Microstructure, Texture, Electrical and Mechanical Properties of AA-6063 Processed by Multi Directional Forging

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Article Microstructure, Texture, Electrical and Mechanical Properties of AA-6063 Processed by Multi Directional Forging

Alireza Dashti 1, Mohammad Hossein Shaeri 1,* , Reza Taghiabadi 1, Faramarz Djavanroodi 2,3, Farzaneh Vali Ghazvini 4 and Hamid Javadi 4 1 Department of Materials Science and Engineering, Imam Khomeini International University (IKIU), Qazvin 3414916818, Iran; [email protected] (A.D.); [email protected] (R.T.) 2 Mechanical Engineering Department, Prince Mohammad Bin Fahd University, Al Khobar 31952, Saudi Arabia; [email protected] 3 Department of Mechanical Engineering, Imperial Collage London, London SW7, UK 4 Ecole de Technologie Supérieure, Department of Mechanical Engineering, Montréal, QC H3C 1K3, Canada; [email protected] (F.V.G.); [email protected] (H.J.) * Correspondence: [email protected]; Tel.: +98-283-390-1190

Received: 30 October 2018; Accepted: 27 November 2018; Published: 29 November 2018

Abstract: In current research, the effect of the multi-directional forging (MDF) process on the microstructure, texture, mechanical and electrical properties of AA-6063 under different heat treatment conditions at various MDF temperatures was studied. The annealed AA-6063 alloy was processed up to three passes of MDF at ambient temperature. Three passes of this process were also applied to the solution-treated AA-6063 at ambient temperature and 177 ◦C. Microstructural investigations demonstrated that the MDF process led to a signiﬁcant reduction in the average grain size as well as a considerable increase in the fraction of low angle grain boundaries. Texture analysis revealed that copper and Goss textures were mainly developed within the annealed and solution-treated samples of AA-6063, respectively. The hardness and shear strength values of all processed samples also showed a sizeable improvement compared to the initial heat-treated samples. For example, the hardness and shear yield strength value of the solution-treated sample MDFed for three passes showed more than 100 and 70% increase, respectively. The effect of the MDF process on the electrical conductivity of AA-6063 under different heat treatment conditions at various temperatures was negligible. So, it can be concluded that the MDF process increased the mechanical properties without an appreciable decrease in electrical conductivity.

Keywords: MDF process; AA-6063 aluminum alloy; microstructure; texture; mechanical properties; electrical properties

1. Introduction Severe Plastic Deformation (SPD) has proved to be a reliable method for producing ultrafine-grained (UFG) metals and alloys. Multi-directional forging (MDF) as one of the most attractive SPD processes, was first applied in the 1990s to develop UFG structure in bulk materials. Despite the lower strain homogeneity achieved by MDF compared with other common SPD processes like equal channel angular pressing (ECAP) and high-pressure torsion (HPT), this method is well-suited to generating a UFG structure in rather brittle materials considering its relatively low specific load and the possibility to conduct the process at elevated temperatures. Other advantages of MDF include its high efficiency and low processing cost. Choosing appropriate temperatures and strain rates can lead to a desired nanocrystalline structure even for large billets [1–5].

Materials 2018, 11, 2419; doi:10.3390/ma11122419 www.mdpi.com/journal/materials Materials 2018, 11, 2419 2 of 17

AA-6063 is one of the most commonly used Al alloys in the heat treatable 6xxx series. This moderate-strength alloy is highly extrudable and shows excellent weldability, brazeability, and fair machinability as well as a prominent resistance to all types of corrosion. Because of these outstanding properties, the alloy has found a varied range of applications including architectural extrusions, pipe, truck and trailer flooring, furniture, road transport, rail transport, doors, windows, and irrigation pipes [6]. Considering the numerous applications of this widely used alloy which mostly require proper mechanical characteristics, it is interesting to produce it with a UFG microstructure by SPD processes. It is well known that heat-treatable aluminum-based alloys such as the 6xxx and 7xxx series can be strengthened appreciably by precipitation hardening. In addition, grain refinement and strain hardening during the SPD process can deliver a significant improvement to the alloy’s strength [2,7,8]. Previous reports revealed that SPD processes, such as ECAP, caused a considerable improvement in the mechanical properties of 6xxx series alloys [9–13]. For instance, the refined and homogeneous microstructure can be achieved during ECAP process of the AA-6063 alloy at an optimum die channel angle of 90◦. The process also led to a significant increase in the ultimate compressive strength, yield strength, compression modulus, and microhardness of this alloy compared to the as-received material [9]. In another research, a combination of pre-ECAP solid-solution with the post-ECAP aging treatment of AA-6061 was demonstrated to be more effective at strengthening the alloy than pre-ECAP peak-aging [10,11]. AA-6061 samples processed by multi-directional forging also exhibited high strength and high ductility. Processing this alloy subjected to prior solutionizing treatment through MDF at cryogenic temperature yielded a homogeneous microstructure with ultrafine grain morphology. 6061 aluminum alloy was also subjected to the MDF process at liquid nitrogen temperature that led to an improvement in hardness and tensile strength as a result of equiaxed sub-grain structure formation and the presence of high dislocation density [12,13]. A few investigations have been carried out on multi-directional forging of 6xxx aluminum alloys, but AA-6063 has not been among relevant researches. There are multiple points that make this research worth studying. First, the researches on the texture and specifically electrical properties of aluminum alloys processed by SPD processes are very limited. Second, the effect of the MDF process in particular on the electrical properties and texture of AA-6063 has not been studied before at all. Considering the fact that AA-6063 alloy has some applications in electrical devices, such as electrical components, conduits, and conductors, that require reasonable conductivity as well as other lightweight and strength demanding applications including the finer details of aircrafts and aerospaces and also extreme sports equipment, knowing the impact of the MDF process on the microstructure, mechanical, and electrical properties and texture development of the alloy can be very interesting. It is worth mentioning that the MDF process is much simpler and cheaper compared with other SPD processes in many cases. Therefore, due to the low alloying elements of AA-6063, by applying the MDF process as a cheap and simple SPD process, a high-strength AA-6063 alloy with reasonable electrical conductivity could be fabricated. The fabricated alloy can replace some high-strength 6xxx aluminum alloys containing higher alloying elements [14–16]. The present investigation was intended to examine the effect of the MDF process on the microstructure, texture, and electrical and mechanical properties of AA-6063 in the annealed and solution-treated states at various temperatures and pass numbers. The core objective was to study grain structure and how it affects the above-mentioned properties. Therefore, electron back scatter diffraction (EBSD) was employed to investigate the evolution of grain structure and texture. In addition, shear strength and microhardness tests, as well as the eddy current method, were used to measure the mechanical and electrical properties.

