Investigation of oxidation and wear behaviour of electron beam melted and conventionally fabricated ti6al4v alloys after pack aluminizing

Abstract

In this thesis, the microstructural characterization, phase analysis, mechanical properties evaluation, wear and oxidation resistance of the pack aluminized EBM and conventionally fabricated Ti6Al4V alloy were investigated. Additive manufacturing is a rapid manufacturing method consisting of layer-by-layer addition and fusion of metal powders to produce 3D parts. Generally, a laser or electron beam is used to melt the powder locally. Additive manufacturing is a very attractive manufacturing method for sustainable product development due to its positive features such as flexibility, reduction of waste material and short production time. Titanium and titanium alloys are fabricated by the traditional method, but the electron beam melting method can also be applied to these alloys. The electron beam melting method enables the successful production of components used in the aviation, biomedical and automotive sectors due to production of complex parts, shortening of production time, control of process parameters and the ability to change mechanical properties. Parts produced with electron beam melting are generally denser than parts obtained with other additive manufacturing methods, but they still have lower wear resistance than their counterparts obtained by traditional method. For this reason, surface treatments are widely applied. Titanium and titanium alloys are preferred as engineering materials in many fields due to their properties such as lightweight, high strength, biocompatibility and corrosion resistance. Titanium and titanium alloys have some disadvantages as well as their advantages. The most important disadvantages of these engineering materials are their low wear and high temperature oxidation resistance. The poor surface properties of titanium and titanium alloys against wear and oxidation limits their use in applications related to wear and oxidation. The low tribological properties of titanium and titanium alloys depend on parameters such as electron configuration, lubrication properties and crystal structure. In order to prevent excessive wear and increase material resistance, some surface properties need to be changed and developed. The most important factor limiting the use of titanium and titanium alloys in high temperature applications is the high affinity of Ti atoms to oxygen. When titanium and titanium alloys are exposed to high temperatures, their mechanical properties begin to deteriorate rapidly due to the non-protective oxide layer formed on their surfaces. In addition, oxygen penetrating into the material causes the material structure to become brittle, reducing the life of the material. In order to increase the high temperature oxidation and wear resistance of titanium and titanium alloys without changing their microstructure and mechanical properties, some surface modifications must be applied. The most commonly used method to improve the oxidation resistance of titanium and titanium alloys is diffusion barrier coatings. The pack aluminization method, which is the subject of this thesis, is one of the diffusion barrier coating applications. The pack aluminization method is an alternative coating method to be applied to protect the substrate material since it has various advantages such as ease of application to complex shaped parts, low cost and good adhesion of the coating layer to the substrate material. In terms of material behavior, when titanium and titanium alloys are are treated at high temperatures, the microstructure of the material changes due to phase transformations and its mechanical properties change accordingly. In order to minimize these changes and protect the substrate from the negative effects of temperature, an effective coating method that can be applied at relatively low temperatures must be selected. During the pack aluminization process, the separation of Al atoms from the AlCl3 vapor in the pack component accumulates on the surface of the material and reacts with Ti atoms through solid state diffusion to form the TiAl3 phase with a tight diffusion bond. The TiAl3 phase has a structure with high hardness and wear resistance. The layer thickness of the TiAl3 formed depends on the process time and temperature. Al atoms are one of the most important elements of thermal diffusion. The formation of a layer by Al atoms on the surface with the thermal diffusion with Al promotes the selective oxidation of Al atoms and ensures the formation of a protective Al2O3 layer on the surface of the material. According to the optical micrographs and coating analysis of the samples of EBM and conventionally fabricated Ti6Al4V alloy aluminized at 600, 650, 700 and 750 C, the coating structure obtained in the 700 C aluminized EBM sample and in the 600 °C aluminized conventionally fabricated sample is compact, uniform and dense. Microhardness measurements were made on the coatings formed on each of the EBM and conventionally fabricated Ti6Al4V samples aluminized at 600, 650, 700, 750 °C and mechanical properties of samples were evaluated. According to the results of microhardness and microstructure analyses, although there was no significant difference between the coating hardness of EBM and conventionally fabricated samples, it was concluded that the aluminization coating applied to the EBM fabricated samples protected the substrate material better from the negative effects of temperature. According to the wear test results of EBM and conventionally fabricated Ti6Al4V samples aluminized at 600, 650, 700 and 750 °C, it was concluded that the coating of the EBM fabricated samples showed the best performance. For both fabricating methods, the aluminization temperatures where the coating structure was best were selected and subjected to oxidation tests for 48 and 96 h. After the oxidation test was completed, as a result of microstructure, SEM/EDS and XRD analysis, it was concluded that the EBM Ti6Al4V sample aluminized at 700 °C showed the best resistance against oxidation.