Microstructural Defects Research Articles

Additive Manufacturing Processes in Selected Corrosion Resistant Materials: A State of Knowledge Review

Additive manufacturing is an important and promising process of manufacturing due to its increasing demand in all industrial sectors, with special relevance in those related to metallic components since it permits the lightening of structures, producing complex geometries with a minimum waste of material. There are different techniques involved in additive manufacturing that must be carefully selected according to the chemical composition of the material and the final requirements. There is a large amount of research devoted to the technical development and the mechanical properties of the final components; however, not much attention has been paid yet to the corrosion behaviour in different service conditions. The aim of this paper is to deeply analyze the interaction between the chemical composition of different metallic alloys, the additive manufacturing processing, and their corrosion behaviour, determining the effects of the main microstructural features and defects associated with these specific processes, such as grain size, segregation, and porosity, among others. The corrosion resistance of the most-used systems obtained by additive manufacturing (AM) such as aluminum alloys, titanium alloys, and duplex stainless steels is analyzed to provide knowledge that can be a platform to create new ideas for materials manufacturing. Some conclusions and future guidelines for establishing good practices related to corrosion tests are proposed.

Stress sensitivity origin of extended defects production under coupled irradiation and mechanical loading

The production of extended and mechanically highly consequential defects are investigated under concurrent driving forces of irradiation and stress. By using two-temperature model to account for realistic exchange of energy between the electronic subsystem and the atomic subsystem during cascades, and by controlling mechanical loading in the principal stress space to remove the dependance on a specific coordinate system, we build stress sensitivity maps for various types of irradiated dislocations. Frank loops are discovered to be dictated by volumetric strains, while Shockley partials are sensitive to volume-conserving shear loads. By tracking the microstructural evolutions during coupled irradiation and external loading, and by scrutinizing the temporal-spatial variation of local von Mises stresses near the center-collision region, the underlying mechanisms responsible for those different responses/sensitivities are elucidated. The implications of these findings in regard to a new strategy to control microstructural defects and materials performance are also discussed.

Fatigue-Damage Initiation at Process Introduced Internal Defects in Electron-Beam-Melted Ti-6Al-4V

Electron Beam Melting (EBM) is a widespread additive manufacturing technology for metallic-part fabrication; however, final products can contain microstructural defects that reduce fatigue performance. While the effects of gas and keyhole pores are well characterized, other defects, including lack of fusion and smooth facets, warrant additional investigation given their potential to significantly impact fatigue life. Therefore, such defects were intentionally induced into EBM Ti-6Al-4V, a prevalent titanium alloy, to investigate their degradation on stress-controlled fatigue life. The focus offset processing parameter was varied outside of typical manufacturing settings to generate a variety of defect types, and specimens were tested under fatigue loading, followed by surface and microstructure characterization. Fatigue damage primarily initiated at smooth facet sites or sites consisting of un-melted powder due to a lack of fusion, and an increase in both fatigue life and void content with increasing focus offset was noted. This counter-intuitive relationship is attributed to lower focus offsets producing a microstructure more prone to smooth facets, discussed in the literature as being due to lack of fusion or cleavage fracture, and this study indicates that these smooth flaws are most likely a result of lack of fusion.

Improved strength-ductility synergy of directed energy deposited AZ31 magnesium alloy with cryogenic cooling mode

ABSTRACT Motivated by the effect of cooling rate on microstructure, standard air cooling (AC) and novel cryogenic cooling (CC) were employed in the wire-arc directed energy deposition process of Mg-alloy. The influences of cooling modes on macrostructure, defects, microstructure, mechanical properties and deformation behaviour were systematically investigated. Compared with AC component, CC component exhibits improved performance with yield strength, ultimate tensile strength and elongation increased by 50%, 58% and 174%, respectively. The improved strength-ductility synergy can be attributed to the decreased porosity, profuse nano-precipitates, finer equiaxed grain within inter-layer and the transformation of columnar to equiaxed grain within intra-layer. The enhancement of strain hardening rate by highly coherent nano-precipitates impeding dislocations glide, dynamic Hall-Petch effect of multiple twins and numerous 3D obstacle networks of dislocations should be responsible for the significant elongation improvement. This study thus introduces new insights into defect alleviation, microstructure modification and performance enhancement of additive manufacturing Mg-alloys.

