Eutectic Al alloys with tailored solidification path to probe fundamental aspects of solidification in laser based AM
Dr. Markus Apel (Access e.V.)
Prof. Dr. Michael Schmidt (Friedrich-Alexander-Universität Erlangen-Nürnberg)
Additive Manufacturing methods, especially powderbed based laser beam melting, are attracting more and more academic and industrial researchers due to its manifold and flexible applicability.
The process allows the manufacturing of complex three-dimensional structures and the tailoring of microstructure and thus also of mechanical properties. While a few materials like AlSi10Mg, 1.4404 stainless steel
or Ti6Al4V have become de facto industry standard and a range of processing parameters is known, the academic focus shifts to the usage of more and new alloys. We want to introduce (hyper-) eutectic Al-Ni and Al-Ni-Ce
alloys to SLM with the aims to study fundamental aspects of the solidification kinetics and microstructure selection in SLM by tailoring the solidification path via alloy composition. As this alloy system offers high-strength,
excellent thermal stability and good castability it is also a promising candidate for a new class of Al-based AM alloys. In order to reduce experimental efforts we will develop and establish an analytical model for the quick prediction
of temperature distribution, meltpool geometry and processing windows. In addition, we will develop and establish a predictive, material sensitive multi-scale simulation toolchain by combining process and phase-field simulations to
describe process dynamics and microstructure formation for eutectic alloys. With these tools at hand we will derive scaling laws for the correlation between processing parameters and microstructure for SLM solidification conditions and
investigate the effect of composition changes on microstructure and growth kineticsWe will validate the modelling results via experiments with binary and ternary alloys having different solidification intervals.A correlation between
process strategy and solidification behavior is a first step towards the ability to create tailored microstructures. In order to utilize the full potential of tailored microstructures the correlation between process, microstructure and
mechanical properties must be understood. Therefore, we want to continue the development of Al-Ni and Al-Ni-Ce alloys during the second period of the priority program and focus on the correlation to mechanical properties and demonstrate
the full potential of the Al-Ni and Al-Ni-Ce alloy systems.By our proposal, we see contributions mainly to two different working groups of this priority program, namely (i) calorimetry and crystallization and (ii) theory and modeling.
As the process models developed within this project are adoptable to other alloys, they can help other researchers in the priority program to understand the solidification behavior of their materials as well.
Dispersion effects of nanocomposites to improve melting and resolidification behavior during SLS with CO2 and diode lasers
Prof. Dr. Stephan Barcikowski (Universität Duisburg-Essen)
Prof. Dr. Michael Schmidt (Friedrich-Alexander-Universität Erlangen-Nürnberg)
The DFG priority program "Materials for additive manufacturing" (SPP 2122) addresses the development of new powder materials for laser-based 3D additive manufacturing. The proposed project follows this call by synthesizing, formulating, material-analytically characterizing and laser-processing new nanoparticle-polymer composite powders for selective laser sintering (SLS). Scalable laser synthesis and processing of colloidal nanoparticles and the adsorption of these onto dispersed polymer micropowders directly in aqueous solution will generate new nanoparticle-polymer composite powders and qualify them for SLS. To fully exploit the potential of this innovative approach, based on successful joint preliminary work, it explores how nanoparticles interact along the process chain from synthesis and supporting on polymer micro-particles, addressing the requirements of the laser-based manufacturing process for improved melting and solidification behavior as well as laser absorption. For this purpose, for the first time, quantitative determinants of the nanoparticle-polymer-composite powder quality (nanoparticle surface dispersion) are systematically correlated with the nanoparticle volume dispersion in the polymer matrix produced by SLS via material analysis (particle size, surface occupation density, interparticle distance, calorimetry, etc.). For the fundamental mechanistic elucidation, three different nanoparticle materials from the representatives of the metals, oxides, and carbon materials are comprehensively investigated for the first time. The nanoadditive studies are initially carried out on the model powder material polyamide 12 and the transferability of the resulting results to the nano-additivation of high-density polyethylene and thermoplastic elastomer powder is investigated. The proposed project thus contributes to an improved understanding of material behavior, in particular, nanoparticle dispersion and thermophysical processes during melting and crystallization, as well as nanoparticle dispersion in SLS processing. To address these research questions, the project is being worked on in tandem by two applicants with complementary expertise in materials and laser processing science. In addition, there are concrete links with other research topics within the framework of the Priority Program.