2. Materials and Methods The AA-6063 used in this study has a nominal composition of (as follows:) Al-0.5 Mg-0.39 Si-0.17 Fe-0.01 V-0.01 Ga (wt. %). Rectangular parallelepiped samples with dimensions of 15 × 10 × 10 mm3 Materials 2018, 11, 2419 3 of 17 were machined from the as-received cast billet. Before MDF, the specimens were subjected to 2 different heat treatments to produce different initial metallurgical states. The annealed samples (AS) were produced via a 2-h heat treatment at 415 ◦C (furnace cooling). The solid solution-treated samples (SS) were solution heat treated at 520 ◦C for 2 h and water quenched to form a super saturated solid Materials 2018, 11, x FOR PEER REVIEW 3 of 17 solution (SSSS). The solution-treated samples were immediately transferred to a −15 ◦C freezer to preventdifferent the aging heat process treatments at room to produce temperature different initial (RT). Themetallurgical samples states. were The then annealed processed samples by MDF(AS) for up to 3 passes.were The produced annealed via samplesa 2‐h heat were treatment only processedat 415 °C (furnace at room cooling). temperature, The solid while solution the solution-treated‐treated samplessamples were process(SS) were at solution room and heat 177 treated◦C temperatures.at 520 °C for 2 h To and study water the quenched dynamic to agingform a duringsuper MDF, saturated solid solution (SSSS). The solution‐treated samples were immediately transferred to a −15 177 ◦C was selected for a high-temperature MDF process as this temperature is the T6 heat treatment °C freezer to prevent the aging process at room◦ temperature (RT). The samples were then processed◦ aging temperatureby MDF for up (solution to 3 passes. treatment The annealed at 520 samplesC + water were quenchingonly processed + peak at room aging temperature, at 177 C) while of AA-6063 alloy [6].the solution‐treated samples were process at room and 177 °C temperatures. To study the dynamic Theaging MDF during die MDF, was 177 made °C was of selected H13 for tool a high steel,‐temperature and its MDF central process cavity as this hastemperature dimensions is of 40 × 15the× 10T6 mmheat 3treatment. The MDF aging die temperature and the (solution related toolingtreatment used at 520 in °C this + water process quenching were + presented peak in our previousaging at work 177 °C) [5]. of A AA 100-ton‐6063 alloy hydraulic [6]. press with 0.5 s−1 strain rate was used to strain the samples The MDF die was made of H13 tool steel, and its central cavity has dimensions of 40 × 15 × 10 by 0.5. The imposed strain in each pass was veriﬁed and calculated as follows [17]: mm3. The MDF die and the related tooling used in this process were presented in our previous work −1 [5]. A 100‐ton hydraulic press with 0.5 s strain2 rate Hwas used to strain the samples by 0.5. The imposed strain in each pass was verified andε = calculated√ ln ( as) follows [17]: (1) 3 W 2 ln (1) where H and W are the height and width of the sample,√3 respectively [17]. Aswhere shown H and in FigureW are the1, height the samples and width were of the rotated sample, respectively by 90 ◦ around [17]. the direction perpendicular to the side 3As after shown each in Figure pass of1, the the samples process were [17 rotated]. They by were 90° around pressed the updirection to a maximumperpendicular of to 3 passes becausethe cracking side 3 after and each segmentation pass of the process were observed [17]. They towere occur pressed during up to the a maximum fourth pass of 3 of passes MDF in the because cracking and segmentation were observed to occur during the fourth pass of MDF in the solution-treated specimens. Graphite-based lubricant “Moly Coat 1000 Paste” was used for the solution‐treated specimens. Graphite‐based lubricant “Moly Coat 1000 Paste” was used for the lubricationlubrication of the of die the and die and samples. samples.

FigureFigure 1. Schematic 1. Schematic view view of the of the multidirectional multidirectional forging (MDF) (MDF) process process applied applied in current in current research. research.

EBSD analysesEBSD analyses were performedwere performed on a on field a field emission emission scanning scanning electron electron microscope microscope (FE-SEM, (FE‐SEM, Hitachi high technologiesHitachi high corporation,technologies corporation, Tokyo, Japan) Tokyo, HITACHI Japan) HITACHI SU-70 SU operating‐70 operating at an at an accelerating accelerating voltage voltage of 15 kV. EBSD scans were acquired via HKL CHANNEL5 (Oxford Instruments, Hobro, of 15 kV. EBSD scans were acquired via HKL CHANNEL5 (Oxford Instruments, Hobro, Denmark). Denmark). The size and resolution of the maps vary from one process condition to another The sizedepending and resolution on the size of the of the maps grains. vary Before from EBSD one characterization, process condition the samples to another were depending cut in half along on the size of the grains.their length, Before and EBSD the central characterization, square cross the‐sections samples were were manually cut in polished half along down their to 1 length, μm via and the central squarediamond cross-sections polishing followed were by manually mechano‐chemical polished polishing down with to 1 µa m0.05 via μm diamond silica colloidal polishing solution followed by mechano-chemicalusing a BUEHLER polishing Vibrometer with (Buehler a 0.05 Company,µm silica Lake colloidal Bluff, IL, solution USA) for using 24 h. a BUEHLER Vibrometer (Buehler Company,Shear punch Lake and Bluff, microhardness IL, USA) tests for 24were h. carried out to document the mechanical properties of the samples. Vickers microhardness measurements with 0.5 kg force and a dwell time of 15 s were Shearperformed punch according and microhardness to the standard tests ASTM were E‐384. carried The hardness out to document of the middle the square mechanical cross‐section properties of the samples.of the samples Vickers along microhardness their length measurementswas measured at with least 0.5five kg times, force and and the a average dwell timevalues of are 15 s were performedreported. according Shear topunch the standardtesting (SPT) ASTM is a E-384. suitable The test hardness to measure of thethe middlemechanical square properties cross-section of of the samplessmall‐ alongsized samples their length [5,17,18]. was Square measured Sheets with at least a thickness five times, of 0.8 and mm thewere average cut from valuesthe middle are of reported. Shear punchthe samples testing along (SPT) their is length. a suitable The sheets test towere measure polished the down mechanical to a thickness properties of 0.7 mm of small-sizedand punched using a shear punch fixture with a 6.2 mm diameter flat cylindrical punch and a 6.25 mm samples [5,17,18]. Square Sheets with a thickness of 0.8 mm were cut from the middle of the samples diameter receiving hole. All the shear punch tests were performed at the room temperature using a along their length. The sheets were polished down to a thickness of 0.7 mm and punched using a shear Materials 2018, 11, 2419 4 of 17 punch fixture with a 6.2 mm diameter flat cylindrical punch and a 6.25 mm diameter receiving hole. All the shear punch tests were performed at the room temperature using a Zwick/Roell Z100 Universal Testing machine (ZwickRoell GmbH & Co., Ulm, Germany). The experiments were conducted at a constant cross-head speed of 10–3 mm/s−1. The shear test was repeated at least 3 times for each sample and the average values are reported. The schematic of equipment used for the shear punch test is illustrated in our previous studies [5,18]. After obtaining the load displacement curves in this test, the shear stress was calculated as follows [18–21]:

P τ = (2) 2πravet where τ, P, and t represent the shear stress, applied force and specimen thickness, respectively and ravg is obtained using the following relation:

rpunch + rdie r = (3) ave 2 SPT curves were obtained by plotting shear strength vs. normalized punch displacement. The following equation was used for measuring normalized punch displacement [18–21]:

h d = (4) t A SIGMOR 100 conductivity meter (New Chapel Electronics, Fairford, UK) was also used to measure the electrical conductivity changes using the eddy current method according to standard ASTM E1004. The electrical conductivity results are reported as the percentage of conductivity relative to the International Annealed Copper Standard (% IACS). The conductivity measurements were performed at room temperature. All measurements were repeated 3 times, and the average value is reported for each one.

