Interlayer Rotation Research Articles

Influence of the scanning strategy on the microstructure and the tribological behavior of a Ni-based superalloy processed by additive manufacturing

The ABD-900AM is a newly developed nickel-based alloy specifically designed for additive manufacturing, and it can be printed using a wide range of process parameters. This alloy, combined with the L-PBF process, allows for the production of dense parts with reference microstructures that are multi-scale, and facilitates the study of the links between these microstructures and their tribological behaviors. To study the relationships between microstructures and tribological behaviors, the parts are built vertically without interlayer rotation. The plane studied are obtained with three scanning directions : 0°, 45° and 90°. These angles allow to generate different microstructures on the planes on which are conducted the microstructural characterizations and tribological tests, for which the sliding direction is always parallel to the building direction. This choice of parameter enables access to different crystallographic textures (200) and (111), and cellular structures with varying morphologies and homogeneity within the melt pools. Tribological tests were performed on these microstructures in a ball-on-flat configuration with alternating motion, without lubrication, and at room temperature. For the three laser angles (0°, 45°, and 90°), the wear volumes are 0.60 ± 0.12 mm3, 0.81 ± 0.15 mm3, and 1.00 ± 0.09 mm3, respectively. The wear resistance is primarily related to the amount of oxide layers formed on the wear track of the samples.

Effect of forming strategies on the microstructure and mechanical properties of thin-walled CuCrZr alloy fabricated by selective laser melting

Thin-walled structures are widely used in industrial fields such as radiators and critical aerospace components due to their advantages in lightweight design. However, traditional manufacturing techniques struggle to meet the high-precision requirements for complex components. Selective Laser Melting (SLM) technology, characterized by high design flexibility and high geometric production freedom, is a promising alternative. This study employs SLM technology to fabricate CuCrZr alloy thin walls and systematically investigates the effects of interlayer rotation angles and placement strategies on the forming quality and mechanical properties of the thin walls produced by SLM. The results indicate that different interlayer rotation angles and placement strategies lead to variations in defect types and shapes. When the interlayer rotation angle is 0° and the specimens are placed perpendicular to the recoater blade, numerous lack-of-fusion (LOF) defects are observed on the specimens. These lack-of-fusion defects are caused by factors such as low scanning line flatness and uneven energy distribution when the interlayer rotation angle is 0°. In contrast to the 0° interlayer rotation angle, the specimens with a 67° rotation angle exhibit significantly more curved columnar grains in the growth direction. The thin-walled specimens with a 67° interlayer rotation angle and placed parallel to the recoater blade show the best mechanical performance, with a maximum tensile strength of 254.19 MPa and an elongation of 48.81 %.

Effects of scanning strategy and scanning speed on microstructures and mechanical properties of NiTi alloys by laser powder bed fusion

In this work, the influence of processing parameter including laser scanning strategy and scanning speed on grain morphology and crystalline orientation of NiTi alloys fabricated by laser powder bed fusion (L-PBF) was comprehensively investigated from the perspective of crystal growth. The results show that as the interlayer rotation angle increases from 0 to 90°, the grain morphology changes from the coarse columnar to columnar-and-equiaxed shapes, accompanied by the texture intensity first rising then reducing. Besides, increasing the scanning speed leads to a transition in grain morphology from columnar to equiaxed crystals, as well as a reduction in grain size and texture intensity. The recovery mechanism caused by different thermal histories under various scanning speeds was discussed, with a particular focus on dislocation density and its distribution. The results demonstrate that the dislocation density gradually increases with the increase of scanning speed, leaving behind more dislocations within the matrix. Furthermore, the effects of scanning speed and building direction on the compressive behavior were investigated through identifying the difference of microstructure in the NiTi alloys by L-PBF. This study highlights the relationship among processing, microstructure, and mechanical performance, paving the way for tailoring and designing microstructures and mechanical properties for the NiTi alloys by L-PBF.

