Soğuk sprey eklemeli imalat yöntemiyle üretilen ınconel 718 süperalaşımının yüzey özelliklerine kutu alüminyumlama işleminin etkisi
Effect of pack aluminizing process on surface properties of inconel 718 superalloy produced by cold spray additive manufacturing
Yazar: OSMAN KAHRAMAN
Danışman: PROF. DR. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Inconel 718, 650 C sıcaklığa kadar yüksek sıcaklık dayanımı sayesinde gaz türbin motorlarında geniş uygulama alanlarına sahip demir-nikel süperalaşımıdır. Alaşım geleneksel olarak dövme ve haddeleme yöntemleri ile üretilse de son yıllarda alternatif üretim yöntemleri uygulanmaktadır.
Alternatif uygulamalardan biri aslen bir yüzey kaplama metodu olan soğuk sprey yöntemidir. Yüzey kaplama uygulamalarında kullanılan yönteme benzer olarak kaplama malzemesinin yüksek hızlarda ve düşük sıcaklıklarda püskürtülmesine dayanan yöntem, kütlesel parça üretilmesi için de kullanılmaktadır. Uygulama sıcaklığının düşük olması altlık malzemesinin başlangıç mikroyapısının korunması için önemli bir avantaj oluşturmaktadır. Sprey ile birikim sağlanarak püskürtülen katmanların üzerine yöntem uygulanmaya devam ettikçe yeni katmanlar oluşması sağlanır. Bu şekilde hedef parçanın ana gövdesi oluşana kadar birikim süreci tekrarlanır. Soğuk sprey yöntemi ergime ve sinterleme gibi adımları barındırmayan bir eklemeli imalat metodu olarak tanımlanmaktadır.
Inconel 718 süperalaşımının üretildiği haldeki yüksek sıcaklık mekanik özellikleri ve oksidasyon direnci yüksek olsa da alaşıma uygulanan farklı yüzey kaplamaları sayesinde bu özelliklerin iyileştirilmesi ve servis ömrünün arttırılması mümkündür. Uygulanan kaplama yöntemlerinden biri kutu alüminyumlama yöntemidir. Bu yöntem katı hal difüzyon mekanizması sayesinde altlık malzemenin yüzeyinde çeşitli nikel – alüminid fazları oluşmasını sağlar.
Bu çalışmada soğuk sprey yöntemi ile üretilmiş Inconel 718 süperalaşımının kutu alüminyumlama yöntemi kullanılarak kaplama uygulamasına uygunluğu incelenmiştir. Bu kapsamda 500 C, 550 C, 600 C, 650 C, 700 C olmak üzere 5 farklı sıcaklık parametresi için deneyler gerçekleştirilmiştir. Tüm sıcaklık parametreleri için deneyler 5 saat süreyle uygulanmıştır. Kaplama işlemi için çalışma sıcaklıklarının nispeten düşük belirlenmesi ile hem uygulama maliyetlerinin düşürülmesi hem de altlık malzemesinin mikroyapısının korunması hedeflenmiştir. Deneyler sonrasında kaplama ve altlık mikroyapıları optik mikroskop ile incelenmiştir. 700 C sıcaklık için yaklaşık 61 µm, 650 C sıcaklık için yaklaşık 30 µm, 600 C sıcaklık için yaklaşık 20 µm kaplama kalınlığı oluştuğu gözlenmiştir. Kaplama bölgesinde oluşan fazların belirlenmesi için XRD analizleri gerçekleştirilmiştir. Ek olarak kaplama bölgesinin morfolojisini incelemek için SEM – EDS analizleri gerçekleştirilmiştir.

Abstract:
Inconel 718 is a versatile iron-nickel superalloy widely recognized for its exceptional mechanical and thermal properties, making it an indispensable material in industries such as aerospace and energy. One of its most notable attributes is its remarkable high-temperature strength, which remains robust up to 650 °C. These properties have solidified its role in critical applications, particularly in gas turbine engines where both reliability and performance at elevated temperatures are paramount. Traditionally, this alloy has been manufactured using established methods like forging and rolling, which have proven effective in producing components with consistent quality and desirable properties. However, recent advancements in materials science have spurred interest in exploring alternative production techniques, with the aim of optimizing performance, reducing costs, and enhancing material properties.