Fatigue behavior and fracture characteristics of 718Plus nickel-based superalloy at high temperature

ABSTRACT The paper describes high cycle rotational bending fatigue (HCF) performance and fracture characteristics of 718Plus alloy at 600°C and 700°C, where the relationship between maximum cyclic stress (S) and fatigue cycles (N) at 600°C and 700°C is explored. We drew S-N curves, and established mathematical model, with simulating S-N curve function. The crack initiation of smooth specimens focus on the surface or subsurface, where the stress concentration caused by inclusions, carbides and microstructure defects, leading to tearing, and inclusion is the main form of crack initiation. In addition, the high cycle fatigue life of 718Plus alloy is greatly affected by the microstructure defects. According to the analysis of high cycle fatigue fracture morphology, at 700°C, the alloy fatigue crack growth is faster, and the number of secondary cracks is more than 600°C. Thus, the growth behaviour is greatly affected by temperature.

Microstructure and defect evolution of nuclear graphite under temperature-dependent ion irradiation

Nuclear graphite plays a vital role in GEN-IV reactors and is expected to function in the harsh environments such as high temperature and high-dose irradiation. The ion irradiation behaviour of ET-10 nuclear graphite in comparison with highly orientated pyrolytic graphite (HOPG) was reported in this work, to explore the combined effects of ion irradiation and temperature. Ion irradiation experiments were carried out using 190 keV H+ and 1.8 MeV Ar+ at different doses. SRIM simulation, scanning electron microscopy (SEM) and Raman spectroscopy were employed to characterize the microstructural changes and defect evolution of graphite specimens under different irradiation conditions. The vein structure in HOPG graphite and the vertical scale-like structure in ET-10 graphite caused by ion irradiation were observed. The effects of temperature on ion irradiation could be divided into two stages of recovering and saturation according to mutual analysis of SEM and Raman. It was also found that the recovery degree of the half-width at half maximum (HWHM) for the G band was greater than that of ID/IG, probably because the deformed or ruptured graphite layers were difficult to recover at rising temperatures and/or could even be enhanced. HWHM(G), ID/IG, and La was comprehensively discussed, giving an overall perspective of defect evolution and crystallite size changes under temperature-dependant ion irradiation.

New fabrication of nonmodel supported catalysts for cellulose to low carbon chemicals with the assistance of mechanochemical abrading

The controllable conversion of cellulose on heterogeneous catalysts is a promising strategy in biomass utilization. However, the catalysts in this field are fabricated by a conventional, calcination/hydrothermal method, both of which are time-consuming, high cost and limit their large-scale applications. In this study, we developed the mechanical force as an auxiliary method to fabricate nonmodel supported catalyst with the assistance of chitosan (CTS). The input kinetic energy facilitated strain refinement and microstructure defects generation by shear force and plastic deformations, which stimulated the metal precursors anchored in situ in the CTS matrix. Based on the interactions between metals (Ni and Sn) and nitrogen atoms, the single metal nanoparticles and intermetallic compounds (NixSny) were well dispersed with 152 μmol/g of Lewis acid and 91 μmol/g of base amount by interaction with nitrogen sites. 62.8% of C2,3 polyols were obtained from cellulose on 15%Ni–15%W/CTS catalyst. More importantly, this catalyst prepared by mechanochemical abrading demonstrated perfect stability with less than 2.0 ppm of etched metal cations for the eighth use in acidic hydrothermal conditions. In accordance, the nitrogen sites in the carbon matrix offered strong covalent bonds regarding as anchoring rope for stabilizing Ni species. This work potentially offers a facile but effective method for the development of supported Ni–Sn catalysts with possible large-scale production.

