G.Vedabouriswaran,S.Aravindan

Department of Mechanical Engineering,Indian Institute of Technology Delhi,India

Abstract Surface metal matrix composite is produced on the as cast Magnesium Rare Earth alloy–RZ 5 by single pass friction stir processing using various micro/nano sized reinforcement particles namely Boron Carbide(B4C),Multi Walled Carbon Nano Tubes(MWCNTs),and a mixture of ZrO2+Al2O3 particles.Fine grained metal matrix composites having the grain size ranging 0.8μm to 1.87μm are achieved.Grain boundary pinning by the reinforcement particles has resulted in the transformation of coarse grained(～81μm)base material into fin grained(＜1μm)metal matrix composite.Finer grain structure and the presence of reinforcements at the stir zone have resulted in increased and improved mechanical properties of the developed composites.Microhardness ranging between 125 HV and 403 HV is achieved.Uni-axial Tensile Testing of the developed composites exhibited improvement in tensile strength.Metal matrix composites developed using various reinforcements exhibited an increase in strength ranges between 250MPa and 320MPa.

Keywords:Friction stir processing;Magnesium Rare Earth alloy–RZ 5;B4C;MWCNT;ZrO2+Al2O3;Microhardness;Grain boundary pinning.

1.Introduction

Magnesium alloys owing to their light weight are findin high technology applications in automotive and aerospace industries.Various types of magnesium alloys were subjected to friction stir processing for the generation of surface composites[1,2].Friction stir processing(FSP)[3]is employed for the purpose of development of surface composites using various reinforcement particles[4–9].Being a solid state processing technique,simplicity and versatile in its methodology,FSP has gained a lot of momentum and it has induced intense research,and experimentation studies on a wide range of materials.FSP variants,namely,single/multiple passes,and multiple pass with overlapping have resulted in enhancement of mechanical properties of various materials.The surrounding conditions with which the FSP is performed such as ambient atmosphere and in submerged conditions using various fluid such as water,dry ice,and cryogenic flui have proved to play a major role/factor during dynamic recrystallization of the plasticized material generated during FSP at the stir zone[10,11].

Selection of tool parameters in FSP namely the tool rotation speed(ω)and the traverse speed of the tool(v)in conjunction with the pin diameter and shoulder diameter ensures requisite amount of heat generation on the work piece during the FSP[6,12].Tool dwelling for a short duration at the stir zone is necessary and subsequent traverse motion of the tool along the entire length of the work piece result in the generation of defect free,fin grained metal matrix composite.

Nano Al2O3particles were dispersed in AZ 91 alloy by FSP using a tool pin profil of square cross section.It is reported that at 40mm/min traverse speed of the tool,better particle distribution is observed at the stir zone of the alloy[5].Multi pass FSP is carried out on AZ 91 alloy for the introduction of nano SiO2particles and concluded that selection of process parameters are vital in the distribution of the reinforcements within the matrix.Increasing the number of FSPpasses resulted in the improved and uniform distribution of SiO2particles at the stir zone.Moreover,the microhardness of the developed composites showed increase in the hardness with the increase in the number of FSP passes.The grain size at the sir zone decreased with the increase in the traverse speed of the tool.Refinemen of grains is attributed to the fact of presence of nano SiO2particles along the boundaries of the grain and thereby restrictions are provided during grain growth.[6].

Nano SiO2is introduced in the matrix of AZ 61 alloy to develop metal matrix composites by performing 4 pass FSP.Grain refinemen is achieved and the hardness is increased twice as that of the parent material[7].MWCNTs were dispersed in AZ 31 alloy by FSP.It has been understood that the dispersion of the MWCNTs in the metal matrix is governed by the FSP tool traverse speed and its rotational speed.A higher tool speed of 1500rpm with a tool traverse speed of 25mm/min resulted in excellent dispersion of MWCNT reinforcements in the metal matrix of AZ 31 alloy with no particle aggregation.The amount of heat generated due to friction is dependent on the tool traverse speed of the tool.Additionally,these multi walled CNTs promoted grain refinin of the alloy and grains of 500nm size have been achieved[9].Use of Al2O3and CNT particles in various mixing proportion were used in the development of composites on AZ 31 magnesium alloy by FSPmethodology.It has been reported that a particular mixing proportion of Al2O3and CNT particles enhanced the wear characteristics of the developed composites and were free of any defects with uniform distribution of these reinforcements were reported[13].

The influenc of providing heat sink during FSPand hence its role on the formation of microstructure and grain size of AZ 91 magnesium alloy has been studied and was found that controlling the heat sink temperature plays a vital role in improving the refinemen of the grains[14].FSPon AZ 61 alloy in combination with a heat sink has resulted in the formation of ultra fin grains of 300nm and the average microhardness of the processed alloy is increased twice that of the base material.The heat sink is made of Copper with provisions for the fl w of liquid nitrogen during FSP.The recrystallized grains present at the bottom most stir zone of the AZ 61 alloy were ultra fin and nano sized[15].Similarly methanol at?20°C is used as a cooling medium and fl wn underneath AE 42 alloy during single and double pass FSP thereby fin grains are achieved[2].FSPon cold rolled AZ 31 alloy specimens exhibited tensile characteristics that are governed by the grain size and are influence by the basal slip and twinning action.The tensile characteristics varied with the orientation of tensile specimen prepared from the processed zone[16].Two pass FSP on die cast AZ 91 alloy demonstrated an extraordinary high strain rate super plastic behavior at 330°C due to the formation of ultra fin grains 0.5μm.Higher Aluminum in AZ 91 formsβ-Mg17Al12intermetallics which plays a vital role in the thermal stability of the alloy[17].

