Formation Mechanism Of Defects Research Articles

Formation mechanism and optimization strategy of surface back-end defects in miniature complex hollow extruded profile

Back-end defects present a crucial problem in the extrusion of aluminum alloy profiles, particularly in miniature complex hollow profiles. In this study, the formation mechanism and optimization strategy of surface back-end defects in miniature complex hollow extruded profile were revealed for the first time. The flow behavior of the billet skin, back-end defects, and effect of the porthole outer radius were studied via numerical simulation. The microstructure, chemical composition, and mechanical properties of the discarded billet and back-end defects were experimentally investigated. The results showed that the corrosion resistance and mechanical properties of the back-end defects were poor. Unlike regular-sized profiles, a rare forward flow of the billet skin dominates the extrusion of miniature hollow profiles, resulting in the premature formation of severe surface back-end defects. The primary cause of the forward flow is the significant weakening of the radial flow of billet metal under an ultra-large extrusion ratio (e.g., 137). Nevertheless, a small porthole outer radius effectively delays the entry of billet skin into the profile along the forward flow path owing to a larger storage choke zone that increases the blocking effect and storage capacity of billet skin. In summary, a new dimensionless parameter η was formulated to evaluate the ability of the extrusion parameters (e.g., porthole outer radius) in delaying the formation of surface back-end defects in miniature hollow profiles. The larger the value of η, the later the back-end defects form.

Atomistic simulation on the generation of defects in Cu/SiC composites during cooling

The coefficient of thermal expansion (CTE) mismatch between the reinforcement and the matrix results in thermal residual stresses and defects within metal-matrix composites (MMCs) upon cooling from the processing temperature to ambient temperature. The residual stresses and thermally induced defects play an important role in the mechanical properties of MMCs, it is critical to understand the mechanism of defect formation and evolution. This study provides atomistic simulations to reveal the generation of thermal residual stresses, dislocation and incomplete stacking fault tetrahedron (ISFT) during cooling in the idealized Cu/SiC composites. We found that dislocations are generated explosively in a certain temperature range during cooling, which results in a non-linear relationship between dislocation density and temperature. The combined effect of the stresses induced by CTE mismatch and the thermodynamic state of the metal leads to the rapid generation of dislocations. The Shockley partial and the highly stable stair rod are the two dominant dislocation structures. The immobile stair-rod dislocations and the highly stable ISFTs formed in the initial high temperature stage inhibit further development of plastic deformation. The present results provide new insights into the defect formation mechanism and the dislocation strengthening mechanism of MMCs caused by thermal mismatch between constituents.

Towards understanding metallurgical defect formation of selective laser melted wrought aluminum alloys

The formation of balling, porosity and cracking defects is a vital obstacle in selective laser melting of wrought Al alloys. However, it lacks systematic research on the origins of these imperfections. Herein, the formation mechanisms and avoidance methods of metallurgical defects in slective laser melting (SLM)-processed Al–Cu–Mg alloy were investigated by numerical simulation and microstructure characterization. Process optimization by altering laser energy density can effectively suppress balling and porosity, thus enhancing relative density. Cracking results from the stress concentration and columnar grains arise due to the rapid cooling process during SLM. The methods that promote the columnar-to-equiaxed grain transition, such as microalloying by Sc/Zr/Ti elements, co-incorporation of ceramic particles and introducing ultrasound, can effectively enhance the cracking resistance and mechanical properties of wrought Al alloys. The formation of balling, porosity and cracking defects is a vital obstacle in selective laser melting of high strength aluminum alloys. Numerical simulation was used to reveal the metallurgical behavior evolution of the molten pool. The defect formation mechanisms and avoidance methods in selective laser melting process of Al-Cu-Mg alloy are discussed both experimentally and computationally.

