Thin Powder Layers Research Articles

Melt pool monitoring for laser beam melting of metals: inline-evaluation and remelting of surfaces

Laser Beam Melting of Metals (LBM-M) is an additive manufacturing technique that is applied successfully for the production of complex parts, small batch series and prototypes. In LBM metallic parts are generated layer by layer directly from sliced CAD data. For each layer a thin powder layer is deposited and subsequently irradiated by a focused laser beam which is guided by a galvanometer scanner. Due to variations in the powder stock, plain parameter sets and unresolved machine maturity problems, quality fluctuations are a major problem for a future series production. Surface roughness of parts during manufacturing is a quality issue. In this paper, we address it with a coaxial melt pool monitoring system and show a method for inline evaluation of surfaces by means of melt pool monitoring and the results of applied remelting strategies.

Melt pool monitoring for laser beam melting of metals: assistance for material qualification for the stainless steel 1.4057

Laser Beam Melting of Metals (LBM-M) is an additive manufacturing technique that is successfully applied for the manufacturing of complex parts, small batch series and prototypes. In LBM metallic parts are generated layer by layer directly from sliced CAD data. For each layer a thin powder layer is deposited and subsequently, irradiated by a focused laser beam, which is guided by a galvanometer scanner. A major drawback of this relatively new manufacturing technology is a limited material portfolio. Today, the whole procedure of material qualification is time-consuming and resource intensive because a fast and reliable modelling of the process is not yet possible. In this work we are going to show the results of a qualification procedure applying a coaxial melt pool monitoring system to determine suitable processing parameters for a material, which has not yet been processed. In this work, a stainless steel material was exemplarily used to proof the feasibility. Furthermore, we are discussing the usability of this methodology for the material qualification in LBM-M in general.

FACILE SYNTHESIS OF SEMICONDUCTING NANOSIZED 0D AND 2D LEAD OXIDES USING A MODIFIED CO-PRECIPITATION METHOD

Nano-sized lead has many versatile applications that could be applied in daily life. In the current study, we report a comprehensive study for preparation of nanosized lead oxide using the co-precipitation method and optimization of reaction parameters to obtain lead oxide (PbO) nanoparticles with homogeneously distributed size, shape and structure. When aqueous solution of lead (II) acetate reduced with sodium hydroxide at elevated temperatures, alpha form of lead oxide nanoparticles, with spherical shape were achieved. Decreasing the ratio of sodium hydroxide to lead (II) acetate concentration at moderate temperatures resulted with a gradual change in crystal structure from quasi -spherical α-PbO nanoparticles and two dimensional nanoflakes of beta PbO with a thickness below 100 nm was synthesized for the first time. The obtained particles in both α and b forms were characterized by X-ray powder diffraction (XRD), dynamic light scattering equipment (Zetasizer), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Finally, thin layers of freeze-dryer α-PbO and β-PbO particle powder on glass and filter paper were formed by the help of nail polisher and conductivity measurements were performed using four-point probes method. Produced layers β-PbO particles with a below 100 nm showed higher conductivity on both supports as compared to the ones produced from spherical α-PbO nanoparticles. This altered conductivity of the material in the semiconducting zone, which is probably due to a more effective electron transfer facilitated by 2D alignment of the molecules, could rose the potential use of this material in voltaic and catalysis.

Joining of mild steel pipes using microwave hybrid heating at 2.45 GHz and joint characterization

The present experimental study was carried out to explore the possibility of joining mild steel (MS) pipes using microwave hybrid heating (MHH) technique. Candidate MS pipes, with a thin sandwich layer of Ni powder in between, were exposed to microwaves inside a multimode applicator at 2.45GHz and 900W. The principles of microwave joining and the role of MHH in context to the pipe joining have been discussed. Microwave absorption in the material system and mechanism of heating the candidate layers of the MS pipes are presented. Effect of the interface powder particle size on heating of the faying surfaces has been analyzed. Experimental results imply a dense joint zone in scanning electron micrographs due to good metallurgical bonding between the MS pipes and Ni powder. The elemental study reveals that joint zone is nickel and iron rich. Average micro indentation hardness of the joint zone, interface and base metal were observed to be 572 HV, 519 HV and 397 HV, respectively. It was found that joint efficiency mainly depends upon the microwave exposure time. The joints developed through microwave exposure upto 480s were observed more efficient (65%) than the joints obtained at 420s (40%) and 510s (57%).

