| Description |
For the last several years the hydrogen sintering phase transformation (HSPT) process has been the focus of a great amount of research due to its ability to produce powder metallurgy titanium components with mechanical properties that compete with traditional wrought processing. The HSPT process takes advantage of unique phase transformations in the Ti-H system which allows the formation of unique microstructures that lead to exceptional mechanical properties in the as-sintered state. As HSPT is a relatively new process, the majority of research has focused on understanding the phase transformations that occur during sintering and heat treatment. Additionally, a great deal of research has focused on developing a robust understanding of the mechanical response, specifically fatigue. Therefore, the focus of this dissertation can be divided into three main topics which further expand the understanding of HSPT processing: 1. Fatigue crack initiation - for this topic the effects of pores and microstructural features are examined in relation to the fatigue life of HSPT processed TI-6AL-4V. A theoretical model is developed in which the minimum pore size required to cause fatigue crack initiation is predicted based on common material properties. Additionally, microstructural features causing fatigue crack initiation are examined and theorized on how they reduce the fatigue strength. 2. Dehydrogenation - after sintering dehydrogenation has been used to remove residual hydrogen from the components. However, dehydrogenation can be used as a method to manipulate the microstructure thus producing a range of mechanical properties. The kinetics, phase transformations, and mechanical properties are examined. 3. Gaseous Isostatic Forging (GIF) - In an effort to close residual pores in powder metallurgy components GIF has been used as an alternative to hot isostatic pressing. A custom designed GIF was built at the University of Utah in order to study the mechanisms by which it works. Various models are presented that can be used to predict the forging pressure from GIF in addition to experimental data on its effects on HSPT Ti-6Al-4V, such as densification and microstructural changes. |
