Hybrid Approaches for Selective Laser Sintering by Building on Dissimilar Materials

materials

Article Hybrid Approaches for Selective Laser Sintering by Building on Dissimilar Materials

Babette Goetzendorfer *, Thomas Mohr and Ralf Hellmann Applied Laser and Photonics Group, University of Applied Sciences Aschaﬀenburg, Wuerzburger Str. 45, 63743 Aschaﬀenburg, Germany; [email protected] (T.M.); [email protected] (R.H.) * Correspondence: [email protected]

Received: 20 September 2020; Accepted: 17 November 2020; Published: 22 November 2020

Abstract: We introduced a new approach in selective laser sintering for hybrid multicomponent systems by fabricating the sintered polyamide 12 (PA12) part directly onto a similar (PA12) or dissimilar (polyamide 6 (PA6) and tool steel 1.2709) joining partner. Thus, the need for adhesive substances or joining pressure was completely circumvented, leading to the possibility of pure hybrid lightweight bi-polymer or metal–polymer systems. By taking advantage of the heating capabilities of the sinter laser regarding the substrate surface, diﬀerent exposure strategies circumvented the lack of overlapping melting temperatures of dissimilar polymers. Therefore, even sintering on non-PA12 polymers was made possible. Finally, the transfer on metallic substrates—made up by selective laser melting (SLM)—was successfully performed, closing the gap between two powder-based additive processes, selective laser sintering (SLS) and SLM.

Keywords: hybrid additive manufacturing; selective laser sintering; multicomponent process

1. Introduction Hybrid components consisting of metal and polymer have recently gained considerable attention in numerous sectors of industry such as automotive [1–3], electronics [4], medical [5], or aerospace engineering [6,7]. By combining the advantages of both materials, entirely new functional characteristics are attainable [8,9]. In accordance with the intended application, for example, lightweight components, different aspects such as weight, improved mechanical behavior, adapted thermal conductivity, and lower production cost might be considered [8,10]. Several approaches have been made for developing multicomponent parts, most of which assemble two finished elements by joining them, e.g., through mechanical fastening or the use of an adhesive [11–13]. Unfortunately, mechanically joined hybrid components using processes such as riveting, screwing, or jacking often exhibit disadvantageous load transmission, which may destabilize the joining area [14,15]. Adhesive technologies may circumvent this drawback. However, introducing adhesives is a sensitive issue, for example, in biomedical applications, and increases production complexity and costs. Laser-based joining has been put forth as an alternative to adhesives [16,17]. However, as this joining process requires specific joining conditions such as an initial surface treatment [18] and the application of pressure [19], complex structures for, e.g., sophisticated lightweight applications cannot be processed by using this technique. A possible approach to realizing hybrid, adhesive-free, lightweight components is additive manufacturing (AM) for at least one of the joining partners [20]. Employing the advantages of AM [21–24], several attempts at hybrid fabrications have been made and reviewed for multi-material applications by Vaezi et al. [25]. Additionally, a combination of additive metal manufacturing and conventional substractive processes such as milling has been demonstrated [26,27]. Moreover,

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different additive technologies have been combined in hybrid approaches, such as stereolithography twowith di selectivefferent additive laser melting technologies (SLM) have [28,29], been combinedone‐photon in hybrid and two approaches,‐photon polymerization such as stereolithography [30], and withmultijet selective combined laser melting with Fused (SLM) Deposition [28,29], one-photon Modelling and (FDM) two-photon [29]. Furthermore, polymerization FDM [30 ],on and a metallic multijet combinedsurface has with been Fused demonstrated Deposition to obtain Modelling hybrid (FDM) objects [29 [31].]. Furthermore, In general, hybrid FDM on structures a metallic consisting surface hasof metal been and demonstrated polymer elements to obtain reveal hybrid a objectssignificant [31]. dependence In general, hybrid of the joining structures quality consisting on the of surface metal andstructure polymer of elementsthe metallic reveal mating a significant part as dependence shown in of[25,27,29,32–34], the joining quality as onthe the connection surface structure area is ofcharacterized the metallic by mating mechanical part as binding shown effects. in [25,27 ,29,32–34], as the connection area is characterized by mechanicalAgainst binding this background, effects. one aim of this study is to investigate the suitability of SLM‐built metal devicesAgainst as part this of background,hybrid polymer–metal one aim of objects, this study as this is technology to investigate offers the the suitability possibility of to SLM-built produce metalcontrolled devices surface as part geometries of hybrid of various polymer–metal types [35,36]. objects, Pretreatment as this technology processes o toff ersmodify the possibility conventional to producemetallic controlledsurfaces can surface thus geometriesbe circumvented, of various facilitating types [35 experimental,36]. Pretreatment investigations processes of to modifysurface conventionalbinding correlations. metallic surfaces can thus be circumvented, facilitating experimental investigations of surfaceAs bindinga polymer correlations. partner, (SLS) as an industrially well‐established additive manufacturing technologyAs a polymer was introduced partner, (SLS) as an as anAM industrially process to well-established produce hybrid additive structures. manufacturing First, SLS of technology standard waspolyamide introduced (PA12) as anon AMsimilar process material to produce was performed. hybrid structures. Then, the First,process SLS was of standardadapted polyamidein order to (PA12)combine on different similar material polyamides was performed. (PA12 on PA6). Then, After the process that, the was application adapted in was order extended to combine to dissimilar different polyamidesmaterial partners (PA12 onby PA6).sintering After PA12 that, on the metal application parts wasthat, extended in turn, had to dissimilar been built material by selective partners laser by sinteringmelting. PA12 on metal parts that, in turn, had been built by selective laser melting. 2. Materials and Methods 2. Materials and Methods SLS was performed using an EOS Formiga P110 (EOS GmbH, Krailling, Germany) sintering SLS was performed using an EOS Formiga P110 (EOS GmbH, Krailling, Germany) sintering PA12 polyamide standard material (PA2200, EOS GmbH, Krailling, Germany) with a 30 W CO2 laser PA12 polyamide standard material (PA2200, EOS GmbH, Krailling, Germany) with a 30 W CO2 laser (spot diameter, 500 µm). A custom-built ground plate allowed for the implementation of different (spot diameter, 500 μm). A custom‐built ground plate allowed for the implementation of different basic substrates (PA12, PA6 (smapla GmbH, Koblenz, Germany), SLM part) into the building chamber basic substrates (PA12, PA6 (smapla GmbH, Koblenz, Germany), SLM part) into the building (Figure1). First, the substrate was mounted onto the custom-built ground plate (Figure1a) to generate chamber (Figure 1). First, the substrate was mounted onto the custom‐built ground plate (Figure 1a) a defined and fixed basic area. Second, the remaining space was filled with PA12 powder to level to generate a defined and fixed basic area. Second, the remaining space was filled with PA12 powder out the total build plate for the first building layer. Figure1b shows a mounted PA6 substrate (black) to level out the total build plate for the first building layer. Figure 1b shows a mounted PA6 substrate embedded in PA12 powder, prepared for starting the hybrid process at layer height zero. (black) embedded in PA12 powder, prepared for starting the hybrid process at layer height zero.

