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Journal of Materials Processing Technology 224 (2015) 117–134

Contents lists available at ScienceDirect

Journal of Materials Processing Technology

jo ur nal ho me page: www.elsev ier .com/ locate / jmatprotec

urface composites by friction stir processing: A review

ipin Sharma, Ujjwal Prakash, B.V. Manoj Kumar ∗

epartment of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee, India

r t i c l e i n f o

rticle history:eceived 17 October 2014eceived in revised form 19 March 2015ccepted 17 April 2015vailable online 2 May 2015

eywords:riction stir processingurface compositesetal matrix composites

ight alloysurface modification

a b s t r a c t

Surface composites are suitable materials for engineering applications encountering surface interactions.Friction stir processing (FSP) is emerging as a promising technique for making surface composites. FSP canimprove surface properties such as abrasion resistance, hardness, strength, ductility, corrosion resistance,fatigue life and formability without affecting the bulk properties of the material. Initially, FSP was usedfor making surface composites in aluminum and magnesium based alloys. Recently surface compositesincluding steel and titanium based alloys have also been reported. While influence of process parametersand tool characteristics for FSP of different alloys has been considerably reviewed during the last decade,surface composites fabrication by FSP and the relation between microstructure and mechanical propertiesof FSPed surface composites as well as the underlying mechanisms have not been wholesomely reviewed.The present review offers a comprehensive understanding of friction stir processed surface composites.The available literature is classified to present details about effect of process parameters, reinforcementparticles, active cooling and multiple passes on microstructure evolution during fabrication of surfacecomposites. The microstructure and mechanical characteristics of friction stir processed surface micro-composites, nano-composites, in-situ composites and hybrid composites are discussed. Considering theimportance of tool wear in FSP of high melting point and hard surface composites, a brief note on toolmaterials and the limitation in their usage is also provided. The underlying mechanisms in strengthening
of friction stir processed surface composite are discussed with reported models. This review has revealedfew gaps in research on surface composites via FSP route such as fabrication of defect-free composites,tailoring microstructures, development of durable and cost effective tools, and understanding on thestrengthening mechanisms. Important suggestions for further research in effective fabrication of surfacecomposited by FSP are provided.
© 2015 Elsevier B.V. All rights reserved.

. Introduction

Surface composites exhibit enhanced characteristics of compos-tes on the surface while retaining properties of the base material.riction stir processing (FSP) is one of the techniques for fabri-ating surface composites and modifying microstructural featuresMishra et al., 2003; Ni et al., 2014). FSP was introduced by Mishrat al. (1999) as an adaptation of friction stir welding (FSW), a pro-ess invented at The Welding Institute (TWI), UK in 1991. Initially,SP was used for producing superplastic aluminum alloys with ultrane grain size and high grain boundary misorientations (Mishra
t al., 1999; Ma, 2008).
Numerous studies have demonstrated that severe plastic defor-ation (SPD) is an effective method of producing ultrafine-grained

∗ Corresponding author. Tel.: +91 8755746590.E-mail addresses: [email protected] (V. Sharma), [email protected]

U. Prakash), [email protected] (B.V.M. Kumar).

ttp://dx.doi.org/10.1016/j.jmatprotec.2015.04.019924-0136/© 2015 Elsevier B.V. All rights reserved.

materials (Valiev et al., 1993; Sabirov et al., 2013). There aremany well established SPD techniques for grain refinement likeequal-channel angular pressing (Valiev and Langdon, 2006), high-pressure torsion (Sakai et al., 2005), multi-directional forging (Sakaiet al., 2008), accumulative roll-bonding (Saito et al., 1999) etc.,while FSP is a relatively late entrant in this list (Kwon et al., 2002).The microstructure evolution during FSP is unique with dynam-ically recrystallized microstructure possessing a large number ofhigh angle grain boundaries (Kapoor et al., 2010). Further, most SPDtechniques modify bulk properties. In contrast, SPD by FSP involvesonly surface modification while the bulk material structure andproperties are retained.

FSP in its simplest form consists of a non-consumable rotatingtool, which is plunged into a work piece and then moved in thedirection of interest. The schematic illustration of FSP is shown in
Fig. 1. The tool serves two primary functions: (a) heating and (b)deformation of work-piece material. The heat is generated mainlyby the friction of the rotating shoulder with the work-piece, whilethe rotating probe or pin stirs the heated material. The heated
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118 V. Sharma et al. / Journal of Materials Processing Technology 224 (2015) 117–134

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Fig. 2. Common methods for placing reinforced particles in the fabrication of surface

Fig. 1. Schematic illustration of FSP technique.

aterial softens and flows around the rotating pin. It then fills theavity at the rear of the tool (Mironov et al., 2008). The materialhat flows around the tool is subjected to severe plastic deforma-ion and thermal exposure, which leads to a significant refinementf microstructure in the processed zone. Stir zone (SZ) refers to theone stirred by the tool probe (Mishra and Ma, 2005). The dynamicecrystallization (DRX) is the main mechanism for the generationf a fine and equiaxed grains in the SZ. However, in high stackingault energy materials such as aluminum and its alloys, the dynamicecovery precedes DRX (McNelley et al., 2008).

Initially, FSP was employed for microstructural refinement ofluminum and magnesium alloys. FSP development has further ledo the successful processing of alloys of copper (Barmouz et al.,011a), titanium (Shamsipur et al., 2011) and steel (Ghasemi-ahrizsangi and Kashani-Bozorg, 2012). FSP has also exhibited

ts efficiency in homogenizing powder metallurgy processed alu-inum alloys (Berbon et al., 2001), microstructural modification

f metal matrix composites (Gan et al., 2010). FSP effectively elimi-ates casting defects (Sun and Apelian, 2011), breaks up or dissolvesecond phase particles and lead to the considerable improvementn properties (Argade et al., 2012).

In the last decade, Mishra et al. (2003) explored the potential ofSP technique in fabricating silicon carbide (SiC) reinforced surfaceomposite layer on aluminum (Al) 5083 alloy. Since then, a vari-ty of surface composites based on magnesium, copper, titaniumnd steel have been developed. However, comprehensive coveragef surface composites prepared by FSP is very limited. The presenteview paper is focused on nano, in-situ and hybrid surface com-osites fabricated by FSP. In this review, recent developments ofSP in fabricating surface composites are discussed. This is followedy discussion on the effect of process parameters of FSP such asool rotational speed, tool traverse speed, number of FSP passes,SP cooling, and tool geometry on microstructure and resultantechanical properties. The challenges and future direction of FSP

re summarized.

. Fabrication of surface composites by FSP

Conventional techniques for fabricating surface compositesnvolves liquid phase processing at high temperatures such as laser

elt treatment and plasma spraying, which may lead to the deteri-ration of composite properties due to interfacial reaction betweeneinforcement and the metal matrix (Pantelis et al., 1995;Mengt al., 2013). Moreover, precise control of processing parameterss required to obtain desired microstructure in surface layer after
olidification. FSP has demonstrated its potential in fabrication of allariants of surface composites with little or no interfacial reactionith the reinforcement. In earlier studies, surface composites were
abricated by applying a layer of ceramic particles slurry in a volatile

composites (a) by groove (b) by drilled holes (c) by using cover plate.

medium (Mishra et al., 2003). Presently, the ceramic particles arereinforced through a machined groove in the specimen plate (Aroraet al., 2011). The major approaches in surface composite fabricationvia FSP are presented schematically in Fig. 2(a–c).

The various steps in groove method are explained in Fig. 2(a).In the first step, the groove is machined on the plate and the rein-forcement particles are filled in the groove. The second step consistsof applying tool without probe (pin) on the groove. The groove iscompletely packed in this step. In the last step, the tool with probeis applied on the packed groove. The dimension, shape and num-ber of grooves can be varied to achieve required volume fractionof the second phases. Surface composite fabrication by drilled-hole method is shown in Fig. 2(b). Li et al. (2013) showed that theintermediate step of packing the groove by probe-less tool can beeliminated by incorporating reinforcement through drilled holes
in fabrication of TiC/Ti-6Al-4V surface composites. The blind holesof 1 mm diameter with varying depth from 0.5-2 mm were drilledon work-piece to accommodate the reinforcement particles. Theloss of reinforced particles during FSP was limited due to sealing

Proces

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alloys like magnesium, copper and titanium alloys heat genera-tion in the processed zone is dependent on rotational and traversespeeds (Azizieh et al., 2011; Barmouz et al., 2011a; Li et al., 2013).

V. Sharma et al. / Journal of Materials

f holes by the half portion of shoulder ahead the travelingin. Akramifard et al. (2014) used two arrays of drilled holes inabricating SiC/Cu surface composites with reduced agglomeratesf SiC particles. A uniform distribution of reinforcement particlesn SZ was observed due to the use of holes. In some studies, a thinheet was covered over the groove or drilled holes to avoid thejection of reinforcement particles as shown in Fig. 2(c).

Lim et al. (2009) used a 1.1 mm thick sheet of AA6111 alloyover plate in multi-walled carbon nano-tubes (MWCNTs) rein-orced AA7075 alloy during processing. Similarly, Avettand-Fènoëlt al. (2014) also used cover plate with thickness of 0.2 mm duringSP in Cu plate reinforced with Y2O3 particles. Huang et al. (2014)sed an approach of direct friction stir processing (DFSP) to fabri-ate surface composite layer on AZ31 matrix. They used hollow androbe-less tool pre-filled with SiC particles. The particles flowedut through the through-hole of 8 mm diameter in the centre ofFSP tool and entered the enclosed space between concave shoul-er and AZ31 work piece. The particles were then pressed into theork piece as the rotating tool transverses along the processingirection.

