Research Article Comparison of Microstructure and Mechanical...

Research ArticleComparison of Microstructure and Mechanical Properties ofA356/SiC Metal Matrix Composites Produced by Two DifferentMelting Routes

Shashi Prakash Dwivedi,1,2 Satpal Sharma,2 and Raghvendra Kumar Mishra2

1 Noida Institute of Engineering and Technology, Greater Noida, Gautam Buddha Nagar, Uttar Pradesh 201310, India2 School of Engineering, Gautam Buddha University, Greater Noida, Gautam Buddha Nagar, Uttar Pradesh 201310, India

Correspondence should be addressed to Shashi Prakash Dwivedi; shashi [email protected]

Received 14 June 2014; Revised 1 October 2014; Accepted 2 October 2014; Published 21 October 2014

Academic Editor: Shia-Chung Chen

Copyright © 2014 Shashi Prakash Dwivedi et al.This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in anymedium, provided the originalwork is properly cited.

A356/SiC metal matrix composites with different weight percent of SiC particles were fabricated by two different techniques suchas mechanical stir casting and electromagnetic stir casting.The results of macrostructure, microstructure, and XRD study revealeduniform distribution, grain refinement, and low porosity in electromagnetic stir casing samples. The mechanical results showedthat the addition of SiC particles led to the improvement in tensile strength, hardness, toughness, and fatigue life. It indicates thattype of fabrication process and percentage of reinforcement are the effective factors influencing the mechanical properties. It isobserved that when percentage of reinforcement increases in electromagnetic stir casting, best mechanical properties are obtained.

1. Introduction

The main challenge in the development and processingof engineering materials is to control the microstructure,mechanical properties, and cost of the product. The alu-minum metal matrix composite materials are the combina-tion of two or more constituents in which one is matrixand other is reinforcement [1–3]. If the casting processesof the melted metal are applied directly in the state ofmelted Al matrix with reinforcement, the reinforced metalmatrix composite parts with the complicated shape can beproduced. However, it is hard to get the products for thereinforcement to be distributed uniformly because of thedifference of densities of matrix and reinforcement in metalmatrix composite [4–6].

The mechanical properties of MMCs are sensitive tothe processing technique used to fabricate the materials.Considerable improvements may be achieved by applyingscience-based modeling techniques to optimize the process-ing procedure. Several techniques have been employed to

prepare the composites including powder metallurgy, melttechniques, and squeeze casting [7–10].

Investigation of mechanical behavior of aluminum alloysreinforced by hard particles such as SiC is an interestingarea of research. Therefore, the aim of the present work is toinvestigate the effects of different factors such as: (i) weightpercentage of the SiC particles (ii) type of fabrication process(mechanical stir and electromagnetic stir casting) on themicrostructure, mechanical properties and wear behavior ofthe metal matrix composites [11, 12].

On the basis of literature review, the compositions of rein-forcement selected in amultiplication of 5 and the percentageof reinforcements are varied from 0 to 15% weight fractionin metal matrix. If the weight percentage of reinforcement’sincreases more than 15% there is no more effect occuring inphysical and chemical properties of metal matrix composite.This work aims to compare the result of aluminum matrixcompositematerial reinforced by (0, 5, 10, and 15wt.%) siliconcarbide particles using electromagnetic stir casting methodand mechanical stir casting method.

Hindawi Publishing CorporationInternational Journal of Manufacturing EngineeringVolume 2014, Article ID 747865, 13 pageshttp://dx.doi.org/10.1155/2014/747865

2 International Journal of Manufacturing Engineering

3500

3000

2500

2000

1500

1000

500

0

20 40 60 80

Rela

tive i

nten

sity

SiC

SiC

SiC

SiCSiC

SiC

SiC

SiC

SiC

SiC SiC

Angle (2-theta)

Figure 1: DRX patterns of SiC particles.

Table 1: Chemical composition of A356 alloy (wt%) [13, 14].

