Toz metalurjisi yöntemleri ile üretilen çeşitli W-Ni-Fe ve W-Ni-Cu ağır alaşımların mikroyapısal ve fiziksel karakterizasyonus

Abstract

Sunulan tez çalışmasında toz metalürjisi (T/M) yön temleri kullanılarak hazırlanan volfram esaslı ağır ala şım numuneleri ile kompozisyon farklılıklarının, mikro- yapı ve mekanik özellikler üzerindeki etkileri incelen miştir. Bu amaçla ilk olarak farklı Ni:Fe ve Ni:Cu oranlarında U-Ni-Fe ve U-I\li-Cu alaşım tozları hazırlan mış ve yüksek devirli top değirmeninde homojenize edil miştir. Hazırlanan ağır alaşım tozları tek eksenli preslerde ve rijid kalıplarda preslendikten sonra elde edilen kompaktlar iki aşamada sinterlenmiştir. Sinter- leme deneyleri sırasında sıvı fazın oluştuğu sıcaklık larda indirgeyici atmosfer olarak hidrojen gazı kullanıl mıştır. Bu deneyler sonucunda elde edilen numunelerin mikroyapıları optik mikroskop ve taramalı elektron mik roskobu (S. E. M) ile, elementel faz dağılımlarıda E. D. S cihazıyla incelenmiştir. Üretilen ağır alaşım numune lerinin fiziksel ve mekanik özelliklerinin belirlenmesi ne yönelik deneyler sırasıyla yoğunluk ölçümleri, mikro- sertlik ölçümleri, çekme deneyleri şeklinde gerçekleşti rilmiştir. Çekme deneyleri sonucunda oluşan kırık yüzey ler üzerinde taramalı elektron mikroskobu ile yapılan incelemelerle alaşımlarda ne tür kırılmaların meydana geldiği araştırılmıştır. Literatürdeki bilgilerle bir likte bu deneyler sonucunda, alaşımlarda meydana getiri len kompozisyon farklılıklarına bağlı alarak değişen mikroyapılar ve elde edilen mekanik özellikler belirlen miştir.

Powder metallurgy is a metal forming process for producing a variety of sturctural parts and bearings. In powder metallurgy, metal powders, that is metals in finely divided form rather than molten metal, are the starting material. The powders are consolidated into products with a given shape. The basic steps in powder metallurgy are therefore powder production and powder consolidation. The most common sequence in powder consolidation includes pressing the powder in a die into a compact and sintering the compact, which means heating it to a temperature below the melting point of the metal or alloy to give it the desired physical, mechanical and chemical properties. Pressing is most often done in rigid dies made of tool steel or cemented carbides. Pressures in the range from 70 to 700 MPa are used. The compacts so produced, called "green compacts", are strong enough so that they can be ejected from the die and handled. At this stage, they are porous and have a lower density than cast and wrought parts of the same metal. Powder metallurgy parts often have complex shapes. In order to produce such parts economically, systems of automatic compacting presses and tools, isostatic pressing in flexible molds, powder rolling, powder extrusion and powder injection molding methods have been usud. When powders are pressed into compacts at room temperature or are shaped by powder rolling or extrusion, the resultant products have insuffi cient strength and ductility for most applications. In order to make them useful they have to be sintered. Many powder metallurgy products are sintered in Many powder metallurgy products are sintered in continuous furnaces, in which the compacts are transported through a preheat, a high-heat and a cooling zone using pusher, endless belt, roller hearth or walking beam mechanisms. VI A protective atmosphere, or sometimes vacuum, must be maintained in the furnaces to prevent undesirable reaction with the compacts during heating and cooling. Sintering proces of compacts from a mixture of metal powders can be performed at temperatures where liquid phase is formed. The amount of liquid phase must be small enough to be held by capillary forces within the skeleton of the remaining solid phase so that the compacts do not warp or lose shape. This process is technically important and the mechanisms involved in it have been intensively studied. In prehistoric and historic times up to the early years of this century, powder metallurgy was a technology for producing wrought products from metals which could not be melted because their melting point was too high. It began with iron in the form of sponge iron produced by reduction with charcoal in charcoal fired furnaces from iron oxide in the form or more or less pure ores. This sponge was then forget into solid iron. Unless the sponge was exceptionally pure the sponge iron would contain large amounts of non-metallic impurities. Since ancient iron and stell were often remarkably free of inclusions, many scientists believe that the process was modified by certain African tribes. After reduction steps this sponge was broken up into powder particles, washed and hand picked to remove as much of the gangue and slag as possible and the powder was either compacted or loose sintered into a porous material, which was then forged. Powder metallurgy again came to the forefront between 1 75D and 1850, when demand arose for platinum in wrought form. Since pure platinum could not be melted, several rather similar processes to produce wrought platinum were developed in spain, England and Russia. Platinum powder was pressed and the compacts sintered and not forged. The processes were described in the scientific