Sıcak daldırma yöntemiyle aluminyum kaplanmış inconel 718 süperalaşımının oksidasyon direncinin incelenmesi
Investigation of oxidation resistance of hot dip aluminized inconel 718 superalloy
Yazar: TOLGAY KALAYCI
Danışman: PROF. DR. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Daldırma kaplama yöntemi = Dipping coating method ; Mikrosertlik = Microhardness ; Seramik kaplama = Ceramic coating ; Termal bariyer kaplama = Thermal barrier coating ; Termik oksidasyon = Thermal oxidation ; Yüzey kaplama = Surface coating ; Yüzey kaplama malzemeleri = Surface coating materials ; Yüzey kaplama yöntemleri = Surface coating methods

Özet:
Havacılık endüstrisinde oldukça popüler malzemeler olan nikel esaslı süperalaşımlar, yüksek sıcaklıklarda gösterdikleri iyi oksidasyon ve korozyon dirençleri nedeniyle özellikle uçakların türbin motor parçalarında en çok tercih edilen malzemelerdendir. Nikel esaslı süperalaşımlar arasında en çok kullanılan alaşımlardan biri olan çökelme sertleşmesi ile dayanımı artırılmış Inconel 718 süperalaşımı, pahalı bir malzeme olmasına rağmen sahip olduğu yüksek mekanik özellikleri (akma, çekme ile kopma dayanımları) nedeniyle sıkça tercih edilmektedir.
Inconel 718 süperalaşımı, diğer süperalaşımlardan farklı olarak yapısında γ″-Ni3Nb fazını bulundurur ve yapıya asıl dayanımı kazandıran mekanizama bu γ″ parçacıklarıdır. Fakat Inconel 718 süperalaşımı, yüksek sıcaklıklarda çok iyi mekanik özelliklere sahip olmasına rağmen ~649 °C sıcaklıktan sonra mekanik özelliklerinde düşüşler meydana gelmektedir ve daha yüksek sıcaklıklarda kullanılabilmesi için kaplama işlemlerine gereksinim duyulmaktadır.
Yapılan bu çalışmada Inconel 718 süperalaşımı; sıcak daldırma yöntemi ile aluminyum kaplanarak kaplama sıcaklığının ve süresinin kaplama morfolojisine etkisi incelenmiştir. Aluminyum kaplama işleminin optimizasyonu için 3 farklı sıcaklıkta (680 °C, 755 °C ve 830 °C) ve 4 farklı zamanda (30 s, 1 dk, 5dk ve 10 dk) kaplama işlemi yapılmıştır.
Daha sonra kaplanmış ve kaplanmamış numunelere yüksek sıcaklıkta oksidasyon işlemi uygulanarak oksidasyon dirençleri gözlemlenmiştir. Yüksek sıcaklıkta oksidasyon için farklı sıcaklıklarda 5'er dakika ile kaplanmış numuneler seçilerek 1, 6, 48, 192 ve 336 saat boyunca 1000 °C'de bekletilerek işlem gerçekleştirilmiştir. Oksidasyon işleminden sonra numuneler oda sıcaklığında soğumaya bırakılıp ilk ve son ağırlık değişimleri belirlenmiştir. Kaplama parametresine bağlı olarak oksidasyon zamanının kaplama yapısındaki değişimini gözlemlemek için numuneler kesitten mikroskop altında incelenerek EDS analizleri yapılmıştır. Kaplamanın farklı kısımlarından ölçümler yapılarak sertlik değişimleri incelenmiştir.
Ayrıca farklı bir oksidasyon işlemi olan mikro ark oksidasyon işlemi 5'er dakika ile farklı sıcaklıklarda aluminyum kaplanmış numunelere uygulanarak oksidasyona uğratılmıştır. Tüm yüzeyi mikro ark ile oksit kaplanmış numuneler 1000 °C'de 336 saat boyunca yüksek sıcaklıkta oksidasyona maruz bırakılmıştır. Bu sayade sıcak daldırma yöntemiyle aluminyum kaplanmış malzemelerin mikro ark oksidasyon işleminden sonra yüksek sıcaklıkta oksidasyona karşı direnci incelenmiştir.

Abstract:
Superalloys, especially Ni base superalloys are the most popular materials because of their good oxidation and corrosion resistance at high temperature. Inconel 718 superalloy is a precipitation hardenable nickel based superalloy (FCC) which is used for a high temperature application such as turbine blades, power generation, aircraft turbines, combustion chamber, turbine discs etc. in which exposed to extremely tough condition. Inconel 718 superalloy is strengthened with Ni3Nb particules which is be found as a γ″ content. It has good mechanical properties (tensile, yield and rupture strength etc.) up to about 650 °C.
This superalloy also contains chromium to provide oxidation resistance through the formation of a protective surface film of chromium oxide (Cr2O3). Also, other oxides scales Fe,Ti(NbO4) and (Ni,Cr,Fe)2O3 can be found in the oxide layer. But at higher than about 900 °C temperature, sometimes it can be at 800 °C - 850 °C in service condition, protective of chromium oxide forming is limited by reason of the formation of volatile CrO3. Aluminium oxide (Al2O3) is more protective layer as an oxidation resistance compare to Cr2O3 and observe better resistance to spalling up to 1350 °C. Moreover, Al2O3 has a slow growing ability on Ni base superalloys under oxidative conditions due to oxygen diffusivity of alumina is the lowest in oxides layer.
