Mechanical Properties of 3D-Printed Maraging Steel Induced by Environmental Exposure

metals

Article Mechanical Properties of 3D-Printed Maraging Steel Induced by Environmental Exposure

Troy Y. Ansell 1,* , Joshua P. Ricks 1, Chanman Park 1, Chris S. Tipper 2 and Claudia C. Luhrs 1

1 Mechanical and Aerospace Engineering Department, Naval Postgraduate School, Monterey, CA 93943, USA; [email protected] (J.P.R.); [email protected] (C.P.); [email protected] (C.C.L.) 2 Marine Corps Logistics Command, Marine Depot Maintenance Command, Albany, GA 31704, USA; [email protected] * Correspondence: [email protected]

Received: 11 January 2020; Accepted: 31 January 2020; Published: 4 February 2020

Abstract: Changes in the mechanical properties of selective laser melted maraging steel 300 induced by exposure to a simulated marine environment were investigated. Maraging steel samples were printed in three orientations: vertical (V), 45◦ (45), and horizontal (H) relative to the print bed. These were tested as-printed or after heat-treatment (490 ◦C, 600 ◦C, or 900 ◦C). One set of specimens were exposed in a salt spray chamber for 500 h and then compared to unexposed samples. Environmental attack induced changes in the microstructural features and composition were analyzed by scanning electron microscopy and energy dispersive spectroscopy respectively. Samples printed in the H and 45◦ directions exhibited higher tensile strength than those printed in the V direction. Corrosion induced reduction in strength and hardness was more severe in specimens heat-treated between 480 ◦C and 600 ◦C versus as-printed samples. The greatest decrease in tensile strength was observed for the 45◦-printed heat-treated samples after exposure. A comparison between additive and subtractive manufactured maraging steel is presented.

Keywords: additive manufacturing; selective laser melting; corrosion; environmental testing

1. Introduction Additive manufacturing (AM) is transforming how organizations manufacture and/or acquire products. The various rapid prototyping technologies: fused deposition model, stereolithography, selective laser melting, direct laser metal deposition, etc., are all changing not only how organizations prototype new parts but also how manufacturing of production and replacement parts are completed [1]. Selective laser melting (SLM) is a mature AM technology and is used for the direct fabrication of complex and functional metal components by laser melting a bed of metal powder layer by layer until a fully formed metal part is achieved. The SLM technique has been validated for a number of important metal alloys including Ti-6Al-4V [2], 316 stainless steel [3], AlMg10Si [4], and maraging steel [5,6]. Microstructure and mechanical properties of AM metals have been investigated and often compared to subtractive manufactured metals. Little is known, however, on how additively manufactured materials will behave in corrosive environments and how their properties compare to materials produced by traditional subtractive technologies. The aim of this paper is to present such a comparison for a maraging steel exposed to a simulated marine environment. Maraging steel (a portmanteau of martensite and aging) is a class of high alloy, low carbon steel developed for structural applications requiring high strength such as rocket booster casings and pressure vessels [7,8]. These steels exhibit high strength and toughness and good ductility and are sought after for the alloys’ excellent resistance to fatigue loading and weldability [9,10]. The excellent mechanical properties of maraging steel are attributed to the presence of Ni and low C content

Metals 2020, 10, 218; doi:10.3390/met10020218 www.mdpi.com/journal/metals Metals 2020, 10, 218 2 of 11

(<0.03 wt%) contributing to the formation of an iron-nickel martensitic microstructure. Precipitation hardening occurs when applying an appropriate heat-treatment; Ni-based precipitates (Ni3Mo and Ni3Ti) are responsible for the strengthening [10,11]. Further increases in strength are accomplished by the addition of Co and Mo, which are part of alloys classified as 200, 250, 300, and 350 [7]. Maraging steel alloys contain between 15 wt% and 25 wt% Ni; however, the highest tensile and yield strength were measured in alloys with 18 wt% Ni [12]. The 18 wt% Ni alloys, 18Ni-300 exhibits high strength, hardness, and ductility. Multiple studies have been published regarding additively manufactured maraging steel: fully dense parts made from the laser processing of 18Ni-300 powders were accomplished by Stanford et al. on an EOS M250 extended platform [13] and the effects that powder size and printing parameters (e.g., scan speed and layer thickness) have on the mechanical properties and microstructure of 18Ni-300 studied by Yasa et al., Kempen et al. and others [5,6,14–16]. Jägle et al. investigated the properties of heat-treated 18Ni-300 and found three Ni-based precipitates form and observed austenite reversion after aging [17]. Additional work on the behavior of aged 18Ni-300 was performed confirming austenite reversion and its deleterious effects on mechanical properties [16,18–20]. Print direction also affected the material properties of printed 18Ni-300 as observed by Tan et al. [21]. In that work, horizontally printed parts exhibited higher strength and hardness compared to vertically printed parts and aging relieved stresses caused by the layer-wise construction of specimens. Corrosion behavior of 18Ni-300 was also studied by Tan et al. using potentiodynamic polarization tests in 3.5% NaCl room-temperature solution [21]. The latter, however, only looked at differences in galvanic corrosion between as-printed and aged (heat-treated) samples printed along one direction. In this paper, the changes in mechanical properties of printed 18Ni-300 along diverse print orientations and various heat-treated states due to exposure to a simulated marine environment were investigated.

