Effect of Friction Stir Processing on Microstructural, Mechanical

Article Eﬀect of Friction Stir Processing on Microstructural, Mechanical, and Corrosion Properties of Al-Si12 Additive Manufactured Components

Ghazal Moeini 1,*, Seyed Vahid Sajadifar 2 , Tom Engler 3, Ben Heider 3 , Thomas Niendorf 2 , Matthias Oechsner 3 and Stefan Böhm 1 1 Department for Cutting and Joining Processes, University of Kassel, Kurt-Wolters-Str. 3, 34125 Kassel, Germany; [email protected] 2 Institute of Materials Engineering, University of Kassel, Moenchebergstraße 3, 34125 Kassel, Germany; [email protected] (S.V.S.); [email protected] (T.N.) 3 Center for Structural Materials (MPA-IfW), Technical University of Darmstadt, Grafenstraße 2, 64283 Darmstadt, Germany; [email protected] (T.E.); [email protected] (B.H.); [email protected] (M.O.) * Correspondence: [email protected]; Tel.: +49-561-804-7443

Received: 12 December 2019; Accepted: 31 December 2019; Published: 3 January 2020

Abstract: Additive manufacturing (AM) is an advanced manufacturing process that provides the opportunity to build geometrically complex and highly individualized lightweight structures. Despite its many advantages, additively manufactured components suffer from poor surface quality. To locally improve the surface quality and homogenize the microstructure, friction stir processing (FSP) technique was applied on Al-Si12 components produced by selective laser melting (SLM) using two different working media. The effect of FSP on the microstructural evolution, mechanical properties, and corrosion resistance of SLM samples was investigated. Microstructural investigation showed a considerable grain refinement in the friction stirred area, which is due to the severe plastic deformation and dynamic recrystallization of the material in the stir zone. Micro-hardness measurements revealed that the micro-hardness values of samples treated using FSP are much lower compared to SLM components in the as-built condition. This reduction of hardness values in samples treated with FSP can be explained by the dissolution of the very fine Si-phase network, being characteristic for SLM samples, during FSP. Surface topography also demonstrated that the FSP results in the reduction of surface roughness and increases the homogeneity of the SLM microstructure. Decreased surface roughness and grain size refinement in combination with the dissolved Si-phase network of the FSP treated material result in considerable changes in corrosion behavior. This work addresses the corrosion properties of surface treated additive manufactured Al-Si12 by establishing adequate microstructure-property relationships. The corrosion behavior of SLM-manufactured Al-Si12 alloys is shown to be improved by FSP-modification of the surfaces.

Keywords: additive manufacturing; friction stir processing; microstructure; corrosion

1. Introduction Technology to produce components with high eﬃciency and functional properties in a relatively short time and at lower costs is the main demand of most industries. Additive manufacturing (AM) as a direct manufacturing technology provides the opportunity to produce high-performance components characterized by complex geometry directly from computer-aided design (CAD) data. Selective laser melting (SLM) is the powder bed fusion additive manufacturing technique most frequently used for AM of aluminum alloys [1]. SLM can produce near-net-shaped objects with superior dimensional accuracy