Several other approaches have also been applied to fabri-ate surface composites. Friction stir surfacing was used byany researchers to fabricate surface composite coating over the

ubstrate. In this process, a consumable tool is filled with rein-orcement particles and is consumed to form layer of compositen the work-piece (Miranda et al., 2013; Gandra et al., 2014).andra et al. (2013) fabricated the functionally graded surfaceomposite layer of Al-SiC by friction stir surfacing. The SiC rein-orcement particles were packed in blind drilled holes in a 20 mmiameter rod of AA6082-T6. The particle packed rod was used forriction stir surfacing of AA2024-T3 alloy. Reinforced layers wereound to be free of inclusions and defects due to non-occurrencef tool wear. However, these layers were found to exhibit vari-ble thickness because the tool consumption increased with rise inemperature.

Applying composite powder coating on the work-piece and sub-equent FSP on the coating is also a new approach for surfaceomposite fabrication. In groove method, the reinforced area isimited to SZ as the distribution of particles is governed by pinather than shoulder. Additionally, there are possibilities of defectsike worm hole or unfilled groove after FSP. The distribution ofarticles into a wider region can be achieved by application of com-osite coating on the alloy plate and subsequent FSP. Zahmatkeshnd Enayati (2010) utilized air plasma spraying technique for depo-ition of 200 m thick composite coating of Al-10% nano-sized Al2O3n AA2024 alloy plate. The coated plates were then subjected toSP to fabricate nano- surface composites. The average thickness ofhe surface composite layer was measured to be 600 �m and Al2O3articles were found uniformly distributed. The maximum micro-ardness of composite layer was measured to be 230 Hv, which isuch higher than that of the AA2024 substrate hardness of 90 Hv.azaheri et al. (2011) also used coating and FSP to fabricate the

l2O3/A356 surface composites. In this study, the A356 chips andl2O3 powder particles were mixed to achieve A356-5 vol. % Al2O3omposition. The composite powders after milling were depositednto grit blasted A356-T6 plates by high velocity oxy-fuel (HVOF)praying. Subsequently, the plates with composite coatings wereubjected to FSP. After FSP, the surface composite layers were wellonded to the aluminium alloy substrates, and no defects werebserved. Hodder et al. (2012) coupled cold spraying and FSP toabricate surface composites. Al-Al2O3 powder mixture was coldprayed on AA6061 alloy and FSP was done on this coated substrate.he highest volume fraction of Al2O3 obtained in the compositesas 48 wt. % and the hardness of this coating increased from 85
V to a maximum observed hardness of 137 HV when FSP waserformed.
sing Technology 224 (2015) 117–134 119

3. Effect of process variables

These fall into three categories of machine variables, tool designvariables and material properties (Fig. 3). The mechanical proper-ties of base materials are decisive in selecting process variables.High heat input is required for high melting point materials likesteel, titanium alloys, copper alloys etc. (Heidarzadeh et al., 2014).Balasubramanian (2008) stated that yield strength (YS), ductilityand hardness of base material are important mechanical proper-ties that control plastic deformation during FSW. In aluminiumalloys with different mechanical properties, it was found that thematerial with lower YS, lower hardness and higher ductility can beprocessed easier than those with higher YS, higher hardness andlower ductility.

Thermal properties of material govern the peak temperatureduring processing. In high thermal conductivity materials, moreheat input is required to obtain defect free processing (Xu et al.,2014). The high thermal conductivity of material would allow moreheat loss by conduction (Khandkar et al., 2003). To manipulate ther-mal properties a backing plate beneath the work-piece was used inFSW (Upadhyay and Reynolds, 2012; Rosales et al., 2010)”.

3.1. Machine variables

Major machine variables are tool rotating rate and tool traversespeed. Tool rotational and transverse speeds determine amount ofheat generated in the work piece (Dolatkhah et al., 2012). Tool tiltangle and penetration depth also affect the formation of SZ, butgenerally these are kept constant. Interaction of rotating tool withwork-piece generates heat due to friction and plastic deformation.The heat input in SZ influences material flow and microstructureevolution which directly affects mechanical and tribological prop-erties. Tool rotational speed and traverse speed determine amountof heat input in the processed zone (Khayyamin et al., 2013). Themaximum temperature observed for various aluminum alloys hasbeen reported to be in a range of 0.6–0.9 Tm. Sufficient amount ofheat generation in SZ is necessary for the formation of defect-freeprocessing zone (Mishra and Ma, 2005). Similarly, in FSP of other

Fig. 3. Classification of FSP process Variables.

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20 V. Sharma et al. / Journal of Materials

Based on experimental observations for Friction Stir WeldingFSW) of aluminum alloys, the following empirical relationshipetween the maximum temperature (Tmax) in SZ and the processingarameters of rotational speed (ω) and traverse speed (�) has beenroposed (Arbegast and Hartley, 1998):

Tmax

Tm= K

(ω2

v × 104

)˛

(1)

here the constant K is reported between 0.65 and 0.75, the expo-ent ranges from 0.04 to 0.06 and Tm (◦ C) is the melting pointf alloy. The ratio between ω2 and � is considered as pseudo heatndex. In FSW of thin AA2024 sheet, Fu et al. (2013) demonstratedhat the relationship between peak temperature (Tmax) and heatndex value is in accordance with Eq. (1), i.e. the peak temperatures proportional to the heat index value. The average heat input pernit length is estimated as per the following according to the modely Frigaard et al. (2001).

= 43

�2 �PNR3

�(2)

here, � denotes friction coefficient, P is the pressure (Pa) Nenotes rotation per second and R is the tool radius (m). Accordingo Eq. (2), the heat input directly relates with the rotational speednd inversely relates to traverse speed. For the given ˛, �, R and Palues, Eq. (2) can be modified as Eq. (3).

∝ ω

�(3)

Chen and Kovacevic (2003) considered both shoulder as well asrobe radius and proposed that rate of heat generation over thentire interface is given by Eq. (4).

= 2�ω�P(Ro3 − ro

3)3

(4)

here, � is coefficient of friction, ω is angular velocity, P is axialressure, R0 is the tool shoulder radius and r0 is the tool pin radius.

.1.1. Effect of tool speedsRotational speed and traverse speed (ω and �) determine the

mount of heat input in the SZ, which in turn affects the microstruc-ure and resultant properties. Lower the heat input, more is therain refinement and vice-versa, but there must be sufficient heatnput to plasticize or soften the material (Moghaddas and Kashani-ozorg, 2013). In surface composite fabrication, higher rotationalpeed is required for distribution and breaking up of clusters of rein-orcement particles. However, high rotational speed affects grainefinement due to high heat input (Azizieh et al., 2011; Barmouznd Givi, 2011b). Thus, rotational and traverse speed must be opti-ized to achieve a defect-free SZ and reduced grain size.In fabrication of SiC/AZ91 composite, it was reported that an

ncrease in rotational speed leads to an increase in grain size whilencrease in traverse speeds leads to a decrease in grain size. This

as explained by noting that increasing traverse speed leads to decrease in the time of exposure to the process heat (Asadit al., 2010). Material flow in SZ enhances with increase in rota-ional speed. As the rotational speed increases, the alumina particleluster size in the Al2O3/AZ31 surface composite also decreasedAsadi et al., 2011). Increased heat input due to high rotationalpeed increases the grain size but nano-particles of alumina areomogeneously distributed due to shattering effect of rotation. Theuccessful combinations of rotational and traverse speeds reportedor surface composites are listed in Table 1.

Kurt et al. (2011) showed that increasing rotating tool speed andraverse speed caused a more uniform distribution of SiC particlesn AA1050 alloy surface composite. However, at very high trans-erse speeds, the surface composite layer was weakly bonded with

sing Technology 224 (2015) 117–134

the aluminum alloy substrate. Khayyamin et al. (2013) used tra-verses speeds of 20, 40 and 63 mm min-1 in fabrication of SiO2/AZ91nano-composite. They observed that the increase in traverse speedrefined the grain size in the SZ and resulted in an increase in hard-ness.

Morisada et al. (2006a) investigated the role of traverse speedin fabrication of MWCNTs reinforced AZ31 alloy surface compos-ites. The tool rotating speed was kept constant at 1500 rpm and thetravel speed was varied from 25 to 100 mm/min. At traverse speedof 100 mm/min, the MWCNTs were not dispersed uniformly andremained entangled. As traverse speed decreased to 25 mm/min, animproved dispersion of the MWCNTs was achieved without aggre-gated MWCNTs. This was attributed to a more suitable viscosity inthe AZ31 matrix at a lower traverse speed. Shahraki et al. (2013)observed that the distribution of ZrO2 nano-particles in the SZ ofAA5083 alloy was not uniform at low rotational speeds or hightraverse speeds. A combination of low rotational speed and hightraverse speed results in poor plastic flow of the material, poor dis-tribution of particles, and formation of porosities. Further, at lowrotational speeds, the amount of heat produced is not sufficient forthe material to become soft enough to attain high traverse speeds.