Si Fe Cu Mn Mg Zn Ti Al6.5–7.5 0.2 0.2 0.1 0.25–0.45 0.1 0.1 Balance

Table 2: Properties of A356 alloy [13, 14].

Liquidus temperature 615∘CSolidus temperature 555∘CDensity (g/cm3) 2.685Tensile strength (MPa) 230Hardness (BHN) 75Toughness (joule) 6Fatigue strength (1 × 107 MPa) 120

2. Materials and Methods

2.1. Selection of the Material

2.1.1. Matrix Alloy. In this study, A356 alloy is selected. Ithas very good strength, ductility, hardness, fatigue strength,fluidity, and machinability. The chemical composition andproperties of A356 are shown in Tables 1 and 2 [7].

2.1.2. Reinforcement. Silicon Carbide (SiC) is composed oftetrahedral of carbon and silicon atoms with strong bondsin the crystal lattice. This produces a very hard and strongmaterial. Silicon carbide is not attacked by any acids or alkalisor molten salts up to 800∘C. The high thermal conductivitycoupled with low thermal expansion and high strength givesthis material exceptional thermal shock resistant qualities.Chemical purity, resistance to chemical attack at temperature,and strength retention at high temperatures have made thismaterial very popular as reinforced material in the research.The properties and DRX pattern of SiC particles are shown inTable 3 and Figure 1, respectively [7].

Table 3: Properties of silicon carbide [15].

Average particle size 15𝜇mMelting point temperature 2700∘CHardness (Vickers) 3100Density (g/cm3) 3.2Crystal structure Hexagonal

Table 4: Mechanical stir casting process parameters.

Casting parameters Parameter settingStirring temperature 700∘CStirring speed 650Time to hold/stirring time 10 minutesBlade angle 45∘

2.2. Fabrication of Metal Matrix Composites

2.2.1. Mechanical Stir Casting Setup and Procedure. Mechan-ical stir casting is a liquid state method for the fabrication ofcompositematerials, inwhich a dispersed phase ismixedwitha molten matrix metal by means of mechanical stirring. Thestir casting setup is shown in Figure 2(a).Three combinationsof reinforcement (5%, 10%, and 15% of SiC) are fabricatedwith aluminiummetal matrix.Themetal matrix is reinforcedwith SiC particle having average particle size 15𝜇m. Siliconcarbide is preheated at 500K for 1 h prior to introduction intothe melt. The temperature inside the furnace is controlledto about 700∘C in order to minimize the chemical reactionbetween the substances. The temperature is controlled byconnecting the relay from the furnace and thermocouple.The function of relay is to cut off the power supply whentemperature goes beyond the 700∘C.The mechanical stirringis used to disperse the silicon carbide particles in matrixalloy. The preheated particles of reinforcement is added tothe melt and stirred at 650 rpm for 10 minutes. The stirringis continued before the composite reaches in mushy zone.The cooling is done in the furnace. Four samples of the metalmatrix composites are shown in Figure 3. Mechanical stircasting process parameters are given in Table 4.

2.2.2. Electromagnetic Stir Casting Setup and Procedure.Figure 2(b) shows the electromagnetic stirring setup for theprocessing of A356/SiC metal matrix composite. A356 alloyis cleaned and loaded in the graphite crucible and heatedto above its liquidus temperature in muffle furnace. Whentemperature of the muffle furnace was recorded 750∘C, theliquid A356 aluminum alloy at a given temperature waspoured into a stainless steel crucible which was packedvery well with the help of glass wool (between crucible andwinding) inside the motor. SiC particles with an average sizeof 15 𝜇m were chosen as the reinforcement particles. Siliconcarbide is preheated at 500K for 1 h prior to introduction intothe melt. The amount of silicon carbide is varied from 0% wtto 15% wt in each matrix. The SiC reinforcing particles wereadded on the surface of themolten liquidA356 at 700∘C to thecrucible.The SiC particles disperse into themeltmaterial.The

International Journal of Manufacturing Engineering 3

Electric motor StirrerSpeed

regulator

Relay

Temperaturecontroller

Muffle furnaceMechanical stir casting setup

(a)

Three phase power

Three phaseVoltage

Vacuum box

Control panel Muffle furance

Glass wool

Three phasemotor

Metal matrixcomposite

stirringtemperature recorder

regulator

supply

transformer

Electromagnetic stir casting setup

(b)

Figure 2: Fabrication of metal matrix composites: (a) by mechanical stir casting (b) by electromagnetic stir casting.

