Evolution of microstructure, texture and corrosion properties of additively manufactured AlSi10Mg alloy subjected to equal channel angular pressing (ECAP)
Abstract
In the selective laser melting process (SLM), the region irradiated by the laser beam is melted and quickly solidified, forming solidification lines (laser scan tracks) with symmetrical shapes. Because of the unique (rapid) crystallization conditions, the subgrain structures, typically observed inside these solidification lines, could also have variable geometric symmetrical patterns, e.g., cellular, pentagonal, or hexagonal cellular. The existence of such distinctive microstructures in SLM-made alloys has a significant impact on their superior mechanical and corrosion properties. Thus, any modification of this symmetrical microstructure (due to post-processing) can degrade or improve the properties of SLM-fabricated alloys. This study presents the experimental results on the effects of heat treatment and ECAP on microstructure modification and corrosion behavior of SLM-fabricated AlSi10Mg alloy. Light microscopy, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD) were used for microstructural analysis. The corrosion properties of the given samples were determined using open-circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. EBSD observations showed that the imposed strain resulted in an obvious reduction in grain size to similar to 1.42 mu m and similar to 0.24 mu m after the first and second ECAP passes, respectively. Electrochemical tests revealed that the corrosion resistance of the ECAP-processed AlSi10Mg alloy improved significantly, which was confirmed by a nobler E-corr and lower I-corr values, and higher polarization resistance. The final results indicated that the strain-induced crystalline defects provided more nucleation sites for the formation of a denser and thicker oxide film, thus enhancing the corrosion resistance of the AlSi10Mg alloy.
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Citation: Snopi´nski, P.; Wo´zniak, A.; Łukowiec, D.; Matus, K.; Ta´nski, T.; Rusz, S.; Hilšer, O. Evolution of Microstructure, Texture and Corrosion Properties of Additively Manufactured AlSi10Mg Alloy Subjected to Equal Channel Angular Pressing (ECAP). Symmetry 2022,14, 674. https://doi.org/10.3390/ sym14040674 Academic Editor: Zine El Abiddine Fellah Received: 9 March 2022 Accepted: 22 March 2022 Published: 24 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). symmetry S S Article Evolution of Microstructure, Texture and Corrosion Properties of Additively Manufactured AlSi10Mg Alloy Subjected to Equal Channel Angular Pressing (ECAP) Przemysław Snopi´nski 1,* , Anna Wo´zniak 2, Dariusz Łukowiec 2, Krzysztof Matus 2, Tomasz Ta´nski 1, Stanislav Rusz 3and Ondˇrej Hilšer 3 1Department of Engineering Materials and Biomaterials, Silesian University of Technology, 18A Konarskiego Street, 44-100 Gliwice, Poland; [email protected] 2Materials Research Laboratory, Silesian University of Technology, 18A Konarskiego Street, 44-100 Gliwice, Poland; [email protected] (A.W.); [email protected] (D.Ł.); [email protected] (K.M.) 3 Faculty of Mechanical Engineering, VSB-TU Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic; stanislav[email protected] (S.R.); ondr[email protected] (O.H.) *Correspondence: przemyslaw[email protected] Abstract: In the selective laser melting process (SLM), the region irradiated by the laser beam is melted and quickly solidified, forming solidification lines (laser scan tracks) with symmetrical shapes. Because of the unique (rapid) crystallization conditions, the subgrain structures, typically observed inside these solidification lines, could also have variable geometric symmetrical patterns, e.g., cellular, pentagonal, or hexagonal cellular. The existence of such distinctive microstructures in SLM-made alloys has a significant impact on their superior mechanical and corrosion properties. Thus, any modification of this symmetrical microstructure (due to post-processing) can degrade or improve the properties of SLM-fabricated alloys. This study presents the experimental results on the effects of heat treatment and ECAP on microstructure modification and corrosion behavior of SLM-fabricated AlSi10Mg alloy. Light microscopy, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD) were used for microstructural analysis. The corrosion properties of the given samples were determined using open-circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. EBSD observations showed that the imposed strain resulted in an obvious reduction in grain size to ~1.42 µ m and ~0.24 µ m after the first and second ECAP passes, respectively. Electrochemical tests revealed that the corrosion resistance of the ECAP-processed AlSi10Mg alloy improved significantly, which was confirmed by a nobler E corr and lower I corr values, and higher polarization resistance. The final results indicated that the strain-induced crystalline defects provided more nucleation sites for the formation of a denser and thicker oxide film, thus enhancing the corrosion resistance of the AlSi10Mg alloy. Keywords: selective laser melting; equal-channel angular pressing; AlSi10Mg alloy; microstructure; corrosion behavior 1. Introduction In recent years, ultrafine-grained materials (UFG) have attracted considerable attention from specialists in materials science. This is because they are characterized by unique properties such as high strength, superplasticity, and improved fatigue life [ 1 , 2 ]. UFG materials are commonly fabricated using bulk severe plastic deformation methods (SPD). These methods include high-pressure torsion (HPT) [ 3 ], equal-channel angular pressing (ECAP) [ 4 ], accumulative roll bonding (ARB) [ 5 ], and some others [ 6 , 7 ]. Consequently, processing using the above-mentioned methods results in significant microstructure modifications. In addition to microstructure refinement, there are also changes in the grain Symmetry 2022,14, 674. https://doi.org/10.3390/sym14040674 https://www.mdpi.com/journal/symmetry
