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A.O. Shiba, S. S. Mohamed, T.S. Mahmoud

Influence of Friction STIR Processing on the Microstructural,Hardness and Tribological Characteristics of A356 CastAluminium AlloyFriction stir processing (FSP) is a surface treatment technology that can eliminatecasting defects locally by refining microstructures, thereby improving the mechanicalproperties of material. The present work investigates the effect of FSP processparameters, typically, the tool rotational and transverse speeds on the microstructure,hardness and tribological behavior of A356 cast aluminum alloy. Three differenttransverse speeds, typically, 37.5, 60 and 75 mm/min and three different tool rotationalspeeds, typically, 750, 950 and 1180 rpm were used for FSP. The results revealed thatFSP improved significantly the wear resistance of the surfaces of A356 alloy. It hasbeen found that friction stir (FS) processed regions showed finer grain size, betterhardness and wear resistance than the as-cast A356 alloy. Increasing the tool rotationaland/or the transverse speeds reduces the size α-Al primary grains and increases thehardness and wear resistance of the A356 alloy surfaces.

1. INTRODUCTION

Friction stir welding (FSW), a solid-statejoining process invented at the WeldingInstitute (TWI), UK in 1991, is a techniquefor joining Al alloys [1]. Friction stirprocessing (FSP) is a variation of FSW. Itapplies the same principle; however, itdoes not join materials but modifies themicrostructure of specific areas of thesurface to improve its microstructural andmechanical properties [1,2]. FSP hasshown significant microstructuralrefinement and improved mechanicalproperties of both hypo- and hypereutecticcast Al–Si alloys [3,4]. It has beenreported that, FSP almost eliminates thecasing porosity and cavities, refines thecoarse acicular Si particles in the eutecticstructure as well as the primary Siparticles by breaking them up anddispersing them into the aluminum matrix[3-6]. Such observations show that FSP isan effective technique that can be used tomodify the microstructure in cast Al–Sialloys.

Although there are large number ofinvestigations were conducted to evaluatethe effect of FSP on the mechanical andmicrostructural characteristics of cast

Al–Si alloys [3-8], a fewer investigationswere conducted to evaluate the effect ofFSP on the tribological characteristics ofcast Al–Si alloys [9,10]. For example,Mahmoud et al. [9] reported that thefriction stir (FS) processed samplesexhibited lower wear rate and coefficientof friction than the as-cast hypereutecticA390 Al-Si alloy. Both the wear rate andthe coefficient of friction were found to bereduced by reducing the tool rotationalspeed and/or increasing the tooltransverse speed. Increasing the numberof passes reduces the wear rate as wellas the coefficient of friction. Also, it hasbeen reported that, the FS processedzones exhibited better mechanicalproperties as well as dry sliding wearresistance than the as-cast A413 Al-Sibase alloy.

The aim of the present investigation is touse FSP to modify the surface structure ofA356 hypoeutectic Al-Si cast alloy. Theinfluence of the FSP parameters, typically,the tool rotational speed and tooltransverse speed on the tribologicalbehavior of the modified surfaces wasinvestigated.

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2. EXPERIMENTAL PROCEDURES

The base material used in the currentstudy is A356 cast Al-Si alloy. The A356aluminum alloy has a nominal chemicalcomposition of (in wt.-%) 7.2% Si, 0.29%Mg, 0.02% Cu, 0.01% Mn, 0.18% Fe, 0.01% Zn, 0.12% Ti, 0.02% Ni, 0.05% Sn,0.1% Pb and 92. % Al. The A356 alloywas received in the form of ingots andmachined into thick plates havingdimensions of [50 mm (width) × 200 mm(length) × 10 mm (thickness)]. The toolwas made from H13 tool steel. Aschematic illustration of the used tools isshown in Figure 1. The FSP wasconducted using a vertical CNC millingmachine using three different toolrotational speeds (ω), typically, 750, 950and 1180 rpm and three differenttransverse speeds (υ), typically, 37.5, 60and 75 mm/min. In all experiments, thetool angle was fixed to 3o and the frictionpressure was held constant.