(a) (b)

Figure 1.1. (a): Polyamide Polyamide 12 12 (PA12) substratesubstrate ﬁxedfixed ontoonto thethe custom-builtcustom‐built groundground plate.plate. ((bb):): PA6 substrate embeddedembedded inin PA12.PA12.

To access access and and evaluate evaluate the the joining joining strength strength of sinteredof sintered material material on di onfferent different base plates,base plates, tensile tensile shear testsshear in tests end in lap end adhesive lap adhesive sealing geometrysealing geometry were performed were performed according according to DIN EN to 1465 DIN [37 EN], as1465 illustrated [37], as inillustrated Figure2a,b. in Figure The DIN 2a,b. standard The DIN is standard mainly used is mainly for comparative used for comparative purposes andpurposes therefore and gavetherefore the opportunitygave the opportunity to judge and to evaluatejudge and the evaluate different the hybrid different processes hybrid within processes this study. within Nevertheless, this study. theNevertheless, described hybrid the described approach hybrid was notapproach defined was in a not standard defined procedure in a standard until procedure now, so that until adaption now, so of adhesivethat adaption standards of adhesive was necessary. standards was necessary. Tensile shear tests were performed using a universal testing machine (Xplus, Shimadzu AG, Shimadzu Deutschland GmbH, Duisburg, Germany). A test method analogue to tensile shear tests

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(a) (b) (c)