2. Materials and Methods Test specimens were printed using Electro-Optical Systems (EOS) MaragingSteel MS1 powder [22] in an EOS M400 printer (EOS, Krailling, Germany). The powder had the composition of Maraging Steel 300 (18Ni-300, US; 1.2709, European; X3NiCoMoTi 19-9-5, German). The printer parameters are listed in Table1. Three groups of samples were printed in three di ﬀerent orientations (group names given in parentheses): in the XY-plane of the print stage (H); perpendicular to the print stage (V), and in a 45◦ direction relative to the print stage (45). Printed specimens were then machined into tensile testing specimens following the ASTM standard E8/E8M-16a [23]. Completed samples were then tested “as-printed (AP)” or heat-treated following several diﬀerent treatment conditions: 490 ◦C for six hours (HT490), 600 ◦C for six hours (HT600), or 900 ◦C for 45 min (HT900). All heat-treatments were completed in an argon atmosphere and air-cooled (with between 30–60 min cool-down time). Printed specimens were compared to 18Ni-300 maraging steel processed by the vacuum induction melting-vacuum arc remelting (VIM-VAR) method (MSC Industrial Supply Co., Cleveland, OH, USA), that was then machined following ASTM E8/E8M, hereafter referred to as CNC.

Table 1. Parameters used in Electro-Optical Systems (EOS) M400 to print 18Ni-300 samples.

Volume Rate Layer Height Powder Dosing Pressure Recoating Speed (mm2/s) (µm) (%) (Pa) (mm/s) 5.5 50 120 to 130 280 260

An MX-9204 Salt Spray Chamber (Associated Environmental Systems, Acton, MA, USA) was used for exposing samples to a simulated marine environment (a schematic is shown in Figure1a). First, a solution of 3.5 g NaCl per 100 mL of distilled water was prepared to approximately simulate the salinity found on average in the world’s oceans [24]. The salt solution was then added to the salt solution reservoir. Compressed air was pumped into the saturation tower where pressure was maintained between 82.7–103 kPa (12–15 psi) while temperature was held between 41–43 ◦C during Metals 2020, 10, 218 3 of 11

Metals 2020, 10, x FOR PEER REVIEW 6 of 11 the test run. Tap water was pumped through a demineralizer before ﬂowing into the saturation tower. Demineralizedtower. Demineralized water was water then routedwas then to anrouted atomizer. to an atomizer. Saltwater Saltwater from the from reservoir the reservoir was siphoned was throughsiphoned a ﬁlter through assembly a filter (containing assembly a nylon(containing mesh a strainer) nylon mesh up to strainer) the atomizer. up to the The atomizer. atomizer The mixed and atomizedatomizer themixed two and water atomized sources the and two sprayed water sources the mixture and sprayed into the the salt mixture spray into chamber. the salt spray chamber. (a)

(b) (c)

FigureFigure 1. Schematic 1. Schematic of theof the salt salt spray spray chamberchamber (“salt (“salt fog”). fog”). In the In sketch the sketch (a), the (a ),saturation the saturation tower and tower salt solution reservoir are labeled 1 and 2, respectively. The flow of air and water are indicated. and salt solution reservoir are labeled 1 and 2, respectively. The flow of air and water are indicated. Samples are seen hanging from 3D printed sample holders in (b). Visible “fog,” in (c), generated from Samples are seen hanging from 3D printed sample holders in (b). Visible “fog,” in (c), generated from the atomized salt solution located in the reservoir. the atomized salt solution located in the reservoir. One end of the samples was threaded in custom‐printed sample holders made of either carbon Onefiber end reinforced of the samplesnylon (CarbonX™ was threaded by 3DX in custom-printedTechnology, Grand sample Rapids, holders MI, USA) made or ofpolyethylene either carbon fiber reinforcedterephthalate nylon (Esun, (CarbonX Shenzhen,™ China).by 3DX These Technology, were hung Grandin the salt Rapids, spray chamber. MI, USA) Figure or polyethylene 1b shows terephthalatesamples placed (Esun, in Shenzhen, the salt spray China). chamber These (colloquially were hung “salt in fog”) the salthanging spray down chamber. from printed Figure sample1b shows samplesholders. placed Salt in fog the is salt visible spray in Figure chamber 1c. Samples (colloquially were exposed “salt fog”) for three hanging weeks down in total from (approximately printed sample holders.500 Salt h). fog is visible in Figure1c. Samples were exposed for three weeks in total (approximately 500 h). The naming scheme for samples was chosen based on whether samples were exposed to a salt Thefog namingenvironment scheme (simulated for samples marine was environment) chosen based or on not, whether their samplesprinting wereorientation, exposed and to aheat salt‐ fog environmenttreatment (simulated (or lack thereof). marine For environment) example; samples or not, exposed their to printing a salt fog orientation, environment and for three heat-treatment weeks, printed perpendicular to the stage, and heat‐treated at 600 °C for six hours are called 3V600 (time of (or lack thereof). For example; samples exposed to a salt fog environment for three weeks, printed exposure in weeks, print orientation, and temperature of heat‐treatment in °C). Another example; perpendicularsamples with to the no stage, exposure, and printed heat-treated horizontally at 600 and◦C with for six no hours heat‐treatment, are called are 3V600 called (time 0HAP. of exposure in weeks, printTensile orientation, test specimens and temperaturewere then installed of heat-treatment in a tensile in tester◦C). Another(Instron 5982 example; Tensile samples Tester, with no exposure,Norwood, printed MA, USA). horizontally The tensile and test with speed no for heat-treatment, all samples was are 2 mm/min. called 0HAP. Fractured samples were Tensilethen prepared test specimens for further were analysis then installed using instandard a tensile metallurgical tester (Instron processing 5982 Tensile involving Tester, cutting, Norwood, MA, USA).mounting, The tensilepolishing, test and speed etching. for all Microhardness samples was 2 measurements mm/min. Fractured were completed samples were on a then Durascan prepared for furtherMicrohardness analysis usingTester (Struers, standard Ballerup, metallurgical Denmark). processing involving cutting, mounting, polishing, and etching.Optical Microhardness imaging of samples measurements was performed were completedon an Epiphot on 200 a Durascan reflective Microhardnessoptical microscope Tester (Nikon, Tokyo, Japan). Microstructural characterization was done on mounted specimens using a (Struers, Ballerup, Denmark). Optical imaging of samples was performed on an Epiphot 200 reflective optical microscope (Nikon, Tokyo, Japan). Microstructural characterization was done on mounted specimens using a Metals 2020, 10, 218 4 of 11 Metals 2020, 10, x FOR PEER REVIEW 6 of 11