Metals 2020, 10, 85; doi:10.3390/met10010085 www.mdpi.com/journal/metals Metals 2020, 10, 85 2 of 11 and an overall higher quality compared to other AM techniques. However, due to the layer-wise nature of additive manufacturing, the components show poor surface quality. To improve the surface quality, different aspects related to AM processing as well as post-processing techniques were studied [2–12]. Calignano et al. [2] studied the effect of the direct metal laser sintering (DMLS) process parameters such as scan speed, laser power and hatching distance on the surface roughness of AlSi10Mg components. Results showed that laser scan speed plays an important role in the surface roughness of as-built parts. Townsend et al. [3] observed that the disappearance of laser tracks on the side surface of the AlSi10Mg SLM parts leads to lower values of surface roughness as compared to that of the top surface. Furthermore, diverse thermal post-processing and surface treatment techniques, e.g., shot peening, were used to enhance the surface quality. Shot peening leads to localized plastic deformation on the surface layer promoting the evolution of compressive residual stress. This eventually improves the mechanical properties and fatigue strength of AM components [5,8,9]. The effect of laser shot peening on the fatigue properties of AM 316L samples was investigated elsewhere [4]. It was reported that the deep level of plastic deformation introduced by the laser shot peening process can considerably enhance the fatigue strength. Maamoun et al. [5] demonstrated the improvement of surface integrity, microstructure homogeneity and mechanical properties of AlSi10Mg SLM surfaces after applying shot peening treatments. The effective depth of shot peening was reported to be in the range of 100 to 200 µm. Establishing an adequate post-processing technique to simultaneously achieve high strength, high surface quality, minimal porosity and high corrosion resistance in AM parts has always been the aim of research. Zakay et al. [6] studied the impact of post-process heat treatments on the corrosion behavior of AlSi10Mg SLM samples. They studied the general corrosion behavior by salt spray testing and electrochemical impedance spectroscopy as well as the stress corrosion cracking behavior by low cycle corrosion fatigue. It was reported that the heat treated samples (200–300 ◦C for 2 h) showed a relatively enhanced corrosion resistance. This was attributed to the preservation of the fine Si cellular morphology embedded in the aluminum matrix that was achieved during SLM processing. Friction stir processing (FSP) is a commonly applied post-treatment technique for microstructural modification and improvement of surface quality in a localized area [13,14]. In FSP, a rotating tool with pin and shoulder is inserted into the material and traversed along the desired path in order to extensively mix the material in the processing area. The influence of FSP on the microstructural modification and mechanical properties of as-cast Al-Si-Mg alloys was numerously studied, e.g., in [15,16]. The results revealed that FSP can be used to improve the mechanical properties of components by grain refinement and reduction of casting porosity. Another study [16] also showed that the FSP improves the intergranular corrosion resistance of Al5083 alloy by refining the microstructure and partial dissolution of precipitates in the FSP treated area. Macias et al. [17] investigated the noticeable effect of FSP on microstructural refinement, porosity reduction and fatigue enhancement of SLM AlSi10Mg. Maamoun et al. [10] applied FSP as a localized surface treatment for additively manufactured AlSi10Mg. Their investigations revealed that the FSP is able to break up the fibrous Si network being characteristic for the as-built SLM samples into nano-scale particles, resulting in a more homogeneous distribution of nano-scale Si particles. Additionally, FSP can efficiently reduce porosity in the stir zone and also can have a pronounced in-depth effect [10]. In light of the presented findings, FSP can be regarded as an effective post-treatment technique to improve the surface quality and promote microstructure refinement. However, there are still only a few studies dealing with the effect of FSP on the mechanical and corrosion behavior of aluminum alloys and Al-12Si, respectively, fabricated via SLM. Therefore, the main objective of this study is to establish a bridge between FSP, microstructural variations and corrosion behavior of Al-Si12 fabricated via the SLM process.

2. Materials and Methods Samples of Al-Si12 alloy were manufactured using a commercially available SLM system SLM 280HL, distributed by SLM Solutions, Lübeck, Germany. The machine is equipped with a 400 W Metals 2020, 10, 85 3 of 11 ytterbium fiber laser with a spot size of 85 µm. The commercially available powder material Al-Si12 (Wt.%) was used to manufacture samples in an inert environment using argon gas. The parameters used for manufacturing the sample volume were: scanning speed 1200 mm/s, laser power 350 W, layer thickness 50 µm and hatch spacing 190 µm. The SLM samples with a size of 60 mm 25 mm 5 mm × × were friction stir processed. An FSP tool with a 10 mm shoulder and no pin was used in this study. FSP was conducted employing parameters as follows: rotational speed 1800 rotations per minute, travel speed 160 mm/min and 0.1 mm tool penetration. The Friction stir process was carried out with appropriate clamping and fixing settings. FSP was conducted in two types of working media, in air and under water. In the latter case, the samples were fixed in a water container. The water level was about 25 mm above the sample surface. Processing direction was parallel to the SLM build direction (BD). To distinguish between samples, the samples were labeled as follows: AC for FSP in air and UW for FSP in water. Standard mechanical grinding and polishing procedures were used to prepare samples for microstructural analysis. Optical microscopy (OM) and a scanning electron microscope (SEM)/(Carl Zeiss AG, Jena, Germany), equipped with an electron backscatter diffraction (EBSD) unit were employed to characterize the microstructure of the samples. For EBSD measurements, samples were prepared by 24 h of vibro-polishing in a colloidal silica solution (OPS). Vickers micro-hardness testing was performed using a KB 30S Vickers hardness tester (KB Prüftechnik, Hochdorf-Assenheim, Germany) with a diamond indenter in the shape of a pyramid. A force of 0.3 N was applied for 10 s on the surface of the samples according to the standard, ASTM: E384-11e1. Optical topography measurements of the SLM surfaces before and after the FSP were performed using MicroProf® (FRT GmbH, Bergisch Gladbach, Germany). To investigate the influence of FSP on corrosion properties, the samples were tested according to DIN EN ISO 11846:2008-08, which is the standard to be used for determination of the resistance against intergranular corrosion of solution heat-treatable aluminum alloys. After initial preparation through etching in 5% sodium hydroxide (NaOH) at 50 ◦C for 2.5 min and subsequent neutralizing in 70% nitric acid (HNO3) at ambient temperature for 2 min, the samples were immersed in a solution of 3% sodium chloride (NaCl) and 1% hydrochloric acid (HCl) for 24 h at ambient temperature. The volume to surface ratio was 5 ml/cm2. After the test, corrosion products were removed by etching using nitric acid. Macroscopic documentation was performed using a Keyence VHX-6000 digital microscope (Keyence Corporation, Osaka, Japan). Metallographic investigations were performed on the Cross-sections of the studied samples.