Numerical and experimental investigations of dimensionless material parameters in laser additive manufacturing of polymers for accelerated material development and process optimization
Dr. Class Bierwisch (Fraunhofer-Institut für Werkstoffmechanik)
Dr. Marieluise Lang (Süddeutsches Kunststoff-Zentrum)
There is a high growth of additive manufactured parts in the polymer industry. Nevertheless, especially for laser sintering (LS) the material selection is very limited in comparison to other polymer production processes like, for example, extrusion or injection molding. One major reason for that problem is that there is almost no knowledge of which material properties are important for the process and how the material parameters affect the product quality. This is particularly disadvantageous because there are many potential influencing factors for polymer sintering or metal melting processes. Thus, a profound understanding of all relevant mechanisms of the LS process is considered to be lacking. In particular, there is no link between process parameters, material data and component quality. This causes significant difficulties in developing new materials, as it is for example not known which combination of viscosity and surface tension of the melt lead to the desired product quality in terms of dimensional accuracy.To address this problem, a link between material data, process parameters and component quality should be developed in this project. For this purpose, 3D numerical simulations, experimental investigations and analytical considerations are combined to develop dimensionless characteristic numbers (DCN). These numbers shall be based on the dimensionless volumetric energy density, the initial porosity, the Capillary number, the visco-thermal Prandtl number and a sintering model of Mackenzei & Shuttleworth. Analytical functions of these characteristic numbers shall give predictions for the final porosity of built structures and shall thereby provide thresholds for the processability of a given material. By calculating the characteristic numbers for newly developed materials, an accelerated estimation of the processability of the material can then be made. For example, it can be estimated quantitatively which bulk density, viscosity and specific melting enthalpy are necessary to stay below a given porosity. This would enable material developers to design materials with the required material properties. Furthermore the numbers support the accelerated optimization of the processing conditions for new materials by estimating the influence of a parameter variation on the product quality. Thereby the DCN can be used by material developers, powder production companies, LS machine manufacturers and producers of LS parts to get faster to better products. Besides, by refining the 3D simulation methodology a further tool for the material and process development will be available.
Disentangled UHMWPE composites for warp-free SLS parts
Prof. Claus Emmelmann (Fraunhofer-Institut für Additivie Produktionstechnologie)
Prof. Gerrit A. Luinistra (Universität Hamburg)
The proposed project targets the preparation and use of tailor-made polymer composite particles in SLS, allowing to generate SLS parts with extended properties, in particular with the aim to overcome the current limited interlayer adhesion. The combination of in-situ polymerization techniques for the production of functional powders and simulation supported process optimization for generating SLS parts is the approach to reach improved mechanical and dimensional stability of the resulting parts. In a first approach, single-walled CNT nanocomposites powders with disentangled UHMWPE will be prepared and sintered. This would lead to parts based on UHMWPE; latter was chosen as material for the parts on account of the outstanding mechanical and chemical stability. It is expected that the formation of entanglement in the sintering process will lead to a much higher adhesion between the particles, also given by the lower viscosity of the starting composite.A numerical simulation will be established validated and adapted by data recorded from a thermal camera. Then the temperature distribution assessed by the simulation will be correlated to the material-specific crystallization behavior. Based on this correlation inhomogeneous crystallization and thus warpage will be avoided by means of post-build temperature adjustments.Subsequently, the material properties of SLS parts will be determined, compared against reference materials like commonly molded UHMWPE nanocomposites. A feedback and optimization loop will be thus established regarding material properties and process parameter.
Surface Inoculation of Aluminium Powders for Additive Manufacturing guided by Differential Fast Scanning Calorimetry
Prof. Dr. Guido Grundmeier (Universität Paderborn)
Prof. Dr. Mirko Schaper (Universität Paderborn)
Dr. Evgeny Zhuravlev (Universität Rockstock)
Prof. Dr. Olaf Keßler (Universität Rostock)
Due to the continuous progression of Rapid Prototyping and comparably fast build rates and an declining number of constrains, additive manufacturing (AM) develops into a manufacturing technique, allowing complex designs and great weight saving for low volume productions. A widely used AM method for metal powders is laser beam melting (LBM). With a mechanical and thermal process similar to common laser welding, decades of research can be utilized and good mechanical properties can be achieved. However, due to difficulties in the solidification process, like crack formations and porosity, welding and additive manufacturing of high-strength metal and especially aluminium is still limited to a restricted set of alloys.To improve the solidification process of laser beam melting (LBM), and thus enable the printing of hard-to-weld high-strength aluminium alloys, the project is designed to modify the powder surface by adding nanoparticles for a guided nucleation. In-situ testing of the fast melting and solidification process of single particles will be performed by using differential fast scanning calorimetry (DFSC). DFSC results will be transferred to LBM of aluminium alloys and used to adjust parameter settings. Specimens will be printed, to check crack formations and physical as well as mechanical material properties by suitable characterisation techniques. The aluminium alloy EN AW-7075 (EN AW-AlZnMgCu1.5) is the most common high strength aluminium alloy used in mechanical engineering, aircraft industry and, due to new developments like press quenching, also in the automobile industry. Due to the importance of Al7075, this project will focus on inoculating its powder particles and thus, enable crack and porosity free 3D-printing.
