Hidrasyon/dehidrasyon (HDH) yöntemiyle geri dönüştürülen Ti6Al4V talaşından titanyum esaslı kaplama üretimi
Producing of titanium based coating from recycled Ti6Al4V turnings by hydrogenation/dehydrogenation (HDH) technique
Yazar: DİLARA ÇELİK
Danışman: PROF. DR. HÜSEYİN ÇİMENOĞLU
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Metaller doğaları gereği dönüştürülebilir kaynaklardır. Üretimleri sırasında oluşan ve kullanım ömürlerini tamamlamaları sonucunda meydana gelen kayıplar geri dönüşüm proseslerini elzem kılmaktadır. Geri dönüşüm ile birlikte kaynak kullanımı, enerji kullanımı ve kayıpları, doğaya verilen zarar azalmakta aynı zamanda ekonomik olarak da fayda sağlanmaktadır. 
Titanyum ve alaşımlarının sahip oldukları düşük yoğunluk, yüksek dayanım değerleri ve üstün korozyon direnci uzay-havacılık, askeri, medikal ve spor malzemeleri gibi alanlarda tercih edilmelerine neden olmaktadır. Ancak bu üstün özelliklerinin yanı sıra yüksek maliyet, üretim ve şekillendirme zorluğu kullanılabilirliğini kısıtlamaktadır. Üretim ve şekillendirme prosesleri sırasında meydana gelen kayıpların %70 seviyelerinde oluşu yüksek maliyet değerlerini daha da arttırmaktadır bu yüzden titanyumun sahip olduğu oksijen, karbon ve azota olan ilgisi göz önünde bulundurularak geri dönüşümünün yapılması maliyet değerlerini ve malzeme kaybını önemli ölçüde azaltabilmektedir.
Titanyum ve alaşımlarının geri dönüşümü için toz metalürjisi yönteminin (PM) döküme göre çok daha ekonomik, elverişli, ürün özellikleri kontrol edilebilir, ürün kaybı az olan ve ekstra işlem gerektirmeyen bir yöntemdir. Genellikle süngerden titanyum tozu elde etmek için kullanılan hidrasyon-dehidrasyon (HDH) yöntemi, hidrojene ilginin fazla olduğu beta fazına sahip titanyum alaşımları için daha elverişlidir. Bu yöntemde, uygun sıcaklık ve basınç değerleri ile birlikte hidrojen atomu kafesi modifiye ederek oda sıcaklığında kırılgan ve oksijene ilgisi olmayan hidrat fazlarının oluşturmaktadır. Böylece koruyucu atmosfer olmaksızın istenilen tane boyutunda toz üretilebilmekte ve uygun denge basıncının altında ki bir basınçta hidrojen atomlarının kafesi terk etmesiyle dönüşüm tamamlanmaktadır. Elde edilen toz, yöntemin sağladığı avantajlar sayesinde yüksek saflıkta olmaktadır. Yöntemi elverişli hale getiren, herhangi bir koruyucu ortama gerek kalmadan hidrat yapının sağladığı güvence ile koruyucu atmosfere gerek kalmadan istenilen prosesin uygulanabilmesidir. Kolay uygulanabilir oluşu ve proses sırasında malzeme kayıplarının yok denebilecek kadar az oluşu yöntemin diğer artılarındandır. 
Soğuk püskürtme (cold spray) prosesi diğer termal püskürtme proseslerine göre ergime olmaksızın katı halde parçacıkların çok yüksek hızda ve düşük sıcaklıkta püskürtülerek daha yoğun kaplama üretimine olanak sağlamaktadır. Günümüzde soğuk püskürtme prosesi güvenilir ve çevre dostu bir teknoloji olup, özellikle metalik esaslı kaplamaların endüstriyel uygulamalarında birçok acıdan önemli fırsatlar sunmaktadır. Yöntemin düşük sıcaklıklarda yapılıyor oluşu oksitlenme risklerini azaltmakta buda elde edilen ürünün kalitesini arttırmaktadır. Elde edilen kaplamalarda oksit, porozite ve artık gerilmeler oldukça düşük seviyededir.
Bu çalışmada, Ti6Al4V talaşının HDH yöntemi kullanılarak geri dönüştürülmesi sonucunda üretim kayıplarını azaltmak ve sonucunda elde edilen ürünün kullanılabilirliğinin soğuk püskürtme yöntemi ile kanıtlamak amaçlanmıştır.

Abstract:
Ti6Al4V alloy (Ti, 6wt% Al, 4 wt% V) is α+β phase titanium alloy and very commonly used due to its high strength/weight ratio, corrosion resistance, bio compatibility, processability, toughness and low thermal expansion behaviour. The application areas of this alloy are mostly aerospace, automotive and medical sectors, occupies about 50% of the total titanium market. 
As the market of this alloy has been growing in last years and large amount of scrap has been founded for recycling. It has been estimated that the size of the global titanium scrap market is 80,000 mt for higher-quality aerospace titanium scrap, plus an additional 80,000 mt for lower-quality, mixed ferrotitanium scrap.  
Ti6Al4V alloys have relatively high prices and in addition, the utilization rate is quite low. In other words, up to 70-80% of the input material is lost while processing the material. Powder metallurgy of Ti6Al4V alloy can be an alternative method either to use powder as the starting material or to recycle the new scrap which coming from semi-fabrication and manufacturing operations as the powder with improved purity.