These applications need an aluminide coating for protection against oxidation and corrosion. The aluminium based diffusion coating provide extended lifetime to high temperature application. The main factor of coating performance is relevant determined composition and microstructure of coating layer.
It has been shown that diffusion coating process has a good adhesion between the substrate and inner layer such as chemical vapor deposition (CVD), pack aluminizing, hot dip aluminizing, slurry process. Mostly; diffusion coating process is used in aerospace industry as a bond coating.
One of the diffusion coating processes is hot dip aluminizing which the surface of metallic component is coated with a layer of aluminum. Aluminizing process provides a better intermetallic layer between the material and its aluminum coating. Aluminized Inconel 718 superalloy has better resistance to oxidation and corrosion than uncoated alloy. The material is immersed in a molten aluminum alloy for a certain period of time at specific temperature throughout the HDA process. Hot dip aluminizing is more cost-effective technique and coated the material in a short time easily than other coating methods. The rate of deposition of an aluminum layer on the substrate is very fast and average coating thickness of 0.4 mm could be obtained in around 10 minutes depend on process temperature. This aluminum layer is changed to alumina oxide at service temperature which provide oxidation and corrosion resistance. So, morphology and composition of bond coating which must involve sufficient high ratio aluminum, is very important because of supporting the continued growth of thermally grown oxide layer. Aluminum is used from protective aluminum oxide and substrate alloy-coating interdiffusion, thus spallation is probably occurred if the aluminum content is consumption with the elapsing time.
Micro-arc oxidation process (MAO), also known as plasma electrolytic oxidation (PEO), is a new technique which is used on light metals such as Mg, Al, Ti etc. and their alloys for transform oxide scale. This process is used for thick and hard ceramic coatings. Its provide good wear and corrosion resistance and protection against high temperature oxidation. This ceramic coating process is usually used in automotive, aerospace and medicine application. This process can not be applied to steel surface directly so materials must be coated with light metals.
In this study, Ni base superalloy Inconel 718 was coated by hot dip aluminizing process and ceramic surface was obtained on the aluminum layer by using MAO. Then, all of the specimens was exposed high temperature oxidation to studied their oxidation resistance. Hot dip aluminizing and micro arc technique of behaviour was optimized. The phase composition, the thickness of coated intermetallic layers and the cross sectional hardness were investigated.
Circular specimens of Inconel 718 superalloy were used as a base metal for hot dip aluminizing in the present work. The specimens were cut to diameters of 8 mm and 8 mm length and then prepared for aluminizing proces by grinding 120 mesh to 400 mesh with abrasive paper. All of the surfaces cleaned ultrasonically to remove grease in phosphoric acid and acetone for 15 min separately and then dried before the aluminizing process. All specimens were tied with stainless steel wire for dipping process.
Pure aluminum (Al>99.967) was used in this process. Aluminum was melted in graphite crucible using electric melting furnace at 680 °C, 755 °C and 830 °C. Then, flux which is containing 40 wt % KCI and 60 wt % NaCl, was added on to melted aluminum and cleaned the surface after 15 min when the temperature reached the dipping condition. Temperature is controlled by K-type thermocouple before every coating process. Specimens were immersed manually into graphite crucible for 30 s, 1 min, 5 min and 10 min and when the time is reached process condition, coated specimens was taken out slowly. After the hot dip aluminizing, specimens were cooled in air. The temperature is the most important parameters than times or others dipping conditions. Because the phase is only changed by the temperature. But coating thickness is increase with both times and and temperature. The bath composition is the other important parameters because it changed phase composition so intermetallic zones changed, too.
Micro arc oxidation test was applied 450 V positive and 80 V negative for 5 min to aluminized specimens at 680 °C, 755 °C and 830 °C which were aluminized at 5 min, for the top of the Al layer transform to Al2O3 which is protective oxide layer and provide more hardness surface than aluminium. The solution which contains 4.5 g/l NaAlO2 and 2 g/l KOH, prepared in the distilled water.
All of the aluminized specimens and the bare substrate were oxidized at 1000 °C for 1 h, 6 h, 48 h, 192 h and 336 h in resistance furnace as a high temperature oxidation process. Also aluminized specimens which were oxidized by micro arc, oxidized at 1000 °C for 336 h to compare other oxidized specimens. Then the specimens were cooled down to the room temperature in air.
Micro hardness measurements were conducted by a micro Vickers hardness tester at the maximum load of 25 g. In every layer, five measurement were done and then averaged.
The specimens were mounted for metallographic examination. The morphology of the coating and oxide layer was studied by SEM; phase composition was characterized by EDS. Top of the coating layer was analyzed by XRD. Micro Vickers hardness measurement were applied from the cross sections of the coatings to characterize different layers. For the high temperature oxidation process, all of the oxidized samples measured weight changes to determine the oxidation resistance.