Doctor of Philosophy
School of Mechanical, Materials and Mechatronics Engineering
Ma, Yan, Fabrication of gamma titanium aluminide alloys by gas tungsten arc welding-based additive layer manufacturing, Doctor of Philosophy thesis, School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, 2015. http://ro.uow.edu.au/theses/4372
Although intermetallic titanium aluminide alloys have extensive practical industrial applications owing to their unique characteristics, their usage has been limited by the difficulties and high cost of fabrication.
This research proposes an alternative manufacturing approach to fabricating titanium aluminide alloys, which combines in-situ alloying and additive layer manufacturing (ALM) using gas tungsten arc welding (GTAW). This new fabrication process promises significant time and cost saving in comparison to traditional methods. Since the chemical composition, microstructural features and mechanical performance of the asfabricated components can be greatly influenced by the manufacturing parameters, the effect of several important variables including arc current, interpass temperature and wire feed rate have been investigated in this study. Suitable parameters for producing crack-free components have been determined.
Apart from the process parameters, it is also important to evaluate the microstructural evolution and the variation of mechanical properties in relation to the location within one build. The typical additively layer manufactured γ-TiAl based alloy consists of comparatively large α2 grains in the near-substrate zone, fully lamellar colonies with various sizes and interdendritic γ structure in the intermediate layer bands, followed by fine dendrites and interdendritic γ phases in the top region. Microhardness measurements and tensile testing results indicate relatively homogeneous mechanical characteristics throughout the deposited material. The exception to this homogeneity occurs in the near-substrate zone immediately adjacent to the pure Ti substrate used in these experiments, where the alloying process is not as well controlled as in the higher regions. The tensile properties are also different for the vertical (build) direction and horizontal (travel) direction because of the differing microstructure in each direction.
Various post production heat treatments have been carried out on the as-fabricated components. The contour method was used to measure the residual stresses, which were clearly reduced by applying the low temperature heat treatment. The degree of stress relief is greater as the treatment temperature is increased from 400 to 500 °C. Investigations were also conducted to assess the influence of various heat treatment conditions on the microstructure evolution, phase transformations and mechanical properties. Post production heat treatment at 1200 °C for 12 hours led to full transformation of the dendritic structure, while interdendritic areas were still visible in the heat-treated samples at 1060 °C for the same duration. A different microstructure was finally produced by heat treatment at 1200 °C and 1060 °C for 24 h, exhibiting equiaxed γ grains and fully lamellar colonies respectively. The heat-treated samples at 1200 °C / 24 h possessed higher ductility but lower strength compared with the asfabricated samples, while the 1060 °C / 24 h treatment resulted in the highest strength but poor ductility (0.4 %). Additionally, the relatively homogenous microstructure in the majority region after all post production heat treatments effectively eliminated the anisotropy of mechanical properties for the vertical and horizontal directions.
Finally, the GTAW-based ALM process with twin wire in situ alloying was used to successfully produce functionally graded TiAl alloy with a compositional gradient ranging from pure Ti at the substrate to Ti-50 at.% Al at the maximum height of the deposition. The microstructure and chemical composition were characterised using optical microscopy and electron microscopy. Microstructural features across the graded composition correspond to those typically observed in conventional α-Ti, Ti3Al- and TiAl-based alloys. The microhardness profile with the composition gradient showed a linear variation of hardness as a function of composition, with the range of 25~33 at.% Al exhibiting the highest microhardness value with only a slight fluctuation of hardness value throughout this range. The morphology and amount of α2 phase could offer a reasonable explanation for the above phenomenon.