| dc.description.abstract | A recently developed metastable β-titanium alloy, Ti-5Al-5V-5Mo-3Cr (Ti-5553), is a potential replacement to existing aircraft landing gear materials, offering a wider processing window along with excellent heat treatability. Laser powder-bed fusion (LPBF), a class of metal additive manufacturing (AM) known for building nearly fully dense parts with high geometrical complexity from 3D CAD data, could be a solution to eliminate the traditional difficulties faced in manufacturing titanium alloy parts. LPBF of Ti-5553 is a relatively new area, where any detailed investigations on the microstructural evolution and the corresponding mechanical properties achievable are not available in the open literature.
In the first part of this thesis, the processing parameters corresponding to the laser conduction mode melting were identified, resulting in near full-density parts consistently printed at scan speeds > 750 mm/s. On subsequent mechanical testing, even though the as-printed specimens reached a notable total elongation of 30±5 %, they exhibit poor tensile strength and poor impact toughness properties. The combination of an outstanding ductility coupled with poor impact toughness was attributed to the presence of intragranular non-lamellar α particles. Fracture surfaces examined under scanning electron microscopy indicate a bi-modal fracture mode.
In the second part, post-processing heat treatment was carried out to enhance the mechanical properties of the printed parts. LPBF-made Ti-5553 specimens were subjected to a wide range of heat treatment cycles to modify their corresponding microstructure in terms of the morphology and distribution of α particles. Solutionizing at the upper α+β region (800 °C) offered a good combination of finer grain size and a satisfactory α dissolution. On aging in the range of 500 to 700 °C for 0.5 to 4 hours, growth of α particles with different morphologies were observed. Subsequently, microhardness measurements ranged 300 to 500 HV depending on the applied heat treatment cycle, and the corresponding tensile strength increased from 780±10 to 1640±6.29 MPa, compared to the as-printed specimens. Fracture surface revealed intergranular dominant and ductile dominant failures depending on the aging conditions.
The heat treatment study was extended to characterizing the detrimental α case caused by the diffusion of elemental oxygen into the material at high temperatures. Irrespective of the heat treatment parameters, a 20 to 50 µm α case followed by a 200 to 250 µm thick transition zone, comprising coarse α rich grain boundaries deleterious to the tensile performance were observed in Ti-5553 specimens. Microhardness measurements revealed a hard and brittle outer case whose depth approximately matched the α case and the transition zone identified earlier. Removing the α case and the transition zone by surface turning after heat treatment substantially improved the ultimate tensile strength by ~200 %, maintaining an acceptable ductility of ~10 %.
In the final part, the effect of anisotropy caused by directional solidification in LPBF-made Ti-5553 parts on its tensile properties was studied. Specimens printed normal (vertically printed), and parallel (horizontally printed) to the building direction were subjected to interrupted uniaxial tensile tests. In comparison, samples printed parallel to the building direction exhibit a significantly higher tensile strength of 846±6 MPa, whereas the specimens printed normal to the building direction reached 780±10 MPa. Electron backscatter diffraction results indicate that the grain boundaries act as favourable locations for fracture initiation, particularly when aligned perpendicular to the loading direction. Conversely, fracture in specimens printed parallel to the building direction were predominantly transgranular, which could be a major contributing factor for the higher tensile strength observed. | en |