(a)

0% SiC + 100% A356

(a)

(b)

5% SiC + 95% A356

(b)

(c)

10% SiC + 90% A356

(c)

(d)

15% SiC + 85% A356

(d)

Figure 3: Fabricated metal matrix composites by mechanical stir casting.

prepared samples are shown in Figure 4. Electromagnetic stircasting process parameters are given in Table 5.

2.3. Percent Porosity. Porosity (𝑃) is the percentage of thepores volume to the total volume with the volume of asubstance. It is defined by:

𝑃 = (1 −

𝑉Theoretical𝑉Experimental

) × 100%

or, 𝑃 = (1 −𝜌Experimental

𝜌Theoretical) × 100%,

(1)

where 𝑃 is Porosity, 𝑉 is Volume, and 𝜌 is Density.Porosity and characteristics of pores (including size,

connectivity, and distribution, etc.) affect the properties ofmaterials greatly. Generally, for the same material, the lowerthe porosity is, the less the connected pores are. Thus, thestrength will be higher, the water absorption will be smaller,and the permeability and frost resistance will be better, butthe thermal conductivity will be greater.

Table 5: Electromagnetic stir casting process parameters.

Sample number Parameters Values set as1 Voltage supply 180V2 Current 18 Ampere3 Stirring speed 215 rpm4 Stirring time 3 minutes5 Stirring temperature 700∘C

The density measurements were carried out to determinethe porosity levels of the samples. This was achieved bycomparing the experimental and theoretical densities of eachvolume percent SiC reinforced composite. The experimentaldensity of the samples was evaluated by weighing the testsamples. The measured weight in each case was divided bythe volume of respective samples.The theoretical density wasevaluated by using the rule of mixtures given by:

𝜌A356/SiC𝑝 = Vol.A356 × 𝜌A356 + Vol.SiC × 𝜌SiC, (2)

where 𝜌A356/SiCpis density of composite.


(a)

0% SiC + 100% A356

(a)

(b)

5% SiC + 95% A356

(b)

(c)

10% SiC + 90% A356

(c)

(d)

15% SiC + 85% A356

(d)

Figure 4: Fabricated metal matrix composites by electromagnetic stir casting.

4mm4mm

4mm

SiC particles

Porosity

(a) (b)

(c)

Figure 5: Macrostructure of A356/SiC MMC produced by mechanical stir casting process.

4mm4mm

4mm

SiC particles

Porosity

(a) (b)

(c)

Figure 6: Macrostructure of A356/SiC MMC produced by electromagnetic stir casting.


SiC particlesSiC particles

(a) (b)

(c)

100x100x

100x

Figure 7: Microstructure of A356/SiC MMC produced by mechanical stir casting process.

SiC particles

SiC particles

(a) (b)

(c)

100x100x

100x

Figure 8: Microstructure of A356/SiC MMC produced by electromagnetic stir casting.

3. Results and Discussions

3.1. Macrostructure Analysis. Macrostructure of the compos-ites of mechanical stir casting samples (MSCS) and electro-magnetic stir casting samples (ESCS) is shown in Figures5 and 6, respectively. In Figure 5, the macrostructure ofA356/SiCmetalmatrix composites for different percentage ofreinforcement can be seen clearly. The white spots in Figures5(a), 5(b), and 5(c) are the porosity. Metal matrix compositeof electromagnetic stir casting samples (Figure 6) appears tobe well-bonded, clean, and with little evidence of porosity. So

from the point ofmacrostructure, it can be concluded that theelectromagnetic stir casting samples are well-bonded.