Dislocation Distribution, Crystallographic Texture Evolution, and Plastic Inhomogeneity of Inconel 718 Fabricated by Laser Powder Bed Fusion

Plastic inhomogeneity, particularly localized strain, is one of the main mechanisms responsible for failures in engineering alloys. This work studies the spatial arrangement and distribution of microstructure (including dislocations and grains) and their influence in the plastic inhomogeneity of Inconel 718 fabricated by additive manufacturing (AM). The bidirectional scanning strategy with no interlayer rotation results in highly ordered alternating arrangements of coarse Goss‐like {110}<001> textured grains separated by fine Cube‐like {100}<001> textured grains. The bidirectional strategy also results in an overall high density of geometrically necessary dislocations (GNDs) that are particularly dense in the fine grains. Although the Cube‐like texture desirable for isotropy is dominant, it gradually weakens during plastic deformation and the undesirable Goss‐like component (second most dominant in the as‐built microstructure) increases. The highly clustered and bimodal distribution of fine and coarse grains, textures, and GND densities causes fast localized roughening during deformation, particularly along the line row of fine Cube‐like grains. However, the chessboard strategy results in a lower GND density and a comparatively more random distribution of crystallographic texture and GNDs, with a dominant Cube‐like component (and much lower Goss‐like texture) that remains stable throughout plastic deformation. This results in more uniform deformation, reducing plastic inhomogeneity.

Theoretical prediction and regulation of interlayer stable angle in twisting bilayer graphene

Recent researches showed that interlayer rotation, as a control method, can effectively regulate the mechanical, electrical, and optical properties of two-dimensional (2D) materials. The control of the twisting angle and its stability influence the properties of rotation-tunable electronics, so it has attracted much attention. In this research, based on the Lennard-Jones (L-J) potential and energy density method, the energy landscape of twisted bilayer graphene (tBLG) is studied. The dependence of the locally stable twisting angle and the number of potential energy barriers on the size of the top layer graphene is analyzed. It is found that the size effect is related to the vertical distance between carbon atoms and the rotation axis. The large-sized top layer graphene has a larger energy barrier and higher stability under the same conditions, which is conducive to the structural stability of the rotation-tunable electronics. The vacancy defect and the shape of the top layer graphene have an effect on the energy landscape, based on which the regulation of stable twisting angle can be achieved. In addition, the effect of the position of the top layer graphene on the energy landscape is studied, and it is found that the energy landscape has a 2D periodicity with the change of the position, which is related to the lattice structure of graphene. There are two main modes of energy landscape with the variation of the position of top layer graphene, and the changing trend is similar under the same mode. Our research predicts the interlayer stable twisting angle theoretically and provides theoretical support for the design of rotation-tunable electronics.

Influence of Scanning Strategy and Post-Treatment on Cracks and Mechanical Properties of Selective-Laser-Melted K438 Superalloy

The feasibility of manufacturing high-performance components with complex structures is limited due to cracks in some superalloys fabricated by selective laser melting (SLM). By controlling the main process parameters such as scanning strategy, the adverse effects of cracks can be effectively reduced. In this paper, the effects of two different SLM scanning strategies with island and ‘back-and-forth’ and post-heat treatment on the cracks and mechanical properties of selective-laser-melted (SLMed) K438 alloy were investigated. The results show that the SLM method of the ‘back-and-forth’ scanning strategy had better lap and interlayer rotation angles and a more uniform distribution of laser energy compared with the island scanning strategy. The residual stress accumulation was reduced and crack formation was inhibited under this scanning strategy owing to the cooling and shrinkage process. In addition, the dislocation motion was hindered by the formation of uniformly dispersed MC carbides and γ’ phases during the SLM K438 alloy process, which resulted in the density of the as-built SLMed K438 alloy being up to 99.34%, the hardness up to 9.6 Gpa, and the tensile strength up to 1309 MPa. After post-heat treatment, the fine secondary γ’ phases were precipitated and dispersed uniformly in the Ni matrix, which effectively improved the Young’s modulus and tensile strength of the alloy by dispersing the stress-concentrated area.