However,work on surface metal matrix composite on magnesium alloy(RZ 5)containing rare earth elements is scarce.Surface metal matrix composites were developed onRZ 5 magnesium alloy through FSP using micro/nano sized particle reinforcements namely B4C,MWCNT,and a mixture of ZrO2+Al2O3particles.The developed composites are subjected to metallurgical examinations to understand the nature of grain structure evolved.

Table 1 Composition of various elements of RZ 5 magnesium alloy.

Fig.1.(a)Work specimen cut from the bulk as cast Mg RZ 5 alloy as per dimension.(b)Groove is made on the top surface of the work specimen by milling operation.(c)Work specimen after groove closure operation.

2.Experimental method and materials

Fig.2.(a)FSW/P machine set up;(b)FSP in progress with tool plunged into the work piece and traversing along the longitudinal direction;(c)Work piece after completion of FSP and key hole is generated at the end point of the work piece due to tool retrieval.

Friction stir processing was performed on as cast RZ 5 magnesium alloy.The main constituents of RZ 5 magnesium alloy are Zinc(Zn),Zirconium(Zr),and rare earth elements(REE)namely Neodymium(Nd),Lanthanum(La)and Cerium(Ce)and the rest being magnesium(Mg).The percentage weight compositions of these individual elements are tabulated in Table 1.To perform FSP experimentation,work specimen of size 180mm(length),40mm(width),and 10mm in thickness was cut from the as cast RZ 5 magnesium alloy block.On the fla surface of this cut work specimen,a groove of size 1.5mm(width)and 3mm in depth was made by milling operation.This groove was cleaned using acetone so as to remove any foreign particles or debris present inside the groove.After cleaning the groove,reinforcement particles were packed manually in this groove.After packing the groove with reinforcement particles,a groove closure operation was done by traversing a cylindrical rotating tool(diameter 25mm)along the entire length of the work specimen.Fig.1(a)–(c)depicts the step by step procedure involved in the preparation of work specimen until groove closure operation performed sequentially on the work specimen.Now the work specimen was subjected to single pass friction stir processing for the development of surface metal matrix composites.

Single pass friction stir processing(FSP)was carried on to the work specimen by using a rotating tool which has a pin/probe(Fig.2(a)–(c)).During FSP,the rotating tool was inserted on to the work specimen with an axial force.The axial force during this tool plunging action ranged between 5000 and 7500N.After plunging,the rotating tool was made to dwell at that start position on the work specimen for about 3–5s.The purpose of dwelling is to establish generation of sufficien amount of heat and plasticized material.The mechanical stirring action of the tool pin causes friction at that zone of tool insertion and it generates heat which in turn softens the material.This plasticized material is made to fl w and fil the entire processed zone when the work table is made to traverse.A tool tilt angle of 2°was maintained during FSP.The table traverse speed was set at an optimized speed of 50mm/min and a tool rotation speed of 900rpm.After FSP,the tool was retracted out of the work specimen.The FSP tool material is a hardened tool steel with an average hardness of 65 HRC.The shoulder diameter of the tool is 25mm and the pin length is 5mm.

The reinforcement particles used in the experimentation are boron carbide(B4C),Multi Walled Carbon nano tubes(MWCNT)and a mixture of ZrO2+Al2O3particles.Metallographic specimens were prepared from the processed zone for the purpose of study of microstructure.These specimens prepared were polished using various grades of SiC emery sheet of grits ranging from(220 to 1200 grit).Polishing is done using 220,400,600,800,1000,and 1200 grit followed by fin polishing using Al2O3paste(grades I–III).The specimen was cleaned ultrasonically using acetone for 5 min to remove any debris.The fin polished specimen was etched using aceticpicral as etchant.The etchant(acetic-picral)comprises of amixture of 5g of picric acid,10ml acetic acid,20ml distilled water,and 100ml ethanol.Etching was done on the polished surface for 3 s for the analysis of microstructure.Microstructure of the specimen was observed in optical microscope with digital image capturing facility.In order to study the tensile property of the processed material,tensile specimens were prepared by CNC wire cut EDM from the processed zone.The tensile specimens prepared out of the processed zone conform to B557M–14 ASTM standards.These tensile specimens were 3mm in thickness.Uni-axial tensile testing of the specimens was carried out at ambient temperature.

Table 2 Characterization of reinforcement particles.

2.1.Reinforcement particles

Commercially available MWCNTs,B4C,ZrO2(Zirconia),and Al2O3(Alumina)were used as a reinforcement in this experimentation.The carbon nano tubes(CNTs)were multiwalled with＞95%purity.The outer and inner diameters of the MWCNTs were 5–20nm and 2–6nm,respectively,while the length is 1–10μm.A ratio of 80:20(by weight)is maintained for the preparation and use of mixture of ZrO2+Al2O3reinforcement particles,wherein 80% by weight is ZrO2particles and the remaining 20%by weight is Al2O3particles.The Zirconia powder has 3 mol%Yttria.Table 2 presents the particle size details of the reinforcements and their corresponding SEM images.

