this is surface energy of particles. Two or more external particles join together to form one grain boundary, thereby reducing the total surface area. Solid state sintering proceeds in three stages. In the initial stages, interpaticle bridges are established and neck formation occurs due to plastic deformation and Vlll migration of atoms. The individuality of powder particles is generally retained. In the second stage, neck growth intensifies, separate particles loose their identity and grain growth occurs rapidly. The pores form coherent network and the grain boundary runs from pore to pore. Most of the shrinkage takes place during this stage, where up to 90 % of theoretical density is achieved. During the third stage, isolated pores become spherical and densif ication proceeds slowly. Densif ication results from material transport between pores and grain boundaries by lattice or boundary diffusion and the grain growth is controlled by the pore mobility which limited by surface, lattice or vapor transport. Pore shrinkage occurs by vacancy diffusion from pores to grain boundaries and dislocations, whereas pore growth is due to either pore coalescence of several pores dragged by the migrating grain boundaries or Gstwald ripenning of pores by a con current diffusion of vacencies from smaller pores to larger ones. This predicts that the extent of grain coarsening and the achievable final density depends on the rate controlling mechanisms in coarsening and densi- fication kinetics. In particular, smaller particle size and high green density can lead to high sintered density with minimal grain growth as a result of enhanced grain boundary diffusivity at the expense of surface diffusivity and vapor transport. The above discussion clearly indi cate that finer particles aid quicker densif ication. Presence of agglomerates promotes pore widening. Liquid phase sintering process usually consists of three stages. In the initial stage, the mixed powders are heated to a temperature where melting of one or more components of the material takes place. During this period, rapid densif ication occurs due to a capillary force exerted by the wetting liquid on the solid particles The elimination of pores occurs as the system tries to minimize its surface energy. During rearrangement, the compact behaves as a viscous solid. But, the densifica- tion rate continuously decreases with reduction in pores. The degree of densif ication will depend on the amount of liquid phase, particle size and solubility of the solid in the liquid phase. Usually finer particles give better rearrangement. But rearrangement process gets inhibited by high green density or irregular particle shape. The particle contacts resulting from compaction form solid state bonds during heating prevent rearrangements. During the rearrangement stage, there are various other events which get overshadowed because of fast kinetics. As densif ication by rearrangement becomes slow, diffusion and solubility effects become dominant. This second stage of classic liquid phase sintering is generally known as "solution-reprecipitation". In this stage, microstructural coarsening takes place. The solubility IX of a particle in its surrounding liquid phase is inversely proportional to its size. The difference in solubilities establishes a concentration gradient in the liquid. Mass is transferred from small particles to large particles by diffusion. The process is called coarsening or Oswald ripening, which ultimately results in larger grains with wider spacing. Solution-reprecipitation contributes to both grain coarsening and densif ication. The last stage of classical liquid phase sintering is referred to as solid state controlled sintering. Densif ication is very slow because of the existence of a solid skeleton. The rigidity of solid skeleton inhibits further rearrangement, although microstructural coarsening continues by diffusion. The residual pores will enlarge if they contain an entrapped gas, giving rise to swelling of the compact. In general, properties of most liquid pahse sintered materials are degraded by prolonged final stage sintering. In liquid phase sintering, more homogeneous is the formation of liquid, greater is the densif ication. A homogeneous powder mixture aids rapid alloying during liquid phase sintering and gives improved sintered properties. Generally, distribution of additives improves with smaller particles. Thus milling the powder mixture priour to compaction is beneficial. One of the first uses of liquid phase sintering for metals is attributed to the ancient incas who converted platinum grains into a consolidated form by use of gold bonds. It is thought that the gold was molten during the sintering cycle. Artifacts, from this process indicate its use over 400 years ago. The development of modern liquid phase sintering technology is traced to the production of cemented carbides. Considerable effort went into the development of tool and machining materials in the 1900 to 1930 time period. By the early 1920's carbides with metallic binder alloys were patented. For these compositions sintering is with a liquid phase typically formed from iron, nickel or cobalt. The liquid phase sintering approach permits the formation of dense, pore-free carbides with properties superior to any previously known cutting materials. Today the cemented carbides are an integral component of industrial operations including mining, machining, metal forming, grinding, drilling, and cutting. Their widespread use results from the composite properties of high strength, high hardness, low thermal expansion coefficient, and reasonable toughness. From a technical paint of view, the major advantage of liquid phase sintering is the result of faster sintering. The liquid phase provides for faster atomic diffusion than the concurrent solid state processes. The capillary attraction due to a wetting liquid gives rapid compact densif ication without the need for an external pressure. The liquid also reduces the interparticle friction, thereby aiding rapid rearrangement of the solid particles. In addition, liquid dissolution of sharp paticle edges and corners allows more efficient packing. Grain size control is possible during liquid phase sintering; thus, processing effects can