NeonNeon 40 40 Dual-beam Dual‐beam (Zeiss, (Zeiss, Oberkochen, Oberkochen, Germany) Germany) scanning electronscanning microscope electron (SEM).microscope Microanalysis (SEM). wasMicroanalysis conducted was by energy-dispersive conducted by energy spectroscopy‐dispersive (EDS) spectroscopy using an Octane (EDS) using Plus EDS an Octane detector Plus (EDAX, EDS Mahwah,detector (EDAX, NJ, USA). Mahwah, Samples NJ, were USA). prepared Samples for were SEM prepared/EDS analysis for SEM/EDS by first analysis mounting by first specimens mounting in epoxyspecimens mounts. in epoxy Mounted mounts. samples Mounted were samples then placed were inthen an placed automatic in an polisher automatic (Buehler, polisher Lake (Buehler, Bluff, IL,Lake USA) Bluff, and IL, ground USA) and using ground 300-, 600-,using and 300 1200-grit‐, 600‐, and paper 1200 at‐grit a wheel paper speed at a wheel of 26.2 speed rad/ sof (250 26.2 RPM) rad/s and(250 a RPM) force ofand 22.2 a force N (5.0 of lbs. 22.2 f) N for (5.0 15–60 lbs. minf) for at 15–60 each step.min at After each grinding, step. After samples grinding, were samples polished were in thepolished sample in automatic the sample polisher automatic using polisher long-napped using synthetic long‐napped polishing synthetic cloth (Buehlerpolishing Microcloth cloth (Buehler PSA) andMicrocloth polishing PSA) fluid, and 1 polishingµm alumina fluid, suspension 1 μm alumina (Buehler suspension MicroPolish). (Buehler Prior MicroPolish). to SEM microstructural Prior to SEM analysis,microstructural samples analysis, were etched samples with awere Nital etched etching with solution a Nital (methanol etching was solution the alcohol (methanol in this was case). the alcohol in this case). 3. Results and Discussion 3. ResultsRaw powders and Discussion used for printing consisted of mostly spherical particles with sizes ranging from 1 µm up to 84 µm and an average size of 14 11 µm. A few of the particles observed in the SEM were Raw powders used for printing consisted± of mostly spherical particles with sizes ranging from irregularly shaped due to being formed by agglomerates that sintered. Those types of particulates are 1 μm up to 84 μm and an average size of 14 ± 11 μm. A few of the particles observed in the SEM were typically observed in powders produced by atomization. The composition of the powders, printed and irregularly shaped due to being formed by agglomerates that sintered. Those types of particulates heat-treated (at 490 C) parts were studied using EDS. The elements found, in wt%, in the powders are typically observed◦ in powders produced by atomization. The composition of the powders, and printed parts are shown in Table2. The powder samples were rich in Ni and Co, but low in Mo as printed and heat‐treated (at 490 °C) parts were studied using EDS. The elements found, in wt%, in compared to the composition of 18Ni-300 found in the literature [6,22,25]. The printed parts matched the powders and printed parts are shown in Table 2. The powder samples were rich in Ni and Co, the expected composition. It is worth noting that EDS is only a semi-quantitative method of analysis but low in Mo as compared to the composition of 18Ni‐300 found in the literature [6,22,25]. The that renders data from localized sections rather than a bulk analysis. printed parts matched the expected composition. It is worth noting that EDS is only a semi‐ quantitative method of analysis that renders data from localized sections rather than a bulk analysis. Table 2. Composition of maraging steel samples used to produce test samples in this work. All values in wt%. Table 2. Composition of maraging steel samples used to produce test samples in this work. All values Samplein wt%. Fe Ni Co Mo Ti Al Cr Si [22]Sample balance Fe 17–19Ni 8.5–9.5Co 4.5–5.2Mo Ti 0.6–0.8 Al 0.05–0.15 Cr 0.5 maxSi 0.1 max Powders balance 17 9.1 2.9 1.3 0.1 0.7 0.1 [22] balance 17–19 8.5–9.5 4.5–5.2 0.6–0.8 0.05–0.15 0.5 max 0.1 max HT490 balance 18 9.2 4.1 0.7 0.1 0.2 0.2 Powders balance 17 9.1 2.9 1.3 0.1 0.7 0.1 HT490 balance 18 9.2 4.1 0.7 0.1 0.2 0.2 Printed metals exhibit a unique microstructure as a result of SLM printing in the form of track marksPrinted as seen metals in Figure exhibit2. The a direction unique microstructure of the track marks as a indicates result of the SLM orientation printing ofin athe printed form sampleof track inmarks relation as seen to the in build Figure plate. 2. The Unaged direction maraging of the steel track printed marks in indicates the V (Figure the 2orientationa), 45 ◦ (Figure of a2 b),printed and Hsample (Figure in2 c)relation directions. to the Arrows build plate. are placed Unaged in themaraging figures st toeel indicate printed the in generalthe V (Figure direction 2a), of 45° the (Figure apex of2b), laser and track H (Figure marks. 2c) During directions. SLM Arrows printing, are the placed high-power in the figures laser melts to indicate the powders the general deposited direction on the of buildthe apex plate. of As laser the laser track passes marks. any During one point SLM in theprinting, build plate, the high the melted‐power powders laser melts begin the to coolpowders and fusedeposited together. on the Once build the plate. printer As completes the laser passes one layer, any theone next point layer in the of build powder plate, is deposited, the melted and powders the laserbegin begins to cool its and next fuse pass. together. Once the pr