Influence of Anvil Properties on RDX Thin Layer Ignition Behavior

AbstractWe report on a drop‐impact protocol that arrests sample radial flow to isolate how anvil properties influence ignition in a thin layer of RDX powder. To eliminate sliding friction as a probable heating mechanism, flow arrestment was provided by a waxed weighing paper that shielded the RDX layer from direct contact with the impact surfaces. RDX reaction sensitivity under bare and shielded conditions for the standard O1 hardened steel anvil was compared with that for two deformable anvil types: 1018 steel and C110 copper. Profilometer measurements of anvil deformation and paper impressions quantified anvil plastic work and final radial flow displacement. Post‐test particle analyses correlated particle size distribution to ignition results. Experiments indicated that the impact energy absorbed by the anvils was varied and inhibited ignition accordingly. For the standard anvil, ignition was not inhibited under flow arrestment, suggesting that significant radial sliding or flow is not essential for thin layer ignition.

Effects of ultraviolet aging on the surface properties and tribological performance of polyformaldehyde

ABSTRACTUltraviolet aging restricts the outdoor application of polyformaldehyde as the friction material. In this work, ultraviolet aging processes were applied to polyformaldehyde specimens and effects of the processes on their surface properties and tribological performance were evaluated. Surface morphology results show that a thin layer of white powder and micro‐cracks with further ramifications in other directions were observed on the surfaces after 400 h of ultraviolet exposure, while it is not detectable for the unaged specimens. Analysis of aging surface indicates that ultraviolet leads to the increase of micro‐cracks and the degree of crystallinity. All the tribological test results demonstrate that with the extension of ultraviolet aging time, (i) the process of damage on polyformaldehyde surface induced by ultraviolet aging is first slowly and faster afterward; (ii) the fluctuation of the friction coefficients of polyformaldehyde/GCr15 rubbing pairs increases; and (iii) the wear rate of polyformaldehyde markedly increase after aging test 400 h. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44684.

Kinetics of powder layer shrinkage during electron-beam treatment

When layerwise synthesis of new material occurs, only thin surface area, thin melt film or thin powder layer undergo essential changes. To describe the behaviour of thin layer where temperature and composition change, a model similar to shallow-water theory is suggested. The equations for thin layer behaviour are deduced for the Maxwell model of viscoelastic body in the approximation of incompressibility. The hydrodynamic part of the problem is coupled with thermal-kinetical one.

ALD Coating of Selective Laser Melted 3-D Printed Stainless Steels

Selective laser melting (SLM), also known as 3-D printing or additive manufacturing process, has been developed for over 2 decades. This technique exhibits the capability to fabricate three-dimensional metal objects of complex geometries & structures from metal powders used in aerospace or medical orthopedics. This additive manufacturing technology has received a lot of attention, because various metals and metal alloys can now be processed to manufacture metal parts or structures, which includes the technologically important metals of stainless steel, cobalt chrome, titanium, tungsten and various other alloys. Typically SLM selectively melts thin layers of fine metal powder above each other by using a focused laser beam with high thermal energy on a substrate plate under argon atmosphere, resulting in production of almost full density metal or alloy parts. In this fashion successive layers of material are formed according to the computer edited 3-D-CAD design. Thereby, SLM could be applied to make functional end-use and geometrically complex products with cost-effect process especially in industry. Due to the geometrically complex form of the SLM manufactured structures, atomic layer deposition (ALD) is uniquely suitable to control the composition, thickness and conformability of thin films in the large scale and complex structures. In this study we report ALD coatings of Al2O3 of SLM stainless steel with the objective of providing corrosion protection by an alumina diffusion barrier and ALD ZrO2 coatings. During the course of this work we found the parts fabricated by 3-D printers tend to be porous and exhibit a rough texture. Figures 1 (a) show FE-SEM micrographs at progressively larger magnification of the surface of the 3-D printed stainless steel samples. As can be seen, the surface is very rough and contain many beads or spheres produced during the SLM manufacturing process, which is a direct result of the laser melting of the metal powder and rapid quenching. In order to reduce the surface roughness of the 3-D printed samples, electropolishing was used to smoothen and streamline the rough surface of the 3-D samples. After electropolishing, the surface of the 3-D printed samples is bright, clean and microscopically smooth and featureless. There are no beads or spheres remaining on the samples as shown in Figures 1 (b). We used SLM 316 stainless steel samples in a cross-flow ALD reactor with TMA and H2O vapor to attempt ALD coatings, while TDMA-Zr and H2O were used as precursors for ALD ZrO2. The analysis of the ALD Al2O3 and ZrO2 films revealed satisfactory chemisorption on the 3-D printed stainless steel surface. The ALD Al2O3 and ZrO2 films were uniformly grown on the 3-D printed stainless steel surface as shown in Figure 2. Figure 1

