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Additive manufacturing encompasses all the proce-dures which build up a component layer by layer onthe basis of a three-dimensional computer model. Ad-ditive building-up permits complex geometrical shapeswhich are subject to hardly any restrictions and thuscould not be implemented with conventional manu-facturing procedures or only at great expense. Similarto selective laser beam melting, selective electron beammelting (SEBM) is a powder-bed-based additive manu -facturing process in which the powder particles are lo-cally melted completely and consolidated by the beam.The electron beam permits the processing of high-melt-ing and reactive metals in a vacuum as well as a highconstruction rate for additive manufacturing processesdue to high power densities and quick deflectionspeeds. This article not only describes the installationsetup and the process sequence during SEBM but alsointroduces the material classes investigated until now,selected applications and the numerical simulation ofthis procedure.

1 Introduction For decades, technological progress and high com-petitive pressure have led to the shortening of the timespan from the product idea to the placing of a product onthe market. This contrasts with a continuous increase inthe complexity and with the current trend towards the in-dividualisation of components and systems. In this fieldof tension, conventional manufacturing procedures areincreasingly reaching their limits while additive manu-facturing is opening up new possibilities for process in-novations and the implementation of totally new productproperties. Additive manufacturing technologies are suitable formanufacturing technically sophisticated products, aboveall as single items and in small-scale series. On the basisof three-dimensional virtual models, the manufacturingcan be carried out at any time in nearly any location, es-pecially close to the place of use too. Additive building-up permits complex geometrical shapes which are subjectto hardly any restrictions. Due to this property, it is possi-ble to manufacture components and finished parts whichcould not be implemented with conventional manufac-turing technologies or only at great expense. Thus, topo-logically optimised and functional components may, forexample, raise the efficiency and productivity of meansof transport and technological installations. In the case of additive manufacturing, componentsare manufactured by joining in layers. In particular, pow-der-bed-based procedures in which individual powderlayers are locally melted completely and consolidated bya beam source are suitable for the processing of metals[1]. The most widespread beam source is the laser (selec-

Additive manufacturing usingselective electron beam melting

tive laser melting, SLM [2]). However, the electron beam(selective electron beam melting, SEBM [3]) which permitsquicker process management due to its higher power den-sity stands out for the processing of high-melting and re-active metals [4]. Experts from industry and science from all over theworld discussed the great potential of this technology atthe “1st International Conference on Electron Beam Ad-ditive Manufacturing“ EBAM 2016. The conference sub-jects range from the preliminary processing of the pow-der via process management and process observationright up to the post-processing of the components. Thisarticle takes up the idea of this conference and links thegreat potential of selective electron beam melting withresearch-related applications. The main topics of thisarticle are the description of the SEBM technology in-cluding possible materials, applications and numericalsimulations.

KEYWORDSElectron beam melting, additive manufacturing, simulation andcalculation, material questions

Dr.-Ing. Matthias Markl is the Head of the Nu-merical Simulation Group at the Chair of MaterialsScience and Engineering for Metals at theFriedrich-Alexander-Universität Erlangen-Nürn-berg/Germany.

Dr.-Ing. Matthias Lodes is the Managing Direc-tor of the Joint Institute of Advanced Materialsand Processes at the Friedrich-Alexander-Univer-sität Erlangen-Nürnberg.

Dr.-Ing. Martin Franke is the Head of the Addi-tive Manufacturing Group at Neue MaterialienFürth GmbH in Fürth/Germany.

Prof. Dr.-Ing. habil. Carolin Körner is the holderof the Chair of Materials Science and Engineeringfor Metals and is responsible for the collegialmanagement of the Joint Institute of AdvancedMaterials and Processes at the Friedrich-Alexan-der-Universität Erlangen-Nürnberg. Moreover, sheis responsible for the scientific management ofthe Additive Manufacturing Group at Neue Ma-terialien Fürth GmbH.