(a) (b) (c) Z

Figure 2. Optical images of as‐printed maraging steel samples printed in (a) V, (b) 45°, and (c) H Figure 2. Optical images of as-printed maraging steel samples printed in (a) V, (b) 45◦, and (c)H orientations.orientations. The The red red arrow arrow indicates indicates the orientation the orientation of samples of samples when imaged when in imaged the optical in microscope.the optical Themicroscope. scale is 10 Theµm. scale is 10 μm.

Shown in Figure 3 are measurements of the ultimate tensile strength (UTS) of as‐printed and heat‐treated maraging steel with a V orientation. The as‐printed samples met EOS benchmark values Metals 2020, 10, 218 5 of 11

MetalsShown 2020, 10, inx FOR Figure PEER3 REVIEWare measurements of the ultimate tensile strength (UTS) of as-printed6 and of 11 heat-treated maraging steel with a V orientation. The as-printed samples met EOS benchmark values andand exceededexceeded the the values values of of traditionally traditionally fabricated fabricated CNC CNC parts parts measured measured here. here. Samples Samples aged aged at 490 at 490◦C (0V490)°C (0V490) saw saw an an increase increase in in UTS UTS close close to to 2000 2000 MPa. MPa. Tensile Tensile strength strength decreased decreased below below this this maximummaximum whenwhen printed printed samples samples were were heat-treated heat‐treated at higher at higher temperatures. temperatures. At 600 At◦C, 600 the 0V600°C, the sample 0V600 exhibited sample aexhibited UTS of 1370 a UTS MPa. of0 When137 MPa. heat-treated When atheat 900‐treated◦C, the at 0V900 900 sample°C, the showed0V900 sample a further showed decrease a further in UTS downdecrease to 950 in UTS MPa; down a value to 950 which MPa; was a value lower which than UTS was forlower as-printed than UTS or for CNC as‐ samples.printed or The CNC behavior samples. of yieldThe behavior strength of closely yield matchedstrength theclosely behavior matched of UTS the forbehavior all samples. of UTS for all samples.

Figure 3. Ultimate tensile strength of as-printed and heat-treated specimens printed in the V orientation andFigure unexposed. 3. Ultimate Values tensile are compared strength toof CNCas‐printed maraging and steel heat and‐treated EOS benchmarkspecimens valuesprinted [22 in]. the V orientation and unexposed. Values are compared to CNC maraging steel and EOS benchmark values These[22]. results are similar to those seen by Yasa and Kempen [5,6]. The tensile properties of as-printed 18Ni-300 exceeded those of wrought maraging steel by several hundred MPa while Young’s modulusThese and results ductility are similar were comparable to those seen and by toughness Yasa and values Kempen were [5,6]. reduced. The tensile The microstructure properties of as of‐ printedprinted maraging18Ni‐300 exceeded steel was those found of to wrought be different maraging from the steel traditional by several microstructure. hundred MPa while When Young’s printed samplesmodulus were and aged,ductility hardness were comparable and tensile and strength toughness were comparable values were to reduced. aged wrought The microstructure maraging steel of whileprinted ductility maraging and steel toughness was found were to reduced. be differen Agedt from or heat-treated the traditional maraging microstructure. steel properties When printed closely matchedsamples thosewere aged, of aged hardness wrought and or othertensile traditionally strength were fabricated comparable maraging to aged steel wrought [6]. Figure maraging3 shows steel the same,while thatductility printed and 18Ni-300 toughness exhibited were reduced. higher strength Aged or than heat‐ CNCtreated 18Ni-300 maraging steel. steel properties closely matchedStrength those is of reduced aged wrought when or the other heat-treatment traditionally temperature fabricated maraging exceeds 500steel◦ [6].C. MaragingFigure 3 shows steel isthe precipitation same, that printed hardened 18Ni‐ to300 obtain exhibited high-strength. higher strength In than traditionally CNC 18Ni fabricated‐300 steel. maraging steel, lath martensiteStrength is will reduced form during when coolingthe heat from‐treatment an initial temperature melt. Age hardening exceeds 500 causes °C. theMaraging transformation steel is ofprecipitation martensite tohardened austenite to if obtain the temperature high‐strength. is increased In traditionally above the fabricated austenite startmaraging temperature, steel, lath AS. Howmartensite high thewill temperature form during iscooling above from AS and an initial how melt. rapid Age the ha coolingrdening will causes determine the transformation the amount ofof austenitemartensite formation to austenite and if whetherthe temperature precipitates is increased form during above cooling.the austenite If the start aging temperature, temperature AS. isHow set betweenhigh the temperature 455 ◦C and 510 is above◦C, the A transformationS and how rapid to the austenite cooling iswill slow determine enough the to allow amount precipitates of austenite to formformation out of and solution whether [10]. precipitates At temperatures form during higher thancooling. 510 If◦C, the the aging transformation temperature rate is set of martensite between 455 to austenite°C and 510 is increased°C, the transformation and precipitates to doaustenite not form. is slow enough to allow precipitates to form out of solutionMicrostructural [10]. At temperatures analysis of higher vertically than printed 510 °C, specimens the transformation with or without rate of heat-treatment martensite to austenite is shown inis increased Figure4. The and intercellular precipitates structure do not form. revealed in 0VAP samples (Figure4a) is believed to be caused by theMicrostructural rapid solidification analysis of material of vertically following printed laser re-melting.specimens As-printed with or without samples heat exhibited‐treatment a multi is “domain”shown in structureFigure 4. madeThe intercellular of a combination structure of fine revealed dendrites in 0VAP with short samples secondary (Figure arms, 4a) is cellular believed regions, to be andcaused columnar by the regions rapid similar solidification to the ones of previouslymaterial following reported by laser [5,6, 17re–‐19melting.]. The onset As‐printed of laser-induced samples rapidexhibited solidification a multi “domain” prevents thestructure formation made of of lath a combination martensite [ 6of,18 fine] usually dendrites associated with short with secondary maraging steel.arms, Thiscellular cellular regions, structure and columnar is responsible regions for similar the improved to the ones strength previously of as-printed reported maraging by [5,6,17–19]. steel as The onset of laser‐induced rapid solidification prevents the formation of lath martensite [6,18] usually associated with maraging steel. This cellular structure is responsible for the improved strength of as‐ printed maraging steel as compared to traditionally fabricated maraging steel. Break‐up of the cellular structure is seen in a sample of 0V490 (Figure 4b). This is caused by the formation of austenite Metals 2020, 10, 218 6 of 11