Fig.3.Microstructure of as cast Mg RZ 5 alloy.

3.Results and discussion

3.1.FSPed zones of developed metal matrix composite

The metal matrix composite made by FSP comprises of these zones namely:(i)stir zone(SZ),(ii)thermo mechanically affected zone(TMAZ),and(iii)heat affected zone(HAZ).Table 3 depicts the microstructure observed at various zones of the magnesium alloy processed by FSPwith the use of aforementioned reinforcement particles.

From the above micro structural images obtained at various zones,it is evident that the grain sizes at these zones are having different size and shape and orientations.The TMAZ zone has elongated and irregular grain orientation due to the fact that this zone which is in the near vicinity of the stir zone experiences the influenc of heat generation and the mechanical agitation induced by the tool pin in the SZ.The heat input and stirring action are insufficien to generate fin grained microstructure.The plasticized fl w of the processed material in the SZ from the advancing side to the retreating side influence the orientation of the grains toward that direction.Heat affected zone experiences insufficien heat input and this is highly insufficien to bring in dynamic recrystallization.Insufficien heating with no severe plastic deformation are the characteristics that prevail in HAZ.However,stir zone is fully characterized by the development of fin grained microstructure.

3.2.Microstructure of base material–as cast Mg alloy(RZ 5)

The microstructure of the as cast Mg RE alloy is depicted in Fig.3.The microstructure clearly depicts large and coarse magnesium grain with a clear and distinct grain boundary.The average grain size of the as cast RZ 5 magnesium alloyis 81μm.The estimation of the grain size is based on the principle of linear intercept method.

At the junctions of these coarse magnesium grains,intermetallics(Mg12RE)are identified The intermetallics(Mg12RE)were distributed at the boundaries of the grain and were identifie to be RE-rich precipitates.EDAX analysis of the base material substantiates the presence of RE elements as depicted in Fig.4.

3.3.Microstructure achieved at the stir zone after single pass FSP

Friction stir processing is performed on the as cast Mg RZ 5 alloy without introduction of any particles reinforcements into the matrix.Fig.5 shows the microstructure obtained at the stir zone after single pass FSP.Using the principle of linear intercept method,the average grain size is estimated to be 5.5μm.During FSP,due to severe plastic deformation of the material at the stir zone,the grains are subjected to dynamic recrystallization and this result in the formation of fine grain structure.The elongated and coarse Mg12RE intermetallics are shattered and made into fine particle during mechanical stirring action and are dispersed within the matrix of the materials.The irregular shaped and randomly oriented Mg12RE intermetallics present in the base material,after single pass FSP,becomes fine particles.A fin grained microstructure is obtained at the stir zone,after single pass FSP without use of any particle reinforcements.

3.4.Microstructure of the developed surface metal matrix composites using different reinforcements

For the development of surface metal matrix composites,friction stir processing is done on the base material(RZ 5 alloy)by introduction of particle reinforcements thereby three different types of surface metal matrix composites are made in the current study.These developed surface metal matrixes are designated as MMC:A,MMC:B,and MMC:C.The microstructure at the stir zone of these surface metal matrix composites were fin grained.In general,the stir zone of all these composites exhibited uniform distribution of particle reinforcements and fin grained microstructure.The coarse grain of the base material underwent dynamic recrystallization during the severe plastic deformation induced at the stir zone.The mechanical stirring action of the tool pin during FSP resulted in the dispersion of reinforcements at the stir zone of all MMCs.Presence of micron and nano level particle reinforcements along the grain boundary aided in grain refinemen owing to grain boundary pinning action.

Fig.4.EDAX analysis of RZ 5 magnesium alloy.

Fig.5.Microstructure achieved at the stir zone after single pass FSPwithout use of particle reinforcement.

3.4.1.Use of B4C reinforcements

The stir zone of the processed surface composite using B4C as reinforcements is shown in Fig.6.Use of micron sized B4C particles resulted in the generation of fine grain size.During FSP,these reinforcement particles were dispersed into the metal matrix uniformly and there is no agglomeration of reinforcement particles in the stir zone.Effective frictional stirring,coupled with uniform fl w of plasticized metal from the advancing side to the retreating side during FSP and no generation of flashe attribute to defect free stir zone.Presence of micron sized B4C particles in the metal matrix plays a vital role in controlling the grain growth during dynamic recrystallization.These particles,dispersed into the matrix,act as a barrier during the growth of the grain and hence results in fin grained microstructure.The average grain size obtained at the stir zone is 1.87μm.The microstructure also reveals presence of shattered and fin shaped Mg12RE particles dispersed uniformly in the matrix.Similar report is available on B4C particle reinforcements into Cu matrix through FSP resulted in the generation of fine grains at the stir zone[18].

3.4.2.Use of MWCNT reinforcements

Fig.7 depicts the stir zone of the processed surface composite using MWCNT as reinforcements.The average grain size at the stir zone is 1.38μm.A good dispersion of the MWCNT particles in the stir zone and fin grained microstructure is achieved.