After processing, the test pieces were cutalong the transverse direction andmounted to analyze their microstructurescharacteristics. The microstructuralcharacteristics of the FS-processedregions were investigated using bothoptical metallurgical (OM) microscope andscanning electron microscope (SEM).

Vickers hardness profile of the FS-processed regions were measured, on thecross-section perpendicular to theprocessing direction, using a Vickersindenter with 10 kg load for 10s. Weartests were performed on a pin-on-ringmachine shown schematically in Figure 2.All tests were run under dry slidingconditions. The wear samples have 10mm (length), 10 mm (width) and a heightof 10 mm. The counterface disc wasmade of 316 stainless steel, of nominalcomposition (by wt.-%) 0.08% C, 1% Si,12% Ni, 17% Cr, 2.5% Mo and balanceFe, with a hardness of 184 VHN. Allsamples were tested for 30 minutes at aload of 20 N. The rotational speed of thedisc and wear track diameter were keptconstant at 100 rpm and 200 mm,respectively. A standard wear testprocedure was employed. Before eachtest, the cylinder was cleaned withacetone to remove any possible traces ofoil, grease and other surfacecontaminants. The specimen, which wasalso cleaned with ethanol, was weighedbefore and after the wear tests using anelectronic balance with accuracy of0.0001 g. The dry sliding weight loss wascomputed using the weight loss beforeand after the experiments. The data forthe wear tests was taken from theaverage of three measurements. Frictioncoefficient measurements were conductedusing a force transducer to measure thefrictional force developed on the pinholder and caused by the ring rotation.The coefficient of friction was computedby dividing the frictional force (F) by thenormal load (W). The worn surfaces of thewear samples were examined using SEM.

3. RESULTS AND DISCUSSION

3.1 Macro- and MicrostructuralCharacteristics

Figure 3 shows typical opticalmacrographs of the transverse cross-sections of A356 alloy specimens after

Figure 1: Schematic illustration of the tool used in thepresent investigation (Dimension in mm)

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FSP at different tool rotational andtransverse speeds. The stirred zones (SZ)are clearly visible in the macrograph. Insome specimens, for example in thespecimen shown in Figure 3a, smalltunnel defect (cavity) was observed at thebottom of the advancing sides (AS). Nodefects were observed at the retreatingsides (RS) of the FS- processed regions.

The tunnel defect was observed inspecimens FS-processed at the lowesttool transverse speed (i.e. at 37.5mm/min) and/or the highest tool rotationalspeed (i.e. at 1180 rpm). It is believedthat the formation of the cavities in theFS-processed zone may attribute to thehigh heat input during FSP [3,9,10].Increasing the tool rotational speed and/ordecreasing the transverse speedincreases the heat input. The high heatinput causes turbulent flow of the metal

around the tool pin due to excessplasticization of base metal under the toolshoulder. In this case, defective FS-processed regions are produced. ForFSP, to produce defect free FS-processedzones sufficient frictional heat is to begenerated in the process zone. In thepresent investigation, at the lowest tooltransverse speed (i.e. 37.5 mm/min)and/or at the highest tool rotational speed(i.e. 1180 rpm), the amount of heatgenerated by friction was most likely to behigh which cases tunnel defect. Figure 4shows optical micrographs of themicrostructure of the A356 as-cast alloy. Itis clear that the structure of the A356 alloyconsists of the primary α-Al dendrites andeutectic structure. It has been found thatthe as-cast A356 alloy suffers fromcavities. Two types of cavities, typically,porosity cavities (not shown) and

Figure 2: (a) A schematic diagram of the pin-on-ring wear tester (b) the pin/ring arrangement

Figure 3: Typical macrostructure developed by FSP attool transverse speed and rotational speed of (a) 37.5mm/min and 1180 rpm; (b) 75 mm/min and 750 rpm

Figure 4: The microstructure of the A356 as-cast alloy

(a) (b)

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shrinkage cavities (shown in Figure 4)with irregular shapes were observed inthe microstructure.