literature several years after they had been developed and used commercially. The English process was described by LJollaston in 1 B29 and the Russian one by Sobolevsky in 1934. The powder metallurgy method Df producing platinum became obsolete, when suitable furnaces and refractories for melting platinum were developed. Vll Powder metallurgy entered the stream of modern metals technology early in the 20th century. Again a metal which could not be melted and which, in addition, could be made malleable only with great difficulty, that is tungsten, was the key material in the early P/M developments of the 20th century. The interest in Tungsten arose from the demand for filaments for incandescent Lamps more stable than the Edison carbon flament. Numerous powder metallurgy methods were developed between 1900 and 1910 for producing tungsten wire. The only commercially succesful method was the one developed by William Coolidge in a process first described in 1910 and patented in 1913. Relatively few changes have been mode over the years in coolidge's procedure and it is still the standard method of producing in candescent Lamp filaments used all over the world. For other applications, Large compacts of tungsten, molybdenum and molybdenum alloys are isostatically pressed from powder and then sintered in furnaces with molybdenum or tungsten heating elements. They are then rolled into sheep or forged or extruded into the desire shape. In addition to pure tungsten and molybde num, certain alloys of the refractory metals were developed for special applications. They include alloys for heavy duty electircal contact materials, in which the refractory mBtals, tungsten or moybdenum, which have high hardness and Low rates of material transfer during making and breaking contact, are combined with copper or silver with their high electrical and thermal conductivity. A common method of producing these contact materials is by infiltrating a porous sintered skeleton of the refractory metal with liquid copper or silver. Currently, there exists other techniques in addition to infiltration, which exploit tungsten into viable commerical products. The so-called "heavy alloys" developed by Price, Smithells and Williams in 1937 are compacted from a mixture of tungsten powder with less than 10 % of either nickel and copper of nickel and iron powders. They may be sintered to theoretical density and are used because of their high density. vixi The combination of high density, strength, ductiliy, and corrosion resistance makes tungsten heavy alloys important. Alloys of the Ni/Fe type have a tungsten content of at least 9G %. They are fabricated from a powder mixture by liquid phase sintering at about 1470°C where the nickel-iron powders melt and join the tungsten grains together. During the initial stage of liquid phase sintering the liquid penetrates the tungsten particle agglomerates. The subsequent coalescence process and solution and reprecipitation of particles cause the grains to grow. Hydrogen-reduced tungsten powder, with particle sizes ranging from 3 to 5 jjm, is mixed with nickel and iron pqwders prepared from the metal carbonyl. Ball mills or mixers are used to blent these materials. The introduction of work hardening to the powders or gene ration of new surfaces, common to ball or attrition milling, is not necessary to achive good results in processing these alloys. The metal powder mixtures develop reasonable green strength when compacted with binders or Lubricants. Polyvinyl Alcohol and parafin are commonly used as binders and lubricants. Uniaxial or isostatic pressing at 20Q MPa pressure provides green strength that is adequate for handling and shaping prior to sintering. Binders, which decompose during presintering, may leave carbon residue that degreades mechanical properties of sintered material. For this reason dewaxing, or binder removal, requires slow heating to prevent the expansion of vapor from fracturing the parts. The furnace tem perature control system must be specifically designed for accurate control from room temperature to 350 C during this phase. Sintering is almost exclusively performed in molybdenum resistance type electric furnaces with a haydrogen or nitrogen-hydrogen atmosphere. Densifica- tion occurs rapidly, but sintering continues to promote grain growth associated with good mechanical properties. During sintering the 3 to 5 urn tungsten grains enlarge to 50 to 90 ^im. Grain growth apparently occurs by preferential solution of one grain and precipitation onto another across a thin matrix film between adjacent particles. The energy level associated with the crystal orientation of the faces of the grains most likely determines which grain grows. IX Upon coaling, tungsten retains only a trace af nicel and iron. The matrix, however, retains a substantial amount of tungsten in solution up to 25 wt %, which may be controlled by the addition of other alloying elements, notably copper in nickel-iron alloys. Nickel-copper alloys are prone to void formation in the matrix on rapid cooling from the sintering temperatures. Ductility of both alloys system is affected by cooling rate. At the onset of sintering and cooling cycle, the particle composite consists of spherical tungsten single