Additive Fertigung
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Paper des Monats
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Additive manufacturing (AM) offers an effective solution to the medical sector. It enables the production, on demand, of customised implants which match the patient’s anatomy, with grafts that promote bone growth, as well as surgical guides that help the surgeons. The objective of this project is to provide a comprehensive basis to enable the safe use of medical AM products with traceable and reliable dimensionalmeasurements. This will guarantee the reliability of medical AM products to notified bodies and facilitate acceptance of AM in the medical sector for a better quality of life.
Synchrotron X-ray diffraction is a powerful non-destructive technique for the analysis of the material stress-state. High cooling rates and heterogeneous temperature distributions during additive manufacturing lead to high residual stresses. These high residual stresses play a crucial role in the ability to achieve complex geometries with accuracy since they can promote distortion of parts during manufacturing. Furthermore, residual stresses are critical for the mechanical performance of parts in terms of durability and safety.
In the present study, Ti-6Al-4V bridge-like specimens were manufactured additively by selective laser melting (SLM) under different laser scanning speed conditions in order to compare the effect of process energy density on the residual stress state. Subsurface residual stress analysis was conducted by means of synchrotron X-ray diffraction in energy dispersive mode for three conditions: as-built on base plate, released from base plate, and after heat treatment on the base plate. The quantitative residual stress characterization shows a correlation with the qualitative bridge curvature method. Computed tomography (CT) was carried out to ensure that no stress relief took place owing to the presence of porosity. CT allows obtaining spatial and size pores distribution which helps in optimization of the SLM process.
High tensile residual stresses were found at the lateral surface for samples in the as-built conditions. We observed that higher laser energy density during fabrication leads to lower residual stresses. Samples in released condition showed redistribution of the stresses due to distortion.
Influence of deposition hatch length on residual stress in selective laser melted Inconel 718
(2018)
The present study aims to evaluate the bulk residual stresses in SLM parts by using neutron diffraction measurements performed at E3 line -BER II neutron reactor- of Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. Together with microstructure characterization and distortion measurements, it is possible to describe the stress state throughout the whole sample. The sample was measured in as-build condition (on a build plate) and after releasing from the build plate. The used material is the nickel based superalloy 718. This alloy is widely used in aerospace and chemical industries due to its superior corrosion and heat resistant properties.
Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component. The normal and transversal component exhibits a rather compressive behavior while the longitudinal was tensile in the center part of the sample and became compressive towards the tip. As expected, the absolute values of all stress components decreased after releasing the sample from the building plate. A surface scan utilizing a coordinate-measuring machine (CMM) allowed us to present top surface distortion before and after releasing. The top surface showed a distortion around ±80µm after releasing. Microstructure evolution in the scanning-building cross-section is largely dominated by columnar grains. In addition, many small random orientated grains are prominent in the regions of a laser overlap during SLM.
In summary, for the sample of superalloy 718 manufactured by SLM, a small distortion occurred when removing the sample from the build plate whereby the residual stress state decreases. Moreover, the observed columnar grains in the building direction could give a reason for the lowest stress values in that normal direction. However, the most important parameter controlling the residual stresses is the temperature gradient. Hence, future investigations are planned for a different scan strategy to distribute the laser impact in a more homogenous manner.
Laser Metal Deposition (LMD) as a technology for additive manufacturing allows the production of large components outside of closed working chambers. Industrial applications require a stable process as well as a constant deposition of the filler material in order to ensure uniform volume growth and reproducible mechanical properties. This paper deals with the influence of travel path strategies on temperature profile and material deposition. Meandering and spiral hatching strategies are used in the center as well as in the edge of a specimen. The temperature is measured with thermocouples attatched to the backside of the specimen. The tests are carried out on the materials S235JR and 316L. The results show a strong dependence of the maximum temperatures on the travel path strategy and the welding position on the component.
Sintered bioactive glass scaffolds of defined shape and porosity, e.g. made via additive manufacturing, must provide sufficient bioactivity and sinterability. As higher bioactivity is often linked to high corrosion and crystallization tendency, a certain compromise between sintering ability and bioactivity is therefore required. Groh et al. developed a fluoride-containing bioactive glass (F3), which allows fiber drawing and shows a bioactivity well comparable to that of Bioglass®45S5.
To study whether and to what extent the sinterability of F3 glass powder is controlled by particle size, coarse and fine F3 glass powders (300-310µm and 0-32µm) were prepared by crushing, sieving and milling. Sintering, degassing and phase transformation during heating were studied with heating microscopy, vacuum hot extraction (VHE), DTA, XRD, and SEM.
For the coarse glass powder, sintering proceeds slowly and is limited by surface crystallization of primary Na2CaSi2O6 crystals. Although the crystallization onset of Na2CaSi2O6 is shifted to lower temperature, full densification is attained for the fine powder. This finding indicate that certain porosity might be tuned via particle size variation. Above 900°C, intensive foaming is evident for the fine powder. VHE studies revealed that carbon species are the main foaming source.
Microstructure characterisation of advanced materials via 2D and 3D X-ray refraction techniques
(2018)
3D imaging techniques have an enormous potential to understand the microstructure, its evolution, and its link to mechanical, thermal, and transport properties. In this conference paper we report the use of a powerful, yet not so wide-spread, set of X-ray techniques based on refraction effects. X-ray refraction allows determining internal specific surface (surface per unit volume) in a non-destructive fashion, position and orientation sensitive, and with a nanometric detectability. We demonstrate showcases of ceramics and composite materials, where microstructural parameters could be achieved in a way unrivalled even by high-resolution techniques such as electron microscopy or computed tomography. We present in situ analysis of the damage evolution in an Al/Al2O3 metal matrix composite during tensile load and the identification of void formation (different kinds of defects, particularly unsintered powder hidden in pores, and small inhomogeneity’s like cracks) in Ti64 parts produced by selective laser melting using synchrotron X-ray refraction radiography and tomography.