Development of surface tailored metal powders for increased production efficiency at the laser powder-bed fusion additive manufacturing process
Prof. Dr. Arno Kwade (Technische Universität Braunschweig)
Prof. Dr. Jan Sehrt (Ruhr-Universität Bochum)
Laser powder-bed fusion of metals (L-PBF-M) is one the most requested additive manufacturing processes for the direct manufacturing of metal parts in the industry. In addition to the multitude of advantages for L-PBF-M, there are also disadvantages such as the low productivity. Recently, more and more L-PBF-M machines are equipped with bigger built envelopes, multiple lasers and/or high power laser systems in order to increase process speed, but there are other ways to increase the speed of L-PBF-M. Most important at this point is a significant improvement of the exposure parameters such as scanning speed and hatching distance. While usually the machine setup allows to further increase scanning speeds and hatching distances, a fully dense and crack-free microstructure cannot be guaranteed anymore. However, this involves the development of new materials which have a higher absorption rate of the laser radiation compared to commercial powder materials. Hence, the main aim in this project is to develop powders for L-PBF-M which have an increased absorptivity of the laser light and improved heat transfer capabilities during the process. Prerequisites for this research project are the investigation of the powder-laser interaction and how particle surface tailoring contributes to the absorption behaviour at L-PBF-M and how these surface modifications are generated. For the planned investigations three different feedstock powders (stainless steel, tool steel and aluminium alloy) are chosen. Based on preliminary investigations and literature review SiC, graphene and iron oxide black are selected for surface tailoring primarily to achieve an advanced absorbance of laser energy. The commercial available additive materials are formulated (especially by nanomilling) to achieve nanosized particles and coated onto the metal powders by wet (fluidized bed drying) and dry (high intensity mixer) processes. Beside material effects the influences of particle size and loading of the additives are investigated systematically. All tailored metal powders are characterised regarding absorbance, thermal conductivity, coverage of the coating, particle size and shape, flowability and L-PBF-M processability. Subsequently, the most promising tailored powders are produced in a higher amount and analysed with advanced methods such as wear resistance of coated metal powders, segregation behaviour and also observation of powder deposition in μCT. For these powders the L-PBF-M process is carried out to find suitable and more economic parameters and the printed specimen are characterised. This includes the determination of the density, hardness, microstructure, micromechanics and tensile strength. All results are applied to develop material–process–property relationships in depth. Moreover, a DEM-simulation is set up to describe the spreading, heat transfer and later in the project absorption behaviour of the surface tailored powders.
Tailor made Magnesium Alloy for Selective Laser Melting: Material Development and Process Modelling
Prof. Dr. Hans Jürgen Maier (Gottfried Wilhelm Leibniz Universität Hannover)
Prof. Dr. Ludger Overmeyer (Laser Zentrum Hannover e.V.)
