Feasibility of surface-coated friction stir welding tools to join AISI 304 grade austenitic stainless steel

A.K.LAKSHMINARAYANAN*,C.S.RAMACHANDRAN,V.BALASUBRAMANIAN

aDepartment of Mechanical Engineering,SSN College of Engineering,Kalavakkam,603 110,Chennai,Tamil Nadu,India

bCentre for Materials Joining&Research(CEMAJOR),Department of Manufacturing Engineering,Annamalai University,Annamalai Nagar,608 002, Tamil Nadu,India

Feasibility of surface-coated friction stir welding tools to join AISI 304 grade austenitic stainless steel

A.K.LAKSHMINARAYANANa,*,C.S.RAMACHANDRANb,V.BALASUBRAMANIANb

aDepartment of Mechanical Engineering,SSN College of Engineering,Kalavakkam,603 110,Chennai,Tamil Nadu,India

bCentre for Materials Joining&Research(CEMAJOR),Department of Manufacturing Engineering,Annamalai University,Annamalai Nagar,608 002, Tamil Nadu,India

An attempt is made to develop the tools that are capable enough to withstand the shear,impact and thermal forces that occur during friction stir welding of stainless steels.The atmospheric plasma spray and plasma transferred arc hardfacing processes are employed to deposit refractory ceramic based composite coatings on the Inconel 738 alloy.Five different combinations of self-fuxing alloy powder and 60%ceramic reinforcement particulate mixtures are used for coating.The best friction stir welding tool selected based on tool wear analysis is used to fabricate the austenitic stainless steel joints.

Friction stir welding;Atmospheric plasma spraying;Plasma transferred arc hardfacing;Stainless steel

1.Introduction

Friction stir welding(FSW)fnds applications in aerospace and defence industries due to its capability of joining aluminium and light weight materials with superior properties compared to the conventional fusion welding processes. Friction stir welding tool used to weld high melting temperature materials,such as steels and titanium alloys,must be capable enough to withstand the high shear and impact forces as well as high wear resistance at elevated temperature.In recent years,friction stir welding of steels were carried out using sintered proprietary tools made out of materials such as tungsten molybdenum alloys,tungsten rhenium alloys,tungsten alloys embedded with carbide particles and super abrasives such as polycrystalline cubic boron nitride.

Though successful welds could be made using the above mentioned tool materials,the costs of such high end materials are very high due to diffculty in manufacturing of these tools. Furthermore,the utilisation of these proprietary FSW tools always ends up in the generation of tool debris in the weld nugget,which in turn compromises the mechanical and corrosion resistance properties.

Packer et al.[1]presented polycrystalline cubic boron nitride(PCBN)tool geometries and process parameters required to friction stir welded steels,stainless steels,and nickel base alloys.They found that the PCBN tools were prone to thermal damage and brittle failure.Park et al.[2]reported that borides were formed during friction stir welding due to reaction of tool material with the base metal and subsequently infuenced the properties of the welded joints.

Though most of FSW tools are manufactured through powder metallurgy routes,few investigators have also tried conventional manufacturing techniques to develop FSW tools to weld high melting point materials.Sato et al.[3]developeda new FSW tool that enables welding of high-softeningtemperature materials,such as steels,and titanium alloys. The new tool was made of a Co-based alloy strengthened by precipitating intermetallics,Co3(Al,W),with an Ll2structure at high temperatures.The Co-based alloy tool can be manufactured at a low cost through a simple production method consisting of casting,heat-treatment,and then machining.It exhibited yield strength higher than 500 MPa at 1000°C, hence this tool might have great potential as a FSW tool for joining hard materials.

Ouyang et al.[4]developed a WC-based cermet/tool steel functionally graded material(FGM)FSW-tool to weld higher melting temperature materials and highly abrasive materials, such as stainless steels,titanium alloys and metal matrix composites.They controlled the amount of constituent powders(WC-based cermet/tool steel)under the optimised laser cladding condition and successfully synthesised FGM based FSW tool layer by layer.They reported that the FGM exhibits an expected gradient structure,a high bonding strength,and a distribution in functionality of synthesised layers between a superhard surface with an excellent wear resistance and an inside core with a combination of toughness and strength at elevated temperatures.