Laser metal deposition (LMD) is an established technology for two-dimensional surface coatings. It offers high deposition rates, high material flexibility, and the possibility to deposit material on existing components. Due to these features, LMD has been increasingly applied for additive manufacturing of 3D structures in recent years. Compared to previous coating applications, additive manufacturing of 3D structures leads to new challenges regarding LMD process knowledge. In this paper, the process steps for LMD as additive manufacturing technology are described. The experiments are conducted using titanium alloy Ti-6Al-4Vand Inconel 718. Only the LMD nozzle is used to create a shielding gas atmosphere. This ensures the high geometric flexibility needed for additive manufacturing, although issues with the restricted size and quality of the shielding gas atmosphere arise. In the first step, the influence of process parameters on the geometric dimensions of single weld beads is analyzed based on design of experiments. In the second step, a 3D build-up strategy for cylindrical specimen with high dimensional accuracy is described. Process parameters, travel paths, and cooling periods between layers are adjusted. Tensile tests show that mechanical properties in the as-deposited condition are close to wrought material. As practical example, the fir-tree root profile of a turbine blade is manufactured. The feasibility of LMD as additive technology is evaluated based on this component.
While additive manufacturing (AM) is blossoming in nearly every industrial field, and the most different process are being used to produce components and materials, little attention is paid on the safety concerns around AM materials and processes.
Leveraging on our leading expertise in non-destructive testing (NDT) and materials characterization, we approach AM at BAM under two important viewpoints: first the on-line monitoring of the process and of the product, second the evolution of the (unstable) microstructure of AM materials under external loads.
These two subjects are the core of the two new-born internal projects ProMoAM and AGIL, respectively.
A detailed view of the goals and the organization of these two projects will be given, together with the expected output, and some preliminary results.
The influence of the support structure on residual stress and distortion in SLM Inconel 718 parts
(2018)
The effect of support structure and of removal from the base plate on the residual stress state in selective laser melted IN718 parts was studied by means of synchrotron X-ray diffraction. The residual stresses in subsurface region of two elongated prisms in as-built condition and after removal from the base plate were determined. One sample was directly built on a base plate and another one on a support structure. Also, the distortion on the top surface due to stress release was measured by contact profilometry. High tensile residual stress values were found, with pronounced stress gradient along the hatching direction. In the sample on support, stress redistribution took place after removal from the base plate, as opposed to simple stress relaxation for the sample without support. The sample on support structure showed larger distortion compared to sample without support. We conclude that the use of a support decreases stress values but stress-relieving heat treatments are still needed.
Wollastonite (CaSiO 3 ) – diopside (CaMgSi 2 O 6 ) glass-ceramic scaffolds have been successfully fabricated using two different additive manufacturing techniques: powder-based 3D printing (3DP) and digital light processing (DLP), coupled with the sinter-crystallization of glass powders with two different compositions. The adopted manufacturing process depended on the balance between viscous flow sintering and crystallization of the glass particles, in turn in fluenced by the powder size and the sensitivity of CaO – MgO – SiO 2 glasses to surface nucleation. 3DP used coarser glass powders and was more appropriate for low temperature firing (800 – 900 °C), leading to samples with limited crystallization. On the contrary, DLP used finer glass powders, leading to highly crystallized glass-ceramic samples. Despite the differences in manufacturing technology and crystallization, all samples featured very good strength-to-density ratios, which bene fit theiruse for bone tissue engineering applications. The bioactivity of 3D-printed glass-ceramics after immersion in simulated body fluid and the similarities, in terms of ionic releases and hydroxyapatite formation with already validated bioactive glass-ceramics, were preliminarily assessed.
The Layerwise Slurry Deposition is a technology for the deposition of highly packed powder layers. A powder bed is achieved by depositing and drying layers of a ceramic suspension by means of a doctor blade. This deposition technique was combined with the binder jetting technology to develop a novel Additive Manufacturing technology, named LSD-print. The LSD-print was applied to a porcelain ceramic. It is shown that it was possible to produce parts with high definition, good surface finish and at the same time having physical and mechanical properties close to those of traditionally processed porcelain, e.g. by slip casting.
This technology shows high future potential for being integrated alongside traditional production of porce-lain, as it is easily scalable to large areas while maintaining a good definition. Both the Layerwise Slurry Deposition method and the binder jetting technologies are readily scalable to areas as large as > 1 m2.
A shape evolution approach based on the thermally activated self-organization of 3D printed parts into minimal surface area structures is presented. With this strategy, the present communication opposes currently established additive manufacturing strategies aiming to stipulate each individual volumetric element (voxel) of a part. Instead, a 3D structure is roughly defined in a 3D printing process, with all its advantages, and an externally triggered self-organization allows the formation of structural elements with a definition greatly exceeding the volumetric resolution of the printing process. For enabling the self-organization of printed objects by viscous flow of material, functionally graded structures are printed as rigid frame and melting filler. This approach uniquely combines the freedom in design, provided by 3D printing, with the mathematical formulation of minimal surface structures and the knowledge of the physical potentials governing self-organization, to overcome the paradigm which strictly orrelates the geometrical definition of 3D printed parts to the volumetric resolution of the printing process. Moreover, a transient liquid phase allows local programming of functionalities, such as the alignment of functional particles, by means of electric or magnetic fields.