THE AUTHORS

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2 Selective electron beam melting Selective electron beam melting is, just like all theother layer-based technologies, integrated into theprocess chain of additive manufacturing. This encom-passes the elaboration of a three-dimensional virtualcomputer model of the component to be manufactured,the preliminary processing of this data by dividing it intoindividual layers with a constant layer thickness, themanu facturing and the post-processing. In the post-pro-cessing, the manufactured components are removed fromthe powder bed by means of sandblasting with the samepowder. The powder recovered in this way can be reusedfor the process. Depending on the application of the com-ponents, the surface can then be post-treated even fur-ther, e.g. in order to reduce the roughness inherent in theprocess. The possible material diversity of selective electronbeam melting is restricted to electrically conductive ma-terials, i.e. metals. Since electrons interact with the at-mosphere, the manufacturing takes place in a vacuumchamber which is usually regulated to a pressure in therange of 2 · 10-3 mbar using a controlled helium inflow.The electron beam gun with a typical power of up to 3 kWand an accelerating voltage of 60 kV is mounted on thevacuum chamber. Beam diameters of 100  to  1,000  µmare achieved depending on the quality and type of thecathode and on the utilised power. Electromagnetic coilscan be used in order to move the electron beam at de-flection speeds of up to 10,000 m/s without any inertia.Two powder tanks and one rake are installed in the inte-rior of the vacuum chamber in order to apply the layersin the construction space with a mobile process platform.The components of the vacuum chamber and the indi-vidual process steps of the manufacturing are illustratedon Fig. 1.

In a first step, a defocused electron beam scans theentire surface of the powder layer several times in orderto set the preheating temperature. Depending on theutilised material, this varies from 300°C for pure copper[6] right up to 1,100°C for some nickel-based alloys [7].This process step is extremely important for the processstability. One challenge facing this procedure is the chargedissipation of the electrons via the powder bed. If this isnot guaranteed, it results in the so-called “smoke“ phe-nomenon with which the top powder layer, so to speak,explodes because powder particles are thrown out of thelayer. This inevitably leads to the termination of theprocess. Due to the slight sintering of the particles duringthe preheating, it is possible to guarantee a sufficient elec-trical conductivity. The inflow of helium up to 2 · 10-3 mbarsupports the stable process management. The heliumatoms are positively ionised by the electron beam andcan subsequently dissipate the negative charge from thepowder bed. Furthermore, the entire powder bed supportsthe component due to the sintering. In a second step, the regions belonging to the com-ponent geometry are melted completely with a focusedelectron beam. Depending on the geometry, differentprocess strategies which may be composed of a combi-nation of contours and hatches at typical scanning speedsof 4 m/s and 10 m/s are used here. In this respect, thecontours can optionally be processed in the so-calledmultibeam mode in which up to 100 contour positionsare melted completely in quasi-simultaneous operation.This is possible due to the high maximum deflectionspeed of the electron beam. A hatch describes an areawhich is melted completely in a meandering shape withcertain line spacing, typically between 25 µm and 150 µm.As standard, this scanning pattern is rotated by 90° ineach layer.

Fig. 1 • Process chain of selective electron beam melting: After the preheating of the entire powder layer with a defocused electronbeam (1), component regions are completely melted selectively according to the computer model (2). After the consolidation andthe solidification, the process platform is lowered by one layer thickness (3) and a new powder layer is applied (4); cf. [5].

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After the solidification, the process platform movesone layer thickness downwards. Depending on the powdersize and the material, it is possible to implement layerthicknesses between 50 µm and 150 µm. A new powderlayer is applied in the last step. For this purpose, the rakefirstly moves in the powder pile in front of the powdertank. This makes a sufficient quantity of powder fall on tothe other side of the rake. These particles are now distri -buted in the build tank. For the next layer, this processtakes place inversely at the other powder tank. The re-sulting material properties of the manufactured compo-nents are essentially dependent on the quality of the pow-der. A high relative density of the powder layer is desiredin general and is guaranteed by a high flowability of thepowder. For this reason, spherical powder particles withsizes between 40 µm and 105 µm from an atomisation orrotation process are used in most cases. The proportionof the fine fraction under 40 µm and on satellites (minuteparticles adhering to normal powder particles) should bekept as low as possible since these drastically reduce theelectrical conductivity and the flowability.