In fabrication of in-situ surface composites, the temperature andduration of high temperature exposure are decisive parameters forthe chemical reactions which are dependent on rotational and trav-erse speed. Lower rotational speed provides less amount of heat andhigh traverse speed reduces the duration of high temperature expo-sure. Chen et al. (2009) opted for lower rotational speed of 500 rpmin the fabrication of in-situ composites from Al-CeO2 system dueto exothermic reaction. A lower traversing speed resulted in a finerand uniform size of in-situ formed particles due to increase in thestirring period. High traverse speed provides less strength whichcan be related to the relatively larger size and lower amount ofin-situ formed particles. This was attributed to the shorter reac-tion time and lower reaction temperature associated with a highertool traversing speed. Zhang et al. (2014) demonstrated that theeffect of traverse speed is more significant than rotational speedin the fabrication of in-situ surface composites in Al-TiO2 system.At a relatively high traverse speed, only partial Al-TiO2 reactiontook place due to lower temperatures and smaller strains in the SZ.The number of Al3Ti and Al2O3 particles increased with decreasingtraverse speed. On the other hand, rotational speed caused littleinfluence on the size and number of particles. The lower trav-erse speed resulted in increased temperature which aided diffusionand also increased the stirring time. Similarly, You et al. (2013a)reported that chemical reaction in Al-SiO2 system was enhancedby reducing the tool traverse speed as it provides longer processingtime.

Barmouz et al. (2011a) showed that effect of traverse speed ongrain size was reversed in the fabrication of Cu/SiC surface com-posites as compared to FSP of the material. The SiC particles tendto agglomerate in the SZ on increasing traverse speed. As traversespeed decreased, the grain size decreased in composite, whereasin FSP of Cu without SiC particles grain size increased with thedecrease in traverse speed because of increase in heat input. Thismay be attributed to the uniform dispersion of SiC particles in lowertraverse speeds which enhance the pinning effect of SiC particlesin the SZ. Salehi et al. (2012) applied Taguchi design approach todetermine the factors which influenced ultimate tensile strength(UTS) of AA6061/SiC nano-composites produced by FSP. It wasfound that rotational speed is the most influential process parame-ter with 43.70% contribution followed by transverse speed 33.79%,pin profile 11.22% and tool penetration depth 4.21% respectively.

3.1.2. Effect of multiple passesClustering of reinforcement particles is a major issue in compos-

ite fabrication as it adversely affects the strength of composites.

V. Sharma et al. / Journal of Materials Processing Technology 224 (2015) 117–134 121

Table 1Successful combination of rotational and traverse speed for surface composite fabrication.

Surface composite Rotational speed (rpm) Traverse speed (mm/min.) Ref.

(SiC+MoS2)/A356 1600 50 Alidokht et al. (2011)SiC/AZ91 900 63 Asadi et al. (2010)Al2O3/AZ31 800 45 Azizieh et al. (2011)SiC/Cu 900 40 Barmouz et al. (2011a)Nano-clay/Polymer 900 160 Barmouz et al. (2011c)TiC/Al 1000 60 Bauri et al. (2011)SiC/AA5052 1120 80 Dolatkhah et al. (2012)(SiC + Al2O3)/AA1050 1500 100 Mahmoud et al. (2009b)Al2O3/AZ91 1600 31.5 Faraji et al. (2011)HA/Ti-6Al-4V 250 16 Farnoush et al. (2013)TiC/Steel 1120 31.5 Ghasemi-Kahrizsangi and Kashani-Bozorg (2012)SiO2/AZ91 1250 63 Khayyamin et al. (2013)SiC/AA1050 1000 15 Kurt et al. (2011)MWCNT/AA1016 950 30 Q. Liu et al. (2013)Ni/AA1100 1180 60 Qian et al. (2012)(TiC + B4C)/AA6360 1600 60 Rejil et al. (2012)SiC/AA6061 1600 40 Salehi et al. (2012)B4C/Cu 1000 40 Sathiskumar et al. (2013)SiC/Ti 800 45 Shamsipur et al. (2011)(SiC+MoS2)/AA5083 1250 50 Soleymani et al. (2012)

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Al2O3/AA2024 800 25

Al2O3/AA6082 1000 135

Cu/AA5083 750 25

lustering is more pronounced in nano-sized reinforcementsSharifitabar et al., 2011). Multi-pass FSP leads to a reduction in sizef cluster and uniform distribution of reinforcement particles andhus decreases the grain size of matrix. In FSPed surface composite,he reinforcement particle distribution mainly depends on num-er of FSP passes (Zohoor et al., 2012). The electron backscatteredEBSD) images of TiC/aluminum in-situ composite, (Fig. 4(a and b))ecessarily indicate the influence of number of FSP passes on grainefinement. This can be attributed to the higher grain boundary areavailable after each FSP pass as grain boundaries and interfacesetween the particles and the matrix are sources of dislocations.he deformation during subsequent FSP pass generates more dis-ocations from these sources, while high stacking fault energy (SFE)f aluminum leads to dynamic recovery and formation of low angleub-grain boundaries (Bauri et al., 2011).

In SiO2/AZ61 nano-composites fabrication, Lee et al. (2006)bserved that the particles cluster size ranged from 0.1 to 3 �mfter one pass of FSP. After four passes of FSP, the cluster size
educed to 150 nm and grain size decreased to 0.8 �m. TheS increased to 225 MPa in nano-composite layer, as comparedo 140 MPa in as-received AZ61. In Al2O3/AZ91 surface nano-omposite fabrication, Asadi et al. (2011) stated that the Al2O3
ig. 4. Effect of passes on refinement of grains as indicated by EBSD images of the Al-TiC i

Zahmatkesh and Enayati (2010)Shafiei-Zarghani et al. (2009)Zohoor et al. (2012)

cluster size in surface composite decreased to about 500 nm aftersix passes of FSP as compared to cluster size of 7 �m after one passof FSP. The UTS of the composite after eight passes increased to244 MPa, as compared to 128 MPa of as received alloy. Similarly,Sharifitabar et al. (2011) observed the particle clusters of meansize of 650 nm in Al2O3/AA5052 surface nano-composite after onepass FSP. After four passes of FSP, the mean cluster size reduced to70 nm, and showed improvement in YS and UTS of the composite.Barmouz and Givi (2011b) showed that after one pass of FSP thebonding between SiC particles and Cu matrix was weak and the SiCparticles were surrounded by pores. Formation of pores increasedthe porosity of composite to 19% as compared to 2.9% as that ofmatrix. The porosity content decreased on increasing the numberof passes. After eight passes of FSP, the porosity of the compositereduced to 5% due to better interfacial bonding.

The in-situ reactions in the fabrication of surface compositeby FSP are highly dependent on heat generated and duration ofprocess (Qian et al., 2012). In fabrication of nickel powder rein-
forced AA1100 alloy surface composite, Qian et al. (2012) observedthat formation of Al3Ni in-situ particles increased with increase inthe number of passes. The increased number of passes enhancedthe in-situ reaction owing to increase in processing time. By
n situ composite subjected to (a) single and (b) double pass FSP (Bauri et al., 2011).

1 Processing Technology 224 (2015) 117–134

mtd1tFi(

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ultiple-passes, the contact area of Al-Ni interface increased dueo break-up of the original Ni particles. After six passes, the uniformispersion of Al3Ni particles in the composites resulted in 271% and87% increase in the hardness and UTS of the composites, respec-ively. In surface composites fabrication, increasing the number ofSP passes leads to a better distribution of reinforcement particlesn the matrix, finer grains, higher hardness, strength and elongationAsadi et al., 2011).

.1.3. Direction of tool rotationIn surface composites, microstructural inhomogeneity has been

bserved as particle segregation with banded structure in the SZegion. Guo et al. (2014) showed that in nano-sized Al2O3 rein-orced in AA6061 alloy by FSP the particle distribution remainedon-uniform after two passes of FSP. The nano Al2O3 particles tendo form macro-bands, with the areas between those bands are freef particles. Dolatkhah et al., 2012 observed that nano SiC parti-les tend to migrate towards advancing side due to asymmetricaterial flow pattern in FSP of 5052 Al alloy. The microstructural

nhomogeneity in FSP can be minimized by changing the directionf rotation of tool after each pass. Asadi et al. (2012) investigatedhe role of tool direction microstructure and mechanical prop-rties of FSPed AZ91. Results showed that a more homogeneousicrostructure can be obtained by changing rotational direction

RD) of tool. Reversing the RD decreases the difference betweenhe advancing side (AS) and the retreading side (RS) hardness val-es as the microstructural homogeneity is increased. Similarly,ejil et al. (2012) have applied two passes in opposite directionso achieve better distribution of ceramic particles in fabricationf (TiC+B4C)/AA6360 hybrid surface composite layer. Izadi anderlich (2012) found that a layer of unreinforced material (flowrm) transferred from the retreating to the advancing side afterwo passes in the fabrication of MWCNT/AA5059 alloy surfaceomposites. A third pass was performed with the tool rotationeversed to suppress the formation of the flow arm and homog-nize the distribution of reinforcing material. Mahmoud et al.2009b) observed a difference in reinforcement density in the SZ ofSiC + Al2O3)/AA1050 hybrid surface composites. After single pass,here was an increase in number of particles on the top portion ofS while RS and the central top portions were observed with lessmount of reinforcement particles. The reinforcement density onhe advancing side became almost similar to that on the retreatingide after three passes with opposite direction of tool rotation inhe second pass.