Tricalcium phosphate (b-TCP) can be used as bone graft, exhibiting suitable bioabsorption and osteoconduction properties. The presence of silica may induce the formation of a hydroxyapatite layer, enhancing the integration between implant and bone tissue. Preceramic polymers present silicon in their composition, being a source of SiO2 after thermal treatment. Using the versatility of 3D printing, b-TCP and a polysiloxane were combined to manufacture a bulkb-TCP with a silica coating. For the additive manufacturing process, PMMA powder was used as passive binder for the b-TCP particles, and polymethylsilsesquioxane (MK), dissolved in an organic solvent, was used both as a printing binder (ink) and as the source of SiO2 for the coating. Five distinct coating compositions were printed with increasing amounts of MK. The structures were then submitted to heat treatment at 1180 °C for 4 h. XRD and FTIR showed no chemical reaction between the calcium phosphate and silica. SEM allowed observation of a silicon-based ating on the structure surface. Mechanical strength of the sintered porous structures was within the range of that of trabecular bones.
Distortions in Additive Manufacturing (AM) Laser Metal Deposition (LMD) occur in the newly-built component due to rapid heating and solidification and can lead to shape deviations and cracking. This paper presents a novel approach to quantify the distortions experimentally and to use the results in numerical simulation validation. Digital Image Correlation (DIC) is applied together with optical filters to measure in-situ distortions directly on a wall geometry produced with LMD. The wall shows cyclic Expansion and shrinking with the edges bending inward and the top of the sample exhibiting a slight u-shape as residual distortions. Subsequently, a structural Finite Element Analysis (FEA) of the experiment is established, calibrated against experimental temperature profiles and used to predict the in-situ distortions of the sample. A comparison of the experimental and numerical results reveals a good agreement in length direction of the sample and quantitative deviations in height direction, which are attributed to the material model used. The suitability of the novel experimental approach for measurements on an AM sample is shown and the potential for the validated numerical model as a predictive tool to reduce trial-and-error and improve part quality is evaluated.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-build condition (on a build plate) and after removal from the build plate.
The absolute values of all stress components decreased after removal from the build plate. Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion. Obtained results indicated different residual stress states for each of the transversal, longitudinal and normal component depending on the thermal gradient in the respective direction.
Build-up strategies for temperature control using laser metal deposition for additive manufacturing
(2018)
The track geometry created with laser metal deposition (LMD) is influenced by various parameters. In this case, the laser power has an influence on the width of the track because of an increasing energy input. A larger melt pool is caused by a rising temperature. In the case of a longer welding process, there is also a rise in temperature, resulting in a change of the track geometry. This paper deals with the temperature profiles of different zigzag strategies and spiral strategies for additive manufacturing. A two-color pyrometer is used for temperature measurement on the component surface near the melt pool. Thermocouples measure the temperatures in deeper regions of a component. The welds are located in the center and in the edge area on a test part to investigate the temperature evolution under different boundary conditions. The experiments are carried out on substrates made from mild steel 1.0038 and with the filler material 316L. The investigations show an influence on the temperature evolution by the travel path strategy as well as the position on the part. This shows the necessity for the development and selection of build-up strategies for different part geometries in additive manufacturing by LMD.
Additive Manufacturing (AM) through the Selective Laser Melting (SLM) route offers ample scope for producing geometrically complex parts compared to the conventional subtractive manufacturing strategies. Nevertheless, the residual stresses which develop during the fabrication can limit application of the SLM components by reducing the load bearing capacity and by inducing unwanted distortion, depending on the boundary conditions specified during manufacturing. The present study aims at characterizing the residual stress states in the SLM parts using different diffraction methods. The material used is the nickel based superalloy Inconel 718. Microstructure as well as the surface and bulk residual stresses were characterized. For the residual stress analysis, X-ray, synchrotron and neutron diffraction methods were used. The measurements were performed at BAM, at the EDDI beamline of -BESSY II synchrotronand the E3 line -BER II neutron reactor- of the Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin. The results reveal significant differences in the residual stress states for the different characterization techniques employed, which indicates the dependence of the residual state on the penetration depth in the sample. For the surface residual stresses, longitudinal and transverse stress components from X-ray and synchrotron agree well and the obtained values were around the yield strength of the material. Furthermore, synchrotron mapping disclosed gradients along the width and length of the sample for the longitudinal and transverse stress components. On the other hand, lower residual stresses were found in the bulk of the material measured using neutron diffraction. The longitudinal component was tensile and decreased towards the boundary of the sample. In contrast, the normal component was nearly constant and compressive in nature. The transversal component was almost negligible. The results indicate that a stress re-distribution takes place during the deposition of the consecutive layers. Further investigations are planned to study the phenomenon in detail.
Additive manufacturing (AM) offers an effective solution to the medical sector. It enables the production, on demand, of customised implants which match the patient’s anatomy, with grafts that promote bone growth, as well as surgical guides that help the surgeons.
The objective of this project is to provide a comprehensive basis to enable the safe use of medical AM products with traceable and reliable dimensionalmeasurements. This will guarantee the reliability of medical AM products to notified bodies and facilitate acceptance of AM in the medical sector for a better quality of life.