Tuning Size and Reducibility of Copper Oxide Particles Supported on SBA‐15

AbstractCopper citrate was deposited on SBA‐15 at various loadings ranging from 10.9 to 27.1 wt.%. Thermal decomposition resulted in copper oxide particles supported on SBA‐15. In order to study the effect of varying calcination conditions on the resulting particles, precursors were calcined both in a thin powder layer and in a thick powder layer. SBA‐15 and copper oxide particles were characterized using N2 physisorption, XRD, DR‐UV/Vis spectroscopy, and TEM. Depending on layer thickness during calcination, copper oxide particles of various sizes were observed in samples with the same chemical composition. Copper oxide particles were examined by temperature‐programmed reduction. Amorphous copper oxide particles showed a single H2 consumption peak and were reduced to copper at 240 °C. Larger crystalline copper oxide particles showed H2 consumption peaks with a shoulder at higher temperatures and were reduced to copper at 300 °C.

Tailoring the texture of IN738LC processed by selective laser melting (SLM) by specific scanning strategies

Selective laser melting (SLM) is an emerging technology of additive manufacturing, which is used to directly produce metallic parts from thin powder layers. This study aims at correlating laser scanning strategies with the resulting textures and corresponding anisotropy of the elastic behavior of bulk materials. Tensile test specimens made of the γ’-containing Ni-base superalloy IN738LC were built with the loading direction oriented either parallel (z-specimens) or perpendicular to the build-up direction (xy-specimens). Their bulk mechanical properties were determined at room temperature and at 850°C. Specimens were investigated in the ‘as-built’ condition and after recrystallization heat treatment. SEM-based electron backscatter diffraction (EBSD) was applied to measure their crystallographic preferred orientations (texture) and to correlate the anisotropy of Young's modulus with the texture of the material. It is shown that the applied laser scanning strategies allow to tailor the crystallographic texture locally. The possibility to switch from transverse anisotropic to transverse isotropic properties and reverse is demonstrated for triple layered tensile samples. A recrystallization heat treatment reduces the degree of crystallographic texture and thus the elastic anisotropy by abundant annealing twinning. Predictions of Young's modulus calculated from the measured textures compare well with the data from tensile tests.

Investigation of Deviations Caused by Powder Compaction During 3D Printing

Powder bed based 3D printing technologies are of great interest for the production of prototypes and low quantity production lots because they allow the realization of highly customized and complex geometries. Moreover, significant benefits of 3D printing include an expedited manufacturing and a decreased number of necessary tools in the process chain.In contrast to traditional production methods, where material is subtracted or molded, thin layers of powder are successively added and solidified by locally applying binder based on a digital model. By continuously depositing material, the powder bed experiences an additional compaction through the increasing weight of the material on top. The consequences are dimensional deviations of the component in build-up direction and occasionally even avalanche-like collapses of material.The objective of this study is the investigation of these effects and their influencing factors. For this reason, occurring deviations and their underlying causes are examined. Test objects were manufactured and examined with the objective of identifying the main influencing factors by varying the length, position and surrounding conditions of the objects within the building chamber. Significant parameters were analyzed in greater detail in order to generate rules, regarding their effect on the occurrence and magnitude of the identified deviations. Finally, an approach for the compensation of these deviations prior to the production by a pre-deformation of the digital model was implemented.

