Denuded Zone Research Articles

Operando tomographic microscopy during laser-based powder bed fusion of alumina

Laser-based Powder Bed Fusion (LPBF) of oxide ceramics enables fabrication of objects with complex three-dimensional shapes. However, mechanical properties of dense LPBF-manufactured ceramics are poor due to large amount of structural defects. Here, we perform the operando tomographic microscopy during LPBF of a magnetite-modified alumina to gain a deeper understanding of the underlying mechanisms. The effect of the laser energy density on the surface roughness, powder denudation zone and porosity formation mechanisms are investigated. Increasing laser power results in significant increase of the melt pool width, but not its depth and no melt pool depression is observed. Forces due to the recoil pressure are not seen to significantly influence the melt pool dynamics. Increasing power allows to avoid fusion porosity but enhances formation of spherical porosity that is formed by either reaching boiling point of liquid alumina, or by introducing gas bubbles by injection of hollow powder particles into the liquid.

Suppression of denudation zone using laser profile control in vacuum selective laser melting

A vacuum selective laser melting method with a beam profile control was developed to fabricate the high quality 3D object. To investigate the quality of 3D fabrication, the denudation zone (DZ) as a quality indicator was evaluated to make a single bead. The main challenge is to clarify the correlation between the DZ and beam profiles. The beam profiles, such as a Gaussian mode, a doughnut mode, and a flat-top mode, were formed in several modes, by a dioptric system with a beam shaper. A stainless steel 316L (SS316L) powder was irradiated in arbitrary pressure to form a single bead, and then the DZ was measured. As a result, it was found that the flat-top mode recorded the minimum value of the DZ under atmospheric pressure. Thus, the dynamics of the metal vapor while the laser irradiation were observed by the Schlieren imaging technique under atmospheric pressure. The average velocity of metal vapor with flat-top mode was the slowest, i.e., 0.199 m/s. It was found that the DZ becomes small depending on the dynamics of metal vapor. The ambient pressure was reduced to 300 Pa in order to reduce the gas movement, and the DZ with flat-top mode was improved to record a minimum value of 0.048 mm, 1/3 of the DZ at atmospheric pressure.

CFD–DPM Simulation Study of the Effect of Powder Layer Thickness on the SLM Spatter Behavior

Selective Laser Melting (SLM) has significant advantages in manufacturing complex structural components and refining the alloy microstructure; however, spatter, as a phenomenon that accompanies the entire SLM forming process, is prone to problems such as inclusions, porosity, and low powder recovery quality. In this paper, a Computational Fluid Dynamics–Discrete Particle Method (CFD–DPM) simulation flow for predicting the SLM spatter behavior is established based on the open-source code OpenFOAM. Among them, the single-phase flow Navier–Stokes equation is used in the Eulerian framework to equivalently describe the effect of metal vapor and protective gas on the flow field of the forming cavity, and the DPM method is used in the Lagrangian framework to describe the metal particle motion, and the factors affecting the particle motion include particle–particle collision, particle–wall collision, fluid drag force, gravity, buoyancy force, and additional mass force. In addition, the equivalent volume force and fluid drag force are used to characterize the fluid–particle coupling interaction. For the spatter behavior and powder bed denudation phenomenon, the calculation results show that the spatter height and the drop location show a clear correlation, and the powder bed denudation phenomenon is caused by the high-speed gas flow, causing the surrounding gas to gather in the forming area, which in turn drives the motion of the powder bed particles. For the effect of powder layer thickness on spatter and powder bed denudation, the calculation results show that the effect of powder layer thickness on the number of spatters is large (when the thickness was increased from 50 μm to 100 μm, the number of spatters increased by 157%), but the effect on spatter height and drop location distribution is small. When the powder layer thickness is small, the width of the denudation zone is significantly larger, but when the powder layer reaches a certain thickness, the width of the denudation zone does not show significant changes. It should be noted that the presented model has not been directly validated by experiments so far due to the difficulty of tracking the large-scale motion of SLM spatter in real time by current experimental means.

