ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2024 12 Wire arc directed energy deposition (WDED) offers a robust alternative for metal printing on a large scale with high deposition rates of up to 9.5 kg/hr[1]. WDED achieves excellent material and energy efficiency, boasting a rate of over 90% and significantly reducing wasted material compared to conventional manufacturing processes such as casting and machining. However, investigating WDED involves several processing para- meters and boundary conditions that must be thoroughly navigated to comprehend their impact on the microstructural characteristics and materials properties. The inherent phase transformations and complex thermal history cycles along with robotics tool planning can introduce anisotropy, underscoring the importance of a deeper scientific understanding of WDED to unlock its full potential in metal production. Optical microscopy can assist in this effort. Commercially pure titanium (cp-Ti) is a notable candidate for WDED due to its remarkable properties, including high yield strength (~180-480 MPa), low specific weight (4.5 g/cm3), and excellent corrosion resistance. These attributes have long positioned cp-Ti as a material of choice across diverse industries from aerospace to automotive to chemical applications. However, as demand for large-scale additive manufacturing of complex components continues to rise, it becomes imperative to grasp the intricate interplay between manufacturing processes and the resulting microstructure and mechanical properties of WDED cp-Ti. Investigating the mechanical properties of anisotropic WDED cp-Ti has underscored the importance of comprehending plastic deformation across different conditions. When subjecting samples to tension and compression (along the same direction), different stressstrain curves are often obtained, a phenomenon known as tensile- compressive asymmetry (TCA)[2]. The present study employs standard and advanced techniques to derive stress-strain curves, coupled with optical microscopy, to investigate the behavior of WDED cp-Ti under tension and compression. MATERIAL PROCESSING AND EXPERIMENTAL METHODS A 1-mm diameter cp-Ti wire with a chemical composition of 99.8 Ti, 0.14 O, 0.04 Fe, 0.003 N, 0.008 C, and 0.002 H wt% was used for the wire arc directed energy deposition. The 3D printed block resulted from the deposition of eight vertical layers overlapping each other on the substrate and 16 parallel tracks using a raster pattern (Fig. 1a). Analysis was conducted along the build direction because the current study focuses on understanding the thermal gradient’s effect on the mechanical response of the bulk. Deformation mechanisms were investigated along the samples after testing subjected them to tensile and compressive (indentation) loads. Uniaxial tensile tests were conducted at room temperature according to the ASTM E8/E8M standard on a universal testing machine (MTS Criterion Model 43) with a calibrated load cell of 30 kN. Tests were performed in displacement controlled mode at a 0.1 mm/min rate. Yield strength was calculated at a 0.2% offset strain. A novel indentation technique, profilometry-based inden- tation plastometry (PIP), was implemented to subject the samples to compressive stresses. The employed indentation plastometer (Plastometrex, version 1.0) is comprised of multiple components: a silicon nitride spherical 1-mm radius indenter, a linear variable differential transducer (LDVT) with 0.3 µm resolution, a Taylor Hobson (Talysurf) profilometer including a contacting stylus with 0.4 µm resolution, a 9 kN load cell with accuracy to 0.1 N, and Corsica 4.0 software to conduct calculations and generate the stress-strain curves[3]. PIP uses the material’s elastic properties, maximum indentation load, and residual indent profiles to feed into the model. In this way, the elastic constants that represent the material’s elastic behavior, load displacement response, and residual indent profiles that describe the plasticity response of the material are integrated into the FEM model, which uses the Voce plasticity equation for the iterations. In this equation, σs is the von Mises equivalent stress, σy denotes the current yield stress, and (-ε/ε0) denotes the equivalent plastic strain. Plasticity parameters are adjusted iteratively in the FEM simulation to mimic the indentation test. Optimal convergence is reached, yielding best-fit plasticity parameters. True stress and strain values reflecting the material’s plasticity are also derived. Finally, stress-strain curves for a representative volume are obtained from the processed FEM model. The general schematic of the PIP is presented in Fig. 2. Fig. 1 — Schematic of (a) bulk commercially pure titanium deposit, sample locations, and orientation; (b) macro image of reference dog bone sample for uniaxial tensile test acquired using a 6K resolution camera; and (c) polarized light microscopy stitched images of reference sample for PIP indentation. (a) (b) (c)
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