.2. Tool design variables

.2.1. Tool geometryTool geometry mainly includes shoulder diameter, shoulder fea-

ure, probe shape, probe size and probe feature. Flow of plasticizedaterial in processed zone is affected by tool geometry as well as

raverse and rotational motion of the tool (Palanivel et al., 2012;ohoor et al., 2012). Tool geometry is an important aspect of FSP as itffects heat generation, material flow and resultant microstructureMishra and Ma, 2005). General tool geometry of FSP tool is shownn Fig. 5. Commonly, the shoulder of tool having concave shapedrofile is used as it serves as an escape volume or reservoir for theisplaced plasticized material from the probe. Tilt angle is neces-ary to maintain the material reservoir beneath the tool and enableshe trailing edge of the shoulder tool to extrude the processed mate-ial. Tilt angle of 1–3◦ is required for effective processing of the
aterial. The tool shoulder is responsible for heat generation. Use of
arge diameter shoulders led to high heat generation and enhancedaterial flow whereas smaller diameters resulted in formation of

efects in the composite (Elangovan and Balasubramanian, 2008).

Fig. 5. Tool geometry of FSP tool.

Some critical issues related to FSW or FSP tools are briefly discussedin previous review articles (Rai et al., 2011; Y. Zhang et al., 2012a,b).

Various tool probe geometries like square, triangular, cylindri-cal, threaded, conical etc. have been adopted for surface compositefabrication (Fig. 6). Threaded cylindrical, square and triangu-lar probe profiles have been used more extensively. The probeouter surfaces can also have different shapes and features includ-ing threads, flats or flutes. Thread-less probes are suitable forprocessing of harder alloys or metal matrix composites as thethreaded features can be easily worn away.

Elangovan et al. (2007) observed in FSW of AA6061 alloy thata square pin profile produces more pulse/sec as compared totriangular pin profile and no such pulsating action was observed incylindrical, tapered and threaded pin profiles. The square pin pro-file produces 80 pulses per second (pulses/s = rotational speed inseconds × number of flat faces) and triangular pin profile produces60 pulses per second at the tool rotational speed of 1200 rpm.More pulsating action generated by square pin profile can producesmaller grains with uniformly distributed fine precipitates whichin turn yield higher strength and hardness. However, Faraji et al.(2011) observed that the grain size and particle size in the speci-men produced by the triangular tool is smaller than that of squaretool in the fabrication Al2O3 reinforced as-cast magnesium alloyAZ91 composite. They attributed this to the sharp edges of triangu-lar pin which results in better stirring of the material. Mahmoudet al. (2009a) showed that in surface composite fabrication ofSiC reinforced aluminum alloy, the square probe resulted in more
homogeneous distribution of SiC particles than the cylindrical andtriangular shape tools. The tool wear rate of flat faces (square andtriangular) probes was higher than that of circular probe. It was


F tom pw

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ig. 6. The tool probe geometry of commonly used tools in FSP (a) conical round botith flutes (e) triangular probe (f) square probe.

lso observed that sharp corners of the probe were deterioratedapidly and the tool worn material reacted with the aluminumo form fine iron aluminides. Padmanaban and Balasubramanian2009) found that a pin with a screw thread generates more heathan the pin without a screw thread in FSW of AZ31 alloy. More heatnput can improve the material flow in the SZ. Moreover, the screwhreaded probe exerts an extra downward force that will be usefulo accelerate the flow of the plasticized material. The effect of toolhoulder (D) to probe diameter (d) ratio i.e. D/d was also studied,nd a D/d ratio of 3 was found to be better in producing defect-ree SZ. Similarly, Azizieh et al. (2011) in fabrication of Al2O3/AZ31ano-composite found that threaded columnar probe tool pro-uced composite without defects as compared to non-threadednd three-fluted columnar probe tools due to better material flow.u et al. (2011) developed a three-dimensional transient compu-ational fluid dynamics (CFD) model to investigate the materialow and heat transfer during FSP with the threaded/non-threadedin in AZ31B magnesium alloy. A comparison of the threaded andon-threaded models showed that the thread strongly influencedhe temperature distribution, material flow velocity and strain rateear the tool pin within the SZ.

.2.2. Tool material and wearIn FSP, high temperature and load experienced by tool results in

ignificant material damage of tool. Tool wear is the most criticalssue in fabrication of surface composites due to hard reinforcementarticles. Tool materials generally used for light alloys are variousard steels like H13 steel, whereas tungsten based alloys, cermetsWC-Co) and Poly cubic boron nitride (pcBN) are used for FSPf harder materials. Some other tool materials including iridium-henium (Ir-Re), tungsten-rhenium (W-Re), cobalt (Co) alloys andungsten carbide (WC) are also successfully employed. pcBN is pre-erred friction stir tool material for hard alloys such as steels anditanium alloys due to its high mechanical and thermal perfor-

ance. However, high cost and low fracture toughness of pcBNequires immediate attention to develop cost effective and durableools (Rai et al., 2011).

Ghasemi-Kahrizsangi and Kashani-Bozorg (2012) observed thatungsten carbide (WC) tool utilized for fabricating TiC/steel sur-ace composite showed significant tool wear (20% reduction inimensions) just after 400 mm of processing. Farias et al. (2013)eported that the severe tool wear caused loss of surface qual-ty and inclusions of tool material (WC) inside the work-piece inSW of Ti-6Al-4V. They found adhesion of the work-piece mate-ial on the tool pin and shoulder surface, and diffusion of tool and
he work-piece materials constituents. Morisada et al. (2010) suc-essfully FSPed thermally sprayed cemented carbide (WC-CrC-Ni)ayer on SKD61 steel by using a sintered cemented carbide (WC-o) tool. After processing the defects in the cemented carbide layer
robe (b) columnar probe (c) threaded columnar probe (d) threaded columnar probe

disappeared and the hardness of the cemented carbide layerincreased to ∼2000 HV, which was nearly 1.5 times higher thanthat of the as-sprayed cemented carbide layer. Swaminathan et al.(2009) utilized tungsten based alloy Densimet (W-7% Ni, Fe) tool forFSP of NiAl bronze without any significant wear of tool. Grewal et al.(2013) FSPed hydroturbine steel (13Cr-4Ni) by using a WC tool andno wear of tool material was observed. Feng et al. (2008) indicatedthat a deleterious phase of Cu2FeAl7 was formed due to wear of steeltool in FSW of SiC/AA2009 bulk composite. Even slight wear of toolresulted in a decrease in strength of the friction stir welded com-posite after T4 condition due to formation of the Cu2FeAl7. Londonet al. (2005) used tools made of pcBN and tungsten-rhenium alloy(W-Re) for FSP of Nitinol (49.2% Ti, 50.8% Ni). No noticeable wearof the tools was observed.

Xue et al. (2014) successfully FSPed nickel (Ni) by utilizing thetool made of common tool steel (M42) using additional water cool-ing. The FSP of Ni exhibited defect-free processing surface and noobvious tool abrasion was observed. However, when the FSP pro-cess was performed without water cooling, the tool failed quicklyafter the shoulder touched the work-piece due to greatly increasedtemperature. The tool wear occurred more on the specific locationsin FSW of Al-Si alloy matrix composite reinforced with 30 vol. % ofSiC by the threaded tool of WC-Co hard alloy (Liu et al., 2005). Itwas observed that at lower welding speed the wear rate of toolis more as compared to higher welding speed. The wear of thepin was different at different lengths of the pin, and the maxi-mum wear was found at about one-third of pin length from the pinroot. Prado et al. (2003) reported that in FSW of AA 6061-20%SiCmetal matrix composite (MMC) the wear of tool steel threaded pintool changed the tool into a smooth self-optimized shape (withoutthreads). They observed that the worn tool or self-optimized shapewith no threads resulted in homogenous and integral welds with-out further significant tool wear. These observations suggest thattool wear can be greatly reduced even for MMCs when using theoptimized tool shape.

Rai et al. (2011) pointed out that the tungsten, molybdenum,and iridium are suitable options of tool materials as they possesshigh hardness, low reactivity with oxygen and high temperaturestrength. These tool properties can be enhanced further by theaddition of alloying elements or coating the tool with a hard andwear resistant material. Tool wear in the FSW of titanium andhigh strength steel was minimized by the use of tungsten-rhenium(W-25%Re) alloys, but high tool costs due to the rare rhenium com-ponent limits its applicability. Thompson and Babu (2010) studiedthe tool life of W-1% La2O3, W-25%Re and W-20%Re-10%HfC for
joining high strength steel. Tool material with HfC was found tobe more resistant to tool degradation as HfC particles effectivelyprevent softening of the tool material and supports the surround-ing W-Re grains. Miyazawa et al. (2011) investigated iridium alloy

1 Proces

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ased tool material for FSW of SUS304 stainless steel with additionf various high melting point metals. A higher softening temper-ture (recrystallization temperature) for Ir-1 at% Re was observedhan any other alloy. Additives such as Ta, W, Nb and Mo lower the

elting points of the Ir alloys. Further, high temperature hardnessnd compressive strength increase with increasing Re concentra-ion. The Ir-10 at% Re tool material exhibits less wear comparedo other tool materials. Buffa et al. (2012) studied tungsten basedlloys for FSW of Ti-6Al-4 V alloy. They used tool material of WC-.2%Co, WC-12%Co and a W-25%Re alloy in this study. The tool maderom WC-4.2% Co exhibited high hardness, but the tool failed veryarly by fracture. In WC-12%Co tool, deformation was observed inhe shoulder and at the pin, together with adhesive wear degra-ation. W-25%Re tool showed the lowest wear degradation and

longer life. Sato et al. (2011) developed a Co-based tool alloyor FSW of high strength materials such as steel and titanium.he Co-based tool was strengthened by precipitating intermetallico3(Al, W) at high temperatures. In FSW of Ti-6Al-4V alloy withhis tool, the tool wear was low and the good quality weld wasbtained.