Symmetry 2022,14, 674 2 of 19 boundary misorientation angles, crystallographic texture, defect density, and/or the size and distribution of intermetallic phases. Although in recent years a large number of investigations have been reported on the fundamental understanding of superior mechanical properties for UFG Al–Si alloys [ 8 – 14 ], the reports on their corrosion behaviors are rather limited. To date, scientists have examined mainly the corrosion properties of severely deformed hypoand hyper-eutectic Al–Si alloys. In this context, Jiang et al. [ 15 ] studied the corrosion resistance of a hyper-eutectic Al-26% Si alloy processed by ECAP. They found that the homogeneous UFG structure contributed to higher pitting resistance. Gebril et al. [ 16 ] investigated the microstructure evolution and corrosion properties of the ECAP-processed A356 aluminum alloy. They reported a decrease in the corrosion rate due to particle refinement through ECAP. In another study, Wang et al. [ 17 ] used a similar hypo-eutectic Al-7Si alloy to investigate the effect of grain refinement on its corrosion behavior in a solution of 3.5 wt % NaCl by electrochemical impedance spectroscopy. They reported enhanced corrosion resistance as a result of a decrease in the number of active sites, microstructure homogeneity, breakage, and uniform distribution of brittle coarse silicon and intermetallic phases. Furthermore, Cheng et al. [ 18 ] investigated the corrosion properties of the Al-7Si-0.3Mg alloy subjected to spinning deformation. They discovered that the susceptibility to corrosion of the interdendritic region dropped as a result of the refinement of the eutectic silicon phase. Compared to traditional manufacturing methods, unique conditions during the SLM process [ 19 ] favor the formation of distinct microstructures composed of the melting pool boundary network [ 20 ], the grain boundary network [ 21 ], and the cellular network [ 22 ]. These conditions also induce defects, such as porosities, microcracks, and rough surfaces. Along with the boundaries of the molten pool [ 23 ], defects formed during the fabrication process can eventually act as preferred sites for localized corrosion, thus degrading the corrosion performance of AM products. Considering the impact of these imperfections on the corrosion performance of AM Al– Si alloys, significant attention has recently been devoted to studying the effects of thermal post-processing on AM parts. In this context, Cabrini et al. [ 24 ] and Zakay et al. [ 25 ] studied the effects of heat treatments on the corrosion resistance of the laser powder bed fusion (L-PBF) AlSi10Mg alloy. Both studies have shown that the corrosion resistance of the as-fabricated samples was enhanced by heat treatment at 200–300 ◦ C due to stressrelieving. Nonetheless, the corrosion resistance of the built samples was reduced as the heat-treated temperature increased to 400–500 ◦ C, which was related to the detrimental effect on the corrosion performance of the coarse Si particles and Mg 2 Si. However, it should be noted that, despite improving corrosion resistance, the heat treatment process is not currently designed to remove any porosity. AM defects can be minimized by employing specific approaches, such as hot isostatic pressing (HIP) [ 26 ], shot peening [ 27 ], or SPD processing [ 28 ]. The HIP is a very effective secondary process that reduces the amount of porosity in the parts and improves their corrosion resistance. However, this is generally achieved at the expense of mechanical properties [ 29 ]. The shot peening process is also capable of reducing process-induced pores, refining grain sizes, inducing high compressive residual stresses, increasing surface hardness, and fatigue life of the SLM samples [ 27 ], but it often results in deterioration of the sample surface quality. On the other hand, HPT processing can significantly reduce porosity, and refine and homogenize the microstructure, resulting in improved mechanical and corrosion properties [ 3 , 28 , 30 ]. This process has serious limitations—the sample shape is in the form of a disk, which is not appropriate for industrial applications. Regardless of the promising capability of the HPT method in modifying the microstructure and improving the corrosion properties of L-PBF alloys, there is still limited information available in the open literature on the bulk severe plastic deformation processes, such as ECAP as a post-printing procedure on AM parts. In particular, there is still no solid experimental evidence addressing the following two questions: (i) How do the defects formed (grain boundaries) modify the reactivity of the passive layer? (ii) How
Symmetry 2022,14, 674 3 of 19 does the change in the unique solidification structure of AM alloy due to ECAP affect its corrosion resistance? Therefore, a systematic investigation and characterization of the microstructure and corrosion behavior of an ECAP-processed AM Al–Si alloy merits attention, both for its possible engineering applications and to extend the current knowledge of the ECAP process. Consequently, the objective of the article is to investigate the effects of microstructural modifications caused by repetitive pressings through the 120 ◦ ECAP die on the corrosion behavior of the additively manufactured AlSi10Mg alloy. In this work, the corrosion performance was measured by conducting electrochemical tests in a sequence of opencircuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Our results indicated that the melt pool boundary fraction, Si particle, and grain size significantly affected the electrochemical responses of the ECAP processed AlSi10Mg alloy; however, despite the beneficial effect of the grain refinement, we identified that the melt pool boundaries emerged as preferential sites for localized pitting corrosion. The justification for the use of the additively manufactured AlSi10Mg alloy in this study lies in the fact that the unique cellular microstructure offers substantially different deformation mechanisms from conventional counterparts [ 22 ], thus providing suitable starting points for severe grain refinement. According to studies by Zhang et al. [ 31 ] and Kim et al. [ 32 ], the basic deformation mechanism in rapidly solidified Al–Si alloys includes plastic incompatibility across the Al/Si interface (accommodated by geometrically necessary dislocations during straining) and the deformation twinning of the Si phase. 