The scientific objective of the project proposal is the development of a magnesium alloy in combination with suitable process parameters (process gas, laser power and hatch patterns) specifically tailored to meet the requirements of the selective laser melting process. The fundamental issue in processing magnesium alloys with SLM is the strong refractory magnesium oxide layer along with the high vapour pressure of metallic magnesium. Controlling the oxidation behaviour is key in improving the processability of magnesium alloys; hence, a combined strategy involving three different approaches will be developed in the proposed project. First, alloying elements like neodymium (Nd) and yttrium (Y) are used to reduce magnesium oxidation and form a modified oxide layer with a lower melting point. Additionally, the strength at elevated temperatures has to be decreased for the oxide layer to break far below the melting point. Lastly, the oxide layer thickness has to be reduced, which will facilitate breaking the oxide layer as well. Strontium (Sr) has the desired effect of minimizing the oxide layer thickness of magnesium without thermodynamically reducing magnesium oxide. The governing mechanisms will be studied in the proposed project. In the course of the project, powder atomization and melting during the SLM process are emulated by means of model experiments in order to induce an oxide layer that approximates the one in the actual process. Subsequent measurements of the oxide layer thickness of the alloys allow a determination of the influence of the elements Nd, Y and Sr on the processability of the magnesium alloys. As the processability is also strongly influenced by the process conditions, it will be investigated to what extent vacuum and the reducing effect of hydrogen-containing shielding gases affect the final part’s quality.Common applications for magnesium alloys are lightweight structures and biomedical implants, for which the mechanical properties and possibly the degradation behaviour have to be adapted too. It is already known that Nd, Y, Sr and Zirconium (Zr) further enhanced the strength. Firstly, the mechanical properties of binary alloys will be determined. Subsequently, ternary and quaternary alloys with a combination of the elements Nd, Y, Sr and Zr will be cast to further improve the mechanical properties. Moreover, the mechanical properties can be increased by tailoring the microstructure through thermal treatments. The temperatures, used in the thermal treatments, will be chosen based on the phase diagrams. Unavailable phase diagrams will be calculated separately. Moreover, the elements used are biocompatible and especially Nd and Y are already applied as alloying elements in magnesium implants.
New high stiffness materials for light weight constructions using ultrafast additive manufacturing
Prof. Dr. Stefan Nolte (Friedrich Schiller Universität Jena)
Prof. Dr. Markus Rettenmayr (Friedrich-Schiller-Universität Jena)
Novel construction materials with superior stiffness combining high elastic moduli with low mass densities will be developed. Two binary aluminum alloy systems are in the focus: the Al-Li system, where additions of Li not only reduce the density, but also significantly increase the elastic modulus, and the Al-Si system where Si, besides reducing the density, is known to distinctly improve the mechanical properties, if a two-phase structure of fine Si particles in an Al matrix can be generated. The alloys are to be processed by ultrashort pulse selective laser melting (USP-SLM). Compared to conventional continuous wave SLM, ultrashort pulsed lasers deliver very high peak power in extremely short times.
High Pressure Spray Process for Polymer Particles applicable for Laser Polymer Deposition (LPD)
Prof. Dr. Andreas Ostendorf (Ruhr-Universität Bochum)
Prof. Dr. Marcus Petermann (Ruhr-Universität Bochum)
The aim of the project is the development and the basic understanding of two processes for the production and processing of polymer powders. The technologies developed in this project will allow to process new polymer materials, thus creating the basis for a broader base of raw materials in 3D additive manufacturing. With a high-pressure spray process polymer particles are produced with smooth surfaces, which cannot be produced with the established manufacturing processes or only very cost intensive. By changing the process parameters of the high-pressure process, particle sizes, particle morphologies and crystallinities of the polymer powders produced are deliberately changed and adjusted. The generated particle systems are processed in a newly developed laser polymer deposition (LPD) process, which is based on the established laser metal deposition (LMD). In contrast to the established selective laser sintering (SLS) technology, which uses CO2 laser, LPD technology relies on cost-effective and more energy efficient solid-state lasers. Therefore, the optical properties of the polymer particles will be optimized with regard to the absorption at the laser wavelength of around 1 µm by adding additives. The new processing technology in the field of polymer systems enables higher processing speeds and lower material usage. Process development and optimization for 3D additive manufacturing is based on systematic analysis of the micro- and macro-scale properties of the polymer components and powders produced. Differential calorimetric and X-ray diffractometric investigations will provide information about the crystallinity and the thermal processing ranges of the polymer powders. Tensile and bending tests show the processability of the polymer powders and help to optimize the laser process as well as the particle generation. With a thermoplastic polyurethane an elastomer will be developed for additive manufacturing.For the second project period of the priority program, a polylactic acid shall be used to test a biodegradable polymer and polypropylene as a thermoplastic mass product with the newly developed LPD process. In addition, the LPD process will be extended to the production of hybrid polymer/metal components.