3. Results and Discussion

3.1. Microstructure Figures2 and3 depict some microstructural aspects of the annealed AA-6063 alloy before and after three passes of the MDF process, respectively. These aspects include EBSD grain orientation maps, related grain boundary maps, and misorientation angle distribution. The unit triangle which determined the grain colors by the orientation of each grain is also depicted in Figure2. The unit triangle is the same for all grain color maps. The EBSD analysis results showed that the average grain size of the annealed samples decreased from about 150 µm to less than 800 nm after being processed for up to three passes of MDF. The area fraction method was applied to measure the average grain size and the grain boundaries with a misorientation angle of more than 15◦ considered as the effective grain boundaries. It also clear that most of the grains are elongated in the direction of shear bands after the MDF process. Materials 2018, 11, x FOR PEER REVIEW 5 of 17 Materials 2018, 11, x FOR PEER REVIEW 5 of 17 confirm the replacement of cells and subgrains formed at relatively lower strains by ultrafine grains confirm the replacement of cells and subgrains formed at relatively lower strains by ultrafine grains Materials 2018as the, 11 , 2419gradual increase of dislocation density in the cell walls transform them into an 5 of 17 as the gradual increase of dislocation density in the cell walls transform them into an ultrafine‐grained structure [27–30]. ultrafine‐grained structure [27–30].

Figure 2. Microstructure features of initial annealed AA‐6063 alloy before MFD process: (a) the Figure 2.FigureMicrostructure 2. Microstructure features features of initial of initial annealed annealed AA-6063 AA‐6063 alloy alloy before before MFD MFD process: process: ( a()a) the the electron electron back scatter diffraction (EBSD) grain orientation map, (b) the EBSD grain boundary map, back scatterelectron diffraction back scatter (EBSD) diffraction grain (EBSD) orientation grain orientation map, (b) map, the EBSD (b) the grain EBSD boundary grain boundary map, map, and (c) the and (c) the misorientation angle distribution diagram. misorientationand (c) the angle misorientation distribution angle diagram. distribution diagram.

Figure 3. Microstructure features of the annealed AA‐6063 alloy after three passes of MFD process at Figure 3. Microstructure features of the annealed AA‐6063 alloy after three passes of MFD process at Figure 3.roomMicrostructure temperature (RT): features (a) the of EBSD the annealed grain orientation AA-6063 map alloy (IPF map) after according three passes to direction of MFD 2, process(b) at room temperature (RT): (a) the EBSD grain orientation map (IPF map) according to direction 2, (b) room temperaturethe EBSD grain (RT): boundary (a) the map, EBSD and grain (c) the orientation misorientation map angle (IPF distribution map) according diagram. to direction 2, (b) the the EBSD grain boundary map, and (c) the misorientation angle distribution diagram. EBSD grain boundary map, and (c) the misorientation angle distribution diagram. The macrostructure of the samples processed by MDF will show an X‐shaped pattern across The macrostructure of the samples processed by MDF will show an X‐shaped pattern across their rectangular cross‐section (the plane perpendicular to the direction z in Figure 1) as a result of Thetheir grain rectangular boundary cross maps‐section in (the Figures plane2 perpendicularand3 show theto the upsurge direction of z lowin Figure angle 1) grainas a result boundaries of MSBs accumulation as presented by Moghaddam et al. [31]. MSBs can contribute to the formation of (LAGBs)MSBs in the accumulation processed as sample presented where by Moghaddam magenta, green,et al. [31]. and MSBs red can lines contribute represent to the the formation boundaries of with sub‐boundaries within the HAGBs [27,31]. EBSD images were taken from the central square the misorientationsub‐boundaries angle within of higherthe HAGBs than [27,31]. 15◦, 5–15 EBSD◦, andimages 2–5 were◦, respectively. taken from the The central fraction square of LAGBs cross‐section of the samples along their length (the plane perpendicular to the direction y in Figure cross‐section of the samples along their length (the plane perpendicular to the direction y in Figure after three1), where passes the represented accumulated more MSBs than would 65% appear compared like a toribbon only (considering 9% for the initialthe abovementioned material. Similarly, 1), where the accumulated MSBs would appear like a ribbon (considering the abovementioned a significantX‐like increase pattern) close in the to surface the center density of the ofspecimens’ high angle cross grain‐section. boundaries The obtained (HAGBs) results canincluding be observed X‐like pattern) close to the center of the specimens’ cross‐section. The obtained results including when comparinggrain refinement Figure and2b withthe significant Figure3 b.increase It can of be LAGBs deduced can thatbe attributed the surface to the density accumulation of HAGBs of and grain refinement and the significant increase of LAGBs can be attributed to the accumulation of MSBs that also leads to the elongation of the grains along these deformation bands mostly in the LAGBsMSBs increases that also considerably leads to the after elongation three passesof the grains of MDF. along these deformation bands mostly in the central area of the aforementioned cross‐section. Thecentral microstructural area of the aforementioned developments cross in‐section. metals and alloys during the SPD process is related to the evolution of the dislocation substructures. The density of dislocations increases significantly with the applying strain at the initial stages of deformation, leading to the development of a cellular substructure. As the applied strain increases, the dislocation cells turn into cell blocks, and dislocation sub-boundaries are formed within these blocks generating LAGBs [22–24]. A variety of deformation bands including micro-shear bands (MSBs), start to appear at medium strains, resulting in the fragmentation of original grains. Finally, the LAGBs transform into HAGBs, and an ultrafine-grained structure progressively develops [25–27]. Previous studies of SPD processes also confirm the replacement of cells and subgrains Materials 2018, 11, 2419 6 of 17 formed at relatively lower strains by ultrafine grains as the gradual increase of dislocation density in the cell walls transform them into an ultrafine-grained structure [27–30]. The macrostructure of the samples processed by MDF will show an X-shaped pattern across Materials 2018, 11, x FOR PEER REVIEW 6 of 17 their rectangular cross-section (the plane perpendicular to the direction z in Figure1) as a result of MSBs accumulationThe lower imposed as presented strain in by each Moghaddam pass of the MDF et al. process [31]. compared MSBs can to contribute the other common to the SPD formation of sub-boundariesprocesses such within as ECAP the can HAGBs be considered [27,31 ].as EBSDthe main images reason wereof high taken fraction from of LAGBs the central in the square cross-sectionMDFed of sample. the samples As a result along of a their relatively length low (the imposed plane strain, perpendicular accumulated to strain the direction after three y passes in Figure 1), of MDF (ε = 1.5) is presumably inadequate to transform LAGBs into HAGBs [13,32]. where the accumulated MSBs would appear like a ribbon (considering the abovementioned X-like The microstructure features of the solution‐treated AA‐6063 alloy before and after three passes pattern)of close the MDF to the process center at both of the room specimens’ temperature cross-section. and 177 °C are Thedemonstrated obtained in results Figures including4–6. Two grain refinementdifferent and EBSD the significant magnifications increase were ofused LAGBs to confirm can be the attributed formation toof thevery accumulation small grains in of some MSBs that also leadsregions to the of elongationMDFed sample of the at grains177 °C. alongThe EBSD these analysis deformation results of bands the solution mostly‐treated in the samples central area of the aforementionedshowed that after cross-section. three passes of MDF at RT and 177 °C, the average grain size of the initial samples Thehas lower been imposedreduced from strain 200 μ inm each to 1000 pass and of 1500 the MDFnm, respectively. process compared The grain toboundary the other map common of the SPD MDFed samples at both temperatures indicate a considerable increase in the LAGBs and HAGBs processes such as ECAP can be considered as the main reason of high fraction of LAGBs in the MDFed density that can be justified according to the previously mentioned analyses for the annealed sample.samples. As a result As can of abe relatively seen in the low misorientation imposed strain, angle distribution accumulated diagrams, strain more after than three 80% passes of the of MDF (ε = 1.5)grain is presumably boundaries are inadequate LAGBs in tothe transform MDFed samples LAGBs at both into temperatures, HAGBs [13, 32while]. only 14% of them Theare microstructure LAGBs in the initial features sample. of the The solution-treated results including grain AA-6063 refinement alloy and before a significant and after increment three passes of the MDFin process the fraction at both of LAGBs room and temperature grain elongation and can 177 be◦C ascribed are demonstrated to the accumulation in Figures of MSBs4–6 .which Two differentis EBSD magnificationssimilar in the annealed were usedsamples. to confirmIt should be the pointed formation out that of the very very small fine grainsequiaxed in grains some were regions of formed after three◦ passes of MDF at the center of the specimens’ cross‐section as a result of the high MDFedimposed sample strain at 177 andC. accumulated The EBSD MSBs analysis in those results regions of the (Figure solution-treated 6b,d). samples showed that after ◦ three passesThe of MDFother key at RTissue and that 177 needsC, to the be discussed average is grain the higher size of fraction the initial of LAGBs samples and average has been grain reduced from 200sizeµ min the to 1000sample and MDFed 1500 at nm, a high respectively. temperature (177 The °C) grain compared boundary to the mapsample of processed the MDFed at RT. samples at both temperaturesAs the processing indicate temperature a considerable increases and increase the flow in localization the LAGBs decreases, and HAGBs the applied density strain that by can be justifiedshear according bands and to the the number previously of dislocation mentioned cells analyses decrease, leading for the to annealed a higher average samples. grain As size can [33]. be seen in On the other hand, MDF at high temperatures is accompanied by dynamic recovery that could lead the misorientation angle distribution diagrams, more than 80% of the grain boundaries are LAGBs in to grain growth and larger average grain size. The formation of new grains in aluminum alloys is the MDFedusually samples a result atof bothLAGBs temperatures, transformation whileinto HAGBs only which 14% of is controlled them are by LAGBs the recovery in the rate. initial The sample. The resultstransformation including of grain LAGBs refinement into HAGBs and occurs a significant due to the increment addition inof themobile fraction dislocations of LAGBs into the and grain elongationgrain can boundaries be ascribed during to plastic the accumulation deformation. On of the MSBs other which hand, isthe similar annihilation in the of annealed dislocations samples. It shouldwithin be pointed the grains out happens that the as very a result fine equiaxedof dynamic grains recovery were at formedhigher temperatures, after three passesand fewer of MDF at the centerdislocations of the specimens’ will join the cross-section boundaries consequently. as a result of Thus, the highthe increase imposed of the strain misorientaion and accumulated angle at MSBs higher temperatures is more difficult to achieve relative to room temperature. Therefore, the in those regions (Figure6b,d). processed sample at 177 °C has a higher fraction of LAGBs [34–37].