The grain refinemen at the stir zone is due to the fact of dynamic recrystallization[9].Presence of MWCNT particles at the stir zone during FSP played a vital role in bringing the pinning effect.These particles impregnated along the magnesium grain boundary retarded the growth of the grain of the RZ 5 magnesium matrix.This pinning effect resulted in a microstructure having grains of 1.38μm as compared to initial grain size of 81μm of the base RZ 5 magnesium alloy.The stir zone is free from defects and voids and there is no agglomeration of the reinforcements.Effective stirring action induced by the rotating tool resulted in uniform distribution of the reinforcements at the stir zone.

Fig.7.Stir Zone microstructure after single pass FSP using MWCNT particles.

Fig.8.Stir zone microstructure after single pass FSP using ATZ particles.

3.4.3.Use of mixture of(ZrO2+Al2O3)reinforcements

Fig.8 depicts the stir zone of the processed material using ZrO2+Al2O3as the reinforcement particle.Nano sized Zirconia(100nm)and micron sized Alumina(50μm)were mixed in a ratio of 80:20(by%weight)and used in the development of metal matrix composite.The average grain size at the stir zone is estimated as 0.8μm.Thus,hybrid surface metal matrix composites are developed by the use of a mixture of two different types of reinforcement particles.

The stir zone of this surface metal matrix composite made by using a mixture of Zirconia and Alumina particle reinforcements exhibited very fine grain structure.The initial coarse grain of the base material(～81 μm)has been transformed into a fin grained matrix of grain size of 0.8μm.Reinforcement particles helped in grain boundary pinning at the stir zone.It is evident from the microstructure that the reinforcement particles were uniformly distributed without any agglomeration at the stir zone.The stir zone possessed fine Mg12Re particles.Table 4 summarizes the grain size achieved at the stir zone of the various metal composite developed by FSP using different reinforcement particles.

It is evident that by the use of a mixture of ZrO2+Al2O3reinforcement particles generated a very fin grain of 0.8μm.Based on the grain size achieved at the stir zone,it can be concluded that the grain size of MMC:C＜MMC:B＜MMC:A..The particle reinforcements used to develop MMC:A,MMC:B and MMC:C were B4C,MWCNT and a mixture of ZrO2+Al2O3respectively and the particle size were～12μm(for B4C),～100nm(for ZrO2)and ～50μm(for Al2O3).Upon comparison of these grain sizes achieved in various MMCs along with their particle size,there exists a trend line in respect of the grain size achieved at the stir zone.The grain size at the stir zone was the least(0.8μm)while using the smallest sized(nano sized)reinforcement particles.This is due to the fact that more number of smaller sized particles(nano sized)is available at any instant along the coarse Mg grain boundary and hence effective grain boundary pinning action happened.

In the current study,the FSP tool used has a square cross sectioned pin which is plunged into the metal to bring in mechanical stirring at the stir zone thereby plasticized material is produced.Square tool brings in agitation of the plasticized material by pulsating stirring action.This stirring coupled with pulsating action intensifie the distribution of particle reinforcement within the metal matrix.Hence in these entire metal matrix composites developed in the present study,the stir zone exhibited uniform distribution of the reinforcements and no agglomeration of particles.It has been reported that nano Al2O3particles has been uniformly distributed in AZ 91 alloy by the use of a square cross sectional tool pin than a cylindrical pin since square cross sectional tool produced more pulsation action during mechanical stirring of the tool pin[5].

4.Phase analysis

X ray diffraction analysis is carried out in order to understand/identify formation of any intermetallic phases in the metal matrix composites during FSP.The 2θrange for the XRD analysis is set between 10°and 80°for all the metal matrix composites and for the base material(RZ 5)magnesium alloy.FSP being a solid state processing technique involving dynamic recrystallization through mechanical stirring,formation of any intermetallic phases due to introduction of reinforcement particles has not occurred.Intermetallic phases normally observed with fusion processing due to high temperature reactions are not observed with friction stir processing.The maximum temperature obtained during FSPis 450°C.Since FSP is in solid state,the reactionof reinforcement with the magnesium matrix is not observed.Distinctive primary peaks corresponding to Mg have been identifie in the X ray diffraction in all of the metal matrix composite specimens alongside with other peaks identifying the presence of the reinforcement particles introduced into the metal matrix during FSP.Fig.9 is XRD done on the base material(RZ 5)magnesium alloy depicting the d spacing,and the corresponding 2 theta position.Major peaks at 2 theta position of 32°,34°and 36°corresponds to the element Mg.

Table 4 Grain size achieved at the stir zone of various metal matrix composites developed by single pass FSP on RZ 5 magnesium alloy.

Fig.9.XRD of RZ 5 magnesium alloy(base material).

Figs.10–12 correspond to the XRD spectrum performed on the metal matrix composite:A to C containing B4C,MWCNT and ZrO2+Al2O3reinforcement particles,respectively.Distinctive peaks characterize the presence of relevant reinforcement particles namely B4C,MWCNT and ZrO2+Al2O3in the metal matrix composites.In Fig.10,at 2theta angle positions of 34.485°and 63.214°,characterizes for the presence of B4C particles in the metal matrix composite A.