Glass-ceramics are noted for their unusual combination of properties and manifold commercialized products for consumer and specialized markets. Evolution of novel glass and ceramic processing routes, a plethora of new compositions, and unique exotic nano- and microstructures over the past 60 years led us to review the Definition of glass-ceramics. Well-established and emerging processing methods, such as co-firing, additive manufacturing, and laser patterning are analyzed concerning the core requirements of processing glass-ceramics and the Performance of the final products. In this communication, we propose a revised, updated definition of glass-ceramics, which reads “Glass-ceramics are inorganic, non-metallic materials prepared by controlled crystallization of glasses via different processing methods. They contain at least one type of functional crystalline phase and a residual glass. The volume fraction crystallized may vary from ppm to almost 100%”.
Having been introduced almost two decades ago, Additive Manufacturing (AM) of metals has become industrially viable for a large variety of applications, including aerospace, automotive and medicine. Powder bed techniques such as Selective Laser Melting (SLM) based on layer-by-layer deposition and laser melt enable numerous degrees of freedom for the geometrical design. Developing during the manufacturing process, residual stresses may limit the application of SLM parts by reducing the load bearing capacity as well as induce unwanted distortion depending on the boundary conditions specified in manufacturing.
The residual stress distribution in IN718 elongated prisms produced by SLM was studied non-destructively by means of neutron (bulk) and laboratory X-ray (surface) diffraction. The samples with different scanning strategies, i.e. hatching length, were measured in as-built condition (on a build plate) and after removal from the build plate.
While surface stress fields seem constant for AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements.
Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e. with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the baseplate.
Together with surface scan utilizing a coordinate-measuring machine (CMM), it is possible to link the stress release to the sample distortion.
We finally propose an explanation of those stress profiles based on the deposition strategy.
We investigated lattice structure manufactured by laser beam melting with computed tomography on difference scales, such as powder scale, strut scale and lattice scale.
The raw powder has been evaluated by means of synchrotron computed tomography (CT) at the BAM-Line (HZB Bessy II, Berlin). Therefore, the particle size distribution and even the pore size distribution was investigated and compared with results received by the producer by means of sieving. Studies with laboratory X-ray CT of porosity and roughness of manufactured struts in dependence of the build angle exhibited the tendency that elongated pores appear solely in a certain range near the edge. The integrity and load-bearing capacity of a lattice structure was investigated by means of in-situ CT during compression. The lattice structure was compressed by 10 % in height with an applied maximum force of 5 kN. We applied digital volume correlation algorithm on volumes of different load steps to quantifies the displacement within the structure.
Im Fokus dieser Arbeit steht die computertomographische (CT) Untersuchung (Synchrotron- und Labor-CT) von IN625-Pulver und den daraus gefertigten Streben, welche wiederum zu Gitterstrukturen zusammengesetzt werden. Aufgrund der Filigranität wurde zur Fertigung dieser Proben das pulverbettbasierte selektive Laserschmelzen verwendet.
Porositätsanalysen und Größenverteilungen wurden für das Pulver bei einer rekonstruierten Voxelgröße von 0,5µm ermittelt. 6,0mm lange Streben variierten im Aufbauwinkel von 30° bis 90° zur Bauplattform und zeigten so den Unterschied zwischen Up- und Down-Skin hinsichtlich der Rauigkeit und Porenverteilung. Die Gitterstrukturen konnten in-situ mit bis zu 5,0kN belastet werden, um deren Verformung computertomographisch zu erfassen.
The topic of the presentation consists in some basic considerations on the application of fracture mechanics to fatigue live and strengh prediction of metallic componends manufatured by additive manufacturing. These are based on an approach developed at BAM which comprises elements such as the elastic-plastic modelling of the cyclic crack driving force, a physically meaningfull determination of the initial crack size and multipile crack initiation and propagation due to variations of the local geometry and material charactaristics. Spezial emphasis is put to spezific aspects of materials composed by selectiv laser melting such as surface roughness, porosity and gradiants in the microstructure.
Most additive manufacturing processes which produce dense ceramics are nowadays limited in size because of inevitable post-processing steps like for example binder removal in stereolithography. The additive manufacturing of voluminous ceramic parts is realized by powder bed based processes which, however, generate parts with residual porosity. Via infiltration these parts can be processed to dense parts like for example SiC but this is not possible for all ceramics like for example Si3N4. There is a lack of methods for the additive manufacturing of dense voluminous parts for most ceramics.
We have developed a new additive manufacturing technology, the Laser Induced Slip casting (LIS), based on the layerwise deposition of slurries and their local drying by laser radiation. Laser Induced Slip casting generates ceramic green bodies which can be sintered to dense ceramic components like traditional formed ceramic powder compacts. We will introduce the LIS technology, green bodies and sintered parts will be shown and their microstructure and mechanical properties will be discussed.
Most additive manufacturing processes which produce dense ceramics are nowadays limited in size because of inevitable post-processing steps like for example binder removal in stereolithography. The additive manufacturing of voluminous ceramic parts is realized by powder bed based processes which, however, generate parts with residual porosity. Via infiltration these parts can be processed to dense parts like for example SiC but this is not possible for all ceramics like for example Si3N4. There is a lack of methods for the additive manufacturing of dense voluminous parts for most ceramics.