The hard coatings processed by physical vapor depositionnd chemical vapor deposition techniques are efficient means toncrease the durability of tools made from cemented tungsten car-ide. The hard material coated layer improves wear resistance,educes adhesion between the tool material and work piece, andcts as a diffusion barrier (Batalha et al., 2012). Batalha et al. (2012)valuated the wear performance of a physical vapor depositionoating of (Al, Cr) N material on cemented carbide (WC-Co) toolor FSW of Ti alloy sheets. The coating was not found on the toolfter FSW and also tool material exhibited wear. Lakshminarayanant al. (2014) studied the effect of various refractory ceramic basedomposite coatings on the Inconel 738 alloy tool for FSW of AISI 304rade austenitic stainless steel. The impact and wear resistance of0% B4N hard faced tool by plasma transferred arc were superiorompared to other hard faced tools, which resulted in debris freeeld nugget.

.3. Effect of cooling

Nano-sized grains can be achieved by proper cooling arrange-ents. Cooling during FSP also serves the additional function of

educing tool wear (Xue et al., 2014). In a study of FSPed intersti-ial free (IF) steel, FSP followed by quenching resulted in formationf a nano-grain layer of 150 �m thickness and 50–100 nm averagerain size. The post-FSP cooling resulted in effective control of grainize by arresting grain growth (Chabok and Dehghani, 2012). Therain size of AA6061-T6 reduced from 50 �m to 100–200 nm byubmerged friction stir processing (SFSP) (Hofmann and Vecchio,005). SFSP was performed in a tank filled with water at a highraverse speed to refine grains to nano-regime. The faster coolingate achieved through SFSP is believed to retard grain growth.

While few studies relating to use of coolant during FSP of variouslloys are reported, investigations on coolant application in fabri-ation of surface composites are rarely available. Najafi et al. (2008)bserved grain size of 800 nm in SiC/AZ31 surface composite fab-icated by single FSP pass using a coolant mixture of methanol andry ice. The grain size of 1 �m was observed in FSPed AZ31 with-ut SiC particles. AZ91 based hybrid composites with Al2O3 andiC as reinforcements showed increased hardness nearly by 100%.ooling was achieved by circulating coolant below the workpiece.hile FSPed composite without cooling showed grain size of nearly

.1 �m, FSP using undersurface cooling resulted in an average grain
ize of 2.4 �m (Arora et al., 2012). Zhang et al. (2011) applied twodditional passes in flowing water on FSPed in-situ Al3Ti and Al2O3ano-particles reinforced Al composites. Four passes were applied
n air and then two passes were applied in flowing water. They


found that the average grain size was decreased to 602 nm afteradditional passes by application of water. Initially, the average grainsize after four passes of FSP in air was 1285 nm. Thus, the effectof coolant on grain refinement in composites is limited probablybecause of the fact that reinforcement also plays a major role inrestricting grain growth.

Sharifitabar et al. (2011) did not cool the sample to room tem-perature after each FSP pass in the fabrication of Al2O3/AA5052surface nano-composite. A uniform distribution of particles as wellas reduction of cluster size was achieved, but grain growth alsoaccompanied. The sample was heated in the first FSP pass and inthe subsequent passes, sample was in preheated condition. Pre-heated sample was softer which allowed more stirring action thatcaused better distribution of particles and cluster size reduction.The average grain size of the matrix was 940 nm which was higheras compared to that reported in similar studies but cluster sizereduced. Asadi et al. (2012) showed that water coolant favouredthe formation of oxides on the surface of AZ91 during FSP and theseoxides dispersed into SZ after subsequent passes.

4. Effect of reinforcement particle on grain refinement

Surface composites show the synergistic effect of grain refine-ment by FSP and reinforcement particles. Zener pinning dictatesthat in the presence of dispersed second phase particles, the move-ment of grain boundaries that are migrating due to recrystallizationand grain growth may be pinned by the small second phase parti-cles (Rohrer, 2010). The Zener limiting grain size (dz) expressed inEq. (5) below:

dz = 4r/3Vf (5)

where, r and Vf are the radius and volume fraction of second phaseparticles, respectively.

According to Eq. (5), the grain refinement by reinforcement par-ticles increases with the decrease in particle size and increase involume fraction of the particles. The schematic representation ofpinning by reinforcement particles is shown in Fig. 7. Shamsipuret al. (2011) found that nano-sized SiC particle reinforcement intitanium refined grains up to average size of 400 nm, as com-pared to 4 �m without SiC particles. Morisada et al. (2006b) alsoobserved reduction of grain size in SZ of AZ31alloy reinforced withSiC particles to 2 �m size. Without SiC particles the grain refine-ment was limited to 5 �m for the same processing conditions. TheSiC particles also restrict abnormal grain growth. As depicted inthis study, the FSPed sample without SiC particles shows abnormalgrain growth (AGG) above 573 K whereas the FSPed sample withSiC particles resists AGG up to 673 K due to pinning effect of SiCparticles.

Sathiskumar et al. (2013) reported that in Cu/B4C surface com-posite, the SZ area with B4C particles was reduced as compared tothe SZ area in FSP of copper without particles. The area of the SZwithout particles was found to be 44 mm2 and reduced to 24 mm2

at 24 vol. % of B4C particles. SZ area reduction was due to resis-tance offered to the flow of plasticized copper by non-deformableB4C particles. Surface composites reinforced with different carbonnano-structures exhibited significant reduction in grain growthduring FSP. Morisada et al. (2006c) reported ∼100 nm size grainsfor AZ31/C60 surface composite fabricated by FSP. The C60 moleculewas uniformly dispersed in the matrix. The FSP and pinning effectrefines grain size which improves hardness up to three times ascompared to the base alloy. Increasing the volume fraction and
employing multi-pass FSP enhanced the pinning effect (Liu et al.,2012).
Shafiei-Zarghani et al. (2009) related the experimentallyobserved grain size with the Zener limiting grain size (dz) in


grain growth by reinforcement particles.

AgcptiHsIrppogAtc

5

spcgsehcmnm(ctscf

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Fig. 8. The improvement in micro-hardness of surface composites fabricated viaFSP.

Fig. 7. Schematic of pinning of the

A6082/Al2O3 nano-surface composites fabricated via FSP. Therain size in the composite was found to be higher than dz as aertain level of local clustering is inevitable, and also not all nano-articles can effectively pin the grain boundary migration. Further,he difference in the composite grain size and dz was much highern single pass due to non-uniform distribution of nano-particles.owever, Ghasemi-Kahrizsangi and Kashani-Bozorg (2012) found

teel/TiC nano-composite grain size to be consistent with the dz.n steel/TiC surface composite, the TiC nano-sized particle of 70 nmefined grains to 600 nm as compared to grain size of 5 �m withoutarticle addition. Asadi et al. (2011) showed the effect of differentarticles and their distribution behavior on the grain refinementf the AZ91 reinforced with nano-size SiC and Al2O3 particles. Therain size after eight passes of FSP was 800 nm for SiC and 1.3 �m forl2O3 reinforced surface composite. This was attributed to the bet-

er wetting behavior and distribution of particles without forminglusters in case of SiC particles.

. Strengthening mechanisms

Strengthening mechanisms operative in surface composites areimilar to those in bulk metal matrix composites. Lloyd (1994)ointed out the four major strengthening mechanisms in parti-le reinforced metal matrix composites: Orowan strengthening,rain and sub-grain boundary strengthening (Hall-Petch relation-hip), strengthening from difference in coefficient of thermalxpansion (CTE) between the particles and the matrix, and workardening resulting from the strain misfit between the parti-les and the matrix. Similarly, Kim et al. (2013) indicated threeajor mechanisms for strength enhancement in metal matrix

ano-composites (MMNCs): Orowan strengthening, grain refine-ent, and CTE mismatch strengthening. Sanaty-Zadeh and Rohatgi

2012) experimentally validated the model for magnesium nano-omposites reinforced with Al2O3 and Y2O3 particles. It was foundhat Hall-Petch strengthening is the most important factor fortrength contribution. The CTE and Orowan strengthening effectontribution to strength increases with an increase in the volumeraction of reinforced ceramic particles.

The enhancement of mechanical properties exhibited by surfaceomposites is shown in Figs. 8 and 9. Fig. 8 shows that the increase inicro-hardness of the surface composites fabricated via FSP. This is

ttributed to grain refinement and incorporation of reinforcementarticles. The increase in strength of surface composites is shown

n Fig. 9, which highlights the potential of FSP in enhancing thetrength. The observed decrease in UTS for SiC/Cu composite wasttributed to the non-uniform distribution and agglomeration ofiC particles (Barmouz et al., 2011a). Fig. 9. The ultimate tensile strength of surface composites fabricated via FSP.

