Influence of Pulsed Exposure Strategies on Overhang Structures

Article Inﬂuence of Pulsed Exposure Strategies on Overhang Structures in Powder Bed Fusion of Ti6Al4V Using Laser Beam

Jonas Grünewald 1,* , Pirmin Clarkson 1,2 , Ryan Salveson 2, Georg Fey 2 and Katrin Wudy 1

1 Professorship of Laser-based Additive Manufacturing, Department of Mechanical Engineering, School of Engineering & Design, Technical University of Munich, Boltzmannstraße 15, 85748 Garching, Germany; [email protected] (P.C.); [email protected] (K.W.) 2 AMCM GmbH, Petersbrunner Straße 1b, 82319 Starnberg, Germany; [email protected] (R.S.); [email protected] (G.F.) * Correspondence: [email protected]

Abstract: Manufacturing structures with low overhang angles without support structures is a major challenge in powder bed fusion of metals using laser beam (PBF-LB/M). In the present work, various test specimens and parameter sets with continuous wave (cw) and pulsed exposure are used to investigate whether a reduction of downskin roughness and overhang angle can be achieved in PBF-LB/M of Ti6Al4V. Starting from cw exposure, the limits of overhang angle and surface roughness at the downskin surface are investigated as a reference. Subsequently, the inﬂuence of laser power,

scanning speed, and hatch distance with ﬁxed pulse duration (τpulse = 25 µs) and repetition rate (υrep = 20 kHz) on surface roughness Ra is investigated. Pulsed exposure strategies enable the ◦ ◦ manufacturing of ﬂatter overhang angles (≤20 instead of ≥25 ). Furthermore, a correlation between the introduced volume energy density and the downskin roughness can be observed for pulsed

Citation: Grünewald, J.; Clarkson, P.; exposure. As the reduction in volume energy density causes an increase in porosity, the combination Salveson, R.; Fey, G.; Wudy, K. of pulsed downskin exposure and commercial cw infill exposure is investigated. The larger the Influence of Pulsed Exposure gap in volume energy density between the infill area and downskin area, the more challenging it is Strategies on Overhang Structures in combining the two parameter sets. By combining cw infill and pulsed downskin exposure, flatter Powder Bed Fusion of Ti6Al4V Using overhang structures cannot be manufactured, and a reduction in roughness can be achieved. Laser Beam. Metals 2021, 11, 1125. https://doi.org/10.3390/ Keywords: additive manufacturing; powder bed fusion of metals using laser beam; PBF-LB/M; met11071125 Ti6Al4V; Ti64; power modulation; pulsed exposure; overhang structure; roughness

Academic Editor: Aleksander Lisiecki

Received: 21 June 2021 1. Introduction Accepted: 12 July 2021 Published: 15 July 2021 In laser-based powder bed fusion of metals (PBF-LB/M), continuous wave (cw) and vector-based exposure strategies are used as a standard exposure strategy nowadays. This

Publisher’s Note: MDPI stays neutral cw exposure with high-line intensities implies high temperature gradients as well as cooling with regard to jurisdictional claims in rates in and around the melt pool, which determines the process dynamic and resulting published maps and institutional affil- component microstructure. In the literature, the advantages of pulsed compared with iations. cw exposure have been discussed for several materials over the past decades. Different beneficial aspects were addressed using pulsed exposure in PBF-LB/M: Minimizing thermal distortion [1]; Increasing spatial resolution for thin structures [2,3]; Copyright: © 2021 by the authors. Tailoring microstructure [1,3]. Licensee MDPI, Basel, Switzerland. These advantageous changes result primarily from the more controlled energy input. This article is an open access article In investigations on pulsed energy input in PBF-LB/M, low repetition rates in the field of distributed under the terms and Hz were applied and the aim was a reduction of top and side surface quality [4,5]. To sum conditions of the Creative Commons up these investigations: with high repetition rates, increased overlap, and reduced scan Attribution (CC BY) license (https:// speed, the top surface roughness is reduced, but at the same time, balling effects occur and creativecommons.org/licenses/by/ thus side surface roughness is increased [4,5]. 4.0/).

Metals 2021, 11, 1125. https://doi.org/10.3390/met11071125 https://www.mdpi.com/journal/metals Metals 2021, 11, 1125 2 of 15

Modern fiber lasers, which are mainly used in PBF-LB/M, provide the possibly to ap- ply fast power modulation, and can thus realize a pulsed energy input with pulse durations down to the range of microseconds and pulse repetition rates up to multiple kHz [6]. Re- cent investigations on continuous and pulsed energy input for different materials prove an increase of porosity and a decrease of dimensional accuracy for large parts [2]. If thin parts, like in lattice structures, should be manufactured, pulsed energy input shows great advantages [2]. Caprio et al. [7] have proven an increase in resolution, which goes along with a decrease in efficiency for stainless steel (316L). Laitinen et al. [8] investigated the influence of pulse duration in PBF-LB/M of stainless steel (316L). Shorter pulse durations result in narrower and shallower melt pools and, therefore, require larger overlap to produce continuous melt paths [8]. Karami et al. [3] analyzed the difference between continuous and pulsed PBF-LB/M and their effect on microstructure of Ti6Al4V lattice structures. The use of pulsed exposure strategies implies a more homogenous microstructure, and thus higher compressive mechanical properties and yield stress [3]. In addition, the use of pulsed energy input allows a tailoring of the resulting microstructure, demonstrated in [1] for AlSi12 and AlSi10Mg, in [9] for AlSi10Mg, and in [10] for IN738LC. Biffi et al. [9] investigated for AlSi10Mg a slightly finer microstructure using pulsed wave laser, while the melt pool track is larger for the continuous wave mode, which leads to higher mechanical properties. The focus of the investigation was mostly on the analysis of the microstructure, the porosity, or the resolution when using pulsed exposure in the PBF-LB/M. The investigations presented aim at the manufacturing of flatter overhang structures in the PBF-LB/M. Overhang structures are especially challenging in PBF-LB/M because of heat accumulation and consequent distortion incurred [11]. In order to eliminate distortions, external support structures are usually used to reinforce the overhang structure during the PBF-LB/M process. The use of support structures leads to time-consuming post-processing. Investigations from Calignano [12] show that overhang structures of AlSi10Mg and Ti6Al4V up to an angle of 30◦ can be manufactured, although the surface roughness is negatively affected. Optimized support geometry leads to an improvement of part quality [12]. According to Han et al. [11], the melt pool in the dross zone is larger compared with the zone where the part is supported by underlying layers, which is caused by the lower thermal conductivity of the powder bed, and leads to higher distortion. Therefore, Han et al. [11] suggest an increased surface contact area between the part and the support structures to significantly reduce the distortion. Chen et al. [13] investigated the roughness on downskin surfaces in Ti6Al4V for angles ≥40◦; the roughnesses became larger with decreasing overhang angles. ◦ For overhang angles of 40 , roughnesses of Ra ≥ 40 µm could be achieved. It is hardly possible or impractical to avoid overhang structures in additive manufacturing. The support structures required for this in PBF-LB/M have to be removed with great effort. From an AM perspective, therefore, avoiding support structures for overhang structures is favorable as this eliminates the need for time-consuming post-processing. The state-of-the-art shows quite well that overhang structures tend to distortion owing to an unbalanced thermal budget. Investigation into the use of pulsed exposure strategies leads to control of thermal budget, which leads to the following question: How flat can overhang structures be manufactured without additional support structures by using pulsed energy input in PBF-LB/M? Therefore, the objective of this study is to extend the limits of overhang angles with reduced roughness of downskin surfaces by reducing heat accumulated in the process zone through pulsed exposure strategies. Because a reduction in introduced energy implies an increase in porosity [14], double exposures are tested to reduce porosity. Finally, selected pulsed parameter sets as downskin parameter are combined with commercially available cw infill parameters to produce parts with the highest relative densities possible. Metals 2021, 11, x FOR PEER REVIEW 3 of 15