Modeling the electrical characteristic of InGaN/GaN blue-violet LED structure under electrical stress

The role of deep defects and their physical origin in InGaN/GaN LED are still widely investigated and debated, in particular their impact on the electrical and optical characteristics and on the reliability of the device. In this paper we evaluate the electrical and optical behavior of a single quantum well InGaN/GaN 420 nm LED during a medium-term accelerated ageing test. From these measurements, an increase in the sub-turn on current was observed: this process was ascribed to the stress-induced generation of defects in proximity of the active region and thus related to an increase of the correlated trap assisted tunneling. Subsequently we modelled the device electrical characteristics, evidencing the fundamental role of deep defects in determining the shape of the I-V curve in low forward bias. We reproduced with high accuracy the experimental current-voltage curves acquired during the ageing test, demonstrating that the increase in forward leakage can be entirely ascribed to the increase in concentration of a sole defect deep level located near midgap. These results provide a better understanding about the defect formation mechanism, that can be used to model and understand the behavior of the devices during ageing.

Defect formation mechanism of laser additive manufacturing joint for large-thickness Ti6Al4V titanium alloy with Y2O3 nanoparticles

Laser additive manufacturing is a method, through which parts are joined by laser melting deposition (LMD) layer by layer as opposed to the conventional welding processes. It presents superior advantages in joining large-size structural parts in aerospace due to its flexibility in technology and equipment. Owing to the large size of the welded joint and the complex welding process, the defect is easily produced in joining large structural parts. Given its detrimental influence on mechanical properties, defects are a major problem for the connection of large thickness components. This study aims to explore the formation mechanism of defects in joining 80 mm thick Ti6Al4V titanium alloy using laser additive manufacturing. The microstructure and element distribution detection of the welded joint is analyzed to verify the defect formation mechanism. It is indicated that defects during laser melting deposition (LMD) of large thickness Ti6Al4V titanium alloy can be classified into two types, porosity and lack of fusion (LOF). It is revealed that the irregular-shaped pores with a size of 1–5 μm can be observed in the joint area without nanoparticles due to larger dendrite spacing. Additionally, nanoparticles can reduce the porosity, but excessive content of nanoparticles will lead to metallurgical porosity, which decreases the performance of the weld joint. The LOF defects are caused by insufficient energy, which mostly appears between the layers or near the fusion line and has sharp edges.

Particle scale modelling of powder recoating and melt pool dynamics in laser powder bed fusion additive manufacturing: A review

Particle scale modelling of the process physics involved in laser powder bed fusion (LPBF) is a recent research hotspot, and many efforts have been made in the literature. However, a comprehensive review of the physics in LPBF and the effects of key variables such as powder- and operation-related parameters, and the mechanisms of defects formation is still lacking. This paper aims to offer a state-of-the-art review on multi-physics related to metal powder recoating and further melting and solidification process. The studies on powder bed recoating explored by discrete element method are presented first, including model theory and validation, effects of process parameters, and physics of particle flow and size segregation. Then powder melting approach based on computational fluid dynamics and the involved phenomena such as melt pool dynamics, keyhole dynamics, defects formation mechanisms of pores, and balling and spattering are described. The needs for future research are also discussed. • Powder bed recoating explored by discrete element method (DEM) is reviewed. • Physics of powder flow and size segregation in powder spreading is presented. • Powder melting based on computational fluid dynamics (CFD) is summarized. • Melt pool flow dynamics and defects formation are discussed.

Multi-scale defects in powder-based additively manufactured metals and alloys

• Multi-scale defects in AMed metals and alloys are classified. • The root causes and the key factors of the multi-scale defects are discussed. • The detection and modeling of the multi-scale defects are briefly introduced. • The effects of the multi-scale defects on mechanical properties are reviewed. • Various control and mitigation methods for the defects are proposed and discussed. Defect formation is a critical challenge for powder-based metal additive manufacturing (AM). Current understanding on the three important issues including formation mechanism, influence and control method of metal AM defects should be updated. In this review paper, multi-scale defects in AMed metals and alloys are identified and for the first time classified into three categories, including geometry related, surface integrity related and microstructural defects. In particular, the microstructural defects are further divided into internal cracks and pores, textured columnar grains, compositional defects and dislocation cells. The root causes of the multi-scale defects are discussed. The key factors that affect the defect formation are identified and analyzed. The detection methods and modeling of the multi-scale defects are briefly introduced. The effects of the multi-scale defects on the mechanical properties especially for tensile properties and fatigue performance of AMed metallic components are reviewed. Various control and mitigation methods for the corresponding defects, include process parameter control, post processing, alloy design and hybrid AM techniques, are summarized and discussed. From research aspect, current research gaps and future prospects from three important aspects of the multi-scale AM defects are identified and delineated.

