Toz metalurjisi yöntemleriyle geliştirilen 90W7Ni3Fe ağır alaşımlarının sıvı faz sinterlemesi süreçleriyle üretilmesi ve karakterizasyonu

Abstract

Sunulan tez çalışmasında Toz Metalürjisi üretim yöntemleriyle 9ÜU-7Ni-3Fe ağır alaşımlarının üretim süreçleri toz karışımı hazırlama, presleme ve sıvı fazsinterlemesi aşamaları anlamında incelenmiş ve her bir aşama sonunda elde edilen ürünler karakterizasyon çalışmalarına tabi tutulmuşlardır. Bu amaçla ilk etapta farklı özelliklere sahip elementel U,Ni ve Fe tozları değişik ortamlarda karıştırılarak ve öğütülerek elde edilen toz karışımları tane boyutu, tane boyut dağılımı ve kimyasal bileşim açılarından karakterize edilerek, daha sonraki presleme ve sinterleme aşamaları için uygun toz karışımları seçilmiştir. Uygun olan toz karışımları tek eksenli preslerde ve rijit kalıplar içinde preslendikten sonra değişik sıcaklıklarda ve değişik ortamlarda sinterlenmişlerdir. Sinterleme işlemleri, sıvı fazın oluşmadığı sıcaklıklarda "önsinterleme" ve sıvı fazın oluştuğu sıcaklıklarda "sıvı faz sinterlemesi" olmak üzer iki farklı deney seti şeklinde gerçekleştirilmiştir. Sinterlenen numuneler, sinterleme koşullarının etkisini belirlemek amacıyla, mikroyapısal ve kimyasal analizler ile yoğunluk ve mikrosertlik ölçümleri şeklinde karakterize edilmişlerdir, Mikroyapı analizleri, literatürde yer alan mikroyapısal gelişim mekanizmaları ile karşılaştırıl arak- 90U7Ni3Fe alaşımında etkin olan mekanizmalar belirlenmiştir. Kimyasal analizler, sinterleme işlemleri sonucunda oluşan fazlar ve elementel metallerin fazlar arasındaki dağılımları anlamında incelenerek, sonuçlar literatür sonuçlarıyla karşılaştırılmış ve sinterleme koşullarıyla bağlantıları kurulmuştur. Aynı şekilde yoğunluk vemik rosertlik ölçüm sonuçları da sinterleme koşullarına bağlı olarak literatürdeki sonuçlarla karşılaştırılmıştır.