3 Materials Currently, there is just one manufacturer which com-mercially offers installations for selective electron beammelting (Arcam AB, Sweden). Therefore, the commercialavailability of materials which can be processed withSEBM is still extremely restricted at present. However, inthe research field, a large number of alloys have alreadybeen successfully processed into totally leak-tight com-ponents. In this respect, the number of available materialsand material systems is constantly increasing even furtherand is thus extending the potential application possibilitiesof this procedure. Table 1 gives an overview of the mate-rials already processed with SEBM today. At the beginning of the introduction of the procedureat the end of the 1990s, interest primarily centred on toolsteel, e.g. in order to implement casting shapes with cool-ing operations close to the contours. Today, attention isfocusing on applications in medical technology or in avi-ation. In association with these, the materials mainlyprocessed with SEBM have also been developed awayfrom steels (316L and H13) towards Co alloys(Co-Cr-Mo-C) and, above all, towards titanium (pure ti-tanium and TiAl6V4). Moreover, nickel-based alloys (e.g.

IN718) and, to an increasing extent, titanium aluminidesare playing major roles nowadays. The great advantage of the SEBM procedure is beingshown with regard to the extension of the material range,precisely for complex material systems, since the high lo-cal cooling rates result in very fine and homogeneousstructures. This leads to outstanding material properties.In this respect, the structural fineness may be two ordersof magnitude below that of cast alloys [7]. Moreover, thereare approaches for using suitable beam manipulationstrategies for the targeted setting of the structure accordingto the requirements [17]. For example, the typically colum-nar structure in the building direction can be transformedinto an equiaxed structure by the intended formation ofnew nuclei, Fig. 7 (left). In general, material properties which are the absoluteequal of or frequently even considerably better than thoseof conventional manufacturing technologies can beachieved nowadays with the correct process strategy. Theproduction of completely leak-tight components is a matterof course in the meantime. The powder quality tends toconstitute a problem since gas pores introduced into the

Fig. 2 • Various cellu-lar structures made ofTiAl6V4, in part withauxetic strain behav-iour (negative trans-verse contraction)(left), skull implantmade of TiAl6V4 withan open-pore structurefor medical technology(right); source:www.arcam.com.

Table 1 • Overview of the investigated material groups for theSEBM process; cf. [3].

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powder by the atomisation process can be found in thecomponent after the SEBM process. That explains a fre-quently arising residual porosity of max. approx. 0.5% which,however, can be removed, for example, by downstream hotisostatic pressing (HIP), in so far as this is necessary.

4 Applications One major advantage of additive manufacturing is thealmost unrestricted design freedom. This is particularlygreat in the case of SEBM since the powder bed not onlyserves as a support for any overhangs but also has a highmechanical stability due to the slight initial sintering inthe preheating step, e.g. also in comparison with SLM. Inconnection with the low inclination to distortion as a resultof the high preheating temperature, hardly any supportingstructures are needed for SEBM and, apart from totallyfinished cavities (from which the powder can then nolonger be removed), there are unlimited freedoms withregard to the component designing. In this respect, thegreat variability of the electron beam permits the simul-taneous production of the geometry and the structure andthus the setting of the material properties. However, it isnecessary to take account of the in-situ heat treatmentdue to the increased process temperature and its influ-ence, for example, on precipitation processes. A few pos-sible applications of SEBM are shown below. The SEBM procedure makes it possible to manufac-ture cellular structures which could not be manufacturedin this way by conventional means, Fig. 2. Thus, hybridstructures consisting of solid and porous regions are ap-plied in medical technology today, e.g. as implants whichare adapted to the patient and can be supplied in an ex-act-fitting form within just a few days on the basis of ascan of the patient’s bone structure. That increases thesuccess of the treatment and reduces the operation stressfor the patient. Moreover, metamaterials are easy to pro-duce with the SEBM procedure. These materials receivetheir properties from their structures. For example, it ispossible to manufacture materials with negative Poisson’sratios, so-called auxetic structures, or even materials withan acoustic band gap which completely eliminate me-chanical oscillations in a certain frequency range. Moreover, cellular structures can be utilised in orderto design structured reactors with adjusted reactions [23].One example of this is illustrated on Fig. 3 (left). The re-actor was manufactured from TiAl6V4 with SEBM andserves as the carrier material for a subsequent catalyst