Fatigue crack growth behavior and mechanical properties of additively processed EN AW-7075 aluminium alloy

Selective Laser Melting (SLM®), an additive manufacturing (AM) technology, allows manufacturing of geometrically complex metallic parts directly. In the SLM technology, a high energy laser beam is applied to melt a thin layer of the metallic powder according to the information provided by CAD files. This layer-wise manufacturing offers the opportunity to create complex parts for application areas e.g., aerospace and automotive industries where the lightweight design has been and still is a priority for material development in recent years. Therefore, the materials such as aluminium alloys come into focus due to their low density and high mechanical characteristics. In view of these aspects, previously unused high strength aluminium alloy EN AW-7075 powder was produced by gas atomization and processed by SLM® as presented in this paper. Initially, specimens were produced to examine monotonic and fracture mechanical properties in different building directions. The tensile tests and the fracture examinations show an anisotropic material behaviour. The fatigue crack growth curves have the double S shape, which is typical of aluminium. Mechanical characteristics obtained from the experiments are lower in comparison to the conventionally manufactured aluminium alloy properties.

A multiscale method for including fine particle effects in DEM models of grinding mills

A multiscale model for including interstitial powder or fine particles in DEM simulations of grinding mills is proposed. This consists of a traditional DEM model at the macroscale which includes only grinding media and potential coarser fractions of feed and product. Microscale models are embedded within this macroscale model. These can be sufficiently small that the fine powder can be included in a computationally affordable manner. The direct inclusion of the fine particles in the model allows predictions to be made of the effect of the local grinding environment on these fine particles. A shear cell is a good choice for the microscale model as it can well represent the local flow conditions at different points within the mill macroscale model. Averaging the macroscale flow allows the local collisional environments to be characterised and provides estimates of the shear rate and normal stress at each of the microscale locations which then controls the configuration of each microscale shear cell. A 1-way coupled implementation of this multiscale model is demonstrated for a simple cement ball mill. The relative importance of each region of the flow is determined with the toe region being the dominant contributor to the grinding. The grinding action produced by the shearing of thin layers of powder between adjacent layers of media flowing over each other is clearly demonstrated by the behaviour predicted in the microscale models. Methods for calculating power draw that include the effect of powder and for constructing collision energy spectra for the powder are described. Finally, the importance of the cushioning effect of high powder loads on the flow behaviour of the media is demonstrated.

Investigation of the laser–powder–atmosphere interaction zone during the selective laser melting process

Selective laser melting (SLM) consists in manufacturing parts in a chamber filled with a protective gas, by melting successive thin powder layers using a laser. This study aims, at first, to understand the laser–material–atmosphere interactions that involve various complex and simultaneous physical phenomena such as heat transfer in the target, melting and vaporization of the material, and expansion of the generated vapor. A modeling approach of the process was developed to investigate the effect of some process parameters, such as the exposure time (texpo) of the laser beam on each point along its trajectory, the pressure of the protective gas (argon) and the spatial thermal energy distribution on the dimensional characteristics of the melted track.

Reduction in mechanical anisotropy through high temperature heat treatment of Hastelloy X processed by Selective Laser Melting (SLM)

Selective Laser Melting (SLM) is an additive manufacturing technology used to directly produce metallic parts from thin powder layers. To evaluate the anisotropic mechanical properties, tensile test specimens of the Ni-base alloy Hastelloy X were built with the loading direction oriented either parallel (z-specimens) or perpendicular to the build-up direction (xy- specimens). Specimens were investigated in the “as-built” condition and after high temperature heat treatment. Tensile tests at room temperature and at 850°C of “as-built” material have shown different mechanical properties for z- and xy-specimens. The anisotropy is reflected in the Young's modulus, with lower values measured parallel to the build-up direction. It is shown that the anisotropy is significantly reduced by a subsequent recrystallization heat treatment. The characterization of microstructural and textural anisotropy was done by Electron Back Scatter Diffraction (EBSD) analysis. Predictions of Young's modulus calculated from the measured textures compare well with the data from tensile tests.