Metals 2020, 10, x FOR PEER REVIEW 6 of 11 compared to traditionally fabricated maraging steel. Break-up of the cellular structure is seen in a within the cells and through the cell boundaries in accordance with prior studies [17,19]. Further sampleMetals of 2020 0V490, 10, x (Figure FOR PEER4b). REVIEW This is caused by the formation of austenite within the cells and6 throughof 11 removal of the cells and enlarging of the austenite phase occurred with 600 °C and 900 °C heat‐ the cell boundaries in accordance with prior studies [17,19]. Further removal of the cells and enlarging treatmentswithin as the seen cells in and Figure through 4c,d, the which cell boundaries are images in of accordance 0V600 and with 0V900, prior respectively. studies [17,19]. Further of the austenite phase occurred with 600 C and 900 C heat-treatments as seen in Figure4c,d, which removal of the cells and enlarging of the◦ austenite ◦phase occurred with 600 °C and 900 °C heat‐ are images of 0V600 and 0V900, respectively. treatments as seen in Figure 4c,d, which are images of 0V600 and 0V900, respectively. (a) (b)

(a) (b)

FigureFigureFigure 4. MicrostructureMicrostructure 4. Microstructure (SEM) (SEM) (SEM) of of ofV V-printed ‐Vprinted‐printed samples samplessamples with with differentdifferent diﬀerent heat heat heat-treatments:‐treatments:‐treatments: (a) 0VAP;(a (a) )0VAP; 0VAP; (b) ( (bb)) 0V490;0V490;0V490; ( (cc)) 0V600; 0V600; (c) 0V600; and and ( (d)) ( 0V900.d) 0V900. The The The scale scale scale is is is 1.0 1.0 1.0 μ μµm.m.

The printing orientation also affected the tensile and yield strength of maraging steel. Shown in TheThe printing printing orientation orientation also also affected aﬀected the the tensile tensile and and yield yield strength strength of of maraging maraging steel. steel. Shown Shown in Figure 5, samples printed in the H or 45° orientations exhibited increased tensile strength over V‐ Figurein Figure 5, 5samples, samples printed printed in inthe the H Hor or45° 45 orientationsorientations exhibited exhibited increased increased tensile tensile strength strength over over V‐ printed samples. In printed samples where ◦layering is perpendicular to the direction of the applied printedV-printedload, samples. samples.failure occurs In Inprinted printed before samples samples samples whereprint whereed layering such layering that islayers is perpendicular perpendicular are parallel to to thethe the loading direction direction direction of of the the (H).applied applied load,load, Tanfailure failure et al. occurs occurs found before beforethat similar samples samples to observations print printeded such such in thatthis that paper,layers layers theare are horizontally parallel parallel to to theprinted the loading loading maraging direction direction steel (H). (H). Tan etexhibited al. found found higher that that tensile similar strength to to observations then vertically in in printedthis paper, specimens the horizontally [21]. printed maraging steel exhibitedexhibited higher tensile strength then vertically printed specimens [[21].21].

Figure 5. Effect of printing orientation on the ultimate tensile strength (top line) and yield strength (bottom line). Figure 5. Eﬀect of printing orientation on the ultimate tensile strength (top line) and yield strength FigureThe 5. Effectchanges of inducedprinting inorientation tensile strength on the afterultimate heat tensile‐treatment strength of V samples(top line) were and alsoyield seen strength in H (bottom line). (bottomand 45° line).printed tensile specimens. Seen in Figure 6, tensile strength increased for samples printed in all three orientations after the 600 °C heat‐treatment. Then the tensile strength decreased for all print The changes induced in tensile strength after heat-treatment of V samples were also seen in H and Theorientations changes upon induced the 90 in0 °C tensile heat‐ treatment.strength after heat‐treatment of V samples were also seen in H and45◦ printed45° printed tensile tensile specimens. specimens. Seen Seen in Figure in Figure6, tensile 6, tensile strength strength increased increased for samplesfor samples printed printed in all in allthree three orientations orientations after after the the 600 600◦C °C heat-treatment. heat‐treatment. Then Then the the tensile tensile strengthstrength decreaseddecreased for all print orientationsorientations upon the 90 9000 °C◦C heat heat-treatment.‐treatment. Metals 2020, 10, x FOR PEER REVIEW 6 of 11 Metals 2020, 10, 218 7 of 11 Metals 2020, 10, x FOR PEER REVIEW 6 of 11