We have developed a new additive manufacturing technology, the Laser Induced Slip casting (LIS), based on the layerwise deposition of slurries and their local drying by laser radiation. Laser Induced Slip casting generates ceramic green bodies which can be sintered to dense ceramic components like traditional formed ceramic powder compacts. We will introduce the LIS technology, green bodies and sintered parts will be shown and their microstructure and mechanical properties will be discussed.
3D-printing or additive manufacturing has many promising and unique advantages. Especially low cost molten polymer Deposition Printers are increasingly populär in the private and educational sector.
Their environmental friendliness can be questioned due to recently reported ultrafine particle and suspected VOC emissions, To further investigate 3D-printing as a potential indoor air pollution source we characterized fine and ultrafine particle emissions from a molten polymer deposition printer producing a 3D object with ten marketable polymer filament materials under controlled conditions in a test chamber. VOC emissions from the filaments have also been compared. Using a straightforward emission model time dependent and averaged particle emission rates were determined. The results indicate that under comparable conditions some filament materials produce mainly ultrafine particles up to an average rate of 1013 per minute. This value is in the upper ränge of typical indoor ultrafine particle sources (e.g. Smoking, frying, candle light, laser printer). The observed material-specific rates differ by five Orders of magnitude. Filament-specific gaseous emissions of organic compounds such as bisphenol A, styrene and others were also detected.
Our results suggest a detailed evaluation of related risks and considering protective measures such as housing and filtering.
In-Situ SAXS Techniques
(2018)
Our project's aim is to enhance the capabilities of additive manufacturing techniques, where enabling a Two-Photon-Polymerization (TPP) 3D printer of producing arrays of precisely aligned nanoparticles is of an enormous value. As heterogeneous functional nanostructures with arrays of oriented nanoparticles are very promising in many fields; electrochemistry, energy storage, nanoelectronics among other vital fields.
The feasibility and the convenience of orienting nanoparticles using magnetic, electric fields and ultrasonic vibrations will be systematically investigated, using Small Angle X-ray Scattering (SAXS), since SAXS can provide detailed information about the orientation characteristics of nano-Ensembles. Corresponding to our prerequisites, a set ad hoc functional sample holders, sample stages and other In-Situ SAXS solutions were developed, and incorporated to be compatible with a state-of-the-arts SAXS machine, called Multi-scale Analyzer for Ultrafine Structures (MAUS).
The MAUS has been customized and engineered to serve as a miniaturized synchrotron, and that is exactly what we need.
Experiments attempting to orient superparamagnetic nanoparticles will be discussed, where the outcomes will not only help in understanding the mechanics of field-particle interactions, it will also help in further developing the adequate needed set of corrections to the SAXS data, that is especially regards oriented samples.
Due to rapid, localized heating and cooling, distortions accumulate in additive manufactured laser metal deposition (LMD) components, leading to a loss of dimensional accuracy or even cracking. Numerical welding simulations allow the prediction of these deviations and their optimization before conducting experiments. To assess the viability of the simulation tool for the use in a predictive manner, comprehensive systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to Cluster these products in new assembly oriented product families for the optimization.
Additive manufacturing (AM) offers a range of novel applications. However, the manufacturing process is complex and the production of defect-free parts with a high reliability is still a challenge. Thermography is a valuable tool for process surveillance, especially in metal AM processes. The high process temperatures allow one to use cameras usually operating in the visible spectral range. Here, we compare the results of first measurements during the manufacturing process of a commercial laser metal deposition (LMD) setup using a MWIR camera with those from a VIS high-speed camera with band pass filter in the NIR range.
An overview of the main Non-Destructive activities conducted at BAM for the microstructural characterization of Additive Manufacturing (AM) materials will be presented. Focus is made on the study of the influence of Selective Laser Melted (SLM) scan strategies on the defect population of a Ti-6Al-4V alloy using the Analyzer Based Imaging (ABI) technique available at the BAMline (BESSY II synchrotron facility). ABI technique takes advantage of X Ray refraction at interfaces to enable the determination of porosity features (orientation, homogeneity, etc.) within large volumes, otherwise not possible to image by means X ray micro Computed Tomography.
Additionally, for a SLM IN718 material, Energy Dispersive X Ray Diffraction (subsurface measurements at 100 µm depth) and neutrons diffraction (internal measurements at 3 mm depth) are combined with distortion measurements to produce a 3D description of the Residual Stress distribution throughout the entire sample.
Laser metal deposition (LMD) has been applied as a coating technology for many years. Today, the technologies capacity to produce 3D depositions leads to a new field of application as additive manufacturing method. In this paper, 3D laser metal deposition of titanium alloy Ti-6Al-4V is studied with special regard to the demands of additive manufacturing. Therefore, only the coaxial LMD powder nozzle is used to create the shielding gas atmosphere, which ensures high geometric flexibility. Furthermore, specimen with high aspect ratio and hundreds of layers are manufactured, which represent typical features in additive manufacturing. The presented study contains the following steps: First, cylindrical specimens are manufactured with a standard shell-core build-up strategy and mechanical properties as well as fracture mechanisms are determined. Based on the results, experiments are conducted to improve the build-up strategy and new tensile test specimens are built with the improved strategy. The improved strategy incorporates variable track overlap ratios to achieve a constant growth in the shell and core area. As blanks, lean cylinders comprising more than 240 layers and a height of more than 120mm are manufactured. The specimens are analyzed by X-ray inspection for material defects. Fractured surfaces are observed via scanning electron microscopy and the composition of the surfaces is determined using energy dispersive X-ray spectroscopy. The tensile test results prove mechanical properties close to ASTM F1108 specification for wrought material.