3.2. Microstructure Analysis. Figures 7 and 8 show six rep-resentative microstructures for the SiC reinforced A356composites produced by mechanical stir casting process andelectromagnetic stir casting process. The microstructure ofthe composites revealed that SiC particles were not dis-tributed evenly and regional clusters of particles existed inthe matrix, fabricated by mechanical stir casting process.


A356 alloySiC

Porosity

Bonding between

A356 and SiC

1000x

(a)

Good bond SiC

A356 alloy

1000x

(b)

Figure 9: Optical micrograph of A356/SiC composite at higher magnification indicating bond between the matrix alloy (A356) and SiCparticle; (a) mechanical stir casting sample (b) electromagnetic stir casting sample.

2.421.87

3.15 3.33

0.55 0.390.96 1.16

00.5

11.5

22.5

33.5

4

0 5 10 15

Poro

sity

Weight fraction SiC

MSCSESCS

Figure 10: Porosity of MSCS and ESCS.

1000

800

600

400

200

0

20 40 60 80

Angle (2-theta)

Rela

tive i

nten

sity

SiC

SiC Si

CSiC

SiC

Al

Al

Al

Si Si SiAl 4

C 3

(a)

1000

800

600

400

200

0

20 40 60 80

Angle (2-theta)

Rela

tive i

nten

sity

SiC

SiC

SiCSi

C

Al

Al

Al

Si Si Al 4

C 3

Al 4

C 3

(b)

Figure 11: XRD of ESCS and MSCS for 5% of reinforcement.


R46

36

60

10

8 8

Figure 12: Dimension of tensile test specimen [16].

Figure 13: Tensile test samples.

But, in electromagnetic stir casting process, SiC particleswere distributed evenly. In mechanical stir casting processthe nonuniform distribution of the particles is mainly asso-ciated with the segregation effects of SiC particles duringsolidification of the composite. But, in electromagnetic stircasting process, the matrix alloy tends to deform in a plasticmanner.This indicates that the technique electromagnetic stircasting process utilized for the production of the compositeis efficient.

Further, the microphotographs of A356/SiCmetal matrixcomposites produced by electromagnetic stir casting processreveal an excellent bond between the matrix alloy and thereinforcement particles (Figure 9(b)). Figure 9(b) shows thatmatrix alloy tends to deform in a plastic manner in theelectromagnetic stir casting process and produce good bondbetween A356 alloy and SiC. Figure 9(a) shows that bondbetween the matrix alloy and the reinforcement particles isnot strong like electromagnetic stir casting samples. Someporositywas also observed inmechanical stir casting samples.

3.3. Porosity Analysis. Porosity of specimens with differentweight percentages of silicon carbide particles formechanicalstir casting samples (MSCS) and electromagnetic stir castingsamples (ESCS) IS shown in Table 6. It was observed thatby increasing the percentage of reinforcement more than5%, porosity increases. The least porosity was measured in5% reinforcement of SiC in A356 alloy, while the highestporosity was measured in 15% reinforced A356. Based onTable 6, Figure 10 was plotted to show the variation ofporositywith SiCparticles content. Averagely, the porosity forelectromagnetic stir casting samples was lower compared tothe mechanical stir casting samples.

When the external object (stirrer) is placed into themoltenmetal matrix composite in themechanical stir castingprocess, some air penetrates into themoltenmetal along withthe stirrer forming air bubbles.When stirrer is taken out fromthe metal matrix composite after the stirring, at the sametime, air again penetrates into the metal matrix compositeresulting in blow holes to be formed. This formation of airbubbles into the metal matrix composite was because of

232.66

274.23

290.63

309.83

231.33

266.96

280.13

298.2

220230240250260270280290300310

0 5 10 15

Tens

ile st

reng

th (M

Pa)

Weight fraction of SiC

ESCSMSCS

Figure 14: Tensile strength of MSCS and ESCS.

