Showing papers on "Texture (crystalline) published in 2021"

Critical role of scan strategies on the development of microstructure, texture, and residual stresses during laser powder bed fusion additive manufacturing

Abstract: Laser based powder bed fusion additive manufacturing offers the flexibility to incorporate standard and user-defined scan strategies in a layer or in between the layers for the customized fabrication of metallic components. In the present study, four different scan strategies and their impact on the development of microstructure, texture, and residual stresses in laser powder bed fusion additive manufacturing of a nickel-based superalloy Inconel 718 was investigated. Light microscopy, scanning electron microscopy combined with electron backscatter diffraction, and neutron diffraction were used as the characterization tools. Strong textures with epitaxially grown columnar grains were observed along the build direction for the two individual scan strategies. Patterns depicting the respective scan strategies were visible in the build plane, which dictated the microstructure development in the other planes. An alternating strategy combining the individual strategies in the successive layers and a 67° rotational strategy weakened the texture by forming finer microstructural features. Von Mises equivalent stress plots revealed lower stress values and gradients, which translates as lower distortions for the alternating and rotational strategies. Overall results confirmed the scope for manipulating the microstructure, texture, and residual stresses during laser powder bed fusion additive manufacturing by effectively controlling the scan strategies.

On the relationship between cutting forces and anisotropy features in the milling of LPBF Inconel 718 for near net shape parts

Abstract: The versatility and potential applications of additive manufacturing have accelerated the development of additive/subtractive hybrid manufacturing methods. LPBF processes are exceptionally efficient at producing complex-shaped, thin-walled, hollow, or slender parts; however, finishing machining operations are necessary to ensure part assembly and surface quality. Rapid solidification during LPBF processes generates columnar grain structures in alloys. This is associated with crystalline textures and anisotropy, and therefore, mechanical properties are highly dependent on space directions, thus affecting cutting force and its variability. In this study, theoretical and experimental analyses examined the effects of LPBF parameters on cutting forces and the anisotropy of alloys. Therefore, an oblique cutting Taylor based model was proposed to quantify the crystallographic effects on the shear strength. For this, the tool geometry, tool position, and laser scanning strategy were considered along with the microstructures, crystallographic textures and grain morphologies of two samples with different layer thicknesses (low-volumetric energy density (VED) and high-VED) using scanning electron microscopy and electron backscatter diffraction. Peripheral milling operations had been performed under 54 experimental conditions to evaluate the interactions between the machining parameters along with the layer thickness and the microstructural characteristics of printed alloys. The analysis revealed a significant interaction between the direction of the plane of the shear band and the grain orientation along the main axis. Three milling configurations were evaluated. The effects of the layer thickness on the evolution of the cutting force were elucidated. Additionally, the low-VED sample exhibited higher anisotropy in the cutting force compared to the high-VED one. The anisotropy in the latter corresponds to a high, dense ring-like texture; however, the crystallographic effect is lower in the low-VED sample. A good correlation between the cutting force fluctuation and the predicted Taylor factor was obtained. Lastly, the grain boundary density was acceptably correlated with the level of cutting force for both the printed cases.

Mechanical anisotropy of additively manufactured stainless steel 316L: An experimental and numerical study

Abstract: The underlying cause of mechanical anisotropy in additively manufactured (AM) parts is not yet fully understood and has been attributed to several different factors like microstructural defects, residual stresses, melt pool boundaries, crystallographic and morphological textures. To better understand the main contributing factor to the mechanical anisotropy of AM stainless steel 316L, bulk specimens were fabricated via laser powder bed fusion (LPBF). Tensile specimens were machined from these AM bulk materials for three different inclinations: 0 ° , 45 ° , and 90 ° relative to the build plate. Dynamic Young’s modulus measurements and tensile tests were used to determine the mechanical anisotropy. Some tensile specimens were also subjected to residual stress measurement via neutron diffraction, porosity determination with X-ray micro-computed tomography ( μ CT), and texture analysis with electron backscatter diffraction (EBSD). These investigations revealed that the specimens exhibited near full density and the detected defects were spherical. Furthermore, the residual stresses in the loading direction were between − 74 ± 24 MPa and 137 ± 20 MPa , and the EBSD measurements showed a preferential 〈 110 〉 orientation parallel to the build direction. A crystal plasticity model was used to analyze the elastic anisotropy and the anisotropic yield behavior of the AM specimens, and it was able to capture and predict the experimental behavior accurately. Overall, it was shown that the mechanical anisotropy of the tested specimens was mainly influenced by the crystallographic texture.

