Nanoindentations were performed in the vicinity of grain boundaries (GBs) in polycrystalline tungsten at room temperature, observing in some cases a secondary pop-in (known as GB pop-in) in the ...load–displacement curve. The dislocation microstructure in the plastic zone of the residual impression was analysed using sequential polishing, electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) analysis on and below the surface. For some indentations, the interaction of the dislocations within the plastic zone and the GB leads to a localized GB movement on or below the surface. The occurrence and magnitude of GB movement are found to be strongly influenced by misorientation between the adjacent grains, the orientation of the indenter, as well as the applied load and the distance to the GB. The results show that the localized GB movement under an inhomogeneous stress field at room temperature is a possible deformation mechanism for tungsten.
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Among metal additive manufacturing technologies, additive friction stir deposition stands out for its ability to create freeform and fully-dense structures without melting and solidification. Here, ...we employ a comparative approach to investigate the process-microstructure linkages in additive friction stir deposition, utilizing two materials with distinct thermomechanical behavior—an Al-Mg-Si alloy and Cu—both of which are challenging to print using beam-based additive processes. The deposited Al-Mg-Si is shown to exhibit a relatively homogeneous microstructure with extensive subgrain formation and a strong shear texture, whereas the deposited Cu is characterized by a wide distribution of grain sizes and a weaker shear texture. We show evidence that the microstructure in Al-Mg-Si primarily evolves by continuous dynamic recrystallization, including geometric dynamic recrystallization and progressive lattice rotation, while the heterogeneous microstructure of Cu results from discontinuous recrystallization during both deposition and cooling. In Al-Mg-Si, the continuous recrystallization progresses with an increase of the applied strain, which correlates with the ratio between the tool rotation rate Ω and travel velocityV. Conversely, the microstructure evolution in Cu is found to be less dependent on Ω, instead varying more with changes to V. This difference originates from the absence of Cu rotation in the deposition zone, which reduces the influence of tool rotation on strain development. We attribute the distinct process-microstructure linkages and the underlying mechanisms between Al-Mg-Si and Cu to their differences in intrinsic thermomechanical properties and interactions with the tool head.
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This research provides the first description of the process parameter relationship to the microstructure/nanostructure and mechanical properties of Aluminum Alloy 6061 AFS-D deposits. The ...solid-state, additive friction stir-deposition, process provides a new path for coating, joining, and additively manufacturing materials with high deposition rates (20–30 lb/hour) while avoiding liquid-solid phase transformation defects. As-deposited samples revealed the AFS-D process reduced the average grain size by an order of magnitude over a range of processing conditions. Transmission Electron Microscopy (TEM) and Atom Probe Tomography (APT) analysis established β″ precipitates were dissolved by the AFS-D process and re-precipitated as Mg–Si solute clusters.
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The corrosion and microstructure of the Al-Cu-Li alloy AA2050 were investigated as a function of artificial ageing. The work herein seeks to provide a consolidated overview of the corrosion of ...AA2050, in particular the influence of T1 (Al2CuLi) precipitates on the observed behaviour. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed to characterise the microstructural evolution of AA2050 during ageing. Accelerated immersion testing revealed that ageing was accompanied by the evolution of three unique stages of resultant corrosion morphologies – which were not correlated with the fine T1 precipitate evolution. Electrochemical testing also revealed that AA2050 showed no discernible metastable pitting from potentiostatic testing. This phenomenon suggests that T1 precipitates may remain too fine to induce localised corrosion in the matrix, and that T1 precipitates in AA2050 are unlikely the controlling factor leading to the evolution of corrosion morphologies observed in AA2050. An elaboration of factors influencing the intergranular corrosion evolution and the role of grain boundary chemistry is also presented.
The use of laser-based additive manufacturing for the fabrication of parts exposed to very high temperature in the aerospace and energy sectors is still very limited. Indeed, non-weldable superalloys ...tend to crack during their processing by Selective Laser Melting (SLM). Inconel 738 LC processed by SLM is subjected to solidification cracking. In this study, attention was focused on the influence of size and shape of the melt pools involved in the fabrication on the occurrence of cracking. This approach was motivated by the fact that the size and shape of the melt pools greatly affect the solidification conditions. Samples were fabricated with various fabrication parameters, leading to different melt pool sizes and shapes and different cracking intensity. Cracking was shown to be minimal when the fabrication used narrow melt pools and a strong overlap between adjacent melt pools. These observations were discussed in the light of solidification theory. As the cracks are known to appear primarily at high-angle grain boundaries, the effect of grain structure was investigated. These observations allowed the fabrication of a dense and crack-free material.
