Enhancement of the Mechanical Properties in Al–Si–Cu–Fe–Mg Alloys with Various Processing Parameters

materials

Article Enhancement of the Mechanical Properties in Al–Si–Cu–Fe–Mg Alloys with Various Processing Parameters

Su-Seong Ahn 1, Sharief Pathan 1, Jar-Myung Koo 1, Chang-Hyun Baeg 2, Chan-Uk Jeong 2, Hyoen-Taek Son 3, Yong-Ho Kim 3, Kap-Ho Lee 4 and Soon-Jik Hong 1,*

1 Division of Advanced Materials Engineering, Kongju National University, Cheonan-si 331-717, Korea; [email protected] (S.-S.A.); [email protected] (S.P.); [email protected] (J.-M.K.) 2 Dongyang A.K Korea Co. Ltd., 3rd plant/R&D center 70, Wonhapgang 1-gil, Yeondong-myeon, Sejong 30067, Korea; [email protected] (C.-H.B.); [email protected] (C.-U.J.) 3 Korea Institute of Industrial Technology, Gwangju, 61011, Korea; [email protected] (H.-T.S.); [email protected] (Y.-H.K.) 4 Department of Materials Science & Engineering, Chungnam National University, Daejeon 34134, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-41-521-9387

Received: 18 September 2018; Accepted: 22 October 2018; Published: 1 November 2018

Abstract: In this research, various processing conditions were implemented to enhance the mechanical properties of Al-Si alloys. The silicon content was varied from hypoeutectic (Si-10 wt.%) to eutectic (Si-12.6 wt.%) and hypereutectic (Si-14 wt.%) for the preparation of Al-XSi-3Cu-0.5Fe-0.6 Mg (X = 10–14%) alloys using die casting. Subsequently, these alloys were hot-extruded with an optimum extrusion ratio (17:1) at 400 ◦C to match the output extruded bar to the compressor size. An analysis of the microstructural features along with a chemical compositional analysis were carried out using scanning electron microscope along with energy dispersive X-ray spectroscopy and transmission electron microscope. The SEM micrographs of the extruded samples displayed cracks in primary Si, and the intermetallic (β-Al5FeSi) phase was fragmented accordingly. In addition, the silicon phase was homogenously distributed, and the size remained constant. The mechanical properties of the extruded samples were enhanced by the increase of silicon content, and consequently the ductility decreased. By implementing proper T6 heat treatment parameters, coherent Al2Cu phases were formed in the Al matrix, and the Si phase was gradually increased along with the silicon content. Therefore, high tensile strength was achieved, reaching values for the Al-XSi-3Cu-0.5Fe-0.6Mg (X = 10–14%) alloys of 366 MPa, 388 MPa, and 420 MPa, respectively.

Keywords: Al-Si alloy; extrusion; microstructure; T6 heat treatment; silicon content

1. Introduction Nowadays, the automobile and aerospace industries are fascinated by lightweight materials that possess superior mechanical properties and consume less fuel thus possibly mitigating the pollution crisis. To achieve these requirements, aluminium (Al) alloys are in a primary position, having additional properties such as high specific strength, good corrosion resistance, low coefficient of thermal expansion, and high strength-to-weight ratio [1–5]. Aluminium alloys with a high weight percentage of silicon (Si) are widely used in the automotive industry, and Si is the alloying element that sustains the aluminium casting industry. The high volumes of silicon in Al-Si alloys can consistently enhance the alloys’ mechanical properties and fluidity, reducing cracking and improving feeding to minimize shrinkage porosity. Moreover, Al-Si alloys are ubiquitous, inexpensive, and lightweight because of the high quantity of silicon [6–10]. The mechanical properties of Al-Si alloys re dependent

Materials 2018, 11, 2150; doi:10.3390/ma11112150 www.mdpi.com/journal/materials Materials 2018, 11, 2150 2 of 12 on the size, shape, and distribution of eutectic and primary silicon particles [4,6,11]. To achieve these required silicon characteristics and to refine the Al-Si matrix, hot extrusion is an effective plastic deformation technique that makes the size of silicon particles smaller and can improve their mechanical strength [11–15]. Srivastava et al. [14] reported that the silicon and intermetallic phases were effectively fragmented to the nano-scale, resulting in enhanced mechanical properties by the hot extrusion process. Especially, the hot extrusion process was used for the production of aluminium alloys to reduce the porosity of the alloys and produce a severe shear stress, that are beneficial for plastic deformation [11,16]. It also considers other features, like temperature, die size, and hot-extrusion ratio, which help modify the resulting microstructure to improve the mechanical properties. In most cases, to achieve better mechanical properties, a high extrusion ratio that exerts high strain and pressure was used to effectively refine the final microstructure [8,13,15–19]. The effective strain equation (ε) is related to the extrusion ratio as follows [16]: √ ε = 2 ln ER; ER = Ac/AE (1) where ER, AC, and AE are extrusion ratio, area of container, and area of extruded bar, respectively. However, the output material from the extruded machine has to maintain the required application area to produce an effective design and to reduce scraps. Hence, the extrusion ratio is fundamental when producing automobile parts, especially compressor parts. Therefore, a combination of optimum extrusion ratio and higher mechanical properties has to be preserved to obtain high-strength compressor parts in industrial applications. Moreover, microstructure modifications and high strength in Al-Si alloys can be achieved by adding elements (Zn, Mg, Cu, Mn, Fe, Ni, Ti) and by heat treatment [1–4]. Each doping element has their own unique properties, and heat treatment may improve the doping elements’ properties. For example, magnesium makes an alloy strong, hard, and responsive to heat treatment. Copper increases the room and high-temperature properties of an alloy by forming Al2Cu intermetallic phases with heat treatment. Iron provides strength, improves wear resistance and thermal stability by precipitating the intermetallic compound Al5FeSi, and, at low concentrations, acts as a microstructure refiner [3,4,12,20,21]. Toenhance the mechanical properties, a precise heating parameter, has to be selected. T6 heat treatment was carried out by three methods: solution heat treatment, quenching, and age hardening [2,15,22]. A Cu- and Mg-containing Al-Si alloy should be maintained at precise heat-treatment conditions for the appropriate length of time for the dissolution of Cu and Mg to realize the full aging potential and to obtain a homogenous supersaturated solution. Depending upon the quenching rate, the amount of solute in the supersaturated solid solution can be decided. With a high cooling rate, higher concentrations of solute are retained in solution for better mechanical properties. By choosing precisely the aging temperature and time, it is also possible to strengthen an Al-Si alloy through the coherent precipitates [7,22]. Therefore, in this research, our aim was to produce high-strength Al-XSi-3Cu-0.5Fe-0.6Mg (X = 10–14%) alloys for compressor part production. To achieve this requisite, we changed the silicon volume percentage aiming at high strength and chose a 17:1 extrusion ratio that reduced wastage. Each Al-Si billet was plastically deformed, and T6 heat treatment was also carried out to enhance the mechanical properties. Precise T6 heat treatments were performed considering a range of aging times to improve the mechanical properties. With precise T6 heat treatment conditions and an optimum extrusion ratio, we achieved the highest tensile strength of 420 MPa for a 14 wt.% Si sample as a result of the formation of coherent Al2Cu phases in the Al-matrix.