Thermal stability, microstructure and texture evolution of thermomechanical processed AlCoCrFeNi2.1 eutectic high entropy alloy

Abstract: The microstructural and texture evolutions of as-cast AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) have been investigated in the course of thermomechanical processing at the temperature range of 25–500 °C. Interestingly, compared with other conventional casting structures, significant strength-ductility ratio has been achieved at room temperature. In addition, the volume fractions of the constituent phases: soft FCC (face-centered cubic), and the hard BCC (body-centered cubic) phases, do not significantly change from room to elevated deformation temperatures. In fact, the strength and ductility have not been decreased at higher temperatures which represent the mechanical stability of the alloy in the examined temperature range. From room temperature up to 300 °C, the dendrites have been stretched and broken with a slight deviation from the load direction, whereas at higher temperature of 500 °C the dendrites have been rotated relative to the direction of load before fracture. Texture examination reveals the formation of a random texture in the initial and deformed states due to simultaneous contribution of different influencing factors such as stretching of dendrites during deformation, the dendrite morphology changes, and the presence of hard and soft phases and their interaction with each other.

Effect of subgrain and the associated DRX behaviour on the texture modification of Mg-6.63Zn-0.56Zr alloy during hot tensile deformation

Abstract: The individual effect of DRXed grains, subgrains and deformed grains on the basal texture of Mg–Zn–Zr alloy during hot tensile deformation was systematically studied by extracting corresponding grains. The results show that the final texture was the result of compromise between the randomized texture of DRXed grains and preferred texture of subgrains and deformed grains. The texture attribute of subgrain did not change fundamentally compared with that of deformed grain, both of which dominated the final texture profile. Prismatic dislocations exerted a profound effect on the compatible deformation of unDRXed grains, giving rise to a sharp [ 01 1 ‾ 0 ] fiber. Few basal dislocations could be detected in the unDRXed grains due to the premature exhaustion of such slip operation and subsequent substitution by non-basal slips. Subgrains, in terms of DDRX, were caused by the interaction between basal and non-basal dislocations, resulting in the protrusions segmented by the subgrain boundaries (sub-GBs) whose misorientation increased gradually as trapping dislocation proceeded. As a result, these protrusions were segregated from the parent grains and transformed into DRXed grains. CDRX proceeded by high activity of profuse dislocations. In this case, sub-GBs came from the realignment and incorporation of the accumulated dislocations in the vicinity of dislocation forest, which subdivided one grain into several subgrains that could be converted into DRXed grains in situ via the transformation from LAGBs to HAGBs by consecutively absorbing dislocations. Besides, twinning deformation was an auxiliary way of compatible deformation, which could reorientate parent grains so as to reactivate the slip systems. The contribution of resulting { 10 1 ‾ 2 } twins to the final texture was not significant yet. Ultimately, the selection of { 10 1 ‾ 2 } twin variants was also discussed.

Understanding and Tuning the Electrical Conductivity of Activated Carbon: A State-of-the-Art Review

Abstract: During the last decades, there has been a growing interest and research activity in the use of activated carbon (AC) and related materials as electrodes in electrochemical energy conversion and sto...

Textured Electrodes: Manipulating Built-In Crystallographic Heterogeneity of Metal Electrodes via Severe Plastic Deformation.