An Overview of Laser Metal Deposition for Cladding: Defect Formation Mechanisms, Defect Suppression Methods and Performance Improvements of Laser-Cladded Layers.

With the development of society and the economy, there is an increasing demand for surface treatment techniques that can efficiently utilize metal materials to obtain good performances in the fields of mechanical engineering and the aerospace industry. The laser metal deposition (LMD) technique for cladding has become a research focus in recent years because of its lower dilution rate, small heat-effect zone and good metallurgical bonding between the coating and substrate. This paper reviews the simulation technology for the melt pool’s grain growth mechanism, temperature and stress distribution that are directly related to defect formation in LMD technology. At the same time, the defect suppression method and the performance improvement method of the cladded layer in LMD technology are introduced. Finally, it is pointed out that the active selection of materials according to the required performance, combined with the controllable processing technology, to form the corresponding microstructure, and finally, to actively realize the expected function, is the future development direction of LMD technology.

Formation mechanism and control method of multiple geometric defects in conical-section profiled ring rolling

In the conical-section profiled ring rolling (CSPRR), due to the asymmetrical geometric features and uncoordinated deformation of the ring along the axial direction, multiple geometric defects may occur, such as diameter error (i.e., the diameter of big and small ends cannot simultaneously reach the target values), roundness error and tilting. In this work, a growth velocity model, describing the ring diameter growing behavior at different axial positions, is developed through the theoretical analysis. Based on the growth velocity model, the largest growth velocity difference along the axial direction is expressed by a mathematical function which relates to the ring geometry parameters (including inner slope angle α1, outer slope angle α2, ring height H) and the mandrel feed rate vf. Further, combined with a mass of FE simulations, it is found that the growth velocity difference along the axial direction is the formation mechanism of multiple geometric defects in CSPRR process. By adjusting α1 or α2, reducing H or reducing vf, the largest growth velocity difference along the axial direction can be made close to zero, which is beneficial to improving geometric accuracy. Following the above understanding, three methods to avoid geometric defects are proposed, including the improved design of target rolled ring (forging drawing), the reduction of vf and the improved design of ring blank, which are applied in the process design for a complex conical-section casing ring. The corresponding simulations and industrial experiment indicate that the proposed methods work well to avoid multiple geometric defects.

Influence of micron and nano SiCp on microstructure evolution and mechanical properties of laser metal deposition AlSi10Mg alloy

In this work, laser metal deposition (LMD) technique was applied to fabricate SiCp/AlSi10Mg composites. Innovatively, the effect of SiCp of different scale (micron, submicron and nano scale) on the microstructure and mechanical properties was studied. With the decrease of SiCp size, the reaction between SiC and aluminum matrix was more intense, and the volume fraction of Si precipitates and grain boundary Al-Si eutectic phase increased. Few micro-SiCp and agglomerated nano-SiCp were all unevenly distributed in the aluminum matrix, which caused the bimodal distribution of grain size in varying degrees. The mechanisms of defect formation, phases evolution and mechanical property enhancement were also discussed. Due to the agglomeration of nano-SiCp and its decomposition under laser irradiation, the seriously uneven microstructure of nano-SiCp/AlSi10Mg limited the mechanical properties. Moreover, the addition of SiCp increased the Marangoni flow instability of molten pool, leading to the high porosity and reducing the tensile strength of composites, which was more prominent in the samples containing micron and nano SiCp. Submicron-SiCp/AlSi10Mg presented uniform equiaxed crystals and the best comprehensive mechanical properties (microhardness 118.37HV, wear rate 1.60 ×10−3 mm3N−1m−1 and UTS 269.08 MPa). Based on the experimental results, adding multi-scale reinforcements will be more conducive to the performance optimization of composites. This study provides theoretical guidance for the engineering application of SiCp/Al composites.

