Interlayer Zone Research Articles

Process study for directed energy deposition of 316L stainless steel with TiB2 metal matrix composites

In addition to laser powder bed fusion, directed energy deposition (DED) is also gaining interest as an effective metal additive manufacturing technique. Due to its system configuration, it is more efficient and flexible for materials development. Therefore, it can be used for processing of metal matrix composites (MMCs) through the use of powder mixture as feedstock. 316L stainless steel has high corrosion resistance, biocompatibility, and ductility. Several studies have shown the feasibility of using DED to process 316L stainless steel. The material properties of 316L stainless steel can be improved using reinforcement particles such as TiB2 to form MMCs. In this study, the effects of process parameters on microstructure and mechanical properties of 316L stainless steel reinforced with TiB2 (316L/TiB2) MMC were studied. The process parameters, including laser power, scanning speed, and hopper speed, were varied and analyzed using Taguchi L9 array. It was found that the process parameters have insignificant effect on the bulk density of the samples produced. Through this study, it is also found that tumble mixing was not suitable for the powder feedstock preparation for MMCs to be processed by DED. The microstructure of DED 316L/TiB2 MMC samples consists of columnar and equiaxed grains. Columnar grains were located within the layers while equiaxed grains were located at the interlayer zones. Fine sub-grains were also observed within these grains and their boundaries were enriched with molybdenum and chromium segregations. Precipitates containing titanium were also observed to segregate at the sub-grain boundaries. Finally, the Vickers microhardness of the DED 316L/TiB2 MMC was found to be similar to pure 316L stainless steel produced by DED.

Microstructural examination of carbonated 3D-printed concrete.

The recent interest in 3D printing with concrete has generated great interest on how inhomogeneities arise and affect performance parameters, in particular strength and durability. With respect to durability, of particular interest is how 3D‐printed layer interfaces can impact transport of species of interest, such as moisture, chlorides or carbon dioxide in carbonation processes. This is of particular interest considering that the primary use case of 3D‐printed concrete has been as a lost formwork for a cast structural concrete, and thus it is of interest to determine the carbonation resistance. This study consists of a preliminary look at the microstructure after accelerated carbonation of a 3D‐printed concrete used as a lost formwork. Preferential carbonation is observed in the layer interfaces compared to the bulk of the printed filaments, possibly related to porosity from air voids or a locally high capillary porosity corresponding to the lubrication layer.

Relationship between water transport behaviour and interlayer voids of 3D printed concrete

3D printed concrete is a rising technology recently developed for construction industry. Unlike conventional concrete, since 3D printed elements are built by using a layer-deposition process, the distinct void generated between the printed concrete filaments considerably reduce mechanical strength and durability of 3D printed concrete. Using X-CT technology combined with CsCl enhancement, this study aims at investigating the water transport characteristics at different interlayer zones of 3D printed concrete, cast concrete as the reference. Meanwhile, the void characteristics of different interlayer zones are analyzed to establish quantitatively the relationship between the void distribution and the water transport process. It can be found that those voids at the interlayer zone led to a faster water transport along the printed concrete’ interlayer zone than that of cast concrete. In addition, the percentages of 0.1 mm-0.6 mm and 1 mm-4 mm voids size are observed to have an opposite effect on the water.

Structural levels of process modeling the formation and wear of high-strength thermal spray coatings

Modeling of physical processes at different structural levels provides an effective solution to a number of physicochemical problems connected with production and application of thermal spray coatings. The use of structure simulation methods allows one to optimize the structure of coatings for particular service conditions. A bidirectional scale of structural levels for modeling the properties of strengthening thermal spray coatings on machine parts is considered. Moving along the scale in a positive direction, we can distinguish the following structural levels: groups of several sprayed particles that form separate microareas of a coating, extended reinforcing elements in the form of wires, rods or grid, transitional interlayer zones, zones between the coating and the substrate, individual layers obtained in one pass, thick transition zones in gradient coatings with a variable composition across the thickness, blocks of several monolayers in multilayer coatings, island elements in discrete coatings, macroelements of the block structure of coatings with controlled distribution of microcracks, individual macrostripes of the coating, continuous single-layer or multi-layer coating, structural coating. Moving from the zero level in the opposite direction, we have the following structural elements: composite particles of conglomerated grains, cladding shells, zones of different phase composition, individual domains of sprayed particles, individual elements sprayed particles and groups of grains, grains, dendrites, shear zones and slip systems, fragments of grains, packets of martensite laths, disclination cells, loops and dipoles, bands in the substructure, kink bands, microtwins, disclination clusters, dislocation pile-ups and tangles, shear bands, dislocation walls, slip systems, linear defects in the form of dislocations, ledges at grain boundary and crowdions, kinks and jogs formed due to intersection of dislocations, point defects in the real structures of solids, crystal structure, electronic structure of matter, electronic structure of individual atoms.