be carried over into microstructure manipulations to optimize properties. Finally, in many liquid phase sintering systems the higher melting phase is also the harder phase. This often results in sintered two-phase composite materials with ductile behavior in spite of a large quantity of hard phase. However, there are some disadvantages. A common problem is compact slumping (shape distortion) which occurs when too much liquid is formed during sintering. Also, the same parameters which control the sintered microstructure often control the final properties. Separation of these effects is sometimes difficult. The tungsten heavy alloys are two phase composites formed by persistent liquid phase sintering of mixed powders. Transition metals such as Ni,Co,Fe and Cu in various combinations are used to form the liquid phase through eutectic reactions with tungsten. The most popular alloys are based on nickel and iron additions in the ratio of 7:3. Other alloy compositions approximate Ni:Co ratios of 1:1 and Ni:Cu ratios of 2:1 The nickel is important to solubility and wetting during sintering. Typically at least 1D % liquid exists during the sintering cycle. This is controlled by the amount of additive, tungsten solubility in the liquid, and sintering temperature. Mixed elemental powders are sintered at temperatures typically 20 to ^0aC over the eutectic temperature for times of 30 to kQ minutes. Cooling from the sintering temperature must be slow to avoid solidification pores and strains in the matrix. Additionally, postsintering heat treatments are used to relieve strains, remove hydrogen and minimize impurity segregation. The resulting combina tion of density, strength, ductility, atomic number, melting temperature, and toughness is unique. The proporties of the heavy alloys are sensitive to pores, intermetallic phases, imprurities, residual hydrogen, thermal strains, and microstructure. The fracture path shows dramatic changes due to changes in XI the processing cycle. The alloys exhibit a classic trade off between strength and ductility. Fine grain sizes give higher strengths but lower ductilities. Furthermore as the tungsten content increases, the ductility decreases because of an increasing contiguity. The applications for the tungsten heavy alloys typically rely on the combination of high sintered densities and good mechanical properties. Thus, they are used as radiation shields, counterbalance weights, vibra tion dampers, gyroscope components, and inertial control devices. Another use is for armor piercing projectiles projectiles because of the large kinetic energy associated with the high density. Additionally, tungsten heavy alloys are used to improve the performance of sporting equipment such as golf clubs and darts. High temperature uses include welding rod holders, electrial contacts, die casting tools, extrusion tools, and spark erosion tools for electrode discharge machining. The heavy alloys have high absolute densities, good strength and ductility in a high temperature material. These attributes make them unique alloys. In this dissertation work, effects of initial powder characteristics on densif ication processes via liquid- phase sintering in tungsten based heavy alloys were investigated. Investigations were performed in three different phases which were seperated from each other according to initial powder characteristics. Standard metalic volfram, nickel, iron and copper powders were used in the all phases. These powders were imported from different European powder suppliers. Also, same heavy alloy compositions (90U7l\li3Fe, 9DUJ7Ni3Cu and 9DU5Ni5Cu) and same sintering temperatures (1490 C and 1450 C) were used, in the investigation of each phase. In the first phase standard metallic powders directly were mixed to consitute heavy alloy compositions which were mentioned above. In the second phase, these heavy alloy mixtures were milled in attritor medium to grain size refinement and homo jenization. In the third phase only the additive parts of the heavy alloy system (Ni,Fe and Cu powders) were ball-milled one by one in order to reach fine grain sizes. After this, all powder mixtures were homojenized in dry ball-mill and binder additions were also realized in this step. Powders were then compacted into both xix cylindirical and rectangle prisms shapes using one action and cold isostatic presses. After compaction green densities of the compacts were determined by taking physical dimensions of the samples by a caliper. After this step, compacts mere liquid phase sintered in the atmosphere controlled sintering furnace. The densities of sintered samples were measured in both distilled water and ethyl alcohol according to Archimeds principle. After than, the samples were prepared for metallographic analyses using standard techniques and after final polishing, modified, Murakami solution were used as a etcher. Optical and scanning electron microscope investiga tions were performed on this samples and characteristic two phase heavy alloy microstructure was observed, consisting of rounded tungsten grains (bcc) which are embedded in the (fee) matrix solid solution of U-Cu-Ni or üJ-Fe-l\li. Moreover, selected samples were characterized to obtain quantitative grain size and grain size distribu tion data. Also, semi-quantitative elemental phase analyses by means of 5EM-EDS system were realized. Tungsten heavy alloys have been proven technologically and economically important in a variety of industrial and defense applications. The increasing interest in these materials is based on the unique properties of tungsten heavy alloys which are needed to meet the extreme thermal, mechanical and chemical requirments in the design and development of advanced material systems. Although there have been significant advances in Turkish P/M industry during the past years, yet Turkey has an unsatisfied P/M market for both civilian and defense P/M products. Especially, demands for defense industry are met by import in the form of kinetic energy penetrators, aeroplanes, helicopters and tanks. It is hoped that the materials and processes investi gated in this thesis work would create significant driving force towards exploitation of tungsten ores and subsequent production of tungsten based materials in Turkey.