FigureFigure 2. 2. GeometryGeometry of of sintered sintered test test specimen specimen on on basic basic substrate substrate plate. plate. (a ()a Top) Top view. view. (b ()b Perspective) Perspective view.view. After After sintering, sintering, the the hybrid hybrid print print plate plate is is separated separated into into ten ten single single test test specimens. specimens. (c ()c Reference) Reference samplesample as as benchmark benchmark object object (binding (binding layers layers 5 5× 7 mm7 mm2 grey).2 grey). × ForTensile comparison shear tests reasons, were reference performed PA12 using test a universal structures testing were machinedeveloped (Xplus, and characterized. Shimadzu AG, FigureShimadzu 2c depicts Deutschland the geometry GmbH, Duisburg,of these specimens, Germany). which A test methodwere printed analogue with to tensilestandard shear building tests for parameterslaser-joined en specimen bloc in contrast (5 mm/ min)to the was aimed chosen hybrid due process to the very (basic similar plates geometric thickness: structure. 5 mm; joining The resulting area: 5 maximum× 7 mm2). forceA tensile was shear normalized strength to theof 25.9 joining MPa area (σ of= 0.58) 5 7 was mm 2obtained,. which can be defined as × maximumFor comparisonvalue for the reasons, given geometry, reference as PA12 the continuous test structures building were process developed should and lead characterized. to optimal mechanicalFigure2c depicts properties. the It geometry is worthwhile of these to stress specimens, that all which tested werehybrid printed‐built specimens with standard broke building outside theparameters latter contact en bloc area in of contrast the hybrid to the objects aimed (highlighted hybrid process grey (basic layers plates in Figure thickness: 2c), 5proving mm; joining that the area: specific5 7 mm test2 geometry). A tensile of shear the specimen strength does of 25.9 not MPa represent (σ = 0.58) a predetermined was obtained, breaking which canpoint be itself. defined as × maximumAs the shear value strength for the givenof hybrid geometry, components as the is continuous directly linked building with processthe quality should of the lead joining to optimal area, themechanical building parameters properties. within It is worthwhile the very first to stress layers that are all determinative, tested hybrid-built and thus, specimens the energy broke density outside ofthe these latter layers contact was experimentally area of the hybrid varied. objects For (highlightedSLS build parts, grey the layers mechanical, in Figure e.g.,2c), tensile, proving properties that the arespecific sensitive test to geometry and depend of the on specimen the deposited does not energy represent absorbed a predetermined by the powder breaking during pointlaser itself.scanning [38,39].As The the amount shear strength of deposited of hybrid energy components E during is the directly CO2‐ linkedlaser process with the can quality be calculated of the joining by the area, appliedthe building laser average parameters power within P divided the very by firstthe product layers are of determinative, scanning speed and v and thus, hatch the energydistance density H [38]. of Inthese this study, layers waswe kept experimentally the hatch distance varied. For constant SLS build at 0.2 parts, mm, the since mechanical, on the one e.g., hand tensile, larger properties distances are leadsensitive to porous to and textures depend with on the inferior deposited mechanical energy properties, absorbed by and the on powder the other during hand laser smaller scanning distances [38,39 ]. resultThe amountin deformation of deposited of the energy built structure. E during Several the CO 2studies-laser process have analyzed can be calculated the dependence by the appliedof the meltinglaser average temperature power on P divided the energy by the density product in ofthe scanning SLS process speed [40–44], v and hatch providing distance typical H [38 ].energy In this densitiesstudy, we in keptthe range the hatch of 16–50 distance mJ/mm constant2 for stable at 0.2 mm,sintering since processes. on the one While hand largervarying distances the scanning lead to speedporous up textures to 3500 mm/s with inferiorand adapting mechanical the laser properties, power to and maintain on the the other applied hand energy smaller density distances leads result to comparablein deformation results of thein terms built structure.of part quality Several and studies mechanical have analyzedproperties, the we dependence chose 1500 of mm/s the melting as an intermediatetemperature scanning on the energy speed densitythat facilitates in the SLSsufficiently process good [40–44 results], providing by varying typical only energy the laser densities power in andthe thereby range of covers 16–50 the mJ preferable/mm2 for stable energy sintering density. processes. While varying the scanning speed up to 3500In mm our/ sstudy, and adapting the mechanical the laser strength power to of maintain the hybrid the parts applied was energy determined density by leads the quality to comparable of the contactresults zone in terms between of part substrate quality and and molten mechanical‐layered properties, PA12 powder, we chose analogue 1500 mm to /singles as an material intermediate SLS partsscanning described speed in that [45]. facilitates With a PA12 sufficiently layer height good results of 100 μ bym varying and the only penetration the laser depth power of and the thereby CO2‐ lasercovers (emission the preferable wavelength energy 10.6 density. μm) in the order of 120 μm for standard PA12 [46], the notable bindingIn area our study,comprised the mechanical the surface strength of the basic of the plate hybrid and parts the wasfirst determined three sintered by thelayers quality at most. of the Startingcontact from zone layer between 4, the substrate sintering and parameters molten-layered did not PA12 affect powder, the binding analogue area to any single further, material so that SLS parameterparts described variation in [45 ].was With not a PA12required layer heightand standard of 100 µm building and the penetration parameters depth (EOS of thevalues CO2 -laserare unpublished)(emission wavelength lead to constant 10.6 µm) properties in the order within of 120 theµ testm for specimen. standard PA12 [46], the notable binding area comprisedAs the layer the surface thickness of the of basicthe very plate first and layer the firstcould three be reduced sintered to layers 50 μ atm, most. the impact Starting of fromthe laser layer on4, the the very sintering first contact parameters layer did could not be aff additionallyect the binding increased. area any This further, lower so limit that of parameter 50 μm was variation given by the machine accuracy and the PA12 powder distribution [47]. The particle size distribution (PSD) Materials 2020, 13, 5285 4 of 11 was not required and standard building parameters (EOS values are unpublished) lead to constant properties within the test specimen. As the layer thickness of the very first layer could be reduced to 50 µm, the impact of the laser on the very first contact layer could be additionally increased. This lower limit of 50 µm was given by theMaterials machine 2020, accuracy13, x FOR PEER and REVIEW the PA12 powder distribution [47]. The particle size distribution (PSD)4 of 12 of the used PA12 powder was investigated with a Retsch Camsizer X2, based on dynamic digital image of the used PA12 powder was investigated with a Retsch Camsizer X2, based on dynamic digital analysis methods according to ISO 13322-2. The found diameters D10, D50, and D90 corresponded, image analysis methods according to ISO 13322‐2. The found diameters D10, D50, and D90 respectively, to the particle sizes equal or lower to which the 10%, 50%, and 90% of the PSD was corresponded, respectively, to the particle sizes equal or lower to which the 10%, 50%, and 90% of the µ µ µ enclosed.PSD was Here, enclosed. D10 (36.7Here, D10m), D50(36.7 (51.7 μm), D50m), and(51.7 D90 μm), (69.4 and D90m) were (69.4 in μm) well were agreement in well withagreement previous µ PA2200with previous powder PA2200 characterizations powder characterizations [48], underlining [48], thatunderlining enough that particles enough smaller particles than smaller 50 mthan were available50 μm were to create available a 50 toµ mcreate layer. a 50 μm layer. InIn Figure Figure3, the3, the schematic schematic view view on theon connectionthe connection layers layers is illustrated. is illustrated. Variation Variation of the of first the binding first layerbinding height layer from height 50, from 80, 100, 50, and80, 100, 120 andµm 120 resulted μm resulted in reproducible in reproducible binding binding effects effects for 50 for and 50 and 80 µ m. Thicker80 μm. layers Thicker in thelayers range in the of therange penetration of the penetration depth prevented depth prevented reliable bindingreliable duebinding to insu dueffi tocient energyinsufficient density energy on the density contact on area. the contact Therefore, area. a Therefore, first layer a height first layer of 50 heightµm was of 50 chosen μm was for chosen the following for experimentalthe following study. experimental To additionally study. To evaluate additionally the evaluate influence the of influence the applied of the fluence, applied the fluence, tensile the shear strengthtensile shear was determined strength was for determined energy densities for energy in thedensities range in between the range 17 between and 83 mJ 17/ mmand 283. mJ/mm2.