Microstructure evolution and mechanical properties of the wire + arc additive manufacturing Al-Cu alloy

Understanding and clarifying the formation and evolution mechanism of the microstructure in wire + arc additive manufacturing (WAAM) Al-Cu alloy is primary and essential for achieving the higher and more isotropic mechanical properties. In this study, the evolution of grain structure, precipitated phases and corresponding mechanical properties of the as-deposited WAAM processed 2219 Al-Cu alloy are investigated. The microstructure exhibits the heterogeneous band characteristics between the fine equiaxed α-Al grains in inter-layer zone (ITZ) and the columnar α-Al grains in inner-layer zone (INZ). Based on the re-melting experiment and the time-dependent nucleation theory, the occurrence of equiaxed grains is attributed to the heterogeneous nucleation of α-Al caused by the prior precipitation of Al3Zr particles at the molten pool boundaries. The relationship between the solidification conditions and the number of the prior Al3Zr nucleation sites is established. A large amount of spherical θ″-Al2Cu phases precipitate in Cu-rich regions around the α-Al+θ-Al2Cu eutectics and they agglomerate and transform to flaky-like θ′ phases under the in-situ thermal effect during WAAM deposition. The hardness of the ITZ (81 ± 2.7HV) is approximately 12% higher than that of the INZ (74 ± 3.6HV) which is mainly due to the finer grains and the more dispersive precipitates in ITZ.

Study on Non-Darcian Flow in Interlayer Shear Weakness Zones in the Basalt by High-Pressure Packer Tests

The interlayer shear weakness zone (ISWZ) is a special structural plane with different widths and spacing in stratified rock masses, it has higher permeability compared with surrounding rocks which is a risk factor for the safety of the hydropower station project. The high-pressure packer test (HPPT) by step injection is always applied to characterize the permeability of ISWZ. However, the non-Darcian flow is easy to appear under high pressure, which makes the Darcy law model no longer applicable. In this study, two non-Darcian flow analytical methods for confined aquifer were proposed to investigate the non-Darcian flow permeability parameters. The equivalent permeability coefficients of different non-Darcian models were derived as well. The in situ tests were conducted on the ISWZs at the Baihetan hydropower station to verify the proposed methods. The results indicate that the flow is non-Darcian flow in the test section from integrity to destruction during the whole HPPT process. Izbash’s law has a better fit than Forchheimer’s law in this complicated test situation. The equivalent permeability coefficients after destruction are one or two orders of magnitude larger than those before. Meanwhile, it is necessary to pay attention to the increased difference of two expressions of the equivalent permeability coefficients under higher gradient (i) or velocity (v). In general, these methods can be used to evaluate the characteristic of ISWZ to analyze the impact on engineering stability.

Analysis of large deformation mechanism of the underground powerhouse at the Baihetan hydropower station

Baihetan hydropower plant has the widest underground powerhouse and the largest scale of underground openings in the world. The surround rock mass of huge caverns is characterized by complex geological conditions and high in-situ stress of 30MPa. The monitoring deformation of the roof arch of the right bank powerhouse reached 165mm and the deformation of the side wall reaches 192mm, which had become the focus of the feedback analysis. In the paper, several field test methods, such as extensometer monitoring, grating fiber displacement monitoring, acoustic test and borehole image test, have been adopted to revealing the fracture expansion and time-dependent deformation of surrounding rock. Moreover, FLAC3D numerical simulation has been conducted integrated the monitoring results to analysis of large deformation mechanism. The results show that the combination of interlayer zone and high stress factors is the fundamental cause of large deformation caused by excavation. Finally, the reinforce measures and reinforcing range had been determined, and the stability of the powerhouse cavern has been evaluated according to the monitoring results for nearly 2 years after excavation. The studies, integrated the research approach of field geological survey, monitoring and numerical simulation, not only have shed light on the large deformation mechanism of excavation response of huge caverns, but also has provided a basis for dynamic optimization design and construction.