Figure 4.FigureMicrostructure 4. Microstructure features features of of initial solution solution-treated‐treated AA‐6063 AA-6063 alloy before alloy MFD before process: MFD (a) process: (a) the EBSDthe EBSD grain grain orientation orientation map, map, ( (bb)) thethe EBSD grain grain boundary boundary map, map, and and(c) the (c )misorientation the misorientation angle angle distribution diagram. distribution diagram.

Materials 2018, 11, 2419 7 of 17

Materials 2018, 11, x FOR PEER REVIEW 7 of 17

Figure 5. Microstructure features of solution-treated AA-6063 alloy after three passes of MFD process FigureFigure 5. Microstructure5. Microstructure features features of of solution solution‐‐treatedtreated AA‐‐60636063 alloy alloy after after three three passes passes of ofMFD MFD process process at RT: (aat) the atRT: EBSDRT: (a )( athe) grain the EBSD EBSD orientation grain grain orientation orientation map, (b )map,map, the EBSD ((b) the grain EBSDEBSD boundary graingrain boundaryboundary map, andmap, map, ( cand) theand (c misorientation) (cthe) the angle distributionmisorientationmisorientation diagram. angle angle distribution distribution diagram. diagram.

Figure 6. MicrostructureFigure 6. Microstructure features features of solution-treated of solution‐treated AA AA-6063‐6063 alloy alloy after after three three passes passesof MFD process of MFD process ◦ at 177 °C: (a,c) the EBSD grain orientation map, (b,d) the EBSD grain boundary map, and (e) the at 177 FigureC: (a,c 6.) Microstructure the EBSD grain features orientation of solution map,‐treated (b AA,d)‐ the6063 EBSDalloy after grain three boundary passes of MFD map, process and (e) the misorientation angle distribution diagram. misorientationat 177 °C: angle (a,c) distributionthe EBSD grain diagram. orientation map, (b,d) the EBSD grain boundary map, and (e) the 3.2.misorientation Mechanical Properties angle distribution diagram. The other key issue that needs to be discussed is the higher fraction of LAGBs and average grain Figure 7 presents the hardness of the annealed and solution‐treated samples of AA‐6063 alloy 3.2. Mechanical Properties ◦ size in thewith sample respect MDFed to the pass at numbers. a high temperature The hardness of (177 the annealedC) compared samples (31 to HV) the reached sample much processed high at RT. As the processingvaluesFigure after 7 temperature presents three passes the hardness increasesof MFD of (48 the and HV) annealed the representing ﬂow and localization solution a 50%‐ treatedincrease. decreases, samples The hardness of the AA applied‐6063 of the alloy strain by shear bandswithsolution respect and‐ thetreated to numberthe samplespass numbers. of (43 dislocation HV) The after hardness three cells passes decrease,of the at RTannealed and leading 177 samples °C showed to a (31 higher HV)much reached higher average increases much grain high size [33]. values after three passes of MFD (48 HV) representing a 50% increase. The hardness of the On the other hand, MDF at high temperatures is accompanied by dynamic recovery that could lead solution‐treated samples (43 HV) after three passes at RT and 177 °C showed much higher increases to grain growth and larger average grain size. The formation of new grains in aluminum alloys is usually a result of LAGBs transformation into HAGBs which is controlled by the recovery rate. The transformation of LAGBs into HAGBs occurs due to the addition of mobile dislocations into the grain boundaries during plastic deformation. On the other hand, the annihilation of dislocations within the grains happens as a result of dynamic recovery at higher temperatures, and fewer dislocations will join the boundaries consequently. Thus, the increase of the misorientaion angle at higher temperatures Materials 2018, 11, 2419 8 of 17 is more difﬁcult to achieve relative to room temperature. Therefore, the processed sample at 177 ◦C has a higher fraction of LAGBs [34–37].