3. Results and Discussion

3.1. Microstructural Evolution From the SEM micrographs of the samples, focusing on the SLM as-built condition as well as the stir zone and the thermomechanically affected zone (TMAZ) upon FSP (Figure1), it can be deduced that a Si-rich network of cellular shape is present in the SLM aluminum matrix. However, upon FSP, the Si network is completely dissolved and transformed into Si-rich particles. The microstructure of the AC-FSP state is characterized by spheroidized Si-rich particles, with relatively high inter-particle distance, while in the UW-FSP state the Si-rich particles are significantly finer and different in shape, i.e., mostly acicular. Dissolution of the Si-rich network was also seen after friction stir welding (FSW) of SLM Al-Si12 elsewhere [18]. Severe plastic deformation, fragmentation, and dynamic recrystallization during FSP and FSW, respectively, are known to be able to dissolve the network formed previously in the SLM process. Metals 2020, 10, 85 4 of 11 Metals 2020, 10, x FOR PEER REVIEW 4 of 11

FigureFigure 1. 1. MicrographsMicrographs (SE (SE contrast) contrast) of of SLM, SLM, AC-FSP AC-FSP and UW-FSP samples.

The microstructuremicrostructure ofof Al-Si12 Al-Si12 samples samples in diinff differenterent conditions conditions was was further further characterized characterized via EBSD. via EBSD.EBSD micrographsEBSD micrographs (inverse (inverse pole figure pole figure (IPF) (IPF) maps) maps) of the of alloy the alloy before before and and after after FSP FSP processing processing are areshown shown in Figurein Figure2. The2. The step step sizes sizes of of EBSD EBSD measurements measurements for for SLM, SLM, AC-FSP AC-FSP and and UW-FSP UW-FSP samplessamples were 200 nm,nm, 500500 nmnm andand 20 20 nm, nm, respectively. respectively. Evidently, Evidently, the the FSP FSP processing processing of SLMof SLM processed processed Al-Si12 Al- Si12leads leads to grain to grain refinement. refinement. Grain Grain refinement refinement is due tois bothdue to severe both plastic severe deformation plastic deformation and dynamic and dynamicrecrystallization recrystallization of the material of the during materi FSPal during [13]. It FSP is also [13]. worth It is also noting worth that noting the level that of the grain level refinement of grain refinementis more pronounced is more forpronounced the sample for processed the sample underwater processed (UW-FSP) underwater compared (UW-FSP) to the samplecompared processed to the samplein air (AC-FSP). processed Since in air heat (AC-FSP). transfer Since is enhanced heat transfer in the is waterenhanced environment, in the water there environment, is less time there for the is lessgrowth time of for newly the growth recrystallized of newly grains recrystallized in UW-FSP. grains Therefore, in UW-FSP. lower Therefore, grain size lower and agrain higher size level and ofa highergrain refinement, level of grain respectively, refinement, are respectively, seen upon underwater are seen upon FSP underwater treatment. FSP treatment. Metals 2020, 10, 85 5 of 11 Metals 2020, 10, x FOR PEER REVIEW 5 of 11

Figure 2. EBSD micrographs of SLM, AC-FSP and UW-FSP samples; color coding is in line with the Figurestandard 2. EBSD triangle micrographs shown to the of lowerSLM, right.AC-FSP and UW-FSP samples; color coding is in line with the standard triangle shown to the lower right. 3.2. Hardness and Topography 3.2. Hardness and Topography Vickers micro-hardness was measured on the surface of the diﬀerently treated samples, Table1 andVickers Figure3 .micro-hardness A decrease in thewas Vickers measured micro-hardness on the surface values of the of differently AC samples treated was samples, found. This Table can 1 andbe rationalized Figure 3. A decrease based on in the the microstructural Vickers micro-hardness evolution, values i.e., the of dissolutionAC samples ofwas the found. initial This Si network can be rationalizedfollowed by based coarsening on the of microstructural the Si precipitates. evolution, Most importantly, i.e., the dissolution due to the of high the coolinginitial Si rate network in the followedcase of UW-FSP by coarsening samples, of thethe micro-hardnessSi precipitates. Most remains importantly, on a medium due levelto the as high compared cooling to rate the in other the casetwo conditions.of UW-FSP Bysamples, using waterthe micro-hardness as the convective rema medium,ins on a frictional medium heat level can as ecomparedﬀectively be to dissipated.the other twoThis conditions. hampers microstructural By using water changes as the and convective coarsening medium, of microstructural frictional features heat can in theeffectively FSP treated be dissipated.area to a certain This hampers degree. Eventually, microstructural underwater changes FSP and can coarsening improve theof microstructural mechanical properties. features in the FSP treated area to a certain degree. Eventually, underwater FSP can improve the mechanical properties. Table 1. Vickers micro-hardness values of SLM, AC-FSP and UW-FSP samples (HV 0.3).