Stability prediction and analysis for large underground cavern controlled by weak interlayer zone under high geostress

Due to the special engineering geological characteristics of weak interlayer zone (WIZ) with the initial shear, the pre-consolidation, the water softening and clay generation, that occurs between different rock strata (e.g., tuff and basalt), it tends to represent potential threats to the overall stability of large underground cavern under high geostress, such as the large deformation of upper and lower rock masses, structural stress-induced collapse and the time-dependent plastic squeezing-out failure, which could never be neglected, and requires more widespread concern. Focusing on the prediction and analysis of such stability issues induced by WIZ, a novel constitutive model for WIZ, based on the unearthed macro-meso mechanical response of WIZ under complicated unloading stress paths, has been first established, and well embed into the effective numerical calculation platform, to realize the quantitative descriptions of the mechanical effects of stress path, the particle breakage as well as the mechanical parameter evolution of WIZ. Then the rock failure index for WIZ (RFDwiz) was derivated, to help determine the cracking scale, depth and degree of surrounding rock masses with WIZ. Based on the proposed model, the RFDwiz and the cracking-restraint design method, the prediction of the failure position and degree, as well as the dynamic optimization design of the reasonable supporting parameters of rock masses with WIZ in underground cavern, were ultimately realized. This study will provide theoretical basis for the analysis and prediction of the instability of deep underground projects controlled by rock masses with WIZ.

Diffusion Bonding of FGH 98 and CoCrNi-Based Medium-Entropy Alloy: Microstructure Evolution and Mechanical Tests

The superalloy FGH98 was successfully diffusion bonded (DB) with medium-entropy alloy (MEA) Al3Ti3(CrCoNi)94 using pure Ni as the interlayer at a temperature range of 1050–1170 °C for 1 h under 5 MPa. The microstructure and mechanical properties of joints were investigated. The diffusion bonding seam was composed of an interlayer zone (IZ) and two diffusion-affected zones (DAZ). The IZ and DAZ beside the FGH98 consisted of cubic Ni3(TiAl)-type γ′ phases due to the diffusion of Ti and Al atoms. Meanwhile, the DAZ adjacent to the MEA consisted of spherical γ′ phases. Both of the γ′ phases with different morphology kept the coherent relationship with the matrix. Moreover, increase of bonding temperature led to the morphology of interlayer γ′ phase to transform from sphere to cube. Due to the strengthening effect of a mass of γ′ phase distributed evenly in IZ and the DAZ beside the FGH98, the microhardness and Young’s modulus of these two zones were higher than that of DAZ near the MEA. The maximum shear strength of DB joint, 592 MPa, was achieved in the joint bonded by 1150 °C, which was the typical ductile fracture feature confirmed by the shear dimples.

Microstructure, Mechanical Properties, and Galvanic Corrosion of 10CrNi3MoV Fabricated by Wire Arc Additive Manufacturing

Wire arc additive manufacturing (WAAM) technology is widely used in the fields of aerospace, shipbuilding, and automobile industry due to the advantages of fast forming speed, high material utilization and low production cost. WAAM is extremely sensitive to parameters, and different processes and materials produce different deposition effects and properties. Therefore, it is of great significance to study the WAAM formability of various materials. Herein, the microstructure, mechanical properties, and galvanic corrosion behavior of the low-carbon high-strength steel (10CrNi3MoV) fabricated by cold metal transfer (CMT) WAAM technology were investigated. The single-channel multilayer deposition parts were prepared by reciprocating deposition, and the forming parts were divided into six zones by observing the different positions of the structure: matrix, heat-affected zone, remelting zone, initial deposition zone, interlayer zone, and interlayer bonding zone. Electron backscattered diffraction (EBSD) analysis showed that the amount of recrystallization and substructure in the deposition layer had no obvious change, and the texture phenomenon was the most obvious in the initial deposition zone of the pole map reaction. The texture phenomenon gradually weakened with the increase of the deposition layers. The microhardness from the matrix to the deposition stable zone was tested. The hardness of the matrix changed smoothly, that of the heat-affected zone was the largest, and that of the deposition layer was 221–282 HV0.2. The tensile properties were tested in different directions and angles, and the yield strength and tensile strength of the deposited layer were more than 550 MPa and 760 MPa, respectively. The galvanic corrosion behavior between the deposited layer and the matrix was investigated, and the polarization curve showed that the corrosion potential of the deposited layer was lower than that of the matrix, and the corrosion current density of the deposited layer was higher than that of the matrix.