crystals in a matrix of Ni-Fe-Ul or Ni-Cu-U solid solution The tungsten grains hava a bcc structure with small amounts of iron and nickel in solution. The matrix has an fee structure. The ductility of the composite is very good compared with pure tungsten, which is brittle at room temperature. The elongation obtained at tensile tests can reach up to 25 %. In order to maximize ductility, it is necessary to reduce process dependence on the sintering coaling rate by using a secondary heat treatment. Phosphorus, which is uniformly distributed over the interface boundaries, adheres preferentially to the matrix or binder phase side of the interface. Consequently, phosphorus con centration is Lower after solution annealing, whereas sulfur concentration is Less uniformly distributed and is found equally on the tungsten and matrix sides of the interface. Typical hardness values of heavy alloys is in the vicinity of 3G HRC, which remains almost unchanged by annealing. Trength in tension incerases slightly, however, to about 9DG MPa. This hardness may be considered too low to effectively penetrate hardened steel armor. Heavy allays are strain-rate sensitive, however, and at impact velocity they appear to have higher hardness and strength, with correspondingly reduced ductility. Id-base heavy alloys may readily be cold worked by swaging to incerase strength and hardness. Typical values are ultimate tensile strength of 1 300 MPa and hardness of ^3 HRC, with a corresponding decrease in ductility after cold working. Id-base heavy alloya machine readily in all metallurgical conditions. Continuous chips form only for materials tuith extreme ductility. A class 2 cemented carbide is used to machine with or without coolant, typically at 150 m/min. Feed rate and depth of cut are limited only by surface finish, component rigidity, and available power. Collection of machined chips in a clean condition allows Low-cost reprocessing to powder by comminution or by oxidation-reduction processes, rather than by more costly chemical recovery processes. Tungsten base heavy metals are used as balance weights in aeroplanes, in gyro-compases, as anti-vibra tion holders for tools, as extruding tools for wires and bars, as radiation shields, as contact materials and as welding electrodes. Tungsten heavy metals were introduced as armour piercing penetrator materials during the 1960's. Originally, a tungsten-nickel-copper alloy was used for a relatively short and thick penetrator body. That alloy, however, has a poor ductility so, when the Length/Caliber ratio was increased during the 1970's, the more ductile tungsten-nickel-iron alloys were generally preferred. Much effort has been devoted to the study of liquid phase sintering ant to explain the different steps in the sintering mechanism. Tungsten heavy metals have been of special interest in these studies because of their ideal properties concerning partricle growth, matrix wettability, and tungsten solubility in the matrix. The strength of the interfacial boundary between matrix and particles is crucial for the ductility of the composite. The interfacial strength can deteriorate due to impurities enriched in the boundaries. The behaviour during plastic deformation and the mechanical proporties of particle composites depend upon the interaction between the particles and the matrix. The capability of the matrix to deform plastically and there by Smooth out the internal stresses acting on the less ductile particles, is the prime mechanism that makes particle composites into succesful materials. Morover, tungsten particle composites are excellent for studying mechanical properties of two phase materials. XI In this dissertation work, the influence of composi tion differences on the developed microstructure and mechanical properties in heavy alloys, were investigated. For this purpose, the compacts, mere produced from the powder mixture, which mere prepared in U/Ni/Fe and ld/Ni/Cu systems at thB different ratio of Nl:Fe and Ni:Gu, homo genized in ball-mill, pressed in suitable dies using 2G0 MPa pressures. these particular compacts mere sintered with the liquid phase sintering mechanism in the furnaces in which hydrogen gas was used as a reducing atmosphere. The influences of selected sintering processes and the alloy composition differences, on obtained micro- structures, were observed by examining tungsten base heavy alloy samples via optical microscope and scanning electron microscope. In this study, elemental phase analysis for each alloy war a also performed by means of EDS which was connected to the scanning electron microscope. Following this, rnicrohardness and density values of six different compositions of heavy alloy samples were measured. Tensile tests followed all of these studies above. Finally the fractured surfaces as a result of tensile tests, were analized. Scanning electron microscope was used fracture modes. Tungsten base heavy alloys are not being produced in our country and have strategical importance beacause of their usage for military purposes. This dissertation work aims at determining production conditions via P/M routes. For this purpose, within,this particular study, the changes in compositions the properties of the final products were investigated.