Depending on processing conditions, laser powder bed fusion (LPBF) is known to have two operational regimes – conduction mode and keyhole mode. Heat conduction is the dominant heat transfer mechanism ...for conduction mode melting, whereas heat convection is the dominant heat transfer mechanism for keyhole mode melting. In addition, there exists a transition mode, which lies between the conduction and keyhole mode, wherein the dominance of conduction or convection depends upon the processing conditions. In this work, normalized processing diagrams are obtained to visualize the three melting modes - conduction mode, transition mode, and keyhole mode. The normalized processing diagrams obtained from this work are shown to be independent of material for specific classes of materials, of LPBF system, of laser modulation, and of powder layer thickness. Additionally, an analytical model is proposed to robustly predict the threshold between the three melting modes for two different classes of materials, (i) materials with low reflectivity and low thermal conductivity such as titanium, ferrous, and nickel alloys, and (ii) materials with high reflectivity and high thermal conductivity such as aluminium alloys. The normalized processing diagrams, alongside the identified melting mode thresholds, can provide a useful tool in diagnosing the origins of porous defects and enable accelerated process optimization efforts towards tailoring material properties in LPBF.
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Owing to melting and solidification, serious issues arise in fusion-based metal additive manufacturing, such as solidification porosity, columnar grains, and large grain sizes. Recently, additive ...friction stir deposition has been demonstrated to overcome these issues via high-temperature, rapid plastic deformation, which can result in fully-dense as-printed material with equiaxed, fine grains. However, the deformation fundamentals underlying this process—e.g., the strain magnitude, its influence on dynamic microstructure evolution, and material flow details—remain poorly understood. Here, we explore the deformation fundamentals of additive friction stir deposition by employing tracer-based feed material (Al-Cu tracer embedded in Al-Mg-Si matrix). This allows us to unravel: (i) the path of plastic deformation, and (ii) concurrent grain structure evolution along the deformation path. X-ray computed tomography is used to directly observe the plastic deformation paths of center and edge tracers. In both cases, the millimeter-scale cylindrical tracer undergoes extrusion- and torsion-like deformation followed by shear-induced thinning, which eventually results in micro-ribbons piling up along the deposition track. Microstructure mapping along the deformation path reveals significant grain refinement during initial material feeding via geometric dynamic recrystallization but no further grain refinement during steady-state deposition. By analyzing the strain components associated with extrusion, torsion, and shear-induced thinning, we estimate the total strain to be on the order of 101 and establish a quantitative relationship between the strain and tracer grain size. While this work focuses on a specific process, the methodology and findings may provide the basis for developing future deformation processing-based additive technologies.
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The state-of-the-art alloys for load-bearing implant applications lack the necessary functional attributes and are largely a compromise between biocompatibility and mechanical properties. While ...commercial alloys pose long-term toxicity and detrimental stress shielding effects, the newly developed alloys are closing in on the gaps, however, falling short of the desired elastic modulus necessary to rule out stress shielding. In this work, we report the fabrication of a low modulus β-Ti alloy, Ti–35Nb–7Zr–5Ta (TNZT), by selective laser melting (SLM) with optimized laser parameters. The as-prepared SLM TNZT shows a high ultimate tensile strength (~630 MPa), excellent ductility (~15%) and a lower elastic modulus (~81 GPa) when compared to the state-of-the-art cp-Ti and Ti-based alloys. The mechanical performance of the as-printed TNZT alloy has been examined and is correlated to the microstructure (grain structure, phase constitution and dislocation density). It is proposed that a high density of GND (geometrically necessary dislocations), resulting from rapid cooling, in the as-prepared condition strengthens the alloy, whereas the single phase β-bcc crystal structure results in lowering the elastic modulus. High grain boundary area and a preferred crystal orientation of {200} planes within the bcc crystal lattices contribute to an additional drop in the elastic modulus of the alloy. It is shown that the TNZT alloy, processed by SLM, demonstrates the best combination of strength and modulus, illustrating its potential as a promising biomaterial of the future.
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Recently, additive manufacturing has got tremendous attention due to ease in the production of complex metallic parts for different applications i.e. aerospace, petrochemical etc. However, there is a ...scarcity of literature, addressing the corrosion behavior of additive manufactured (AM) alloys. This study presents, the chemical composition and corrosion response of the passive oxide film formed on the AM 316L stainless steel in acidic regime (pH ≤ 3) and its comparison to wrought counterpart, by applying X-ray photoelectron spectroscopy (XPS) and electrochemical analysis, respectively. Microstructural characterization of AM specimen revealed the presence of nanometer-ranged ripples type sub-granular structure confined within the macro grains. XPS analysis indicated the formation of mono layered and bi layered passive oxide film in pH 1 and 3 electrolytes, respectively. Interestingly, higher charge transfer resistance (50 times) and significantly decreased corrosion current density (2 order of magnitude) in aggressively acidic solution (pH 1) has been observed by AM specimens compared to conventional wrought 316L stainless steel. The higher corrosion resistance has been attributed to the development of fine sub-granular structure, which most likely regulates the stability of the passive oxide film and the raid solidification rate (approx. 107 K/s) involved in the additive manufacturing process rationalizing the reduction of MnS inclusions. In comparison, a significantly higher corrosion resistance by the AM 316L stainless steel in highly acidic environment (pH ≤ 3) has been recoded, surpassing the conventional wrought material.
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