75.66

84.16

96.13

104.66

75.3381.16

92.33

102

707580859095

100105110

0 5 10 15

Har

dnes

s (BH

N)


ESCSMSCS

Figure 15: Hardness of MSCS and ESCS.

placing and taking out stirrer. While in electromagnetic stircasting process, no external object is used. Therefore, thereare minimum chances of porosity observed in electromag-netic stir casting.

3.4. XRD Analysis. In the present work, X-ray diffraction(XRD) was utilized to identify the phases present in the


Table 6: Reduced percentage Porosity of A356/SiC MMC by electromagnetic stir casting process.

Metal matrix composite(A356/SiC)

Theoretical density(g/cm3)

Percentage porosity ofmechanical stir casting samples

Percentage porosity ofelectromagnetic stir casting samples Reduced

percentageporosity

Experimentaldensity(g/cm3)

Percentageporosity

Experimentaldensity(g/cm3)

Percentageporosity

100% A356 + 0% SiCp 2.685 2.62 2.42% 2.67 0.55% 77.27%95% A356 + 5% SiCp 2.71075 2.66 1.87% 2.70 0.39% 79.14%90% A356 + 10% SiCp 2.7365 2.65 3.15% 2.71 0.96% 69.52%85% A356 + 15% SiCp 2.7622 2.67 3.33% 2.73 1.16% 65.16%

Figure 16: Impact strength specimen.

metalmatrix composite. XRDplots between relative intensityand 2-theta (degree) diffraction angle for electromagnetic stircasting sample (ESCS) and mechanical stir casting sample(MSCS) are shown in Figures 11(a) and 11(b). Each figureshows the variation of relative intensity with 2𝜃 diffractionangle for composition of 5% SiC (for 5% SiC, minimumpercent porosity was observed for both ESCS and MSCS).Figure 11(a) clearly shows the peak points corresponding toSiC and Al, respectively. Figure 11(b) shows that the peakpoints corresponding to SiC are very low; it indicates thatdistribution of SiC reinforcement in matrix alloy is notuniform. In the electromagnetic stir casting samples, onlyaluminum and silicon peaks were present. Very few Al

4C3

peaks were found in electromagnetic stir casting sample(ESCS).

3.5. Tensile Strength Analysis. For tensile testing of A356/SiCmetalmatrix compositesmaterial, twenty four samples (threesamples for each percentage of ESCS and MSCS samples)have been prepared as per specification which is shown inFigures 12 and 13. The diameters of the sample preparedare 6mm and gauge length is 36mm. The tensile testingwas carried out on universal testing machine linked to acomputer where data were stored and analyzed by preparingthe specimen of various compositions (Table 7). The tensilesamples were tested at room temperature.

Table 8 shows the effect of SiC particle on the tensilestrength for electromagnetic stir casting samples (ESCS)and mechanical stir casting samples (MSCS). The tensileproperties of the A356/SiC metal matrix composites fordifferent weight fractions (0%, 5%, 10%, and 15%) at ambienttemperature reveal an increase in tensile strength. The graphof the tensile strength of the composites according to the vol-ume fraction of SiC is shown in Figure 14. Results show thatthe tensile strength of electromagnetic stir casting samples

Table 7: Technical data of computerized universal testing machine.

Sample number Parameters Values set as1 Gauge length 25–50mm2 Maximum extension 5mm3 Maximum load 10 T4 Specimen diameter 0.5–30mm5 Strain rate 10−4–10−1 s−1

(ESCS) are higher than that obtained for the mechanical stircasting samples (MSCS). Improved tensile strength for 0%,5%, 10%, and 15% of reinforcement by electromagnetic stircasting process can be seen from Table 8, which is 0.57%,2.65%, 3.61%, and 3.75%, respectively.

