Titanyum ilavesinin nikel alüminyum bronzunun yapı ve özelliklerine etkisi
Effect of titanium addition on structure and properties of nickel aluminum bronze
Yazar: TURHAN ÜRÜN KOÇAK
Danışman: DR. ÖĞR. ÜYESİ CEVAT FAHİR ARISOY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Bakır yüksek iletkenliği ve korozyon dayanımı ile tanınan bir metaldir. Ancak düşük mukavemeti bu metalin belirli özelliklerini iyileştirme ihtiyacı doğurmuştur. Bakırın kalay ile alaşımlandırılması sonucu bronz çağı başlamış ve demir çağına kadar bakır alaşımları yaygın olarak kullanılmıştır. Günümüzde ise bakır alaşımları korozyon dayanımı, aşınma dayanımı gibi özellikler gerektiren özel uygulamalarda tercih edilmektedir. Alüminyum bronzu grubunda bulunan nikel alüminyum bronzları üstün korozyon ve aşınma dayanımı özelliklerine yüksek mukavemet ile birlikte sahip olmaları nedeniyle denizcilik başta olmak üzere pek çok alanda kullanılmaktadır.
Literatürde nikel alüminyum bronzlarının bu üstün özelliklerini daha da ilerletmek amacıyla gerçekleştirilmiş çok sayıda çalışma yer almaktadır. Farklı yaklaşımlarla gerçekleştirilen bu çalışmalar içerisinde en düşük maliyetli olan ve yüzey işlemlerine kıyasla alaşımın tamamının özelliklerine iyileştiren çalışmalar farklı alaşım elementi ilaveleri ile gerçekleştirilmiştir. Krom, gümüş, boron ve kurşun gibi ilavelerle malzeme özelliklerinde ilerlemeler raporlanmıştır. Nikel içermeyen alüminyum bronzuna yapılan titanyum ilavesinin aşınma hızını düşürdüğü ve mekanik özellikleri iyileştirdiği literatürde yer almaktadır.
Bu çalışmada farklı oranlarda titanyum ilavesinin nikel alüminyum bronzunun mekanik özelliklerine, mikroyapısına ve aşınma dayanımına olan etkilerinin belirlenmesi amaçlanmıştır. Bu amaçla döküm yöntemi ile iki farklı oranda titanyum içeren nikel alüminyum bronzu üretilmiş, yine çalışma kapsamında üretimi gerçekleştirilen titanyum içermeyen alaşım ile özellikleri kıyaslanmıştır. Döküm malzemeler sıcak dövme işlemine tabi tutulmuş, hem döküm durumda hemde dövme sonrasında alaşımların mikroyapıları incelenmiş, mekanik testler ve aşınma testleri gerçekleştirilmiştir. Çalışma kapsamında alaşımlara normalizasyon, su verme ve yaşlandırma ısıl işlemleri uygulanmış, ısıl işlem sonrası alaşımların mikroyapıları incelenmiş ve sertlik ölçümleri yapılmıştır.
Çalışma sonucunda döküm alaşımlardan düşük miktarda titanyum içeren alaşımın akma mukavemeti ve sertliğinde bir miktar iyileşme olduğu ancak her üç alaşımında aşınma daynımlarının benzer olduğu saptanmıştır. Isıl işlemler sonrasında titanyum içermeyen alaşıma kıyasla düşük miktarda titanyum içeren alaşımın sertliğinin küçük bir miktar daha yüksek olduğu belirlenmiştir. Titanyum içeriğinin yükselmesiyle birlikte alaşım mikroyapısında yumuşak faz miktarının artışı nedeniyle mekanik özelliklerde düşüş görülmüş, yapıda bulunan sert fazların irileşmesi sonucu tokluk değerleri de düşmüştür. Yüksek titanyum oranında mikroyapı kaynaklı aynı etkinin ısıl işlemler sonrasında da devam ettiği belirlenmiştir. Dövme sonucu farklı oranlarda titanyum içeren malzemelerin tamamının çekme dayanımları ve sertlikleri yükselmiştir. Ancak aşınma dayanımlarında kayda değer bir fark görülmemiştir.

Abstract:
Copper was first used nearly 10.000 years ago. Copper is well known with its electrical and thermal conductivity also with its corrosion resistance. But copper's low hardness and strength ingenerated need to improve its certain characteristic. With discovery of alloying of copper, bronze age began. Bronzes are widely used till beginning of iron age. After discovery of electricity copper and its alloys became popular because of their good conductivity.
There are different classifications for copper alloys on different sources. But it's possible to classify copper alloys according to application areas and material characteristics as high strength copper alloys, corrosion resistant copper alloys, high conductivity copper alloys and wear resistant copper alloys. Aluminum bronze alloy group consists of wide range of alloys which are used in different applications with their different properties. Nickel aluminum bronzes are one of these alloys and well known with its wear and corrosion resistance.