2. Experimental Procedure In this research, the Al-XSi-3Cu-0.5Fe-0.6Mg (X = 10, 12.6, 14 wt.%) alloys were fabricated. The parent alloys with a high purity of 99.99% were purchased from R&D Korea Pvt LTD. (Ansan, Korea) and weighed according to the stoichiometric ratio of 2 kg, with various silicon contents (10, 12.6, 14 wt.%). Initially, these alloys were placed in a graphite crucible which was kept inside a high-frequency induction Materials 2018, 11, 2150 3 of 12 furnace, and the temperature was maintained at 750 ◦C to obtain the molten alloy. The molten alloy was transformed into stainless steel mold and rested in air. As-cast cylindrical billets of Al-XSi-3Cu-0.5Fe-0.6Mg (X = 10, 12.6, 14 wt.%) alloys were used for further experiments. According to the Si wt.%, the alloys were differentiated as alloy A (Si-10 wt.%), alloy B (Si-12.6 wt.%), and alloy C (Si-14 wt.%), respectively. The resultant cast billets were preheated for 1 h at 400 ◦C, and subsequently hot extrusion was performed at 400 ◦C with a 500-ton extruder. The obtained extruded Al-Si bars were about 1000 mm long and 16 mm in diameter. The extrusion was carried out under the conditions of an extrusion ratio of 17:1, a ramp speed of 4.5 mm/min, and an output temperature of 410 ◦C. The extruded samples were subjected to T6 aging treatment with solution heat treatment at 500 ◦C for 5 h, quenched with hot water at 60 ◦C, and then artificially aged at 200 ◦C for different times (1, 2, 3, and 4 h). The heat treatment was carried out using an electric vacuum furnace, and the oxidation was disabled by circulating Ar gas. The characterization was carried out along parallel to the extrusion direction. The phase of the T6 heat-treated extruded specimens was characterized by using an X-ray diffractometer (XRD) with high energy monochromatic Cu–Kα radiation in the 2θ range from 20 to 80◦, with a scan speed of 2◦/min (Rigaku, MiniFlex-600, Tokyo Japan). The microstructure of the extruded samples was examined using scanning electron microscopy (SEM- MIRA-LMH II (TESKAN, Brno-Kohoutovice, Czech Republic),), and the T6 annealed specimens were probed using transmission electron microscopy (TEM, Tecnai G2 F30 S-Twin/FEI company, Hillsboro, OR, USA) operated at 200 kV. The samples for the TEM images were prepared by using a focused ion beam (Helios Nanolab 450 F1/FEI company, Hillsboro, OR, USA) equipment. EDS was carried out to examine the chemical composition of the extruded samples. Vickers hardness of the extruded samples was measured at different locations using a Vickers hardness tester with a denting load of 1 kgf for 10 s, and the values were averaged. The tensile strength of the extruded and T6 specimens were meticulously investigated for two times to obtain significant and reliable values, using ASTM A370 small-size samples (D: 4 mm, L: 20 mm) with a load cell of 7.5 KN and a crosshead speed of 2.0 mm/min. The tensile strength was measured at room temperature using a tensile testing machine (UTM-T, R&B, Daejeon, Korea). The density of the casted alloys was measured 15 times using the Archimedes principle and then averaged for better accuracy.

3. Results and Discussion

3.1. Microstructure Investigation of the Extruded Samples In this research, fine extruded bars without cracks were successfully fabricated. Hence, it appears that the employed extrusion parameters were able to prevent the breakage of the extruded bars. Figure1 shows the optical micrograph images of as-cast and extruded alloys. It is clearly seen from Figure1 that the silicon distribution in cast A, B, and C alloys was irregular, and, after extrusion, the silicon phase was well modified and evenly distributed. From the particle analysis, it was evident that the average eutectic silicon size remained almost constant in both cast and extrusion materials for alloy A, alloy B, and alloy C, which was due to the low extrusion ratio. Figure2 shows the SEM micrographs for the extruded bars. From Figure2a–c, it is apparent that, as the silicon content in the Al-Si-Cu-Fe-Mg compound increased from 10 to 14 wt.%, the Al-Si eutectic and Si phase gradually increased. It is also evident that eutectic silicon phase was homogeneously distributed in the Al matrix along the extrusion direction. This was due to the fact that the Al-Si alloy comprised soft Al and hard Si, and, when the hot-extrusion process was initiated, aluminium became softer, and the resulting deformation became uniform [6]. A similar behavior was observed by Wang et al. [13], who reported that, after hot extrusion, the silicon particles progressively matured, and their density increased. From our results, we concluded that the silicon particles were evenly distributed with the optimum extrusion ratio, and hot extrusion appeared as an optimal methodology to refine and distribute the silicon phase. As seen in Figure2d,e, the extruded samples contain primary Si-phase and needle shaped intermetallic compound (β-Al5FeSi) which was formed during the solidification process [10,21]. The compositional analysis was performed by EDS, which was shown in the inset Figure2d,e. From EDS, it was assured that the β-phase and primary silicon were evolved in the matrix. In addition, Materials 2018, 11, 2150x FOR PEER REVIEW 4 of 12 it was assured that the β-phase and primary silicon were evolved in the matrix. In addition, in the in the inset picture of Figure2d,e, the intermetallic ( β-Al5FeSi) phase was fragmented consistently inset picture of Figure 2d,e, the intermetallic (β-Al5FeSi) phase was fragmented consistently and a and a crack was appeared in the primary-Si along the extrusion direction which shows brittle fracture crack was appeared in the primary-Si along the extrusion direction which shows brittle fracture behavior during hot extrusion. The observed crack in the primary-Si was may be due to the extruding behavior during hot extrusion. The observed crack in the primary-Si was may be due to the extruding force and shearing stress. However, Si was not fractured well due to low extrusion ratio and this type force and shearing stress. However, Si was not fractured well due to low extrusion ratio and this type of behavior was also reported by Zuo et al. [1]. of behavior was also reported by Zuo et al. [1].