2. Materials and Methods In this work, selective laser melting fabricated the samples from a spherical gasatomized AlSi10Mg powder supplied by Sigma Aldrich (Germany); Table 1gives its nominal chemical composition. Table 1. Elemental composition of the AlSi10Mg alloy, wt %. Si Mg Fe Ti Zn Mn Ni Co Al 9–11 0.25–0.45 <0.25 <0.15 <0.10 <0.10 <0.05 <0.05 Balance The TruPrint 1000 system from Trumpf (Germany) fabricated the cylindrical samples (length 45 mm, diameter 11 mm). All samples were prepared under an argon atmosphere with H 2 O and O 2 concentrations below 20 ppm. The process details (i.e., model slicing, scanning strategy, component orientation) were planned, using the Materialize Magics software (Materialise, Belgium). The parameter sets utilized for sample fabrication are as follows: •Scanning speed 1400 mm/s; •Laser beam diameter 55 µm; •Layer thickness build-up of 20 µm. More details about the applied scanning strategy and SLM process parameters can be found in our previous studies [33,34]. A short-time low-temperature annealing heat treatment was necessary to relieve the internal residual stresses derived from the manufacturing process and increase the alloy workability prior to ECAP processing at room temperature. To achieve this, SLM samples were placed in a conventional laboratory dryer and heated under an argon atmosphere for 8 min at 300 ◦C. Heat-treated cylindrical samples were machined to their final dimensions of 45 mm long with a 9.95 mm diameter. To reduce friction between the work sample and the die wall, MoS 2 was used as a lubricant. The ECAP process was conducted at room temperature using a die with a channel angle of Φ = 120 ◦ and a curvature angle of ψ = 30 ◦ (see scheme in Figure 1). The work samples were processed by 1 (S2-1 sample) and 2 (S2-2 sample)
Symmetry 2022,14, 674 4 of 19 passes of ECAP via route A. Table 2shows the labeling scheme and detailed information about the heat treatment/ECAP procedure. Symmetry2022,14,xFORPEERREVIEW4of19 wall,MoS 2 wasusedasalubricant.TheECAPprocesswasconductedatroomtempera‐ tureusingadiewithachannelangleofФ=120°andacurvatureangleofψ=30°(see schemeinFigure1).Theworksampleswereprocessedby1(S2‐1sample)and2(S2‐2 sample)passesofECAPviarouteA.Table2showsthelabelingschemeanddetailedin‐ formationabouttheheattreatment/ECAPprocedure. Figure1.SchematicillustrationoftheECAPdieusedinthisstudy. Table2.Labelsusedforrespectivesampleconditionswithdetailsontheappliedprocedure. LabelConditionHeatTreatment/ECAPProcedure S0As‐builtnone S2Heat‐treatedAnnealingat300°Cfor8min S2‐1Heat‐treatedandECAPprocessedAnnealingat300°Cfor8min+1ECAPpass S2‐2Heat‐treatedandECAPprocessedAnnealingat300°Cfor8min+2ECAPpasses AninvertedAxioObserverZ1lightmicroscope(CarlZeissNTSGmbH,Oberkochen, Germany)andaZeissSupra35scanningelectronmicroscope(CarlZeissNTSGmbH, Oberkochen,Germany)equippedwithanenergyspectrometer(EDS)characterizedthe microstructuresofthesamples.Forthisanalysis,sampleswerepreparedfollowingstand‐ ardmetallographicprocedures.ThepolishedsurfaceswerethenetchedusingBarker’s(2 vol.%fluoroboricacidindistilledwater)orKeller’sReagent(2.5vol.%nitricacid1.5vol.% hydrochloricacid,and1.0vol.%hydrofluoricacidindistilledwater). ThestructuralanalysiswasperformedbyX‐raydiffractionusingaPANalytical X’PertProdiffractionsystem(MalvernPanalyticalLtd.,Royston,UK)equippedwitha CoKαradiationsource. Fortheelectronbackscattereddiffraction(EBSD)characterization,sampleswerepol‐ ishedusingthestandardmetallographictechniquesandthenetchedusingKeller’srea‐ gent.Thegrainstructurewasrecordedusingorientationimagingmicroscopy(OIM)ap‐ plyingtheEBSDtechniqueintegratedwithaZeissSupra35SEMcontrolledandanalyzed usingOIMsoftware(EDAX,Inc.,Mahwah,NJ,USA).ThestepsizeinEBSDwassetto0.3 μmforheat‐treated(S2),0.09μmfor1ECAP(S2‐1),and0.06μmfor2ECAP(S2‐2)sam‐ ples.Aneighbororientationcorrelationdatacleaningprocess(level4)wasapplied,fol‐ lowedbyagrainconfidenceindexstandardizationcleanupwithagraintoleranceangle thresholdof2°.Afterdatacleaning,acoincidenceindexfilterof0.05wasusedtoremove anypointsthatwerenotindexedcorrectly. Figure 1. Schematic illustration of the ECAP die used in this study. Table 2. Labels used for respective sample conditions with details on the applied procedure. Label Condition Heat Treatment/ECAP Procedure S0 As-built none S2 Heat-treated Annealing at 300 ◦C for 8 min S2-1 Heat-treated and ECAP processed Annealing at 300 ◦C for 8 min + 1 ECAP pass S2-2 Heat-treated and ECAP processed Annealing at 300 ◦C for 8 min + 2 ECAP passes An inverted Axio Observer Z1 light microscope (Carl Zeiss NTS GmbH, Oberkochen, Germany) and a Zeiss Supra 35 scanning electron microscope (Carl Zeiss NTS GmbH, Oberkochen, Germany) equipped with an energy spectrometer (EDS) characterized the microstructures of the samples. For this analysis, samples were prepared following standard metallographic procedures. The polished surfaces were then etched using Barker’s (2 vol.% fluoroboric acid in distilled water) or Keller’s Reagent (2.5 vol.% nitric acid 1.5 vol.% hydrochloric acid, and 1.0 vol.% hydrofluoric acid in distilled water). The structural analysis was performed by X-ray diffraction using a PANalytical X’Pert Pro diffraction system (Malvern Panalytical Ltd., Royston, UK) equipped with a CoK α radiation source. For the electron backscattered diffraction (EBSD) characterization, samples were polished using the standard metallographic techniques and then etched using Keller’s reagent. The grain structure was recorded using orientation imaging microscopy (OIM) applying the EBSD technique integrated with a Zeiss Supra 35 SEM controlled and analyzed using OIM software (EDAX, Inc., Mahwah, NJ, USA). The step size in EBSD was set to 0.3 µ m for heat-treated (S2), 0.09 µ m for 1 ECAP (S2-1), and 0.06 µ m for 2 ECAP (S2-2) samples. A neighbor orientation correlation data cleaning process (level 4) was applied, followed by a grain confidence index standardization cleanup with a grain tolerance angle threshold of 2 ◦ . After data cleaning, a coincidence index filter of 0.05 was used to remove any points that were not indexed correctly.