The effect of shielding gas on weldability of the AISI 420 martensitic stainless steel

Abstract Most of weld defects occurring in the welding of martensitic stainless steels are caused by the presence of hydrogen. Thus, the effects of hydrogen in the weld zone need to be well-understood to estimate the quality and service life of martensitic stainless steel joints. In the present study, AISI 420 martensitic stainless steel materials were welded by using different combinations of shielding gas via the gas metal arc welding (GMAW) method. It is known that shielding gases also play a critical role in heat input, cooling rate, microstructure of weld seam, weld defects, and mechanical properties besides drying of molten weld pool. Thus, it is important to investigate the effects of shielding gases and gas combinations on the welding of martensitic stainless steels in the welding process. In the present study, 100 % Ar, 97 % Ar + 3 % H2 and 93 % Ar + 7 % H2 gas combinations were employed. The welded sheets were subjected to the metallographic examination as well as hardness, tensile, and bending tests. The effect of the tests and the combination of shielding gas on the mechanical and microstructural properties of AISI 420 stainless steel was investigated. The results indicated that a noticeable grain coarsening occurred in the microstructure of the weld metal and heat affected zones (HAZs) after the addition of H2 into the Ar gas during the welding process. The highest tensile strength was obtained from the joints with 100 % Ar gas. As a result of the tensile test, rupture occurred in the base metal-HAZ transition zone in all the welded samples. In the joints welded with 97 % Ar + 3 % H2 and 93 % Ar + 7 % H2 gas combinations, fracture occurred in the base metal-HAZ transition zone during the bending test.

One stone two birds: Pitch assisted microcrystalline regulation and defect engineering in coal-based carbon anodes for sodium-ion batteries

Coal-based carbons with abundant resources and low cost are regarded as promising anode materials for sodium‐ion batteries (SIBs). However, their ordered carbon microstructure and abundant surface defects often result in low Na-storage capacity and poor initial coulombic efficiency (ICE). Herein, we propose a simple vapor deposition strategy to synthesize coal-based carbons coated with pitch-based soft carbon layer (PCLC) for potential use as anodes for SIBs. The deposition of pitch-based volatile species allows for an efficient cross-linking reaction between hydroxyl‑containing volatile species and oxygen-rich functional groups of coal, thus generating a disordered inner-phase microcrystalline structure with dominant pseudo-graphitic phase in coal-derived carbon. Meanwhile, the exterior soft carbon coating layer substantially reduces surface defects and increases the electrical conductivity of coal-based carbon. Unlike the pristine lignite coal pyrolytic carbon which exhibited a Na-storage capacity of 290.2 mAh g−1 and ICE of 59.9%, the optimal PCLC (PCLC-1) delivered a higher reversible capacity of 312.2 mAh g−1 and a much-improved ICE of 85.3%. In addition, the PCLC-1 electrode also demonstrated excellent cycle stability with 93.4% retention after 1,000 cycles. When coupled with O3-NaNi1/3Fe1/3Mn1/3O2 cathode, PCLC-1 as an anode in a full cell configuration achieved a high energy density of 220.9 Wh kg−1 with excellent cycling and rate performance. The proposed work offers a unique insight into modulating the microcrystalline structure and surface chemistry of high-performance coal-based carbon anode materials for commercial SIBs.

Additive manufacturing of tantalum scaffolds: Processing, microstructure and process-induced defects

Extreme melting point, high density, and ease of oxidation of tantalum (Ta) make its processing rather difficult using conventional methods. Additive manufacturing (AM) serves as an alternative Ta processing technique with unique design flexibility, customization, and minimizing material wastage. However, most additively manufactured parts contain undesired microstructural features or deviations (process-induced defects). This study aimed to assess the processability of the solid and structurally porous scaffolds of Ta through laser powder-bed fusion (LPBF) AM. It will further characterize and evaluate microstructure of LPBF processed Ta, with emphasis on assessing the mechanism of the process-induced defects. The x-ray diffraction (XRD) and microstructural investigation confirmed the presence of BCC Ta phase containing columnar, equiaxed and fine grains with roughness between 3.88 nm to 10.40 nm. The presence of oxygen resulted in the formation of some oxide phases such as Ta2O5 and Ta2O3. Numerous process-induced defects, including solidification-induced micropores, pores-induced and solidification-induced microcracks were identified. The dislodge unmelted or partially fused Ta powder resulted in other form of defects including micro-concaves and microgrooves. The mechanism of pore formation during the LPBF of Ta was assessed through computational simulation and its findings rationalised the experimental results.

Angular error measurement of workpiece repositioning using a full-scale rotation detection method.