(a) (b)

FigureFigure 3. 3.Schematic Schematic view. (( aa)) Perspective Perspective view, view, (b (b) side) side view)) view) on on the the three three binding binding layers layers (dark (dark grey). grey).

AsAs basic basic substrates, substrates, PA12PA12 plates were sintered sintered on on the the used used EOS EOS Formiga Formiga P110 P110 with with standard standard parameters,parameters, whereas whereas PA6PA6 substratessubstrates were injection injection-molded‐molded plates. plates. To To implement implement a metallic a metallic part, part, SLMSLM plates plates (steel (steel tool tool 1.2709)1.2709) were produced produced on on a a DMG DMG Mori Mori LaserTec LaserTec 30 30 (DMG (DMG Mori Mori AG, AG, Bielefeld, Bielefeld, Germany).Germany). By By adjusting adjusting the energy density density on on a a slightly slightly deficient deficient energy energy level, level, leading leading to insufficient to insufficient layerlayer melting, melting, aa stochasticallystochastically porous surface surface structure structure of of the the SLM SLM part part was was obtained. obtained. Optical Optical measurementsmeasurements were were performed performed using a a Keyence Keyence VR VR-3200‐3200 profilometer profilometer (Keyence (Keyence Deutschland Deutschland GmbH, GmbH, Neu‐Isenburg, Germany). Neu-Isenburg, Germany).

3.3. Results Results and and Discussion Discussion In a first attempt, SLS was performed on similar base material (PA12). A seperately sintered In a first attempt, SLS was performed on similar base material (PA12). A seperately sintered PA12 basic substrate was mounted into the build chamber (Figure 1), and the PA12 join partner was PA12 basic substrate was mounted into the build chamber (Figure1), and the PA12 join partner was sintered onto the surface. Figure 4 depicts the tensile shear strength as a function of applied energy sintered onto the surface. Figure4 depicts the tensile shear strength as a function of applied energy density. Hatch distance (0.2 mm) and scan speed (1500 mm/s) were kept constant. Below 33 mJ/mm2, density. Hatch distance (0.2 mm) and scan speed (1500 mm/s) were kept constant. Below 33 mJ/mm2, no binding occurs. no binding occurs. In the studied range, the resulting tensile shear strength revealed only a minor dependence, varying from 23 to 25 MPa, on the applied energy density, indicating no distinct correlation. As the internal reference specimen exhibited maximum shear strength of 25.9 MPa, the obtained values reached near optimal binding behavior. In order to investigate the effect of the scan speed, ten different speed values ranging from 250 to 3500 mm/s with constant energy density of 33 mJ/mm2 were considered. Within this range, the resulting tensile shear strength was 24.7 MPa (σ = 1.52).

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28 27 26 25 24 23 Materials 2020, 13, 5285 5 of 11 Materials 2020, 13, x FOR PEER22 REVIEW 5 of 12 21 2820 27

Tensile Shear Strength [MPa] 19 26 20 30 40 50 60 70 80 90 25 Energy Density [mJ/mm²] 24 23 Figure 4. Tensile shear strength of hybrid produced similar material specimen (PA12 on PA12). 22 In the studied range,21 the resulting tensile shear strength revealed only a minor dependence, varying from 23 to 2520 MPa, on the applied energy density, indicating no distinct correlation. As the

