Structural, Fatigue Behavior, and Mechanical Properties of Zirconium Tungstate-Reinforced Casted A356 Aluminum Alloy

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Article Structural, Fatigue Behavior, and Mechanical Properties of Zirconium Tungstate-Reinforced Casted A356 Aluminum Alloy

Muhammad Raza 1, Hussein Alrobei 2,* , Rizwan Ahmed Malik 1, Azhar Hussain 1, Meshal Alzaid 3 , Mohsin Saleem 4 and Mian Imran 5

1 Department of Metallurgy and Materials Engineering, University of Engineering and Technology, Taxila 47050, Pakistan; razajaﬀ[email protected] (M.R.); [email protected] (R.A.M.); [email protected] (A.H.) 2 Department of Mechanical Engineering, College of Engineering, Prince Sattam bin Abdul aziz University, AlKharj 11942, Saudi Arabia 3 Physics Department, College of Science, Jouf University, Al-Jouf, Sakaka 2014, Saudi Arabia; [email protected] 4 School of Chemical and Materials Engineering, National University of Sciences and Technology (NUST), Islamabad 44000, Pakistan; [email protected] 5 Department of Mechanical Engineering, Institute of Space Technology, Islamabad 44000, Pakistan; [email protected] * Correspondence: [email protected]; Tel.: +966-11-588-8272

Received: 28 September 2020; Accepted: 2 November 2020; Published: 9 November 2020

Abstract: The aim of this study is to investigate the structure–property relationship of the zirconium tungstate-reinforced casted A356 aluminum alloy. The reinforcement ceramic used was zirconium tungstate of the negative thermal coeﬃcient type, which assists in the weldment of crack growth and enhances the fatigue life. The specimens used in this study were casted by stir casting method and prepared according to Compact Tension standard E-399, and microstructural, fatigue behavior, and mechanical properties were investigated systematically. Microstructural analysis showed reduction in porosity by the addition of ZrW2O8 particles. Fatigue results depict the increase in the fatigue life of aluminum reinforced ceramic as compared to the casted base aluminum alloy. Brinell hardness of ZrW2O8 reinforced alloy samples increased 7% as compared to the base aluminum alloy hardness value. Tensile strength also signiﬁcantly improved from 176 MPa for the base A356 alloy to 198 MPa for the ZrW2O8 reinforced composite. Furthermore, addition of ZrW2O8 ceramic powder increased the fatigue life more than 50% of the base alloy. These results suggest that the ZrW2O8 reinforced A356 composites may be potential candidates for aerospace industry, military, transportation and in structural sites.

Keywords: zirconium tungstate; mechanical properties; negative thermal coeﬃcient; low cycle fatigue; microstructure

1. Introduction During the last two decades, a lot of research has been done on Metal Matrix composite (MMC) for improvement in the industrial sector. The combination of high strength, high modulus refractory particles with a ductile metal as a matrix produces a material whose mechanical properties are in between ceramic particle and metal matrix. MMC are being used for the structural applications of transport and aeronautical sector, due to their enhanced mechanical and thermo physical properties. Aluminum matrix composite (AMC) is one the most prominent MMC, as they are cheap and easily

Metals 2020, 10, 1492; doi:10.3390/met10111492 www.mdpi.com/journal/metals Metals 2020, 10, 1492 2 of 15 available alloy. Certain AMC properties like long fatigue life, high specific modulus and corrosion resistance made this material especially prominent in field of aerospace, automobile, transportation, and building industries. Different techniques were used for the preparation of AMC. One of the cheap and simple technique is stir casting method [1–7]. The high strength-to-weight ratio and excellent properties like Al-Si mixture replaced the steel and cast iron from different fields [8,9]. In most cases, aluminum-made machineries deteriorated due to fatigue loading. Researchers have proposed different techniques to solve this matter and to improve the fatigue life and mechanical properties of the aluminum alloy. It was concluded that the existence of second phase particles have positive effect on the fatigue behavior of aluminum alloy matrix. At high temperatures, these particles can be used as precipitate. Mostly particles were composed of Zr, Cr, and Mn. Second phase particles are hard enough which promote planar slip, and also affect crack branching, crack closure, crack deflection to decrease the possibility of fatigue crack growth (FCG) [10]. Elhadari et al. [11] added thermally stable alloying elements: Zr, V, Ti in the aluminum alloy to improve the fatigue life and tensile properties. Silicon carbide (SiC) is one of the mostly used ceramic for the preparation of aluminum composite. SiC particulate enhanced the fatigue life of spray formed aluminum alloy. AMC formed with the SiC particles improved the tensile and yield strength. Fatigue strength and fracture behavior of the matrix was also enhanced [12,13]. Lisa Winter et al. [14] observed the effect if SiC on the fatigue behavior and mechanical properties of 2124 Al alloy. Fatigue and mechanical properties increase with the addition of ceramic particle. Squeeze casted aluminum alloy was aged after the heating process. The fatigue life of aged alloy was increased, after it was tested. Other mechanical properties also shown an improvement in the result [15,16]. Sajjadi et al. [17] formed aluminum matrix composite with the alumina particles, it enhances the mechanical properties of aluminum alloy. Different processing techniques were used for the preparation of alumina reinforced aluminum matrix. Zhu et al. [18] enhanced the fatigue behavior of aluminum matrix with the addition of aluminum borate whiskers, this addition of whiskers was experimented with the method of squeeze casting. J. Jeykrishnan et al. [19] enhances the mechanical properties (hardness, tensile, and impact) of aluminum alloy by addition of titanium di-borate as a reinforcement. Hai Fu et al. [20] enhances the fatigue behavior with the inclusion of ceramic particles. Zirconia oxide was used as biomaterial with unique qualities like transformation toughening which gives it toughness and high strength as compared to other ceramics. It is thermally stable compound with good quality of corrosion resistance and mechanical properties. Zirconia oxide and coconut shell were used as reinforcement for the improvement mechanical properties of aluminum alloy. It enhances the tensile, hardness and impact strength of the alloy [21]. D. Mummoorthi et al. [22] added Fe2O3 and B4C as a reinforcement to enhance the tensile and hardness properties of the aluminum alloy. R. Pandiyarajan et al. [23] enhances the mechanical properties of aluminum by addition of Zirconium oxide and graphite. Madhusudan et al. [24] used copper particles for the preparation of aluminum matrix composite. It helps in the enhancement of the strength and hardness of the composite. A unique type of material has been introduced which contracts upon heating and expand during cooling. Negative thermal expansion (NTE) materials have extraordinarily industrial advantages to control the thermal expansion of different materials. This concept has been introduced in the composite to control the thermal expansion. The NTE build composite will generate new functionalities like light weight, high strength structural material, low expansion, and large scale. High-precision equipment with ultimate thermal expansion control can be built with this concept. Zirconium tungstate (ZrW2O8) is one of the types of NTE, having good qualities as compared to other NTE materials. ZrW2O8 has unique properties of negative thermal coefficient; it gets contract upon heating and expand upon cooling. These properties are considered to be helpful in the crack weldment and crack closure, causes due to the fatigue loading. When crack initiate in the fatigue loading, the temperature of that area increases, so inclusion of these particles may contract the area of crack initiation and helps in crack closure and weldment. Furthermore, ZrW2O8 acts as hindrance in the path of crack opening and can improve the strength of the material. Consequently, mechanical properties of the alloy can be improved Metals 2020, 10, 1492 3 of 15