coating. The reaction to be carried out in it is the releaseof hydrogen from perhydro-N-ethylcarbazole, a liquid or-ganic hydrogen carrier [24]. With regard to the heat trans-port and the pressure loss, the structured reactor is farsuperior to the classic powder bed reactor. Furthermore,only additive manufacturing is feasible for reactor ele-ments such as the illustrated flue gas tube for the quickdissipation of the gaseous hydrogen or even integratedcooling ducts. The combination of the high thermal con-ductivity of pure copper with the design freedom due toSEBM also makes it conceivable to manufacture complexheat exchangers which can be exactly oriented to an ex-isting heat transmission problem, Fig. 3 (right). The SEBM technology shows further advantages dur-ing the processing of high-temperature materials such asnickel-based alloys and titanium aluminides (TiAl). Thepossibility of making the electron beam scan over thepowder bed in a defocused mode at high deflection speedsduring the preheating permits a high energy input withoutmelting the powder completely. Thus, the entire powderbed can be constantly kept at temperatures above 1,000°Cduring the construction phase. With this strategy, residualstresses, particularly in thick-walled components, can bereduced effectively and high-performance materials suchas TiAl can be processed. The use of TiAl for rotating parts, e.g. turbine bladesor turbocharger wheels, is particularly attractive as a resultof the low density and the low moments of inertia. Untilnow, more complex components made of TiAl have beenmanufactured almost exclusively by means of centrifugalcasting. Selective electron beam melting constitutes analternative with various advantages. The SEBM processworks in a vacuum. Therefore, no contamination takesplace in spite of the high affinity of the TiAl materials toreactions with other elements. Moreover, any reactionsof TiAl with the crucible material and thus any possiblechanges in the properties can be avoided since it is set upin the powder bed without any tools. The rapid solidifica-tion leads to extremely fine microstructures and a homo-geneous distribution of the alloying elements. Any demix-ing processes are suppressed. In the case of electron beammelting, supporting structures which turn out to be smallin comparison with the casting residues in the centrifugalcasting procedure are utilised in a targeted way for theformation of distortion-free components. In interplay withthe powder recycling after the manufacturing, this resultsin good material utilisation.

Fig. 3 • Structured reactormade of catalyst-coatedTiAl6V4 in order to releasehydrogen from a liquid or-ganic hydrogen carrier(LOHC) (left), prototype ofa heat exchanger structuremade of pure copper(right).

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The specified advantages can also be transferred tothe processing of nickel-based alloys with high γ contents.In combination with the design freedom, it is possible toimplement turbine blades with optimised cooling struc-tures, Fig. 4. It is disadvantageous that the high construc-tion space temperatures and the associated sinteringprocess lead to a material-locking joint between the indi-vidual powder particles in the powder bed (formation ofsintering necks). These joints prevent the powder fromsimply falling out of the regions with difficult accessibility(cooling structures). Therefore, post-processing is re-quired. Within the framework of the development activi-ties, investigations are currently being conducted intopromising procedures for the post-processing of coolingstructures. Furthermore, high surface roughnesses(Ra > 25 µm) arise during the SEBM process due to theinstallation-specific beam diameter (> 150 µm), the soft-ware for controlling the beam and the utilised powderfraction (45 to 150 µm). Mechanical and electrochemicalpost-processing procedures are utilised for the subsequentreduction in the surface roughness.