Among these innovative techniques is the cold spray method. Originally developed as a surface coating technology, the method has gradually evolved to be explored for the production of bulk materials. This process operates on a unique principle that involves the high-speed spraying of alloy powders onto a substrate. The kinetic energy of the particles, combined with their rapid deformation and adhesion, facilitates the buildup of layers to create a cohesive bulk material. Unlike conventional thermal-based processes, the cold spray method is performed at relatively low temperatures. This characteristic is particularly advantageous as it preserves the initial microstructure of the substrate material, ensuring minimal thermal distortion. As successive layers of powders are deposited, the material grows incrementally, making the method viable for additive manufacturing applications. Hence, cold spray has garnered attention as a promising additive manufacturing approach (commonly referred to as cold spray additive manufacturing or CSAM), where melting or sintering is entirely avoided during production.
The exceptional high-temperature performance of Inconel 718 is derived not only from its intrinsic chemical composition but also from the synergistic effects of heat treatment and its meticulously engineered microstructure. Beyond its bulk mechanical properties, the alloy's surface characteristics such as oxidation resistance can be further improved through specialized coating techniques. Enhancing surface properties is critical for extending the service life of components in aggressive environments. Among various surface treatment methods, pack aluminizing stands out as a cost-effective and efficient technique. This process utilizes solid-state diffusion to form nickel aluminide phases on the alloy's surface, which serve as a protective barrier against oxidation and other surface-related degradations.
In the present study, the feasibility of applying pack aluminizing to Inconel 718 produced via cold spray additive manufacturing was systematically investigated. The study examined the effects of various temperature and time parameters on the coating process, specifically focusing on the formation, thickness, and integrity of the aluminized layer. Pack aluminizing was conducted at temperatures of 500, 550, 600, 650, and 700 °C for a fixed duration of 5 hours, with an additional set of experiments at 600 °C for varying durations of 1, 5, and 10 hours. Results indicated that 600 °C represents an optimal process temperature, balancing cost-efficiency and coating quality. For lower temperatures such as 500 and 550 °C, only a very thin and discontinuous coating layer was observed under optical microscopy. However, at temperatures of 600 °C and above, the coating layer thickness increased significantly, with 20 µm achieved at 600 °C.
To ensure a comprehensive understanding of the coating's characteristics, advanced characterization techniques were employed. Scanning electron microscopy (SEM) was used to evaluate the morphology of the coating layer, while X-ray diffraction (XRD) analysis provided insights into the phases present within the layer. When the XRD test graphs of the samples after coating were examined, it was observed that Ni3Al dominated phases formed in the sample coated at 600 °C for 5 hours and under similar parameters. With the increase in temperature and duration, it was determined that the aluminum content in the Ni-Al phases increased, leading to the formation of NiAl3.Microhardness testing was conducted across the coating and substrate to assess mechanical properties. Furthermore, optical microscopy was used to examine the microstructural features of the coating and substrate, with particular attention given to the interface between the two regions.
At 600 °C, a uniform coating layer of approximately 20 µm thickness was achieved. When the process temperature was raised to 650 °C, the thickness increased to around 30 µm, and at 700 °C, a more substantial layer of approximately 60 µm was observed. Optical microscopy of the samples revealed significant findings regarding the coating interface. Across all examined temperatures, a continuous and homogeneous distribution was noted at the interface, indicating strong adhesion and minimal disruption during the aluminizing process. Although some cracks and defects were identified in the substrate material, these were attributed to the inherent characteristics of the cold spray manufacturing process. Importantly, these defects did not propagate through the coating layer, affirming the integrity of the interface.
The study also highlighted the absence of pore-like defects within the aluminized layer, further supporting the quality and reliability of the coating. This absence, along with the distinct differences in the nature of defects observed in the substrate and coating regions, underscores the suitability of cold spray-produced Inconel 718 for pack aluminizing applications. By carefully controlling temperature and time parameters, the pack aluminizing process not only enhanced the surface properties of the alloy but also preserved its bulk mechanical properties, making it a highly effective strategy for extending the performance and lifespan of components under demanding conditions.
SEM-EDS and XRD analysis results of oxidation samples shows that after pack aluminizing, Al2O3 phase is detected on surfaces of the samples. On the other hand, Cr2O3 is detected for bare samples of Inconel 718. When the weight change per unit surface area graphs were examined, it was observed that the samples coated using the pack aluminizing method exhibited a weight gain, while the uncoated samples showed a weight loss. This is attributed to the protective characteristic of the Al2O3 phase which provides a stable layer and prevents the formation of Cr2O3 present on the surfaces of the coated samples.