No description available.

Selective electron beam melting (SEBM) is a promising powder bed Additive Manufacturing technique for near-net-shape manufacture of high-value titanium components. An extensive research program has been carried out to characterise in 3D the size, volume fraction, and spatial distribution of the pores in model samples, using X-ray computed tomography (XCT), and correlate them to the SEBM process variables. The average volume fraction of the pores (97.5 %) where fatigue cracks would initiate based on the relative stress intensity factor of all the pores. In contrast, crack growth was found to be insensitive to porosity, which was attributed to the much higher stress concentration generated by the crack in comparison to the pores. Some crack diversion was associated with the local microstructure, with prior β grain boundaries often coincident with crack diversion.

No description available.

Hybrid Titanium Composite Laminates (HTCL) have vast potential for future commercial aircraft development. In order for this potential to be properly utilized the HTCLs material properties must first be well understood and obtained through experimentation. Crack initiation and crack growth characteristics of HTCLs are dependent on the heat treatment of the embedded constituent titanium foil. While high strength titanium foils may delay crack initiation, there may be an adverse effect of unsuitable crack growth rates in the HTCLs. Literature has indicated that when properly designed, cracks in HTCLs can arrest due to fiber bridging mechanisms and other crack closure mechanisms. Traditional surface inspection techniques employed on facesheet laminate evaluations will not be able to properly monitor the internal crack growth and damage progression for the internal plies. The main objective of the this joint Georgia Tech/Boeing research project was to determine and compare crack initiation and crack growth characteristics of different heat-treated -Ti 15-3 titanium foil embedded in HTCLs. Georgia Tech utilized a unique capability of x-raying the internal foils of the HTCL specimen in a servo-hydraulic test frame while under load. The titanium foil in this study represented four different heat treatments that result in four increasing levels of strength and decreasing levels of elongation. Specifically, open-hole HTCL coupons were tested at four stress load levels under constant amplitude fatigue cycles to determine a-N curves for the HTCL layups evaluated. The layup evaluated was [45/0/-45/0/Ti/0/-45/0/45]. Crack growth rates were determined once the initiated crack was detected via radiographic exposure. Radiographic delamination analysis and thermoelastic stress analysis techniques were employed to determine additional damage mechanisms in the laminate. Analytical and finite element methods were utilized to determine ply stresses. Additionally, titanium foil properties were determined via dog-bone coupons for each of the four heat treatment conditions.

Metal matrix composites can produce mechanical and physical properties better than those of the monolithic metal. Titanium alloys are widely used matrix materials as they can offer outstanding specific strength, corrosion resistance and other advantages over its competitors, such as aluminium, magnesium and stainless steel. In past decades, titanium matrix composites served in broad areas, including aerospace, military, automobile and biomedical industries. In this project, a revised powder metallurgy method, which contains cold isostatic pressing and hot isostatic pressing, was adopted to refine the microstructure of monolithic titanium. It was also used to manufacture titanium matrix composites. TiH2 powder was selected as the starting material to form Ti matrix and the reinforcements were sub-micron and nano-metric TiB particles. Mechanical properties and microstructure of commercial titanium composites exhaust valves from Toyota Motor Corporation have been studied as the reference of properties of titanium composites manufactured in this project. It has been shown that tensile strength and hardness of exhaust valves increase about 30% than those of similar matrix titanium alloys. Examination on powder starting materials of this project was also carried out, especially the dehydrogenation process shown in the DSC result. Mechanical properties and microstructures of titanium matrix composites samples in this project, as related to the process parameter, have also been investigated. The density of these samples reached 96% of theoretical one but cracks were found through out the samples after sintering. Fast heating rates during the processing was suspected to have caused the crack formation, since the hydrogen release was too fast during dehydrogenation. Hardness testing of sintered samples was carried out and the value was comparable and even better than that of commercial exhaust valves and titanium composites in literature. Microstructure study shows that the size of reinforcements increased and the size of grains decreased as the increasing amount of TiB reinforcements. And this condition also resulted in the increasing amount of the acicular alpha structure.