Production of SLS particles via liquid-liquid phase separation and precipitation
Dr. Jochen Schmid (Friedrich-Alexander-Universität Erlangen-Nürnberg)
The overall aim of this project is the development of novel semi-crystalline polymer powders for selective laser sintering (SLS) via a liquid-liquid phase separation (LLPS) and subsequent crystallization for polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyoxymethylene (POM) and polyvinylidene fluoride (PVDF). Selection criteria for appropriate solvents for LLPS will be deduced by an iterative screening method based on Hildebrand and Hansen solubility parameters. In-situ model LLPS experiments employing dynamic light scattering, optical microscopy and impedance spectroscopy –supported by ex-situ electron microscopy- will allow for deeper insights into the phase separation and particle formation mechanisms. A detailed mechanistic understanding of the underlying mechanism of LLPS, i.e. nucleation, droplet coalescence and growth and elucidation of their dependencies on system composition, time-temperature history will be achieved. LLPS will be implemented on the mini plant scale, where the effect of process parameters (e.g. influence of stirring, cooling regime) on material and bulk solid properties of the obtained polymer particles will be systematically studied. Moreover, in-situ functionalization of the particles with nanoparticulate flowing aids and additive enhancement with thermal stabilizers (antioxidants) will be studied, scale-up criteria will be deduced and powder amounts applicable for studies on SLS processing behavior (several 100 grams to the kilogram scale) will be produced. A detailed structural characterization of the obtained SLS particles with respect to crystallinity, polymorphism, thermal characteristics and morphology will be performed employing amongst others dynamic scanning calorimetry, X-ray diffraction, vibrational spectroscopy (IR, Raman) and electron microscopy. The effect of process parameters on molar mass distributions of the polymer and melt viscosity will be assessed by gel permeation chromatography and melt rheology. Bulk solid characteristics such as the product particle size distribution and the powder flowability which are seen to be most important for processing will be characterized by laser diffraction particle sizing, respectively shear testers and powder application model experiments. The gained understanding of the structure-property relationships and the information on SLS processability provided by the cooperation partners will be utilized for further LLPS process optimization to tailor the desired properties of the novel SLS powders. Novel SLS powders with improved material behavior, i.e. chemical resistance (PET, PBT, PVDF), impact resistance (PBT), high stiffness, excellent dimensional stability (PET, PBT, POM) shall be developed widening the field of application of SLS-manufactured parts.
Qualification of new steel-alloying strategies for LAM powders by combined in-situ additivation, agglomeration and in-/post-process treatments
Prof. Dr. Werner Theisen (Ruhr Universität Bochum)
Prof. Dr. Frank Walther (Technische Universität Dortmund)
Prof. Dr. Hans-Werner Zoch (Leibniz-Institut für Werkstofforientierte Technologien
Main objective of this joint proposal within SPP 2122 is the development of new starting materials and their qualification for processing martensitic-hardenable tool steels or cast irons by SLM. In a first step of our approach, we will consider our strategies on grey cast iron alloy and adjust the alloying concepts, based on this knowledge, to process white cast iron (2nd step) and ledeburitic cold work tool steel (3rd step), the two classes of materials that are not processible by SLM till now. Thereby, the grey cast iron will not be processed by using a pre-alloyed gas-atomized powder. Here we will admix different volume fraction of SiC-particles to cheap low-alloyed steel powder and the powder mixture will be densified by SLM. The aim is to realize a partial melting of particles, thus a “green body” is shaped without high defect density. The specific adjustment of the microstructure and the associated materials properties will be obtained by a further required post-processing via SLM (in-process) or heat treatment (post-process). Thereby, dissolution of admixed metastable components of the bulk powder during SLM-processing and post-processing (hot isostatic pressing, super-solidus-liquid phase-sintering) will be investigated and how the powders have to be modified to realize a full dissolution and different amounts of Fe3C or graphite. In the second and third step, we are adding further ferro-alloy powders to increase the content of certain elements like Cr, Mn, Ni in order to produce white cast iron and ledeburitic cold work tool steels. Finally, the mixtures of ferro-alloy powders and pure elements, which are conditioned by agglomeration to form flowable powders, will be processed in a similar way. It is assumed that similar alloys or chemical compositions, respectively, with similar properties can be achieved in spite of the difference in the processing routes. If this hypothesis can be confirmed, only a few basic powders with different chemical compositions are necessary to generate multiple alloys. This can significantly accelerate the development of new alloys for laser-based additive manufacturing due to economic and processing advantages as microstructure formation process and the related formation of internal defects are decoupled from SLM densification process. In order to process the complex material-oriented research within SPP 2122, all aspects will be addressed in a holistic approach with regard to powder production and powder conditioning (IWT), alloy design and SLM-processing of the designed powders (LWT) as well as microstructural, micro-magnetic and mechanical characterization of the SLM-components (WPT).