2. Theoretical Consideration 2.1. Exposure Parameters State-of-the-art PBF-LB/M systems use fiber lasers as a beam source, which can be Metals 2021, 11, 1125 power modulated at a multiple of 10 kHz. In this study, power modulation is used3 of for 15 pulse generation. The pulses have a rectangle-like shape with pulse peak powers Ppeak corresponding to the set laser power. There is no power superelevation, as is the case with pulsed beam sources with Q-switch or modelocking. The energy delivered per pulse Epulse 2. Theoretical Consideration is determined by the pulse length τpulse and the laser peak power Ppeak according to Exposure Parameters State-of-the-art PBF-LB/M systems𝐸 use 𝑃 ﬁber ∙𝜏 lasers. as a beam source, which can(1) be power modulated at a multiple of 10 kHz. In this study, power modulation is used for The average power Pavg is determined according to pulse generation. The pulses have a rectangle-like shape with pulse peak powers Ppeak corresponding to the set laser power. There𝑃 𝐸 is no power∙𝜐,superelevation, as is the case with(2) pulsed beam sources with Q-switch or modelocking. The energy delivered per pulse E where υrep is the pulse repetition rate. Two more important parameters for the processpulse are is determined by the pulse length τ and the laser peak power P according to the distance between two pulses dpulse (see Figure 1) and the volumepeak energy density Ev, which describes the energy input per volume. The distance between two pulses dpulse is Epulse = Ppeak·τpulse. (1) calculated as a product of the scan speed vscan and the laser off time

The average power Pavg is determined according to 𝑑 𝑣 ∙ 𝜏. (3) Pavg = Epulse·υrep, (2) The volume energy density Ev is the quotient of the average power Pavg and the prod-

whereuct of scanυrep is speed the pulse vscan, hatch repetition distance rate. h Two, and more powder important layer thickness parameters tpowder for: the process are

the distance between two pulses dpulse𝑃(see Figure𝑃1) and∙𝜏 the volume∙𝜐 energy density Ev, 𝐸 . which describes the energy input per volume. The distance between two pulses dpulse(4)is 𝑣 ∙ℎ∙𝑑 𝑣 ∙ℎ∙𝑡 calculated as a product of the scan speed vscan and the laser off time By combining the varied parameter, Ev helps to compare different parameter sets. All data and equations on energies or powers in this1 study refer to the power emitted by the dpulse = vscan· − τpulse . (3) laser. The energy absorbed by the material, anυrepd thus available in the process, is always dependent on the material and the surface properties of the irradiated area.

Figure 1. Schematic view of the overlapping of the laser focus points, adaptedadapted fromfrom [[2].2].

3. MaterialsThe volume and Methods energy density Ev is the quotient of the average power Pavg and the product3.1. Feedstock of scan Material speed vscan, hatch distance h, and powder layer thickness tpowder: In this study, Ti6Al4V is used as feedstock material. This alloy is the most widely Pavg Ppeak·τpulse·υrep used titanium alloy with Ea vmarket= share of 75–=85% [15]. The reason. for this is its properties (4) vscan·h·dpowder vscan·h·tpowder such as good strength, ductility, fracture toughness, and corrosion and temperature re-

sistanceBy combiningwith a low the density varied of parameter, 4.43 g/cm³.E vThhelpsese material to compare properties different make parameter it particularly sets. All data and equations on energies or powers in this study refer to the power emitted by the laser. The energy absorbed by the material, and thus available in the process, is always dependent on the material and the surface properties of the irradiated area.

3. Materials and Methods 3.1. Feedstock Material In this study, Ti6Al4V is used as feedstock material. This alloy is the most widely used titanium alloy with a market share of 75–85% [15]. The reason for this is its properties such Metals 2021, 11, x FOR PEER REVIEW 4 of 15

interesting for aerospace applications [16]. In these fields of application, the avoidance of support structures in favor of the manufacturability of complex components is in demand. Regarding the absorption behavior of Ti6Al4V, Boley et al. [17] performed experi- Metals 2021, 11, 1125 ments on the absorption of different metal powders and substrates at a wavelength of4 of λ 15 ≈ 1 µm. For Ti6Al4V powder, they measured an absorption of 74% at a layer thickness of 100 µm. The absorption of a smooth substrate is estimated to be 39%. The absorption of a roughas good surface strength, (e.g., ductility, a sandblaste fractured substrate toughness, or a and layer corrosion that has and already temperature been melted) resistance is in between,with a low depending density of on 4.43 the g/cm surface3. These properties. material properties make it particularly interesting for aerospaceTi6Al4V (EOS applications Titanium [16 Ti64;]. In [18]) these powder fields is of provided application, by EOS the GmbH avoidance (Krailling/Mu- of support nich,structures Germany). in favor The of powder the manufacturability is produced by of th complexe gas atomization components process. is in demand. It has a particle size ofRegarding D90,3 ≈ 50 µm. the absorption Before each behavior process, of the Ti6Al4V, powder Boley is sieved et al. with [17] performeda 63 µm sieve experiments to ensure thaton the the absorption particle size of distribution different metal remains powders constant and substrates during the at tests. a wavelength of λ ≈ 1 µm. For Ti6Al4V powder, they measured an absorption of 74% at a layer thickness of 100 µm. 3.2.The Test absorption Specimens of a smooth substrate is estimated to be 39%. The absorption of a rough surfaceIn order (e.g., ato sandblasted show geometry substrate limits or and a layer the thatinfluence has already of the beenintroduced melted) energy is in between, as well asdepending the influence on the of surface an extended properties. layer time in cw processes, the test specimens in Figure 2a,b withTi6Al4V overhang (EOS thicknesses Titanium Ti64; of 2.5 [18 mm]) powder and 10 mm is provided are used. by The EOS test GmbH specimens (Krailling/ consist ofMunich, overhang Germany). structures The with powder angles isin producedthe range byof 10–40°, the gas which atomization are varied process. in steps It hasof 5°. a Flatterparticle angles size of are D90,3 arranged≈ 50 µ m.in higher Before z-positi each process,ons to thereduce powder interferen is sievedce in with the amanufacture 63 µm sieve ofto the ensure various that angles. the particle size distribution remains constant during the tests. Based on the results of the first tests, the test specimen in Figure 2c is used for the 3.2. Test Specimens investigation of pulsed exposure strategies. These include adjusted cross sections and an- gularIn increments order to show with geometry lower angles, limits as and overhang the influence structures of the up introduced to an angle energy of 25° as wellcan be as manufacturedthe influence of safely an extended in the cw layer process. time in cw processes, the test specimens in Figure2a,b withFor overhang a final thicknessesinvestigation of of 2.5 the mm combination and 10 mm of are commercial used. The cw test parameters specimens and consist quali- of ◦ ◦ fiedoverhang pulsed structures parameters, with the angles test specimen in the range in Figure of 10–40 2d is, whichused. This are varied test specimen in steps is of ex- 5 . tendedFlatter anglesby angles are arrangedof 25° and in higher30°, whereby z-positions the angle to reduce with interference 10° is not considered in the manufacture further owingof the variousto insufficient angles. manufacturability.