with the incorporation of these particles. Various types of composite can be build using (ZrW2O8) as a reinforcement. Some of the composites are ZrW2O8/aluminum, ZrW2O8/copper, ZrW2O8/phenolic resin, etc. [25,26]. On the contrary, A356 is one of the mostly used alloys due to its good silicon content. Silicon helps in ﬂuidity of casting and is helpful in other mechanical properties. It has good resistance to corrosion and also imparts strength to other mechanical properties. Cu and Mg were added to Al-Si which also added their part in increasing the mechanical properties [27,28]. In this scenario, a novel AMC was developed in this study by using A356 as a base metal and ZrW2O8 as a reinforcement material and its structural, fatigue and mechanical properties were explored. The major aim of this work is to evaluate the fatigue and mechanical behavior of the A356 alloy with the combination of ceramic particles. According to best of our knowledge, the investigated matrix/reinforcement composite is not prepared and studied for mechanical properties.

2. Material Properties and Experimental Procedure

2.1. Sample Preparation The material used in this research was A356 Al alloy, composition of the alloy is given in Table1. 3 The density of the Al alloy is 2.75 g/cm . This alloy is reinforced with particles of ZrW2O8 of 1.86 microns in size; the chemical composition and properties of the ceramic are given in the Table2. Stir casting, one of the low-cost casting techniques with various processing parameters, was used to disperse the ZrW2O8 particles into the melted A356 Al alloy and produce a composite. In a stationary port type pit furnace [29], cut ingots aluminum alloy were melted in a clay graphite crucible at a temperature of 750 ◦C.

Table 1. Chemical composition of aluminum alloy A356.

Si % Cu% Mg% Mn% Ni% Zn% Fe% Ti% Al% 7.5 0.25 0.20 0.35 0.2 0.35 0.8 0.25 Bal.

Table 2. Chemical composition and properties of ZrW2O8.

ZrW2O8 Impurities (%Max) Density Particle Size Appearance 3 (%Min) Fe2O3 SiO2 Mo TiO2 g/cm (D50, µm) Powder 99.6 0.001 0.0071 0.002 0.0062 5.07 1.86 Light Yellowish

After complete melting of Al alloy coverall flux and hexaflourine tablet was added to remove the dust particles from the molten metal and discharge the entrapped gasses from the molten metal respectively. The slag and dust particles were removed from the top of the crucible. The zirconium tungstate particles were pre-heated at 200 ◦C, in order to remove the moisture contents. The parameters of stirring setup were addition rate of ZrW2O8 in 4% amount, stirring speed of 600 rev/min and 5 min stirring time for uniform distribution. The melted composite was then poured into pre-heated permanent mold of 298 mm in length, 143 mm in width, and 10 in thickness and allowed to solidify at an atmospheric condition [30]. The casted samples were cut out into two pieces, as during one-time casting 2 samples were achieved. Samples were machined with the vertical milling machine according to the given parameters of standard ASTM. Two holes of 14 mm diameter were drilled on each of the samples by drilling procedure for gripping in the hydraulic machine (Zwick Roell, Ulm, Germany). Notch was formed with an Electric Discharge Machining (EDM, Neuar Precision Machinery Co. limited, Taipei, Taiwan) using copper electrode of designed notch prepared by wire cutting machine (Suzhou Baoma Numerical Control Equipment Co. Ltd., Jiangsu, China). The finally prepared samples are in the dimension of Metals 2020, 10, 1492 4 of 15

71.5 mm 68.6 mm 6 mm as shown in Figure1a. which is the ASTM standard for fatigue test also Metals 2020, 10, x FOR PEER× REVIEW× 4 of 16 known as Compact Tension C(T) E-399 sample [31].