SLM AC-FSP UW-FSP Table 1. Vickers micro-hardness values of SLM, AC-FSP and UW-FSP samples (HV 0.3). 115-123 HV 78-95 HV 97-112 HV SLM AC-FSP UW-FSP 115-123 HV 78-95 HV 97-112 HV The surface roughness measurements of samples in different conditions (as-build, AC-FSP and UW-FSP) are shown in Figure 4. It can be seen that relatively poor surface quality is found in the case of the as-built SLM sample. On FSP surfaces the surface roughness was significantly reduced and the pattern became quite uniform. Metals 2020, 10, 85 6 of 11 Metals 2020, 10, x FOR PEER REVIEW 6 of 11

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FigureFigure 3. 3. HardnessHardness distribution distribution along along the the FSP FSP treated treated su surfacesrfaces in in AC-FSP AC-FSP and and UW-FSP UW-FSP conditions. conditions. The surface roughness measurements of samples in diﬀerent conditions (as-build, AC-FSP and UW-FSP) are shown in Figure4. It can be seen that relatively poor surface quality is found in the case of the as-built SLM sample. On FSP surfaces the surface roughness was signiﬁcantly reduced and the patternFigure became 3. Hardness quite uniform. distribution along the FSP treated surfaces in AC-FSP and UW-FSP conditions.

Figure 4. Surface topography obtained by optical measurements on the surface of samples in different conditions.

3.3. Corrosion Behavior

Figure 4. Surface topography obtained by optical measurements on the surface of samples in Without FSP-modification of the surface, the as-built SLM-manufactured sample appears intact; diFigurefferent 4. conditions.Surface topography obtained by optical measurements on the surface of samples in different however,conditions. traces of a corrosive attack are observed between the individual laser scanning paths, cf. Figure3.3. Corrosion 5. It seems Behavior that the interfaces between individual layers are preferential corrosion sites. This can3.3. beCorrosion rationalized Behavior based on local changes in microstructural appearance and distribution of Without FSP-modification of the surface, the as-built SLM-manufactured sample appears intact; elements, as can be deduced from the locally different shapes of the Si-rich networks, in each single however,Without traces FSP-modification of a corrosive attack of the aresurface, observed the as between-built SLM-manufactured the individual laser sample scanning appears paths, intact; cf. layer and scan track (Figure 6). The penetration depth of corrosion was found to be about 980 μm. Figurehowever,5. It traces seems of that a corrosive the interfaces attack between are observed individual between layers the are individual preferential laser corrosion scanning sites. paths, This cf. canFigure be rationalized5. It seems that based the on interfaces local changes between in microstructural individual layers appearance are preferential and distribution corrosion of sites. elements, This ascan can be be rationalized deduced from based the locallyon local di ffchangeserent shapes in microstructural of the Si-rich networks, appearance in each and singledistribution layer and of scanelements, track as (Figure can be6). deduced The penetration from the depth locally of different corrosion shapes was found of the to Si-rich be about networks, 980 µm. in each single layer and scan track (Figure 6). The penetration depth of corrosion was found to be about 980 μm. Metals 2020, 10, x FOR PEER REVIEW 7 of 11 Metals 2020, 10, 85 7 of 11 Metals 2020, 10, x FOR PEER REVIEW 7 of 11

Figure 5. 5. As-builtAs-built SLM SLM sample sample after after corrosion corrosion testing. testing. Left: Left:Macroscopic Macroscopic documentation; documentation; Right: Cross- Right: FigureCross-sectionsection 5. perpendicular As-built perpendicular SLM tosample build toafter direction build corrosion direction (top), testing. corrosi (top), Left:on corrosion attack Macroscopic at attackthe large documentation; at su therface large (middle), surface Right: corrosion (middle), Cross- sectioncorrosionattack inperpendicular the attack smaller in the surface to smaller build (bottom). direction surface (bottom). (top), corrosion attack at the large surface (middle), corrosion attack in the smaller surface (bottom).

