ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2024 13 Samples were machined from the bulk WDED cp-Ti using a high-speed diamond saw (Allied High Tech Products, TechCut 5x) for the PIP indentation all along the building direction; indentation locations are shown in Fig. 1a. Modified sub-size (ASTM E8) tensile test specimens were machined using wire electrical discharge machining to ensure precision of the dogbone shape; samples along the buildup are shown in Fig. 1a. Metallography preparation was conducted manually using a rotary polishing wheel at 40-120 RPM. Silicon carbide papers with grit sizes of 400 and 600 [ANSI] were used for grinding. A chemical-mechanical procedure was used with a 7:2:1 mixture of 0.05 µm amorphous colloidal silica suspension, H2O2 with a concentration of 30%, and Kroll’s reagent for polishing to achieve a mirror finish free of deformations. Residual particles were removed through ultrasonic cleaning in methanol for 15 minutes. Microscopic imaging of the samples (before and after uniaxial tensile and PIP tests) was performed using optical microscopy (Axioscope 5, Carl Zeiss Microscopy) along with ZEN core 3.3.92 software for image acquisition. Polarized light microscopy (PLM) and differential interference contrast (DIC) microscopy were used to capture images of deformed grains from the unetched surfaces. Reference images from undeformed tensile and PIP samples are shown in Figs. 1b and 1c, respectively. After testing, images were post-processed in Adobe Photoshop to highlight regions of the deformed twins. Next, the twinned area fraction was computed using ImageJ open-source software. RESULTS AND DISCUSSION Combining metallography sample preparation with optical microscopy techniques enabled microstructural characterization and revealed deformation mechanisms along the WDED cp-Ti buildup. It was observed that the solidified microstructure of the pristine material exhibited a monolithic bimodal grain size (≈1 mm coarse grains with nested finer grains) distribution of alpha-phase titanium (α-Ti), which is characterized by its hexagonal close-packed (HCP) structure[4]. HCP structures enable microscopy observation under polarized light even after Fig. 2 — Schematic of PIP technique for plasticity characterization: This combines the material’s elastic properties, residual indent profiles, and mathematical models implementing the Voce equation and FEM to iterate calculations that result in the generation of stress-strain curves.
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