Figure 2. SchematicSchematic of of the the test test specimens specimens po positionedsitioned on on the the base base plate. plate. (a) ( aTest) Test specimen specimen with with overhang overhang thicknesses thicknesses of 10 of 10mm mm for for cw cw exposure. exposure. (b) ( bTest) Test specimen specimen with with overhang overhang thicknesses thicknesses of of 2.5 2.5 mm mm for for cw cw exposure. exposure. ( (cc)) Test Test specimen with overhang thicknesses of 5 mm for pulsed exposure. (d) Test specimen with overhang thicknesses of 5 mm for combined overhang thicknesses of 5 mm for pulsed exposure. (d) Test specimen with overhang thicknesses of 5 mm for combined pulsed and cw exposure. pulsed and cw exposure.

3.3. PBF-LB/MBased on System the results with ofOptical the first Tomography tests, the test specimen in Figure2c is used for the investigationThe test samples of pulsed were exposure fabricated strategies. on a customized These include PBF-LB/M adjusted system cross sections(M290, EOS and GmbH,angular Krailling/Munich, increments with lower Germany). angles, The as overhang PBF-LB/M structures system was up to upgraded an angle ofwith 25◦ acan 1 kW be fibermanufactured laser (YLR-1000-WC-Y14, safely in the cw process.IPG Photonics®, Oxford, MA, USA) with a wavelength of λ ≈ 1070For anm, final ainvestigation maximum pulse of the repetition combination rate of of commercial υrep = 50 kHz, cw parameters and a minimum and qualified pulse durationpulsed parameters, of τpulse = 10 the µs. test The specimen spot size inon Figure the base2d isplate used. is d Thisspot = test80 µm. specimen is extended by anglesThe system of 25◦ andincludes 30◦, wherebya camera-based the angle optical with tomography 10◦ is not considered (OT) system further (EOS owing GmbH, to Krailling/Munich,insufficient manufacturability. Germany) consisting of a sCMOS camera (16-bit) placed above the process chamber and aligned to the base plate via a deflection mirror. During the manufac- turing3.3. PBF-LB/M process, Systemthe OT withrecords Optical individual Tomography images with a narrowband filtered wavelength The test samples were fabricated on a customized PBF-LB/M system (M290, EOS GmbH, Krailling/Munich, Germany). The PBF-LB/M system was upgraded with a 1 kW fiber laser (YLR-1000-WC-Y14, IPG Photonics®, Oxford, MA, USA) with a wavelength of

λ ≈ 1070 nm, a maximum pulse repetition rate of υrep = 50 kHz, and a minimum pulse duration of τpulse = 10 µs. The spot size on the base plate is dspot = 80 µm. Metals 2021, 11, 1125 5 of 15

The system includes a camera-based optical tomography (OT) system (EOS GmbH, Krailling/Munich, Germany) consisting of a sCMOS camera (16-bit) placed above the process chamber and aligned to the base plate via a deﬂection mirror. During the manufacturing process, the OT records individual images with a narrowband ﬁltered wavelength of ≈900 nm with exposure times of 100 ms. The individual images are combined to form one gray-scale image per slice. The signal qualitatively estimates the process temperature via the thermal emission at ≈900 nm. The data accrue as 16-bit images and are evaluated by averaging the detected data in desired areas and normalizing them. The results shown include the data of exposed cross sections from different z-coordinates. The z-coordinates and evaluated areas are selected depending on the test specimen and overhang angles in a way that the cross-sectional area is in comparable areas of the overhang. Table1 summarizes the z-coordinates and areas used.

Table 1. z-coordinates and areas of evaluated optical tomography (OT) data.

Overhang Thickness Angles z-Coordinate Area in Pixels 5 mm 20◦ 8.40 mm 35 × 105 2.5 mm and 10 mm 25◦ 27.12 mm 36 × 66 and 70 × 179 2.5 mm and 10 mm 30◦ 26.52 mm 33 × 66 and 70 × 170 2.5 mm and 10 mm 35◦ 21.00 mm 28 × 66 and 70 × 150 2.5 mm and 10 mm 40◦ 15.72 mm 23 × 66 and 70 × 117

3.4. Process Parameters The process parameter sets used can be categorized into three different test series: cw exposure, single-pulsed exposure, and double-pulsed exposure. The cw exposure test series is conducted with the commercial parameter set “Ti64_SpeedM291 1.10” of EOS GmbH (Krailling/Munich, Germany). For the overhang thicknesses of 10 mm, the layer time is additionally increased from tL10 mm ≈ 80 s to tML10 mm = 260 s. The layer time describes the time required by the PBF-LB/M for complete processing of one layer. It can be assumed that the time for heat dissipation of an exposed volume between two exposures corresponds on average to the layer time. The additional time allows a longer heat dissipation from the process zone, and thus reduces the average process temperature. The layer time of the manufacturing process with overhang thicknesses of 2.5 mm is tL2.5mm ≈ 50 s. Besides cw exposure, all other experiments are carried out with pulsed laser radiation (single and double exposure). The investigated parameter sets of both strategies are summarized in Table2. The experimental parameters are constant within the respective test specimen. There are no separate contour or downskin parameters. The single-pulsed exposure test series is a screening with a wide variation of laser power Ppeak = 140 ... 640 W with ∆Ppeak = 100 W and scan speed vscan = 750 ... 4750 mm/s with ∆vscan = 500 ... 1000 mm/s at various hatch distance of h = 40, 60, 80, and 120 µm (total of 54 parameter sets). The design results in volume 3 energy densities in the range Ev ≈ 4 ... 73 J/mm . Based on the results of the single-pulsed exposure, the parameters for the double exposure test series are derived. Restricted ranges of power Ppeak = 140 ... 240 W with ∆Ppeak = 20 W and scan speed vscan = 1000 ... 2500 mm/s with ∆vscan = 500 mm/s (total of 72 parameter sets) are chosen. The hatch distance is varied with h = 40, 50, and 60 µm. This series is designed full factorial. Owing to the double exposure, 3 the design results in volume energy densities of Ev ≈ 16 ... 100 J/mm . The energy actually absorbed and thus available in the process is lower owing to the lower absorption of the already melted layer. For this reason, the volume energy densities between single and double exposure are not directly comparable. Metals 2021, 11, x FOR PEER REVIEW 6 of 15

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Table 2. Summarized pulsed process parameter sets.