Figure 6. SEM micrographs of the cross-section of the as-built SLM sample after corrosion testing. Figure 6. SEM micrographs of the cross-section of the as-built SLM sample after corrosion testing. TheFigure FSP 6. zonesSEM micrographs of the samples of the processed cross-section under of the water as-built (UW-FSP) SLM sample and inafter air corrosion (AC-FSP) testing. corroded as The FSP zones of the samples processed under water (UW-FSP) and in air (AC-FSP) corroded as well during 24 h of immersion in 3% NaCl with 1% HCl. However, there are obvious differences in the wellThe during FSP 24zones h of of immersion the samples in processed3% NaCl with under 1% water HCl. (UW-FSP) However, and there in areair (AC-FSP)obvious differences corroded as in corrosion resistance of the different zones in the FSP samples, as well as alongside the direction of tool wellthe corrosionduring 24 resistance h of immersion of the indifferent 3% NaCl zones with in 1% the HCl. FSP However,samples, as there well are as alongsideobvious differences the direction in movement during FSP treatment. Both samples corroded severely at the boundary between the FSP theof toolcorrosion movement resistance during of theFSP different treatment. zones Both in sa themples FSP corrodedsamples, severelyas well as at alongside the boundary the direction between zone and the SLM base metal, while most of the FSP zone is still intact and, in most of the area, without ofthe tool FSP movement zone and duringthe SLM FSP base treatment. metal, while Both mostsamples of the corroded FSP zone severely is still atintact the boundaryand, in most between of the any traces of corrosion damage. The FSP-modified zone of the AC-FSP sample, as shown in Figure7, is thearea, FSP without zone and any the traces SLM of base corrosion metal, whiledamage. most The of FSP-modifiedthe FSP zone iszone still ofintact the and,AC-FSP in most sample, of the as intact up to about 10 mm from the starting position of the FSP treatment. After that, corrosion marks area,shown without in Figure any 7, traces is intact of upcorrosion to about damage. 10 mm fromThe FSP-modifiedthe starting position zone ofof the FSPAC-FSP treatment. sample, After as shown in Figure 7, is intact up to about 10 mm from the starting position of the FSP treatment. After Metals 2020, 10, 85 8 of 11 Metals 2020, 10, x FOR PEER REVIEW 8 of 11 that,appear corrosion in the middle marks of appear the modified in the surfacemiddle areaof th ande modified the corrosive surface attack area between and the thecorrosive FSP zone attack and betweenSLM base the metal FSP on zone the advancing and SLM side base intensifies. metal on Pitting the advancing corrosion occurredside intensifies. within the Pitting FSP zone, corrosion while occurredit seemsMetals that within2020 selective, 10, xthe FOR FSP PEER corrosion zone, REVIEW ofwhile the boundaryit seems that between selective the FSP corrosion zone and of thethe SLMboundary base metal between8 of prevails. 11 the TheFSP latterzone and could the have SLM been base initiated metal prevails. as galvanic The corrosionlatter could due have to the been diff initiatederence in as microstructures galvanic corrosion and that, corrosion marks appear in the middle of the modified surface area and the corrosive attack dueelemental to the distribution,difference in respectively,microstructures of the and two elemen adjacenttal distribution, zones. Progressive respectively, degradation of the two seems adjacent to be between the FSP zone and SLM base metal on the advancing side intensifies. Pitting corrosion further driven by pitting and/or crevice corrosion. With the FSP zone remaining intact, the prevalent zones.occurred Progressive within degradation the FSP zone, seems while toit seemsbe further that selective driven bycorrosion pitting of and/or the boundary crevice betweencorrosion. the With thecorrosion FSPFSP zone zone type andremaining leads the toSLM the intact, base undercutting metal the prevalentprevails. of that The corrosion zone.latter could Maximum type have leads been corrosion to initiated the undercutting depth as galvanic within corrosion theof that FSP zone. zone Maximumis 312dueµm. to corrosionthe difference depth in microstructures within the FSP and zone elemen is 312tal μdistribution,m. respectively, of the two adjacent zones. Progressive degradation seems to be further driven by pitting and/or crevice corrosion. With the FSP zone remaining intact, the prevalent corrosion type leads to the undercutting of that zone. Maximum corrosion depth within the FSP zone is 312 μm.