Symmetry 2022,14, 674 5 of 19 Prior to corrosion test, the specimens were ground using 4000 SiC abrasive paper. All experiments were carried out after a fixed time to stabilize the passive film in laboratory air. Anodic polarization tests were performed using a standard three-electrode corrosion cell setup connected to a PC-controlled Atlas 0531 EU potentiostat, as per the PN ISO 17475:2010 standard [35]. In this configuration, the test sample, a saturated silver chloride electrode Ag/AgCl, and a platinum wire were used as the working electrode reference electrode, and supporting electrode, respectively, and a 3.5 wt % NaCl solution was used as the electrolyte, which is an effective electrolyte for indexing and ranking the corrosion resistance of different materials. The dissociation of chloride from sodium is easy through the electron emission. The dissociation of chloride from sodium is easy through electron emission. The ionic mobility strength of chloride ions in NaCl 3.5 wt %. is more potent than a solution with higher ions concertation (i.e., 9.6 wt %NaCl) and less than medium with lower concertation ions (i.e., 2 wt % NaCl). It is associated with nonlinear inverse functions for the cation mobility and anion mobility of NaCl solution versus molar concentration at a different temperature. Potential scans (E ocp ) were carried out after 1 h of open circuit voltage stabilization (Eocp), with a scan rate of 1 mV/s from the initial value E init =E ocp − 100 mV. EIS tests were conducted using AutoLab’s PGSTAT 302N system (AutoLab, Warsaw, Poland) with a frequency response analyzer (FRA2) and a three-electrode system identical to those used during potentiodynamic tests, after frequency stabilization for 1 h from 10 −3 to 10 4 Hz with a perturbation amplitude of 10 mV. All electrochemical measurements were performed in triplicate to ensure high statistical accuracy. 3. Results 3.1. Microstructure Characterization Figure 2(a1,a2) shows the microstructure of the as-built (S0) sample taken in the XY plane, perpendicular to the building direction. The microstructure featured almost symmetrical discontinuous laser scan tracks, which are typical for metallic materials prepared by SLM. The laser scan track boundaries appeared to be more heavily etched and darker than their interiors, where smaller crystals with diverse crystallographic orientations were also visible. Symmetry2022,14,xFORPEERREVIEW5of19 Priortocorrosiontest,thespecimensweregroundusing4000SiCabrasivepaper.All experimentswerecarriedoutafterafixedtimetostabilizethepassivefilminlaboratory air.Anodicpolarizationtestswereperformedusingastandardthree‐electrodecorrosion cellsetupconnectedtoaPC‐controlledAtlas0531EUpotentiostat,asperthePNISO 17475:2010standard[35]. Inthisconfiguration,thetestsample,asaturatedsilverchlorideelectrodeAg/AgCl, andaplatinumwirewereusedastheworkingelectrodereferenceelectrode,andsupport‐ ingelectrode,respectively,anda3.5wt%NaClsolutionwasusedastheelectrolyte,which isaneffectiveelectrolyteforindexingandrankingthecorrosionresistanceofdifferent materials.Thedissociationofchloridefromsodiumiseasythroughtheelectronemission. Thedissociationofchloridefromsodiumiseasythroughelectronemission.Theionicmo‐ bilitystrengthofchlorideionsinNaCl3.5wt%.ismorepotentthanasolutionwithhigher ionsconcertation(i.e.,9.6wt%NaCl)andlessthanmediumwithlowerconcertationions (i.e.,2wt%NaCl).Itisassociatedwithnonlinearinversefunctionsforthecationmobility andanionmobilityofNaClsolutionversusmolarconcentrationatadifferenttempera‐ ture.Potentialscans(Eocp)werecarriedoutafter1hofopencircuitvoltagestabilization (Eocp),withascanrateof1mV/sfromtheinitialvalueEinit=Eocp−100mV.EIStestswere conductedusingAutoLab’sPGSTAT302Nsystem(AutoLab,Warsaw,Poland)withafre‐ quencyresponseanalyzer(FRA2)andathree‐electrodesystemidenticaltothoseused duringpotentiodynamictests,afterfrequencystabilizationfor1hfrom10−3to104Hzwith aperturbationamplitudeof10mV.Allelectrochemicalmeasurementswereperformedin triplicatetoensurehighstatisticalaccuracy. 3.Results 3.1.MicrostructureCharacterization Figure2(a1,a2)showsthemicrostructureoftheas‐built(S0)sampletakenintheXY plane,perpendiculartothebuildingdirection.Themicrostructurefeaturedalmostsym‐ metricaldiscontinuouslaserscantracks,whicharetypicalformetallicmaterialsprepared bySLM.Thelaserscantrackboundariesappearedtobemoreheavilyetchedanddarker thantheirinteriors,wheresmallercrystalswithdiversecrystallographicorientationswere alsovisible. Figure2.MicrostructuresoftheAlSi10Mgalloy;(a1,a2)XYplaneand(b1,b2)XZplaneofas‐built sample,(c1,c2)XYplaneand(d1,d2)XZplaneofheat‐treatedsample. Figure2(b1,b2)showsthemicrostructureofthesamesampletakenintheXZplane (paralleltothebuilddirection).Asseen,themicrostructureconsistedofsemicircular(fish‐ Figure 2. Microstructures of the AlSi10Mg alloy; ( a1 , a2 ) XY plane and ( b1 , b2 ) XZ plane of as-built sample, (c1,c2) XY plane and (d1,d2) XZ plane of heat-treated sample. Figure 2(b1,b2) shows the microstructure of the same sample taken in the XZ plane (parallel to the build direction). As seen, the microstructure consisted of semicircular (fishscale) patterns along the Z-direction, of which the geometry is attributed to the Gaussian effect of the incident laser beam.