Investigation of the microstructural evolution and mechanical properties of bimodal TiN-reinforced 316L stainless steel composite fabricated by SLM

ABSTRACT Selective Laser Melting (SLM) preparation of TiN-reinforced 316L stainless steel effectively addresses the weaknesses of 316L stainless steel in terms of strength and hardness. While ensuring the flowability of the composite powder, this study employs dual-sized TiN particles to enhance 316L stainless steel. Experimental results indicate that micrometre-sized TiN particles are embedded in the matrix, and other TiN particles dissolve and re-form into cubic TiN particles with a diameter of around 200 nm. Samples produced at high laser energy density exhibit high density, consistently above 99%. The average grain size of the austenite is refined to 2.38 μm, and the tensile samples with a 90° interlayer rotation angle and island size of 60 × 10mm2 exhibit a maximum yield strength of 661 MPa, ultimate tensile strength of 950 MPa and elongation of 35.5%. Samples with lower laser energy density demonstrate the highest microhardness, reaching 365 HV. The corrosion resistance of samples with different scanning strategies is similar to pure 316L stainless steel, with the 67° interlayer rotation angle strategy showing the optimal corrosion resistance.

Angle-Resolved Optical Imaging of Interlayer Rotations in Twisted Bilayer Graphene.

Twisted bilayer graphene (TBG) is a prototypical layered material whose properties are strongly correlated to interlayer coupling. The two stacked graphene layers with distinct orientations are investigated to generate peculiar optical and electronic phenomena. Thus, the rapid, reliable, and nondestructive twist angle identification technique is of essential importance. Here, we integrated the white light reflection spectra (WLRS), the Raman spectroscopy, and the transmission electron microscope (TEM) to propose a facile RGB optical imaging technique that identified the twist angle of the TBG in a large area intuitively with high efficiency. The RGB technique established a robust correlation between the interlayer rotation angle and the contrast difference in the RGB color channels of a standard optical image. The angle-resolved optical behavior is attributed to the absorption resonance matching with the separation of van Hove singularities in the density of states of the TBG. Our study thus developed a route to identify the rotation angle of stacked bilayer graphene by means of a straightforward optical method, which can be further applied in other stacked van der Waals layered materials.

Ferroelectric polarization of graphene/h-BN bilayer of different stacking orders

In recent years, two-dimensional van der Waals materials including graphene and hexagonal boron nitride (h-BN) have attracted extensive interest from researchers of many fields. In this paper, the polarization properties of graphene/h-BN van der Waals heterostructures with different stacking orders are explored by first-principle calculations. Notably, the direction of graphene/h-BN heterostructure polarization can be flipped (from AB stacking to BA stacking) by changing its stacking method. According to differential charge density analysis, the interlayer charge redistribution is the main reason driving the polarization flip. Also, we find that the polarization increases with compression of the layers and tensile strain of the lattice. In addition, we investigated the effect of the twisting angle on the polarization. The results show that polarization flip occurs during interlayer rotation and, interestingly, the magnitude and direction of polarization remain constant over a range of twist angles. The results of this study provide insight into the polarization properties of graphene/h-BN heterostructures, which is important for their potential applications in electronic devices.

Rotation angle dependent Li intercalation and the induced phase transition in bilayer MoTe2