Numerical and experimental investigation into powder entrainment and denudation phenomena in laser powder bed fusion process

This paper investigates the formation and dynamics of the powder entrainment and denudation phenomena in the L-PBF process. A new three-dimensional (3D) coupled Computational Fluid Dynamics (CFD) – Discrete Element Method (DEM) model is constructed to simulate not only the powder entrainment and powder denudation effects, but also the powder absorption effect of the moving melt-pool. Numerical simulations are performed to simulate the denudation width in single-track L-PBF processing. It is clearly found that significant difference between the denudation widths is obtained for different metal vapor ejection angles. To enhance the accuracy of the simulated denudation zone, a pre-defined virtual moving melt-pool is incorporated to consider the powder absorption effect of the moving melt-pool. The simulated results qualitatively have a good agreement with both self-conducted experiments and published experimental results.

Fundamental physics effects of background gas species and pressure on vapor plume structure and spatter entrainment in laser melting

First-principles modeling, based on the direct simulation Monte Carlo (DSMC) method, is applied to quantitatively explain the physics of stainless-steel (SS316L) vapor plumes and jets and powder particle entrainment under conditions relevant to laser welding and laser powder bed fusion additive manufacturing. The DSMC is a particle-based gas kinetic simulation approach, with few built-in assumptions, that resolves the Knudsen and diffusion layers in vapor microjets. The simulations are performed for different background gas species (argon, helium, and neon) at background gas pressures from 0.1 to 5 bar and molten pool temperatures from 3250 K to 4000 K. The simulations capture the evolution of the vapor jet structure from unsteady to steady-state vapor flow, the transition from subsonic flow at low surface temperatures to supersonic flow at surface temperatures close to boiling, as well as the effects of the background gas species and pressure. The simulations show that the vapor jet structure, as well as the magnitude and direction of the entrained gas close to the laser spot, greatly affects the process environment, creating particle spatter and denudation around the laser track. High pressure and lighter background gas can decrease the amount of spatter. At high pressure, the evaporation is reduced, which forms a low-velocity subsonic plume. For lighter gas species, the gas-entrainment velocity increases while the jet radius decreases which forces entrained particles to merge with the melt pool. The powder particle simulations reveal that the width of the denudation zone strongly depends on particle diameter and increases at low pressure and lighter background gas. This study emphasizes that the ambient gas species and the pressure in the build chamber should be treated as process parameters that are as important as controlling laser power and scan speed when trying to prevent defects. • Kinetic modeling reveals strong effect of molecular diffusion in AM vapor microjets. • The laser power and surface temperature stronlgy affect the transition from subsonic to supersonic vapor jet. • Ambient gas chamber species and pressure strongly affect the dynamics (gas, powder) near the laser spot. d. • Strong gas flow (helium) does not necessarily create more spatter as these merge with the melt pool and disappear. • The width of the vapor plume is more strongly dependent on the ambient gas pressure than on the laser beam size.

The role of grain boundaries and denuded zones for tritium retention in high-dose neutron irradiated beryllium

Neutron irradiation of beryllium leads to a lasting material damage. Besides the displacement damage, large amounts of helium and tritium are produced by neutron induced transmutation. In future fusion reactors the accumulation of large amounts of tritium in beryllium must be prevented due to safety concerns and potential waste disposal problems. It is therefore essential to understand where tritium is trapped and how it is released from the sample during irradiation. Using a simple self-developed theoretical model we show that all tritium which is produced and already released during irradiation originates from the area of later bubble denuded zones (DZ) around grain boundaries (GB). In these areas no or only very few bubbles can be observed since grain boundaries are strong sinks for point defects. The formation of precipitation-bubble-pairs in the immediate vicinity of these regions presumably prevents the release of larger amounts at temperatures ≤ 500 °C, as they act as additional tritium traps. The presented model can be used in the future to estimate the tritium retention in irradiated samples using a small number of easily determined experimental parameters, providing important insights for the blanket safety of future fusion reactors.