Figure 7. 7. AC-FSPAC-FSP sample sample after after corrosion corrosion testing. testing. Top: Top:Macroscopic Macroscopic documentation; documentation; Bottom: Bottom: Cross- sections:Cross-sections:Figure perpendicular 7. AC-FSP perpendicular sample to processing after to processing corrosion direction testing. direction with Top:in within Macroscopicthe boundary the boundary documentation; region region (left) (left) Bottom:and andmiddle Cross- middle of the of FSPthe FSPzonesections: zone (right). perpendicular (right). to processing direction within the boundary region (left) and middle of the FSP zone (right). Large areas of the UW-FSP-modified UW-FSP-modiﬁed zone were dissolved during the the corrosion corrosion test test (Figure (Figure 88).). These zonesLarge are areas located of the at UW-FSP-modified the starting point zone of thewere FSP dissolved zone andduring within the corrosion the outer test areas, (Figure where 8). the retreatingThese sidesidezones seemsseems are located to to be be more atmore the deteriorated. starting deteriorated. point Theof Th the middlee FSPmiddle zone of theof and the FSP within FSP zone zonethe seems outer seems to areas, be to fully wherebe fully intact the intact after afterthe corrosion retreatingthe corrosion test. side The seemstest. same The to be holdssame more holds true deteriorated. for true the for end Ththe ofe middleend the FSPof ofthe zone. the FSP FSP At zone. zone these Atseems spots these to there spotsbe fully are there nointact signs are no of after the corrosion test. The same holds true for the end of the FSP zone. At these spots there are no signslocalized of localized corrosion. corrosion. Within the Within transition the transition area between area between the FSP zone the FSP and zone the SLM and the base SLM metal, base a similarmetal, signs of localized corrosion. Within the transition area between the FSP zone and the SLM base metal, acorrosion similar corrosion appearance appearance is observed is inobserved the cross-section in the cross-section as in case of as the in AC-FSW case of the sample; AC-FSW however, sample; the a similar corrosion appearance is observed in the cross-section as in case of the AC-FSW sample; edge of the FSP zone seems to be more severely deteriorated upon UW-FSP. The maximum observed however,however, the theedge edge of theof the FSP FSP zone zone seems seems toto be more severelyseverely deteriorated deteriorated upon upon UW-FSP. UW-FSP. The The µ maximumcorrosionmaximum depth observed observed in the corrosion transition corrosion depth depth area in is in the 260 the transition transitionm. areaarea is is 260 260 μ m.μm.

FigureFigure 8. UW-FSP8. UW-FSP sample sample after after corrosion corrosion testing. testing. Top: Macroscopic Top: Macroscopic documentation; documentation; Bottom: Cross- Bottom: sections perpendicular to processing direction within the boundary region (left) and middle of the FigureCross-sections 8. UW-FSP perpendicular sample after to processingcorrosion testing. direction Top: within Macroscopic the boundary documentation; region (left )Bottom: and middle Cross- of FSP zone (right). sectionsthe FSP zoneperpendicular (right). to processing direction within the boundary region (left) and middle of the FSP zone (right). Metals 2020, 10, 85 9 of 11 Metals 2020, 10, x FOR PEER REVIEW 9 of 11

In the SEM micrographs recorded in the transition area between the FSP zone and the as-builtas-built SLM microstructure, significantsignificant corrosion activityactivity isis seenseen (Figure(Figure9 9).). TheThe FSPFSP surfacesurface isis characterizedcharacterized by a wavy surface surface appearance, appearance, while while the the SLM SLM surface surface is issignificantly significantly rougher. rougher. Previous Previous melt melt pools pools in inthe the as-built as-built SLM SLM material material can canbe perceived be perceived clearly clearly in the in high-resolution the high-resolution image. image. In between In between both areas both areasthere thereis a zone, is a zone, where where the shape the shape of the of themelt melt pools pools is distorted. is distorted. This This zone zone isis affected affected by by the the severe deformation induced by the movement of the FSP tooltool on the surface. While the surface of the FSP zone isis almostalmost freefree ofof corrosion corrosion induced induced changes, changes, the the transition transition zone zone is severelyis severely deteriorated deteriorated upon upon the corrosionthe corrosion test. test. The reasonThe reason for this for is this seen is in seen the microstructuralin the microstructural and compositional and compositional differences differences between thebetween two adjacentthe two adjacent zones: While zones: the While Si-rich the networkSi-rich netw of cellularork of cellular shape shape is intact is intact in the in una theff unaffectedected melt pools,melt pools, i.e., sample i.e., sample regions regions without without any FSP inducedany FSP distortion,induced distortion, the Si-rich networkthe Si-rich is transformednetwork is intotransformed Si-rich particles into Si-rich embedded particles in an embedded aluminum in matrix an aluminum in the FSP zone.matrix Both in microstructuresthe FSP zone. showBoth goodmicrostructures corrosion resistance;show good however, corrosion the resistance; transition however, area, i.e., the the transition TMAZ, where area, i.e., an intermediatethe TMAZ, where state featuringan intermediate ill-defined state network featuring structures ill-defined and network partially structures grown Si-rich and partially precipitates grown prevails Si-rich (cf. precipitates Figure1), isprevails obviously (cf. Figure highly 1), susceptible is obviously to corrosion. highly susceptible to corrosion.

Figure 9. SEM-image of the transition ar areaea between the FSP zone and as as-built-built SLM microstructure of the AC-FSPAC-FSP sample:sample: thethe high-resolutionhigh-resolution micrograph micrograph (right (right hand hand side) side) is is taken taken from from the the area area marked marked by theby the yellow yellow square. square.