In this work, taking twisted bilayer MoTe2 (tb-MoTe2) as an example, density functional theory (DFT) calculations are done to investigate the interlayer rotation angle dependent lithium intercalation as well as the induced 2H → 1T phase transition in two-dimensional layered materials. Both static equipotential energy diagrams and dynamic ab initio molecular dynamics simulations (AIMD) are utilized to obtain the reliable lithium diffusion pathways and barriers. The results reveal that the interlayer rotation leads to increased interlayer separation in tb-MoTe2, and provides a stable environment for Li insertion. Moreover, the diffusion barrier is substantially reduced with increasing twist angle, promoting the fast Li diffusion. At room temperature, the mobility of Li in tb-MoTe2 with the twist angle of θ = 21.79˚ and θ = 60˚ is increased by a factor of 10 and 105, respectively. The energy barrier for the 2H → 1T phase transition is lowered from 1.23 eV in the non-twisted MoTe2 to 0.88 eV in the tb-MoTe2 with a twist angle θ = 21.79˚ for a given concentration of inserted Li. Above all, the enhanced diffusion and lower critical electron concentration facilitate the 2H-1T phase transition in tb-MoTe2. The results provide valuable insights into the non-uniform intercalation and present an opportunity to modulate the phase transition of twisted 2D systems.

Closed-form solutions for two-layer Timoshenko beams with interlayer slip, uplift and rotation compliance

Closed-form solutions for two-layer Timoshenko beams with interlayer slip, uplift and rotation compliance

Adoption of appropriate scanning strategy on selective laser melting of stainless steel 316L to enable healthy mesenchymal stem cells response

In this study, an appropriate scanning strategy in selective laser melting [SLM, also known as laser powder bed fusion (LPBF)] was adopted to enhance the forming quality of stainless steel (SS) 316L for load-bearing implant applications, with a particular focus given to investigate the effect of argon flow velocity inside the build chamber. The biocompatibility of the resulting printed surfaces was evaluated by in vitro culturing of mesenchymal stem cells (MSCs) at different time points up to 96 h. Notably, it is one of the first studies to document the MSC response on SLM 316L surfaces. The results showed that highly dense parts (>99.8% density) can be produced by carefully selecting the interlayer rotation, scan vector length, and hatch distance. Microsized surface defects (i.e., balling) appeared after the SLM process. Their chance of occurrence and size were found to be related to the gas flow velocity inside the build chamber. The resulting printed surfaces were hospitable for MSCs, and healthy cell response was recorded throughout the 96-h culture periods. These findings can be instrumental in optimizing the surface features of SLM in order to improve the cell response.

The multi-dimensional fragility analysis of the underground large-scale frame structure based on the internal relationships of different seismic performance evaluating indexes

In this paper, the seismic performance of the underground large-scale frame structure (ULSFS) is thoroughly investigated aiming at the drift deformation and rotation deformation of vulnerable positions, and the multi-dimensional fragility analysis of the ULSFS is further developed based on the internal relationship of different seismic performance evaluating indexes. Employing the increment dynamic analysis (IDA) method, the seismic performance of the ULSFS is investigated during diverse earthquakes. The interlayer deformation ratio (IDR) and interlayer rotation angle (IRA) are adopted as Damage measures (DM) to assess the drift deformation and rotation deformation, respectively. The internal relationships of two DMs are revealed, and the correlation coefficients are proposed by the correlation analysis. Assuming the DMs subordinated to the joint lognormal distribution, the generalized equations are established to describe performance states of the ULSFS aiming at different seismic failure modes. Considering the seismic thresholds and correlation coefficients of two DMs in different performance limit states, the 2-D fragility curves of the ULSFS are achieved. The results shows that the correlation of seismic evaluating indexes would decrease with the increase seismic intensities due to the structural inelastic damage; and the 2-D fragility curves with considering the reasonable internal relationships of two DMs would be more accurate and conservative. This paper provides the theoretical basis and technical support for the seismic performance assessment of large underground frame structures with complicate seismic failure modes.