Figure 1. (a) Sample after cutting, machining, drilling, and notch making. (b) Fracture sample after Figure 1. (a) Sampletensile test. after cutting, machining, drilling, and notch making. (b) Fracture sample after

tensile test.2.2. Experimental Techniques

2.2. Experimental2.2.1. Techniques Tensile Testing Micro-tensile specimens of D1708 ASTM standard were prepared from the casted aluminum alloy to compare the tensile properties of composite and casted base alloy. Tensile tests were performed up 2.2.1. Tensile Testingto failure on the fully automatic, servo hydraulic machine of Shimadzu (Zwick Roell, Ulm, Germany) equipped with 100 kN load. Figure1b shows the break tensile sample. The strain rate value for the all Micro-tensilethe samples specimens was 1 mm /ofmin. D1708 The gauge ASTM length ofstandard the samples waswere 25 mm.prepared All the tests from were performedthe casted aluminum alloy to compareunder the same tensile constraints. properties of composite and casted base alloy. Tensile tests were performed up 2.2.2.to failure Hardness on the fully automatic, servo hydraulic machine of Shimadzu (Zwick Roell, Ulm, Germany) equippedThe Brinell with hardness 100kN test of load. the samples Figure was 1b performedshows the on break the Optical tensile Hardness sample. Tester The strain rate value for the all(HBRVU-187.5, the samples Time was Group 1 Inc. mm/min. Beijing, China). The ga Theuge size length of the indenter of the was samples 1.5 mm, thewas load 25 mm. All the tests were performedapplied on under each sample the was same 980N, constraints. and the holding time for each load was 30 s. Total of 8 samples were tested, 4 of Al composite and 4 of base Al-Alloy. Average hardness value of samples was reported for result comparison. 2.2.2. Hardness 2.2.3. Fatigue The Brinell Thehardness Compact Tensiontest of C(T) the samples samples of ASTM was E-399 performed standard were testedon the in a ServoOptical Hydraulic Hardness Tester (HBRVU-187.5,Machine Time ofGroup Zwick Roell Inc. (HC-25, Beijing, Zwick China). Roell, Ulm, The Germany) size of underthe indenter control load was of tension–tension 1.5 mm, the load applied with stress ratio of 0.1 at a frequency of 10 HZ. All samples were tested at room temperature of on each sample15 was◦C; the 980N, relative and humidity the holding was in the time range for of 20 ea toch 35%. load Samples was were30 s. tested Total in of the 8 low samples cyclic were tested, 4 of Al compositefatigue and mode 4 (Seeof base Figure Al-Alloy.2a). For Low Average Cycle Fatigue hardness (LCF), the applied value load of valuesamples for fatigue was testing reported for result is usually taken between the range of 50 to 80% of its tensile strength. As fatigue strength of any comparison. material is usually lower than its tensile strength. In this study, four diﬀerent stress values of 40, 35,

2.2.3. Fatigue The Compact Tension C(T) samples of ASTM E-399 standard were tested in a Servo Hydraulic Machine of Zwick Roell (HC-25, Zwick Roell, Ulm, Germany) under control load of tension–tension with stress ratio of 0.1 at a frequency of 10 HZ. All samples were tested at room temperature of 15 °C; the relative humidity was in the range of 20 to 35%. Samples were tested in the low cyclic fatigue mode (See Figure 2a). For Low Cycle Fatigue (LCF), the applied load value for fatigue testing is usually taken between the range of 50 to 80% of its tensile strength. As fatigue strength of any material is usually lower than its tensile strength. In this study, four different stress values of 40, 35, 30, and 25 MPa at load value of 80, 70, 60, and 50% of its tensile strength were applied, respectively. The crack initiation site (notch area) was polished with 900-grit emery paper to observe the crack initiation clearly. Four samples of each composite and base alloy were tested at four different values of stress amplitude (25, 30, 35, and 40 MPa). Samples were tested until failure (Figure 2b).

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30, and 25 MPa at load value of 80, 70, 60, and 50% of its tensile strength were applied, respectively. The crack initiation site (notch area) was polished with 900-grit emery paper to observe the crack initiation clearly. Four samples of each composite and base alloy were tested at four diﬀerent values of Metals 2020stress, 10, x amplitude FOR PEER (25, REVIEW 30, 35, and 40 MPa). Samples were tested until failure (Figure2b). 5 of 16

FigureFigure 2. (a) 2.Fatigue(a) Fatigue testing testing of ofC(T) C(T) sample sample (b(b)) Break Break sample sample after after fatigue fatigue test. test. 2.2.4. Characterization 2.2.4. Characterization The initial microstructure of as-received casted-machined A356 aluminum samples was Thecharacterized initial microstructure by metallurgical of microscope as-received after casted-machined sample preparation andA356 polishing. aluminum The samplesamples was characterizedpreparation by metallurgical was followed by samplingmicroscope with epoxyafter andsample resin inpreparation the round pipe and piece, polishing. then ground The the sample samples with the emery paper of (120–2000) grit, to remove the ﬁnal scratches; samples were polished preparationwith was the alumina followed paste by in sampling order to observe with theepoxy grain and structure resin clearly.in the Theround polished pipe samplespiece, then were ground the samplesthen washedwith the with emery the Keller paper Etchant of (120–2000) (2 mL HF, 3 mL grit HCL,, to 5remove mL HNO 3theand final 190 mL scratches; water) for samples 30 s to were polishedremove with the the alumina dust particles paste from in the order surface to observe [32]. The microstructuralthe grain structure analysis clearly. of the samples The polished was done samples µ µ were thenat 25washedm and with 100 mthe with Keller the optical Etchant microscope (2 mL HF, equipped 3 mL with HCL, image 5 mL analyzer. HNO3 and 190 mL water) for 30 s to removeFracture the dust samples particles of the fatigue from testedthe surfac samplese [32]. were The analyzed microstructural by scanning electron analysis microscope of the samples (SEM, Vega Tescan, Brno, Czech Republic). Sample pieces were cut on hacksaw, approximately 7 mm was donefrom at 25 the μ fracturem and surface.100 μm Samples with the were optical then washed microscope with ethanol equipped to remove with the image dust andanalyzer. foreign Fractureparticles samples and mounted of the onfatigue the stubs tested for SEMsamples analysis. were Fracture analyzed analysis by scanning was done electron on the VEGA microscope (SEM, VegaTESCAN Tescan, SEM Brno, at 20 kVCzech in scattered Republic). electron Sample (SE, Vega pieces Tescan, were Brno, cut Czech on hacksaw, Republic) approximately mode and also 7 mm from thein fracture back scattered surface. mode Samples (BSE, Vega were Tescan, then Brno, wash Czeched with Republic) ethanol for phase to remove contrast the of the dust regions. and foreign particlesEnergy and mounted dispersive X-rayon the spectroscopy stubs for (EDS,SEM Vega analysis. Tescan, Fracture Brno, Czech analysis Republic) was technique done ofon SEM the VEGA was used for the composition analysis of the samples. TESCAN SEM at 20 kV in scattered electron (SE, Vega Tescan, Brno, Czech Republic) mode and also in back scattered3. Results andmode Discussion (BSE, Vega Tescan, Brno, Czech Republic) for phase contrast of the regions.