FigureFigure 1 1.. OpticalOptical micrographs micrographs and and particle particle size size analysis analysis of ofsilicon silicon in both in both casting casting and and extruded extruded Al- Al-XSi-3Cu-0.6Mg-0.5FeXSi-3Cu-0.6Mg-0.5Fe [X = [X (alloy = (alloy A): A):10; 10;(alloy (alloy B): B):12.6 12.6;; (alloy (alloy C): C):14] 14]alloys. alloys.

The density value of Al Al-Si-Si cast alloys is shown in Table 1 1.. TheThe densitydensity valuesvalues slightlyslightly decreaseddecreased when increasing increasing the the silicon silicon content. content. Since silicon has aa low low density density (~2.3 (~2.3 g/cm g/cm3)) compared to aluminium (~2.7(~2.7 g/cm 3),3), the the alloy’s alloy’s density density decreased decreased with with silicon silicon addition. addition.

Table 1.1. NominalNominal chemicalchemical compositioncomposition ofof Al-XSi-3Cu-0.6Mg-0.5FeAl-XSi-3Cu-0.6Mg-0.5Fe [X == (a): 10 (Alloy A), (b): 12.6 (Alloy B),B), (c):(c): 14 (Alloy C)] cast alloys.alloys.

CompositionComposition (wt.%) (wt.%) AlloyAlloy DensityDensity of of Casting Casting Alloy (g/cm(g/cm3)3) SiSi Cu Cu Mg Mg Fe Fe Al Al 4007A4007A 9.0~10.59.0~10.5 2.5~3.52.5~3.5 0.5~0.7 0.5~0.7 <0.5 <0.5 Bal. Bal. - - AlloyAlloy A A 10 10 3 0.6 0.6 0.5 0.5 Bal. Bal. 2.715 2.715 AlloyAlloy B B 12.6 12.6 3 0.6 0.6 0.5 0.5 Bal. Bal. 2.701 2.701 AlloyAlloy C C 14 14 3 0.6 0.6 0.5 0.5 Bal. Bal. 2.688 2.688

3.2. Mechanical Properties of the Extruded Alloys Without T6 Heat Treatment Figure3 3 shows shows thethe micromicro VickersVickers hardnesshardness values values (HV1) (HV1) ofof Al-SiAl-Si extrudedextruded bars.bars. TheThe HV1HV1 valuesvalues of Al Al-Si-Si extruded alloys slightly increased with silicon content increase, and the recorded recorded values were 71, 73, 79 HV1,HV1, respectively. The increase in hardness was due to the enhancement of the Al-SiAl-Si and

Materials 2018, 11, 2150 5 of 12 siliconMaterials phases. 2018, It11 is, x FOR also PEER presumed REVIEW that, since silicon has a low solubility in aluminium, up to5 thisof 12 point it precipitatedsilicon phases. as pure It is siliconalso presumed which isthat, hard since and silicon therefore has a increased low solubility the alloy’sin aluminium, hardness up [ 11to ].this pointOne ofit precipitated the important as pure mechanical silicon which properties is hard thatand therefore should be increased consider the for alloy’s Al-Si hardness alloys is [11]. the tensile strength.One An of increment the important of the mechanical tensile strength properties value that wouldshould allowbe consider the Al-Si for Al applications.-Si alloys is the In tensile this work, we measuredstrength. An the increment tensile strength of the tensile of the strength extruded value alloys would before allow the and Al after-Si applications. T6 heat treatment. In this work, Figure 4 showswe themeasured tensile the strength tensile strength of Al-Si of alloys the extruded measured alloy ats roombefore temperature.and after T6 heat Before treatment. T6 heat Figure treatment, 4 the tensileshows the strength tensile increased,strength of Al and-Si thealloys elongation measured rateat room decreased temperature. simultaneously Before T6 heat by treatment, increasing the siliconthe content.tensile strength The values increased, obtained and the were elongation 242, 267, rate anddecreased 296 MPa simultaneously for alloy A, by alloy increasing Band the alloy C, silicon content. The values obtained were 242, 267, and 296 MPa for alloy A, alloy Band alloy C, respectively, simultaneously, the elongation rates observed were 13.3, 13, and 12.8%, respectively. respectively, simultaneously, the elongation rates observed were 13.3, 13, and 12.8%, respectively.

FigureFig 2.ureSEM 2. SEM Micrographs Micrographs of of Al-XSi-3Cu-0.6Mg-0.5Fe Al-XSi-3Cu-0.6Mg-0.5Fe [X [X = =(a): (a 10):; 10; (b): ( b12.6): 12.6;; (c): 14] (c): extruded 14] extruded alloys alloys;; (d) β-Al FeSi intermetallic compound phase and (e) primary silicon in Al-Si extruded bars. (d) β-Al5FeSi5 intermetallic compound phase and (e) primary silicon in Al-Si extruded bars.