Influence of inter-layer rotation in parallel deposition strategies on the microstructure, texture, and mechanical behaviour of Inconel-625 during directed energy deposition

Influence of inter-layer rotation in parallel deposition strategies on the microstructure, texture, and mechanical behaviour of Inconel-625 during directed energy deposition

Pressure-induced reentrant Dirac semimetallic phases in twisted bilayer graphene

External hydrostatic pressure ($P$) is another controllable and clean knob to tune the energy position of van Hove singularities in twisted bilayer graphene (TBLG), besides the interlayer rotation angle ($\ensuremath{\theta}$). Based on total energy density functional theory calculations, we report here the ground-state properties of high-angle TBLG with two families due to their even and odd sublattice-exchange parity, hereafter parity for simplicity, under vertical hydrostatic $P$. Even (odd) parity refers to rotate one layer with respect to the other in a bilayer graphene with stacking AA (AB). We observed a Dirac semimetal (0--20 GPa) semiconductor (20--70 GPa) phase transition at $\ensuremath{\theta}=21.{8}^{\ensuremath{\circ}}$ and $13.{4}^{\ensuremath{\circ}}$; however, a reentrant Dirac semimetallic phase is observed for $\ensuremath{\theta}=9.{4}^{\ensuremath{\circ}}$ and $P\ensuremath{\ge}70$ GPa whenever TBLG has even parity. Meanwhile, TBLG systems with odd parity and different $\ensuremath{\theta}$ remain metallic but with an enhanced trigonal warping for $P>50$ GPa. Indeed, the semiconductor phase is only present for high $\ensuremath{\theta}$. The phase transition in TBLG with even parity and the metallic character of TBLG with odd parity are due to band inversion effect assisted by a band-to-band repulsion mechanism shown by unfolded bands. This work shows the relevance of external $P$ and parity in TBLG electronic structure and their possible implications in experiments.

Tunable tensile mechanical properties of bilayer graphene through inter-layer rotation

The stacking fashion of two-dimensional material system plays critical roles on their multi-functional properties. In this study, bilayer graphene with or without pre-crack was rotated by different angles and tensile mechanical properties of bilayer graphene were investigated by molecular dynamics simulations. It was found that the fracture strength, Young's modulus and fracture toughness of bilayer graphene are highly dependent on the rotation angle and reach maximum values when θ bottom = θ upper = 0°. And as for bilayer graphene without pre-crack, the mechanical properties of each layer dominate the mechanical behaviors of bilayer graphene. However, when pre-crack exists, fracture is mainly caused by super high stress concentration and the propagation of crack in bilayer graphene is blocked due to the existence of rotation angles to some extent. On the other hand, a continuum laminate-plate model was developed, which could efficiently predict the mechanical properties of bilayer graphene with or without pre-crack under different rotation angles. This work provided an in-depth understanding on the mechanical characteristics of bilayer graphene and laid foundation for the structural designs of advanced graphene-based nanomaterials. • Mechanical properties of bilayer graphene are highly dependent on the rotation angle. • The mechanics of each layer dominates the mechanical behaviors of bilayer graphene. • A continuum laminate-plate model could efficiently predict the mechanics of bilayer graphene.

High specific capacitance and long cycle stability of few-layered hexagonal Ti3C2 free-standing film constructed with spiral chiral Hexagon Ti3AlC2

Two-dimensional Ti3C2 MXene has become a rising star in the field of electrochemical energy storage due to its excellent conductivity and controllable surface functional groups. However, Ti3C2 has the risk of structural collapse caused by stacking of sheets in water-based electrolyte during long cycle charging-discharging. So, hexagon Ti3AlC2, a strong precursor material was synthesized and first visually demonstrated as a spiral chiral shape following a spiral dislocation-driven growth pattern. Then, the few-layered hexagonal Ti3C2 was constructed by “Lewis acid molten salt method first, then microwave assisted etching method” from spiral chiral Hexagon Ti3AlC2 for the first time. Then, it was fabricated into few-layered hexagonal Ti3C2 free-standing film, and the microstructure evolution, electrochemical energy storage properties and mechanism of this film were researched. The results showed that the problem of large attenuation of specific capacitance caused by the lamellar accumulation of Ti3C2 under long cycle was effectively solved. This provides a novel idea for the research and development of energy storage materials for flexible supercapacitors with long period stability and high specific capacitance. This will also lay a structural foundation for the synthesis of single-layered hexagonal MXene materials and the research of magic angle effect of interlayer rotation.