internal reference specimenTensile Shear Strength [MPa] 19 exhibited maximum shear strength of 25.9 MPa, the obtained values reached near optimal binding20 behavior. 30 40 50 60 70 80 90 In order to investigate the effect of theEnergy Density [mJ/mm²] scan speed, ten different speed values ranging from 250 2 to 3500Figure mm/s 4. withTensile constant shear strength energy of density hybrid produced of 33 mJ/mm similar materialwere considered. specimen (PA12 Within on PA12).this range, the resultingFigure tensile 4. Tensile shear shear strength strength was of 24.7 hybrid MPa produced (σ = 1.52). similar material specimen (PA12 on PA12). AsAs the the original original hybrid hybrid approach approach included included two two components components (basic (basic plate plate and and sintered sintered part) part) made made viavia the Inthe samethe same studied SLS SLS process, process, range, a thea reference reference resulting experiment experiment tensile shear was was performed.strength performed. revealed First, First, the theonly basic basic a minor plate plate structure dependence,structure was was sintered;varyingsintered; from then, then, 23 the theto process25 process MPa, was onwas the paused paused applied for for energy 0 /0/60/12060/120 density, min, min, respectively. indicatingrespectively. no Followed Followeddistinct correlation. by by that, that, the the As three three the crucialinternalcrucial binding bindingreference layers layers specimen (Figure (Figure exhibited3, 3, grey) grey) were weremaximum sintered sintered shear with with definedstrength defined building of building 25.9 parameters.MPa, parameters. the obtained Starting Starting values with with layerreachedlayer 4, 4, the nearthe standard standard optimal building buildingbinding parameters behavior. parameters were were used used as as described described above. above. Figure Figure5 depicts5 depicts the the resulting resulting shearshearIn strength strengthorder to values. values.investigate the effect of the scan speed, ten different speed values ranging from 250 to 3500 mm/s with constant energy density of 33 mJ/mm2 were considered. Within this range, the resulting tensile shear strength30 was 24.7 MPa (σ = 1.52). As the original hybrid approach included two components (basic plate and sintered part) made via the same SLS process,27.5 a reference experiment was performed. First, the basic plate structure was sintered; then, the process was paused for 0/60/120 min, respectively. Followed by that, the three crucial binding layers (Figure25 3, grey) were sintered with defined building parameters. Starting with layer 4, the standard building parameters were used as described above. Figure 5 depicts the resulting 22.5 shear strength values. 17mJ/mm² 20 30 33 mJ/mm² 17.5 50 mJ/mm² 27.5 Tensile Shear Strength [MPa] 67 mJ/mm² 15 25 0 30 60 90 120 Build Stop [min] 22.5 Figure 5. Tensile shear strength of reference samples, imitating the hybrid building process. Figure 5. Tensile shear strength of reference samples, imitating17mJ/mm² the hybrid building process. 20 With no interruption to the build process, all four energy densities33 mJ/mm² lead to a stable object. The tensile With no interruption to the build process, all four energy densities lead to a stable object. The shear strength rose with17.5 higher energy densities, implying the50 mJ/mm² known correlation between binding tensile shear strength rose with higher energy densities, implying the known correlation between layer parameters andTensile Shear Strength [MPa] mechanical properties in our hybrid system67 mJ/mm² as well. If a timeout of 60 min was binding layer parameters and mechanical properties in our hybrid system as well. If a timeout of 60 forced before sintering of15 the first binding layer started, an energy density of 17 mJ/mm2 was no longer min was forced before sintering of the first binding layer started, an energy density of 17 mJ/mm2 sufficient to build a stable hybrid0 object. 30 Further, the 60 tensile shear 90 strength at 120 higher energy densities was no longer sufficient to build a stable hybrid object. Further, the tensile shear strength at higher was reduced. For even longer interruptions (120Build Stop [min] min), this effect became more pronounced, with no binding at all for fluences below 50 mJ/mm2. Notably,Figure all 5. samplesTensile shear whose strength build of reference process wassamples, interrupted imitating brokethe hybrid at the building joining process. area, despite relatively high tensile shear strength. We, therefore, assume that interrupting the building process With no interruption to the build process, all four energy densities lead to a stable object. The leads to a kind of predetermined breaking point. This effect may be partially attributed to a shrinkage tensile shear strength rose with higher energy densities, implying the known correlation between of the sintered PA12, having a higher density (0.93 g/cm3) as pulverulent PA12 (0.43 g/cm3). As a binding layer parameters and mechanical properties in our hybrid system as well. If a timeout of 60 result, the height of the first powder layer increases, which in turn will be less fused, leading to an min was forced before sintering of the first binding layer started, an energy density of 17 mJ/mm2 inferior bond. Thus, these findings support our previous assumption that the formation and thickness was no longer sufficient to build a stable hybrid object. Further, the tensile shear strength at higher

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energy densities was reduced. For even longer interruptions (120 min), this effect became more pronounced, with no binding at all for fluences below 50 mJ/mm2. Notably, all samples whose build process was interrupted broke at the joining area, despite relatively high tensile shear strength. We, therefore, assume that interrupting the building process leads to a kind of predetermined breaking point. This effect may be partially attributed to a shrinkage 3 3 ofMaterials the sintered2020, 13, 5285PA12, having a higher density (0.93 g/cm ) as pulverulent PA12 (0.43 g/cm ). 6As of 11a result, the height of the first powder layer increases, which in turn will be less fused, leading to an inferior bond. Thus, these findings support our previous assumption that the formation and thickness of the the first first layer layer has has a major a major influence influence on the on resulting the resulting mechanical mechanical properties properties of the ofbond the of bond the hybrid of the compound.hybrid compound. To transfer the approach to dissimilar materials, materials, subsequently, subsequently, PA12 PA12 was was sintered sintered onto onto PA6 PA6 substrates.substrates. As As the the melting melting point point of ofPA6 PA6 (210–225 (210–225 °C ◦[49,50])C[49,50 is]) significantly is significantly higher higher than than for PA12 for PA12 (178 °C),(178 sintering◦C), sintering laser parameters laser parameters used above used abovefor joining for joining PA12 and PA12 PA12 and could PA12 not could be notapplied, be applied, which ledwhich to a led weak to abond weak with bond a resulting with a resulting tensile shear tensile strength shear strength of only 3.5 of onlyMPa. 3.5 To MPa.vanquish To vanquish this inherent this difficulty,inherent di thefficulty, CO2 sintering the CO2 sinteringlaser was laserused wasas a usedheating as asource heating for source the plain for PA6 the plain surface PA6 to surface generate to agenerate preheated a preheated and molten and surface molten surfacewithout without applying applying a powder a powder layer. layer. This Thisapproach approach compensated compensated for thefor thedifferent different melting melting points points of ofthe the two two polymers, polymers, as as PA12 PA12 powder powder was was applied applied on on a a softened softened surface surface toto enhance the the bonding bonding properties properties of of the the first first powder powder layer. layer. Figure Figure 6 showsshows thethe binding area of PA6 (black) and PA12 (white) with (Figure 6 6b)b) and and withoutwithout (Figure(Figure6 6a)a) pre-creation pre‐creation of of a a local local melting melting pool pool by preceding exposure cycles.