Figure 6. Changes in tensile strength due to heat‐treatment for all three print orientations. Figure 6. Changes in tensile strength due to heat-treatment for all three print orientations. Figure 6. Changes in tensile strength due to heat‐treatment for all three print orientations. Samples placed in the salt fog for three weeks (~500 h) of simulated marine exposure experienced a decreaseSamples in placed tensile in strength. the salt fog Figure for three 7 shows weeks the (~500 change h) of in simulated tensile strength marine exposureof samples experienced before and Samples placed in the salt fog for three weeks (~500 h) of simulated marine exposure experienced aafter decrease exposure. in tensile Testing strength. results Figure for V‐7printed shows samples the change are inshown tensile in strengthFigure 7a; of 45° samples‐printed before samples and in a decrease in tensile strength. Figure 7 shows the change in tensile strength of samples before and afterFigure exposure. 7b; and Testing H‐printed results samples for V-printed in Figure samples 7c. Surface are shown corrosion in Figure degraded7a; 45 the-printed tensile samples strength after exposure. Testing results for V‐printed samples are shown in Figure 7a; 45°‐printed◦ samples in inbetween Figure7 b;1.9% and and H-printed 3.7% for as samples‐printed in specimens Figure7c. (1.9% Surface for corrosion 3VAP and degraded 3.7% for 345AP). the tensile HT600 strength samples Figure 7b; and H‐printed samples in Figure 7c. Surface corrosion degraded the tensile strength betweenexperienced 1.9% andthe 3.7%highest for as-printedreduction specimensin strength (1.9% with for 6.3% 3VAP for and 3H600 3.7% and for 345AP).11.7% reduction HT600 samples for the between 1.9% and 3.7% for as‐printed specimens (1.9% for 3VAP and 3.7% for 345AP). HT600 samples experienced345,600 sample. the highest The smallest reduction reduction in strength in withstrength 6.3% occurred for 3H600 for and HT900 11.7% samples reduction with for thea negligible 345,600 experienced the highest reduction in strength with 6.3% for 3H600 and 11.7% reduction for the sample.reduction The in smallest V samples reduction and a in2.1% strength reduction occurred in UTS for for HT900 the 345,900 samples samples. with a negligible Figure 7d reduction presents inthe 345,600 sample. The smallest reduction in strength occurred for HT900 samples with a negligible Vrelative samples percent and a 2.1% changes reduction in UTS in of UTS all forsamples the 345,900 after 500 samples. h of exposure. Figure7d As presents will be the seen relative later, corrosion percent reduction in V samples and a 2.1% reduction in UTS for the 345,900 samples. Figure 7d presents the changespenetration in UTS in ofsamples all samples causes after the 500formation h of exposure. of cracking As which will be may seen otherwise later, corrosion not appear penetration in pristine in relative percent changes in UTS of all samples after 500 h of exposure. As will be seen later, corrosion samplessamples. causes This the may formation explain of the cracking reduction which in may strength otherwise of samples not appear upon in pristineexposure samples. to a marine This penetration in samples causes the formation of cracking which may otherwise not appear in pristine mayenvironment. explain the reduction in strength of samples upon exposure to a marine environment. samples. This may explain the reduction in strength of samples upon exposure to a marine environment. (a) (b) (a) (b)

FigureFigure 7. Reduction7. Reduction in tensile in tensile strength strength due to threedue weeksto three exposure weeks in exposure a simulated in marinea simulated environment marine (bothenvironment as-printed (both and heat-treated): as‐printed and (a) Vheat printed‐treated): samples; (a) V ( bprinted) 45◦ oriented samples; samples; (b) 45° (c oriented) H printed samples; samples; (c) Figure 7. Reduction in tensile strength due to three weeks exposure in a simulated marine andH printed (d) percent samples; change and in tensile(d) percent strength change of exposed in tensile printed strength samples of exposed compared printed to beforesamples exposure. compared environment (both as‐printed and heat‐treated): (a) V printed samples; (b) 45° oriented samples; (c) to before exposure. H printed samples; and (d) percent change in tensile strength of exposed printed samples compared to before exposure.

Metals 2020, 10, x FOR PEER REVIEW 6 of 11

As evidenced in Figure 7, the as‐printed samples (printed in all three orientations) exhibited relatively good corrosion resistance and little reduction in strength after exposure. Samples heat‐ treated at 600 °C saw larger reductions in strength after exposure. Samples heat‐treated to 900 °C experienced a softening after cooling but also show better corrosion resistance. This was seen by a smaller reduction in strength after exposure as compared to the as‐printed samples. Figure 7d summarizes these results. It was also seen in Figure 7 that samples heat‐treated at 600 °C were more affected by corrosion than as‐printed samples in terms of tensile strength. Heat treatment at 490 °C also increased the susceptibilityMetals 2020, 10, 218 to corrosion in aged samples. Microhardness tests were conducted on AP and HT4908 of 11 samples and shown in Figure 8. Comparisons of hardness between 0VAP and 3VAP (bottom two sets of data) and between 0V490 and 3V490 (top two sets of data) are shown. The locations of hardness measurementsAs evidenced of the in Figure0V490 7sample, the as-printed are shown samples in the (printedinset. Similar in all positions three orientations) were chosen exhibited for the hardnessrelatively measurements good corrosion resistanceconducted and on littlethe other reduction samples in strength where hardness after exposure. was measured. Samples heat-treated at 600There◦C saw was larger no difference reductions in inhardness strength away after from exposure. the surface Samples between heat-treated as‐printed to 900 specimens◦C experienced before anda softening after exposure, after cooling 0VAP but and also 3VAP. show betterThere corrosion was a measurable resistance. difference This was seen(close by to a 10%) smaller in reductionthe near‐ surfacein strength hardness after exposure between asheat compared‐treated toat 490 the as-printed°C samples samples. 0V490 and Figure 3V490.7d summarizes Results of hardness these results. tests alignIt with was the also results seen in of Figure tensile7 thattesting samples seen in heat-treated the increased at 600 rate◦ ofC weredecline more of both aﬀected in samples by corrosion after exposure.than as-printed Therefore, samples heat in‐treatment terms of tensilebetween strength. 490 °C Heat and treatment600 °C led at 490to parts◦C also with increased a greater the susceptibility to corrosion. corrosion inSuch aged an samples. outcome Microhardness is in agreement tests with were previous conducted reports on [26–29] AP and and HT490 it is believedsamples andto be shown a result in Figure of the8. formation Comparisons of intermetallic of hardness between precipitates. 0VAP It and has 3VAP been (bottom shown twothat sets the existenceof data) and of austenite between with 0V490 high and Ni 3V490 content (top leads two to sets preferential of data) are pitting shown. corrosion The locations as shown of in hardness duplex stainlessmeasurements steels used of the for 0V490 welding sample [30]. Recall, are shown during in theheat inset.‐treatments, Similar if positionsthe temperature were chosen is higher for than the Ahardnesss, austenite measurements reversion will conducted take place on [10]. the other samples where hardness was measured.