An Insight into the Excavation-Induced Stress Paths on Mechanical Response of Weak Interlayer Zone in Underground Cavern Under High Geostress

A weak interlayer zone (WIZ) is a poor zonal geotechnical system with loose structure, weak mechanical properties, variable thickness, random distribution, and strong extension, that occurs between different rock strata (e.g., tuff and basalt), due to the intense tectonic movement, representing a potential threat to the overall stability of rock masses with WIZs in large underground cavern excavations. Focusing on the excavation-induced hazards in the weak interlayer zone (WIZ) occurring in underground cavern under high geostress, the mechanical response of WIZ under different loading and unloading stress paths has been well investigated and unearthed, by a series of automatic stress path controlled triaxial tests, as well as through scanning electron microscope (SEM) analysis. Results show that the mechanical characteristics of WIZ are closely related to the initial confining pressure and stress paths. Sudden increases in the circumferential strain and strong dilations of WIZ occur from the start of unloading, among which the unloading total strains are most strongly promoted by stress path II (i.e., axial pressure loading and confining pressure unloading), whereas the unloading stress path IV (i.e., axial pressure and confining pressure unloading) has the greatest promotion on enhancing the plastic volumetric dilatation. The deformation modulus and the Poisson’s ratio follow a deterioration law under the unloading process of confining pressure, and it is the unloading stress path II that most influences the damage of elastic parameters. The ultimate bearing strength, the internal friction angle, and the cohesion of WIZ appear extremely obvious degradation in course of unloading, among which the damage effect of stress path IV is the most remarkable. The macroscopic and mesoscopic analyses reveal that the failure mechanism of WIZ under complicated unloading stress paths lays in the accumulation and propagation of axial and circumferential cracks and fractures, as well as pervasive particle breakage, with the intergranular fractures, transcrystalline fractures, and the shear scratches on a meso level. The research could provide an effective basis and good lessons for the mechanical response and failure mechanism of WIZ under unloading stress paths in underground cavern under high geostress.

Deformation and Failure Analyses of the Surrounding Rock Mass with an Interlayer Shear Zone in the Baihetan Underground Powerhouse

In the process of underground cavern excavation, the existence of the interlayer shear zones or large faults often makes the surrounding rock tend to be unstable or even deformed. Under the influence of interlayer shear zone C2, different degrees of deformation and failure occurred in many parts during the excavation of the Baihetan left bank underground powerhouse. Based on field monitoring and numerical calculation, this paper studies the deformation and failure characteristics of the rock mass with C2 in the whole excavation process and the failure mechanisms are analyzed. The results show that C2 has poor mechanical properties. In the process of excavation, it mainly induces two failure modes: rock collapse and shear deformation, which specifically leads to rock collapses, large deformation and shotcrete cracking in the main powerhouse, and shear deformation in the omnibus bar caves. In addition, the similarities and differences between this study and other studies on the deformation and failure of surrounding rock of underground powerhouse in recent years are discussed, and the relevant treatment measures for C2 are given. The above research results can be a reference for other related studies.