Experimental and computational study on thermal and fluid behaviours of powder layer during selective laser melting additive manufacturing

The movability of powder layer such as spattering and denudation is a major cause of defect formation and quality uncertainty in selective laser melting (SLM) additive manufacturing. Currently, the detailed thermal and fluid behaviours of the powder layer during laser scan process are still not fully understood. In this work, we employ a combination of high-speed imaging and multi-physics modelling to elucidate the thermal and fluid behaviours of the powder layer during the laser scanning process. The particle spattering speed, particle trajectory, and denudation zone of the powder layer are quantified. The sensitivity of particles’ thermo-fluid behaviours to the laser power intensity is also investigated. The multi-physics simulation is used to reproduce the scanning process of the powder layer with various laser power intensities. Detailed physics that result in the observed powder layer motion, such as hot metal vapour flow, ambient gas flow, drag force and heat flux on particles are investigated. This provides deeper insights into the thermal fluid dynamics of the powder layers. Potential ways to reduce the spattering and denudation of the powder layer during SLM process are discussed.

Effect of Germanium Codoping on the Grown‐In Oxide Precipitates in Nitrogen‐Doped Czochralski Silicon

The effects of germanium (Ge)‐codoping at 1018, 1019, or 1020 cm−3 level on the grown‐in oxide precipitates in 1014 cm−3 level nitrogen (N)‐doped Czochralski (NCZ) silicon have been investigated. It is found that Ge‐codoping enhances the formation of relatively larger grown‐in oxide precipitates. However, such enhancement effect weakens with the increasing Ge‐codoping concentration. On this basis, the effect of 1018 cm−3 level Ge‐codoping on the near‐surface denuded zone (DZ) and bulk microdefects (BMDs) associated with oxygen precipitation in NCZ silicon subjected to an anneal at 1100 °C for 4 h (1100 °C/4 h) has been paid close attention. Moreover, the sustainability of the DZ in the 1100 °C/4 h ‐annealed NCZ silicon subjected to a “copper decoration” or a rigorous oxygen precipitation anneal featuring a prolonged nucleation anneal has been addressed. Thus, it is technologically significant to learn that for the 1100 °C/4 h ‐annealed NCZ silicon, the 1018 cm−3 level Ge‐codoping can remarkably increase the BMD density to better satisfy the requirement of internal gettering (IG) and also ensure the existence of a well‐defined DZ. Finally, the mechanism underlying the effect of Ge‐codoping on the formation of grown‐in oxide precipitates in NCZ silicon has been tentatively elucidated.

Development of actual powder layer height depending on nominal layer thicknesses and selection of laser parameters

For an improved understanding and modelling of the melting process and dynamics in laser powder bed fusion the knowledge of the actual powder layer height and its development is essential. According to theoretical calculations the powder layer thickness approaches the quotient of the nominal layer height and the powder layer density after several layers. However, a previous study shows that in this stable state the actual powder layer thickness is much higher than theoretically expected due to denudation and spattering. Based on the approach of past investigations this study examines experimentally the development of the effective powder layer height by varying the nominal layer thickness and the laser parameters. Furthermore, the influence of these process parameters on the formation of the denudation zone was examined. The results show the significant impact of the variation of the laser parameters on the actual powder layer thickness likely caused by denudation and spatter effects. Moreover, for low nominal layer heights the effective powder layer thickness is relatively higher. This development seems to be affected by the selection of the laser parameters. The outcomes also provide a possible explanation for the unexpected high increase in powder consumption, which amounts to at least 98 % compared to theoretical calculations within the experiments.

An analytical model for prediction of denudation zone width in laser powder bed fusion additive manufacturing

Accurately modeling the physics of laser-powder interaction is needed for predicting the probabilities of performance-limiting defects in parts additively manufactured using laser powder bed fusion. In this paper, we present an analytical model for rapid prediction of denudation width in laser powder bed fusion additive manufacturing process. The model is based on the force balance analysis for a powder particle sitting on top of the powder bed. The model relates the laser power and laser scan speed to the vapor jet speed and consequently to the inward entrainment gas velocity. The effects of laser power and laser speed on denudation width are investigated and compared with experimental data. In addition, a criterion for the onset of denudation is presented which accounts for chamber gas, powder particle size and density. The results show that for a given laser power the denudation width increases as the laser speed decreases.