BAM ensures and represents high standards for safety in technology and chemistry. Additive manufacturing (AM) changes the requirements for conventional non-destructive testing (NDT) as new processes of defect creation occur. Especially in safety critical areas, such as aerospace and automotive, new manufacturing processes and materials always require reliability tests and new standards which is a big challenge for NDT.
Modern and energy-efficient materials are essential for innovative designs for aerospace and automotive industries. Current technologies for rapid manufacturing such as additive manufacturing and liquid composite moulding by polymer Extrusion allow innovative ways of creating robust and lightweight constructions. Commercially available printing devices often use polylactide (PLA) or acrylonitrile butadiene styrene (ABS) as raw material. Therefore, parameters like the infill ratio, influencing the ability to resist mechanical stress, may have a beneficial impact on the lifetime of components.
These manufacturing technologies require a good knowledge about materials and even adapted non-destructive testing technologies and methods. Airborne ultrasonic testing has beneficial advantages for testing those lightweight constructions. It is a contact-free testing method, which does not require a liquid couplant. Therefore, it allows fast test cycles without any unwanted alternations of the material properties due to interactions with any coupling liquid. This contribution deals with the characterisation of printed specimens based on PLA by using airborne ultrasound and presents the current edge of non-destructive testing and evaluation using airborne ultrasonic transducers. The specimens, manufactured by polymer extrusion, are printed as thin plates. The infill ratio, as well as the material thickness, were varied to model density imperfections with different geometric shapes and properties. For better understanding of the limits of airborne ultrasonic testing in transmission, we compared own-developed transducers based on different physical principles: on ferroelectrets, on the thermoacoustic effect, as well as a new type of transducers based on gas discharges.
Virtual-lab-based determination of a macroscopic yield function for additively manufactured parts
(2018)
This work presents a method for the yield function determination of additively manufactured parts of S316L steel. A crystal plasticity model is calibrated with test results and used afterwards to perform so-called virtual experiments, that account for the specific process-related microstructure including crystallographic and morphological textures. These simulations are undertaken on a representative volume element (RVE), that is generated from EBSD/CT-Scans on in-house additively manufactured specimen, considering grain structure and crystal orientations. The results of the virtual experiments are used to determine an anisotropic Barlat yield function, that can be used in a macroscopical continuum-sense afterwards. This scale-bridging approach enables the calculation of large-scale parts, that would be numerically too expensive to be simulated by a crystal plasticity model.
Powder bed -based technologies are amongst the most successful Additive Manufacturing (AM) techniques. "Selective laser sintering/melting" (SLS/SLM) and "binder jetting 3D printing" (3DP) especially are leading AM technologies for metals and polymers, thanks to their high productivity and scalability. In this context, the "layerwise slurry deposition" (LSD) has been developed as a layer deposition method which enables the use of SLS/SLM and 3DP technologies for advanced ceramic materials. LSD consists in the layer-by-layer deposition of a ceramic slurry by means of a doctor blade. Each layer is deposited and dried to achieve a highly packed powder layer, which can be used for SLM or for 3DP. This technique offers high flexibility in the ceramic feedstock used, especially concerning material and particle size, and is capable of producing parts with physical and mechanical properties comparable to traditionally shaped parts. In this presentation, the LSD technique will be introduced and several examples of application to porcelain, SiC and alumina products will be reported.
In powder-based Additive Manufacturing (AM) processes, an object is produced by successively depositing thin layers of a powder material and by inscribing the cross section of the object in each layer. The main methods to inscribe a layer are by binder jetting (also known as powder 3D printing) or by selective laser sintering/melting (SLS/SLM).
Powder-based AM processes have found wide application for several metallic, polymeric and also ceramic materials, due to their advantages in combining flexibility, easy upscaling and (often) good material properties of their products.
The deposition of homogeneous layers is key to the reproducibility of these processes and has a direct influence on the quality of the final parts. Accordingly, powder properties such as particle size distribution, shape, roughness and process related properties such as powder flowability and packing density need to be carefully evaluated.
Due to these requirements, these processes have been so far precluded to find commercial use for certain applications. In the following, two outstanding cases will be presented.
A first example is that powder-based AM processes are widely used for many metallic and polymeric materials, but they find no commercial application for most technical ceramics.
This seemingly contradicting observation is explained by the fact that in powder based AM, a dry flowable powder needs to be used. The processing of technical ceramics in fact typically requires very fine and poorly flowable powder, which makes them not suitable for the standard processes. There have been several approaches to adapt the raw materials to the process (e.g. by granulation), but in order to maintain the superior properties of technical ceramics it seems necessary to follow the opposite approach and adapt the process to the raw materials instead.
This was the motivation for developing the Layerwise Slurry Deposition (LSD), an innovative process for the deposition of powder layers with a high packing density. In the LSD process, a ceramic slurry is deposited to form thin powder layers, rather than using a dry powder. This allows achieving high packing density (55-60%) in the layers after drying. It is also important, that standard ceramic raw materials can be used. When coupled with a printing head or with a laser source, the LSD enables novel AM technologies which are similar to 3D printing or selective laser sintering, but taking advantage of having a highly dense powder bed.
The LSD -3D printing, in particular, offers the potential of producing large (> 100 mm) and high quality ceramic parts, with microstructure and properties similar to traditional processing. Moreover, due to the compact powder bed, no support structures are required for fixation of the part in the printing process.