Symmetry 2022,14, 674 6 of 19 Figure 2(c1,c2,d1,d2) shows the microstructure of the heat-treated sample (S2). The light microscopy images did not provide distinct evidence of microstructure evolution with respect to the untreated (S0) sample. Therefore, it was necessary to perform a more detailed analysis using SEM. Figure 3(a1,a2) shows the microstructure of the as-built SLM AlSi10Mg sample, examined using secondary electrons. As indicated in the magnified image of the melt pool boundary, the microstructure of the as-built sample was heterogeneous, showing three different zones. These zones were identified as fine (‘MP fine’), coarse (‘MP coarse’), and a heat-affected zone (‘HAZ’). The ‘MP fine’ zone was composed of fine primary α -Al cells (darker gray) surrounded by a continuous cellular Si network (lighter white). The ‘MP coarse’ zone consisted of a coarser Si network, while the ‘HAZ’ consisted of a partially broken Si network. Symmetry2022,14,xFORPEERREVIEW6of19 scale)patternsalongtheZ‐direction,ofwhichthegeometryisattributedtotheGaussian effectoftheincidentlaserbeam. Figure2(c1,c2,d1,d2)showsthemicrostructureoftheheat‐treatedsample(S2).The lightmicroscopyimagesdidnotprovidedistinctevidenceofmicrostructureevolution withrespecttotheuntreated(S0)sample.Therefore,itwasnecessarytoperformamore detailedanalysisusingSEM. Figure3(a1,a2)showsthemicrostructureoftheas‐builtSLMAlSi10Mgsample,ex‐ aminedusingsecondaryelectrons.Asindicatedinthemagnifiedimageofthemeltpool boundary,themicrostructureoftheas‐builtsamplewasheterogeneous,showingthree differentzones.Thesezoneswereidentifiedasfine(‘MPfine’),coarse(‘MPcoarse’),and aheat‐affectedzone(‘HAZ’).The‘MPfine’zonewascomposedoffineprimaryα‐Alcells (darkergray)surroundedbyacontinuouscellularSinetwork(lighterwhite).The‘MP coarse’zoneconsistedofacoarserSinetwork,whilethe‘HAZ’consistedofapartially brokenSinetwork. Figure3.SEMimagesoftheAlSi10Mgalloymeltpoolboundary;(a1,a2)as‐built(XYplane),(b1,b2) heat‐treated(XYplane),(c1,c2)heat‐treated(XZplane). HeattreatmentsignificantlymodifiedthecellularSinetworkinthe‘MPfine’and ‘MPcoarse’zones(ontheXYplane),Figure3(b1,b2),andcolumnarSinetwork(ontheXZ plane),Figure3(c1,c2).Afterbeingheldat300°Cfor8min,theinitialcontinuouscellular networkofAl/SiwasdissolvedbycoarseningoftheSiprecipitates. Figure4showstheopticalmicrographsoftheECAPprocessedsamples.Amicro‐ structureconsistingofmultiplesemicircularpatternswasrevealedinsampleS2‐1,Figure 4a.ThesepatternsreplacedthediscontinuouslaserscantracksintheXYplane.Thesec‐ ondpassmadesignificantprogressinmicrostructureevolution,whichcanbequantita‐ tivelyevaluatedbymeasuringtheheightofthe‘fishscale’patternsandthefractionofthe boundaryofthemeltpool.Ascanbeseen,the‘fishscale’patternsbecamecompressed andelongatedalongthex‐axis(Figure4b),sotheirheightdecreasedfrom54±6μm(S2‐ 1sample)to16±5μm(S2‐2sample).Additionally,thefractionofmeltpools(MP)bound‐ ariesincreased;seethebrighterareasinFigure4c,d. Figure 3. SEM images of the AlSi10Mg alloy melt pool boundary; ( a1 , a2 ) as-built (XY plane), (b1,b2) heat-treated (XY plane), (c1,c2) heat-treated (XZ plane). Heat treatment significantly modified the cellular Si network in the ‘MP fine’ and ‘MP coarse’ zones (on the XY plane), Figure 3(b1,b2), and columnar Si network (on the XZ plane), Figure 3(c1,c2). After being held at 300 ◦C for 8 min, the initial continuous cellular network of Al/Si was dissolved by coarsening of the Si precipitates. Figure 4shows the optical micrographs of the ECAP processed samples. A microstructure consisting of multiple semicircular patterns was revealed in sample S2-1, Figure 4a. These patterns replaced the discontinuous laser scan tracks in the XY plane. The second pass made significant progress in microstructure evolution, which can be quantitatively evaluated by measuring the height of the ‘fish scale’ patterns and the fraction of the boundary of the melt pool. As can be seen, the ‘fish scale’ patterns became compressed and elongated along the x-axis (Figure 4b), so their height decreased from 54 ± 6 µ m (S2-1 sample) to 16 ± 5 µ m (S2-2 sample). Additionally, the fraction of melt pools (MP) boundaries increased; see the brighter areas in Figure 4c,d.