Micromachining of Alumina Using a High-Power Ultrashort-Pulsed Laser.

We report on a comprehensive study of laser ablation and micromachining of alumina using a high-power 1030 nm ultrashort-pulsed laser. By varying laser power up to 150 W, pulse duration between 900 fs and 10 ps, repetition rates between 200 kHz and 800 kHz), spatial pulse overlap between 70% and 80% and a layer-wise rotation of the scan direction, the ablation efficiency, ablation rate and surface roughness are determined and discussed with respect to an efficient and optimized process strategy. As a result, the combination of a high pulse repetition rate of 800 kHz and the longest evaluated pulse duration of 10 ps leads to the highest ablation efficiency of 0.76 mm/(W*min). However, the highest ablation rate of up to 57 mm/min is achieved at a smaller repetition rate of 200 kHz and the shortest evaluated pulse duration of 900 fs. The surface roughness is predominantly affected by the applied laser fluence. The application of a high repetition rate leads to a small surface roughness Ra below 2 m even for the usage of 150 W laser power. By an interlayer rotation of the scan path, optimization of the ablation characteristics can be achieved, while an interlayer rotation of 90 leads to increasing the ablation rate, the application of a rotation angle of 11 minimizes the surface roughness. The evaluation by scanning electron microscopy shows the formation of thin melt films on the surface but also reveals a minimized heat affected zone for the in-depth modification. Overall, the results of this study pave the way for high-power ultrashort-pulsed lasers to efficient, high-quality micromachining of ceramics.

Influence of Hexagonal Boron Nitride on Electronic Structure of Graphene.

By performing first-principles calculations, we studied hexagonal-boron-nitride (hBN)-supported graphene, in which moiré structures are formed due to lattice mismatch or interlayer rotation. A series of graphene/hBN systems has been studied to reveal the evolution of properties with respect to different twisting angles (21.78°, 13.1°, 9.43°, 7.34°, 5.1°, and 3.48°). Although AA- and AB-stacked graphene/hBN are gapped at the Dirac point by about 50 meV, the energy gap of the moiré graphene/hBN, which is much more asymmetric, is only about several meV. Although the Dirac cone of graphene residing in the wide gap of hBN is not much affected, the calculated Fermi velocity is found to decrease with the increase in the moiré super lattice constant due to charge transfer. The periodic potential imposed by hBN modulated charge distributions in graphene, leading to the shift of graphene bands. In agreement with experiments, there are dips in the calculated density of states, which get closer and closer to the Fermi energy as the moiré lattice grows larger.

Robotic four-dimensional pixel assembly of van der Waals solids.

Van der Waals (vdW) solids can be engineered with atomically precise vertical composition through the assembly of layered two-dimensional materials1,2. However, the artisanal assembly of structures from micromechanically exfoliated flakes3,4 is not compatible with scalable and rapid manufacturing. Further engineering of vdW solids requires precisely designed and controlled composition over all three spatial dimensions and interlayer rotation. Here, we report a robotic four-dimensional pixel assembly method for manufacturing vdW solids with unprecedented speed, deliberate design, large area and angle control. We used the robotic assembly of prepatterned 'pixels' made from atomically thin two-dimensional components. Wafer-scale two-dimensional material films were grown, patterned through a clean, contact-free process and assembled using engineered adhesive stamps actuated by a high-vacuum robot. We fabricated vdW solids with up to 80 individual layers, consisting of 100 × 100 μm2 areas with predesigned patterned shapes, laterally/vertically programmed composition and controlled interlayer angle. This enabled efficient optical spectroscopic assays of the vdW solids, revealing new excitonic and absorbance layer dependencies in MoS2. Furthermore, we fabricated twisted N-layer assemblies, where we observed atomic reconstruction of twisted four-layer WS2 at high interlayer twist angles of ≥4°. Our method enables the rapid manufacturing of atomically resolved quantum materials, which could help realize the full potential of vdW heterostructures as a platform for novel physics2,5,6 and advanced electronic technologies7,8.