Energy dispersive3.1. Fractography X-ray spectroscopy (EDS, Vega Tescan, Brno, Czech Republic) technique of SEM was used for the composition analysis of the samples. Figure3 shows the SEM image of the fatigue break surface of base aluminum alloy. The fracture sample was analyzed at high magnification of the overall crack region. Figure3a shows the overall 3. Resultscrack and region Discussion of base alloy with different planes. Figure3b was observed at high resolution, which shows the pores with their dimensions and different ups and down regions. Figure3c shows the river ridges 3.1. Fractographyof the crack surface. River ridges explain the crack propagation manner of the fatigue crack [33]. Fatigue crack striations and river ridges were observed in most of the images, which indicates that Figurethe samples3 shows were the ductileSEM image in nature. of the It was fatigue also observedbreak surface that most of base of the aluminum fatigue crack alloy. generated The fracture sample wasdue theanalyzed defects andat high porosity magnification area of the samples of the [ 12overall]. Therefore, crack the region. porosity Figu wasre the 3a fatigue shows crack the overall crack regiondomination of base site. alloy However, with indifferent the defect planes. less sample, Figure the crack3b was propagation observed is due at high to slip resolution, bands and which shows the pores with their dimensions and different ups and down regions. Figure 3c shows the river ridges of the crack surface. River ridges explain the crack propagation manner of the fatigue crack [33]. Fatigue crack striations and river ridges were observed in most of the images, which indicates that the samples were ductile in nature. It was also observed that most of the fatigue crack generated due the defects and porosity area of the samples [12]. Therefore, the porosity was the fatigue crack domination site. However, in the defect less sample, the crack propagation is due to slip bands and eutectic structure. In the region of early microscopic crack growth fine and shallow striations were found. It was found that the presence of hard particles in the soft area stops the formation of voids and keeps on the cyclic stressing of the material. Presence of Dimples indicate that there is formation of fine particles due to the addition of the ceramic particles [14]. According to previous studies, river- like ridges were observed in A356 material during the crack initiation process of tension loading, which depicts the ductile nature of the material [29,33]. The striations in the Figure 3c show that sample was ductile in nature. Fatigue loading also causes fracture surface with slip bands (see Figure 3d). Slip bands leads to the formation of intrusions and extrusions.

Metals 2020, 10, 1492 6 of 15 eutectic structure. In the region of early microscopic crack growth ﬁne and shallow striations were found. It was found that the presence of hard particles in the soft area stops the formation of voids and keeps on the cyclic stressing of the material. Presence of Dimples indicate that there is formation of ﬁne particles due to the addition of the ceramic particles [14]. According to previous studies, river-like ridges were observed in A356 material during the crack initiation process of tension loading, which depicts the ductile nature of the material [29,33]. The striations in the Figure3c show that sample was ductile in nature. Fatigue loading also causes fracture surface with slip bands (see Figure3d).

SlipMetals bands 2020, 10 leads, x FOR to PEER the formationREVIEW of intrusions and extrusions. 6 of 16

Figure 3. SEMSEM images images of of fracture fracture surface surface of ofbase base aluminum aluminum alloy: alloy: (a) overall (a) overall image image of the of notch the notch area areaof the of specimen the specimen from from where where crack crack started, started, (b) fracture (b) fracture surface surface showing showing the thepores pores dimension, dimension, (c) (striations/riverc) striations/river ridges ridges depictin depictingg the the crack crack propagation, propagation, and and (d (d) )slip slip bands bands on on the the fracture fracture surface surface of the specimen.

Micro-cracks mostly initiate from the intrusions. Therefore, Therefore, slip slip bands bands has has a main role in the initiation of fatigue cracks [[34].34]. As As it was discussed earlier, the eutectic Si particles have a long and elongated microstructure,microstructure, which which causes causes early early breaking breaking and and crack crack initiation initiation of the of samples. the samples. In the In initial the periodinitial period of low of cycle low fatigue cycle fatigue loading, loading, particle particle cracking cracking occurs. occurs. When Si When particles Si particles crack, it crack, will become it will nucleationbecome nucleation site for othersite for micro-cracks other micro-cracks [11]. In this [11]. manner In this micro-cracks manner micr increaseso-cracks withincreases increasing with fatigueincreasing cycle. fatigue Micro-cracks cycle. Micro-cracks from all the from region all get the combine region get and combine causes forand the causes fracture for ofthe the fracture samples. of Therefore,the samples. it isTherefore, analyzed it in ismost analyzed of the in cases mostthat of th fatiguee cases cracksthat fatigue mostly cracks occur mostly due to occur the breakage due to the of siliconbreakage particles of silicon or particles the porosity or the of porosity the region. of the But region. in the But case in of the composite, case of composite, it was observed it was observed that the ceramicthat the particlesceramic hinderparticles in fronthinder of crackin front propagation of crack sitepropagation and increases site theand fatigue increases performance the fatigue of theperformance sample [35 of]. the Figure sample4 shows [35]. SEMFigure image 4 shows of the SEM crack image surface of the of thecrack composite. surface of Figure the composite.4a is the overallFigure 4a image is the of overall the composite image of fracturethe composite area which fracture shows area the which diﬀerent shows regions. the different Figure regions.4b is the Figure crack initiation4b is the crack and propagationinitiation and region propagation of the composite. region of the The composite. presence ofThe striation presence and of riverstriation like and ridges river is thelike indicationridges is the of indication the stable of crack the stable growth crack in the growth fracture in the surface fracture [35 ]surface (Figure [35]4c). (Figure Fatigue 4c). striations Fatigue andstriations river ridgesand river mostly ridges occur mostly due occur to the due dislocation to the dislocation slips of the slips planes of the in theplanes plastic in the zone plastic region zone of theregion crack of tip.the Crackcrack propagationtip. Crack propagation rate slows downrate slows with thedown presence with the of reinforcementpresence of reinforcement particles and particles and are justified by the presence of dimples (see Figure 4d). The microscopic dimples of fracture analysis describe that the fracture is ductile in nature [21]. From the SEM micrographs, it was also found that one of the key reasons for crack initiation was larger defects near the sample surface, such as large pores, large shrinkages, which cause fatigue cracks.