In Situ Tracking Water Transport at the Interlayer Zone of 3d Printed Concrete Using X-Ct Combined with Cscl Enhancing

In Situ Tracking Water Transport at the Interlayer Zone of 3d Printed Concrete Using X-Ct Combined with Cscl Enhancing

Hybrid manufacturing of γ-TiAl and Ti–6Al–4V bimetal component with enhanced strength using electron beam melting

γ-TiAl-based alloys have been studied for decades due to their attractive properties. However, the applications are limited because of their poor formability. Electron beam melting (EBM) is advanced in fabricating components with high complexity. Studies have shown that γ-TiAl alloys can be successfully fabricated using EBM. To further explore the advantages of EBM, a bimetal component is designed: a turbine blade that containing an EBM-built Ti–48Al–2Cr–2Nb blade and a Ti–6Al–4V fir tree mount. The Ti–48Al–2Cr–2Nb side is directly built using EBM on a wrought Ti–6Al–4V substrate. Different printing strategies are applied in order to generate straight and curved diffusion interlayers. No defect is observed in both of the EBM built parts and the interlayer zones. The interlayers consist of α2-Ti3Al and B2 phases. To evaluate the reliability of the bimetal component, the tensile strength of the bimetal component is assessed. The sample with a curved interlayer exhibits a higher strength than that with a straight interlayer. The average tensile strength of the samples with curved interlayer reachs 389 MPa, which is much higher than that of the brazed TiAl/Ti–6Al–4V joints. The tensile test reveals that the fracture occurred in the interlayer zone. This work provides a promising application of γ-TiAl based alloys and a new method to form a strong bond for bimetal materials.

Interlayer Dilution Zone Elemental Profiling and Microhardness Measurements for Individual Laser Clads

A detailed study of microstructure and microhardness distribution at clad-to-substrate interlayer dilution zone as well as elemental profiling were carried out for individual laser clads. Tungsten carbide (44 712-10) and nickel alloy (1560) powders were used for individual clad synthesis at the surface of low alloy steel by coaxial laser cladding technique. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) revealed partial tungsten carbide (WC) melting and secondary chromium carbides formation near WC grains. Iron and nickel elemental profiles were quantified by EDX technique to estimate interlayer dilution zone dimensions. Interlayer zone depth in the clad center increased from 15 to 80 µm when laser power was changed from 0.8 to 1.5 kW while at the outer clad regions it varied in 3–20 µm range. Interlayer clad-to-substrate zone microhardness study revealed a non-uniform dependence of hardness distribution for single clads synthesized with varying laser power.

Static and cyclic characterization of fouled railroad sub-ballast layer behaviour

This paper investigates the static and cyclic characteristics of a fouled sub-ballast material using static and cyclic triaxial tests. Static triaxial tests were conducted first to determine the peak strength, which was used as the applied stress in the cyclic tests. Moreover, the results of the displacement-controlled static triaxial test were used to assess the shear stiffness and strength during the cyclic stress-controlled loading cycles. Isotropically consolidated drained (CD) compression cyclic triaxial tests were conducted to simulate the long-term response of fouled railway substructure under progressive repetitive loading and unloading cycles. The accumulated plastic strains of the fouled railroad sub-ballast were determined under cyclic loading and unloading conditions. The obtained results indicated that the soil plasticity and effective confining pressure, and relative density are not the only important factors in shaping the damping curves but also the loading rate, and particle size. Both parameters play a major role in determining the behaviour of fine contaminated sub-ballast. The railway operational speed limits were deduced from the stress-strain behaviour of the fouled sub-ballast interlayer zone. It is recommended that the maximum speed of trains in fouled railroad segments be limited to a maximum of 32 km/h. The developed guidelines are aimed to prolong the service lifespan of the fouled track segments, while maintaining safety standards to avoid train derailments, track deterioration, and high maintenance costs.

Orbital Overlapping through Induction Bonding Overcomes the Intrinsic Delamination of 3D-Printed Cementitious Binders.