In the XRD of metal matrix composite:B,(Fig.11)the presence of MWCNT in the matrix is identifie at the 2 theta position of 25.655,41.234,and 52.931.Similarly in the XRD of the metal matrix composite:C,(Fig.12),the presence of Al2O3is identifie at the 2θposition of 57.440.However,no formation of intermetallics is identifie in any of these XRD spectrums pertaining to various metal matrix composites developed by FSP.The Zirconia particles introduced into the MMC is identifie at 2θvalues of 31.500,34.424,50.540,and 61.840.These 2θvalues correlate for the tetragonal phase of ZrO2particles.

5.Elemental analysis

5.1.EDAX analysis

EDAX analysis is performed on the base material(RZ 5)magnesium alloy as well as on other metal matrix composites developed using various reinforcement particles.The EDAX analysis of the base material revealed presence of major elements such as Magnesium,Zinc,and rare earth element such as Lanthanum,Cerium and Neodymium.Table 5 presents the EDAX analysis done on the base material as well on the metal matrix composites and the spectrum obtained.

5.2.Elemental mapping

Fig.10.XRD of metal matrix composite:A–RZ 5+B4C particles.

Fig.11.XRD of metal matrix composite:B–RZ 5+MWCNT particles.

Fig.12.XRD of metal matrix composite:C–RZ 5+(ZrO2+Al2O3)particles.

Table 5 EDAX analysis and its spectrum.

Table 6 Elemental mapping of metal matrix composite:A.

Elemental mapping at the stir zone of various metal matrix composites is carried out.Tables 6–8 relate to elemental mapping performed at the stir zone of metal matrix composite:A to C containing B4C,MWCNT and a mixture of ZrO2+Al2O3particles,respectively.In Table 6,major elements such as Boron and Carbon were identifie during the elemental mapping of metal matrix composite:A.Uniform distribution of B4C within the metal matrix composite is thus characterized.

The elemental mapping done at the stir zone of metal matrix composite:B,is presented in Table 7.Presence of MWCNT in the metal matrix is correlated by the presence of carbon elements which are uniformly distributed at the processed zone.Uniformity in the distribution of MWCNT particles by FSP has already been reported[9].

The elemental mapping of metal matrix composite:C is presented in Table 8.Addition of mixture of Zirconia and Alumina into the metal matrix by FSP and their presence is inferred by the identificatio of Zr,and Al elements during elemental mapping.

Elemental mapping done on all metal matrix composites infer that FSP proves to be a versatile solid state technique wherein reinforcement particles introduced into the metal matrix has been uniformly distributed.

6.Studies on mechanical properties of the base material and metal matrix composites

6.1.Hardness

The microhardness of the base material as well as the other metal matrix composites developed by using various reinforcement particles viz.,B4C,MWCNT and a mixture of ZrO2+Al2O3particles were measured.The base material(RZ 5)magnesium possessed an average microhardness of 81 HV.Table 9 provides the minimum,maximum,and average microhardness possessed by the base material(RZ 5)magnesium alloy before FSPas well as microhardness possessed by the various metal matrix composites developed through FSP.

Fig.13 gives a comparison plot of the microhardness obtained on various FSPed samples against the base material.It can be concluded that by performing FSP,the microhardness of the samples has increased.

Amongst the three types of metal matrix composites developed by FSP,the microhardness of the metal matrix composite A developed by introduction of B4C particles exhibited the highest microhardness of 425.7 HV.Theincrease in the microhardness of the metal matrix composite is attributed to the fact of high hardness possessed by B4C particles.The base material(RZ 5)magnesium alloy exhibited an average microhardness of 81 HV.This is due to the fact of presence of coarse Mg grain of the as cast RZ 5 magnesium alloy.After single pass FSP(without use of any particle reinforcement),the microhardness of the alloy increased to 117 HV.This is due to grain refinemen at the stir zone during FSP.Coarse grain of the base material along with irregular shaped intermetallics(Mg12RE)that were present in the base material undergoes dynamic recrystallization during FSPresulting in the formation of fin grain microstructure(5.5μm).This attributed to the enhancement of the microhardness of the stir zone.

Table 7 Elemental mapping of metal matrix composite:B.

Introduction of various reinforcement particles into the base material resulted in enhanced hardness of the metal matrix composites.These micro/nano sized particles aided in grain refinemen by the phenomenon of grain boundary pinning[8].Severe plastic deformation of the stir zone coupled with dynamic recrystallization and grain boundary pinning and the introduction of the different types of reinforcement particles into the matrix of the base material resulted in the generation of metal matrix composites of varied hardness’s.Presence of hard intermetallics(Mg12RE)and the reinforcement particle acts as a barrier at the grain boundary thereby resulting in development of fin grain microstructure.

It is reported that introduction of MWCNT particles into the stir zone during FSP increased the microhardness[9].Fig.14 depicts the microhardness of various metal matrix composites exhibited at the various zones in comparison with the base material.It is evident that the stir zone possessed the peak hardness.TMAZ and HAZ exhibited a relatively lower hardness owing to the fact that these zones are not fully influence by the phenomenon of grain refinemen and grain boundary pinning.The material at these zones(TMAZ and HAZ)is not subjected to extensive plastic deformation,and mechanical/frictional stirring action(induced by the tool pin).