3.2. Mechanical Properties Figure7 presents the hardness of the annealed and solution-treated samples of AA-6063 alloy with respect to the pass numbers. The hardness of the annealed samples (31 HV) reached much high values after threeMaterials passes 2018 of, 11 MFD, x FOR (48PEER HV) REVIEW representing a 50% increase. The hardness of the solution-treated8 of 17 samples (43 HV) after three passes at RT and 177 ◦C showed much higher increases of 109 and 121% and reachedMaterialsof 109 90 2018and and, 11121%, 95 x FOR HV,and PEER reached respectively. REVIEW 90 and 95 In HV, all respectively. cases, the In increase all cases, the in theincrease hardness in the hardness value8 of 17 is mostly value is mostly achieved after the first pass, while the hardness of the annealed samples remaining achieved after the ﬁrst pass, while the hardness of the annealed samples remaining almost constant ofalmost 109 and constant 121% afterand reachedthe second 90 passand 95and HV, the respectively.solution‐treated In all samples cases, displaythe increase a slight in theincrease hardness after after the secondvaluethe second is passmostly and and achieved third the passes. solution-treated after Thethe firstsamples pass, processed while samples the athardness display177 °C haveof a the slight a annealedhigher increase hardness samples after than remaining the RT second and third passes.almostsamples The constant samplesin all passes. after processed the second atpass 177 and◦C the have solution a higher‐treated hardness samples display than thea slight RT increase samples after in all passes. the second and third passes. The samples processed at 177 °C have a higher hardness than the RT samples in all passes.

Figure 7. Hardness variations of the annealed and solution‐treated samples of AA‐6063 with respect Figure 7. Hardness variations of the annealed and solution-treated samples of AA-6063 with respect to to the MDF pass number. the MDF passFigure number. 7. Hardness variations of the annealed and solution‐treated samples of AA‐6063 with respect to the MDF pass number. The shearThe stress shear versus stress versus normal normal displacement displacement of the the annealed annealed and and solution solution-treated‐treated samples of samples of AA‐6063 is shown in Figure 8 which also includes a heat treated sample under T6 conditions to AA-6063 ismake shownThe a comparisonshear in Figure stress betweenversus8 which normal the also MDFed displacement includes and T6 a‐ treatedof heat the treatedannealed samples. sample andThe solutionextracted under‐treated information T6 conditions samples from of to make AA‐6063 is shown in Figure 8 which also includes a heat treated sample under T6 conditions to a comparisonthese between graphs including the MDFed shear yield and strength T6-treated (τ), ultimate samples. shear The strength extracted (τ), informationhardening ration from these make(τ /aτ comparison) are summarized between in Tablethe MDFed 1. The shearand T6 yield‐treated stress samples. and ultimate The extracted shear stress information of the annealed from graphs including shear yield strength (τY), ultimate shear strength (τUTS), hardening ration (τUTS/τY) thesesample graphs after includingthree passes shear of yieldMDF strengthincreased (τ by), ultimate56 and 30%, shear respectively. strength (τ Shear), hardening punch test ration also are summarized in Table1. The shear yield stress and ultimate shear stress of the annealed sample after (resultedτ/τ) arein a summarized 72% rise in the in Tableshear 1.yield The stress shear andyield a stress24% increase and ultimate in the shearultimate stress shear of the stress annealed for the three passessamplesolution of MDF after‐treated increasedthree samples passes byprocessed of 56 MDF and increasedat 30%, RT. These respectively. by values 56 and are 30%, even Shear respectively. higher punch for the test Shear samples also punch resulted processed test also in at a 72% rise in the shearresulted177 yield °C as in stress thea 72% shear and rise yield in a 24%the and shear increase ultimate yield stressshear in the andstress ultimate a 24%were increase reported shear in stress by the 83% ultimate for and the 27% shear solution-treated higher stress than for thethe samples solution‐treated samples processed at RT. These values are even higher for the samples processed at processedinitial at RT. values, These respectively. values are even higher for the samples processed at 177 ◦C as the shear yield 177 °C as the shear yield and ultimate shear stress were reported by 83% and 27% higher than the and ultimateinitial shear values, stress respectively. were reported by 83% and 27% higher than the initial values, respectively.

Figure 8. Shear stress versus normal displacement for annealed, solution‐treated, and T6 samples of

AA‐6063 alloy. Figure 8. ShearFigure 8. stress Shear versus stress versus normal normal displacement displacement for for annealed, annealed, solution solution-treated,‐treated, and T6 samples and T6 of samples of AA‐6063 alloy. AA-6063 alloy.

Materials 2018, 11, 2419 9 of 17

Table 1. Mechanical testing results for the annealed, solution-treated, and T6 samples of AA-6063.

Shear Yield Stress Ultimate Shear Stress Hardness Sample τ /τ (MPa) (MPa) YUTS Y (HV) Initial sample (annealed) 52 77 1.6 31 3 passes MDFed (annealed) 75 100 1.3 48 Initial sample (solution-treated) 79 130 1.6 43 Heat treated sample (T6) 101 145 1.4 76 3 passes MDFed at RT (solution-treated) 136 162 1.2 90 3 passes MDFed at 177 ◦C (solution-treated) 145 165 1.1 95

The achieved improvement in the mechanical properties of the annealed and solution-treated samples of AA-6063 is closely related to the increase of LAGBs, grain reﬁnement and signiﬁcant increase in the density of dislocations. The Hall–Petch equation also proves that the yield stress and other mechanical properties increase accordingly as the average grain size of all samples decreases. The relationship between yield stress and average grain size (Hall–Petch equation) is as follows [38]:

− 1 σY = σo + kyd 2 (5)

