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Abstract: A number of laser and electron beam-based fabrication processes are under consideration for aerospace components, where the ability to obtain a consistent and desirable microstructure and resulting mechanical properties is of critical concern. To this end, this work employs a combination of analytical and numerical modeling approaches to investigate the effects of process variables and size-scale on solidification microstructure (grain size and morphology) in beam-based fabrication of bulky 3D structures. Thermal process maps are developed for predicting solidification microstructure in any material system, and results are plotted on solidification maps to investigate trends in grain size and morphology in Ti–6Al–4V. The results of this work suggest that changes in process variables (beam power and velocity) can result in a grading of the microstructure throughout the depth of the deposit, with a transition from columnar to mixed or equiaxed microstructure at higher powers. [Copyright &y& Elsevier]

The Laser Directed Energy Deposition (LDED) technique in metal additive manufacturing (MAM) offers intricate geometries while maintaining material properties akin to cast and wrought components. However, challenges persist in fabricating high-strength aluminum alloys like 2xxx, 6xxx, and 7xxx series due to hot cracking during rapid solidification in LDED. This study addresses Al7075 hot cracking issue by introducing 1 wt% Zr. To evaluate this novel approach, the influence of process parameters on track geometry, porosity, microstructure, hardness, and tensile properties of both Al7075 and modified Al7075 (with 1 wt% Zr) was examined using an L27 orthogonal array of experiments. Findings indicate that increased laser power widens bead width and wetting angle. Conversely, higher scan speeds reduce bead height but marginally increase width, impacting wetting angle. Notably, the addition of Zr decreased porosity from 0.07 to 0.032%, indicating enhanced material quality. Microstructural analysis reveals Zr’s role in preventing solidification cracking by enhancing molten metal fluidity during solidification, transitioning the microstructure from columnar to equiaxed fine grain due to Al3Zr precipitates, and promoting grain refinement. This addition of Zr also improved hardness and tensile strength by 11% and 10%, respectively, attributed to Al3Zr precipitates’ role in grain refinement and precipitation strengthening within Al7075. In summary, incorporating 1 wt% Zr into Al7075 via LDED demonstrates promising improvements in microstructure, reducing porosity, enhancing mechanical properties, and mitigating solidification cracking, thereby offering potential enhancements in the fabrication of high-strength aluminum alloys. [ABSTRACT FROM AUTHOR]

The aim of this work is to study the effect of heat treatment on the microstructure, phase transformations, shape memory characteristics and corrosion behaviour of laser deposited equiatomic NiTi alloy. Dense samples of NiTi alloy were fabricated using Laser Engineered Net Shaping (LENS™) with two different laser energy densities by varying the scan speed and laser power. These samples were annealed for 30 min at 500 °C and 1000 °C in flowing argon, followed by furnace-cooling to room temperature. The resulting microstructures and properties were compared with the corresponding as-deposited samples. Microstructural analysis after heat treatment showed needle-shape martensite in the samples processed at lower laser energy density of 20 J/mm 2 , and lenticular or plate-like martensite in the samples processed at 80 J/mm 2 . The XRD results revealed relatively high concentration of martensite (B19′) in heat-treated NiTi alloy compared to as-processed samples. Furthermore, the heat treatment decreased the forward and reverse transformation temperatures of NiTi alloy from 80 – 95 °C to 20–40 °C, presumably due to annihilation of thermally induced defects. Interestingly, the samples annealed at 500 °C showed a measurable increase of 1–2% in the shape memory recovery, from the net recovery of 8% exhibited by the as-processed NiTi alloy. The corrosion resistance of laser-processed NiTi alloy decreased upon annealing. [ABSTRACT FROM AUTHOR]