Figure 5 shows a sample micrograph ofthe FS-processed region, at theadvancing side, for a sample processed ata tool rotational speed 950 rpm and a tooltransverse speed 60 mm/min. Beside thebase alloy, three regions are clearlyvisible in the micrograph, typically, theheat affected zone (HAZ), the thermo-mechanically affected zone (TMAZ), andthe stir zone (SZ). These regions areformed due to intense plastic deformationat high temperature leading to dynamicrecrystallization and complex materialmixing during FSP [1]. The SZexperiences a combination of thermalcycle and extensive plastic deformationresulting in an extra-fine microstructure.The TMAZ experiences both temperatureand deformation during FSP andcharacterized by a highly deformedstructure. The HAZ is the zone that isbelieved to be mechanical unaffected butonly the thermal effects caused by thefrictional heat generated by the shouldand tool pin rotation. Figure 5 shows alsothat the SZ exhibited finer α-Al grainswhen compared with the base alloy. Thestirring action of the tool during FSPresulted in fragmentation of the coarseprimary α-Al grains to finer grains.Moreover, the stirring action eliminatedthe shrinkage cavities observed in thebase alloy by closing them. Figure 6shows the variation of the average grainsize with the transverse speed at differenttool rotational speeds. The resultsrevealed that, at constant tool rotationalspeed, increasing the transverse speedreduces the average grain size. Forexample, at constant tool rotational speedof 1180 rpm, increasing the tooltransverse speed from 37.5 to 75 mm/minreduces the average grin size from 21.45to 11.54 µm. Also, it has been found that,at constant tool transverse speed,

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increasing the tool rotational speedreduces slightly the average size of the α-A1 grains. For example, at constant tooltransverse speed of 37.5 mm/min,increasing the tool rotational speed from750 to 1180 rpm reduces the averagegrain size from 26.61 to 21.45 µm.According to the aforementioned results, itcan be concluded that increasing the tooltransverse speed and/or reducing the toolrotational speed reduce(s) the averagegrain size of the α-A1 grains at the stirredzones.

3.2 Hardness Measurements

The as-cast A356 alloy exhibited averagehardness of about 60 VHN. Figure 7shows typical example of the transversesection hardness profile of FS-processedregions at constant tool rotational speedof 950 rpm and several tool transversespeeds. The vertical dashed lines indicatethe position of the pin during FSP. It hasbeen observed that the FS-processed

regions exhibited higher hardness whencompared with the base as-cast alloy. Thestirred zones exhibited higher hardnessvalues than both of TMAZ and HAZzones. Such observation was noticed forboth retreating and advancing sides.Figure 8 shows the variation of theaverage hardness of the stirred zoneswith tool rotational speed at different tooltransverse speeds. The results revealedthat increasing the tool rotational speedand/or the transverse speed increase(s)the average hardness of the stirred zones.The enhancement of the mean hardnessof the stirred zones in comparison with theA356 as-cast alloy may attribute to themicrostructural enhancement due to FSPsuch as the grain refinement of the α-A1grains as well as the eliminations ofcasting defects such as cavities andpores. The enhancement of the hardnessof the cast Al-Si alloys due to FSP wasreported by many workers [3-10]. Therelationship between the hardness (Hv) inthe stirred zone and the grain size (d) canbe explained using the Hall–Petchequation [11]:

Hv = H0 + HK d−1/2 …(1)

Where H0 and HK are appropriateconstants. It is clear from equation (1) thatHv is proportional to d−1/2. Accordingly,the finer the grain size is, the higher thehardness value is.