치과 SLM용 Ni-Cr 금속분말 특성 관찰

Purpose: The selective laser melting (SLM) process for dentistry, which is one of the additive manufacturing technologies (AM) allows for rapid production of a three-dimensional model with complex shape by directly melting metal powder. This process generates detailed items of a three-dimensional model shape through consolidation of a thin powder layer by utilizing both selective melting and laser beam simultaneously. In regard to SLM process, Fe-base powder, Ti-6AI-4V powder, AI-base powder, etc. have been researched. It is believed that the aforementioned technologies will be widely utilized in manufacturing metal parts using metal powder of raw material. This study chose Ni-Cr-Mo metal powder in order to manufacture metal powder materials that would be used in the selective laser melting for dentistry. Methods: This study manufactured metal powder using mechanical alloying technique (MA) among those metal powder manufacturing techniques. Moreover, this study aimed to utilize the metal powder manufactured after observing the characteristics of powder as preliminary data of Ni-Cr-Mo metal powder. This study could obtain the following conclusions within the experimental limitations. Results: As a result of mechanically alloying Ni-Cr-Mo powder over time, its mean particle size was <TEX>$66.93{\mu}m$</TEX> <TEX>$54.4{\mu}m$</TEX> and <TEX>$45.39{\mu}m$</TEX> at 10h, 20h and 30h, respectively. The gtain form of metal powder by mechanical alloying technique was a sponge-like shape of irregular plate; however, the gtain form manufactured by high-pressure water aromization process had the following three types: globular type, chain type and oval type. Conclusion: This study found <TEX>$37.65{\mu}m$</TEX> as the mean particle size of Ni-Cr-Mo metal powder, which was manufactured using water atomization technique under the following conditions: water atomization flux of 300 liter/min, hydraulic pressure of <TEX>$400kgf/cm^2$</TEX> and injection angle of <TEX>$45^{\circ}$</TEX>. This study confirmed that the grain form of powder (solid particle form) would vary depending on the manufacturing process.

3D printing of ceramic implants

Abstract

Fiber reinforcement during 3D printing

Three-dimensional (3D) printing is an attractive rapid prototyping technology for the fabrication of 3D structures by the localized deposition of a reactive binder liquid onto thin powder layers in predominantly technical applications. A practical limitation is often the low green strength of printed samples, which can lead to a collapse of large and fragile structures during removal from the powder bed and the following depowdering procedure. Fibre reinforcement may improve green mechanical properties of printed samples, which was investigated in this study using a range of different short fibres added to a matrix of cellulose-modified gypsum powder. Mechanical testing of printed samples revealed a bending strength increase of 180% and up to 10 times higher work of fracture values compared to non-reinforced printed samples.

Effects of the Selective Laser Melting manufacturing process on the properties of CoCrMo single tracks

Selective Laser Melting (SLM) is an additive technology that produces solid parts by selectively melting thin layers of metallic powder. SLM can produce significant differences in the final properties due to the melting-consolidation phenomena of the process, which can be controlled by the appropriate parameters. Therefore, the objective of this study was to create a link between the process conditions and the resulting properties by experimenting in an own-developed SLM machine using CoCrMo powder as material. The fabricated samples were characterized by density, hardness and microstructural properties. The experimental results proved the capability of the SLM technique to build high dense samples. The hardness results gave evidence of a superior outcome compared to conventional processes. Finally, it was found that grain size was defined by scanning speed. Based on the results, a better understanding of the processing principles given by the parameters was achieved and improved fabrication quality was promoted.

New Strategies for Powder Compaction in Powder-based Rapid Prototyping Techniques

In powder-based rapid prototyping techniques, powder compaction is used to create thin layers of fine powder that are locally bonded. By stacking these layers of locally bonded material, an object is made. The compaction of thin layers of powder mater ials is of interest for a wide range of applications, but this study solely focuses on the application for powder -based three-dimensional printing (e.g. SLS, 3DP). This research is primarily interested in powder compaction for creating membranes with specific properties.In this paper, methods of powder deposition are discussed and experiments carried out using a specimen powder bed apparatus and a custom powder compaction device, using these methods (doctor blade, forward and backward rotating roller, double action roller) with various parameters. The model of powder compaction is verified in experiments. Insight is gathered to gain a better prediction of powder compaction.