(a)

(a) (b)

Figure 6. 6. BindingBinding area area of of PA12 PA12 (white) (white) on on PA6 PA6 (black) (black) in independence dependence of preceding of preceding laser laser exposure. exposure. (a) Without(a) Without creation creation of a of local a local melt melt pool pool a clear a clear boundary boundary line line is visible. is visible. (b) After (b) After 6 exposure 6 exposure cycles cycles the darkthe dark grey grey fusion fusion domain domain is enlarged is enlarged and the and interfacial the interfacial area becomes area becomes blurred, blurred, increasing increasing the inter the‐ polymerinter-polymer bonding. bonding.

The resulting tensile shear strength increased linearly with the number of preceding exposure cyclescycles as summarizedsummarized inin Figure Figure7 ,7, reaching reaching 11.4 11.4 MPa MPa for for six six exposure exposure cycles cycles prior prior to applying to applying the first the firstjoining joining powder powder layer, layer, corresponding corresponding to an to increase an increase by more by more than than a factor a factor of three. of three. For For more more than than six sixpreceding preceding exposure exposure cycles, cycles, the amount the amount of molten of PA6molten exceeded PA6 exceeded a critical level,a critical modifying level, planaritymodifying of Materialsplanaritythe surface 2020 of, so13 the, muchx FORsurface PEER as to soREVIEW hinder much aas proper to hinder and evena proper coating and with even PA12 coating powder. with PA12 The energy powder. density7 ofThe 12 2 energyvaried fromdensity 33 tovaried 67 mJ from/mm 33, but to 67 as determinedmJ/mm2, but before, as determined the effect before, of energy the fluence effect of variation energy becamefluence variationsuperimposed became by superimposed the dominant impactby the dominant of the binding impact layer of formation.the binding layer formation.

14.0 12.0 PA12 on PA6 10.0 8.0 6.0 4.0 2.0

Tensile Shear Strength [MPa] 0.0 01234567 Number of Exposure Cycles

Figure 7. Tensile shear strength of PA12 sintered on PA6 as a function of preceding exposure cycles. Figure 7. Tensile shear strength of PA12 sintered on PA6 as a function of preceding exposure cycles.

For the hybrid sintering approach to join dissimilar PA12 onto a selectively molten metal substrate, the sintering strategy had to be adapted, as the melting temperature, apparently, differed significantly. Here, the inherent surface roughness, cavities and channels of the SLM‐created metal surface, intentionally created by adjusting the energy density to a deficient energy level, lead to insufficient layer melting, yielding mechanical interlocked bonding between PA12 and the metal surface. In detail, a large hatch distance (0.3 mm compared to 0.115 mm standard hatch) was used in combination with an insufficient melting irradiation by introducing non‐illuminated layers within the building process and diminished laser energy. Therefore, the applied energy was only about 25% of the energy which is needed to obtain full density objects. Figure 8a depicts a 3D view of the metallic surface. Figure 8b shows the top view.

(a) (b)

Figure 8. Surface of selective laser melting (SLM)‐built metal substrate. (a): 3D view. (b): Top view.

The SLM steel 1.2709 substrate was built by DMG Mori LaserTec 30 and manually placed into the SLS machine. The custom‐built ground plate (Figure 1a) facilitated proper fixing and correct levelling of the substrate. Subsequently, a PA12 powder layer was manually applied onto the metallic surface before starting the printing process. The polymer powder trickled into the surface cavities and was then molten by the CO2 laser irradiation. Upon its subsequent solidification, the polymer created a mechanical bonding to the metallic substrate, hooking into the porous structure. As described before, several exposure cycles were performed to establish a tight interconnection. Figure 9 shows the resulting tensile shear strength as a function of the preceding exposure cycles.