Workpiece repositioning error has always been a key factor affecting manufacturing accuracy. The issues become more sensitive when machining microstructures with special morphologies, where the declination error caused by the repositioning may lead to microstructural defects. To solve this practical problem, in this paper, we report the design of a fixture that can detect the plane angular displacement error between the workpiece and the tool, namely the Rotation Correction Fixture (RCF). The fiducial marker referred to as polar microstructure is proposed and placed on the RCF edge. Angular displacement measurement is realized by observing the microstructural changes. Simultaneously, a Full-scale Rotation Detection (FRD) method is proposed to obtain the full-scale and high-precision angular displacement, including coarse extraction based on Fourier transform and fine extraction based on the Fast and Robust Feature-based Positioning method. Template matching is employed to eliminate the phase ambiguity in the Fourier transform. The results show that the proposed method can realize the calibration of the workpiece declination with a standard deviation error of 250.24 seconds, which meets the needs of workpiece precision positioning well.

Influence of cryogenic grinding surface on fatigue performance of carburised 27MnCr5

Automotive transmission components are subjected to cyclic loads and, thus, must have a reliable fatigue performance. Since fatigue cracks nucleate at the surface, it is necessary to guarantee that its surface integrity accomplishes the required specifications. Typically, those components are finished by wet grinding after carburising heat treatment. However, there is an increasing demand to reduce pollutants and hazardous lubricants in the industry, and eco-friendly finishing operations have been highly encouraged. To this end, it is necessary to understand the effect of these novel finishing processes on surface integrity and, consequently, on fatigue behaviour. This study aims to assess the surface integrity and the fatigue performance of cryo-ground surfaces of 27MnCr5 steel, extensively used in fabricating shafts and gears for gearboxes. Fatigue specimens for pure torsion tests were initially case-hardened and afterwards finished using two different cryogenic grinding conditions applying liquid N2 and, as a reference, using the conventional wet grinding process. First, the surface integrity was analysed in terms of texture, residual stresses, microstructure, and microhardness. Second, the batches of specimens were tested under pure torsion fatigue. Surface residual stress relaxation was also measured during fatigue tests. Finally, fracture surfaces were observed to identify crack initiation sites and establish correlations with the surface integrity. Specimens produced by cryogenic and conventional wet grinding did not show microstructural defects or hardness reductions in the carburised layer. All conditions induced compressive residual stresses, and they barely relaxed during fatigue tests. Compressive residual stresses induced by cryogenic grinding were 10–20% lower than those generated by conventional wet grinding. This decrease resulted in a minor reduction of the fatigue resistance (4–6%) compared to the wet grinding. Importantly, this study demonstrates that with a slight geometrical radio correction in the design of the mechanical components (around 2.2%), cryogenic grinding generates pieces with the same fatigue strength as conventional grinding. Therefore, it confirms that cryogenic cooling could be a potential solution to replace pollutant coolant/lubricant fluid in grinding operations.

Achieving Both Ultrahigh Electrical Conductivity and Mechanical Modulus of Carbon Films: Templating-Coalescing Behavior of Single-Walled Carbon Nanotube in Polyacrylonitrile.

Promoting the feasibility of carbon films as electrode applications requires sufficient performances in view of both electrical and mechanical properties. Herein, carbon films with ultrahigh electrical conductivity and mechanical modulus are prepared by high temperature carbonization of polyacrylonitrile (PAN)/single-walled carbon nanotube (SWNT) nanocomposites. Achieving both performances is ascribed to remarkable graphitic crystallinity, resulting from the sequential templating-coalescing behavior of concentrated SWNT bundles (B-CNTs). While well-dispersed SWNTs (WD-CNTs) facilitate radial templating according to their tubular geometry, flattened B-CNTs sandwiched between carbonized PAN matrices induce vertical templating, where the former and latter produce concentric and planar crystallizations of the graphitic structure, respectively. After carbonization at 2500 °C with the remaining WD-CNTs as microstructural defects, the flattened B-CNTs coalesce into graphitic crystals by zipping the surrounding matrix, resulting in high crystallinity with the crystal thicknesses of 27.4 and 39.4nm for the (002) and (10) planes, respectively. For comparison, the graphene oxide (GO) containing carbon films produce a less-ordered graphitic phase owing to irregular templating, despite the geometrical consistency. Consequently, PAN/B-CNT carbon films exhibit exceptional electrical conductivity (40.7× 104 S m-1 ) and mechanical modulus (38.2 ± 6.4GPa). Thus, controlling the templating-coalescing behavior of SWNTs is the key for improving final performances of carbon films.