Nickel aluminum bronzes contain 8 %-11 % aluminum, also nickel, iron and manganese. This alloy group is widely used in industry both in cast and wrought conditions. Aluminum is the most effective alloying element of nickel aluminum bronzes. Its weight percent effects directly the mechanical properties. Iron is used as grain refiner and this refining effect is obvious till 3.5 %. Iron also forms Fe-rich kappa precipitations which improves materials strength. Nickel content improves hardness and corrosion resistance of nickel aluminum bronzes but it reduces the ductility of the material.
Production method of this alloy group is determined by application type and desired material properties. Different casting techniques like sand casting, centrifugal casting, continuous casting, permanent mold casting and hot working methods like forging, extrusion and rolling is used by manufacturers. Heat treatments like annealing, normalizing, quenching and gaining are also applied to improve mechanical properties.
Nickel aluminum bronzes have complex microstructures and it affects material properties to a high degree. Microstructure contains soft alfa matrix, martensitic phase βı and hard intermetallic compounds which are called kappa phase. Mechanical properties and wear rate of nickel aluminum bronzes depend on quantity of soft alfa and hard kappa+ retained beta phase. With increase of volume fraction of alfa phase, strength tends to decrease, with increase of hard kappa precipitates strength of materials tends to increase. Volume fraction of these phases also effect wear mechanism of the alloys. Adhesive wear occurs for nickel aluminum bronzes with high amount of alfa phase, with decrease of alfa phase and increase of hard phases, abrasive wear occurs. It's possible to control volume fraction of those phases with hot forming and heat treatment operations.
Even though nickel aluminum bronzes have superior mechanical and wear properties, there are different approaches on literature to improve its properties even further. One of them is to optimize production parameters. This involves additional processes during casting for grain refining, optimizing deformation degree or temperature during hot forming, optimizing heat treatment parameters. Second approach is severe plastic deformation which has very high effect on the material properties but brings more cost and time loss for manufacturers. Third approach is to use surface modification techniques to improve hardness and wear resistance. It's an effective way to increase materials properties but its effect is limited by the surface. Last approach is additions of different alloying elements, which is effective on bulk properties and cost efficient. Different additions like chromium, silver, cerium were made by different researchers to Nickel Aluminum bronzes. Besides addition of titanium to aluminum bronze gave promising results on mechanical properties. Some researches were conduct by a research group by additions of boron, titanium and lead together to nickel aluminum bronze, results were also promising.
The purpose of this study is to determine effect of titanium addition on properties and structure of nickel aluminum bronze. With this purpose three castings were made with following titanium contents, 0 %, 0.2 % and 0.9 %. Chemical compositions of castings were determined by wet chemical analysis. Macrostructures of castings were observed, microstructures are examined by optical light microscope and phase fractions are determined by an image analysis software. Mechanical tests, notch impact test and wear tests were performed. Samples were heat treated with 4 different conditions; normalizing, quenching and two different aging treatments were done. Microstructures of heat treated samples were examined by optical light microscope and hardness tests were performed.
Macro etching results shows that grain refining was occurred on as cast samples by increasing titanium addition. Typical cast microstructure was observed for the cast sample which not contains titanium. Effect of titanium addition was determined by phase friction analysis. By 0.2 % titanium addition volume fraction of hard kappa phase were increased, while volume fraction of kappa phases decreased and alfa content increased for 0.9 % titanium containing sample. Coarse hard precipitates were also observed on sample with 0.9 % titanium content. These differences on microstructures were effected mechanical properties. Hardness and yield strength of sample containing 0.2 % titanium is increased a bit. Tensile and yield strength, hardness of 0.9 % titanium containing sample were decreased because of high alfa phase content. Meantime elongation of this sample was decreased because of coarse and hard precipitations.
According to wear test results, friction coefficients of cast samples are similar. Friction coefficient of 0.9 % titanium containing sample were a bit higher than other two samples. This difference caused by hard precipitates which are observed on this samples microstructure. Wear profiles of all samples and wear surface morphologies were similar.
Microstructure exemination showed that volume fraction of reteined beta phase were increased from 8-9 % to 30-35 % for each material by hot forging. Also microsturcture became finer after hot forging. Due to this changes, after hot forging tensile strenght and hardness of all materials were increased sucssesfully, while wear loss were decreased.
Microstructures and hardness of cast alloys were examined after heat treatments. βı content was increased after normalizing on all alloys and more Widmenstatten structure was observed on 0.2 % titanium containing sample. βı content was increased further by quenching, no kappa phases observed except 0.9 % titanium containing sample. Quenching were not enough to solve the coarse phases. βı content decreased by aging at 525oC and some κıı were precipitated. When aging temperature was increased to 650oC, more κıı were precipitated and βı content decreased. Volume fraction of alfa were higher than other samples on 0.9 % titanium containing alloy. There were more precipitations observed on the microstructure after aging at 650oC in comparision with other samples.
Hardness of all samples were increased by normalizing and further increased by quenching. Hardness tends to decrease with increasing aging temperature due to dissolution of βı phase to α+κ phases. Hardness of nickel aluminum bronze and nickel aluminum bronze containing 0.2 % titanium were similar while hardness of sample with 0.9 % titanium content were lower than other samples because of higher alfa phase content