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14.0 12.0 PA12 on PA6 10.0 8.0 6.0 4.0 2.0

Tensile Shear Strength [MPa] Tensile Shear Strength 0.0 01234567 Number of Exposure Cycles

MaterialsFigure2020, 7.13 Tensile, 5285 shear strength of PA12 sintered on PA6 as a function of preceding exposure cycles.7 of 11

For the hybrid sintering approach to join dissimilar PA12 onto a selectively molten metal substrate,For the the hybridsintering sintering strategy approach had to be toadapted, join dissimilar as the melting PA12 temperature, onto a selectively apparently, molten differed metal substrate,significantly. the Here, sintering the strategyinherent hadsurface to be roughness, adapted, ascavities the melting and channels temperature, of theapparently, SLM-created di ffmetalered significantly.surface, intentionally Here, the created inherent by surface adjusting roughness, the ener cavitiesgy density and to channels a deficient of the energy SLM-created level, lead metal to surface,insufficient intentionally layer melting, created yielding by adjusting mechanical the energyinterlocked density bonding to a deficientbetween energyPA12 and level, the lead metal to insusurface.fficient In detail, layer a melting, large hatch yielding distance mechanical (0.3 mm compared interlocked to bonding 0.115 mm between standard PA12 hatch) and was the used metal in surface.combination In detail, with aan large insufficient hatch distance melting (0.3 irradiation mm compared by introducing to 0.115 mm non-illuminated standard hatch) layers was usedwithin in combinationthe building process with an and insu diminishedfficient melting laser irradiation energy. Therefore, by introducing the applied non-illuminated energy was layers only about within 25% the buildingof the energy process which and is diminishedneeded to obtain laser energy.full density Therefore, objects. the Figure applied 8a depicts energy a was3D view only of about the metallic 25% of thesurface. energy Figure which 8b isshows needed the to top obtain view. full density objects. Figure8a depicts a 3D view of the metallic surface. Figure8b shows the top view.

(a) (b)

Figure 8. Surface of selective laser melting (SLM)-built metal substrate. ( a): 3D 3D view. view. ( (b):): Top Top view. view.

The SLM steel 1.2709 substrate was built by DMG Mori LaserTec 30 and manually placed into the SLS machine.machine. The The custom-built custom-built ground ground plate plate (F (Figureigure 11a)a) facilitatedfacilitated properproper ﬁxingfixing andand correctcorrect levelling of the substrate. Subsequently, Subsequently, a PA12 powder layerlayer was manually applied onto the metallic surface before starting the printingprinting process.process. The polymer powder trickled into the surface cavities and was then moltenmolten byby thethe COCO22 laserlaser irradiation.irradiation. Upon its subsequentsubsequent solidiﬁcation,solidification, the polymerpolymer Materialscreated 2020 aa mechanical mechanical, 13, x FOR PEER bonding bonding REVIEW to the to metallicthe metallic substrate, substrate, hooking hooking into the into porous the structure.porous structure. As described8 of As 12 before,described several before, exposure several cycles exposure were cycles performed were toperformed establish to a tightestablis interconnection.h a tight interconnection. Figure9 shows Figure the resulting9 shows the tensile resulting shear tensile strength shear as a strength function as of a the function preceding of the exposure preceding cycles. exposure cycles.

10 PA12 on metal (SLM built)

Tensile Shear trength [MPa] 0 01234567 Number of Exposure Cycles

Figure 9. Tensile shear strength of PA12 sintered on SLM steel 1.2709 basic plate as a function of Figurepreceding 9. Tensile exposure shear cycles. strength of PA12 sintered on SLM steel 1.2709 basic plate as a function of preceding exposure cycles. Apparently, the tensile shear strength increased linearly nearly threefold from 3 MPa (two exposure cycles)Apparently, up to 8.5 MPathe tensile (six exposure shear strength cycles). Figureincreased 10 showslinearly a microscopenearly threefold image from of a cross-section3 MPa (two exposure cycles) up to 8.5 MPa (six exposure cycles). Figure 10 shows a microscope image of a cross‐ section through the hybrid part, where the thickness of the interlocking area can be determined at about 300 μm. Below that, no molten PA12 structure was produced.

Figure 10. Cross‐section of SLM basic plate with molten PA12 (6 exposure cycles).

To illustrate the potential of hybrid selective laser sintering of a PA12 lightweight structure onto what may as well be a 3D‐printed metal lightweight substrate (selective laser melting), Figure 11 highlights a dynamic overload clutch as a hybrid metal–polymer device. By producing a conventional metallic basic element by SLM, internal lightweight structures reduce the material usage and the overall weight by a significant amount. Moreover, the surface structure can be optimized for hybrid sintering. In addition, the fins of the upper polymer part can be individually adapted to specific load dimensions. The combination of two powder‐based additive manufacturing processes uses the lightweight advantages to full capacity, considering specific requirements regarding stability and complex design.

Materials 2020, 13, x FOR PEER REVIEW 8 of 12

10 PA12 on metal (SLM built)

Tensile Shear trength [MPa] 0 01234567 Number of Exposure Cycles

Figure 9. Tensile shear strength of PA12 sintered on SLM steel 1.2709 basic plate as a function of preceding exposure cycles.

Materials 2020 13 Apparently,, , 5285the tensile shear strength increased linearly nearly threefold from 3 MPa8 of 11 (two exposure cycles) up to 8.5 MPa (six exposure cycles). Figure 10 shows a microscope image of a cross‐ sectionthrough through the hybridthe hybrid part, part, where where the thickness the thickness of the interlocking of the interlocking area can bearea determined can be determined at about at about300 300µm. μm. Below Below that, that, no molten no molten PA12 PA12 structure structure was produced. was produced.