In this investigation,an attempt was made to modify the surface of the FSW tool material by providing refractory compositecoatingsdepositedusingthetwowell-establishedsurface modifcation techniques,namely the atmospheric plasma spraying(APS)and plasma transferred arc(PTA)hardfacing processes,in order to bring down the cost and increase the durability(increasing the wear resistance of tool and reducing thetooldebrisintheweldnugget)oftheFSWtools.Further,the fabricated tools were used to join AISI 304 austenitic stainless steel and the results are reported in this paper.

2.Experimental work

2.1.Development of FSW tools

In this investigation,the following carbide powders,namely tungsten carbide(WC),chromium carbide(CrC),boron carbide(B4C),titanium carbide(TiC)and boron nitride(B4N), were chosen for coating and hardfacing purpose.The selffuxing alloy powder NiCrFeBSiC(SF)(Cr 17,Fe 12,B 4.0, Si 4.0,C 1.0 and Ni 62(in wt.%))was chosen as the binder. The ceramic reinforcement powders had a size in the range from 10 μm to 45 μm and the self-fuxing powder had a size in the range from 60 μm to 105 μm.The self-fuxing powder and the independent ceramic powder were mixed with a weight ratio of 40 and 60%,respectively,and were mixed thoroughly for 24 h in a ball mill(Make:VBCC:India.Model:VBPBM-11 high energy ball mill)containing zirconia balls and isopropyl alcohol as mixing medium.After mixing,the powders were dried and used for spray/hardfacing purpose.

2.1.1.Atmospheric plasma spraying(APS)

A large number of spray trial runs with different combinations of APS process parameters were carried out on gritblasted Inconel 738 tool material surface to determine the optimal APS parameters.Inconel 738 was chosen as the substrate for coating/hardface deposition,since it has high temperature stability above 850°C and also its high weldability.The plasma spray deposition was carried out using a 40 kW IGBT-based plasmatron APS system(Make:Ion Arc Technologies,India;Model:APSS-II).The SF+WC powder was directly sprayed on to the grit-blasted tool(IN738)surface.Coating thickness for all the deposits was maintained at 500±15 microns.Adhesion strength,shear strength,porosity, and the hardness of the coatings were considered for optimisation.

The optimised APS parameter settings to deposit SF+60% ceramic reinforcement particulates were:power:23 kW;standoff distance:100 mm;primary gas fow rate(Ar):35 lpm; secondary gas fow rate(N2):4 lpm;powder feed rate:24 gpm; and carrier gas fow rate(Ar):11 lpm.The detailed characterisation and optimisation of APS process parameters to deposit dense coatings are dealt with elsewhere[5].

2.1.2.Plasma transferred arc hardfacing(PTA)

The hardfacing of the SF+60%ceramic reinforcement particulates mixture was carried out using an automatic PTA hardfacing system(Make:Primo Automation Systems,India; Model:OMPLAS-PRIMO-010).The experiments were conducted by forming a single layer with direct current electrode negative(DCEN)operation mode.Again a large number of weld trial runs with different combinations of PTA process parameters were carried out to determine the optimum conditions which will yield a low dilution deposit.The optimum parameter settings to hardface the tool material(IN738)with SF+60%ceramic reinforcement particulates were:transferred arc current:160 A;rotational speed:30 rpm;powder feed rate: 30 gpm.The hardfaced tools were then meticulously machined using diamond grinding wheel so as to get the specifed pin profle and the tool dimensions.The freshly machined tools were measured using vernier calliper and the total surface area was measured using digital proflometry.The methodology adopted to optimise and predict the dilution levels in the PTA hardfaced deposits can be referred elsewhere[6].