Symmetry 2022,14, 674 7 of 19 Symmetry2022,14,xFORPEERREVIEW7of19 Figure4.MicrostructureoftheAlSi10Mgalloy(a)and(c)heat‐treated+1ECAPpass,(b)and(d) heat‐treated+2ECAPpasses. Figure5a,bshowstheinversepolefigure(IPF‐Z)orientationandgrainboundary maps(representingtheXYplane)oftheheat‐treatedsample.TheIPF‐Zorientationmap, Figure5a,revealeddistinctareaswithfinegrainsalongacurvingarea(indicatedbydot‐ tedlines)correspondingtothelaserscantrackboundary(heataffectedzone),whichis typicalforSLMalloys[36].Meanwhile,thevicinityofthelaserscantrackwascomposed oflargerequiaxedgrains.Accordingtothegrainboundarymap,Figure5b,andthedata summarizedinTable3,thefractionoflow‐angleboundaries(LABs,definedasboundaries withmisorientationanglebetween3°and15°)oftheheat‐treatedsamplewasapproxi‐ mately8%.ThemeasuredaveragegrainsizefortheXYplanewasapproximately3.2μm. Figure 4. Microstructure of the AlSi10Mg alloy ( a , c ) heat-treated + 1 ECAP pass, ( b , d ) heat-treated + 2 ECAP passes. Figure 5a,b shows the inverse pole figure (IPF-Z) orientation and grain boundary maps (representing the XY plane) of the heat-treated sample. The IPF-Z orientation map, Figure 5a, revealed distinct areas with fine grains along a curving area (indicated by dotted lines) corresponding to the laser scan track boundary (heat affected zone), which is typical for SLM alloys [ 36 ]. Meanwhile, the vicinity of the laser scan track was composed of larger equiaxed grains. According to the grain boundary map, Figure 5b, and the data summarized in Table 3, the fraction of low-angle boundaries (LABs, defined as boundaries with misorientation angle between 3 ◦ and 15 ◦ ) of the heat-treated sample was approximately 8%. The measured average grain size for the XY plane was approximately 3.2 µm. Table 3. Summary of microstructural characteristics for investigated samples. Sample Average Grain Size, [µm] Fraction of Low-Angle Grain Boundaries, % Fraction of High-Angle Grain Boundaries, % S2 (XY plane) 3.24 8.45 91.55 S2 (XZ plane) 10.11 26.71 74.29 S2-1 1.42 29.24 70.76 S2-2 0.25 19.32 80.68 Figure 5c,d presents the inverse pole figure (IPF-Z) orientation and grain boundary maps (representing the XZ plane) for the same sample. The EBSD result shows that the microstructure was composed of columnar grains, Figure 5c, with an average grain size of approximately 10.1 µ m. Such a grain morphology is typical for the side plane of SLMfabricated samples and results from the boundaries of the epitaxial growth from the molten pool [ 37 ]. According to the grain boundary map, Figure 5d, the side (XZ) plane contained a large fraction of LABs than the building (XY) plane (~27% of the total grain boundaries).