3D printing of cementitious materials holds a great promise for construction due to its rapid, consistent, modular, and geometry-controlled ability. However, its major drawback is low cohesion in the interlayer region. Herein, we report a combined experimental and computational approach to understand and control fabrication of 3D-printed cementitious materials with significantly enhanced interlayer strength using multimaterial 3D printing, in which the composition, function, and structure of the materials are programmed. Our results show that the intrinsic low interlayer cohesion is caused by excess moisture and time lag that block the majority of valuable interactions in the interlayer zone between the adjacent cement matrices. As a remedy, a thin epoxy layer is introduced as an intermediator between the adjacent extruded layers, both to improve the interlayer cohesion and to extend the possible time delay between printed adjacent layers. Our ab initio calculations demonstrate that an orbital overlap between the calcium ions, as the main electrophilic part of the cement structure, and the hydroxyl groups, as the nucleophilic part of the epoxy, create strong interfacial absorption sites. These electronic absorptions lead to several iono-covalent bonds between the cement matrix and epoxy, leading to significant improvements in tensile, shear, and compressive strengths as well as ductility of the 3D-printed composites. This is verified by our experimental data, which showed an average of 84% improvement in interlayer bonding. The upward augmentation of interlayer bonding helps 3D printing cementitious material to overcome their intrinsic limitation of weak interlayer cohesion, thereby mitigating/eliminating the key bottleneck of additive manufacturing in constructing materials.

Anisotropic mechanical properties and deformation behavior of low-carbon high-strength steel component fabricated by wire and arc additive manufacturing

Wire and arc additive manufacturing (WAAM) is an efficient technique for fabricating large and complex components that are applied in the manufacturing industry. In this study, anisotropic mechanical properties of a low-carbon high-strength steel component fabricated by WAAM were investigated via mechanical testing, and the transversal and longitudinal deformation behavior of the component were studied using the digital image correlation (DIC) method. Additionally, the features of microstructure, texture, and fracture mode of the inter-layer area and deposited area were also investigated to reveal the mechanism of anisotropy. The results showed the mechanical properties of longitudinal specimens were inferior to that of the transversal specimens. Several strain concentration zones in the longitudinal specimen were relevant to the inter-layer characteristics observed from the fracture surface and macrostructure, which was confirmed by the strain evolution recorded by DIC. The inter-layer areas were proved to be the weak link in the deposited component by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) analysis results, including various phase composition, phase morphology, misorientation angle, grain size, Schmid factor, and texture. Finally, based on the fractography analysis, anisotropy resulted from inter-layer zones is also confirmed via the comparison of transversal and longitudinal fracture morphology.

Mechanism of reduction of damage during helical milling of titanium/CFRP/aluminium stacks

Composite/metal stacks are used extensively in aerospace structures. It is still a challenge to make holes on the composite/metal stacks with high processing quality. To study the mechanism of damage generation during drilling of titanium/carbon fibre-reinforced polymer (CFRP)/aluminium stacks, both conventional drilling and helical milling were used. The glass-transition temperature (Tg) of the CFRP was tested and a method of hole temperature measurement in interlayer zone was presented. Based on the orthogonal experiment method, the results show that high cutting temperature is the main reason for the damage generation during machining of the stacks. Cutting heat generated during drilling Ti alloy conducts to the CFRP and leads to the increase of temperature. High cutting temperature induces the degradation of CFRP properties, which results in the generation of damage during machining of CFRP. The influence of different factor on cutting temperature during helical milling was also analysed based on the range analysis. The cutting force in axial direction during helical milling was weaker than that of conventional drilling, except when the CFRP temperature exceeded the Tg. Moreover, helical milling produced no continuous chip and allowed for chip removal. Helical milling of titanium and aluminium is superior to conventional drilling in terms of hole quality.

Experimental analysis of zero-gap laser welding of zinccoated steels in a lap joint configuration

Automotive manufacturing requires high productive manufacturing technologies to ensure a high-volume efficient production. Joining metal sheets in an overlap joint configuration of zinc-coated steel is an important task for body-in-white production. This paper describes the influence of different spot sizes regarding process stability in laser beam welding. Therefore, welds were performed in an overlap joint configuration with fully penetrated welds. Relevant material combinations for automotive industry were investigated. A broad-based parameter study and the analysis of interaction between spot- and keyhole diameter state a higher process stability. Welds with a smaller spot diameter lead to a sufficient increase of joint quality by using a zero gap overlap configuration. It is shown that the amount of joint instabilities such as pores, pinholes, spatter and further defects can be minimized by using smaller spot diameters. This can be explained by different keyhole regimes. Moreover, the change of isothermal interaction in the interlayer zone caused by the adjusted spot diameter leads to high-quality joints.