During friction stir processing,refinemen of grains occur due to dynamic recrystallization and hence the hardness ofspecimen increases as compared to the base material owing to the fact of presence of refine and smaller sized grains increases the hardness at the stir zone.Upon introduction of reinforcement particles at the stir zone,the hardness increases further as these reinforcements strengthened the composites[9].Hall–Petch relation states that any reduction in the size of the grain attributes to the fact of increase in yield strength and hardness of the material.This find valid in the current study wherein the hardness of the FSPed sample increases with the decrease in the grain size.Eventually it observed that the grain size of MMC:C＜MMC:B＜MMC:A＜Single Pass FSP＜BM and hence the hardness of these composites had a higher value as compared to single pass FSPed sample.It is reported that introduction of nano Al2O3(0.3%)in AZ

31 alloy has increased the hardness of the alloy.Moreover,a mixture of Al2O3with CNT in certain proportion for the purpose of development of hybrid composite on AZ 31 alloy by FSP exhibited significan increase in the hardness of the hybrid composite in comparison with the base AZ 31 alloy[13].

Table 8 Elemental mapping of metal matrix composite:C.

Table 9 Microhardness of base material(RZ 5)magnesium alloy and various metal matrix composites.

Fig.13.Microhardness comparison plot of various metal matrix composites.

Fig.14.Microhardness exhibited by various metal matrix composites developed by FSP.

6.2.Tensile test

Uni-axial tensile test(at ambient temperature)is performed on the tensile specimen prepared from the processed zone.The tensile specimens were prepared from the stir zone of the various metal matrix composites developed by FSP conforming to ASTM B557M–14 standards.Fig.15(a)and(b)depicts the zone and the orientation for the preparation of tensile specimen from the processed stir zone by CNC wire cut as well as the various dimensions of the tensile specimen prepared from the FSP processed zone.This particular orientation is chosen so that the tensile specimen will be essentially of the processed zone in its entirety.

A crosshead speed of 2.5mm/min is maintained during the uni-axial tensile test for all the specimens prepared out of the developed metal matrix composites A–C.Fig.16(a)and(b)represents the CNC wire cut tensile specimen prepared from the base material before and after the conduct of tensile test.

Stress vs%strain plot(Fig.17)is drawn for all metal matrix composites A–C made by FSPand compared with the tensile characteristics of the base material(RZ 5)magnesium alloy.The base material(RZ 5)magnesium alloy exhibited UTS of 203MPa and%strain of 5.5.Lower UTS of the base material is due to the coarse grain(81μm)and presence of the Mg12RE precipitates at the grain junctions.These precipitates hence become favorable sites for initiation of cracks.These Mg12RE precipitates leads to weaker grain bonding strength and becomes preferential for crack initiation and its propagations near these precipitates.

The tensile specimen of single pass FSPed(without use of any reinforcement particle)exhibited UTS of 230MPa and a%strain of 5.9.Table 10 tabulates the UTS in MPa and%strain exhibited by various types of tensile specimen prepared from the stir zone of all metal matrix composites A–C.A significan percentage increase in UTS ranging from 25%(with the use of MWCNT particles)to 60%(with the use of B4C particles)has been achieved.Metal matrix composite developed on AZ 61 alloy with the introduction of nano SiO2reinforcements has reported to exhibit high strain rate super plastic behavior.The uniform presence of nano SiO2increased the hardness of the composite[7].

The tensile characteristic depends predominantly by fine grain structure and dislocation strengthening.Exhibition of super plastic behavior of metal is governed by the presence of ultra fin grains.It is reported that refinemen of grains by severe plastic deformation resulted in exhibition of high strain rate super plasticity characteristics[14,17].Severe plastic deformation on AZ 91 by FSP resulted in ultra fin grains.These ultra fin grained AZ 91 alloy exhibited high strain rate super plasticity.Exhibition of super plastic behavior is mainly due to grain boundary sliding mechanism[14,17].

Generally,in accordance with the Hall–Petch relations,σy= σo+kyd?1/2, the yield strength characteristics are governed by the grain size.σyrepresents yield stress,σorepresents friction stress,kyrepresents stress concentration factor and d represents average grain size.In the current study,the base material having coarse grains exhibited lower UTS of 200MPa as compared to other samples.The ultimate tensile strength of other samples in the current study showed an increased strength values on comparison with the base material and this increased strength characteristics obeyed the Hall–Petch relation.

Fig.15.(a)Zone and orientation for preparation of tensile specimen by CNC wire cut.(b)Dimensions of the tensile specimen prepared from the FSP processed zone by CNC wire cut.(All dimensions are in mm).

Fig.16.(a)Tensile specimen of base material(RZ)magnesium alloy(b)tensile specimen after the conduct of uni-axial tensile test.

Table 10 Tensile properties exhibited by the various metal matrix composites.

Fig.17.Stress vs%strain plot of various metal matrix composites in comparison with the base material.

Fig.18.SEM image at the tensile fracture surface of base material(RZ 5)magnesium alloy.