Quantitative characterization of defects in stereolithographic additive manufactured ceramic using X-ray computed tomography

Ceramics have been widely fabricated by additive manufacturing (AM). Compared to conventional technologies, the strength of additive manufactured ceramic is relatively low owing to the formation of defects during manufacturing process. These defects have significant effects on the microstructure and mechanical properties of additive manufactured ceramics. However, systematic research on defects, including defect geometrical features, quantitative statistics, and formation mechanism, as well as the intrinsic relationship with mechanical properties, need to be studied in depth. In this work, Al2O3 ceramics were prepared from photosensitive slurries with different solid loadings by using stereolithographic (SL) additive manufacturing. The defects, including their sizes and distributions, in both green and sintered bodies were investigated by using scanning electron microscopy (SEM) and X-ray computed tomography (X-CT). Geometrical features and quantitative statistics of the defects were evaluated and discussed to reveal their formation mechanism. Moreover, the intrinsic relationship between defects and mechanical properties of the additive manufactured ceramic was revealed. This study can give some fundamental understanding of the defects in additive manufactured ceramics.

Numerical and experimental investigation on the defect formation in lithium-ion-battery electrode-slot coating

• Three types of defects in lithium-ion-battery electrode-slot coating are found. • Defect formation is related to static or dynamic contact line location movement. • A larger maximum coating speed can be obtained for a lower coating gap. • The maximum coating speed of the underbite die configuration increases by 25.6%. Defect-free slot coating is important in the design and manufacturing of lithium-ion-battery electrodes. However, the defect formation in electrode-slot coating is not clearly understood because of the involvement and coupling of many parameters. Here, we investigate the defect formation mechanisms in electrode-slot coating by using numerical and experimental methodologies. Three defects, namely, striped, spotted, and irregular, are observed. The striped defect is related to downstream menisci contraction; however, the spotted and irregular defects are associated with the unstable development of the sawtooth dynamic contact line. In addition, three regions for the capillary number and minimum-thickness-based coating window are found, and the dominant force in each region is clarified. The results indicate that a higher maximum coating speed can be obtained for a lower coating gap, and the maximum coating speed of the underbite die configuration increases by 25.6% compared with that of the normal die configuration.

Characterizing defect structures in AM steel using direct electron detection EBSD

The mechanical properties of additive and traditionally manufactured alloys are largely dependent on the characteristics and distribution of dislocation cell networks that develop during the fabrication process. This work demonstrates the ability to quantitatively characterize these dislocation structures by high angular resolution electron backscatter diffraction analysis using a direct electron detector. The defect structures are characterized in terms of the geometrically necessary dislocation density and the associated Burgers vector and line direction. The results are discussed in terms of potential defect formation mechanisms.

In Situ Structural Dynamics of Atomic Defects in Tungsten Oxide.

Atomic defects are critical to tuning the physical and chemical properties of functional materials such as catalysts, semiconductors, and 2D materials. However, direct structural characterization of atomic defects, especially their formation and annihilation under practical conditions, is challenging yet crucial to understanding the underlying mechanisms driving defect dynamics, which remain mostly elusive. Here, through in situ atomic imaging by an aberration-corrected environmental transmission electron microscope (AC-ETEM), we directly visualize the formation and annihilation mechanism of planar defects in monoclinic WO3 on the atomic scale in real time. We captured the atomistic process of the nucleation dynamics of the dislocation core in the [010] direction, followed by its propagation to form a planar defect. Corroborated by density functional theory-based calculations, we rationalize the formation of dislocation through O extraction from bridge sites followed by an atomic channeling process. These in situ observations shed light on the defect dynamics in oxides and provide atomic insights into forming and manipulating defects in functional materials.