(f)

(b)

Figure 8. Results of microhardness testing in as-printedas‐printed and heat-treatedheat‐treated V samples, inset indicates indent location.

MicrostructuralThere was no di fferenceanalysis in hardnessrevealed awaythe fromdifferences the surface in susceptibility between as-printed to general specimens corrosion before dependingand after exposure, on heat 0VAP‐treatment and 3VAP.There (or lack thereof). was a measurable SEM analysis diff erenceof V printed (close to maraging 10%) in the steel near-surface samples bothhardness unaged between (Figure heat-treated 9a) and heat at 490‐treated◦C samples at 600 °C 0V490 for six and hours 3V490. (Figure Results 9b) of revealed hardness pitting tests align and withcorrosion the results by‐products. of tensile Figure testing 9 shows seen in the the differences increased rate in corrosion of decline susceptibility of both in samples between after as exposure.‐printed Therefore,and heat‐treated heat-treatment maraging between steel. Note 490 ◦theC and deeper 600 ◦penetrationC led to parts of withcorrosion a greater in the susceptibility aged sample. to Observedcorrosion. on Such the an left outcome‐hand side is in of agreement Figure 9b, with is a previous contrast reportsdifference [26 –between29] and itthe is believedbase metal to beand a corrosionresult of the affected formation metal. of The intermetallic contrast difference precipitates. is due It has to exposure been shown to the that underlying the existence microstructure. of austenite withIn effect, high the Ni contentenvironment leads toin preferential the salt spray pitting chamber corrosion etched as shownpart of in the duplex metal stainless near the steels surface. used The for weldingincreased [30 corrosion]. Recall, susceptibility during heat-treatments, of heat‐treated if the (490 temperature °C and is600 higher °C) samples than As ,is austenite likely due reversion to the will take place [10]. Microstructural analysis revealed the differences in susceptibility to general corrosion depending on heat-treatment (or lack thereof). SEM analysis of V printed maraging steel samples both unaged (Figure9a) and heat-treated at 600 ◦C for six hours (Figure9b) revealed pitting and corrosion by-products. Figure9 shows the di fferences in corrosion susceptibility between as-printed and heat-treated maraging steel. Note the deeper penetration of corrosion in the aged sample. Observed on the left-hand side of Figure9b, is a contrast di fference between the base metal and corrosion affected metal. The contrast difference is due to exposure to the underlying microstructure. In effect, the environment in the salt spray chamber etched part of the metal near the surface. The increased corrosion susceptibility of Metals 2020, 10, 218 9 of 11 Metals 2020, 10, x FOR PEER REVIEW 6 of 11 Metals 2020, 10, x FOR PEER REVIEW 6 of 11 existence of precipitates and multiple phases. For samples heat‐treated at 900 °C, corrosion heat-treated (490 ◦C and 600 ◦C) samples is likely due to the existence of precipitates and multiple existencesusceptibility of precipitateswas like as‐ printedand multiple samples phases. due to theFor existence samples of heat a one‐treated‐phase atonly 900 (austenite °C, corrosion in this phases. For samples heat-treated at 900 ◦C, corrosion susceptibility was like as-printed samples due to susceptibilitycase). was like as‐printed samples due to the existence of a one‐phase only (austenite in this case).the existence of a one-phase only (austenite in this case).

(a) (b) (a) (b)