Failure analysis of an aeroengine intermediate case splitter

The intermediate case splitter (splitter) is a kind of thin-walled welded component installed on the aeroengine. During the service of the aeroengine, the crack initiation and growth occurred on the weld joints of the splitter, which seriously threatens the safe operation of the aeroengine. In this study, the fracture morphology characteristics of the crack surface are analyzed in details by scanning electron microscope (SEM), based on the observation of the macro crack of the splitter. The results show that the cracks always appear in the welding zone at the edge of the outer panel of the splitter. The fracture morphology analysis shows that there are obvious fatigue striations in the stable crack growth zone. The microstructure defects such as pores and lack of fusion formed by welding are the direct causes of the fatigue failure of the splitter. In addition, the stress analysis of the splitter under random vibration load shows that the stress concentration in the V-shape slot of the splitter can promote the initiation of fatigue cracks.

Micromagnetic simulation of neutron scattering from spherical nanoparticles: Effect of pore-type defects

We employ micromagnetic simulations to model the effect of pore-type microstructural defects on the magnetic small-angle neutron scattering cross section and the related pair-distance distribution function of spherical magnetic nanoparticles. Our expression for the magnetic energy takes into account the isotropic exchange interaction, the magnetocrystalline anisotropy, the dipolar interaction, and an externally applied magnetic field. The signatures of the defects and the role of the dipolar energy are highlighted and the effect of a particle-size distribution is studied. The results serve as a guideline to the experimentalist.

Corrosion Performance of Wire Arc Additive Manufacturing of Stainless Steel: A Brief Critical Assessment.

To enhance the products fabricated from wire arc additive manufacturing (WAAM) processes, it is very important to implement a critical assessment of the corrosion performance of additively manufactured stainless steel (SS) for the application of additive manufacturing parts widely used in industries. The common defects in metal additive manufacturing, which include porosity, poor surface finish, oxidation, environmental factor, residual stress, and microstructural defects, are known to significantly influence the corrosion behavior of WAAM-processed SS components prepared to be used under different corrosive and marine environments. This article reviews the recently published works on WAAM-processed SS and provides a critical overview method to improve the corrosion performance of SS components built with the WAAM processes. It also documents some significant factors that determine the corrosion resistance of WAAM-processed SS and identifies key areas for future work.

Impact toughness and fractography of diverse microstructure in Al-Cu alloy fabricated by arc-directed energy deposition

To investigate the effects of heterogeneous microstructure, precipitates, and pore defects on dynamic impact toughness of Al-Cu alloy fabricated by arc-directed energy deposition (Arc-DED). Samples with diverse microstructures were prepared by adjusting the forming parameters, namely air cooling (AC), water cooling (WC), and the post-treatment, hot isostatic pressing (HIP), and heat treatment (T6) samples. The impact fracture behavior of samples with Charpy V-notched was studied by an instrumented drop hammer impact testing machine which can provide a load-displacement curve. The important load parameters and the absorbed energy at different stages can be determined by the load-displacement curve. The results show that the impact absorbed energy of the AC/WC/HIP/T6 sample was 1.33 ± 0.20, 1.96 ± 0.26, 1.91 ± 0.18, and 2.54 ± 0.21 J, respectively. The impact fracture mechanism was discussed as the corresponding fractographic observations. Brittle fractures with inhomogeneous dimples were observed in all the samples. The key determinant of the impact toughness was the block-shaped brittle θ phase that precipitated along the grain boundaries and pore defects. The equiaxed/columnar heterogeneous microstructure affects not only the coordinated deformation ability of grains but also the distribution of brittle θ phases. Therefore, compared with the AC sample, the impact toughness of the WC sample with a low-volume fraction θ phase, the HIP sample with low porosity, and the T6 sample with a large amount of ductile θ″/θ' phases was significantly improved. This study provides valuable insights for improving the dynamic impact resistance of Arc-DED fabricated Al-Cu alloy.

Engineering Multiple Microstructural Defects for Record-Breaking Thermoelectric Properties of Chalcopyrite Cu1- x Agx GaTe2.