3.6. Hardness Analysis. For hardness testing, the samples ofA356/SiCmetal matrix composites have been prepared as perdimension (10mm × 10mm × 25mm). Hardness testing wasdone on hardness testing machine (Table 9). Brinell hardnessof A356/SiC composite is related to the distribution of SiCparticles in the A356 alloy. Theoretically, the hardness of thecast ingot should be uniform from the top to the bottomof theingot.This is due to the uniform distribution of SiC particles.However, other factors such as cooling rate, gravity effect,and nonuniform distribution of the particles in the ingot willgive different values of hardness. Figure 15 shows that thehardness of the ingot is varied according to the distributionof silicon carbide. It can be seen from Table 10 that averagehardness of mechanical stir casting samples for 0%, 5%, 10%,and 15% of reinforcement is 75.33 BHN, 81.16 BHN, 92.33BHN, and 102 BHN, respectively, while average hardness ofelectromagnetic stir casting samples for 0%, 5%, 10%, and15% of reinforcement is 75.66 BHN, 84.16 BHN, 96.13 BHN,and 104.66 BHN, respectively. Improved hardness for 0%, 5%,10%, and 15%of reinforcement by electromagnetic stir castingwas observed 0.43%, 3.56%, 3.95%, and 2.54% respectively.

3.7. Toughness Analysis. According to EN 10045 standard,three specimens from each percentage of reinforcement(10mm × 10mm × 55mm) have been prepared as shownin Figure 16. Toughness was carried out on Charpy ImpactTesting Machine. According to EN 10045 standard, threespecimens from each percentage of reinforcement were


Table 8: Improved tensile strength of A356/SiC MMC by electromagnetic stir casting process.

A356/SiCMMC

Samplenumber

Electromagnetic stir casting samples (ESCS) Mechanical stir casting samples (MSCS) Improvedtensilestrength(MPa)

Tensilestrength(MPa)

Variation(MPa)

Average tensilestrength(MPa)

Tensilestrength(MPa)

Variation(MPa)

Average tensilestrength(MPa)

100% A356 + 0% SiC1 232

4 232.66235

7 231.33 0.57%2 235 2313 231 228

95% A356 + 5% SiC1 274.6

4.7 274.23268.5

14.1 266.96 2.65%2 271.7 2783 276.4 254.4

90% A356 + 10% SiC1 292

2.5 290.63258.8

36.2 280.13 3.61%2 289.5 286.63 290.4 295

85% A356 + 15% SiC1 310

7.5 309.83318.6

46.6 298.2 3.75%2 313.5 3043 306 272

Table 9: Input parameters for hardness.

Sample number Parameter Values set as1 Load applied 100Kgf2 Diameter Of ball 2.5mm3 Testing time 30 seconds

6.33

8

11.5

13.5

6.167.33

10.16

12.33

56789

101112131415

0 5 10 15

Toug

hnes

s (J)


ESCSMSCS

Figure 17: Toughness of MSCS and ESCS.

prepared of square cross-section of size (10 × 10 × 55) withsingle V-notches.

The toughness of A356/SiC composites fabricated bymechanical stir casting process and electromagnetic stircasting process is listed in Table 11. Average toughness ofmechanical stir casting samples for 0%, 5%, 10%, and 15%of reinforcements was observed 6.16 joule, 7.33 joule, 10.16joule, and 12.33 joule, respectively, while average toughnessof electromagnetic stir casting samples for 0%, 5%, 10%, and15% of reinforcement was found to be 6.33 joule, 8 joule,11.5 joule, and 13.5 joule, respectively. Improved toughness ofelectromagnetic stir casting samples for different percentage

R72

All dimensions are in mm

9

27

18

Figure 18: Dimension of fatigue strength test specimen (ASTME466) [16].

Figure 19: Fatigue test specimen.

of reinforcement can be also seen from Figure 17, which is2.68%, 8.37%, 11.65%, and 8.66%, respectively.

3.8. Fatigue Strength Analysis. Fatigue behavior of cast metalmatrix composite can be correlated to its ductility andon strengthening mechanism. Compared to matrix (unre-inforcement alloy), metal matrix composites have longerfatigue life as the ductility is lower. In these cases, a numberof cycles (usually 107) are chosen to represent the fatiguelife of the material. Three representative specimens fromeach percentage of reinforcement were prepared for thetest. Technical data of fatigue testing machine are given


Table 10: Improved hardness of A356/SiC MMC by electromagnetic stir casting process.