Use of B4C particles in the development of MMC:A has resulted in an increase of 60%in the UTS.Tensile specimen of MMC:B possessing MWCNT particles exhibited an increase of 25%in UTS in comparison with the base material.MMC:C possessing ZrO2+Al2O3reinforcement particles exhibited an increase of about 30%in UTS.The UTS of single pass FSPed sample without use of any reinforcement particles,exhibited an increase in UTS of the order of 15%.It has been reported that in ZK 60A Mg alloy,with the introduction of nano Al2O3particles,the strength and the failure strain has increased significantl[19].

Scanning electron microscope(SEM)images at the fractured surface of all the samples are examined further in order to understand the nature and mode of fracture.Fig 18 shows the SEM image of the fractured tensile specimen of base material.Cracks are identifie predominantly at the fractured surface at many localities.A higher magnificatio of the fractured surface of the base material(Mg RZ 5 alloy)is shown in Fig.19.At higher magnificatio (Fig.19)deeper cracks are seen.Cleavages with shallow dimples were also identifie at the fractured surfaces.Mixed mode of fracture is characterized in the tensile tested surfaces of the as cast base material(RZ 5)magnesium alloy.

Fig.19.SEM image depicting cracks seen at the tensile fractured surface of base material.

The fractured surface of the base material depicts deeper cracks and cleavage fracture characteristics.This is due to the presence of coarse Mg12RE intermetallics at the grain boundaries and these are prone to initiate cracks under tensile deformation of the specimen.Table 11 depicts high resolution SEM images taken at the fractured surface of all the tensile specimens.

6.3.Strengthening mechanism

Metal matrix composites reinforced by particle reinforcements are governed by various strengthening mechanism namely the load bearing effect,Orowan Strengthening Mechanism,enhanced strengthening on account of increased dislocation density induced due to the mismatch of co-efficien of thermal expansion,Hall–Petch strengthening mechanism.

6.3.1.Load bearing effect

Strong bonding of the reinforcements and the matrix material plays a vital role in the load transfer.There happens to be an effective transfer of load from the matrix material to the reinforcements thereby increasing the strength of the MMC.The enhancement of MMCs strength owing to the load bearing effect given by the presence of particle reinforcement is given by ΔσLoad=0.5Vpσyield[20].It is evident from the empirical formula thatΔσLoadis solely dependent upon the quantum i.e.,volume fraction of particle refinforecment available within the metal matrix.In the current study,theMMCs that are developed here by FSP method,the amount i.e.volume fraction of particle reinforcements introduced into the Mg matrix is significantl negligible and hence load bearing effect by the particle reinforcments can never play a significan role in the enhancment of strengthening of the MMCs during uni axial plastic deformation.

Table 11 SEM image at the fractured surface.

6.3.2.Orowan strengthening mechanism

Enhancement in strength of particle reinforced metal matrix composite owing to Orowan strengthening effect,ΔσOrowanis given by the following equation[21–23].

The size of the particle reinforcements used in the development of metal matrix composite has a greater and predominant influenc in increasing the strength of the matrix by Orowan’s strengthening effect.It is reported that in the development of Mg with Al2O3particles and Ti with Y2O3particles,the critical particle size that governed for Orowan’s maximum strengthening effect happens to be 5.44 times Burgers vector or atomic diameter of the matrix material.Hence,Orowan’s strengthening effect is influentia only in the metal matrix composites having nano reinforcements and its effect becomes insignifican and negligible in the case of MMCs having normal and micron sized particle reinforcements[21].

6.3.3.Taylor’s equation

Taylor correlates the role of dislocation density with respect to the enhancement of strength characteristics of a material.Dislocations in a material are due to the effect of residual plastic strain.These residual strain in a material or MMC can be due to(a)the prevalence of huge mismatch of the co-efficien of thermal expansion between the particle reinforcements and the matrix material;(b)the mismatch in the Young’s modulus between the particle reinforcements and the matrix material;(c)Severe plastic deformation operations such as equal channel angular pressing(ECAP),extrusion etc.,carried out on the material brings in work hardening and hence dislocations.These dislocations are defects that are created at the near vicinity of the reinforcement particles.The increase in the strength on account of mismatch of the co-efficien of thermal expansion and due to the prevalence of enormous dislocation density is given byΔσCTE[20]

whereinβrefers to strengthening co-efficien Gmrefers to shear modulus of matrix,b refers to Burgers vectorΔα referes to the difference in the co-efficien of thermal expansion of Mg and the reinforcement particlesΔT refers to the temperature difference between the processing temperature and the test temperature Vprefers to the volume fraction of the particle reinforcement and dprefers to the average particle size of the reinforcements[20,22].B4C particles have a co efficien of thermal expansion of 4.3×10?6K?1whereas for RZ 5 Mg alloy,the thermal expansion co efficien is～27.1×10?6K?1.There exists an enormous mismatch in the thermal expansion coefficien between the reinforcement particles and the matrix material in the case of the MMC:A developed in the present study.This leads to the generation of dislocation defects at the vicinity of the reinforcement particles thus leading to a very high dislocation density near the reinforcement particles and thereby contributing to the increase in the strength of the metal matrix composite.There is an efficien load transfer from the soft Mg to the hard B4C particles under deformation in uni-axial tensile test.