σY, σo, ky, and d are the yield strength, a materials constant for the starting stress for dislocation movement, strengthening coefficient, and average grain size, respectively. Many of the previous researches also confirm the validity of Hall–Petch equation for the metals and alloys processed by SPD methods with ultrafine grains ranging from 1 µm to 100 nm [39–44]. Another basic strengthening mechanism in metals and alloys is strengthening through dislocations. SPD methods increase the density of dislocations, and as a result, the mobile dislocations interact with themselves and impede their own motion. Thus, the increase of dislocation density leads to an improvement in the mechanical properties. According to the Taylor model, the rise of yield strength due to the high density of dislocations during deformation is calculated as follows [45–50]:

1 σd = σo + MαGbdρ 2 , (6)

ρ, α, M, G, and b represent the density of dislocations, a material constant, Taylor factor (three for polycrystalline metals), shear modulus (26 GPa for aluminum), and the magnitude of Burgers vector (0.286 nm for aluminum), respectively. The solution-treated samples of AA-6063 processed by three passes at both temperatures indicated better mechanical properties compared to the T6-treated sample. This can be attributed to the simultaneous effect of grain reﬁnement, work hardening and dynamic aging process. As the precipitation rate accelerates during mechanical working, dynamic aging can occur. The MDF process presumably provides some favorable nucleation sites through increasing the density of dislocations and hastening the diffusion process especially at higher temperatures which leads to a higher precipitation rate. The accumulation of shear bands as high-energy and favorable nucleation sites during the MDF process is also another factor resulting in a higher precipitation rate [51–53]. Since the aging process is affected by temperature and time, processing the solution-treated samples of AA-6063 by three passes at 177 ◦C brings about better mechanical properties compared to the three-pass MDFed samples at RT as higher temperature accelerates the precipitation and aging process. Hence irrespective of the higher average grain size and less work hardening at 177 ◦C compared to the processed sample at RT, age hardening is possibly the main reason behind the more desirable mechanical properties at high temperature. To investigate the extent of aging achieved during the MDF process at RT and 177 ◦C, the three-pass MDFed samples were aged at T6 temperature (177 ◦C). Figure9 demonstrates the changes in the hardness value with respect to the aging time. The obtained results showed that the hardness value of the three-pass MDFed sample at RT after being aged for 3, 6, 9, 12, and 15 h has Materials 2018, 11, 2419 10 of 17 fallen from 90 HV to 82, 79, 78, 77, and 77 HV, respectively. On the other hand, the hardness value of the three-pass MDFed sample at 177 ◦C gradually decreased from 95 HV to 92, 89, 84, 81 and 80 HV after 3,Materials 6, 9, 12, 2018 and, 11, x 15 FOR h PEER of aging, REVIEW respectively. The thermal recovery and possible grain10 growthof 17 are the prevailing factors in contributing to the decrease of the mechanical properties including hardness prevailing factors in contributing to the decrease of the mechanical properties including hardness during post-MDF aging. Additionally, a signiﬁcant part of the aging process also occurs dynamically during post‐MDF aging. Additionally, a significant part of the aging process also occurs duringdynamically the MDF process, during the especially MDF process, at highespecially temperature. at high temperature. Hence Hence the maximum the maximum hardness hardness value is possiblyvalue achieved is possibly during achieved forging during as aforging result as of a dynamic result of dynamic aging, and aging, post-MDF and post‐MDF aging aging just just makes the precipitatesmakes grow the precipitates and lose coherency,grow and lose leading coherency, to lower leading hardness to lower hardness values [ values54]. [54].

Materials 2018, 11, x FOR PEER REVIEW 11 of 17

having a notable decrease in the electrical conductivity. In other words, a better combination of mechanical strength and electrical conductivity can be obtained by processing the AA‐6063 alloy by the MDF process compared to the commercial Al‐Mg‐Si alloy processed via conventional Figure 9.FigureHardness- 9. Hardness aging‐ aging time time curves curves of of the the solution-treated solution‐treated samples samples of AA of‐ AA-60636063 MDFed MDFed for three for three thermo‐mechanical treatments. passes atpasses room at temperatureroom temperature and and 177 177◦C. °C. Furthermore, the effect of the aging time on the electrical conductivity of the solution‐treated 3.3. Electrical3.3.samples Electrical Properties of AA Properties‐6063 MDFed for three passes was investigated. According to Figure 10b, there has been some improvement in the electrical conductivity of the samples with an increase of aging time. SinceThis the Sincehappened AA-6063 the AA as‐6063 alloya result alloy has hasof some thesome electricaldynamic electrical recovery applicationsapplications mechanism in inelectrical electrical and substations the substations formation and electricalof and more electrical componentsprecipitates. Thedue depletionto its proper of alloying electrical elements conductivity, as a result it isof interestingprecipitates to formation study the reduces effect theof componentsdeformation due to processes its proper such electrical as MDF conductivity, on electrical properties it is interesting of this alloy. to study Figure the 10 effect represents of deformation the processesnegative such aseffect MDF imposed on electrical by the alloying properties elements’ of this tension alloy. field Figure on the 10 movement represents of thefree evolution electrons of the evolutionwhich leads of tothe an electrical increase conductivityin conductivity. for Itthe should annealed be noted and thatsolution the ‐solutetreated atoms samples dissolved of AA in‐6063 the electricalwithmatrix conductivity respect are much to the formore number the effective annealed of MDF in passes.scattering and solution-treatedTable electrons 2 also contains and consequently samples a summary of a AA-6063 of reduction the obtained with in electrical results. respect to the numberconductivity of MDFThe electrical passes. compared conductivity Table to 2other also ofobstacles contains both AA [15,59,60]. a‐6063 summary conditions of thedoes obtained not vary significantly results. with MDF. The obtained results showed that the electrical conductivities decrease slightly under both conditions at the first pass suggesting that a considerable increase in plastic deformation is known to cause a disturbance in the movement of free electrons through the crystal structure as dislocations have a dispersive effect on electrons leading to a lower electrical conductivity [55,56]. However, as the number of passes increases, more dislocations join into LAGBs and consequently, the density of dislocations decreases and the free movement route of electrons lengthens and electrical conductivity slightly increases. Thus, the slight increment of conductivity in the second and third passes can be ascribed to a decrease in the density of dislocations as LAGBs transform into HAGBs. It can be concluded that although the impact of MDF process on the grain refinement, as well as enhancement of surface density of LAGHs and HAGBs, is significant (Figures 2–6), it has been almost negligible on the electrical conductivity as grain boundaries are not serious obstacles to the movement of free electrons. It is the dislocations’ interaction with grain boundaries (LAGBs and HAGBs) that results in a huge improvement in the mechanical properties. Electrical conductivity in metals is closely correlated with the microstructure and is highly affected by the structural defects including vacancies, dislocations, grain boundaries, impurities, etc. FigureHence, 10.Figure improvingElectrical 10. Electrical conductivitythe conductivitystrength changesby changes adding of of thealloying the annealedannealed elements, and and solution solution-treatedwork‐ treatedhardening samples samplesand of precipitationAA‐ of6063 AA-6063 withhardening respectwith respect to: of metals (a) to: the ( ais number) thetypically number of associated MDF of MDF passes passes with and and a decrease ((b) aging aging in time. time.conductivity [15,57,58]. Alloying is one of the most common ways to increase the strength of metals as was mentioned earlier. However, it is associatedTable 2.with Electrical some conductivitydownsides, measurementsuch as a fall results in electrical (annealed conductivity. and solution‐ treatedSo it wouldsamples be of very advantageousAA‐6063). to find a way to increase the strength with no tangible fall in conductivity. In the current investigation, the MDF processInitial increased Sample theFirst mechanical Pass Second properties Pass Thirdnotably Pass without an Samples appreciable decrease in the electrical conductivity(IACS) [15,57,58].(IACS) Therefore,(IACS) it can be(IACS) concluded that grain refinement byAnnealed SPD processes is a useful51.5 method to improve49.0 the mechanical50.0 properties50.3 without Solution‐treated at RT 46.0 43.8 44.9 45.0 Solution‐treated at 177 °C 46.0 43.6 44.8 45.3