Figure 1 shows the schematics of the working principle of the LSD-3D print and illustrates some examples of the resolution and features achievable.
The second outstanding case here described is the application of powder-based AM in environments with reduced or zero gravity. The vision is to be able to produce repair parts, tools and other objects during a space mission, such as on the International Space Station (ISS), without the need of delivering such parts from Earth or carrying them during the mission. AM technologies are also envisioned to play an important role even for future missions to bring mankind to colonize other planets, be it on Mars or on the Moon. In this situation, reduced gravity is also experienced (the gravitational acceleration is 0.16 g on the Moon and 0.38 g on Mars).
These environments cause the use of AM powder technologies to be very problematic: the powder layers need to be stabilized in order to avoid dispersion of the particles in the chamber. This is impossible for standard AM powder deposition systems, which rely on gravitation to spread the powder.
Also in this case, an innovative approach has been implemented to face this technological challenge. The application of a gas flow through a powder has a very strong effect on its flowability, by generating a force on each particle, which is following the gas flow field. This principle can be applied in a simple setup such as the one shown in Figure 2.
In this setup, the gas flow causes an average pressure on the powder bed in direction of the arrows, generating a stabilizing effect which acts in the same direction of the gravitational force. This effect can be used in addition to normal gravity on Earth to achieve a better stabilization of 3D printed parts in the powder bed. In this case, even a significant increase of packing density of the powder was measured, compared to the same experimental setup without gas flow. This is due to the fact that the force on each single particle follows the gas flow field, which is guiding the particles to settle between the pores of the powder bed, thus achieving an efficient packing.
The same principle can be applied in absence of gravitation, where the gas flow acts to stabilize the powder layers. It has been shown that ceramic powder could be deposited in layers and laser sintered in µ-gravity conditions during a DLR (Deutsches Zentrum für Luft- und Raumfahrt) campaign of parabolic flights, as shown in Figure 2. A follow-up campaign is dedicated to the deposition of metallic (stainless steel) powder in inert atmosphere and to study the effects of laser melting in µ-gravity.
In conclusion, the description of these two example cases shows how the development of novel technological processes can address some of the limitations of standard powder-based AM, in order to enable the use of new materials, such as technical ceramics, or to tackle the challenges of AM in space.
Microstructure characterisation of advanced materials via 2D and 3D X-ray refraction techniques
(2018)
3D imaging techniques have an enormous potential to understand the microstructure, its evolution, and its link to mechanical, thermal, and transport properties. In this conference paper we report the use of a powerful, yet not so wide-spread, set of X-ray techniques based on refraction effects. X-ray refraction allows determining internal specific surface (surface per unit volume) in a non-destructive fashion, position and orientation sensitive, and with a nanometric detectability. We demonstrate showcases of ceramics and composite materials, where microstructural parameters could be achieved in a way unrivalled even by high-resolution techniques such as electron microscopy or computed tomography. We present in situ analysis of the damage evolution in an Al/Al2O3 metal matrix composite during tensile load and the identification of void formation (different kinds of defects, particularly unsintered powder hidden in pores, and small inhomogeneity’s like cracks) in Ti64 parts produced by selective laser melting using synchrotron X-ray refraction radiography and tomography.
Due to the advantages of additive manufacturing (AM), it has been increasingly integrated into many industrial sectors.
The application of AM materials for safety-critical parts requires the detailed knowledge about their microstructure stability under thermo-mechanical or mechanical load and knowledge on ageing process mechanisms. Ageing processes are characterized by change of the material microstructure that is to be initially investigated. This work deals with the Investigation of 316L stainless steel manufactured by selective laser melting (SLM). Describing Parameters must be defined and applied on the microstructure of these materials in their initial state and after loads were applied. The findings of this work form the basis for the investigation of AM material ageing.
Additive Manufacturing by Selective Laser Melting (SLM) offers an ample scope for producing geometrically complex parts as compared to the traditional subtractive manufacturing strategies. However, the residual stresses (RS) developed during the processing can reduce the load bearing capacity as well as induce unwanted distortion, limiting the life time and the application of SLM parts.
The paper deals with the integration of a light emitting diode (LED) into an additive manufactured metal component. Selective laser melting (SLM) and laser metal deposition (LMD) are used. The material used is the chrome-nickel steel 316L. The basic component is manufactured by means of SLM and consists of a solid body and an area with grid structure. The solid body includes a duct in the shape of a groove with a recess for the positioning of the power cable. The LED is embedded in the grid structure via an inlet from the solid body. In further processing, the groove is filled with LMD. Two strategies with different parameter combinations were investigated. It shows that a high energy input near the power cable leads to its destruction. By using multiple parameter combinations during the manufacturing process, this destruction can be prevented. There was a comparison of both strategies with regard to the necessary number of tracks and duration of welding time.
The paper deals with the integration of a light emitting diode (LED) into an additive manufactured metal component. Selective laser melting (SLM) and laser metal deposition (LMD) are used. The material used is the chrome-nickel steel 316L. The basic component is manufactured by means of SLM and consists of a solid body and an area with grid structure. The solid body includes a duct in the shape of a groove with a recess for the positioning of the power cable. The LED is embedded in the grid structure via an inlet from the solid body. In further processing, the groove is filled with LMD. Two strategies with different parameter combinations were investigated. It shows that a high energy input near the power cable leads to its destruction. By using multiple parameter combinations during the manufacturing process, this destruction can be prevented. There was a comparison of both strategies with regard to the necessary number of tracks and duration of welding time.