It wasIt was presumed presumed that that the the Al-Si Al interface-Si interface was was enhanced enhanced by by increasing increasing the the Si Si percentage. percentage. WhenWhen stress wasstress applied was during applied the during tensile the test,tensile the test, soft the aluminium soft aluminium phase phase deforms deforms before before the the hard hard silicon silicon phase, triggeringphase, dislocations triggering dislocations in the Al in matrix the Al [ 6 matrix]. As the [6]. dislocations As the dislocations moves moves towards towards the Al-Si the Al interface,-Si the siliconinterface, particles the silicon restrict particles the restrict movement. the movement. Hence, Hence, it should it should be be possible possible toto apply apply mo morere stress stress in in the presence of a high silicon percentage, overcoming dislocations which cause plastic the presence of a high silicon percentage, overcoming dislocations which cause plastic deformation. deformation. Therefore, the tensile strength was improved by increasing the silicon content. The Therefore,decrement the tensileof the elongation strength wasrates improvedwith the increase by increasing of the silicon the siliconweight content.percentage The was decrement due to the of the elongationplastically rates brittle with nature the increaseof silicon ofin the matrix silicon and weight the cumulative percentage volume was of due the to hard the silicon plastically phase. brittle nature of silicon in the matrix and the cumulative volume of the hard silicon phase. Materials 2018, 11, 2150 6 of 12 Materials 2018, 11, x FOR PEER REVIEW 6 of 12 Materials 2018, 11, x FOR PEER REVIEW 6 of 12

Figure 3. HV1 values of Al-XSi-3Cu-0.6Mg-0.5Fe [X = (A): 10; (B): 12.6; (C): 14] extruded alloys with FigureFig 3.ureHV1 3. HV1 values values ofAl-XSi-3Cu-0.6Mg-0.5Fe of Al-XSi-3Cu-0.6Mg-0.5Fe [X[X = ( (A):A): 10 10;; (B (B):): 12.6 12.6;; (C (C):): 14] 14] extruded extruded alloys alloys with with and without T6 heat treatment. and withoutand without T6 heat T6 heat treatment. treatment.

Figure 4. Comparison of tensile strength between the extruded Al-XSi-3Cu-0.6Mg-0.5Fe [X = (A): 10, Figure 4. Comparison of tensile strength between the extruded Al-XSi-3Cu-0.6Mg-0.5Fe [X = (A): 10, Figure( 4.B):Comparison 12.6, (C): 14] alloys of tensile before strength heat treatment. between the extruded Al-XSi-3Cu-0.6Mg-0.5Fe [X = (A): 10, (B): 12.6,(B): (C):12.6, 14](C): alloys 14] alloys before before heat heat treatment. treatment. 3.3. SEM Analysis of Tensile Fractured Surfaces Without T6 heat Treatment 3.3. SEM3.3. SEM Analysis Analysis of Tensile of Tensile Fractured Fractured Surfaces Surfaces Without Without T6T6 heat Treatment Figure 5 displays the SEM morphology and cross-sectional microstructure of the fractured Figure 5 displays the SEM morphology and cross-sectional microstructure of the fractured Figuretensile5 displaysspecimens the before SEM heat morphology treatment. andFigure cross-sectional 5a shows the schematic microstructure diagram of of the fractured fractured tensile tensile tensile specimens before heat treatment. Figure 5a shows the schematic diagram of fractured tensile specimensspecimen befores’ heatsurface treatment.s and illustrates Figure5 howa shows the themicrostructure schematic diagram was examine of fracturedd. The morphology tensile specimens’ and specimens’ surfaces and illustrates how the microstructure was examined. The morphology and surfacesmicrostructure and illustrates of how the cracked the microstructure portion (red- wasshaded examined. portion in The Figure morphology 5a) in the and three microstructure samples are of microstructure of the cracked portion (red-shaded portion in Figure 5a) in the three samples are shown in Figure 5b–g. It is observed from Figure 5e–g that the coarse silicon decreased, and the fine the crackedshown in portion Figure(red-shaded 5b–g. It is observed portion from in FigureFigure 55a)e–g in that the the three coarse samples silicon aredecreased shown, and in Figure the fine5b–g. silicon phase tended to increase by increasing silicon content. The marginal increment of silicon It is observedsilicon phase from tended Figure to5e–g increase that theby increasingcoarse silicon silicon decreased, content. The and marginal the ﬁne silicon increment phase of silicon tended to phases with silicon content was due to the Lever rule [11]. As a result, the mechanical properties, increasephases by increasingwith silicon siliconcontent content. was due Theto the marginal Lever rule increment [11]. As ofa result, silicon the phases mechanical with siliconproperties content, such as tensile strength and hardness, gradually increased in alloy C compared to alloy A and alloy B. was duesuch to as thetensile Lever strength rule and [11 ].hardness As a result,, gradually the mechanicalincreased in alloy properties, C compared such to asalloy tensile A and strength alloy B. and As seen in Figure 5e–g, the silicon phases which were distributed in the Al matrix showed As seen in Figure 5e–g, the silicon phases which were distributed in the Al matrix showed hardness,dissimilar gradually shapes increased, i.e., roundish, in alloy spheroid C compared, and ligament. to alloy A and alloy B. dissimilar shapes, i.e., roundish, spheroid, and ligament. As seenFigure in Figure 5h–m shows5e–g, the morphology silicon phases and microstructure which were distributed in the middle in of the the Altensile matrix specimen showeds. Figure 5h–m shows the morphology and microstructure in the middle of the tensile specimens. dissimilarAs seen shapes, in Figure i.e., roundish, 5h–m, the spheroid,crack was differentially and ligament. located in Si-phases with different shapes. The As seen in Figure 5h–m, the crack was differentially located in Si-phases with different shapes. The Figure 5h–m shows the morphology and microstructure in the middle of the tensile specimens.

As seen in Figure5h–m, the crack was differentially located in Si-phases with different shapes. The crack in all primary silicon phases was an indication of good interfacial bonding arising between Materials 2018, 11, 2150 7 of 12 Materials 2018, 11, x FOR PEER REVIEW 7 of 12 the primarycrack in all silicon primary particles silicon andphases the was aluminium an indication matrix of good [11], interfacial which was bonding attributed arising to between the significant the refiningprimary effect silicon of hot particles extrusion. and By the applying aluminium a force matrix through [11], which the tensile was attributed test, cracks to the gradually significant started in therefining Si phases. effect There of hot wasextrusion. a fluctuation By applying in the a force tensile through strength the tensile signal test, in the cracks extruded gradually alloys. started This Si phasein fracturethe Si phases mechanism. There wa iss consistenta fluctuation with in the the tensile report strength by Hong signal et al. in [the5] thatextruded the ruin alloys. and This failure Si of phase fracture mechanism is consistent with the report by Hong et al. [5] that the ruin and failure of Al-Si alloys are in general related to development of cracks in the primary Si phase. Al-Si alloys are in general related to development of cracks in the primary Si phase.