Hatch Dis- Powder Layer Pulse Dura- Pulse Repeti- Peak Power TableScanning 2. Summarized Speed pulsed process parameter sets. Number of tance Thickness tion tion Rate υrep Hatch Distance PeakPpeak Power in W Scanningvscan Speed in mm/sPowder Layer Thickness Pulse Duration Pulse Repetition ExposuresNumber of h in µm tpowder in µm τpulse in µs in kHz h in µm Ppeak in W vscan in mm/s tpowder in µm τpulse in µs Rate υrep in kHz Exposures 40 140, 240 750 60 25 20 1 40 140, 240 750 60 25 20 1 40 140,140, 240, 240, 340, 340, 440 1250, 2250 60 25 20 1 40 1250, 2250 60 25 20 1 140, 240,440 340, 440, 60 140, 240, 340, 2250,2250, 3250, 3250, 4250, 4250, 4750 60 25 20 1 60 60 25 20 1 440,540, 540, 640 640 4750 80 140,140, 240, 240, 340, 340, 440 750, 1250 60 25 20 1 80 750, 1250 60 25 20 1 140, 240,440 340, 440, 140, 240, 340, 120120 750, 1250750, 1250 6060 25 20 20 1 1 440,540, 540, 640 640 140, 160, 180, 1000, 1500, 2000, 40, 50, 60 140, 160, 180, 200, 60 25 20 2 40, 50, 60 200, 220, 240 1000,2500 1500, 2000, 2500 60 25 20 2 220, 240

TheAll reduction test samples of volume are built energy with adensity pulse repetitionleads to increased rate of υrep porosities= 20 kHz, [14]. a pulse However, duration for ofmostτpulse applications,= 25 µs, and it ais powder necessary layer that thickness components of tpowder have= a 60 highµm. density to ensure high quality Theand reductiongood mechanical of volume properties energy density [19]. Therefore, leads to increased three parameter porosities sets [14 with]. However, low downskinfor most roughnesses applications, and it is various necessary powers, that components hatches, scan have speeds, a high and density volume to ensure energy high densitiesquality are and combined good mechanical with the propertiescommercial [19 cw]. Therefore,infill parameter three parameterset “Ti64_SpeedM291 sets with low 1.10”downskin qualified roughnesses to produce high and densities. various powers, The parameter hatches, sets scan Downskin speeds, and1 and volume 2 are taken energy fromdensities the double are combined pulsed exposure with the commercialtest series. Downskin cw inﬁll parameter 3 is based set on “Ti64_SpeedM291 the single-pulsed 1.10” exposurequaliﬁed test to series, produce but highreceives densities. an adjust Theed parameter scan speed. sets With Downskin these downskin 1 and 2 are parameter taken from setsthe (shown double in pulsed Table 3), exposure single and test double series. Downskinexposures 3and is based the variation on the single-pulsed of downskin exposurethick- test series, but receives an adjusted scan speed. With these downskin parameter sets ness are investigated. The areas to which the various parameter sets are assigned are (shown in Table3), single and double exposures and the variation of downskin thickness shown schematically in Figure 3. are investigated. The areas to which the various parameter sets are assigned are shown Tableschematically 3. Summarized in Figureprocess3 .parameters for the combination of cw and pulsed exposure.

Table 3. Summarized processScanning parameters forPowder the combination Layer Pulse of cw andDura- pulsedPulse exposure. Repe- Volume Energy Hatch Distance Peak Power Name Speed Thickness tion tition rate Density Hatch Scanning Powder Layer h in µm PeakPpeak Power in W Pulse Duration Pulse Repetition Volume Energy Name Distance h in Speed v in Thickness t in P in W vscanscan in mm/s tpowderpowder in µm ττpulsein inµs µs Rateυrep υin inkHz kHz EDensityv in J/mm³E in J/mm 3 µm peak mm/s µm pulse rep v Downskin 1 40 140 2500 60 25 20 12 Downskin 1 40 140 2500 60 25 20 12 DownskinDownskin 2 2 60 60 240 240 2000 2000 60 60 25 25 20 20 17 17 Downskin 3 120 340 850 60 25 20 28 DownskinInﬁll 3 120 Ti64_SpeedM291 340 1.10 850 60 60 cw 25 20cw 28 38 Infill Ti64_SpeedM291 1.10 60 cw cw 38

FigureFigure 3. Schematic 3. Schematic of parameter of parameter combination combination in the in theinfill inﬁll and and downskin downskin area. area.

3.5.3.5. Build Build Job JobDesign Design TheThe build build job job design design for for the the different different test test sp specimensecimens from from Figure Figure 11 is is shown shown in in Figure Figure 4. 4. InIn accordance accordance with VDIVDI 34053405 PartPart 2 2 [ 20[20],], the the test test specimens specimens are are placed placed at anat an azimuth azimuth angle ◦ angleof 45of 45°to theto the coating coating and and gas gas ﬂow flow direction direct withion with the openthe open angle angle ends ends oriented oriented in the in coating the coatingdirection. direction. This arrangementThis arrangement prevents preven protrudingts protruding test specimens test specimens from being from approached being head-on by the coater blade and increases process robustness. The twisted arrangement prevents scan paths in the direction of the shielding gas ﬂow during contour runs or runs

Metals 2021, 11, x FOR PEER REVIEW 7 of 15

Metals 2021, 11, 1125 7 of 15 approached head-on by the coater blade and increases process robustness. The twisted arrangement prevents scan paths in the direction of the shielding gas flow during contour runs or runsparallel parallel to the to contour.the contour. In this In way,this way, undesirable undesirable interactions interactions between between discharged dis- process charged processemissions emissions and the and laser the radiation laser radiation are reduced. are reduced.

(a) (b)

Figure 4. DesignFigure of 4. buildDesign jobs of with build test jobs specimens: with test specimens:cw exposure cw (a exposure) and pulsed (a) andexposure pulsed (b exposure). (b).