Defect engineering for vacancies, holes, nano precipitates, dislocations, and strain are efficient means of suppressing lattice thermal conductivity. Multiple microstructural defects are successfully designed in Cu1- x Agx GaTe2 (0≤x≤ 0.5) solid solutions through high-ratio alloying and vibratory ball milling, to achieve ultra-low thermal conductivity and record-breaking thermoelectric performance. Extremely low total thermal conductivities of 1.28W m-1 K-1 at 300 K and 0.40Wm-1 K-1 at 873 K for the Cu0.5 Ag0.5 GaTe2 are observed, which are ≈79% and ≈58% lower than that of the CuGaTe2 matrix. Multiple phonon scattering mechanisms are collectively responsible for the reduction of thermal conductivity in this work. On one hand, large amounts of nano precipitates and dislocations are formed via vibrating ball milling followed by the low-temperature hot press, which can enhance phonon scattering. On the other hand, the difference in atomic sizes, distorted chemical bonds, elements fluctuation, and strained domains are caused by the high substitution ratio of Ag and also function as a center for the strong phonon scattering. As a result, the Cu0.7 Ag0.3 GaTe2 exhibits a record high ZTmax of ≈1.73 at 873K and ZTave of ≈0.69 between 300-873K, which are the highest values of CuGaTe2 -based thermoelectric materials.

High temperature thermomechanical properties of a microcracked model refractory material: A silica-doped aluminium titanate

An aluminium titanate based (AT) material doped with silica was investigated as refractory model material in order to highlight its thermomechanical properties through various techniques of characterization operating at high temperature such as ultrasonic pulse echography technique operating in long bar mode, acoustic emission, dilatometry and tensile test measurements up to 1400 °C. Young's modulus (MoE) as a function of temperature evolves in the form of a large hysteresis loop with a maximum value of about 170 GPa due to the healing of diffuse microcracks during heating. A sharp decrease in MoE occurs on cooling at about 780 °C, corresponding to the re-opening of the microcracked network due to a high level of stress around AT grains. In addition, during cooling, the dilatometric analysis shows a quasi-linear shrinkage followed by a sudden non-linear expansion from 750 °C. The thermal expansion coefficient value determined between 1100 °C and 750 °C is about 8.8 10−6 °C−1. By recording the evolution of the cumulative number of hits as a function of temperature, the results of the acoustic emission clearly confirm the resurgence of microstructural defects at 780 °C. The incremental tensile loading test performed at 1400 °C shows a greater degree of nonlinearity suggesting a higher flexibility of the studied AT due to both the microcracks network and the low viscosity of intergranular glassy phase. Symmetric alternating loading tests have highlighted that the viscous contribution in the viscoelastic behaviour of such materials is increasing from 850 H °C to 1400 H °C as the viscosity of the silica-riched amorphous phase is decreasing. These results are very useful to understand the more sensitive parameters involved in the high thermal shock resistance of aluminium titanate.

Room temperature cyclic creep behaviour of equimolar CoCuFeMnNi high entropy alloy

The room temperature ratcheting response of equiatomic FCC CoCuFeMnNi high entropy alloy containing Cu-rich nanoclusters in HEA matrix at different mean stress and stress amplitude indicated the critical role of interaction among dislocations as well as dislocations and Cu-rich nanoclusters in determining the deformation response and damage accumulation contributing to final failure. Detailed microstructural investigation using EBSD and TEM capture the evolution of complex defect microstructure for different cyclic loading conditions. Perennial network of Cu-rich nanoclusters transforming into entangled Cu-rich nanofibers during ratcheting was observed in the three-dimensional elemental maps obtained using APT. This can be attributed to shearing of spherical clusters followed with enhanced short-range diffusion contributing to the decoration of dislocation structure formed by cyclic loading with copper atoms. Further, an increase in the concentration of copper in the solid solution matrix contributes to additional solid solution strengthening, compensating for cluster strengthening and leading to a lower steady-state creep rate with better ratcheting performance. Thus, synergistic operation of cluster and solid solution strengthening in improved ratcheting response of CoCuFeMnNi HEA showcases the unique ability of HEAs to offer excellent mechanical properties beyond superior strength-ductility combination.