The maximum corrosion depth was significantly reduced through FSP-modification of the The maximum corrosion depth was significantly reduced through FSP-modification of the SLM- SLM-manufactured samples. However, corrosion still occurred on the surface of the FSP-modified manufactured samples. However, corrosion still occurred on the surface of the FSP-modified samples. By direct comparison of the final appearance of the differently treated samples, it seems to be samples. By direct comparison of the final appearance of the differently treated samples, it seems to possible to deduce the mechanisms leading to the differing kinds of corrosive attack. Assuming that be possible to deduce the mechanisms leading to the differing kinds of corrosive attack. Assuming lower cooling rates and, eventually, higher heat input are e ective during AC-FSP, it is expected that that lower cooling rates and, eventually, higher heat input areff effective during AC-FSP, it is expected largerthat larger TMAZ TMAZ and heatand aheatffected affected zones zones (HAZ) (HAZ) prevail, prevail, finally finally increasing increasing the undercutting the undercutting depth belowdepth thebelow rather the rather undamaged undamaged FSP zone. FSP zone. Furthermore, Furthermore, the continued the continued heat inputheat input during during FSP seemsFSP seems to lead to tolead sensitization to sensitization of the of already the already modified modified surface. surface. In UW-FSP In UW-FSP the higher the cooling higher ratescooling and rates absorbed and excessabsorbed heat excess seem heat to improve seem to theimprove corrosion the corrosion resistance resistance of the FSP ofzone. the FSP The zone. reason The for reason this mayfor this be, amongmay be, others, among the others, smaller the smaller size and size larger and distance larger di betweenstance between the Si-rich the precipitates.Si-rich precipitates. However, However, due to thedue smallerto the smaller TMAZ andTMAZ HAZ, and the HAZ, gradient the gradient in electrochemical in electrochemical potential ispotential higher, eventuallyis higher, eventually leading to increasedleading to dissolutionincreased dissolution of the boundary of the region,boundary severe region, undercutting severe undercutting and strongly and promoted strongly dissolution promoted ofdissolution the FSP-modified of the FSP-modified surface. This surface. is, presumably, This is, presumably, the reason forthe thereason areal for dissolution the areal dissolution in vicinity ofin thevicinity starting of the point starting of the point FSP of zone. the FSP For in-depthzone. For analysis in-depth of analysis the underlying of the underlying mechanisms mechanisms and their interplay;and their however,interplay; further however, work further is needed, work focusing is needed, for example focusing on for microstructural example on homogeneitymicrostructural in thehomogeneity FSP affected in zonesthe FSP as wellaffected on direct zones determination as well on ofdirect local determination electrochemical of potentials. local electrochemical Results will bepotentials. published Results in follow-up will be published studies. in follow-up studies. 4. Conclusions 4. Conclusions Friction stir processing (FSP) technique was employed to enhance the surface quality and Friction stir processing (FSP) technique was employed to enhance the surface quality and homogenize the microstructure of Al-Si12 samples produced by selective laser melting (SLM). FSP was homogenize the microstructure of Al-Si12 samples produced by selective laser melting (SLM). FSP carried out in two different environments i.e., water and air, to investigate the effect of cooling rate was carried out in two different environments i.e., water and air, to investigate the effect of cooling rate during processing. Microstructural evolution, as well as surface roughness and corrosion Metals 2020, 10, 85 10 of 11 during processing. Microstructural evolution, as well as surface roughness and corrosion behavior of both FSP treated and SLM as-built samples were explored. The following conclusions can be drawn from the results presented:

1. SEM analysis revealed that the microstructure of SLM processed samples is characterized by Si-rich networks in the Al matrix. These networks were dissolved upon FSP treatment and replaced with spheroidized Si-rich particles. The size of these particles is larger for the sample FSP treated in air as compared to the condition processed in water. 2. EBSD measurements confirmed that grain refinement took place during FSP of additively manufactured Al-Si12. The degree of refinement was seen to be higher for the sample processed in water. 3. The corrosion behavior of SLM-manufactured AlSi12 alloys can be improved by the FSP-modification of the surfaces. The corrosion performance within the FSP treated zone is thought to be affected by several mechanisms, among others by the anodic dissolution of the Al matrix due to the occurrence of Si-rich precipitates in these zones. 4. Higher cooling rates being characteristic for UW-FSP seem to lead to a microstructure with higher corrosion resistance as compared to AC-FSP.

More detailed investigations with a more localized exposition to corrosive media are required to fully understand the interaction between the diﬀerent zones induced by FSP in corrosive environments.

