Pack aluminizing of high Si-Mo cast iron and investigation of high temperature oxidation behavior

Nowadays, with the development of technology, especially in the automotive and aerospace industry, it becomes more and more difficult to manufacture without compromising quality and safety in order to meet customer demands. While working to meet customer demands, keep costs at a minimum and develop environmentally sensitive systems forces the materials to work on heavier conditions. Many materials used in the automotive and aircraft industries are expected to show high performance in high temperature, high pressure and corrosive environments. Since producing new materials and establishing new facilities for production increase the costs, instead of producing new materials, it is a more attractive option to upgrade the currently used materials and to adapt them to working conditions. Metallic materials frequently used in the industry are heat treated to improve various properties. Through heat treatment, the mechanical properties of the metallic material can be increased to the desired level. However, it is not possible to increase high temperature behaviors, wear and corrosion resistance at desired levels by heat treatment. Surface coating techniques are known to improve the surface properties of metallic materials and also, which are used to protect metallic materials from environmental influences. Cast iron is preferred due to its high sound damping and cost-effectiveness in engine parts which are frequently use in the automotive industry and exposed to high temperature. However, high temperature oxidation behavior and low fatique resistance of cast iron cause the crack formation. These cracks reduce the strength and life of cast irons. Alloying elements add to increase the strength of cast iron at high temperatures and to prevent cracks against expansion-shrinkage. For this reason, High Si-Mo cast irons are used in engine parts such as exhaust manifold. Environmental constraints, such as system developed for increased fuel efficiency and exhaust emissions in recent years have cause engine temperatures to rise. Alloying elements added to cast irons provide the requirements such as strenght; however, they need to be developed due to their weakness in corrosion and oxidation. Surface modification are frequently used to improve the surface properties of iron-based materials in industry. Fe-Al intermetalic compounds have low density and show high properties againts to high temperature oxidation. Its known that aluminum coatings is applied to iron based materials by various aluminizing methods (Chemical Vapor Deposition (CVD), Physical Vapor Derposition (PVD), Plasma Spraying, Thermal Spraying, Hot Dipping and Pack Aluminizing). Hot dipping and pack aluminizing methods are preffered for homogenous coatings of parts with complex geometry. In hot dipping process, additional heat treatment is required to form the desired intermetallic phase in the coated materials. However, intermetallic phase can be formed in pack aluminizing process without the necessity of a secondary process. In this study, high Si-Mo added spheroidal graphite cast iron which used for engine parts is aluminized by pack aluminizing method. The effects of different temperature, time and pack composition on Fe-Al intermetallics phase and intermetallic thickness have been investigated. The effects of Fe-Al intermetallics phases on high temperature oxidation resistance have been examined. Intermetallics were investigated using scanning electron microscopy (SEM), optical microscope and X-ray diffraction (XRD) analysis. These samples which were subjected pack aluminizig process were applied high temperature oxidation tests. The effects of different Fe-Al intermetallic phases and different coating thickness on high temperature oxidation behavior has been studied. Pack aluminizing process was applied at 750, 850 and 950 °C. 6, 12 and 18 hours pack aluminizing process were performed to determine the effect of the process time. Pack powder composition containing 15 wt% and 30 wt% pure Al powder were used to determine the effect of Al content in powder mixture used in pack aluminizing process. Aluminized samples were exposed to oxidation at 750 and 850 °C for 24 hours. Fe2Al5 intermetallic phase was observed in the samples which were aluminized at 750 °C. Also, it was observed that the intermetallic layer thickness increased with the rise of experimental time. Similarly, it has been detected that the excess amount of pure Al in powder composition increases the intermetallic layer thickness. Intermetallic formation was not observed in the samples which were aluminized at 850 °C. Hovever, it has been noticed that Fe –Al solid solution is formed considering the shift in the XRD peaks. Both Fe-Al intermetallic phase and solid solition observed in the samples which were aluminized at 950 °C. The increase in the amount of pure Al in powder mixture and prosess time rised intermetallic layer thickness. The samples performed to the pack aluminizing process were exposed to oxidation at 750 and 850 °C for 24 hours. 70- 150 µm oxide film was observed in the high Si-Mo iron without any treatment. Samples with pack aluminizing applied 750 and 950 °C have intermetallic layer and the oxide film thickness did not exceed ~10 µm. However, since the intermetallic coating was not formed in the samples which were aluminized at 850 °C, ~50 -150 µm oxide film detected on the surface. It has been determined that resistance to oxidation is elevated on the surfaces forming intermetallic coatings.