2.1.3.Coating microstructural analysis

Thespecimensformetallographicexamination were sectioned using a slow speed metallurgical sample saw(Make: Ducom,India;Model:MSS-10)equipped with resin bonded diamond cutting disc.The sectioned specimens were polished using different grades of emery papers.Final polishing was done using the diamond compound(1 μm particle size)slurry in a disc-polishing machine.

A solution containing 5 g ferric chloride,2 ml hydrochloric acid and 95 ml alcohol was used to etch the coatings and hardfaced deposits for 20-30 s.The porosity present in the APS coatings was analysed at seven different locations on the polished cross section of a coating under a 1000×magnifcation using an optical microscope(Make: Meiji,Japan.Model:MIL-7100)equipped with image analysing system as per ASTM B 276 standard.

The microstructures of hardfaced deposits were analysed using scanning electron microscope (Make: Quanta, Switzerland.Model:3D FEG-I).The phases present in the deposits were determined by X-ray diffraction analyser (Make:Rigaku,Japan.Model:ULTIMA-III).

2.1.4.Evaluation of mechanical properties of the coatings and hardfaced deposits

The microhardness measurement of all the deposits were made using Vickers microhardness tester(Make:Shimadzu, Japan;Model:HMV-2T).A load of 300 g and a dwell time of 15-s were used to evaluate the hardness.Hardness values were measured at ten random locations on the polished cross sections of the coatings and hardfaced deposits.To evaluate the coating properties,three geometries of coated substrates were used:(i)25.4 mm×25.4 mm(diameter×height)cylindrical specimens for tensile bond strength test; (ii) 25.4 mm×50.8 mm×3 mm coupons for lap shear bond strength test;and(iii)25.4 mm ×25.4 mm coupons for metallographic examination and hardness measurement.The tensile bond strength test and the lap shear bond strength test for the APS coatings were carried out using an universal testing machine(Make:FIE BlueStar,India;Model: UNITEK-94100)as per ASTM C 633 and EN 1465 standards. A commercially available heat-curable epoxy was used as an adhesive to test the coated specimens.The tensile bond strength and lap shear bond strength values of the epoxy were found to be 65 MPa and 34 MPa,respectively.The analysis and interpretation of the bond strength test results of the APS coatings are available in Ref.[7].

2.1.5.Plunge and shear test

The coatings and hardfaced deposits made over the tool (IN738)were subjected to shear and plunge tests using an indigenously developed FSW machine(Make:R.V.Machine Tools Pvt.Ltd.,India:Model:EFWM-FSW-01).The base material used for FSW trials was AISI 304 austenitic stainless steel.The APS process parameter selected was highly reproducible in all the considered powder combinations,and APS Coating yielded a porosity level of 4±2 Vol%.The hardness was only factor which got varied(in the range of 637-873 HV0.3)with respect to SF+60%reinforcement combinations.

In post deposition,the coated tool was fxed in the FSW machine and was plunged into the base plate.Traverse motion was not given and the tool was retracted after dwell time of 30 s.It was found that the coating got delaminated from the tool and some of the coating debris got incorporated into the weld,as shown in Fig.1.The optimised PTA parameters led to a 7%dilution level in the tool material.A sample of PTA hardfaced FSW tool and its corresponding cross-sectional macrostructure are shown in Fig.2.

Fig.1.APS coated tool and delaminated coating after plunge test.

The developed PTA hardfaced tools were also subjected to plunge test,where the tool was allowed to plunge into the base metal at different plunge rates,and the profles of tools were analysed.After each trial,the tool was remachined or redressed to the original pin profle and the tool dimensions were characterised using digital proflometry.The tool with minimum wear at macro level is subjected to shear test where the tool is moved at different welding(traverse)speeds at the end of plunging stage.The micro level tool debris in the stir zone of FSW joints were classifed and quantifed using image analysis as per the ASTM E562 guidelines.The tools which had suffcient hot strength and wear resistance to withstand the shear loads that acts during the traverse movement were selected based on shear test and used for further investigation. Some of the FSW bead profles produced after shear test are shown in Fig.3.