Abstract: Control of crystallography of metal electrodeposit films has recently emerged as a key to achieving long operating lifetimes in next-generation batteries. It is reported that the large crystallographic heterogeneity, e.g., broad orientational distribution, that appears characteristic of commercial metal foils, results in rough morphology upon plating/stripping. On this basis, an accumulative roll bonding (ARB) methodology-a severe plastic deformation process-is developed. Zn metal is used as a first example to interrogate the concept. It is demonstrated that the ARB process is highly effective in achieving uniform crystallographic control on macroscopic materials. After the ARB process, the Zn grains exhibit a strong (002) texture (i.e., [002]Zn //ND). The texture transitions from a classical bipolar pattern to a nonclassical unipolar pattern under large nominal strain eliminate the orientational heterogeneity of the foil. The strongly (002)-textured Zn remarkably improves the plating/stripping performance by nearly two orders of magnitude under practical conditions. The performance improvements are readily scaled to achieve pouch-type full batteries that deliver exceptional reversibility. The ARB process can, in principle, be applied to any metal chemistry to achieve similar crystallographic uniformity, provided the appropriate temperature and accumulated strains are employed. This concept is evaluated using commercial Li and Na foils, which, unlike Zn (HCP), are BCC crystals. The simple process for creating strong textures in both hexagonal and cubic metals and illustrating the critical role such built-in crystallography plays underscores opportunities for developing highly reversible thin metal anodes (e.g., hexagonal Zn, Mg, and cubic Li, Na, Ca, Al).

Comparative study on crystallographic orientation, precipitation, phase transformation and mechanical response of Ni-rich NiTi alloy fabricated by WAAM at elevated substrate heating temperatures

Abstract: In this investigation, a Ni-rich NiTi alloy was in-situ deposited with different substrate heating temperatures and the evolution of crystallographic orientation, precipitation, phase transformation, and mechanical responses were evaluated. The experimental results indicated that with the increment of substrate heating temperature from 150 °C to 350 °C, the average B2 grain size and the high angle grain boundaries (HAGBs) gradually increased from 53.44 μm to 85.38 μm and 53.6%–62.4%, respectively. The crystallographic texture exhibited a dominant, strong (001) orientation with comparatively weak (111) and (101) orientations in all conditions and the intensity of {100} increased slightly as the substrate heating temperature increased. Moreover, Ni4Ti3 precipitates with an inhomogeneous size distribution were identified within the B2 NiTi matrix. Increasing the substrate heating temperature coarsened the Ni4Ti3 precipitates. All the phase transformation temperatures increased when the substrate heating temperature increased, indicating that the martensitic transformation is more likely to occur. As the substrate heating temperature increased from 150 °C to 350 °C, the yield stress and ultimate tensile stress decreased from 683.9 to 513.1 MPa and 855.2 to 743.8 MPa, respectively, and the ductility decreased from 6.90% to 6.13%. In addition, a remarkable eir, poor recovery ratio and a broad stress hysteresis were obtained during the initial deformation of the cyclic loading-unloading tension. The highest recoverable strain (ere), recovery ratio and elastic energy storage efficiency (ƞ) were obtained in samples processed with the lowest substrate heating temperature. These findings provide useful references concerning process optimization in fabricating Ni-rich NiTi components by WAAM with acceptable microstructure and mechanical properties.

Enhanced electrical conductivity and mechanical properties in thermally stable fine-grained copper wire

Abstract: The rapid development of high-speed rail requires copper contact wire that simultaneously possesses excellent electrical conductivity, thermal stability and mechanical properties. Unfortunately, these are generally mutually exclusive properties. Here, we demonstrate directional optimization of microstructure and overcome the strength-conductivity tradeoff in copper wire. We use rotary swaging to prepare copper wire with a fiber texture and long ultrafine grains aligned along the wire axis. The wire exhibits a high electrical conductivity of 97% of the international annealed copper standard (IACS), a yield strength of over 450 MPa, high impact and wear resistances, and thermal stability of up to 573 K for 1 h. Subsequent annealing enhances the conductivity to 103 % of IACS while maintaining a yield strength above 380 MPa. The long grains provide a channel for free electrons, while the low-angle grain boundaries between ultrafine grains block dislocation slip and crack propagation, and lower the ability for boundary migration. Strength and electrical conductivity are generally mutually exclusive properties in copper wires, both of which are required in applications such as high-speed rail. Here, rotary swaging is used to manufacture copper wires that combine high strength and conductivity, and are thermally stable.