3.4. Texture Analysis Texture evolution of the annealed and solution‐treated samples of AA‐6063 MDFed at both temperatures was also studied. In this regard, the typical orientation and ideal texture of a face centered cubic material are shown in Figure 11 in {111} and {200} pole figures. Copper {112} <111>, brass {011} <211>, S {123} <634>, cube {001} <100>, and Goss {011} <100> are the components included in Figure 11 [31]. {111} and {200} pole figures of the annealed and solution‐treated samples of AA‐6063 before and after three passes of MDF and their related texture intensity indicators are depicted in Figures 12 and 13. Both the annealed and solution‐treated samples of AA‐6063 show almost no texture before the MDF process. Figure 12 shows the formation of a textured fiber <111> including copper texture components in the annealed sample when is processed up to three passes of MDF. The fiber texture is displayed by more intense texture intensity on the northern and southern poles. On the other hand, Goss texture components were mainly formed in the solution‐treated samples subjected to

Materials 2018, 11, 2419 11 of 17

Table 2. Electrical conductivity measurement results (annealed and solution-treated samples of AA-6063).

Initial Sample First Pass Second Pass Third Pass Samples (IACS) (IACS) (IACS) (IACS) Annealed 51.5 49.0 50.0 50.3 Solution-treated at RT 46.0 43.8 44.9 45.0 Solution-treated at 177 ◦C 46.0 43.6 44.8 45.3

The electrical conductivity of both AA-6063 conditions does not vary significantly with MDF. The obtained results showed that the electrical conductivities decrease slightly under both conditions at the first pass suggesting that a considerable increase in plastic deformation is known to cause a disturbance in the movement of free electrons through the crystal structure as dislocations have a dispersive effect on electrons leading to a lower electrical conductivity [55,56]. However, as the number of passes increases, more dislocations join into LAGBs and consequently, the density of dislocations decreases and the free movement route of electrons lengthens and electrical conductivity slightly increases. Thus, the slight increment of conductivity in the second and third passes can be ascribed to a decrease in the density of dislocations as LAGBs transform into HAGBs. It can be concluded that although the impact of MDF process on the grain refinement, as well as enhancement of surface density of LAGHs and HAGBs, is significant (Figures2–6), it has been almost negligible on the electrical conductivity as grain boundaries are not serious obstacles to the movement of free electrons. It is the dislocations’ interaction with grain boundaries (LAGBs and HAGBs) that results in a huge improvement in the mechanical properties. Electrical conductivity in metals is closely correlated with the microstructure and is highly affected by the structural defects including vacancies, dislocations, grain boundaries, impurities, etc. Hence, improving the strength by adding alloying elements, work hardening and precipitation hardening of metals is typically associated with a decrease in conductivity [15,57,58]. Alloying is one of the most common ways to increase the strength of metals as was mentioned earlier. However, it is associated with some downsides, such as a fall in electrical conductivity. So it would be very advantageous to find a way to increase the strength with no tangible fall in conductivity. In the current investigation, the MDF process increased the mechanical properties notably without an appreciable decrease in the electrical conductivity [15,57,58]. Therefore, it can be concluded that grain refinement by SPD processes is a useful method to improve the mechanical properties without having a notable decrease in the electrical conductivity. In other words, a better combination of mechanical strength and electrical conductivity can be obtained by processing the AA-6063 alloy by the MDF process compared to the commercial Al-Mg-Si alloy processed via conventional thermo-mechanical treatments. Furthermore, the effect of the aging time on the electrical conductivity of the solution-treated samples of AA-6063 MDFed for three passes was investigated. According to Figure 10b, there has been some improvement in the electrical conductivity of the samples with an increase of aging time. This happened as a result of the dynamic recovery mechanism and the formation of more precipitates. The depletion of alloying elements as a result of precipitates formation reduces the negative effect imposed by the alloying elements’ tension field on the movement of free electrons which leads to an increase in conductivity. It should be noted that the solute atoms dissolved in the matrix are much more effective in scattering electrons and consequently a reduction in electrical conductivity compared to other obstacles [15,59,60].

3.4. Texture Analysis Texture evolution of the annealed and solution-treated samples of AA-6063 MDFed at both temperatures was also studied. In this regard, the typical orientation and ideal texture of a face centered cubic material are shown in Figure 11 in {111} and {200} pole ﬁgures. Copper {112} <111>, Materials 2018, 11, x FOR PEER REVIEW 12 of 17

three passes of MDF at both temperatures. The comparison of texture intensity indicators in Figures 12 and 13 reveals that strong texture forms during MDF process at both temperatures. In FCC metals with a medium to high stacking fault energy (SFE), grains have a tendency to form a copper‐type texture as the plastic deformation increases [61]. The presence of high amount of precipitates in the annealed sample is probably the reason of formation different texture during MDF in annealed samples compared to solution‐treated samples. Itʹs likely that the aforementioned precipitates hinder the free rotation of grains. The formation of different textures in differently Materials 2018, 11, 2419 12 of 17 heat‐treated aluminum alloy after MDF process has been reported before as well. It was ascribed to the distinct microchemistry of the matrices and the high density of deformation bands in the solution treated samples since the higher solute content retards dynamic recovery as reported for brass {011} <211>, S {123} <634>, cube {001} <100>, and Goss {011} <100> are the components included MDFed over‐aged and solution treated Al‐Cu‐Mg alloy [62]. Materials 2018, 11, x FOR PEER REVIEW 12 of 17 in Figure 11 [31]. three passes of MDF at both temperatures. The comparison of texture intensity indicators in Figures 12 and 13 reveals that strong texture forms during MDF process at both temperatures. In FCC metals with a medium to high stacking fault energy (SFE), grains have a tendency to form a copper‐type texture as the plastic deformation increases [61]. The presence of high amount of precipitates in the annealed sample is probably the reason of formation different texture during MDF in annealed samples compared to solution‐treated samples. Itʹs likely that the aforementioned precipitates hinder the free rotation of grains. The formation of different textures in differently heat‐treated aluminum alloy after MDF process has been reported before as well. It was ascribed to the distinct microchemistry of the matrices and the high density of deformation bands in the solution treated samples since the higher solute content retards dynamic recovery as reported for MDFed over‐aged and solution treated Al‐Cu‐Mg alloy [62].