Metals 2020, 10, 1492 7 of 15 are justiﬁed by the presence of dimples (see Figure4d). The microscopic dimples of fracture analysis describe that the fracture is ductile in nature [21]. From the SEM micrographs, it was also found that one of the key reasons for crack initiation was larger defects near the sample surface, such as large

Metalspores, 2020 large, 10,shrinkages, x FOR PEER REVIEW which cause fatigue cracks. 7 of 16

Figure 4. SEMSEM images images of of fracture surface of composite: ( (aa)) overall overall image image of of the the notch notch area area of the specimen from where crack started, ( b)) crack initiation site site of the composite from where the crack originated, ((cc)) planeplane surfacessurfaces showing showing the the slip slip bands bands and and curved curved pattern pattern depicts depicts the the river river ridges ridges of the of thecrack crack pattern, pattern, and and (d) dimples(d) dimples and and voids voids shows shows that that the crackthe crack was was rupture rupture in nature. in nature.

3.2. Microstructure Microstructure Analysis Microstructure ofof simplysimply castedcasted base base aluminum aluminum alloy alloy and and ceramic ceramic reinforced reinforced Al-alloy Al-alloy is shownis shown in inthe the Figure Figure5. Porosity5. Porosity and and shrinkage shrinkage pores pores were were also also observed observed in in the the material material because because ofof ineineffectiveffective feeding and cooling rate during the casting (Figure5 5a).a). The bright region in the figure depicts Al-dendrite structure and the dark gray regions are the eutectic Al-Si dendrite structures (Figure5b). The cell-shaped Al-dendrites are the secondary dendrite cells. The characteristics of the eutectic Al-Si dendrite phase are three-dimensional in nature. The fatigue crack propagation path faces the Al-Si eutectic structures. Eutectic silicon particles were more sensitive to damage during cyclic loading. It was noticed that with the addition of ceramic particles, the number of pores decreased in Al-composite as compared to the base alloy which does not have ceramic inclusion (Figure5c) [ 36]. It was also observed from the Figure5d that increasing cooling rate refines the eutectic particles and also changes their morphology from large plates to long fibers. With the addition ZrW2O8 ceramic powder, the eutectic structure of Al-Si structure becomes more refined [37]. It may be due to increased solidification distance which solidify the melt slowly and particles become coarser and elongated. However, it has also some disadvantage too, as there are more chances of cracking of large elongated particles [35]. The amount of Mg content has a main role in the particle cracking because of the enlarged size of eutectic Al-Si and Fe rich intermetallic particles [34]. Figure5d shows the dispersion of ZrW 2O8 ceramic particles in the aluminum matrix [22]. The particle size of the ceramic was 1.86 µm.

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Figure 5. Optical micrographs of base Al-alloy and ceramic-reinforced composite: (a) low Figure 5. Opticalmagnification micrographs of base ofalloy base displaying Al-alloy the distribution and ceramic-reinforced of porosity, (b) high magnification composite: of base (a )alloy low magnification of base alloy displayingshowing thethe microstructure distribution of eutectic of porosity, silicon particles (b) high and magnification aluminum dendrites, of base (c) alloylow showing the microstructure ofmagnification eutectic of silicon composite particles showing andthe eutectic aluminum structure, dendrites,and (d) high magnification (c) low magnification of composite of composite showing the distribution of ZrW2O8 into aluminum alloy. showing the eutectic structure, and (d) high magnification of composite showing the distribution of The bright region in the figure depicts Al-dendrite structure and the dark gray regions are the ZrW2O8 into aluminum alloy. eutectic Al-Si dendrite structures (Figure 5b). The cell-shaped Al-dendrites are the secondary dendrite cells. The characteristics of the eutectic Al-Si dendrite phase are three-dimensional in nature. Figure6 showsThe fatigue the crack SEM propagation micrographs path faces with the Al-Si the eu supporttectic structures. of an Eutectic EDS silicon line particles scan of were di fferent phases present in the basemore alloy.sensitive The to damage EDS during results cyclic of theloading. base It was Al-alloy noticed that only with consist the addition of the of ceramic A356 alloy elements; particles, the number of pores decreased in Al-composite as compared to the base alloy which does the presence ofnot copper have ceramic in the inclusion Al-Si (Figure alloy 5c) gives [36]. It itwas strength also observed through from the theFigure formation 5d that increasing of Al 2Cu and also with the combinationcooling rate of refines Al-Fe-Si the eutectic structures. particles and Magnesium also changes their also morphology has its from own large role plates in to the long matrix. It forms a structure Mgfibers.Si and With strengthens the addition ZrW the2O8 ceramic overall powder, alloy. the Iron eutectic can stru becture easily of Al-Si chosen structure duringbecomes the melting of 2more refined [37]. It may be due to increased solidification distance which solidify the melt slowly aluminum andand inevitably particles become present coarser in and cast elongated. aluminum-silicon However, it has also alloy some [11 disadvantage] (see Figure too, as6a,b). there Metalsare more 2020 ,chances 10, x FOR ofPEER cracking REVIEW of large elongated particles [35]. The amount of Mg content has a 9main of 16 role in the particle cracking because of the enlarged size of eutectic Al-Si and Fe rich intermetallic particles [34]. Figure 5d shows the dispersion of ZrW2O8 ceramic particles in the aluminum matrix [22]. The particle size of the ceramic was 1.86 μm. Figure 6 shows the SEM micrographs with the support of an EDS line scan of different phases present in the base alloy. The EDS results of the base Al-alloy only consist of the A356 alloy elements; the presence of copper in the Al-Si alloy gives it strength through the formation of Al2Cu and also with the combination of Al-Fe-Si structures. Magnesium also has its own role in the matrix. It forms a structure Mg2Si and strengthens the overall alloy. Iron can be easily chosen during the melting of aluminum and inevitably present in cast aluminum-silicon alloy [11] (see Figure 6a,b).