Pores in additive manufactured metal parts occur due to different reasons and affect the part Quality negatively. Few investigations on the origins of porosity are available, especially for Ni-based super alloys. This paper presents a new study to examine the influence of common processing Parameters on the Formation of pores in parts built by laser metal Deposition using Inconel 718 powder. Further, a comparison between the computed tomography (CT) and the Archimedes method was made. The Investigation Shows that CT is able to identify different kinds of pores and to give further Information about their distribution. The identification of some pores as well as their shape can be dependent on the Parameter Setting of the Analysis tool. Due to limited measurement Resolution, CT is not able to identify correctly pores with Diameters smaller than 0.1 mm, which leads to a false decrease on Overall porosity. The applied Archimedes method is unable to differentiate between gas porosity and other Kinds of holes like internal cracks or lack of Fusion, but it delivered a proper value for Overall porosity. The method was able to provide suitable data for the statistical Evaluation with design of Experiments, which revealed significant Parameters ont he Formation of pores in LMD.
This book fulfills its intention to provide a comprehensive overview on 3D-printing of metals.
The interested reader can get a lot of information about the topics one should deal with when working with additively manufactured metallic parts. It gives a general roadmap where to start, what to learn and how it fits together.
The quality of components made by laser beam melting (LBM) additive manufacturing is naturally influenced by the quality of the powder bed. A packing density < 1 and porosity inside the powder particles lead to intrinsic voids in the powder bed. Since the packing density is determined by the particle size and shape distribution, the determination of these properties is of significant interest to assess the printing process. In this work, the size and shape distribution, the amount of particle’s intrinsic porosity as well as the packing density of micrometric powder used for LBM have been investigated by means of synchrotron X-ray computed tomography (CT). Two different powder batches were investigated: Ti-6Al-4V produced by plasma atomization and Stainless Steel 316L produced by gas atomization. Plasma atomization particles were observed to be more spherical in terms of the mean anisotropy compared to particles produced by gas atomization. The two kinds of particles are comparable in size according to the equivalent diameter. The packing density is lower (i.e. the powder bed contains more voids in between particles) for the Ti-6Al-4V particles. The comparison of the tomographic results with laser diffraction, as another particle size measurements techniques, proved agreement.
Virtual-lab-based determination of a macroscopic yield function for additively manufactured parts
(2018)
This work presents a method for the yield function determination of additively manufactured parts of S316L steel. A crystal plasticity model is calibrated with test results and used afterwards to perform so-called virtual experiments, that account for the specific process-related microstructure including crystallographic and morphological textures. These simulations are undertaken on a representative volume element (RVE), that is generated from EBSD/CT-Scans on in-house additively manufactured specimen, considering grain structure and crystal orientations. The results of the virtual experiments are used to determine an anisotropic Barlat yield function, that can be used in a macroscopical continuum-sense afterwards. This scale-bridging approach enables the calculation of large-scale parts, that would be numerically too expensive to be simulated by a crystal plasticity model.
Additive Manufacturing (AM) through the Selective Laser Melting (SLM) route offers ample scope for producing geometrically complex parts compared to the conventional subtractive manufacturing strategies. Nevertheless, the residual stresses which develop during the fabrication can limit application of the SLM components by reducing the load bearing capacity and by inducing unwanted distortion, depending on the boundary conditions specified during manufacturing.
The present study aims at characterizing the residual stress states in the SLM parts using different diffraction methods. The material used is the nickel based superalloy Inconel 718. Microstructure as well as the surface and bulk residual stresses were characterized. For the residual stress analysis, X-ray, synchrotron and neutron diffraction methods were used. The measurements were performed at BAM, at the EDDI beamline of -BESSY II synchrotron- and the E3 line -BER II neutron reactor- of the Helmholtz-Zentrum für Materialien und Energie (HZB) Berlin.
The results reveal significant differences in the residual stress states for the different characterization techniques employed, which indicates a dependence of the residual state on the penetration depth in the sample. For the surface residual stresses, longitudinal and transverse stress components from both X-ray and synchrotron agree well and the obtained values were around the yield strength of the material. Furthermore, synchrotron mapping disclosed gradients along the width and length of the sample for the longitudinal and transverse stress components. On the other hand, lower residual stresses were found in the bulk of the material measured using neutron diffraction. The longitudinal component was tensile and decreased towards the boundary of the sample. In contrast, the normal component was nearly constant and compressive in nature. The transversal component was almost negligible. The results indicate that a stress re-distribution takes place during the deposition of the consecutive layers. Further investigations are planned to study the phenomenon in detail.
The residual stress distribution in IN718 elongated prisms produced by Selective Laser Melting was studied by means of neutron (bulk) and laboratory X-ray (surface) diffraction.
Two deposition hatch lengths were considered. A horizontal plane near the top surface (perpendicular to the building direction) and a vertical plane near the lateral surface (parallel to the building direction) were investigated. Samples both in as-built (AB) condition and removed (RE) from the base plate were characterized.
While surface stress fields seem constant for AB condition, X-ray diffraction shows stress gradients along the hatch direction in the RE condition. The stress profiles correlate with the distortion maps obtained by tactile probe measurements.
Neutron diffraction shows bulk stress gradients for all principal components along the main sample directions. We correlate the observed stress patterns with the hatch length, i.e. with its effect on temperature gradients and heat flow. The bulk stress gradients partially disappear after removal from the baseplate.