Author Contributions: Conceptualization, G.M.; Methodology, S.V.S., G.M., B.H. and T.E.; Validation, G.M.; Investigation, S.V.S., B.H., T.E., G.M.; Writing-Original Draft Preparation, G.M.; Writing-Review & Editing, G.M., T.N.; Visualization, G.M.; Supervision, S.B., T.N. and M.O. All authors have read and agreed to the published version of the manuscript. Funding: The authors are grateful for the support from the Hessen State Ministry for Higher Education, Research and the Arts-Initiative for the Development of Scientific and Economic Excellence (LOEWE) for the ALLEGRO project. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Frazier, W.E. Metal additive manufacturing: A review. J. Mater. Eng. Perform. 2014, 23, 1917–1928. [CrossRef] 2. Calignano, F.; Manfredi, D.; Ambrosio, E.P.; Iuliano, L.; Fino, P. Influence of process parameters on surface roughness of aluminum parts produced by DMLS. Int. J. Adv. Manuf. Technol. 2013, 67, 2743–2751. [CrossRef] 3. Townsend, A.; Senin, N.; Blunt, L.; Leach, R.K.; Taylor, J.S. Surface texture metrology for metal additive manufacturing: A review. Precis. Eng. 2016, 46, 34–47. [CrossRef] 4. Hackel, L.; Rankin, J.R.; Rubenchik, A.; King, W.E.; Matthews, M. Laser peening: A tool for additive manufacturing post-processing. Addit. Manuf. 2018, 24, 67–75. [CrossRef] 5. Maamoun, A.; Elbestawi, M.; Veldhuis, S. Influence of Shot Peening on AlSi10Mg Parts Fabricated by Additive Manufacturing. J. Manuf. Mater. Process. 2018, 2, 40. [CrossRef] 6. Zakay, A.; Aghion, E. Effect of Post-Heat Treatment on the Corrosion Behavior of AlSi10Mg Alloy Produced by Additive Manufacturing. JOM 2019, 71, 1150–1157. [CrossRef] 7. Lamikiz, A.; Sánchez, J.A.; López de Lacalle, L.N.; Arana, J.L. Laser polishing of parts built up by selective laser sintering. Int. J. Mach. Tools Manuf. 2007, 47, 2040–2050. [CrossRef] 8. AlMangour, B.; Yang, J.-M. Improving the surface quality and mechanical properties by shot-peening of 17-4 stainless steel fabricated by additive manufacturing. Mater. Des. 2016, 110, 914–924. [CrossRef] 9. Uzan, N.E.; Ramati, S.; Shneck, R.; Frage, N.; Yeheskel, O. On the effect of shot-peening on fatigue resistance of AlSi10Mg specimens fabricated by additive manufacturing using selective laser melting (AM-SLM). Addit. Manuf. 2018, 21, 458–464. [CrossRef] 10. Maamoun, A.H.; Veldhuis, S.C.; Elbestawi, M. Friction stir processing of AlSi10Mg parts produced by selective laser melting. J. Mater. Process. Tech. 2019, 263, 308–320. [CrossRef] Metals 2020, 10, 85 11 of 11

11. Kumbhar, N.N.; Mulay, A.V. Post Processing Methods used to Improve Surface Finish of Products which are Manufactured by Additive Manufacturing Technologies: A Review. J. Inst. Eng. Ser. C 2018, 99, 481–487. [CrossRef] 12. Ahmed, H.; Stephen, C. Post-processing of the additively manufactured AlSi10Mg parts produced by Selective Laser Melting. In Proceedings of the The 7th International Conference on Virtual Machining Process Technology, Hamilton, ON, Canada, 7–9 May 2018; pp. 5–6. 13. Lathabai, S.; Migeon, R.; Tyagi, V.K.; O’Donnell, R.G.; Estrin, Y. Friction Stir Processing: A Technique for Microstructural Refinement in Metallic Materials. Mater. Sci. Forum 2009, 618–619, 63–67. [CrossRef] 14. Mishra, R.S.; Ma, Z.Y. Friction stir welding and processing. Mater. Sci. Eng. R Rep. 2005, 50, 1–78. [CrossRef] 15. Ma, Z.Y.; Sharma, S.R.; Mishra, R.S. Microstructural modification of As-cast Al-Si-Mg alloy by friction stir processing. Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 2006, 37, 3323–3336. [CrossRef] 16. Abdi behnagh, R.; Besharati Givi, M.K.; Akbari, M. Mechanical properties, corrosion resistance, and microstructural changes during friction stir processing of 5083 aluminum rolled plates. Mater. Manuf. Process. 2012, 27, 636–640. [CrossRef] 17. Santos Macias, J.G.; Van Hooreweder, B.; Maire, E.; Adrien, J.; Jacques, P. Friction stir processing of additive manufactured AlSi10Mg parts to improve mechanical behaviour. In Proceedings of the FSWP 2017—5th International Conference on Scientific and Technical Advances on Friction Stir Welding & Processing, Metz, France, 11–13 October 2017. 18. Moeini, G.; Sajadifar, S.V.; Wegener, T.; Brenne, F.; Niendorf, T.; Böhm, S. On the low-cycle fatigue behavior of friction stir welded Al–Si12 parts produced by selective laser melting. Mater. Sci. Eng. A 2019, 764, 138–189. [CrossRef]

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