FigureFigure 11. The 11. typical The typical orientation orientation and and ideal ideal texturetexture of of a aface face centered centered cubic cubic material material in: (a) {111} in: (a and) {111} and (b) {200}(b pole) {200} ﬁgures. pole figures.

{111} and {200} pole figures of the annealed and solution-treated samples of AA-6063 before and after three passes of MDF and their related texture intensity indicators are depicted in Figures 12 and 13. Both the annealed and solution-treated samples of AA-6063 show almost no texture before the MDF process. Figure 12 shows the formation of a textured fiber <111> including copper texture components in the annealed sample when is processed up to three passes of MDF. The fiber texture is displayed by more intense texture intensity on the northern and southern poles. On the other hand, Goss texture components were mainly formed in the solution-treated samples subjected to three passes of MDF at both temperatures. The comparison of texture intensity indicators in Figures 12 and 13 Figure 11. The typical orientation and ideal texture of a face centered cubic material in: (a) {111} and reveals that strong texture forms during MDF process at both temperatures. (b) {200} pole figures.

Figure 12. {111} and {200} pole figures of the annealed AA‐6063 alloy: (a) before and (b) after three passes of MDF and their related texture intensity indicators.

Figure 12.Figure{111} 12. and{111} {200}and {200} pole pole ﬁgures figures of of the the annealed annealed AA AA-6063‐6063 alloy: alloy: (a) before (a) before and (b and) after (b )three after three passes ofpasses MDF of and MDF their and their related related texture texture intensity intensity indicators. indicators.

Materials 2018, 11, 2419 13 of 17 Materials 2018, 11, x FOR PEER REVIEW 13 of 17

Figure 13.Figure{111} 13. {111} and and {200} {200} pole pole ﬁgures figures ofof the the solution solution-treated‐treated AA‐6063 AA-6063 alloy: alloy:(a) before (a )(b before) and (c) ( b) and (c) afterafter three three passes passes of MDFof MDF at at room room temperature temperature and and 177 177 °C,◦ C,respectively, respectively, and their and theirrelated related texture texture intensity indicators. intensity indicators. 4. Conclusions In FCC metals with a medium to high stacking fault energy (SFE), grains have a tendency to form a copper-typeThe effect texture of the asMDF the process plastic on deformation the microstructure, increases texture, [61]. mechanical The presence and ofelectrical high amount properties of both annealed and solution‐treated samples of AA‐6063 was investigated. According to of precipitates in the annealed sample is probably the reason of formation different texture during the results obtained, the main conclusions of this investigation are as follows: MDF in annealed(1) EBSD samples analyses compared showed the toaverage solution-treated grain size of both samples. annealed It’s and likely solution that‐treated the aforementioned samples precipitateshas decreased hinder thenoticeably free rotation during the of grains.MDF process. The formationFor instance, of the different average texturesgrain size in of differentlythe heat-treatedsolution aluminum‐treated sample alloy MDFed after MDF at room process temperature has been reduced reported from 200 before μm for as the well. initial It sample was ascribed to 1 to the distinctμm after microchemistry three passes. ofIn theaddition matrices to grain and refinement, the high density all samples of deformation also indicated bands a considerable in the solution increase in the density of LAGBs and most of the grains are elongated along the deformation bands. treated samples since the higher solute content retards dynamic recovery as reported for MDFed (2) The hardness of both annealed and solution‐treated samples demonstrated an appreciable over-agedincrement and solution after being treated processed Al-Cu-Mg by MDF. alloy For [ instance,62]. the hardness value of the solution‐treated samples MDFed at RT and 177 °C experienced more than 100% increase compared to the initial 4. Conclusionsmicrostructure. The increase in the hardness value is much more noticeable after the first pass and Thebecome effect less of the efficient MDF for process the next on passes. the microstructure, Development of UFG texture, structure, mechanical work hardening and electrical (as a result properties of the increased density of dislocations) and the dynamic aging process are the main factors behind of both annealed and solution-treated samples of AA-6063 was investigated. According to the results the improvement of hardness. obtained, the(3) main Shear conclusions punch testing ofresults this of investigation the annealed sample are as after follows: three passes of MDF showed 56 and (1)30% EBSD increase analyses in shear showed yield thestrength average and ultimate grain size shear of strength, both annealed respectively. and The solution-treated solution‐treated samples has decreased noticeably during the MDF process. For instance, the average grain size of the solution-treated sample MDFed at room temperature reduced from 200 µm for the initial sample to 1 µm after three passes. In addition to grain reﬁnement, all samples also indicated a considerable increase in the density of LAGBs and most of the grains are elongated along the deformation bands. Materials 2018, 11, 2419 14 of 17

(2) The hardness of both annealed and solution-treated samples demonstrated an appreciable increment after being processed by MDF. For instance, the hardness value of the solution-treated samples MDFed at RT and 177 ◦C experienced more than 100% increase compared to the initial microstructure. The increase in the hardness value is much more noticeable after the first pass and become less efficient for the next passes. Development of UFG structure, work hardening (as a result of the increased density of dislocations) and the dynamic aging process are the main factors behind the improvement of hardness. (3) Shear punch testing results of the annealed sample after three passes of MDF showed 56 and 30% increase in shear yield strength and ultimate shear strength, respectively. The solution-treated samples processed at RT and 177 ◦C also showed 72 and 83% rise in the yield strength and 24 and 27% increases in the ultimate shear strength, respectively. These improvements could also be attributed to the above-mentioned reasons for the hardness increase. (4) Post-MDF aging of three-pass MDFed AA-6063 samples at RT and 177 ◦C led to a reduction in the mechanical properties. Because the maximum hardness value was achieved during the MDF process as a result of dynamic aging and post-MDF aging just makes the precipitates grow and lose coherency. (5) Effect of the MDF process on the electrical conductivity of both solution-treated and annealed samples of AA-6063 was found to be negligible. The electrical conductivity decreases negligibly after the first pass and shows a slight increase in the second and the third passes. (6) Texture analyses revealed that a relatively strong copper-type texture component was formed in annealed samples of AA-6063 alloy MDFed up to three passes, while, a relatively strong Goss texture component was formed in solution-treated samples after three passes of the MDF process at both room and 177 ◦C temperatures. The reason for this difference lies in the different amount of precipitates and distinct microchemistry of the matrices in the annealed and solution-treated samples.

Author Contributions: Conceptualization, M.H.S.; Investigation, A.D., H.J. and F.V.G.; Project administration, M.H.S.; Resources, R.T., F.D. and H.J.; Supervision, M.H.S.; Writing—original draft, A.D.; Writing—review and editing, M.H.S., R.T. and F.D. Funding: This research received no external funding. Acknowledgments: The authors are very grateful to Alumtek Corp Iran for providing the AA-6063 billets used in this work. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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