3.6. Component3.6. Component Properties Properties To assess Tothe assess manufacturability the manufacturability of flat overhang of flat overhangstructures, structures, the roughness the roughness of the of the downskin downskinareas of the areas test ofspecimens the test specimens is measured. is measured. As very large As very roughness large roughness values are values are expected atexpected the downskin at the downskinsurfaces owing surfaces to the owing geometry to the geometryand the process, and the the process, specimens the specimens are declared not manufacturable from a roughness R > 150 µm, which is about twice as are declared not manufacturable from a roughness Ra > 150 µm,a which is about twice as high as thehigh reference as the values reference shown values later. shown Porosity later. analyses Porosity are analysesperformed are for performed selected pa- for selected rameter sets,parameter because sets, the focus because of the the optimi focus ofzation the optimization is on the reduction is on the of reductionroughness. of roughness. 3.6.1. Roughness Measurement 3.6.1. Roughness Measurement A 3D profilometer (Keyence VR-3200, Osaka,¯ Japan) is used to measure the roughness A 3D profilometer (Keyence VR-3200, Ōsaka, Japan) is used to measure the rough- on the downskin surfaces of the test specimens via fringe projection. By means of the ness on the downskin surfaces of the test specimens via fringe projection. By means of the fringe projection method, an unambiguous topography measurement and high resolution fringe projectioncan be method, achieved an [21 unambiguous]. In order to to establishpography comparability measurement with and otherhigh resolution measurements, the can be achieved [21]. In order to establish comparability with other measurements, the established arithmetic average roughness Ra according to ISO 4287 [22] is evaluated from establishedthe arithmetic raw data average obtained roughness by 31 homogeneously Ra according to distributed ISO 4287 [22] single is evaluated measurement from sections of the raw data obtained by 31 homogeneously distributed single measurement sections of 20 mm in length. No low-pass filter λs is used in the evaluation, and the high-pass cut-off 20 mm in length. No low-pass filter λs is used in the evaluation, and the high-pass cut-off wavelength is set to λc = 2.5 mm. wavelength is set to λc = 2.5 mm. 3.6.2. Porosity 3.6.2. Porosity In order to compare the porosities of single and double exposure on one example In orderparameter to compare set, micrographs the porosities are of created. singleFor and this double purpose, exposure the specimens on one example are prepared in an parameteracrylic set, micrographs cold mounting are created. system, For then th groundis purpose, with the a 220 specimens grit SiC are foil prepared and polished in in three an acrylic coldstages mounting with cloths system, and then polishing ground suspensions with a 220 grit (9 µ SiCm, 1foilµm, and 0.04 polishedµm). Subsequently,in three the stages withmicrographs cloths and arepolishing recorded suspension with a 5×s objective(9 µm, 1 andµm, a 0.04 pixel µm). size ofSubsequently, approximately the 1.4 µm/px. micrographs are recorded with a 5× objective and a pixel size of approximately 1.4 µm/px. 4. Results and Discussion 4. Results 4.1.and ContinuousDiscussion Exposure Strategies 4.1. Continuous InitialExposure tests Strategies with commercial cw process parameters show the limits of manufacturing overhang structures using cw parameters and the potential to reduce the roughness by Initial tests with commercial cw process parameters show the limits of manufactur- reducing heat accumulation in the specimen as a reference. According to Calignano [12], ing overhang structures using cw parameters and the potential to reduce the roughness manufacturing of overhang structures with angles of 30◦ using PBF-LB/M is possible. by reducing heat accumulation in the specimen as a reference. According to Calignano With the commercial cw parameters in this study, structures with overhang thicknesses [12], manufacturing of overhang structures with angles of 30° using PBF-LB/M is possible. of 2.5 mm and 10 mm with angles up to 25◦ can be manufactured. With the commercial cw parameters in this study, structures with overhang thicknesses Figure5a shows that the reference test specimens with overhang thicknesses of of 2.5 mm and 10 mm with angles up to 25° can be manufactured. 10 mm have roughnesses of Ra > 70 µm on the downskin surface for all angles shown. The test specimens with overhang thicknesses of 2.5 mm exhibit, with Ra < 55 µm, significantly lower roughnesses for all angles. The extension of the layer time from

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Figure 5a shows that the reference test specimens with overhang thicknesses of 10 mm have roughnesses of Ra > 70 µm on the downskin surface for all angles shown. The test specimens with overhang thicknesses of 2.5 mm exhibit, with Ra < 55 µm, significantly lower roughnesses for all angles. The extension of the layer time from tL10 mm ≈ 80 s to tML10 mm = 260 s with Δt10 mm ≈ 180 s in the production of the reference test specimens with overhang thicknesses of 10 mm also leads to a significant reduction of the roughness Ra < 50 µm for all angles shown. These values fit well with studies by Chen et al. [13], who achieved roughnesses on downskin surfaces of Ra ≥ 40 µm at 40° angles, Ra ≥ 30 µm at 50° angles, and Ra ≥ 15 µm at 60° angles. Reducing the overhang thickness from 10 mm to 2.5 mm reduces the volume and area per layer to be melted. This reduces the energy introduced into the overhang structure. Less energy introduced results in less heat accumulated in the specimen. Extending the layer time by 3 min for test specimens with overhang thickness of 10 mm enables heat to be dissipated from the processing zone into the specimen and base plate for 3 min longer, thus reducing the heat accumulated. Lower accumulated heat results in lower process temperatures, and thus a reduction of thermal radiation at 900 nm, which can be detected by the OT system (see Figure 5b). Both adjustments in the cw exposure slightly reduce the average OT signal, confirming a reduction in the process temperature. Metals 2021, 11, 1125 8 of 15 This lower process temperature tends to lead to lower roughness values on the overhang structures. The reasons for this can be less adhering powder particles, less melt pool dynamics at the overhang structures, as well as the formation of smaller melt pools in the tdrossL10 mm zone≈ 80 as sshown to tML10 in mm[11].= The 260 high s with standard∆t10 mm deviations≈ 180 s of in the the OT production signals in of Figure the ref- 5b erenceindicate test high specimens melt pool withdynamics, overhang resulting thicknesses in more ofirregular 10 mm melt also tracks leads at to the a signiﬁcant downskin reductionsurface, and of thus the causing roughness the Rcomparativelya < 50 µm for high all roughness angles shown. values. These In addition values to ﬁt reduc- well withing accumulated studies by Chenheat, reducing et al. [13 ],melt who pool achieved dynamics roughnesses through pulsed on downskin exposure surfaces strategies of ◦ ◦ ◦ Roffersa ≥ 40 theµ mpotential at 40 angles, to reduceRa ≥downskin30 µm at roughness. 50 angles, and Ra ≥ 15 µm at 60 angles.

100 140 Ti6Al4V tpowder = 60 µm overhang thickness Ti6Al4V tpowder = 60 µm overhang thickness dspot = 80 µm 10 mm 90 dspot = 80 µm 10 mm 120 Δ Δ t10 mm = 180 s 2.5 mm t10 mm = 180 s 2.5 mm 10 mm t 80 10 mm t 100 ML ML

70 80 / µm a R 60 60 OT-Signal / % / OT-Signal 40 50 not manufacturable manufacturable not manufacturable not

0 0

20 30 40 20 30 40 angle / ° angle / ° (a) (b)

Figure 5. Measured roughnessesFigureRa 5.(a Measured) and OT signal roughnesses (b) with R commerciala (a) and OT cw signal process (b) with parameters commercial with varyingcw process specimens, parameters overhang angles, and layer times.with varying specimens, overhang angles, and layer times

4.2. PulsedReducing Exposure the overhang Strategies thickness from 10 mm to 2.5 mm reduces the volume and area per layerThe pulsed to be melted. exposure This test reduces series (see the energySection introduced 3.4.) confirms into that the the overhang reduction structure. of the Lessintroduced energy volume introduced energy results density in less Ev heatalso accumulatedcauses a reduction in the specimen.of roughness Extending even with the layerpulsed time exposure. by 3 min for test specimens with overhang thickness of 10 mm enables heat to be dissipatedFigure 6 fromshows the the processing results of zone the single-pulsed into the specimen exposure and base test plateseries. for The 3 min measured longer, thus reducing the heat accumulated. Lower accumulated heat results in lower process roughness values Ra at the downskin surfaces of 20° overhang structures are presented as temperatures, and thus a reduction of thermal radiation at 900 nm, which can be detected by the OT system (see Figure5b). Both adjustments in the cw exposure slightly reduce the average OT signal, conﬁrming a reduction in the process temperature. This lower process temperature tends to lead to lower roughness values on the overhang structures. The reasons for this can be less adhering powder particles, less melt pool dynamics at the overhang structures, as well as the formation of smaller melt pools in the dross zone as shown in [11]. The high standard deviations of the OT signals in Figure5b indicate high melt pool dynamics, resulting in more irregular melt tracks at the downskin surface, and thus causing the comparatively high roughness values. In addition to reducing accumulated heat, reducing melt pool dynamics through pulsed exposure strategies offers the potential to reduce downskin roughness.