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.1. Strengthening models

Liu et al. (2012) proposed a model for strengthening inNT/AA2009 alloy surface composite by considering load bearingnd grain refinement strengthening effect. The Orowan effect wasot considered in this model due to CNTs dispersion along therain boundaries. The YS (�c) of the composites with individuallyispersed CNTs can be expressed by their model is given in Eq. (6):

c = (�0 + kd−1/2)[Vf (s + 4)/4 + (1 − Vf )] (6)

here, k is a constant, d is the average grain size, s is the aspectatio of the CNTs, Vf is the volume fraction of the CNTs.

They claimed that predicted results by the Eq. (6) were inood agreement with the experimental results. The discrepanciesetween the experimental and calculated results were below 5%or the CNT/Al alloy composites. They later incorporated the effectf clustering of CNTs in their later model. The clustering reduceshe effective volume fraction and also induces the porosity in theomposites (Liu et al., 2014a). They proposed a modified equationo estimate the tensile strength of CNT/AA2009 composites as givenelow in Eq. 7:

L = (�0 + kd−1/2)[(Vf − Vcluster)(s + 4)/4

+ (1 − (Vf − Vcluster))] exp(−��) (7)

here, �L is the strength of composite with pores, Vcluster is theolume fraction of the CNT clusters, � is the volume fraction oforosity and � is a constant for material with porosity.

Lee et al. (2011) used modified shear lag model to estimatehe YS (�cy) of silicon reinforced aluminum composites fabricatedy FSP combined with powder metallurgy. They considered therowan, Hall Petch and CTE strengthening to estimate the YSccording to Eqs. (8) and 9.

cy = �my (1 + 0.5SV) (8)

my = �o + �OR + �g + �CTE (9)

here, �my is the matrix YS, V is the volume fraction of the particles, is related to aspect ratio, and S = 1 for equiaxed particles, �o ishe initial matrix strength, �OR is the strength contribution fromrowan strengthening, �g is the grain size strengthening and �CTE

s the CTE mismatch strengthening. The estimated results at varyingercentage of silicon were overestimated in the range of 10–15% asompared to experimental results.

Guo et al. (2014) demonstrated by using models and experi-ental results that grain refinement and Orowan strengthening

ontributed to the strengthening of Al2O3/AA6061 alloy surfaceano-composites. The estimated contribution of grain refinementnd Orowan strengthening showed that Orowan strengtheningontribution was nearly four times more as compared to the grain

ig. 10. Micrograph of SiC/A206 surface composite fabricated by FSP (a) strong bondinarticles in the matrix (Sun and Apelian, 2011).


refinement contribution. They used grain refinement contributionto strength and modified Orowan equation, as given in Eqs. (10)and (11):

�O = M0.81Gmb

2�(1 − �)1/2�ln(

2√

2/3r/r0

)(10)

� =[√

�

Fv− 2

](√23

)r (11)

where M = 3 is the Taylor factor for a face centered cubic polycrystal,Gm is the matrix shear modulus, b is the Burgers vector magnitude,� is Poisson’s ratio, r is the radius of particles, r0 is the dislocationcore radius, and Fv is the volume fraction of particles.

6. Classification of FSPed surface composites

Commonly used reinforcement in fabrication of surface com-posites are micron or nano size SiC, Al2O3, B4C, TiC etc. andcarbon nano-structures (Arora et al., 2011). The surface compos-ites fabricated by FSP route exhibit a good bonding and uniformdistribution of reinforcement particles as shown in Fig. 10(a andb). Li et al. (2013) observed reduction in the size of micron size TiCreinforcement particles by FSP in fabrication of TiC/Ti-6Al-4V sur-face composite. Some of the ∼5.5 �m size TiC reinforcement wasreduced to nano-size after one pass of FSP. Al2O3 reinforcement(30 �m, 300 nm and 30 nm) were used in fabricating Al2O3/AZ91surface composites by FSP by Faraji et al. (2011). They observedthat the FSP with the nano-sized Al2O3 particles was more effec-tive in grain refinement of the AZ91 matrix. Dolatkhah et al. (2012)investigated the effect of micron and nano size SiC particles in fab-rication of SiC/AA5052 surface composites. They found that 50 nmsized SiC particles reduced grain size more than two times as com-pared to 5 �m size SiC particles. Incorporation of nano-particlesreduced average grain size to 0.9 �m, whereas grain size reductionin 5 �m size particles limited to 2.2 �m.

6.1. Surface nano-composites

Dispersion of the nano-sized reinforcements in a uniform man-ner in metal matrix is a major challenge. Nano-particles due to theirhigh surface area tend to agglomerate to reduce their overall energy(Tjong, 2007). Agglomeration induces pores and also increasesinter-particle spacing which in turn decreases the strengthening ofnano-composites. FSP is a newer technique for fabrication of sur-
face nano-composites and it is advantageous in ease of dispersionand elimination of clusters of nano-particles. The uniform distribu-tion of nano-sized reinforcement particles in steel matrix is shownin Fig. 11.
g between the composite layer and matrix (b) uniform distribution of reinforced

V. Sharma et al. / Journal of Materials Proces

Fig. 11. SEM micrograph of uniformly distribution nano-size TiC reinforced in steelb

r(sFbgcstorttarw

micro-hardness of the composite after three passes was increasedto more than twice as compared to base alloy. Q. Liu et al. (2013)

F(

y FSP (Ghasemi-Kahrizsangi and Kashani-Bozorg, 2012).

Several studies were reported on nano-surface composite byeinforcing nano-size ceramic particles and carbon nano-tubesCNTs). Dispersion and distribution of nano-size particles in theurface composite can be successfully achieved after multi-passSP. One important feature of surface nano-composites fabricatedy FSP is that they are defect free without voids. Also a homo-eneous particle distribution is achieved (Liu et al., 2013). Thisan provide exceptional combinations of strength and ductility inurface nano-composites. Shafiei-Zarghani et al. (2009) showedhat Al2O3/AA6082 nano-composites can achieve hardness valuesf 295HV after four pass FSP. The nano-sized Al2O3 particles alsoeduced the grain size of the matrix in which some grains were lesshan 300 nm. The high micro-hardness obtained was attributed tohe Orowan mechanism and ultrafine grain size of the aluminumlloy matrix. The surface composite exhibited improved wear
esistance as compared to as-received alloy. The mechanism ofear observed was a combination of abrasion and adhesion.
ig. 12. Carbon nano-structures reinforcement in surface composites (a) TEM image of Mb) good bonding as showed in the interface between MWCNT and AA1016 alloy (Q. Liu e

sing Technology 224 (2015) 117–134 127

Farnoush et al. (2013) deposited coating of nano-hydroxyapatite(HA) on the Ti-HA surface composite. Firstly, the surface compositelayer of CaP-Ti was fabricated via FSP by reinforcing HA powder inTi-6Al-4V alloy. After FSP, the coating of HA was applied on thissurface layer by electrophoretic deposition. Bonding strength wasmore than twice as compared to HA coating on the as received alloy.Increase in bonding strength was attributed to the Ti-CaP innerlayer which reduced the thermal expansion mismatch between Tialloy and the HA coating. Ratna et al. (2014) observed enhanced cor-rosion resistance for nano-hydroxyapatite (nHA) reinforced AZ31magnesium alloy surface composite fabricated by FSP. They foundthat the AZ31-nHA composite have better corrosion resistancecompared to fine grained FSP AZ31 and AZ31 samples due to thecombined effect of reduced grain size and incorporated nHA parti-cles.

Barmouz et al. (2011c) fabricated polymer nano-composite byFSP and demonstrated a three-fold increase in hardness value ascompared to that exhibited by composite fabricated using conven-tional mixing in the melt. High density polyethylene (HDPE) andnano-clay composite was fabricated by FSP by placing the nano-clayin the machined groove in the HDPE. The significant increase in themicro hardness values could be attributed to the good dispersionof nano-clay particles on the surface of polymer.

Carbon nanotubes (CNTs) are the ideal reinforcements for fabri-cating composites because of their extremely high elastic modulusand strength with good thermal and electrical properties (Popov,2004). The exceptional properties of CNTs can be well harnessedby uniform dispersion in metal matrix with minimum damage tothe structure. The good bonding between carbon nano-structureand matrix alloy is shown in Fig. 12(a and b). The survivability ofsingle-walled carbon nanotubes during FSP in AA7075 alloy matrixwas investigated by Johannes et al. (2006). Raman spectroscopyanalysis indicated that CNTs survived the thermal and stress cyclesinvolved in FSP with a tool rotation speed of 400 rpm and a trav-erse speed of 25.4 mm/min. Multiple passes of FSP are required forthe uniform distribution of CNTs, but may lead to damage CNTs to agreat extent (Izadi and Gerlich, 2012; Liu et al., 2013). In fabricationof AA5059 alloy reinforced with MWCNT, Izadi and Gerlich (2012)showed that MWCNT survived after two passes of FSP. However,the uniform distribution of MWCNTs was achieved by three passesof FSP only. The high shear stresses along with the elevated temper-atures in the SZ led to the damage of MWCNT structure. The average

also observed breakage of MWCNTs due to intense stirring effect inFSP. However, a strong interface between the MWCNTs and matrix

WCNT reinforced in AA5059 alloy after two passes of FSP (Izadi and Gerlich, 2012)t al., 2013). Arrows in (a) indicate survived CNTs after two passes.


matio

wRcdot

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ithout intermediate compounds or nano-pores was observed.egions of ultrafine grains (50-100 nm) were also detected in theomposites, which were induced by severe deformation and theynamic recrystallization during FSP. The average micro-hardnessf composite incorporated with 6 vol. % MWCNTs was 2.2 times ofhat of the FSPed aluminum alloy without MWCNTs.