Powder metallurgy is the technology and art of producing metal powders and of the utilization of metal powders for the production of massive materials and shaped objects. Especially the art side of powder metallurgy can be dated up to 3000B.C, long before furnaces were developed, by using sponge iron for making tools in Egypt. Incos also used powder metallurgy practices in making platinum. Powder- metallurgy of platinum has carried the further developments in powder metallurgy technology in the 18 th. and 19 th centuries in Europe. But the first commercial application of powder metallurgy occured when carbon and later zirconium, vanadium, tantalum, and tungsten, was used for incandescent lamp filaments between 1B7B-1B9B. When tungsten was recognized as the best material for lamp filaments the attempts aimed at the field of refractory metals such as tungsten, molibdenum and tantalum in the early 190D's. The need for harder materials to withstand greater wears while drawing process of refraktory metals into wires and f laments. another development took place and cemented carbides and their production technology by powder metallurgy methods become one of the greatest industrial developments of the century. The next development in powder metallurgy was the production of composite metals which consist of refractory metal particles and a cementing metarial with a lower melting point, present in various propor tions, used for special applications such electrods, counter weights etc. Vll This development in powder metallurgy technology is followed by the development of porous metal bearings during the early 1900's. During the period between 19G0, World War I and 1920's developments on powder metallurgy technology advanced through modern developments. Such as infiltration technigues, porous materials, iron powder cores, permanent magnets, W-Ni- Cu heavy metals. After World Ular II powder metallurgy technology developed on the field of otomotiv industry making the use of iron and copper powders in large tonages. Through the 19^0'sf and 1950's iron, copper powders, self lubricating bearings and steel components became the dominont powder metallurgy products. With the advent of aerospace and nuclear technology, developments have been wide spread with regard to the powder metallurgy of refractory and reactive metals such as tungsten, molybdenum, niobium, titanium, tantalum, beryllium, uranium, zirconium, and thorium. A new development was powder metallurgy wrought products in 195D's and 1960's. Hot isostatically pressed super alloys, P/M forgings, P/M tool steels, dispersion-strengthened products etc. are all break throughs of 197G's and 19BG's in powder metallurgy area The major advantages of P/M techniques with respect to ingot metalurqy is elimination of segregations and ensuring of a fully homogeneous, fine-grained, pore- free, high alloy structure. Low sintering temperatures, net design features and dimensional stablization are the other advantages of this technique* These advan tages of this technique ensures a wide spread current application such as cemented carbides, structural components, heavy alloys, superalloys etc. The possibility of geting porous structures by this technique, porous metals like metallic filters and self- lubricating bearings can be also produced. Among these current applications P/M of heavy alloys has a special importance because of geting superior properties. Especially tungsten-based heavy alloys have a combination of high density, strength, ductility and corrosion resistance which make these allays important. vixi Tungsten heavy alloys have been used as counter weights for self-winding wrist watches and various instruments, as gyro rotors, as sheilding against gamma radiation, as counterweights for aircraft and aerospace vehicles as contact materials, as extruding tools for wires and bars, as welding electrodes, and as armour piercing penetrator materials for military ordnance applications. The development of heavy alloys started in the 1930's. Drginally the alloys contained UJ-Ni-Cu but in the 1940's the U-Ni-Fe type was developed, and become dominant. These alloys have a tungsten content of BB-98%. Typical nickel-to-iran and nickel-to-copper rations range from 1/1 to 4/1 and 3/2 to 4/1 respectively. Because of the liquid phase sintering, these alloys achieve essentially full theoretical densities and a characteristic microstructure develope. The charac teristic microstructure of these composite materials consists of nearly spherical grains of body-centered cubic tungsten surrounded by a solidified face-centered cubic matrix phase containing Ni,Fe and U. The superior properties of these alloys are related with this characteristic microsturcture while the tungsten grains suport high dencity and strength, the maxrix phase imperts ductility to the composite. (Figure 1). Tungsten heavy alloys are produced by liquid phase sintering (LPS) process which is a special type of sintering technology. In LPS the liquid phase consists of low melting point metals present during the entire time while the compacts are at the sintering temperature; they are sintered between the solidus and the liquidus of the alloy systems. This process is also called the "heavy alloy mechanizm". Densif ication and growth of the solid particles in the heavy alloy mechanizm involves three stages 1)- The liquid flow or rearrangement stage, 2)- The solition-reprecipitation stage, and 3)- The solid state sintering stage. In the first stage of LPS, during heating to sintering temperature, parts of the compacts form a liquid phose. In conditions of good wetting liquid penetrates into particles densif ication can be achieved. In the second stage of LPS further densif ication and growth of the solid particles is achieved by solution and reprecipitation processes such as Dstwald ripening, contact flattening IX and coalescence. In the last stage of LPS, prolonged holding of the compacts at the" sintering temperature, leads to microstructural changes including further growth of the solid particles and formation of a skeleton of the solid phase. FIGURE : 1. Compact of 92 u".% Uf 7 Ut, % Nif3.. '?.J* % Fe Sintered 1 h at U70 C (2680°F) Murakami's etch. Magnification: 350x) (present work) In the light of these explanations this disserta tion work has aimed at developing a tungsten based heavy alloy via powder metallurgy processing routes using elemental starting UJ,Ni,Fe powders. These powders are mixed homogeneously and grinded by using ball mill. The resultant mixtures of powders are subjected to some characterization analyses such as microstruc tural observations by using optical and scanning electron microscopes. The selected mixtures are pressed via uniaxial presses in rijid dies and sintered at different temperatures. 5intered products are characterized by means of microstructure, chemical composition, density and micron arr'nanf-. The stages of microstructural evolution and densif ication is related with literature and especially the solition precipitation mechanizms such as Dstwald ripening, contact flattening and coalescence are discussed. Chemical compositions of different phases and the formed intermetalics are also investi gated by means of X-ray dif Tactometer and energy dispersion spectroscopy (EDS) of SEM. The resuls of these characterization are related with phase-digrams At least the densif ication and micro hardness measurment results are compared with previous studies.