FigureFigure 10. 10.CrossCross-section‐section of of SLM SLM basic basic plate plate with molten molten PA12 PA12 (6 (6 exposure exposure cycles). cycles).

To illustrateTo illustrate the the potential potential of of hybrid hybrid selective selective laser sinteringsintering of of a a PA12 PA12 lightweight lightweight structure structure onto onto whatwhat may may as well as well be bea 3D a 3D-printed‐printed metal metal lightweight lightweight substratesubstrate (selective (selective laser laser melting), melting), Figure Figure 11 11 highlights a dynamic overload clutch as a hybrid metal–polymer device. By producing a conventional highlights a dynamic overload clutch as a hybrid metal–polymer device. By producing a conventional metallic basic element by SLM, internal lightweight structures reduce the material usage and the metallic basic element by SLM, internal lightweight structures reduce the material usage and the overall weight by a significant amount. Moreover, the surface structure can be optimized for hybrid overallsintering. weight In by addition, a significant the fins amount. of the upperMoreover, polymer the partsurface can structure be individually can be adapted optimized to specific for hybrid sintering.load dimensions. In addition, The the combination fins of the upper of two polymer powder-based part can additive be individually manufacturing adapted processes to specific uses the load dimensions.lightweight The advantages combination to full of capacity, two powder considering‐based specific additive requirements manufacturing regarding processes stability uses and the lightweightcomplex advantages design. to full capacity, considering specific requirements regarding stability and Materials 2020, 13, x FOR PEER REVIEW 9 of 12 complex design.

Figure 11. DynamicDynamic overload clutch fabricated by the new polymer–metal hybrid process.

4. Conclusions This contribution introduced a novel approach for hybrid selective laser sintering on different diﬀerent similar and dissimilar substrate materials. In In particular, particular, sintering on external build PA12 substrates was demonstrated,demonstrated, with with the the resulting resulting tensile tensile shear shear strength strength being being on the on order the of order continuously of continuously sintered PA12sintered reference PA12 reference specimen. specimen. In addition, In addition, sintering PA12sintering on dissimilar,PA12 on dissimilar, non-PA12-polymer non‐PA12 substrates‐polymer withsubstrates divergent with meltingdivergent temperatures melting temperatures was facilitated was facilitated by the development by the development of a new of strategy a new strategy in laser exposure.in laser exposure. The initial The creation initial creation of a local of melt a local pool melt by the pool CO by2 sinteringthe CO2 sintering laser enhanced laser enhanced bonding between,bonding inbetween, our study, in our PA12 study, on a PA12 PA6-substrate on a PA6‐ bysubstrate a factor by of a three, factor as of quantiﬁed three, as quantified by the tensile by the shear tensile strength. shear Finally,strength. this Finally, approach this approach is extended is toextended the formation to the formation of a hybrid of metal–polymer a hybrid metal–polymer object, with object, the metal with the metal substrate itself being additively manufactured by selective laser melting. By melting the first powder layer into the amplified cavities on the metal surface, an interlock system was generated, leading to strong mechanical bonding effects. Appropriate surface structures can easily be obtained by the use of selective laser melting.

Funding: This research received no external funding.

Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

References

1. Grujicic, M.; Sellappan, V.; Omar, M.A.; Seyr, N.; Obieglo, A.; Erdmann, M.; Holzleitner, J. An overview of the polymer‐to‐metal direct‐adhesion hybrid technologies for load‐bearing automotive components. J. Mater. Process. Technol. 2008, 197, 363–373, doi:10.1016/j.jmatprotec.2007.06.058. 2. Carradò, A.; Faerber, J.; Niemeyer, S.; Ziegmann, G.; Palkowski, H. Metal/polymer/metal hybrid systems: Towards potential formability applications. Compos. Struct. 2011, 93, 715–721, doi:10.1016/j.compstruct.2010.07.016. 3. Grujicic, M.; Sellappan, V.; Kotrika, S.; Arakere, G.; Obieglo, A.; Erdmann, M.; Holzleitner, J. Suitability analysis of a polymer–metal hybrid technology based on high‐strength steels and direct polymer‐to‐metal adhesion for use in load‐bearing automotive body‐in‐white applications. J. Mater. Process. Technol. 2009, 209, 1877–1890, doi:10.1016/j.jmatprotec.2008.04.050. 4. Michaeli, W.; Pfefferkorn, T.G. Electrically conductive thermoplastic/metal hybrid materials for direct manufacturing of electronic components. Polym. Eng. Sci. 2009, 49, 1511–1524, doi:10.1002/pen.21374.

Materials 2020, 13, 5285 9 of 11 substrate itself being additively manufactured by selective laser melting. By melting the first powder layer into the amplified cavities on the metal surface, an interlock system was generated, leading to strong mechanical bonding effects. Appropriate surface structures can easily be obtained by the use of selective laser melting.

Author Contributions: Conceptualization, B.G., T.M., and R.H.; methodology, B.G. and T.M.; validation, B.G. and T.M.; formal analysis, B.G. and T.M.; investigation, B.G. and T.M.; data curation, B.G. and T.M.; writing—original draft preparation, B.G. and R.H.; writing—review and editing, B.G., and R.H.; visualization, B.G.; supervision, R.H.; project administration, R.H.; funding acquisition, R.H. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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