Figure 5. (a) Schematic diagram of the fracture mechanism in the tensile test of specimens of Al-XSi- Figure3Cu 5.-0.6Mg(a)-0.5Fe Schematic [X = 10, diagram 12.6, 14] ofextruded the fracture alloy; (b mechanism–d) show low in-magnification the tensile testimages of and specimens (e–g) of Al-XSi-3Cu-0.6Mg-0.5Feshow high-magnification [X =images 10, 12.6, of the 14] fracture extruded surface alloy; at the (b –boundaryd) show of low-magnification the specimens; (h– imagesj) show and (e–g)low show-magnification high-magnification images and images (k–m of) show the fracture high-magnification surface at the images boundary of the of fracture the specimens; surface (ath –thej) show low-magnificationcenter of the specimen imagess and. (k–m) show high-magnification images of the fracture surface at the center of the specimens. Materials 2018, 11, 2150 8 of 12 Materials 2018, 11, x FOR PEER REVIEW 8 of 12

3.4.3.4 Inﬂuence. Influence of of T6 T6 Heat Heat Treatment Treatment on on Al-Si Al-Si Extruded Extruded AlloysAlloys FigureFigure6 shows 6 shows the the XRD XRD data data analysis analysis of of T6 T6 heat-treated heat-treated Al-SiAl-Si alloy alloys.s. As As the the silicon silicon conten contentt was was increased,increased, the the peak peak position position of the silicon silicon phase phase also also increased increased,, and and other other few fewpeaks peaks,, like those like thoseof the of theMg Mg2Si,2Si, θ-θAl-Al2Cu2Cu,, and and β-Alβ-Al5FeSi5FeSi phases phases,, were were also also noticed. noticed. From From Figure Figure 3d and3d andthe theXRD XRD analysis analysis,, it it waswas presumedpresumed that the modification modiﬁcation of of the the ( (ββ-Al-Al5FeSi)5FeSi) phase phase did did not not occur occur even even after after the the T6 T6heat heat treatment.treatment. Thus, Thus, the the T6 T6 heat heat treatment treatment showedshowed aa negligiblenegligible effect on on the the ββ-Al-Al5FeSi5FeSi phase phase,, which which was was alsoalso observed observed by by Costa Costa et et al. al. [2 ][2] and and Wu Wu et et al. al. [23 [23].]. Oxygen Oxygen discrete discrete peaks peaks were were not not observed, observed, since since the oxygenthe ox contentygen content was in was a very in a very narrow narrow range, range undetectable, undetectable by X-rayby X-ray analysis. analysis.

FigureFigure 6. 6XRD. XRD patterns patterns of of Al-XSi-3Cu-0.6Mg-0.5FeAl-XSi-3Cu-0.6Mg-0.5Fe [X [X = =(a ():a 10):; 10; (b): (b 12.6): 12.6;; (c): (14]c): extruded 14] extruded bars after bars T6 after T6treatment treatment..

TheThe HV1 HV1 values values of of the the heat-treated heat-treated Al-SiAl-Si alloysalloys are shown in in Figure Figure 3. After After solution solution heat heat treatmenttreatment and and quenching quenching of of the the solution, solution, the the hardness hardness increased increased by by 50% 50% in in comparison comparison with with the the extrudedextruded alloys, alloys, and and thethe recorded values values were were 108 108 HV1 HV1,, 114 114 HV1 HV1,, 120 H 120V1 HV1for alloy for A, alloy alloy A, B, alloy and B, andalloy alloy C, C, respectively respectively.. This This wa wass du duee to tothe the formation formation of ofprecipitates precipitates which which enhance enhancedd the the hardness hardness.. Here,Here we, we selected selected a a solutionsolution heat heat treatment treatment of of 500 500 °C ◦forC for5 h, 5because, h, because, at a high at a temperature high temperature and with and witha long a long time, time, inceptive inceptive melting melting of θ-Al of2θCu-Al, due2Cu, to due high to thermal high thermal stress, occur stress,red occurred after quenching, after quenching, which whichprompted prompted a deterioration a deterioration of the of the mechanical mechanical properties properties [7,12,22 [7,12]., 22 We]. We affirm afﬁrm that that our our solution solution treatmenttreatment showed showed effective effective strengtheningstrengthening and and,, as as shown shown in inthe the literature literature [22] [,22 the], theaddition addition of Mg of to Mg toan an Al Al-Si-Cu-Si-Cu alloy alloy show showeded a higher a higher strengthening strengthening effect effectat 500 at°C 500than◦ atC 480 than °C at. In 480 addition,◦C. In Costa addition, et Costaal. [2] et al.compared [2] compared treatment treatmentss for 5 and for 58 andh and 8 hdemonstrated and demonstrated that 5 thath was 5 heffective was effective in modifying in modifying the thealloy alloy microstructure microstructure,, which which result resulteded in in improv improveded hardness. hardness. After After solution solution heat heat treatment treatment and and quenching,quenching, age-hardening age-hardening was was validated validated as as an an important important process. process Choosing. Choosing the the proper proper aging aging time time and and temperature is essential to avoid deleterious effects to the mechanical properties of aluminium temperature is essential to avoid deleterious effects to the mechanical properties of aluminium alloys. alloys. In this prospect, we optimized the proper aging time with respect to hardness and performed In this prospect, we optimized the proper aging time with respect to hardness and performed the the Vickers hardness test for different aging periods (1, 2, 3, and 4 h). This kind of experimental Vickers hardness test for different aging periods (1, 2, 3, and 4 h). This kind of experimental procedure procedure was also performed by Gupta et al. [11] to obtain the proper peak aging time, and their was also performed by Gupta et al. [11] to obtain the proper peak aging time, and their results showed results showed a high hardness value after 9 h of aging. a high hardnessAs shown value in Figure after 3 9, the h of maximum aging. hardness was obtained at 3 h with 140, 143, 150 HV1 for alloyAs A, shown alloy inB, and Figure alloy3, theC, respectively. maximum hardnessWith a further was obtainedheat treatment at 3 hfor with 4 h, 140,the hardness 143, 150 tend HV1ed for alloy A, alloy B, and alloy C, respectively. With a further heat treatment for 4 h, the hardness tended to