Powder forging route comprising mixing of CNTs with alu-inum alloy was employed prior to FSP to increase the stability

f CNTs in AA2009 alloy by Liu et al. (2012). CNT clusters werebserved in forged composites. After one pass FSP, the cluster sizeas decreased. Single dispersion of the CNTs was achieved after

our pass FSP and damage to CNTs during FSP was not severe. How-ver, the CNTs in the composites were shortened to some extentnd Al4C3 formed in the matrix. Liu et al. (2013) utilized rollingfter overlapping multi-pass FSP to align the CNTs in 1.5–4.5 vol.

CNT reinforced AA2009 alloy composites. The FSP zone was hotolled at 753 K up to a total reduction in thickness of about 80%. Itas found that CNTs were individually dispersed and directionally

ligned in the matrix. The tube structure of the CNTs was retainednd the CNT-Al interface was bonded without pores.

Zinati et al. (2014) dispersed MWCNTs in Polyamide 6 (PA 6)y FSP and a numerical model based on Lagrangian incrementalormulation was developed to investigate the thermo-mechanicalehavior. Simulation results showed peak temperature at the inter-ace of tool shoulder and work-piece. Study of effective plastictrain showed that material shearing is more on advancing sides compared to retreading side. The effective plastic strain on theop surface is highest as shoulder imparts more plastic deformationn the surface.

Recently, Liu et al. (2014a) reported that by increasing FSPasses, the strength reduced due to CNT length shortening. Theyabricated the 4.5 vol. % CNTs /AA2009 composite by FSP and pow-er metallurgy route. The maximum strength was obtained withhree-passes of FSP due to the elimination of CNTs clustering. Fur-her, when the number of FSP passes increased from three to five,oth the YS and the UTS of the composite decreased due to reduced

ength of CNTs. A CNT shortening model was proposed by Eq.12) to describe the evolution of CNT length during mechanicalrocessing.

1Ln

− 1L0

= n

2kD2ε (12)

here Ln is the length of CNTs after n number of cycles of similarrocessing, L0 is the CNTs length before processing, k is a constantelated to ultimate strain of CNT, D is the diameter of CNT and εs the imposed strain on a CNT during processing. No significanteduction in CNTs diameter was observed even after five passesf FSP. According to Eq. (12), reciprocal of CNT length exhibitsinear relationship with the number of FSP passes. This CNT short-
ning model can also predicts the CNT length shortening by otherechanical processes.Liu et al. (2014b) examined the electric conductivity and ten-
ile properties of CNTs reinforced aluminum and AA6061 alloy

n and dispersion in Al matrix during FSP.

composites fabricated by powder metallurgy and subsequentmulti-pass FSP. Artificial ageing was also done for CNT/AA6061composite after FSP. After FSP, almost no CNT clusters were foundwhich results in increased tensile strength. The electrical con-ductivity of the CNT/Al composite decreased while the electricalconductivity of the CNT/AA6061composite increased as comparedto base alloy. Increase in electrical conductivity was attributed tothe decrease in Mg and Si concentration as they are segregated atthe CNT and matrix interface.

6.2. Surface in-situ composites

In-situ composites offer many advantages such as a defect-free reinforcement-matrix interface, more thermodynamicallystable reinforcements, improved compatibility, and higher bond-ing strength between the reinforcements and the matrix (Tjongand Ma, 2000). The major problem in the fabrication of in-situ com-posites by conventional methods is the segregation of the in-situformed reinforced particles (Bauri, 2014). FSP is an effective routefor fabricating in-situ composites as it provides synergistic effect ofsevere plastic deformation to promote mixing, elevated tempera-ture to facilitate the in-situ reaction and hot consolidation to form afully dense solid (Hsu et al., 2006). FSP route is also attractive as in-situ formed reinforced particles are of nano-sized which contributetowards significant improvement in strength (Tjong, 2007). Fur-ther, the large plastic strain imposed in FSP can effectively removethe in-situ formed particles at the interface. As the particles areremoved rapidly from the interface, the growth of the particles islimited and results in nano-sized particles (Hsu et al., 2006).

Intermetallic compounds are reported to form at the inter-face of aluminum matrix and reinforced element during FSP. Theformation of in-situ particle AlxM during FSP is schematicallydemonstrated in Fig. 13. The reaction during FSP occurs at inter-face of aluminum matrix and reinforced element (M: Ti, Ni, Fe etc.).Further processing results in increased plastic strain leading to dis-persion of AlxM particles in aluminum matrix. Again, the matrixand element M comes in direct contact to form intermetallic com-pound and the process is repeated. Studies are mainly reported onthe fabrication of Al-Al3Ti, Al-Al3Ni, Al-Al3Fe, and Al-Al2Cu nano-composites via reactive FSP of Al-Ti, (Hsu et al., 2006) Al-Ni, (Keet al., 2010) Al-Fe, (Lee et al., 2008) and Al-Cu (Hsu et al., 2005).Formation of additional in-situ nano-size Al2O3 or MgO particlesare also reported in oxide systems like Al-CeO2 (Chen et al., 2009),Al-TiO2 (Zhang et al., 2011), Al-CuO (You et al., 2013b) and Al-Mg-TiO2 (Khodabakhshi et al., 2014). Hsu et al. (2006) demonstratedthat by FSP route, a large amount nearly up to 50 vol. % of nano-sized Al3Ti particles can be reinforced in-situ using Al-Ti elementalpowder mixtures. In this study, the pre-mixed Al-Ti alloy powders
of 40 �m size with 5 to 15 vol. % Ti were cold compacted and sin-tered. For Al-15 vol. % Ti, the volume fraction of Al3Ti was near 0.5after four FSP passes with the average size of 165 nm. The Young’smodulus was 63% higher than that of Al.

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Chen et al. (2009) reported the production of aluminum basedn-situ composites from powder mixtures of aluminum and ceriumxide. The in-situ reinforcing phases were observed in compositesven after sintering, but their distribution was homogenized afterSP. The reinforcing phases were identified as Al11Ce3 with an aver-ge size of 1.4-3.5 �m, and �-Al2O3 with an average size of ∼10 nm.ou et al. (2013b) also applied FSP to produce aluminum based

n-situ composite using powder mixtures of aluminum and cop-er oxide (CuO). The nano-sized particles of Al2O3 and Al2Cu wereormed in-situ during FSP. Lee et al. (2013) investigated the forma-ion of Al-Mo intermetallic particles from the mixture of aluminumnd molybdenum powders via FSP. They found that FSP promoteshe exothermic reaction between aluminum and molybdenum toroduce fine Al-Mo intermetallic particles with an average size of00 nm. FSP effectively removed the intermetallic particles formedt the Al/Mo interface by the shear deformation of rotating tool.ubsequently, intimate contact between Mo and Al could be main-ained and the reaction proceeded without hindrance.

Due to short exposure of temperature and stirring in FSP, theeaction may not be completed and residue constituents remainn-reacted (Ke et al., 2010; Zhang et al., 2011, Khodabakhshi et al.,014). Thus, it becomes necessary to understand the heat expo-ure by sintering during FSP or heat treatment done prior or afterSP. Ke et al. (2010) demonstrated that nickel remain un-reactedfter three passes of FSP. They reinforced nickel powder in drilledole aluminum plate by FSP to fabricate Al-Ni intermetallic com-osite. After FSP heat treatment was performed at 550 ◦C for sixours. Heat treatment enhances the reaction and causes nickel toeact fully to form intermetallic composite. The UTS of compos-te showed 171% increase as compared to FSPed aluminum. Then-reacted constituents were also observed by Khodabakhshi et al.2014) in TiO2 reinforced AA5052 alloy in-situ nano-compositesia FSP. The in-situ chemical reactions were not completed dueo short processing time and more than 55% TiO2 nano-particlesemained un-reacted. It was observed that after annealing at 400 ◦Cor three hours, the fraction of un-reacted TiO2 nano particles signif-cantly reduced. The annealing treatment led to a more than threeimes improvement in the ductility of the nano-composite withoutacrificing their tensile strength and hardness.

The lower temperature and holding duration in hot pressingf Al and Ti powders may not be sufficient to initiate the in-situeactions. At higher temperature and holding duration, Ti reactedith Al to from Al3Ti completely but a coarse particle size range of

.5–3 �m with inhomogeneous distribution in the Al matrix wasbtained. After four passes of FSP, the average size of the Al3Tiecreased and the distribution of the Al3Ti became homogeneousZhang et al., 2011). The particle size influence on in-situ particleormation was studied by Zhang et al. (2012). They fabricated in-itu composites from powder mixtures of Al and TiO2 using hotressing, forging and subsequent four passes of FSP. Decreasinghe size of TiO2 from 450 to 150 nm resulted in the formation of

ore Al3Ti and Al2O3 particles. In the in-situ surface compositeabricated from Al-Ti-Cu systems Zhang et al. (2013) reported liq-id phase formation in Al-Ti-Cu system owing to eutectic reactionetween Al and Al2Cu. The liquid phase formation accelerated theiffusion between Al and Ti resulted in the increased number ofl3Ti particles.