Figure 6. SEM images and EDS spectra show the chemical composition of base aluminum alloy (a). Figure 6. SEMSpectra images on andthe SEM EDS image spectra of the base show alloy the(b) with chemical phases of compositioniron, magnesium, ofzinc, base silicon, aluminum and alloy (a). Spectra on the SEMcopper. image of the base alloy (b) with phases of iron, magnesium, zinc, silicon, and copper.

EDS spectra shows in the (Figure 7a,b) revealed the presence of bright phases of Zirconium (Zr). Tungsten (W) and oxygen (O) were also found in the selected spectrum region which prove the presence of ZrW2O8 (zirconium tungstate) particles in the Al composite. Al-Si-Cu alloy offer good casting characteristics, improved machineability with reduced ductility, higher strength and hardness [28]. During the initial alloy solidification cycle, micro-sized structures combine and develop primary phases. Other than this Si eutectic particles forms structures, i.e., Si-Cu, Si-Mg-Cu, and Si-Mg-Fe.

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EDS spectra shows in the (Figure7a,b) revealed the presence of bright phases of Zirconium (Zr). Tungsten (W) and oxygen (O) were also found in the selected spectrum region which prove the presence of ZrW2O8 (zirconium tungstate) particles in the Al composite. Al-Si-Cu alloy oﬀer good casting characteristics, improved machineability with reduced ductility, higher strength and hardness [28]. During the initial alloy solidiﬁcation cycle, micro-sized structures combine and develop primary phases. Other than this Si eutectic particles forms structures, i.e., Si-Cu, Si-Mg-Cu, and Si-Mg-Fe. Metals 2020, 10, x FOR PEER REVIEW 10 of 16

Figure 7.7. SEMSEM images images and and EDS EDS spectra spectra show show the chemical the chemical composition composition of composite of composite (a). Selected (a). spectrumSelected onspectrum SEM image on SEM of composite image of (b composite) phases of silicon,(b) phases manganese, of silicon, zirconium, manganes tungsten,e, zirconium, and other tungsten, components and ofother A356. components of A356.

3.3. Hardness EEffectﬀect Hardness ofof all the 4 samples of each reinforced and unreinforced alloy was observed by Brinell hardness testtest andand shownshown inin FigureFigure8 8.. The The average average hardness hardness value value of of ZrW ZrW22OO88 reinforcedreinforced alloy samples increasedincreased 7%7% asas comparedcompared to to the the average average hardness hardness value value of of base base aluminum aluminum alloy. alloy. The The improvement improvement in thein the hardness hardness value value of composite of composite as compared as compared to the to base the alloy base is alloy due tois thedue addition to the addition of ceramic of powders ceramic whichpowders act which as hindrance act as hindrance to the motion to the of motion dislocations of dislocations [36,37]. Inclusion [36,37]. ofInclusion ZrW2O 8ofparticles ZrW2O8hindered particles thehindered plastic the formation plastic formation that increased that increased the hardness the ofhardness the composite of the composite [21]. The enhancement[21]. The enhancement of Brinell of Brinell Hardness values in ZrW2O8 reinforced A356 composite is in good agreement with microstructural analysis, where the base alloy contains more amount of porosity as compared to ZrW2O8 reinforced A356 composite. The Brinell hardness value increases with increasing the ceramic inclusions [38]. The ceramic inclusion also has a role in the hardness of the material. The hardness of ceramic reinforced alloy is higher that of the base alloy due to the strengthening of the alloy caused by the particles. These particles help to resist in the dislocation of motion. The enhancement in hardness is also due to the refining in the grain size and eutectic structure of the alloy which can be easily observed by microscopic images. Increasing the hardness with the incorporation of zirconium

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Metals 2020, 10, x FOR PEER REVIEW 11 of 16 Hardness values in ZrW2O8 reinforced A356 composite is in good agreement with microstructural tungstateanalysis, wherecan bethe linked base to alloy the presence contains moreof hard amount particles of porosityin the soft as matrix compared which to ZrWact as2 Obarrier8 reinforced to the A356movement composite. of the Thedislocation Brinell hardness[5]. value increases with increasing the ceramic inclusions [38].

Figure 8. Brinell Hardness value of base aluminum alloy and AlAl/ZrW/ZrW22O8 withwith standard standard deviation.