5 Simulation Today, many different materials can already beprocessed with selective electron beam melting. In part,process windows are available for a stable procedure withsufficient material properties of the manufactured com-ponents. However, these findings are mostly bought at ahigh price in the form of many experiments. For this rea-son, a lot of mechanisms which give rise to certain mate-rial properties have not yet been understood correctlyuntil today. Process observation constitutes a major prob-lem since, for example, temperature measurements canbe taken to an inadequate extent only. Furthermore, manyprocesses, such as the molten pool dynamics, take placeon a very small timescale with which measuring systemshave difficulties with regard to the resolution. For this rea-son, an accompanying numerical simulation is extremelysensible in addition to experimental work. On the onehand, the high-dimensional spaces of the process para -meters such as beam power, scanning speed, line spacing,layer thickness, powder sizes etc. can be delimited to sen-sible subspaces before any experimental work. On theother hand, it is possible to investigate and identify theunderlying mechanisms of the material consolidation andthe resulting material properties. The resulting material properties are dependent on thequalities and compositions of the powder and the powderlayer. For this reason, a mesoscopic simulation approachwhich is capable of ensuring the resolution of the individualpowder particles is ideally suitable for identifying the un-derlying mechanisms. A three-dimensional mesoscopicsimulation with the powder bed, the molten pool includingthe molten pool dynamics and the electron beam is por-trayed on Fig. 5. Furthermore, the most important physicalphenomena during selective electron beam melting whichare considered by the simulation are listed on Fig. 5. The most important aspect during the simulation ofselective electron beam melting is the correct modellingof the thermal balance of the process. Nearly all the modi -fications to the process parameters have influences on

Fig. 4 • Additively manufactured turbine blade made of a nickel-based alloy with a cooling structure (front and rearsides). The blade root has been post-processed mechanically.The blade shows the as-built condition.

Fig. 5 • Essential physical phenomena during selective electron beam melting. Representation of the powder bed (blue), the elec-tron beam (red) and the molten pool (temperature distribution from white to red) including the molten pool dynamics (arrows) [5].

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the heat conduction, the energy input by the electronbeam or the heat loss caused, for example, by heat radia-tion or evaporation. Furthermore, a lot of material pa-rameters are temperature-dependent and thus sensitiveto correct modelling. The complete melting of the materialresults in a molten pool whose dynamics are essentiallydetermined by capillarity, evaporation pressures, wettingeffects, Marangoni flows and gravitation. During the so-lidification, the temperature gradient and the solidificationrate have decisive influences on the occurring microstruc-ture unless this is destroyed once again by solid phasetransformations. By applying numerical simulation [25...27], it was pos-sible to explain different phenomena arising during theSEBM process. One such phenomenon is, for example,the characteristic of tunnel porosity. Any kind of porosityshould be minimised in order to achieve good mechanicalproperties. The porosity structures for an experiment (topleft) and a simulation (top right) with existing tunnelporosity are illustrated on Fig. 6. It is possible to explainthis phenomenon by considering the development of animperfection over many layers in the simulation (bottom).Due to the low power of the electron beam and the sto-chastically unfavourable positions of the particles in thepowder bed, the molten pool can be divided into twosmaller ones. The surface tension curves both moltenpools at the separating point and establishes a small im-perfection. Since the melting depth with the electron beamis not sufficient in order to melt this defect completely,the molten pool is now separated at this defect time andtime again and both parts are drawn to the sides of thedefect by capillary forces. Thus, the imperfection may per-sist over many layers and the tunnel porosity occurs [28]. By choosing suitable parameters, though, it is possibleto manufacture compact and leak-tight componentswhich no longer differ in the transverse sections. However,both columnar-crystalline and polycrystalline structures,as illustrated on Fig. 7, can be recognised on EBSD (elec-tron backscatter diffraction) orientation maps. One aimduring the manufacturing is to be able to set this structurefreely in order to exert targeted influences on materialproperties in a component. Experimental results showthat the solidification direction of the molten pool playsan essential role in this respect. The greater the deviationsfrom the building direction are, the more likely a poly-