A356/SiCMMC

Samplenumber

Electromagnetic stir casting samples (ESCS) Mechanical stir casting samples (MSCS) Improvedhardness(BHN)

Hardness(BHN)

Variation(BHN)

Average Hardness(BHN)

Hardness(BHN)

Variation(BHN)

Average hardness(BHN)

100% A356 + 0% SiC1 76

1 75.6675

1 75.33 0.43%2 76 763 75 75

95% A356 + 5% SiC1 82

4.5 84.1677.5

7.5 81.16 3.56%2 84 853 86.5 81

90% A356 + 10% SiC1 93

5.4 96.1388

9 92.33 3.95%2 98.4 973 97 92

85% A356 + 15% SiC1 101

7 104.6697

10 102 2.54%2 105 1073 108 102

Table 11: Improved toughness of A356/SiC MMC by electromagnetic stir casting process.

A356/SiCMMC

Samplenumber

Electromagnetic stir casting samples (ESCS) Mechanical stir casting samples (MSCS) Improvedtoughness(joule)

Toughness(joule)

Variation(joule)

Average toughness(joule)

Toughness(joule)

Variation(joule)

Average toughness(joule)

100% A356 + 0% SiC1 6.5

0.5 6.336

0.5 6.16 2.68%2 6.5 6.53 6 6

95% A356 + 5% SiC1 7.5

1 87.5

1.5 7.33 8.37%2 8 83 8.5 6.5

90% A356 + 10% SiC1 11

3.5 11.57

5 10.16 11.65%2 10 123 13.5 11.5

85% A356 + 15% SiC1 15

2.5 13.513

8 12.33 8.66%2 13 83 12.5 16

121.33

138

155.66

171.33

120.33

132

148

168.66

110120130140150160

180170

0 5 10 15

Fatig

ue st

reng

th


MSCSESCS

Figure 20: Fatigue strength of MSCS and ESCS.


PorosityCracks

(a) (b)

1000x 1000x

Figure 21: Microstructure of fracture after fatigue test (a) ESCS (A356 with 5% SiC) and (b) MSCS (A356 with 5% SiC).

Table 12: Technical data of fatigue testing machine.

Samplenumber Parameters Values set as

1 Number of cycles 1 × 107

2 Distance of specimenholder Adjustable

3 Cycle speed 1.7Hz4 Test temperature Ambient5 Testing extension ratio 1.6–2.46 Electricity 3 phase, 380 ± 10V, 50/60Hz

in Table 12. The specimens for fatigue test were machinedaccording to ASTM E466 as shown in Figures 18 and 19.Table 13 indicates the fatigue strength of reinforced A356 at0%, 5%, 10%, and 15% SiC particle. The results point outthat higher content of reinforcement particulates producedhigher fatigue strength. Improved fatigue strength for 0%, 5%,10%, and 15%of reinforcement by electromagnetic stir castingprocess can be seen from Figure 20, which is 0.82%, 4.34%,4.92%, and 1.55%, respectively.

The microstructural fracture analysis was carried out toidentify microstructural degradations after fatigue test forESCS (5% SiC) and MSCS (5% SiC). Plastic fractures wereobserved for both ESCS and MSCS, respectively (Figures21(a) and 21(b)).Most of the SiCparticles concentrationswereobserved in the fractured areas for both ESCS and MSCS.However, more porosity and cracks were identified at theplaces of SiC particles for MSCS.

Further, the light microscopy and phase contrast obser-vations were carried out after fatigue test for ESCS (5%SiC) and MSCS (5% SiC), shown in Figures 22 and 23,respectively. From Figures 22 and 23, it can be concluded thatmore discontinuities were observed at the areas of the SiCconcentration for MSCS.