3.4. TensileThe ceramic Properties inclusion also has a role in the hardness of the material. The hardness of ceramic reinforced alloy is higher that of the base alloy due to the strengthening of the alloy caused by the Mechanical properties of Al-ZrW2O8 composite and base Aluminum alloy is shown on the Table particles. These particles help to resist in the dislocation of motion. The enhancement in hardness is 3. Results showed an increase in the tensile, yield and hardness values of the composite as compared also due to the refining in the grain size and eutectic structure of the alloy which can be easily observed to the base aluminum alloy [19,20]. The composite shows 13% increase in the tensile strength as by microscopic images. Increasing the hardness with the incorporation of zirconium tungstate can be compared the base aluminum alloy. The increase in percentage elongation from 1.5% to 1.4% also linked to the presence of hard particles in the soft matrix which act as barrier to the movement of the takes place with reduction in the cross-sectional area. The increase in these values are due to the dislocation [5]. composite formation, reinforcement and good interface (see Figure 9) [39]. Other reason is that the voids3.4. Tensile and porosity Properties contents are more in casted aluminum alloy as compared to the composite, that is in good agreement to microstructural study [24]. Composite like role of Cu- and Fe-containing siliconMechanical particles also properties played of a Al-ZrWgreat role2O 8forcomposite the tensile and strengthening base Aluminum of the alloy samples is shown (Figure on the 3b). Table Low3. degreeResults of showed porosity an level increase in composite in the tensile, also helps yield it and to easily hardness transfer values the ofapplied the composite stress and as enhance compared its to the base aluminum alloy [19,20]. The composite shows 13% increase in the tensile strength as tensile stress [12]. Addition of ZrW2O8 ceramic plays a positive role in the enhancement of the tensile strengthcompared and the elongation base aluminum of the alloy. modified The increase alloy as in compared percentage to elongation the unmodified from 1.5% [37]. to The 1.4% presence also takes of placethese withmicro-sized reduction particles in the cross-sectionalhinders the dislocation area. The du increasering the in tensile thesevalues deformation; are due thus, to the it composite causes to increaseformation, the reinforcement strength of casted and goodcomposite. interface (see Figure9)[ 39]. Other reason is that the voids and porosity contents are more in casted aluminum alloy as compared to the composite, that is in good agreement to microstructuralTable 3. studyMechanical [24]. Compositeproperties of like the roleA356 of and Cu- A356/ and ZrW Fe-containing2O8. silicon particles also played a great role for the tensile strengthening of the samples (Figure3b). Low degree of porosity Yield Tensile level in composite alsoBrinell helps it to easily transfer the applied stress andYoung’s enhance its tensileElongation stress [ 12]. Materials Strength Strength Addition of ZrW2HardnessO8 ceramic plays a positive role in the enhancementModulus of the(GPa) tensile strength(%) and elongation of the modified alloy as compared(MPa) to the unmodified(MPa) [37]. The presence of these micro-sized particlesA356 hinders the70.75 dislocation during 154.52 the tensile deformation; 175.60 thus, it causes 21.7 to increase the 1.5 strength A356- of casted composite.75.12 174.65 198.47 23.8 2 ZrW2O8

Table 3. Mechanical properties of the A356 and A356/ ZrW2O8.

Yield Strength Tensile Strength Young’s Modulus Materials Brinell Hardness Elongation (%) (MPa) (MPa) (GPa) A356 70.75 154.52 175.60 21.7 1.5

A356-ZrW2O8 75.12 174.65 198.47 23.8 2

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Figure 9. Effect of ceramic on the tensile strength of the base alloy and composite with standard Figuredeviation. 9.9. E Effectffect of of ceramic ceramic on on the the tensile tensile strength strength of the of base the alloy base and alloy composite and composite with standard with deviation.standard deviation. It can be seen from the Figure 1010 thatthat thethe tensiletensile strengthstrength ofof ceramicceramic addedadded alloy is higherhigher than the baseIt can alloy.alloy. be seen It is from suggested the Figure that 10 particle that the strengthening, tensile strength and of grainceramic refinementrefinement added alloy may is enhancehigher than the thetensile base strength alloy. It of is the suggested ceramicceramic that reinforcedreinforced particle alloy.alloy. strengthening, The major andenhancement grain refinement may may be maydue enhance to the grain the tensilerefinementrefinement strength as the of particle’sthe ceramic inclusion reinforced act asalloy. hindrance The major to dislocation enhancement and may restrict be themdue to against the grain the refinementapplied strength as the [[29]. 29particle’s]. The strength inclusion of actmaterial as hindrance can alsoalso to be dislocation evaluated byand its restrict porosityporosity them level.level. against It can the be appliedobserved strength from the [29]. microstructural The strength imagesof material that thecan compositecoalmpositeso be evaluated samples by have its lessporosity porosity level. contents It can be as observedcompared from to the the base microstructural alloy. The presence images ofthat pores the resultscomposite in the samples decrease have of less the porositytensile strength contents and as comparedmechanical to properties the base alloy. [[38].38]. The presence of pores results in the decrease of the tensile strength and mechanical properties [38].

Figure 10. EffectEﬀect of ceramic on tensile strength of base aluminum alloy and composite. Figure 10. Effect of ceramic on tensile strength of base aluminum alloy and composite. 3.5. Fatigue Performance 3.5. Fatigue Performance Fatigue tests results are shown below in Figure 1111.. The number of cycles to fracture was drawn againstFatigue thethe alternating alternatingtests results stress stress are amplitude. shown amplitude. below In general, inIn Figuregeneral, the 11. fatigue the The fatigue number life of alife sampleof cyclesof a decreasessample to fracture decreases with was increasing drawn with againsttheincreasing stress the amplitude. the alternating stress amplitude. At stress lower amplitude. stress, At lower the fatigueInstress, general, the life fatigue increasesthe fatigue life and in lifecreases at higherof a and sample stress, at higher decreases the fatiguestress, with lifethe increasingdecreasesfatigue life [ 11thedecreases]. stress As the amplitude.[11]. load As increases the Atload lower from increases 30%stress, tofr omthe 35%, 30%fatigue the to value 35%, life decreasesinthecreases value and fromdecreases at 40,911 higher from to stress, 12,817 40,911 the for to fatiguethe12,817 base for life aluminum the decreases base aluminum alloy [11]. and As fromthe alloy load 67,201 and increases from to 23,981 67,201 from for to30% the 23,981 compositeto 35%, for thethe alloy valuecomposite [40 decreases]. It alloy is also from [40]. evident 40,911It is also that to 12,817 for the base aluminum alloy and from 67,201 to 23,981 for the composite alloy [40]. It is also