Symmetry 2022,14, 674 8 of 19 Symmetry2022,14,xFORPEERREVIEW8of19 Figure5.EBSDIPF‐ZimageandgrainboundaryorientationmapsofAlSi10Mgalloyinheat‐treated condition(a)and(b)XYplane,(c)and(d)XZplane. Table3.Summaryofmicrostructuralcharacteristicsforinvestigatedsamples. SampleAverageGrain Size,[μm] FractionofLow‐AngleGrain Boundaries,% FractionofHigh‐AngleGrainBoundaries, % S2(XYplane)3.248.4591.55 S2(XZplane)10.1126.7174.29 S2‐11.4229.2470.76 S2‐20.2519.3280.68 Figure5c,dpresentstheinversepolefigure(IPF‐Z)orientationandgrainboundary maps(representingtheXZplane)forthesamesample.TheEBSDresultshowsthatthe microstructurewascomposedofcolumnargrains,Figure5c,withanaveragegrainsize ofapproximately10.1μm.SuchagrainmorphologyistypicalforthesideplaneofSLM‐ fabricatedsamplesandresultsfromtheboundariesoftheepitaxialgrowthfromthemol‐ tenpool[37].Accordingtothegrainboundarymap,Figure5d,theside(XZ)planecon‐ tainedalargefractionofLABsthanthebuilding(XY)plane(~27%ofthetotalgrain boundaries). Figure6showstheinversepolefigure(IPF‐Z)orientationandgrainboundarymaps oftheECAPprocessedsamples.Inthesemaps,toreducetheinfluenceofnoiseonthe data,crystallitessmallerthanorequaltofourmappixelswereignored.Asisvisible,the firstECAPpasscausedsignificantmodificationofthemicrostructure.ReferringtoFigure 6a,themicrostructurebecamemorehomogeneousandfiner.Thefractionoflow‐angle Figure 5. EBSD IPF-Z image and grain boundary orientation maps of AlSi10Mg alloy in heat-treated condition (a,b) XY plane, (c,d) XZ plane. Figure 6shows the inverse pole figure (IPF-Z) orientation and grain boundary maps of the ECAP processed samples. In these maps, to reduce the influence of noise on the data, crystallites smaller than or equal to four map pixels were ignored. As is visible, the first ECAP pass caused significant modification of the microstructure. Referring to Figure 6a, the microstructure became more homogeneous and finer. The fraction of lowangle boundaries (LAGBs) increased to ~29.2%. At the same time, the average grain size decreased to 1.425 µm . The second ECAP pass brought significant progress in grain refinement, shown in Figure 6c,d. It appears that the cell boundaries acted as additional obstacles for mobile dislocation; therefore, with the progressive increase in the number of ECAP pressings, these cells formed subgrains. Consequently, the fraction of grains that had low angle misorientation decreased slightly to 19.32%, and the grain size was further reduced from 1.42 to 0.25 µm. Figure 7presents a series of the typical (111) pole figures derived from EBSD data, which were plotted for heat-treated and ECAP processed samples. According to the notation proposed by Toth [ 38 ], the heat-treated sample on the XY plane demonstrated the major {011}<100 > Goss and minor {001}<100 > cube texture components, shown in Figure 7a, which is typical for the bi-directional scanning strategy [ 39 ]. Taking into account the side surface (XZ plane), elongated grains oriented along the building direction formed, shown in Figure 5c, which resulted in an increased preferred fiber texture composed of {011}<112> (brass) orientations, shown in Figure 7b.
Symmetry 2022,14, 674 9 of 19 Symmetry2022,14,xFORPEERREVIEW9of19 boundaries(LAGBs)increasedto~29.2%.Atthesametime,theaveragegrainsizede‐ creasedto1.425μm.ThesecondECAPpassbroughtsignificantprogressingrainrefine‐ ment,showninFigure6c,d.Itappearsthatthecellboundariesactedasadditionalobsta‐ clesformobiledislocation;therefore,withtheprogressiveincreaseinthenumberofECAP pressings,thesecellsformedsubgrains.Consequently,thefractionofgrainsthathadlow anglemisorientationdecreasedslightlyto19.32%,andthegrainsizewasfurtherreduced from1.42to0.25μm. Figure6.EBSDIPF‐ZimageandgrainboundaryorientationmapsofAlSi10Mgalloy(a)and(b) heat‐treated+1ECAPpass,(c)and(d)heat‐treated+2ECAPpasses,thecolorcoderepresenting thecrystalorientation. Figure7presentsaseriesofthetypical(111)polefiguresderivedfromEBSDdata, whichwereplottedforheat‐treatedandECAPprocessedsamples.Accordingtothenota‐ tionproposedbyToth[38],theheat‐treatedsampleontheXYplanedemonstratedthe major011 100 Gossandminor001 100 cubetexturecomponents,shown inFigure7a,whichistypicalforthebi‐directionalscanningstrategy[39].Takingintoac‐ countthesidesurface(XZplane),elongatedgrainsorientedalongthebuildingdirection formed,showninFigure5c,whichresultedinanincreasedpreferredfibertexturecom‐ posedof{011}<112>(brass)orientations,showninFigure7b. Figure 6. EBSD IPF-Z image and grain boundary orientation maps of AlSi10Mg alloy ( a , b ) heat-treated + 1 ECAP pass, ( c , d ) heat-treated + 2 ECAP passes, the color code representing the crystal orientation. Symmetry2022,14,xFORPEERREVIEW10of19 Figure7.(111)polefiguresofAlSi10Mgalloy(a)heat‐treatedXYplane,(b)heat‐treatedXZplane, (c)heat‐treated+1ECAPpass,(d)heat‐treated+2ECAPpasses. AfterthefirstpassofECAP(Figure7c),thetypeoftexturecomponentschanged.We identifiedastrongshear𝐴 ∗|𝐴 ∗111 112 texturecomponentwithamaximuminten‐ sityof~4.6mr(multiplyrandom).Withfurtherincreasingthedeformationstrainto~1.2 (2ECAPpasses),showninFigure7d,the𝐴 ∗|𝐴 ∗111 112 componentsbecame strengthenedand,additionally,the𝐵|𝐵 112 110 texturecomponentappeared. Figure8showsthefullXRDpatternscorrespondingtoeachinvestigatedcondition. Inthesepatterns,weidentifiedthemainreflections,whichcorrespondedtotheFccAl, diamondcubicSi,andcubicMg2Siphases.Ascanbeobserved,theSipeaksfortheheat‐ treatedS2samplehadhigherintensitiesthanthoseoftheas‐builtS0sample,whichmeans thattheheat‐treatmentperiodwaslongenoughtoinitiatetheprecipitationofexcessSi fromtheAlmatrix,thusdecreasingtheSicontentinthesolidsolution.Thisaccordingly causedalatticeparameterincreasefrom4.0495±0.0004Å(S0sample)toamoreequilib‐ riumvalueof4.0509±0.0003Å(S2sample),whichforAlinAlSi10Mgalloysisreported tobe4.0515Å[40].TheinsetinFigure8illustratestheshiftoftheAl(100)reflectionto‐ wardthelower2θangles,suggestedtheminorvariationsintheAllatticeparameterafter ECAP.Infact,thecalculatedAllatticeparameteroftheS2‐1sampledecreasedto4.0476± 0.0003Å;however,itincreasedslightlyto4.0502±0.0003ÅafterthesecondECAPpass (S2‐2sample),whichindicatedvariationsintheSisolidsolubilityduringECAPpro‐ cessing. Figure8.XRDdiffractionpatternoftheAlSi10Mgalloyindifferentinvestigatedconditions. BasedonmodifiedVegard’sEquation(1)[41],wecalculated1.1at%Sisoluteforas‐ built,0.3at%Sisoluteandheat‐treatedsamples,respectively.ThefirstpressingofECAP ledtosolidsolubilityextensionofsiliconinthealuminummatrix.TheamountofSiinthe solidsolutionincreasedto2.2at%(notethatforAlalloys,itis1.65at%);however,during Figure 7. (111) pole figures of AlSi10Mg alloy ( a ) heat-treated XY plane, ( b ) heat-treated XZ plane, (c) heat-treated + 1 ECAP pass, (d) heat-treated + 2 ECAP passes. After the first pass of ECAP (Figure 7c), the type of texture components changed. We identified a strong shear A∗ 1 A∗ 2{111}<112 > texture component with a maximum intensity of ~4.6 mr (multiply random). With further increasing the deformation strain to ~1.2 (2 ECAP passes), shown in Figure 7d, the A∗ 1 A∗ 2{111}<112 > components became strengthened and, additionally, the B B{112}<110 >texture component appeared. Figure 8shows the full XRD patterns corresponding to each investigated condition. In these patterns, we identified the main reflections, which corresponded to the Fcc Al, diamond cubic Si, and cubic Mg 2 Si phases. As can be observed, the Si peaks for the heattreated S2 sample had higher intensities than those of the as-built S0 sample, which means that the heat-treatment period was long enough to initiate the precipitation of excess Si from the Al matrix, thus decreasing the Si content in the solid solution. This accordingly caused a lattice parameter increase from 4.0495 ± 0.0004 Å (S0 sample) to a more equilibrium
Symmetry 2022,14, 674 16 of 19 Several references [ 57 , 58 ] indicated that such particles stimulate α -Al corrosion since they produce local galvanic couples. In fact, a higher magnification (Figure 14c) revealed localized corrosion at the interface between the Si/ α -Al interface. The results of the SEM/EDX analyses (Figure 14d) confirmed that the corrosion pits were located at the melt pool boundaries, where the coarser Si particles are exposed to the corrosive environment. 4. Conclusions Corrosion resistance is one of the most important properties of metallic materials. This is because the corrosion of engineered materials represents a huge industrial problem. This causes untold economic losses and catastrophic damage to technical facilities. Corrosion resistance can be modified by modifying the grain size. Consequently, it is not unreasonable to expect surfaces with relatively high grain boundary densities to exhibit electrochemical behavior different from that of coarser grained surfaces with low grain boundary densities. In this paper, we investigated the effect of ECAP on the microstructure and corrosion properties of the SLM AlSi10Mg aluminum alloy. The corrosion behavior of the as-built and heat-treated samples was compared with those fabricated by the hybrid route that combined selective laser melting, heat treatment, and ECAP processing. The following conclusions can be drawn: • The ECAP-induced microstructural modifications promoted the formation of a layered structure that comprised semi-circular patterns and melt pool boundaries, whose fraction increased with additional ECAP pressings. • The results of the microstructural study via EBSD revealed that ECAP up to two passes led to significant grain size reduction to 1.42 µm after the first pass and 0.24 µm after the second pass. • Different corrosion behaviors occurred between the as-built/heat-treated and ECAP processed samples. Potentiodynamic polarization test results revealed the lowest corrosion current density accompanied by higher R p parameter for ECAP-processed samples, confirming their superior corrosion resistance. • Microstructural examination of corroded surfaces revealed differences in corrosion attacks depending on the processing history. The as-built and heat-treated, which had microstructures composed of laser scan tracks, showed almost uniform pitting with the formation of slightly larger pits along the laser scan track boundaries (melt pool boundaries). The samples subjected to ECAP, having a microstructure composed of semi-circular patterns, showed superior corrosion resistance; however, in the sample subjected to two ECAP passes, the melt pool boundaries showed a more deeply penetrating corrosion that preferentially initiated in the areas where multiple melt-pool boundaries overlap. Nevertheless, globally, for ECAP samples, improved corrosion resistance was observed. Author Contributions: Conceptualization, P.S. and A.W.; methodology, P.S. and A.W.; formal analysis, P.S. and A.W.; investigation, P.S., A.W., O.H., T.T., D.Ł. and K.M.; resources, S.R. and O.H.; data curation, P.S. and A.W.; writing—original draft preparation, P.S. and A.W.; supervision, T.T. and S.R. All authors have read and agreed to the published version of the manuscript. Funding: This article was completed in association with project Innovative and additive manufacturing technology—new technological solutions for 3D printing of metals and composite materials, reg. no. 319 CZ.02.1.01/0.0/0.0/17_049/0008407 financed by Structural Funds of the European Union. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data are available from the corresponding author upon request. Acknowledgments: Przemysław Snopi´nski is a scholarship holder of the Visegrad International Scholarship Grant for the period September 2021 to July 2022. This work was done in the framework of this scholarship.
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