Due to high oxidation tendency of titanium, it is very complex to produce titanium powder. Intensive researches have been performed on powdering titanium melt under vacuum or intert gas atmosphere however, those special technologies such as EIGA (Electrode Induction melting Gas Atomization), VAR (Vacuum Arc Melting), CHM (Cold heart melting) etc. increase the production cost enormously. Besides that, re-melting may cause imbalance in the composition of the alloys. To overcome those disadvantages, HDH (Hydrogenation/Dehydrogenation) can be used as an economical method of titanium powders production.
HDH method benefits from room temperature embrittlement of the hydrate and produces powder size hydrate which must be dehydrated to produce the final powder product. The balance pressure and the temperature of the system are the main parameters for hydride transformation. The typical reaction can be written as M + H_2  ↔〖MH〗_2 and can be reversed by the balance pressure of hydrogen gas. If the pressure is above the equilibrium pressure, hydrogen atoms enter the lattice to form metal hydrate; if it is below that level, hydrogen atoms decompose from the metal to hydrogen gases. During the hydration, some of hydrogen atoms (H) enter the crystal lattice under the critical temperature and pressure. Then while cooling under the room temperature, hydrogen atoms are diffused crystal lattice and occupy the tetrahedral sites. This mostly occurs in beta phase which has more hydrogen solubility than alpha. After the produced Ti-Hydrate phase which provides the embrittlement, dehydration part will be practicable. Finally, the powders must be kept under the protective atmosphere because of the affinity of the titanium. 
Titanium and its alloys could play an important role in improvement of energy efficiency and in emission reductions because of its magnifical potential for weight savings in automobiles and aircrafts. Consequently, it is a great interest for researchers to develop low-cost methods for spray forming of titanium based materials. Cold gas dynamic spray technology, has the potential to quickly fabrication complex parts or additive features with minimal waste. Cold spray is an all-solid state, high kinetic energy, coating and free-form manufacture process that uses high pressure compressed gas to propel micron-size solid particles onto a substrate under atmospheric conditions. This supersonic feed is capable of dragging powder particles to 500 to 1500 m/s directing them toward a substrate where they impact, plastically deform, and adhere. It has been documented that cold spray coating is dependent on numerous factors, such as the morphology of the feedstock powder, the process conditions under which the coating is produced, the preparation of the substrate surface, etc. In addition, cold spray pure titanium and Ti-6Al-4V coatings are reported to be generally porous, except with very high particle velocity obtained by using helium as propelling gas, which in turn, unfortunately, is substantially more expensive than nitrogen gas or air.
Ti6Al4V turnings were placed into a reactor after washing and drying stage for the hydrogenation step. Hydrogenation was performed under 2.5 bar hydrogen atmosphere and the turnings were heated with a heating rate of 400°C/hour up to almost 700 °C. Hydrogenation is an exothermic process. The activation started for the corresponding turnings around 470 °C and temperature increment was observed even after shutting off the furnace. The product after hydration process was milled in the steel ball milling equipment (Retsch GmbH-MM 301) with the frequency of 20 s-1 for up to four minutes and sieved under 100 μm. The sieved powder was placed again in the same reactor and the dehydrogenation was started nearly at the same temperature as the hydrogenation. At the end of the dehydrogenation period, the system was cooled under vacuum down to room temperature. The hydrogen-free powder was taken out from the reactor in a glovebox. After dehydration process, the powder is light sintered under high temperature and vacuum atmosphere is difficult to re-size. Therefore, the dehydrated powder was milled once again in the steel-ball milling unit under argon atmosphere. After a final sieving, stored under argon atmosphere.  
For coating part, the constant parameters used for all experimental tests included air as the propelling gas, nozzle standoff distance of 1 cm, beam distance of 2mm, and gun traverse speed at 1 mm/s. After coating, samples cut from the Cp-Ti were successively ground on 240–1200 mesh SiC abrasive papers. Final polishing was made with silica slurry abrasive to achieve a mirror-like surface finish. Then these samples were subjected to oxidation at 600°C, for different time intervals ranging between 24h, 48h, 72 h.
The characterization was conducted by the following methods: elemental analysis by atomic absorption (for Al, V, Fe) solid state infrared absorption (for oxygen) and thermal conductivity (for hydrogen), phase analysis by XRD (X-ray Diffraction, BrukerTM D8 Advance Series 35 kV 40 mA), flowability measurement by AOR (Angle of Repose), morphology by SEM (Scanning Electron Microscope, JEOLTM JCM-6000Plus NeoScope) and OM (LeicaTM DM6000 M), particle size distribution by laser diffraction (Mastersizer 2000), Bioactiviy was analyzed using SBF tests.  The test was conducted by immersing the samples in SBF solution. Hardness measurements were carried out on the cross-sections of the polished samples with a Vickers indenter by utilising ShimadzuTM HMV microhardness tester, dry and wet (in SBF solution) sliding wear performance of the coatings was tested on a Tribotech™, wear tracks were examined by using a Veeco Dektak 6 M surface profilometer . 
The objective of this research is to decrease the cost and increase the recycling rate of titanium and alloy by HDH (Hydrogenation-Dehydrogenation) process which is known to be the most economical method of producing titanium powders. The recycled product can be applied in cold-spray process which are highly recommended at new age due to perfect processing condition and cost efficiency.