4.2. Pulsed Exposure Strategies The pulsed exposure test series (see Section 3.4.) conﬁrms that the reduction of the introduced volume energy density Ev also causes a reduction of roughness even with pulsed exposure. Figure6 shows the results of the single-pulsed exposure test series. The measured ◦ roughness values Ra at the downskin surfaces of 20 overhang structures are presented as a function of power P (a), scan speed v (b), and volume energy density Ev (c) calculated according to Equation (4). For the sake of clarity, only parameter combinations with hatches h = 60, 120 µm are shown in Figure6a,b. As the hatch distances h are adjusted to power, Metals 2021, 11, x FOR PEER REVIEW 9 of 15

a function of power P (a), scan speed v (b), and volume energy density Ev (c) calculated according to Equation (4). For the sake of clarity, only parameter combinations with hatches h = 60, 120 µm are shown in Figure 6a,b. As the hatch distances h are adjusted to power, scan speed, and volume energy density (see Section 3.4), they are not shown in a separate diagram. Figure 6c summarizes all tested parameter combinations. For volume energy densities Ev < 7 J/mm³, the applied energy is not sufficient to produce adequate material cohesion. Corresponding parameter sets are thus not considered further. Figure 6a,b show increasing roughnesses Ra for increasing laser power Ppeak and decreasing scanning speed vscan. Summarizing the varied and constant process parameters into the volume energy density (see Equation (4)), it can be seen in Figure 6c that volume energy densities Ev and roughnesses Ra are strongly correlated. High volume energy densities Ev lead to high roughnesses Ra. At similar volume energy densities Ev, parameter sets with low power (especially P = 140, 240 W) lead to low roughness values. As published by Demir et al. [2] and Laitinen et al. [8], pulsed exposures lead to narrower melt tracks owing to shorter interaction times of laser and material. This suggests that, without adjusting the hatch to h < 120 µm, there is an insufficient overlap of individual melt tracks. In order to ensure sufficient overlap of adjacent melt tracks, and thus enable low roughness and theoretically dense components, the hatch distances have to be in a sufficiently low range. Assuming that the distance for two adjacent pulses should also not be greater than a suitable hatch distance h ≤ 80 µm, Metals 2021, 11, 1125 9 of 15 Equation (3) shows that, for a pulse duration of τpulse = 25 µs and a pulse repetition rate of υrep = 20 kHz, a scan speed of vscan < 3200 mm/s must be used to achieve sufficient overlap between the pulses and melt tracks. Figure 6b shows that the roughnesses remain constant scanabove speed, this scan and speed volume (with energy the densityexception (see of Section the samples 3.4), they produced are not with shown the in laser a separate power diagram.P = 640 W). Figure This means6c summarizes that there all is testedalready parameter a minimum combinations. of roughness. For Increasing volume the energy scan 3 densitiesspeed aboveEv < this 7 J/mm value, further the applied decreases energy the is volume not sufﬁcient energy to density, produce potentially adequate material leading cohesion.to an unnecessary Corresponding increase parameter in porosity. sets are thus not considered further.

τ τ tpowder = 60 µm pulse = 25 µs angle = 20° tpowder = 60 µm pulse = 25 µs angle = 20° υ υ rep = 20 kHz dspot = 80 µm single exposure rep = 20 kHz dspot = 80 µm single exposure

140 vscan / mm/s h / µm 140 h / µm Ppeak / W 750 60 60 140 120 2250 120 120 120 240 100 3250 100 340 4750 440 80 80 640 / µm / µm / a a

R 60 R 60

40 40

20 20

0 0

0 100 200 300 400 500 600 700 0 1000 2000 3000 4000 5000 Metals 2021, 11, x FOR PEER REVIEW 10 of 15

Ppeak / W vscan / mm/s (a) (b)

140 Ppeak / W h / µm tpowder = 60 µm Ti6Al4V υ angle = 20° 140 40 rep = 20 kHz 120 τ single exposure 240 60 pulse = 25 µs

100 340 80 dspot = 80 µm 440 120 80

/ µm 540 a

R 60 640

0 102030405060708090100 3 Ev / J/mm (c)

◦ Figure 6. Measured roughnessesroughnessesR Raa ofof 2020° anglesangles withwith singlesingle exposure:exposure: RoughnessRoughnessR Raaas as aa functionfunction of of the the laser laser peak peak power power Ppeak (a), scan speed vscan (b) and volume energy density Ev (c) Ppeak (a), scan speed vscan (b) and volume energy density Ev (c).

Figure 67a,b shows show the increasing measured roughnesses OT signals as Raa functionfor increasing of the volume laser power energyPpeak densityand Edecreasingv. As can be scanning seen, the speed introducedvscan. Summarizing volume ener thegy varieddensity and and constant the signal process delivered parameters by OT areinto strongly the volume correlated. energy densityAs the volume (see Equation energy (4)), density it can decreases, be seen in the Figure standard6c that deviation volume ofenergy the OT densities signal alsoEv and decreases. roughnesses ConsequentRa arely, strongly the introduction correlated. of lower High volumevolume energy densities Ev lowerslead to the high average roughnesses temperatureRa. in the overhang structures during pulsed processes,At resulting similar volume in smoother energy processes densities andEv lo, parameterwer roughness sets on with the low downskin power (especiallysurfaces of Pthe= 140,test 240specimens. W) lead toHowever, low roughness the lower values. energy As publishedinput leads by to Demir increased et al. [ 2porosities] and Laitinen (see Figureet al. [8 8).], pulsed Using exposuresthe parameter lead set to narrowershown in melt Figure tracks 8 as owing an example, to shorter it can interaction be found times that doubleof laser andexposure material. has Thisthe potential suggests that,to reduce without porosity adjusting (by the23.8%) hatch with to h increasing< 120 µm, thererough- is nessan insufficient (by 12.9%). overlap For these of reasons, individual the meltdouble tracks. exposure In order test toseries ensure is designed sufficient as overlap described of inadjacent Section melt 3.4. tracks, and thus enable low roughness and theoretically dense components, the hatch distances have to be in a sufficiently low range. Assuming that the distance for 100 τ Ppeak / W h / µm tpowder = 60 µm pulse = 25 µs Ti6Al4V υ angle = 20° 140 40 rep = 20 kHz dspot = 80 µm 80 240 60 single exposure 340 80 60 440 120 540 40 640 OT-Signal / % / OT-Signal

0 102030405060708090100 3 Ev / J/mm Figure 7. Measured OT signals of 20° angles with broad varying powers, scan speeds, and hatch distances.