Materials 2018, 11, 2150 9 of 12 Materials 2018, 11, x FOR PEER REVIEW 9 of 12 decrease,to decrease reaching, reaching values values of 128, of 129, 128, 132 129, HV1. 132 From HV1 these. From results, these itresults, was concluded it was concluded that a 4 h treatmentthat a 4 h causedtreatment over-aging caused over characterized-aging characterized by particle by agglomeration, particle agglomeration leading to, thelead deteriorationing to the deteriorat of strengthion of andstrength hardness. and hardness. A treatment A treatmentof 3 h was of considered 3 h was considered a proper aging a proper time toaging enhance time the to enhanc propertiese the ofproperties aluminium of alloys aluminium through alloys the through formation the of formation coherent precipitates of coherent capable precipitates to cutoff capable aluminium to cutoff dislocationsaluminium [dislocations7]. [7]. FigureFigure7 shows7 shows the the tensile tensile strength strength values values of of the the T6 T6 heat-treated heat-treated samples. samples. The The tensile tensile strength strength waswas greatly greatly enhanced enhanced after after T6 T6 heat heat treatmenttreatment byby meticulouslymeticulously choosing the optimum aging time time as as 3 3h h.. In In comparison comparison to to the the extruded extruded alloys alloys that that did did not not undergo undergo heat heat treatment treatment,, the the tensile tensile strength strength was wasimproved improved by ~45 by ~45% in % all in T6 all extruded T6 extruded samples. samples. TheThe recorded recorded tensile tensile strength strength values values were were 366, 366, 388, 388,and and 420 420MPa MPa for foralloy alloy A, B, A, C B,, respectively, C, respectively, and and the theelongation elongation rates rates were were 10, 10,9.7, 9.7,and and 9.4 9.4%. The %. Theobtained obtained tensile tensile strength strength of 420 of 420 MPa MPa was was greater greater than than those those previously previously reported reported by byWu Wu et al. et [23] al. [ 23and] andKe Keet al. et [8]. al. [8].

FigureFigure 7. 7Comparison. Comparison of of tensile tensile strength strength between between the the Al-XSi-3Cu-0.6Mg-0.5Fe Al-XSi-3Cu-0.6Mg-0.5Fe [X = [X (a): = 10,(a): (b): 10, 12.6,(b): 12.6, (c): 14](c): extruded 14] extruded alloys alloys after T6after heat T6 treatment.heat treatment.

Figure8 shows the TEM analysis of T6 heat-treated extruded alloys. From Figure8a–c TEM Figure 8 shows the TEM analysis of T6 heat-treated extruded alloys. From Figure 8a–c TEM images, it is evident that there are numerous white spots and lines in the Al matrix. In general, images, it is evident that there are numerous white spots and lines in the Al matrix. In general, the the age-hardening response is very efficient in the Al-Si-Cu-Mg alloy [7], and it was shown that age-hardening response is very efficient in the Al-Si-Cu-Mg alloy [7], and it was shown that copper copper has a great influence at high temperatures, increasing the strength of Al-Si alloys through has a great influence at high temperatures, increasing the strength of Al-Si alloys through the the dispersion of the Al2Cu phase in the Al matrix. As shown in Figure8a–c, the Al 2Cu phase was dispersion of the Al2Cu phase in the Al matrix. As shown in Figure 8a–c, the Al2Cu phase was finely finely dispersed at different locations in a definite pattern by meticulously selecting proper aging time dispersed at different locations in a definite pattern by meticulously selecting proper aging time and and temperature. temperature.

Materials 2018, 11, 2150 10 of 12 Materials 2018, 11, x FOR PEER REVIEW 10 of 12

FigureFigure 8.8. TTransmissionransmission electron electron microscope microscope (TEM) (TEM) image image of of Al Al-XSi-3Cu-0.6Mg-0.5Fe-XSi-3Cu-0.6Mg-0.5Fe [X [X= (a =): (10,a): ( 10,b): 0 (12.6b): 12.6;; (c): (c 14]): 14] T6 T6 heat heat-treated-treated extruded extruded bars bars;; (d (d) )high high-resolution-resolution TEM TEM ( (HRTEM)HRTEM) image of of θθ-Al′-Al22CuCu phase.phase. FastFast Fourier Fourier transform transform pattern pattern of theof the (e) white(e) white and and (f) red (f) circles red circles displayed display in theed HRTEMin the HRTEM image shownimage shown in (d). in (d).