Defect‐Dependent Crystal Plane Control on Inorganic CsPbBr3 Film by Selectively Anchoring (Pseudo‐) Halide Anions for 1.650 V Voltage Perovskite Solar Cells

AbstractOne of the key problems to improving the performance of a typical perovskite solar cell (PSC) is to minimize the defect‐determined nonradiative recombination. So far, the cutting‐edge strategies mainly focus on the film‐scale defect manipulation at grain boundaries and surfaces by trial‐and‐error, but the intrinsic passivation mechanism and fundamental guideline in the perspective of crystal lattices are still unclear. Herein, a much lower defect formation energy of PbBr antisite defect at (110) plane than that at (h00) planes is demonstrated by theoretically studying the crystal plane‐dependent defect density in an inorganic CsPbBr3 film, and then the authors precisely control the plane growth by selectively anchoring (pseudo‐) halide anions from ionic liquids to (110) plane. Because of the different adsorption energies on various planes, the growth of defective (110) plane is suppressed, leading to the formation of (h00) plane‐oriented film. Finally, the all‐inorganic CsPbBr3 PSC passivated by ionic liquid EMImCl delivers the best efficiency of 10.71% with an open‐circuit voltage up to 1.650 V. This work provides an in‐depth insight into defect formation and passivation mechanism to stabilize perovskite photovoltaics.

Ultrafast pulsed laser stealth dicing of 4H-SiC wafer: Structure evolution and defect generation

Ultrafast pulsed laser for wafer processing has been promising in the semiconductor industry due to its high precision and low thermal effect. The microstructure and defect during the stealth dicing process is critical to controlling the desired quality of the wafer. The structural evolution and defect formation mechanisms during stealth dicing of 4H-SiC with femtosecond and picosecond lasers were investigated. In order to understand the ultrafast pulsed laser interaction with 4H-SiC wafer, we studied the laser intensity dependence of the nonlinear refractive index, the propagation of laser beam focusing in 4H-SiC, the laser absorption, electron concentration evolution and energy deposition. The effect of two significant processing factors was discussed, including laser fluence and laser scribing times. The propagation paths and energy deposition of femtosecond and picosecond lasers in the internal processing of 4H-SiC were calculated by finite element method. The evolution of electrons temperature, lattice temperature and free electron concentration in the material during laser processing was analyzed. Femtosecond laser stealth dicing had higher processing accuracy due to minimal thermal effects. The defect induced by femtosecond laser was dominated by nanovoids and micro-cracks. The defect structure was affected by laser processing parameters such as scanning passes and laser power density. Multi-focusing of ultrafast pulsed lasers in transparent materials was generally considered to be a dynamic balance of nonlinear self-focusing and plasma defocusing. Picosecond laser processing resulted a higher processing efficiency under the same laser energy density comparing with femtosecond laser processing. However, defects during picosecond laser dicing were dominated by thermal ablation, which impaired the processing accuracy.

Avoiding oxygen induced Pb vacancies for high thermoelectric performance of n-type Bi-doped Pb1-xBixTe compounds

For n-type PbTe based compounds with relatively low optimal concentration (1019 cm−3), the accompanied structural defects play pivotal role on the charge transport upon doping with alien valent atoms. Herein, we report that Oxygen has huge influence on the defect evolution and thermoelectric properties in n-type PbTe for the first time. A boosted thermoelectric figure of merit ZT of ∼1.12 at 673 K and a ZTave of 0.84 in the range of 300–873 K are achieved for vacuum sintered Bi-doped PbTe with the powders ground under the protection of Ar. This is in sharp contrast with the same Bi-doped PbTe material sintered with the powders ground in the air, which reaches a ZTave of 0.52. This distinct thermoelectric properties in Bi-doped PbTe compounds are strongly correlated to Bi dopant, the vacancy defects (VPb″ and VTe⋅⋅) induced during the fracturing process, and oxygen in the air. The coexistence of Bi dopant and oxygen lowers the formation energy of Pb vacancies and induces large number of O–Bi related Pb vacancies in the structure, yielding an acceptor-like effect. Such negatively charged Pb vacancies in the structure exert a strong long-range coulombic repulsion force on the electrons, leading to a low doping efficiency, a low carrier mobility (200 cm2 V−1 s−1), and inferior electronic properties. Whereas, the protection provided by Ar effectively avoids the contamination by oxygen and prevents the formation of O–Bi related Pb vacancies, which appears remarkably effective in enhancing the electronic transport. A high carrier mobility of 1300 cm2 V−1 s−1 is attained in slightly doped Pb0·9995Bi0·0005Te sample and this contributes to an ultra-high power factor of 40 μW cm−1 K−2 for the Pb0·9995Bi0·0005Te sample at room temperature. The high PFave exceeds 25 μW cm−1 K−2 in the range of 300–723 K. The work provides a new insight into the formation mechanism of structural defects and a new avenue for suppressing the cation vacancies, resulting in higher performance of n-type PbTe-based compounds.