crystalline structure is to result from the formation of newgrains on the bottom of the molten pool. However, it isunexplained why this does not coincide with the classicmodel for CET (columnar-to-equiaxed transition). On Fig.7 (right), the solidification rates at the solidification frontare illustrated over the temperature gradient from data inthe numerical simulation for two different sets of param-eters. These results contradict the CET in two aspects.Firstly, the formation of new grains would be expected atthe end of the solidification of the molten pool on thebasis of the results whereas new grains form at the bottomof the molten pool. Secondly, the assignment of colum-nar-crystalline and polycrystalline structures should beprecisely opposite to the process parameters on the basisof the solidification conditions [29]. These contradictionscould be revealed with the aid of numerical simulationand are currently the subject of the research into the mod-elling of the texture characteristic, especially the formationof new grains, during selective electron beam melting.

6 Concluding remarks Selective electron beam melting is suitable for manu-facturing technically sophisticated products and offersthe possibility of processing high-performance alloys. Inthis respect, additive building-up permits complex geo-metrical shapes which are subject to hardly any restric-tions. These possibilities are reflected in the diversity ofthe technological applications. Nevertheless, the furtherindustrial success of this procedure needs continuous re-search into new materials, better process observation andprocess control as well as understanding for process andcomponent optimisation measures. At present, a wide material range of the most diversematerial systems is the subject of the research. In this re-spect, the complexity of the materials and the require-ments on the processing are increasing continuously. Outof numerous application possibilities as individual com-ponents and small-scale series, reactors as a combinationof cellular and solid structures and turbine blades withinternal cooling ducts were described here. In addition to applications of this procedure, simula-tion is an important pillar for better understanding of theprocess. The development of the tunnel porosity or thetexture characteristic was specified as an example butmany other subject fields such as alterations in the alloy

Fig. 6 • Tunnel porosity inthe experiment (top left)

and in the simulation (topright) during the manu-facturing of cube speci-

mens made of TiAl6V4 us-ing hatches without con-tours. Representation of

the origination of thetunnel porosity by the

simulation (bottom) [28].

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composition due to selective evaporation [30] or dimen-sional accuracies resulting from residual stresses and dis-tortion [31] are the subjects of the research here. Apart from these research fields, the connections be-tween material properties, process strategies and processoptimisation must be investigated in greater detail. Withthis knowledge, it should, in the future, be possible to re-place the control of installations with a regulation systemwhose decision algorithms can be obtained by combiningexperiments and simulations, based on process observa-tion, in order to be able to manufacture high-quality prod-ucts in a reproducible way.

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ACKNOWLEDGEMENTSThe authors thank the German Research Federation (DFG) forpromoting the SFB 814: “Additive manufacturing“ (SubprojectB4) and SFB/Transregio 103: “Superalloy single crystals“ pro -jects as well as the “Engineering of advanced materials“ (EAM)excellence cluster, the European Union (EU) for promoting the“SimChain“ and “AMAZE“ projects as well as the BavarianState Ministry of Economic Affairs and Media, Energy and Tech-nology for promoting the Application Centre for Process Engi-neering (VerTec) at the Central Institute for New Materials andProcess Engineering (ZMP).

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[31] Jamshidinia, M., F. Kong and R. Kovacevic: The coupledCFD-FEM model of electron beam melting (EBM). ASMEDistrict F ECTC Proceedings 12 (2013), pp. 163/171.

Welding and Cutting – editorial previewIssue 4 (July/August) Issue 5 (September/October)• Fair Issue “Schweissen & Schneiden 2017“ • Adhesive bonding technology• Examples of applications in welding and cutting technology • Welding and brazing of lightweight constructions

Closing date for editiorial contributions: 14 July 2017 Closing date for editiorial contributions: 12 September 2017Closing date for advertisements: 26 July 2017 Closing date for advertisements: 29 September 2017

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