The role of powder morphology in particle movement behavior in laser powder bed fusion with an emphasis on fluid drag

This study investigates the movement behavior of particles of dissimilar morphology in the powder bed in Laser Powder Bed Fusion. Gas atomized (GA) and water atomized (WA) low alloy steel powders were employed to study their motion around the laser scan path. Particle velocities, entrainment distances and denudation zones were measured for both powders using high-speed imaging. The entrainment of GA powder particles in front of the laser beam towards the process area was initiated 1.6 mm from the edge of the melt pool, whereas the distance was 0.6–0.8 mm for the WA powder. The differences in observed behavior were related to the variations in particle shape of the two types of powder. The processing of WA powder resulted in a 16% narrower denudation zone (for a low volumetric energy density) compared to GA powder. However, the denudation width difference decreased with increasing volumetric energy density, most likely due to a steeper pressure gradient in the process area which diminishes the impact of powder shape. X-ray computed microtomography was utilized to estimate the drag force acting on the powder particles of various morphologies. The results showed that the radial drag force exerted on GA powder was 64% greater than when using WA powder. Moreover, if the WA powder particles were of elongated shape the drag force decreased by almost an order of magnitude, demonstrating the importance of the particle's morphology in the process dynamics.

Investigation of reheat cracking susceptibility in CGHAZ of modified T23 steel

T23 steel (2.25Cr–1Mo–1.6W–0.24V–0.05Nb, in wt.%) has been widely used as water walls in ultra-supercritical (USC) power plants. However, high reheat cracking susceptibility of T23 steel hazarded the safety of USC power plants. It is aimed to improve the reheat cracking susceptibility of T23 steel from the perspective of modifying chemical composition. Gleeble-3800 thermal simulator was used to simulate the coarse-grained heat-affected zone (CGHAZ) in T23 and modified T23 steels via thermal simulation of welding, and then evaluate the reheat cracking susceptibility in CGHAZ of T23 and modified T23 steels by the isothermal slow strain rate tensile test. The microstructure was systematically investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the reheat cracking susceptibility of modified T23 steel is extremely improved. Compared with T23 steel, there are fewer M23C6 particles at the grain boundaries in modified T23 steel, and the degree of intergranular weakening is smaller. At the same time, not only MX in the grain interiors but also the solid-solutioned C and W in the matrix decrease in modified T23 steel, leading to the decline of intragranular strengthening. In addition, small grain size is beneficial to the improvement of the reheat cracking susceptibility of modified T23 steel. The decreased number of M23C6 at grain boundaries helps to retard the formation and propagation of reheat cracks because of decreased denuded zones.

Source‐sink System and Sedimentary Model of Progradational Fan Delta Controlled by Restricted Ancient Gully: An Example in the Enping Formation in the Southern Baiyun Sag, Pearl River Mouth Basin, Northern South China Sea

AbstractTectonic activity occurred during the depositional period of the Enping Formation in the southern Baiyun Sag in the Pearl River Mouth Basin, with a series of synsedimentary faults and a set of large fan delta reservoir bodies developing. There is no data from drilling, cores, etc. for this area, so this paper applies three‐dimensional seismic data to study the structural style of the steep slope zone, the seismic facies characteristics of fan deltas, and the source‐sink system and sedimentary model of the Enping Formation. The control action of tectonic activity on fan deltaic sedimentary systems is studied by combining interpretation of fault systems, dissection of structural styles, seismic reflection structure, seismic facies geometry, and seismic attribute analysis, together with theoretical analysis of the source‐sink deposition process. The Baiyun Sag has experienced tectonic activity since the Eocene, and a series of synsedimentary faults are developed in the southern steep slope zone. Under the common control of multiple large synsedimentary faults, a large ancient gully formed in the steep slope zone in the south, which gradually widened from south to north. The uplift area in the southern part of the sag was exposed for a long time during the deposition of the Enping Formation and consequently suffered weathering and erosion. The resulting sediments were transported through a system of provenance channels composed of slopes and an ancient gully to the depression area, where they were deposited and eventually converged to form a large fan delta. The fan delta presents the overall characteristics of NS strong wedge reflection and EW strong domal reflection—thick in the middle part and thin in the wings. It displays a lobe‐shaped distribution on the plane, with the fan root pointing to the south slope. According to differences in reflection intensity from bottom to top, it can be subdivided into three stages of progradational sedimentary bodies—the southern uplift and denudation zone, the large ancient gully, and the fan delta—which together constitute a complete source‐sink system. This represents a sedimentary model of progradational fan delta under the overall joint control of the restricted ancient gully and syndepositional faults.