Figure 9. SEM images of (a) 3VAP and (b) 3V600 showing differences in corrosion susceptibility. The Figurescale for 9. bothSEMSEM imagesimages images isof of 10 ( (aa μ)) 3VAP 3VAPm. and ( b) 3V600 showing differences diﬀerences in corrosion susceptibility. The scale for both images is 10 μµm. Corrosion induced a 4.7% decrease in the ultimate tensile strength of the as‐fabricated CNC sample.Corrosion This decrease induced was a 4.7% greater decrease than in in the the as ultimate‐printed tensilespecimens strength with ofdecreases the as-fabricatedas‐fabricated in UTS of CNC 1.8% sample.(between This This 0VAP decrease decrease and 3VAP), was was greater greater 2.1% than(0HAP than in in andthe the as as-printed3HAP),‐printed and specimens specimens 3.8% (045AP with with anddecreases decreases 345AP). in in UTS UTSAfter of tensile 1.8% (betweentesting, cross 0VAP ‐sections and 3VAP), were cut 2.1%2.1% from (0HAP(0HAP corroded andand 3HAP),3HAP), as‐printed andand and 3.8%3.8% CNC (045AP(045AP samples. and 345AP). Electron After microscopy tensile testing,was performed cross-sectionscross‐sections on the were were surface cut cut from fromof the corroded corroded cross‐ as-printedsections as‐printed where and and CNC corrosion CNC samples. samples. formed. Electron Electron Microscopy microscopy microscopy of was the performedwasexposed performed samples on the on surfaceshowed the surface of a thegreater cross-sectionsof the volume cross‐ sectionsof where pitting corrosionwhere corrosion corrosion formed. in the formed. Microscopy surface Microscopy of ofthe the as exposed‐ printedof the exposedsamplesspecimens showedsamples (Figure ashowed 10a) greater as compareda volume greater of volumeto pitting the as of corrosion‐fabricated pitting incorrosion CNC the surfacespecimens in the of the surface(Figure as-printed of10b). the Microscopy specimensas‐printed (Figurespecimensobservations 10a) (Figure as indicated compared 10a) aas larger to compared the number as-fabricated to ofthe surface as CNC‐fabricated cracks specimens in CNC as‐ (Figureprinted specimens 10samplesb). (Figure Microscopy compared 10b). observations Microscopy to the CNC observationsindicatedsample. a larger indicated number a larger of surface number cracks of surface in as-printed cracks in samples as‐printed compared samples to compared the CNC sample. to the CNC sample.

(a) (b) (a) (b)

FigureFigure 10.10. Schematic representation of (a)) 3VAP andand ((bb)) 3CNC3CNC (no(no heat-treatment).heat‐treatment). Cross-sectionsCross‐sections cutcut Figure 10. Schematic representation of (a) 3VAP and (b) 3CNC (no heat‐treatment). Cross‐sections cut awayaway fromfrom fracturefracture zones.zones. ScaleScale ofof 100100 μµmm includedincluded withwith eacheach SEMSEM image.image. away from fracture zones. Scale of 100 μm included with each SEM image. DespiteDespite thethe extent extent of of surface surface corrosion corrosion features features identified identified in the in SEMthe SEM study study in the in as-printed the as‐printed parts, Despite the extent of surface corrosion features identified in the SEM study in the as‐printed thoseparts, featuresthose features did not did seem not to beseem greater to be than greater the samples than the manufactured samples manufactured by traditional by methods traditional or parts, those features did not seem to be greater than the samples manufactured by traditional havemethods affected or have the overallaffected UTS the performanceoverall UTS performance under the conditions under the explored conditions herein. explored The 3D herein. printed The aged 3D methods or have affected the overall UTS performance under the conditions explored herein. The 3D specimens;printed aged however, specimens; seemed however, greatly seemed affected greatly indicating affected heat-treated indicating specimens heat‐treated should specimens be protected should printed aged specimens; however, seemed greatly affected indicating heat‐treated specimens should bybe coatingsprotected or by their coatings exposure or their to marine exposure environments to marine environments be limited. It be is limited. worth noting It is worth that thenoting surface that be protected by coatings or their exposure to marine environments be limited. It is worth noting that cracksthe surface found cracks in the found additively in the manufactured additively manufactured parts could be parts exacerbated could be with exacerbated longer exposure with longer times, the surface cracks found in the additively manufactured parts could be exacerbated with longer

Metals 2020, 10, 218 10 of 11 application of stress or more corrosive environments, thus, future studies should include a wider set of those variables.

4. Conclusions Maraging steel 18Ni-300 was printed by the SLM method in an EOS M400 3D metal printer. Samples were printed in three different orientations: V, 45◦, and H relative to the print plate. Results of the OM examination showed directional changes in laser track marks due to printing orientation differences. Printing orientation also affected tensile properties, where unexposed and non-heat-treated H and 45◦ specimens had higher UTS over as-printed V samples. The tensile strength of as-printed V samples was higher than the as-fabricated CNC (VIM-VAR) 18Ni-300. Heat-treatments at 490 ◦C and 600 ◦C led to improved mechanical properties due to precipitation hardening. Heat-treatment at 900 ◦C led to a decrease in mechanical properties. For all three printing directions, heat-treatments at 490 ◦C or 600 ◦C degraded corrosion resistance in relation to the as-printed samples. Exposed as-printed samples exhibited a smaller loss in tensile properties when compared to exposed as-fabricated CNC 18Ni-300. Yet the as-printed samples were more susceptible to surface pitting corrosion.

Author Contributions: T.Y.A. conducted the optical and electron microscopy on exposed and unexposed samples and analysis of the data. J.P.R. carried out the metallographic sample preparation; conducted some of the electron microscopy of corroded samples; and performed microhardness tests. C.P. assisted in the salt spray work and tensile testing. C.S.T. printed the maraging steel parts. C.C.L. secured the funding for the project; directed the overall research; conducted analysis of the data; and assisted in the salt spray work, electron microscopy, hardness testing, and tensile testing. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Naval Postgraduate School through the Naval Research Program, grants NPS-19-M283-A and NPS-19-N180-A. Acknowledgments: The authors would like to express our gratitude to John Mobley for his machining of samples and the sample mounts for tensile testing. The authors acknowledge the assistance of Chantel Lavendar to the microscopy effort. The authors appreciate the assistance in corrosion testing provided by student interns Isaiah Stewman and Tiffany Smith. The authors would also like to thank William Alday for his help during the sample printing stage. Conflicts of Interest: The authors declare no conflict of interest. The views expressed in this document are those of the authors and do not reflect the official policy or position of the Department of Defense or the U.S. Government.

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