Metals 2021, 11, x FOR PEER REVIEW 10 of 15

140 Ppeak / W h / µm tpowder = 60 µm Ti6Al4V υ angle = 20° 140 40 rep = 20 kHz 120 τ single exposure 240 60 pulse = 25 µs

100 340 80 dspot = 80 µm 440 120 80

/ µm 540 a

R 60 640 Metals 2021, 11, 1125 10 of 15 40

0 two adjacent pulses should also not be greater than a suitable hatch distance h ≤ 80 µm, 0Equation 102030405060708090100 (3) shows that, for a pulse duration of τpulse = 25 µs and a pulse repetition rate of υrep = 20 kHz, a scan speed of vscan < 32003 mm/s must be used to achieve sufﬁcient overlap Ev / J/mm between the pulses and melt( tracks.c) Figure6b shows that the roughnesses remain constant above this scan speed (with the exception of the samples produced with the laser power Figure 6. Measured roughnessesP = 640 Ra of W). 20° This angles means with thatsingle there exposure: is already Roughness a minimum Ra as a function of roughness. of the laser Increasing peak power the scan Ppeak (a), scan speed vscan (b) andspeed volume above energy this value density further Ev (c) decreases the volume energy density, potentially leading to an unnecessary increase in porosity. Figure 77 showsshows thethe measuredmeasured OTOT signalssignals asas aa functionfunction ofof thethe volumevolume energyenergy densitydensity Ev. As can be seen, the introduced volume energy density and the signal delivered by OT Ev. As can be seen, the introduced volume energy density and the signal delivered by OT are strongly correlated. As the volume energy density decreases, the standard deviationdeviation of the OT signal also decreases. Consequent Consequently,ly, the introduction of lower volume energy densities Ev lowers the average temperature in the overhang structures during pulsed pro- densities Ev lowers the average temperature in the overhang structures during pulsed cesses,processes, resulting resulting in smoother in smoother processes processes and and lower lower roughness roughness on onthe the downskin downskin surfaces surfaces of theof the test test specimens. specimens. However, However, the the lower lower energy energy input input leads leads to to increased increased porosities (see Figure 88).). UsingUsing thethe parameterparameter setset shownshown inin FigureFigure8 8 as as an an example, example, it it can can be be found found that that double exposure hashas thethe potential potential to to reduce reduce porosity porosity (by (by 23.8%) 23.8%) with with increasing increasing roughness roughness(by 12.9%). (by 12.9%). For theseFor these reasons, reasons, the doublethe double exposure exposure test test series series is designed is designed as describedas described in inSection Section 3.4 3.4..

100 τ Ppeak / W h / µm tpowder = 60 µm pulse = 25 µs Ti6Al4V υ angle = 20° 140 40 rep = 20 kHz dspot = 80 µm 80 240 60 single exposure 340 80 60 440 120 540 40 640 OT-Signal / % / OT-Signal

0 102030405060708090100 3 Metals 2021, 11, x FOR PEER REVIEW Ev / J/mm 11 of 15

Figure 7. Measured OTFigure signals 7. Measured of 20◦ angles OT signals with broad of 20° varying angles powers,with broad scan varyin speeds,g powers, and hatch scan distances. speeds, and hatch distances.

FigureFigure 8. Comparison 8. Comparison of porosities of porosities from from single single exposure exposure (a) to (a double) to double exposure exposure (b). (b).

FigureFigure 9 shows9 shows the theresults results of the of the double-pul double-pulsedsed exposure exposure test test series. series. The The values values for for the thevolume volume energy energy density density are split are split into into two two exposure exposure passes passes with with identical identical parameters. parameters. TheThe calculated calculated total total energy energy introduced introduced is thus is thus twice twice as high. as high. As already As already mentioned, mentioned, the the energyenergy introduced introduced is not is notequal equal to the to theabsorb absorbeded energy energy actually actually present present in the in theprocess. process. In Figure 9, the roughnesses are plotted as a function of laser power Ppeak (a), scan speed vscan (b), and hatch distance h (c) for two levels of the remaining parameters. Figure 9 (d) summarizes all parameter combinations as volume energy density Ev and shows the roughness as a function of Ev. The level of roughness values Ra is significantly lower than in the more widely spread single exposure test series, which can be explained by the lower absorbed energy owing to double exposure. Figure 9a–c show that low powers Ppeak, high scan speeds vscan, and high hatch distances h tend to lead to small downskin roughnesses Ra. Summarizing these process parameters to volume energy density Ev confirms a correlation (see Figure 9d). It can be seen that the lowest roughnesses are achieved from volume energy densities of Ev ≈ 50 J/mm³. A further reduction of the volume energy density does not lead to lower values for roughness Ra. The roughnesses are in the volume energy density range between Ev ≈ 15 … 50 J/mm³ at a level between Ra ≈ 20 … 35 µm. Although double exposures contribute twice as much energy as single exposures, the results show that double exposures can achieve very low roughness values on the downskin surfaces compared with a cw process or single exposures. For the final investigation of the combination of pulsed and continuous exposure strategies, three parameter sets with low roughnesses of different powers, scan speed, hatches, and volume energy densities are selected (see red squares in Figure 9d).

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◦ Figure 9. Measured roughnesses Ra of 20 angles with double exposure: Roughness Ra as a function of the laser peak power Ppeak (a), scan speed vscan (b), hatch distance h (c) and volume energy density Ev (d).

In Figure9, the roughnesses are plotted as a function of laser power Ppeak (a), scan speed vscan (b), and hatch distance h (c) for two levels of the remaining parameters. Figure9d summarizes all parameter combinations as volume energy density Ev and shows the roughness as a function of Ev. The level of roughness values Ra is signiﬁcantly lower than in the more widely spread single exposure test series, which can be explained by the lower absorbed energy owing to double exposure. Figure9a–c show that low powers Ppeak, high scan speeds vscan, and high hatch distances h tend to lead to small downskin roughnesses Ra. Summarizing these process parameters to volume energy density Ev conﬁrms a correlation (see Figure9d). It can be seen that the lowest roughnesses are achieved from volume energy 3 densities of Ev ≈ 50 J/mm . A further reduction of the volume energy density does not lead to lower values for roughness Ra. The roughnesses are in the volume energy density 3 range between Ev ≈ 15 ... 50 J/mm at a level between Ra ≈ 20 ... 35 µm. Although Metals 2021, 11, 1125 12 of 15

double exposures contribute twice as much energy as single exposures, the results show that double exposures can achieve very low roughness values on the downskin surfaces compared with a cw process or single exposures. For the ﬁnal investigation of the combination of pulsed and continuous exposure strategies, three parameter sets with low roughnesses of different powers, scan speed, hatches, and volume energy densities are selected (see red squares in Figure9d).