Impacts of aging treatments on the microstructural evolutions, mechanical properties and strengthening mechanism in 1Cr15Ni36W3Ti

The microstructural evolution, mechanical properties, fracture behaviors and age hardening mechanism in 1Cr15Ni36W3Ti subjected to various thermal treatments involving a preceding solution at 1080 °C and three distinct aging schedules were investigated. Upon aging treated at 870 °C and then 700 °C, discrete globular MC carbides combined with dispersive and fine γ′ particles were formed at grain boundaries and in the γ matrix, respectively, which is beneficial in resisting the propagation of grain-boundary microcracks and dramatically enhancing yield and ultimate strengths. Rather, the second aging strategy conducted at 730 °C after solution resulted in relatively coarse γ′ precipitates and nearly continuous intergranular MC carbides, while the third aging schedule carried out firstly at 780 °C and then 730 °C led to a dual size of γ′ phase and also a nearly continuous distribution of the grain-boundary MC carbides. Moreover, γ′ denuded zones on both sides of grain boundaries were formed in the latter two aging treatments, which were severer in the third age. The relatively coarse γ′ phase and the appearance of precipitate-free zones dramatically lowered the strength, whereas the nearly continuous intergranular carbides with γ′ denuded zones alongside accelerated the propagation of intergranular micro-cracks. Dislocation shearing was dominated. The γ′ coherency strengthening, the introduction of complex stacking faults in the γ′ phase arising from Shockley partial dislocations shearing and the elimination of γ’ denuded zones were responsible for the highest tensile strength when undergoing the first-type age.

Investigation of a high-dose irradiated beryllium microstructure

The present paper studies the evolving Be microstructure after high-dose neutron irradiation at 660 K, 753 K and 873 K up to 34 dpa and generation of 5524 appm He and 596 appm 3H. At all three temperatures gas bubbles were observed whereby their average size is increasing with the temperature. In addition denuded zones (DZ) formed near grain boundaries. Their width increased with irradiation temperature and was found to be narrower for grains with a random orientation compared to those with a close to basal orientation with respect to the grain boundary. Furthermore, perceptions containing elements such as Al, Cr, Fe, Mg, Si as well as U were found inside single grains, often attached to dislocation lines, and in the vicinity of grain boundaries, mainly at 660 K and 753 K.

Spattering and denudation in laser powder bed fusion process: Multiphase flow modelling

To understand the physical mechanisms of the spattering and denudation phenomena in the laser powder bed fusion process, we develop a multiphase flow model by bi-directionally coupling the discrete element method and finite volume method, where the exchanges of both momentum and energy between the powder particles and gases are incorporated. It is the first time that the dynamic behaviours of both the gas phase and powder particles during the spattering and denudation phenomena are reproduced in computational modelling, which agree well with the published experimental observation. The metal vapour spouts out and decelerates sharply along the jetting direction while expanding radially, which induces remarkable vortex flows of the ambient gas. With comparative simulation cases, the vapour jetting and the consequent vortex flow are demonstrated to be dominant in the spattering and denudation phenomena, and the thermal buoyancy effect is proved to be negligible. Moreover, the influence of the jetting angle is investigated: no remarkable effect within the range of 60° - 120°; while larger than 150°, the accumulation zone is eliminated leading to a completely exposed denudation zone.