Metals 2020, 10, 1492 12 of 15 Metals 2020, 10, x FOR PEER REVIEW 13 of 16 evidentaluminum that and aluminum its most ofand the its alloys most do of notthe showalloys a do continuous not show fatigue a continuous limit, but fatigue its fatigue limit, strength but its fatiguedecreases strength continuously decreases with continuously increase in thewith number increase of in cycles the number [15]. of cycles [15].

Figure 11.11. Stress amplitude and fatigue life comparisoncomparison ofof basebase aluminumaluminum alloyalloy andand AlAl/ZrW/ZrW22O8.

Figure 10 showsshows the the result result of of fatigue fatigue life life of of ca castedsted base base aluminum aluminum alloy alloy and and composite, composite, tested tested at stressat stress ratio, ratio, R ( Rσ max (σ /maxσ min/σ) min= 0.1) = and0.1 four and different four different level levelof stress of stress levels: levels: 25, 30, 25, 35, 30, and 35, 40 and MPa. 40 From MPa. microstructuralFrom microstructural analysis, analysis, it was it wasobserved observed that that the themost most of ofthe the cracks cracks nucleated nucleated from from porosity porosity and oxide films.films. Mostly fatiguefatigue crackcrack initiatedinitiated from from single single site. site. But, But, large large amount amount of of porosity porosity may may result result in inmultidirectional multidirectional fatigue fatigue crack. crack. River-like River-like ridges ridges and and striations striations werewere observedobserved inin the fatigue region, which indicate that the samples were ductile in nature.nature. In the overload region, dimples and cracked particles ofof eutecticeutectic siliconsilicon were were observed. observed. Tear Tear ridges ridges and and dimples dimples are are the the sign sign of ductileof ductile nature nature of the of thecrack crack [41 ,[41,42].42]. These These were were the the result result of of tensile tensile rupture. rupture. From From Figure Figure 11 11,, it it was was observedobserved thatthat fatigue life of composite is higher than the base aluminum alloy due to the inclusion of ceramic particles in the aluminum melt. It It is is also also observed observed that addition of 4% ceramic particles increases the fatigue life more than the 50% of the base aluminum alloy [[1144].]. In the composite sample, the pores and defects are less in numbernumber as comparedcompared toto thethe basebase aluminumaluminum alloyalloy (Figure(Figure4 a).4a). Therefore,Therefore, inin defect-freedefect-free samples, crackscracks mostlymostly initiateinitiate fromfrom slip slip bands bands and and eutectic eutectic particles, particles, which which increase increase the the fatigue fatigue life life of ofthe the samples. samples. Porosity and defects have an important role on the crack initiation and propagation. Larger the porosity, largerlarger willwill bebe thethe crackcrack initiationinitiation andand moremore rapidlyrapidly itit propagates.propagates. So, the fatigue life of material is directly proportional to its defects, the larger the defect size the the lower lower will will be be fatigue fatigue life. life. The figurefigure shows that the samplessamples having ceramic inclusionsinclusions have greater fatigue strength than the base aluminum alloy [[43]43] that is well supported by microstructural analysis.analysis.

4. Conclusions • In this study, the eﬀect of a 4% zirconium tungstate-reinforced A356 alloy on the microstructure, • In this study, the effect of a 4% zirconium tungstate-reinforced A356 alloy on the microstructure, fatigue, andand mechanical mechanical properties properties were were investigated. investigated. The ceramic The ceramic inclusion inclusion had shown had noticeable shown improvementsnoticeable improvements in fatigue behavior, in fatigue and behavior, mechanical and properties mechanical (i.e., properties hardness and (i.e., tensile hardness strength). and The existence of ZrW O particles reﬁned the microstructure of the aluminum matrix. • tensile strength). 2 8 • The hardness value of2 composite8 increases by 7% compared to the casted aluminum. • The existence of ZrW O particles refined the microstructure of the aluminum matrix. • TensileThe hardness strength value of the of composite increasesincreases upby to7% 13% compared compared to the to thecasted base aluminum. alloy. • • TheTensile addition strength of ZrW of theO compositecauses improvement increases up in to the 13% fatigue compared life of the to aluminumthe base alloy. alloy; the addition • 2 8 • ofThe 4% addition ceramic of powder ZrW2O increased8 causes improvement the fatigue life in more the fatigue than 50% life of of the the base aluminum alloy. Fatigue alloy; lifethe addition of 4% ceramic powder increased the fatigue life more than 50% of the base alloy. Fatigue life would be further increased if defects were minimized during casting and if aging of the sample was done.

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would be further increased if defects were minimized during casting and if aging of the sample was done.

Author Contributions: Conceptualization, M.R. and A.H.; methodology, M.R. and M.I.; software, M.R. and M.I.; formal analysis, M.R. and A.H.; investigation, M.R., A.H., H.A. and R.A.M.; writing—original draft preparation, M.R., H.A. and R.A.M.; writing—review and editing, M.R., H.A., M.A., M.S. and R.A.M.; supervision, H.A.; project administration, H.A.; funding acquisition, H.A. All authors have read and agreed to the published version of the manuscript. Funding: This project was supported the Deanship of Scientific Research at Prince Sattam bin Abdul aziz University, under the research project no. 2020/01/17063. Conflicts of Interest: The authors declare no conflict of interest.

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