4.3. Combination of Pulsed Downskin and Continuous Wave Inﬁll Parameters When combining pulsed downskin with cw inﬁll parameters (see Figure3), the overhang structures with angles of 20◦ cannot be manufactured. For this reason, the results of the next steeper overhang angle (25◦) are presented. With parameter set Downskin 1 3 ◦ (Ev ≈ 12 J/mm ), 25 angles are also not manufacturable (see exemplary Figure 10). Test 3 specimens with parameter sets Downskin 2 and 3 (Ev ≈ 17, 28 J/mm ) can be manufactured regardless of the downskin thickness. Figure 11 shows the roughness as a function of the Metals 2021, 11, x FOR PEER REVIEW 13 of 15 3 downskin thickness (see Figure3). The commercial cw parameter set ( Ev ≈ 38 J/mm ) is shown for reference as a vertical line and at 0 mm downskin thickness.

3 Figure 10. PhotographFigure 10. of testPhotograph specimen ofwith test combined specimen cw with (Ev ≈ combined38 J/mm³) and cw pulsed (Ev ≈ (38Ev ≈ J/mm 12 ) and pulsed 3 J/mm³) parameters(Ev ≈ 12with J/mm a downskin) parameters thickness with of a downskin2.4 mm. thickness of 2.4 mm.

140 Ti6Al4V #exposure Ev / J/mm³ tPowder = 60 µm υ angle = 25° 1 38 rep = 20 kHz 120 τ 2 17 pulse = 25 µs 28 d = 80 µm 100 spot

80 / µm / a R 60

0,00,51,01,52,02,53,0 downskin thickness / mm

Figure 11. Measured roughnesses Ra of 25° angles with commercial cw infill parameter as a function of downskin thickness with pulsed parameters.

5. Conclusions

In this study, the influence of pulsed exposure strategies (τpulse = 25 µs, υrep = 20 kHz)

on manufacturing of flat overhang structures and◦ the roughness of corresponding downskin surfacesFigure in powder 11. Measured bed fusion roughnesses of Ti6Al4RaVof using 25 angles laser with beam commercial was investigated. cw inﬁll parameter Initially, as a function it was foundof that downskin a reduction thickness in withaccumulated pulsed parameters. heat during cw exposure has the potential to reduce downskin surface roughness. The use of pulsed exposure strategies instead of cw also leads to reduced accumulated heat. The results of this investigation can be summarized in the following key takeaways: • Pulsed exposure strategies with reduced volume energy densities enable manufacturing of flatter overhang structures down to <20° (pulsed) instead of 25° (cw). • In addition, pulsed exposure strategies lead to a reduction in roughness at the downskin surfaces down to Ra ≈ 20 µm (pulsed) instead of Ra ≈ 50 µm (cw). • When combining cw infill and pulsed downskin, heat accumulation and thickness of the downskin area must be considered to enable a decrease in roughness. A reduction of the overhang angle could not be shown owing to the high energy introduced in the infill. In order to achieve low overhang angles and low downskin roughness at high component densities, a further research approach could be to increase the volume energy density from the downskin surface in a graded manner. Thus, the gap in energy densities and porosities between downskin and infill could be reduced.

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As can be seen, the roughness values increase for the combination of pulsed and cw parameters for thin downskin areas, especially for single exposure with Downskin 3 2 (Ev ≈ 17 J/mm ). The roughness values for the combination of cw infill and pulsed 3 downskin with Downskin 3 (Ev ≈ 28 J/mm ) remain at a consistent level above the achievable roughness of cw parameters without separate downskin exposure, regardless of the downskin thickness used. The roughnesses of the downskin surfaces with Downskin 3 2 (Ev ≈ 17 J/mm ) exceed these values up to a downskin thickness of 1.2 mm. From a downskin thickness of ≥1.5 mm, the roughness reaches its minimum with this parameter combination and falls below the roughness values for the cw exposure without a separate downskin parameter. One possible hypothesis is that the cw infill exposure introduces energy into the material at a level at which the heat capacity and thermal conductivity of the downskin region are no longer sufficient to dissipate the heat owing to the increased porosity. As a result, the downskin area overheats, which leads to increased roughness and, in extreme cases, even reduces the manufacturability of the specimen. The larger the gap between the energy densities, the higher the porosity in the downskin area, which intensifies the effect. This hypothesis is supported by the high roughnesses in single exposures with Downskin 3 2 (Ev ≈ 17 J/mm ). In these specimens, the double exposure presumably reduces the porosity, so that the increase in roughness is lower. Above a certain downskin thickness (approximately 1.3 mm in the example shown), the temperature at the interface between the powder bed and the downskin surface is no longer critical for roughness increase. As temperature increases occur in deeper layers, it cannot be detected with the existing system technology.

5. Conclusions

In this study, the influence of pulsed exposure strategies (τpulse = 25 µs, υrep = 20 kHz) on manufacturing of flat overhang structures and the roughness of corresponding downskin surfaces in powder bed fusion of Ti6Al4V using laser beam was investigated. Initially, it was found that a reduction in accumulated heat during cw exposure has the potential to reduce downskin surface roughness. The use of pulsed exposure strategies instead of cw also leads to reduced accumulated heat. The results of this investigation can be summarized in the following key takeaways: • Pulsed exposure strategies with reduced volume energy densities enable manufacturing of flatter overhang structures down to <20◦ (pulsed) instead of 25◦ (cw). • In addition, pulsed exposure strategies lead to a reduction in roughness at the downskin surfaces down to Ra ≈ 20 µm (pulsed) instead of Ra ≈ 50 µm (cw). • When combining cw infill and pulsed downskin, heat accumulation and thickness of the downskin area must be considered to enable a decrease in roughness. A reduction of the overhang angle could not be shown owing to the high energy introduced in the infill. In order to achieve low overhang angles and low downskin roughness at high component densities, a further research approach could be to increase the volume energy density from the downskin surface in a graded manner. Thus, the gap in energy densities and porosities between downskin and infill could be reduced. In this study, pulse repetition rates and pulse durations were kept constant with υrep = 20 kHz and τpulse = 25 µs. The pulse durations, repetition rates, and their combination have an influence on the process and the process result [8]. Studies with varying pulse durations and repetition rates should thus be the subject of further investigations. This study demonstrates the enormous potential that the use of pulsed exposure strategies offers in reducing and avoiding support structures.

Author Contributions: Conceptualization, J.G., R.S., and G.F.; methodology, J.G., P.C., and R.S.; validation, J.G. and P.C.; formal analysis, J.G. and P.C.; investigation, P.C.; resources, G.F.; data curation, J.G.; writing—original draft preparation, J.G.; writing—review & editing, J.G. and K.W.; Metals 2021, 11, 1125 14 of 15

visualization, J.G. and P.C.; supervision, G.F. and K.W.; project administration, J.G. and R.S.; funding acquisition, no additional/external funding. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Data Availability Statement: The data presented in this study are available in the article. Acknowledgments: This study was carried out in close cooperation with AMCM GmbH (an EOS group company). We thank AMCM for providing their infrastructure and materials as well as the great cooperation. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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