New insights into microstructure of irradiated beryllium based on experiments and computer simulations

The microstructural response of beryllium after neutron irradiation at various temperatures (643–923 K) was systematically studied using analytical transmission electron microscope that together with outcomes from advanced atomistic modelling provides new insights in the mechanisms of microstructural changes in this material. The most prominent feature of microstructural modification is the formation of gas bubbles, which is revealed at all studied irradiation temperatures. Except for the lowest irradiation temperature, gas bubbles have the shape of thin hexagonal prisms with average height and diameter increasing with temperature. A high number density of small bubbles is observed within grains, while significantly larger bubbles are formed along high-angle grain boundaries (GB). Denuded zones (DZ) nearly free from bubbles are found along both high- and low-angle grain boundaries. Precipitations of secondary phases (mainly intermetallic Al-Fe-Be) were observed inside grains, along dislocation lines and at GBs. EDX analysis has revealed homogeneous segregation of chromium and iron along GBs. The observed features are discussed with respect to the available atomistic modelling results. In particular, we present a plausible reasoning for the abundant formation of gas bubbles on intermetallic precipitates, observation of various thickness of zones denuded in gas bubbles and precipitates, and their relation to the atomic scale diffusion mechanisms of solute-vacancy clusters.

Carbon effect on the survival of vacancies in Czochralski silicon during rapid thermal anneal

Rapid thermal anneal (RTA) at high temperatures can be employed to introduce vacancies to control oxygen precipitation (OP) behavior in Czochralski (CZ) silicon. Such excessive vacancies survive from the recombination of silicon-interstitials and vacancies (V-I recombination) during the RTA. In this work, we aim to elucidate the carbon effect on the survival of vacancies in CZ silicon during the high temperature RTA by means of gold diffusion in combination with deep-level transient spectroscopy. It is revealed that the existence of ∼1017 cm−3 carbon atoms significantly increases the amount of survival vacancies in the form of vacancy-oxygen (VOm, m ≥ 2) complexes in CZ silicon when subjected to the 1250 °C/60 s RTA. Moreover, such an increase in the number of vacancies becomes more significant with the increase in the cooling rate of RTA. The density functional theory calculations suggest that the V-I recombination is to some extent unfavorable as a carbon atom is close to the silicon-interstitial. Alternatively, it is believed that the substitutional carbon (Cs) atoms tend to trap the silicon-interstitials, thus forming Ci complexes (Cs + I → Ci) in CZ silicon during the RTA. In this context, the V-I recombination is suppressed in a manner, leading to the survival of more vacancies, thus generating more VOm complexes. Furthermore, after the 1250 °C/60 s RTA, the oxide precipitate nucleation based on the VOm complexes is more significant in carbon-rich CZ (CCZ) silicon than in the conventional CZ counterpart. Hence, when subjected to the same OP anneal consisting of the nucleation anneal at 650 or 800 °C for 4 h and the subsequent growth anneal at 1000 °C for 16 h, CCZ silicon possesses a higher density of bulk microdefects and therefore stronger internal gettering capability than CZ silicon. However, the nucleation temperature for OP should be carefully selected as 650 °C for CCZ silicon in order to form an oxide precipitate-free denuded zone.

Opportunities and limitations for ion beams in radiation effects studies: Bridging critical gaps between charged particle and neutron irradiations

Ion beams are indispensible for understanding radiation effects in materials, enabling access to conditions challenging to perform with neutrons. However, ion irradiations can produce potential artifacts from beam rastering, high displacement damage rate effective “temperature shifts”, subthreshold collisions, high ionization rates, and non-monoenergetic primary knock-on atom energies. The influence of near-surface denuded zones and implanted ion effects is analyzed, including diffusional broadening effects. At high ion irradiation energies, “swift heavy ion” effects can lead to enhanced defect production or recovery. Ion energies of 10–50MeV typically offer a good compromise for minimizing near-surface, implanted ion, and swift heavy ion effects.

Reheating cracking susceptibility in the weld heat-affected zone of a reduced activation ferritic-martensitic steel for fusion reactors

Reheating cracking susceptibility in the weld heat-affected zone of reduced activation ferritic-martensitic steel was explored by stress-rupture tests. The HAZ samples were experimentally simulated using Gleeble simulator with heat input of 30kJ/cm. After HAZ simulation, the samples were reheated to three different temperatures which correspond to post-weld heat treatment temperature, and stress-rupture tests were carried out. After the samples ruptured, fracture morphologies and cross-sectional microstructures were carefully observed by scanning electron microscopy and transmission electron microscopy. The results revealed that reheating cracking occurred in the vicinity of intergranular Cr23C6 carbides, due to the formation of soft denuded zone along the grain boundary. In addition, the reheating cracking susceptibility increased with increase in PWHT temperature.