Figure 6: Variation of the size of primary α-A1 grainswith the tool transverse speed at different tool

rotational speed

Figure 5: Optical micrographs of a region FS-processed at tool rotational and transverse speed of

950 rpm and 60 mm/min, respectively

Figure 7: Hardness profiles of transverse section ofregions FS- processed at tool rotational speed of 950

rpm and several tool transverse speeds

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3.3 Tribological Behaviour of FS-Processed Regions

Figure 9 shows typical variation of thecoefficient of friction with sliding distancefor the sample FS-processed at 750 rpmand 37.5 mm/min under drysliding conditions. The A356as-cast alloy exhibited anaverage coefficient of frictionof about 0.135. The FS-processed samples exhibitedlower average coefficient offiction when compared withthe as-cast alloy. Table 1 liststhe values of the coefficient of friction forthe FS-processed samples. The resultsrevealed that at constant tool rotationalspeed increasing the transverse speedreduces slightly the average coefficient offriction. For example, at constant toolrotational speed of 950 rpm, increasingthe transverse speed from 37.5 to 75mm/min reduces the coefficient of frictionfrom 0.138 to 0.133. It has been foundalso that, at constant transverse speed,increasing the tool rotational speed haspractically no effect on the coefficient offriction.

The as-cast A356 alloy showed averagewear rate of about 1.8331×10-3 g/m. TheFS-processed samples exhibited betterwear resistance than the as-cast A356alloy. The maximum average wear ratewas about 1×10-3 g/m for samples FS-processed at 750 rpm and 37.5 mm/min.Figure 10 shows the variation of theaverage wear rate of the FS-processedsamples with the transverse speed atseveral tool rotational speeds. It is clearthat increasing the tool transverse speedand/or the tool rotational speed reducesthe average wear rate. The improvementof the wear resistance of the FS-processed samples compared with the as-cast A356 samples may attribute to the

improvement of the hardness due to FSP.It is well known that increasing thehardness of the materials reduces thevolume of the material removed from thesurface and hence improves the wearresistance of the alloy. In the presentinvestigation, the FS-processed regionsshowed higher hardness that of the as-cast A356 base alloy which assisted inreducing the wear rate (i.e. improving thewear resistance) of the A356 alloy.

Figure 11 shows SEM micrograph of theworn surface of the FS-processed

Transverse

speed (mm/min)

Average coefficient of friction, (µ)

Rotational speed (rpm)

750 950 1180

37.5 0.136 0.138 0.136

60 0.134 0.135 0.134

75 0.132 0.133 0.133

Table 1: Average coefficient of friction for A356 alloy FS- processed samples

Figure 10: Variation of the average wear rate of the FS-processed samples with the transverse speed at

several tool rotational speed

Figure 8: The Variation of the average hardness of thestirred zones with tool rotational speed at different tool

transverse speeds

Figure 9: Variation of the coefficient of friction with thesliding distance sample FS-processed at 750 rpm and

37.5 mm/min

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samples. It has been observed that theworn surfaces FS-processed at high tooltransverse speeds and/or high toolrotational speed exhibited smoother andless damaged (smaller grooves and cracksizes) surfaces (as shown in Figure 11b,11d) than those FS-processed at lowertool rotational and transverse speeds(asshown in Figure 11a, 11c). For example,the grooves on the worn surfaces ofsamples FS-processed at 37.5 mm/minand 750 rpm (Figure 11a) were deep andthe plastic deformation at the edge of thegroove is heavy when compared withsamples FS-processed at the same toolrotational speed but higher tool transversespeed of 75 mm/min (see Figure 11b).and also, the grooves on the wornsurfaces of samples FS-processed at 75mm/min and 1180 rpm (Figure 11d) weresmooth and the plastic deformation at theedge of the groove is small when

compared with samples FS-processed atthe same tool rotational speed but lowertool transverse speed of 37.5 mm/min(see Figure 11c).