FigureFigure8 d8d showsshows thethe high-resolutionhigh-resolution transmission electron electron microscope microscope (HRTEM) (HRTEM) image image of ofθ’- θAl’-Al2Cu2Cu precipitated precipitated with with streaks streaks,, as as observed observed in in Fig Figureure 8c.c. The direction ofof thethe electronelectron beambeam was was parallelparallel toto [100[100]Al]퐴푙, θ,휃0 .′ . The The fastfast FourierFourier transformtransform (FFT)(FFT) patternspatterns inin FigureFigure8 8e,fe,f were were obtained obtained from from the the regionsregions delimited delimited by by the the white white and and red red circles circle ins Figure in Figure8d, corresponding 8d, corresponding to the to matrix, the matrix,α, and αθ,0 phase,and θ′ 0 respectively.phase, respectively. The FFT The pattern FFT confirmedpattern confirm the existenceed the existence of the θ -Alof the2Cu θ phase′-Al2Cu along phase the along[001 ]theθ0 direction. [001]휃′ 0 Thedirection. orientation The relationship orientation relationshipbetween the matrix betweenα andthe θmatrixis known α and to be θ(′ 001 is ) knownα (001) θto0 , [ be001 ](α001[001)훼] θ⫽0 . (001It)휃 was′ , [001 observed]훼 ⫽ [001 that]휃′ the . Al2Cu phases increased with Si content in the Al matrix, which is shown in FigureIt wa8sa–c. observed The increment that the Al in2Cu the phases Al 2Cu increased phases with with Si the content increase in the of Al Si matrix wt.% was, which interpreted is shown asin dueFigure to the8a–c increase. The increment by 20-fold in the of theAl2Cu diffusion phases coefficientwith the increase of copper of Si in wt.% aluminium, was interpreted following as due the increaseto the increase of silicon by content20-fold of in the alloydiffusion [7]. Incoefficient Al-Si alloys, of copper Mg can in predominantlyaluminium, following control the and increas reducee theof silicon formation content of Al 2inCu the phases alloy [ 23 [7].], andIn Al this-Si effectalloys, was Mg compensated can predominantly by increasing control the and silicon reduce content, the whichformation increases of Al the2Cu formationphases [23] of, theand Al this2Cu effect phases. was compensated by increasing the silicon content, whichIn increases this Al-Si-Cu-Fe-Mg the formation composition, of the Al2Cu copper phases (3. wt.%) and magnesium (0.6 wt.%) were added for the precipitationIn this Al-Si hardening-Cu-Fe-Mg effect. composition, According copper to Gowri (3 wt.%) et al. and [20], magnesium a minimum (0.6 3 wt.% wt.%) of copperwere added must for be includedthe precipitat in Al-Si-Cu-Fe-Mgion hardening alloyseffect. forAccording the precipitation to Gowri of et Mg al. 2[20],Si and a minimum Al2Cu phases. 3 wt.% Caceres of copper et al. must [24] proposedbe included that, in Al typically,-Si-Cu-Fe 0.3-Mg to 0.7alloy wt.%s for ofthe Mg precipitation in Al-Si foundry of Mg2Si alloys and Al precipitates2Cu phases. as Caceres Mg2Si, et and, al. upon[24] proposed T6 heat treatment,that, typically this, 0.3 Mg to phase 0.7 wt.% can of dissolve Mg in Al in-Si the foundry solid solution alloys precipitate and actss as as an Mg effective2Si, and, strengtheningupon T6 heat precipitatetreatment, [this7]. AsMg shown phase in can Figure dissolve6, the in XRD the analysis solid solution confirmed and the acts presence as an effective of the Mgstrengthening2Si phase in precipitate alloy A, alloy [7]. B,As and shown alloy in C, Figure and it 6 was, the assumed XRD analysis that the confirmed Mg2Si phase the presence also would of actthe asMg a2 strengtheningSi phase in alloy precipitate. A, alloy B, and alloy C, and it was assumed that the Mg2Si phase also would act as a Hence,strengthening after T6 precipitate. heat treatment, the mechanical properties were improved as a result of the 0 effectiveHence combination, after T6 heat of the treatment, Mg2Si, Al the2Cu mechanical phase with propertiesthe streak-like were shape improved phase as (θ aphase), result which of the wereeffective dispersed combination in the Alof matrix,the Mg2 andSi, Al plausibly2Cu phase the with transformation the streak-like of the shape Si morphology phase (θ′ phase) leading, which to a strengtheningwere dispersed phase in the that Al couldmatrix precipitate, and plausibly during the the transformation heat treatment of [the15]. Si The morphology effective combination leading to a ofstrengthening the Al2Cu phase phase precipitated that could precipitate during heat during treatment the heat and treatment the hard [15]. silicon The slowed effective the combination aluminium of the Al2Cu phase precipitated during heat treatment and the hard silicon slowed the aluminium

Materials 2018, 11, 2150 11 of 12 dislocations [25], and, consequently, the elongation rate decreased progressively compared to the non-heat-treated alloys.

4. Conclusions In this study, Al-XSi-3Cu-0.5Fe-0.6Mg (X = 10, 12.6, 14) alloys were prepared using the die casting and extrusion technique. The effect of Si content on the microstructure and mechanical properties of the extruded bar and of the T6 heat-treated specimens were systematically investigated. The following conclusions were obtained from the experimental results:

1. The Si phase was ﬁnely distributed in the Al matrix along the extrusion direction, and the amount of Si phase increased with the increase of Si content. Cracks in the Si phases and fragmented intermetallic (β-Al5FeSi) phase were observed in the Al matrix. 2. The tensile strength of the extruded bar improved with the increase of Si content, while the ductility decreased. The maximum tensile value obtained was 296 MPa for the 14 wt.% Si-doped sample. 3. To further improve the tensile strength, T6 heat treatment was carried out. The optimum aging time was conﬁrmed by the Vickers hardness values. The HV1 value increased after up to 3 h of aging and reached a maximum value of 150 HV1.

4. The TEM images of T6 heated specimens displayed a coherent Al2Cu phase in the Al matrix. 0 A precipitated θ -Al2Cu phase was conﬁrmed by the FFT patterns. 5. The tensile strength improved by ~45 % compared to non-heat-treated alloys. A maximum tensile strength of 420 MPa was obtained for the Al-14Si-3Cu-0.5Fe-0.6Mg alloy, due to the formation of 0 the θ -Al2Cu phase in the Al matrix.