The mechanism that governs the mechanical strengthening in the case of the development of MMC:B in the present study is also attributed to dislocation strengthening mechanism and stress transfer mechanism.MWCNT has a thermal co efficien expansion of～10?6K?1.RZ 5 while Mg alloy has a thermal co efficien expansion of ～27.1×10?6K?1.This predominant and higher order of mismatch in the thermal co efficien of expansions resulted in dislocation nucleation around MWCNT particles and thereby increasing the overall strength of the metal matrix composite.Moreover,MWCNTs are subjected to a mechanical loading during plastic deformation.Interfacial bonding of the reinforcement particle and the matrix plays a major role in effective load transfer.Strong bonding at the interfaces between magnesium matrix and the MWCNTs ensures substantial transfer of load from the matrix material to the MWCNTs during uniaxial tensile test[24].

Similarly in the case of the MMC:C which is a hybrid composite(since a mixture of ZrO2and Al2O3reinforcement particles are used),the coefficien of thermal expansion of 3 mol%yttria stabilized zirconia particle is～7.5×10?6K?1and that of Al2O3particle is～7.9×10?6K?1while the thermal co efficien expansion of Mg alloy is～27.1×10?6K?1.Dislocations are created at the near vicinity zones of these reinforcement particles on the account of huge mismatch of thermal expansion coefficient and thereby contributing to the enhancement of the strength characteristics of the developed MMC.The mechanism of effective load transfer from the soft Mg matrix to the hard particle s(mixture of ZrO2and Al2O3particle reinforcements)in the case of MMC:C holds little significanc in the current scenario.Studies on the enhancement of strength of Mg MMC developed with the use of nano Al2O3and nano Y2O3particles,it has been understood that the load bearing effect by the reinforcement particles during uni-axial loading has little significanc even with the presence of 10 vol%of these reinforcement introduced into the metal matrix[20].

6.3.4.Hall–Petch strengthening

The Hall–Petch strengthening effect plays a major and predomination role in the enhancement of strength characteristics of metal matrix composites which is contributed on the basis of refine grains prevailing in the metal matrix.Zener equation expresses the grain size of the matrix dmthat can be achieved owing to the concept of grain boundary pinning action exerted by the presence of reinforcement particles.

Here,dprepresents the size of the particles and vprepresents the volume fraction of the particles andαis a constant of proportionality[25].It can be concluded that grain size evolved is governed only by the size of the reinforcement particles and its volume fraction.The decrease in the grain size leads to increases in the yield strength in accordance with Hall–Petch equation given by ΔσHall?Petch[22].

dcompositesand dmatrixare the grain size of the composite and RZ 5 matrix,respectively.K happens to be strengthening coefficient

It is to be noted that the elongation to tensile fracture of the developed composites i.e.MMC:A,MMC:B and MMC:C has improved marginally upon comparison with the base RZ 5 Mg alloy.This exhibition of improved ductility behavior of the developed composite is predominantly attributed to the fact of refinemen of grains and uniform distribution of the reinforcement particles within the matrix.Presence of refine grains in the matrix promotes uniform plastic deformation with less stress concentration during the uni-axial tensile deformation.Presence of reinforcement particles brings in alterations to the direction of crack growth which causes crack bridging,branching,and deflectio [22].Because of this,good amount of energy is absorbed and hence this helps in increasing the resistance to the propagation of crack and subsequently contributing to the increase in elongation to tensile fracture of the composites.It can be concluded that the enhancement in the yield strength of the developed composites is purely governed by Hall–Petch equation owing to the fact that the reduction in the size of the grains of the metal matrix has a direct effect to the increase in the strength characteristics of the tensile specimen subjected to uni-axial deformation.The load bearing effect induced by the presence of reinforcement particles within the matrix has lesser or little influenc due to the fact that the volume fraction of reinforcement particles governs the strength aspects which in the current scenario have little significance Enhancement of strength due to mismatch of co-efficien of thermal expansion of the reinforcement particles and the matrix material plays yet another significan role and also remains as a major contributing factor.Also the size of the reinforcement particle(Orowan strengthening)contributes in the improvement of strength characteristics.

7.Conclusion

Single pass friction stir processing is performed on the RZ 5 magnesium alloy for the development of surface metal matrix composites using various reinforcement particles namely B4C,MWCNT and a mixture of ZrO2+Al2O3.

1.Introduction of these reinforcement particles onto the matrix of as cast magnesium RZ 5 alloy,during FSP produced a defect free metal matrix composites(MMC).

2.The stir zone of the developed MMCs exhibited a fin grain microstructure with grain size ranging between 0.8μm and 1.87μm as compared to the initial coarse grain of 81μm possessed by the base alloy.

3.The developed metal matrix composites had uniform distribution of reinforcement particles at the stir zone.

4.Metal matrix composites exhibited an increased microhardness at the stir zone.The microhardness of the composites ranged between 293 HV and 403 HV as compared to 81 HV microhardness of the base material and no agglomeration of reinforcement particle at the stir zone.

5.Uni-axial tensile tests conducted on the developed metal matrix composites demonstrated improved/increased strength characteristics.Metal matrix composites developed using various reinforcements exhibited an increase in strength ranging between 250MPa and 320MPa.The main mechanism that governed the enhancement of strength in all metal matrix composites developed is by Hall–Petch strengthening on account of generation of fin grains during FSP.