Anvari et al. (2013) studied the wear behavior of Al-Cr-Oano-composites fabricated in-situ by applying FSP on the plasmaprayed coating. In this study, the Cr2O3 powder was applied onA6061-T6 plate by plasma spray. After coating, six passes of FSPas conducted on the plate. During FSP, the Cr2O3 was reducedith aluminum to produce pure Cr and Al2O3. Also as a result of

eaction between Al and Cr, intermetallic compounds including
l13Cr2 and Al11Cr2 were formed. Reciprocating wear test resultshowed improved wear resistance in nano-composites.
sing Technology 224 (2015) 117–134 129

6.3. Surface hybrid composites

Surface composites with more than one type of reinforcementcome in the category of hybrid composites. To achieve desiredproperties in composites, use of more than single reinforcementcan provide satisfactory results. Hybrid composites show enhancedproperties compared with single particles reinforced compositesas it combines the advantages of its constituent reinforcements.(Basavarajappa et al., 2007). In hybrid composites, commonly asofter phase added with hard ceramic particles to achieve bettertribological properties, but on the other hand the softer phase hasdeleterious effect on mechanical properties. So, an optimum ratioof both the constituents is required to achieve better properties inthe hybrid composites (Suresha and Sridhara, 2010).

The properties of hybrid composites depend on the ratio ofreinforcement particles as demonstrated by Mostafapour Asl andKhandani (2013) in a study of (graphite + Al2O3)/AA5083 hybridsurface composite fabricated by FSP. They studied the effect ofhybrid ratio (graphite/Al2O3) of reinforcement on mechanicaland tribological properties of the surface hybrid nano-compositeof aluminum alloy. The nano-composite with hybrid ratio of 3exhibits the lowest wear rate while a hybrid ratio of 1 pro-vides better combination of wear and tensile strength. Devarajuet al. (2013) studied the mechanical and tribological properties of(SiC + graphite)/AA6061 hybrid surface composites. They observedthat increasing the graphite content up to certain limit decreasesthe hardness and wear rate. The graphite forms a mechanicallymixed layer which avoids the direct metal to metal contact and actas a solid lubricant. The lower wear rate was observed at the opti-mum condition of rotational speed of 1120 rpm, 6 vol. % of SiC and3 vol. % of graphite. Further increasing graphite content increasedthe wear rate due to low fracture toughness of composites.

Solid lubricant molybdenum disulphide (MoS2) reduced thewear rate by forming a stable mechanically mixed layer as indicatedby Alidokht et al. (2011) in a tribological study of (SiC + MoS2)/A356hybrid surface composites. They found that the wear resistanceof hybrid composite was higher compared to SiC/A356 compos-ite. The MoS2 particles formed a mechanically mixed layer (MML)which reduces the wear rate of the composite. The unstable MMLformed in the absence of the lubricant phase (MoS2), leads to lowerwear resistance in SiC/A356 composite as compared to the hybridcomposite though the hardness of hybrid composite was lowerdue to the presence of soft MoS2 particles. Similarly, Soleymaniet al. (2012) incorporated MoS2 with SiC particles in the ratio of1:2 in AA5083 alloy to fabricate hybrid surface composite. Thehybrid composite exhibited higher wear resistance as comparedto SiC/AA5083 or MoS2/AA5083 composites. The superior wearresistance of hybrid composite was attributed to presence of MoS2particles as they assisted in the formation and stability of a solidlubricating layer.

7. Defects in FSPed composite surfaces

Improper selection of process parameter results in defects inFSPed zone. FSP is susceptible to similar defects which are observedin FSW. Incorporation of second phase particles may hinder mate-rial flow during fabrication of surface composites. This may alsoincrease chances of defect formation. Table 2 shows common typesof defects observed in FSW. Defects include worm hole, scalloping,ribbon flash, cavity, surface lack of fill, nugget collapse and surfacegalling (Arbegast, 2008).

and rotational and traverse speed are prominent parameters fordefect free processing (Elangovan et al., 2007; Padmanaban and


Table 2Common types of defects observed in FSW of alloys (Arbegast, 2008).

Defect Description Probable reasons

Worm hole An advancing side tunnel of inadequatelyconsolidated and forged material running in thelongitudinal direction.

Excessive travel speed for given rotationalspeedCold weldToo low weld pitch

Ribbon flash Excessive expulsion of material on the top surfaceleaving a corrugated or ribbon like effect along theretreating side.

Excessive forge load or plunge depthExcessively hot weldToo high weld pitch

Surface lack of fill A continuous or intermittent top surface void onthe advancing side.

Insufficient flow arm formationacross top surfaceInsufficient forge pressureImproper backside supportInsufficient plunge depth

Nugget collapse Improper formation of dynamically recrystallizednugget shape.

Excessive Flow Arm Formation and injection ofmaterial into Advancing SideExcessively hot weldToo high weld pitch

Surface galling Galling and tearing of the metal on the top surfaceof the weld beneath the pin tool.

Sticking of metal to pin toolExcessively hot weldToo high weld pitch

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alasubramanian, 2009). Commonly, the defects tend to occur onhe advancing side where an abrupt microstructural transitionccurs from the highly refined SZ to the TMAZ, while the transi-ion is gradual on the retreating side (Nandan et al., 2008). Kimt al. (2006) pointed out three common defects in FSW of alu-inum cast ADC 12 alloy. They observed flash due to the excess

eat input, a cavity like defect (hole) caused by an insufficienteat input and cavity produced by the abnormal stirring. Theool probe is also responsible for defect formation in the SZ asndicated by Elangovan and Balasubramanian (2008) in FSW ofA2219 alloy. Gandra et al. (2011) observed triangular cavitiesear the SZ in SiC/AA5083 surface composites. Particle agglom-ration was also observed. Further, they found that cavity on thedvancing side was almost empty while the cavity on the retreat-ng side was filled with compacted particles under the compressiveorces exerted by the material flow. The defects formation wasue to insufficient material flow influenced by the existence of SiCarticles.

In fabrication of SiC/Ti nano-composites, it was reported byhamsipur et al. (2011) that defect free surface composites can beabricated by utilizing optimum range of rotational (ω) and trav-rse speed (�). At relatively low ratio of ω/� the tunneling defectas observed due to insufficient plastic deformation and flow ofaterial because of less interaction time between tool and mate-

ial. Sharifitabar et al. (2011) also observed large voids, and tunnelsn the SZ of Al2O3/AA5052 surface nano-composite FSPed at low/� ratios. The low tilt angle of the tool caused the formation of
oid even if samples FSPed with high ω/� ratio. This was attributedo low heat input at low ω/� ratios which decreased materials flown the SZ during stirring leading to the formation of large voids andunneling defect.
Insufficient tool penetration depth (PD) is a common cause fordefects in SZ. The PD has to be optimized to supply enough heat tothe material so that defects like longitudinal cracks and tunnelingcavity are not formed in the processing zone (Asadi et al., 2010).Tool geometry is also one of the factors in defect formation in SZ. InFSP of Al2O3/AZ31 surface nano-composites, Azizieh et al. (2011)reported that defect-free SZ was observed with threaded probetool whereas non-threaded and three-fluted probes left cavities andmicro-voids in the specimens due to the less material flow.

8. Summary and future directions

FSP is a versatile technique for fabricating surface composites.The grain refinement achieved by FSP and the solid state nature ofprocessing is the unique advantages of this process. Nanocrystallinegrains have been reported in several surface composites fabricatedby FSP. The surface composites exhibit high hardness as well asincreased wear and corrosion resistance. FSP is a relatively newtechnique for fabrication of surface nano-composites and offersease of particle dispersion. A number of reinforcements includ-ing ceramic and metallic particles and carbon nano-tubes havebeen successfully incorporated in metallic matrix by FSP. Elim-ination of clustering of nano-particles can be achieved. Surfacenano-composites fabricated by FSP are defect free without voidsand have a homogeneous particle distribution. In fabricating in-situ composites, FSP route is advantageous due to rapid removal ofreaction products from interface which enhances further reaction.
Moreover, the in-situ formed particles are in nano-meter regime.Hybrid composites comprising a hard and a soft reinforcement havebeen successfully produced with promising properties. To harnessthe full potential of nano-composites various methodologies to

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chieve uniform distribution have been used in FSP. Initial stud-es pertaining to coating pointed out that application and stabilityf coating can be significantly improved by FSP. Micro alloying withow melting point metals like tin, lead etc. can be incorporatedn the surface composite. FSP of polymers and polymeric compos-tes was initiated recently and the initial results are encouraging,ut further investigation is required due to low melting point andolymeric chain structure arrangement.

The tool wear is important issue in FSP especially in preparingigh temperature melting point materials such as steel, titaniumr ceramic particle reinforced composites. Tools of polycrystallineubic boron nitride, tungsten based alloys etc. are recommendedor FSP of hard materials. However, the high cost and low fac-ure toughness of these tools limits their usage. These limitationsestrict the use of FSP technique to prepare hard surface compos-tes. Most of the surface composites fabricated so far is of aluminumased. Surface composites of harder alloys still await the develop-ent of cost effective and durable tools.

cknowledgement

One of the authors (VS) gratefully acknowledges the financialupport of Department of Science & Technology, Government ofndia received under INSPIRE Fellowship Program.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.jmatprotec.2015.4.019

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1 Proces

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hodabakhshi, F., Simchi, A., Kokabi, A.H., Gerlich, A.P., Nosko, M., 2014. Effects ofpost-annealing on the microstructure and mechanical properties of friction stirprocessed Al-Mg-TiO2 nanocomposites. Mater. Design 63, 30–41, http://dx.doi.org/10.1016/j.matdes.2014.05.065

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