Author Contributions: S.S.A. and S.P. had performed the experiments and investigated the alloys’ properties. H.T.S. and Y.H.K. helped in the experiments by performing the extrusion process. The scanning electron microscope analysis was handled by C.H.B. and C.U.J. K.H.L. performed the transmission electron microscopy. S.S.A. and S.P. wrote the manuscript. J.M.K. scrutinized the article and provided his valuable suggestions in interpreting the results. The whole study was supervised by S.J.H. who provided resources and helped edit this manuscript. Funding: This research received no external funding. Acknowledgments: This research was supported by the Ministry of Trade, Industry & Energy (MOTIE) and the Korea Institute for the Advancement of Technology (KIAT) through the Encouragement Program for The Industries of Economic Cooperation Region (G02A01210011003). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Zuo, K.; Zhang, H.; Qin, K.; Han, X.; Shao, B.; Cui, J. Effects of heat treatment and extrusion on microstructure and properties of A390 alloy hollow billet fabricated via DC casting. Mater Trans. 2015, 56, 1591–1598. [CrossRef] 2. Costa, T.A.; Dias, M.; Gomes, L.G.; Rocha, O.L.; Garcia, A. Effect of solution time in T6 heat treatment on microstructure and hardness of a directionally solidified Al-Si-Cu alloy. J. Alloys Compd. 2016, 683, 485–494. [CrossRef] 3. Sharma, A.; Shin, Y.S.; Jung, J.P. Influence of various additional elements in Al based filler alloys for automotive and brazing industry. J. Weld. Join. 2015, 33, 1–8. [CrossRef] 4. Ye, H.Z. An overview of the development of Al-Si-Alloy based material for engine applications. J. Mater. Eng. Perform. 2003, 12, 288–297. [CrossRef] 5. Hong, S.J.; Suryanarayana, C. Mechanical properties and fracture behavior of an ultrafine-grained Al-20 Wt Pct Si alloy. Metall. Mater. Trans. A 2005, 36, 715–723. [CrossRef] 6. Ma, P.; Jia, Y.; Prashanth, K.; Yu, Z.; Li, C.; Zhao, J.; Yang, S.; Huang, L. Effect of Si content on the microstructure and properties of Al-Si alloys fabricated using hot extrusion. J. Mater. Res. 2017, 32, 2210–2217. [CrossRef] Materials 2018, 11, 2150 12 of 12

7. Mohamed, A.M.A.; Samuel, F.H. A review on the heat treatment of Al-Si-Cu/Mg casting alloys. In Heat Treatment-Conventional and Novel Applications; InTech: Rijeka, Croatia, 2012. 8. Ke, D.; Hengcheng, L.; Qiumin, J.; Yun, T. Effect of hot extrusion on mechanical properties and microstructure of near eutectic Al-12.0%Si-0.2%Mg alloy. Mater. Sci. Eng. A 2010, 527, 887–6892. [CrossRef]

9. Lucas, A.; Caceres, C.H.; Taylor, J.A. The effect of Si content on the size of β-Al5FeSi intermetallics in Al-Si-Cu-Mg casting alloys. In Proceedings of the 9th International Conference on Aluminium Alloys, Brisbane, Australia, 2–5 August 2004. 10. Sivarupan, T.; Taylor, J.; Caceres, C. SDAS, Si and Cu content, and the size of Intermetallics in Al-Si-Cu-Mg-Fe alloys. Metall. Mater. Trans. A. 2015, 46, 2082–2107. [CrossRef] 11. Gupta, M.; Ling, S. Microstructure and mechanical properties of hypo/hyper-eutectic Al-Si alloys synthesized using a near-net shape forming technique. J. Alloys Compd. 1999, 287, 284–294. [CrossRef]

12. Lasa, L.; Rodriguez, J.M. Characterization of the dissolution of the Al2Cu phase in two Al-Si-Cu-Mg casting alloys using calorimetry. Mater. Charact. 2002, 48, 371–378. [CrossRef] 13. Wang, Y.; Liao, H.; Wu, Y.; Chun, B.S. Effect of Si content on microstructure and mechanical properties of Al-Si-Mg alloys. Mater. Des. 2014, 53, 634–638. [CrossRef] 14. Srivastava, A.K.; Srivastava, V.C.; Gloter, A.; Ojha, S.N. Microstructural features induced by spray processing and hot extrusion of an Al-18% Si-5% Fe-1.5% Cu alloy. Acta Mater. 2006, 54, 1741–1748. [CrossRef] 15. Li, R.X.; Yu, F.X.; Zuo, L. Effect of hot extrusion process on microstructure and mechanical properties of hypereutectic Al-Si alloys. China Foundry 2011, 8, 145–149. 16. Saha, P.K. Fundamentals of Extrusion. Aluminium Extrusion Technology; ASM international: Geauga, OH, USA, 2000. 17. Gulay, V.; Khalifa, N.B.; Tekayya, A.E. The effect of extrusion ratio and material flow on the mechanical properties of aluminium profiles solid state recycled from 6060 aluminium alloy chips. AIP Conf. Proc. 2011, 1353, 1609–1614. 18. Chen, Y.; Wang, Q.; Peng, J.; Zhai, C.; Ding, W. Effect of extrusion ratio on the microstructure and mechanical properties of AZ31 Mg alloy. J. Mater. Process. Technol. 2007, 182, 281–285. [CrossRef] 19. Xiong, B.Q.; Zhang, Y.G.; Wei, Q.; Shi, L.K.; Xie, C.G.; Shang, C.J.; He, X.L. The study of primary Si phase in spray forming hypereutectic Al-Si alloy. J. Mater. Process. Technol. 2003, 137, 183–186. 20. Gowri, S.; Samuel, F.H. Effect of alloying elements on the solidification characteristics and microstructure of Al-Si-Cu-Mg-Fe 380 alloy. Metall. Mater. Trans. A 1994, 25, 437–448. [CrossRef] 21. Hong, S.J.; Kim, T.S.; Kim, W.T.; Chun, B.S. The effects of Cr and Zr addition on the microstructure and mechanical properties of rapidly solidified Al-20Si-5Fe alloys. Mater. Sci. Eng. A 1997, 226–228, 878–882. [CrossRef] 22. Sjolander, E.; Seifeddine, S. The heat treatment of Al-Si-Cu-Mg casting alloys. J. Mater. Process. Technol. 2010, 210, 1249–1259. [CrossRef] 23. Wu, C.; Lee, S.; Hsieh, M.; Lin, J. Effects of Mg content on microstructure and mechanical properties of Al-14.5Si-4.5Cu alloy. Metall. Mater. Trans. A. 2010, 41, 751–757. [CrossRef] 24. Caceres, C.H.; Svensson, I.L.; Taylor, J.A. Strength-ductility behavior of Al-Si-Cu-Mg casting alloys in T6 temper. Int. J. Cast Metals Res. 2003, 15, 531–543. [CrossRef] 25. Shabestari, S.G.; Moemeni, H. Effect of copper and solidification conditions on the microstructure and mechanical properties of Al-Si-Mg alloys. J. Mater. Process. Technol. 2004, 153, 193–198. [CrossRef]

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