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Titanium alloys are prone to increased oxidation rates when exposed to higher temperatures during application. As a result, the components suffer mechanical failure due to the formation of the alpha-case layer at 500 °C. To improve its corrosion and oxidation properties, and ultimately its mechanical performance, it is necessary to modify its surface properties. In this study, a LENS 3D-printing system was used to fabricate titanium alloy sample coupons, while surface treatment was performed using laser shock processing (LSP) to improve the surface properties. The characterisation of the samples was performed to establish a basis for the corrosion behaviour of the 3D-printed material and the effect of LSP treatment on the rate of corrosion. The samples fabricated at the moderate laser energy density of 249 J/mm3 showed the best-performing properties as the microstructures that evolved showed elevated hardness profiles, which were associated with material property improvements such as high strength and corrosion resistance. After subjecting the samples to LSP treatment, the properties of the LENS samples showed a further improvement in corrosion resistance. [ABSTRACT FROM AUTHOR]

The effects of micro-alloying gamma titanium aluminide (γ-TiAl) with niobium (Nb) using a laser melt pool as a melting pot are reported. The Optomec LENS machine was used to carry out the laser in situ alloying experiments where Nb, ranging from 6 to 10 (at. %), was added to the stable binary γ-TiAl alloy. The results of this study concluded that when a stable binary γ-TiAl alloy is micro-alloyed with Nb, there is a definite microstructural transformation, anneal twinning, promotion, and retardation of aluminium solubility in the dual and pure γ phases, respectively. Twinning in the as-built in situ alloyed ternary Ti–48Al–xNb was for the first time reported in this study. It was observed that 6 at. % Nb promoted twinning in the as-built sample, which inferred that the sample might have room temperature ductility. In fact, it was shown that the twins formed in the as-built sample dissipated with the addition of Nb. A heat treatment temperature of 1200 °C promoted anneal twinning only in the binary alloy, as confirmed by XRD data. Meanwhile, this twinning was absent in all the ternary alloys when they were heat treated to a temperature of 1200 °C. Anneal twinning was confirmed only for the alloy containing 8 Nb (at. %) at 1400 °C. Stalk faults, dislocations, and dislocation pile-ups were observed in the α2 phase of the alloys. Aluminium solubility was seen to increase in the α2 + γ (±49 at. %) phase alloy and sharply decrease in the pure γ (>49 at. %) phase alloys. Most importantly, this study determined that the laser in situ alloying process was highly exothermic. The heat gained by the reaction was found to increase with the increase in niobium content. [ABSTRACT FROM AUTHOR]

Intermetallics based on gamma titanium aluminide (γ-TiAl) alloys are potential lightweight materials that can be used for high-temperature components. However, poor room temperature ductility hinders their full potential for producing components using conventional manufacturing routes. This work is focused on the study of the microstructure and mechanical properties of ternary Ti-Al-Si alloy fabricated through laser in situ alloying by utilizing laser engineered net shaping (LENS) technology with subsequent heat treatment. The produced samples were analyzed using a scanning electron microscope (SEM) equipped with an energy-dispersed spectroscope (EDS) to investigate the composition and microstructure. Phase identification was investigated with x-ray diffraction (XRD) and electron backscattered diffraction (EBSD). Vickers hardness testing machine was utilized to determine the microhardness values, while the yield strength and tensile strength were calculated from the hardness results. Based on the results obtained, the microstructure of the heat-treated and as-produced ternary Ti-Al-Si alloy comprised mainly of α2-Ti3Al, γ-TiAl and ζ-Ti5Si3 phases. The SEM and EBSD results confirmed the phases formed, especially the fine lath structures of α2 in the colonies of α2/γ lamellae. The quantity of ζ-Ti5Si3 present in the LENS produced ternary alloy contributed majorly to the reported microhardness values. The sample heat-treated at 1150 °C/15 min/air-cooled (AC)/950 °C /6 h/furnace cooled (FC) (sample E1) had the lowest microhardness value of 624 Hv (6118 MPa or 6.12 GPa), with a corresponding tensile strength of 2325 MPa and yield strength of 1022 MPa. This was ascribed to the occurrence of more colonies of α2/γ lamellar in comparison with other samples. [ABSTRACT FROM AUTHOR]

In this study, a full-factorial design of experiments was systematically structured to optimize the laser processing parameters of additively manufactured high entropy alloys by determining the impact of these parameters on the mechanical properties of the amalgams for aerospace applications. Analysis of variance (ANOVA) was employed to determine the conditions that allow the optimization process and to verify the model significance or otherwise. Laser additive manufacturing was used to fabricate the high-entropy alloys on a steel base-plate. To improve the quality of the components built and its mechanical properties, the laser process parameters must be optimized. Thus, the design of experiments was employed as a cost-effective tool for optimization with laser power and scan speed as parametric factors using a response surface methodology (RSM). These parameters were varied simultaneously over a few sets of experimental runs to determine the optimum process parameter for improving the output response. The output response was the micro-hardness properties of the HEAs after the laser deposited components were subjected to a Vickers hardness test using a Matsuzawa Seiki MMT-X series Vickers micro-hardness testing machine. The results showed that the optimum parameters were at a laser power of 1500 and 1600 W with a scan speed of 10 mm/s to give microhardness responses of 450 and 715 HV for the Ti-based and Cu-based high-entropy alloy, respectively. The model revealed that there was a strong correlation between the predicted microhardness response and the actual experimental data. [ABSTRACT FROM AUTHOR]

• Laser-deposition technique was employed for the fabrication. • The influence of laser process parameters on the corrosion resistance was investigated. • The scanning speed had more influence on corrosion behaviour than the laser power. • The Ti-based HEAs was more resistant to corrosion than the Cu-based HEAs. Corrosion is a conservational occurrence that has a large economic impact on most metals and its alloys because it destroys and deteriorates most materials by an electrochemical process through the reaction of these materials with the environment. High Entropy Alloys in aerospace applications react with the environment in applications such as jet engines, especially at elevated temperatures. Thus, the capacity of high entropy alloys to resist corrosion must be investigated to expand the application of this advanced material in the aerospace industry. In this comparative study, AlCoCrFeNiCu and AlCoCrFeNiTi High Entropy Alloy samples were fabricated by Laser Additive Manufacturing, particularly direct energy deposition and the corrosion behaviour of both alloys were examined and compared. The influence of the laser processing parameters on the microstructure and corrosion responses of the high entropy alloys in 3.5 wt% NaCl solution was also investigated. The microstructural morphologies were examined using an X-ray diffraction system (XRD) and Scanning electron microscope (SEM) equipped with Energy Dispersion Spectroscopy (EDS). The results showed that the Scan speed had the most influence on the microstructure and corrosion behaviour of the alloys. There was a strong relationship between the phase structure of the alloys and their susceptibility to localized corrosion. Therefore, it has been proposed in this study that the phase distribution within the alloys also influences the corrosion behaviour of laser deposited high entropy alloys. [ABSTRACT FROM AUTHOR]