Selective laser melted AZ91D magnesium alloy with superior balance of strength and ductility

In the context of global carbon neutrality, the application of lightweight magnesium alloys is becoming increasingly attractive. In this study, selective laser melting (SLM) was employed to achieve nearly full dense and crack-free AZ91D components with fine equiaxed grain structure. The formation mechanism of typical pore defects (gas pore, lack-of-fusion pore and keyhole pore) and melting modes (keyhole mode and conduction mode) were systematically studied by varying the laser power and scanning speed. The morphology and volume fraction of the pores under different processing conditions were characterized. A criterion based on the depth-to-width ratio of the melt pool was established to identify different melting modes. The strength and ductility (tensile strength up to 340 MPa and uniform elongation of 8.9%) of the as deposited AZ91D are far superior to those of the casting components and are comparable to those of its wrought counterparts. The superior balance of strength and ductility of SLMed AZ91D, as well as the negligible anisotropic properties are mainly ascribed to the extremely fine equiaxed grain structure (with average grain size of ∼1.2 µm), as well as the discontinuous distribution of β-Al12Mg17 phases. It thus provides an alternative way to fabricate high-strength magnesium alloys with complex geometry.

Investigation on characteristic and formation mechanism of porosity defects of Al–Li alloys prepared by sand casting

Porosity defects and interfacial reaction are very serious problems in sand casting Al–Li alloy, which greatly affect the quality of castings. In this work, the characteristic and formation mechanism of the porosity defects in the sand casting Al–Li alloys were systematically investigated by scanning electron microscopy, transmission electron microscopy, energy spectrometry, and inductively coupled plasma mass spectrometry, in order to provide a theoretical basis for the prevention and control of the pore defects in the sand casting Al–Li alloys. The results found that the Al–Li alloys prepared by metal mold and graphite mold as casting molds had almost no pore defects and interfacial reaction layers, with the porosities of 5.81% and 5.05%, respectively, while the sand casting Al–Li alloys with furan resin, phenolic resin, and sodium silicate binder as binders all had serious pore defects and interfacial reaction layers, with the porosity of 15.67%, sharply weakening the mechanical properties of the Al–Li alloy casting. The types of the pore defects were mainly reactive pores and intrusive pores, and the interface reaction layer was mainly composed of LiAlO2 and C–H–O organics. The formation mechanism of the pore defects in the sand casting Al–Li alloys was caused by the reaction between Li and the hydroxyl groups in the binder of the sand mold to generate a large amount of hydrogen, which intruded into the melt or was sucked into the melt.

Factors influencing the structure of the complex-defects in AF2: RE3+ (A= Ca, Sr and Ba): A first-principles study

As aliovalent doping materials, RE3+ doped AF2 (A = Ca, Sr and Ba), which show not only high thermal conductivity and low nonlinear refractive index ascribed to fluorides, but also unique spectroscopic properties related to the RE3+ ions, are promising ultra-strong and ultra-fast laser crystal candidates. It is noted that the stability of the C3v complex-defect, in which Fi− compensates at the next-nearest interstitial site of RE3+, increases with the ratio of the lattice parameter to the dopant ionic radius. In this work, systematic first-principles calculations are performed to resolve factors influencing the structure of the RE-Fi monomers. The formation energy, the reorientation energy and the local structure of the monomers are obtained, and the Coulomb interaction and the sub-lattice distortion are quantified. It is concluded that the sub-lattice distortion determines the defect structure even though the Coulomb interaction is the fundamental basis for the complex-defect formation in aliovalent doping materials. On this basis, the cluster aggregation process in AF2: RE3+ are elucidated and the octahedral arrangement of the hexamer is specially explained. Our calculation results and analytical methods not only provide new insights for the defect formation mechanism in AF2: RE3+, but also guide the improvement of aliovalent doping materials.