4. Conclusions

A356/SiC (15 𝜇m) composites at different percentage of rein-forcement (0%, 5%, 10%, and 15%) were fabricated at 700∘C

via direct melt reaction method with mechanical stirringand electromagnetic stirring. By studying the properties ofA356/SiC composites produced by mechanical stirring andelectromagnetic stirring, the following conclusions can bedrawn.

(1) Compared with the macrostructure and microstruc-tures obtained bymechanical stirring, themacro- andmicrostructures obtained by electromagnetic stirringwere homogenous.

(2) Measured data with big deviation were observed inmechanical stir casting samples but, in electromag-netic stir casting samples, deviation of measured datawas not so high. This is because of porosity observedin mechanical stir casting samples.

(3) The percentage porosity of mechanical stir castingsamples for the reinforcement of 0%, 5%, 10%, and15% is 2.42, 1.87, 3.15, and 3.33, respectively, whilethe percentage porosity of electromagnetic stir castingsamples are 0.55, 0.39, 0.96, and 1.16, respectively.77.27% (0% SiC), 79.14% (5% SiC), 69.52% (10% SiC),and 65.16% (15% SiC) percentage porosity reduced byelectromagnetic stirring.

(4) The tensile strength by mechanical stirring at 0%,5%, 10%, and 15% of reinforcement is 231.33MPa,266.96MPa, 280.13MPa, and 298.2MPa, respectively,while that by electromagnetic stirring is 232.66MPa,274.23MPa, 290.63MPa, and 309.83MPa, respec-tively. 0.57%, 2.65%, 3.61%, and 3.75% tensile strengthimproved by electromagnetic stir casting process.

(5) From the results, hardness of mechanical stir castingsamples for 0%, 5%, 10%, and 15% of reinforce-ment is75.33 BHN, 81.16 BHN, 92.33 BHN, and102 BHN, respectively. By electromagnetic stirring,0.43%, 3.56%, 3.95%, and 2.54% hardness improved.

(6) The value of toughness of A356/SiC composites bymechanical stirring for 0%, 5%, 10%, and 15% ofreinforcement is 6.16 joule, 7.33 joule, 10.16 joule, and12.33 joule, respectively. By electromagnetic stirring,2.68%, 8.37%, 11.65%, and 8.66% toughness improved.


Table 13: Improved fatigue strength of A356/SiC MMC by electromagnetic stir casting process.

A356/SiCMMC

SampleNumber

Electromagnetic stir casting samples (ESCS) Mechanical stir casting samples (MSCS) Improvedfatiguestrength at1 × 107(MPa)

Fatiguestrength at1 × 107(MPa)

Variation at1 × 107(MPa)

Average fatiguestrength at1 × 107(MPa)

Fatiguestrength at1 × 107(MPa)

Variation at1 × 107(MPa)

Average fatiguestrength at1 × 107(MPa)

100% A356 + 0% SiC1 117

9 121.33118

24 120.33 0.82%2 121 1143 126 129

95% A356 + 5% SiC1 125

29 138114

46 132 4.34%2 135 1223 154 160

90% A356 + 10% SiC1 138

37 155.66132

39 148 4.92%2 175 1413 154 171

85% A356 + 15% SiC1 152

50 171.33144

69 168.66 1.55%2 202 1493 160 213

(a)

25x

(a)

25x

(b)

(b)

Figure 22: Light microscopy of fracture after fatigue test (a) ESCS (A356 with 5% SiC) and (b) MSCS (A356 with 5% SiC).

(a)

200x

(a)

200x

(b)

(b)

Figure 23: Phase contrast of fracture after fatigue test (a) ESCS (A356 with 5% SiC) and (b) MSCS (A356 with 5% SiC).


(7) The significant improvements in fatigue strengthwereobserved by electromagnetic stir casting process.Improved fatigue strength for 0%, 5%, 10%, and 15%ofreinforcement by electromagnetic stir casting processis 0.82%, 4.34%, 4.92%, and 1.55%, respectively.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

References

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