4. CONCLUSIONS

From the present investigation, thefollowing important conclusions arederived:

1. The microstructural characteristics ofthe A356 alloy was significantlyimproved using friction stir processing(FSP). FSP assisted in eliminating thestructural cast defects such as porosityand shrinkage cavities. FSP producesfiner α-Al primary grains whencompared with the as-cast A356 alloy.Increasing the tool rotational and/ortransverse speeds reduces the size ofthe α-Al primary grains.

2. The friction stirs (FS) processedregions exhibited higher average

Figure 11: SEM micrograph of the worn surface of samples FS-processed at different tool rotational and transverse speeds(arrows indicate the sliding direction)

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hardness and better wear resistancethan the as-cast A356 alloy. Increasingthe tool rotational and/or transversespeeds increases the hardness andimproves the wear resistance of theA356 alloy.

3. The FS-processed samples exhibitedlower average coefficient of frictionwhen compared with the as-castsamples under dry sliding conditions.Increasing the transverse speedreduces slightly the average coefficientof friction while increasing the toolrotational speed has practically noeffect on the coefficient of friction.

REFERENCES1. R. S. Mishra, Z. Y. Ma, “Friction stir welding and

processing”, Mater. Sci. Eng., R 50, 2005, pp. 1–78.2. R. S. Mishra, M. W. Mahoney, “Friction Stir Welding

and Processing”, ASM International, 2007.3. T. S. Mahmoud, O. M. Shaban, H. Zakaria, T. A.

Khalifa, “On the effect of FSP on the microstructuraland mechanical characteristics of A390 hypereutecticAl-Si alloy”, Mater. Sci. Technol., 26 (9), 2010, pp.1120-1124.

4. Z. Y. Ma, S. R. Sharma, R. S. Mishra, “Effect ofmultiple-pass friction stir processing on microstructureand tensile properties of a cast aluminium–siliconalloy”, Scr. Mater., 54, 2006, pp. 1623–1626.

5. L. Karthikeyan, V. S. Senthilkumar, K. A.Padmanabhan, “On the role of process variables in thefriction stir processing of cast aluminum A319 alloy”,Mater. Design, 31, 2010, pp. 761–771.

6. Z. Y. Ma, S. R. Sharma, R. S. Mishra, “Effect of

friction stir processing on the microstructure of castA356 aluminium”, Mater. Sci. Eng. A, A433, 2006, pp.269–278.

7. K. Nakata, Y.G. Kim, H. Fujii, T. Tsumura, T.Komazaki, “Improvement of mechanical properties ofaluminum die casting alloy by multi-pass friction stirprocessing”, Mater. Sci. Eng. A, 437, 2006, pp.274–280.

8. F. Y. Tsai, P. W. Kao, “Improvement of mechanicalproperties of a cast Al–Si base alloy by friction stirprocessing”, Mater. Letters, vol. 80, 2012, pp. 40–42.

9. T.S. Mahmoud, "Surface modification of A390hypereutectic Al–Si cast alloys using friction stirprocessing", Surface & Coatings Technology, 228,2013, pp. 209–220.

10. T.S. Mahmoud, S. S. Mohamed, “Improvement ofmicrostructural, mechanical and tribologicalcharacteristics of A413 cast Al alloys using friction stirprocessing”, Materials Science and Engineering A,2012, 558, pp. 502–509.

11. Yutaka S. Sato, Seung Hwan C. Park and HiroyukiKokawa, "Microstructural Factors Governing Hardnessin Friction-Stir-Welds of Solid-Solution-Hardened AlAlloys", Metal. Mater. Trans. A, 32A-12, 2001, pp.3033-3042.

Authors

A.O. Shiba (a_naser_o@yahoo.com)Production Technology Department, Faculty of Industrial

Education, Helwan University, Cairo, Egypt

S. S. MohamedMechanical Engineering Department, Shoubra Faculty

of Engineering, Benha University, Cairo, Egypt

T.S. MahmoudMechanical Engineering Department, Shoubra Faculty

of Engineering, Benha University, Cairo, Egypt

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