2.2.FSW experiments and mechanical testing of welds

After a series of feasibility tests,the following FSW parameters were selected.A welding tool made of PTA hardfaced SF+60%boride nitride,consisting of a shoulder with a diameter of 20 mm and a tapered pin with a length of 2.7 mm, was used to fabricate the joints.The pin was tapered from 9 mm at the shoulder to 6 mm at the pin tip.The tool rotation speed was varied from 400 rpm to 1200 rpm and the welding speed was varied from 50 mm/min to 130 mm/min.The tool was not tilted,and the joints were fabricated under a position control mode by maintaining the penetration depth at 2.7 mm using servomotors and an automated control system during FSW.The photograph taken during friction stir welding ofAISI 304 austenitic stainless steel is shown in Fig.4.It can be observed from Fig.4 that there is no change in shape of pin profle after the withdrawal.

Fig.2.PTA hardfaced FSW tool and its corresponding macrostructure.

Fig.3.Beads after shear test.

The fabricated welded joints were sliced using abrasive cutting and then machined to the required dimensions for preparing tensile,impact,metallographic and bend test specimens.Un-notched smooth tensile specimens were prepared to evaluate transverse tensile properties of the joints,such as tensile strength and fracture elongation.Charpy impact specimens were prepared to evaluate the impact toughness of weld metal with the notch placed(machined)at weld centre.As the plate thickness is small,the subsized specimens were prepared.ASTM E23-06(2006)specifcations were followed for preparing the impact specimens.Bend tests were performed as per ASTM E190-03 specifcations.

3.Results

3.1.Microstructure of APS coating

The cross-sectional microstructure of the APS deposited SF+60%WC coating is shown in Fig.5.It can be seen from Fig.5 that the mechanical bonding with coating contains pores.

3.2.Microstructures of hardfaced deposits

The microstructures of the hardfaced deposits are shown in Fig.6.From Fig.6a-e,it is evident that all the deposits have the respective reinforcing particulates evenly distributed throughout the SF matrix.The light grey areas represent the SF matrix,and the dark blocky and angular particulates are the reinforcements.The deposition effciency of the PTAW process is close to 88%.Hence,the possibility of volume change and distribution of ceramic particles in the microstructure of hardfaced deposits is very less.The interface characteristic of the hardfaced deposit can be seen in Fig.6a which displays very low dilution along with fne anchorage between the hardfaced deposit and the substrate(IN738).

3.3.Tool wear mechanisms

As reported by Gan et al.[8],in friction stir welding,tool wear takes place by plastic deformation and by physical wear (abrasive or adhesive)process.During FSW,the plastic deformation(geometrical change)of tool occurs due to large stress developed at high temperature.The deformation is commonly referred to as“mushrooming”,which results as the height of tool during FSW gets depressed with the resultant increase in the width of tool.Though it was reported that plastic deformation was the major cause of the tool degradation,in this study the developed tools failed mainly due to the abrasive and adhesive wear compared to the failure due to plastic deformation(Fig.7).

Fig.4.Photograph taken during FSW of AISI 304 stainless steel.

Fig.5.Microstructure of hardfaced deposits made over IN738 substrate.

Abrasion takes place by direct contact(i.e.two-body)of the base materials with the tool material.This mechanism also often changes to three-body abrasion as the wear debris then acts as an abrasive between the base metal and tool shoulder surface.When steel is welded,usually tool failure occurs by adhesive wear.Abrasive wear occurs by the diffusion of elements present in the base material into the tool surface during friction stir welding of high melting point materials.Thesediffused materials can form brittle intermetallics on the surface of the tool and easily break off during high temperature deformation.The profles of developed tools after FSW trials are shown in Fig.7.From Fig.7a-d,it can be well understood that a majority of the wear occurs in the pin and a small amount of wear occurs at the shoulder.Most of failures are due to abrasive and ploughing as observed in Fig.7e-i.Sticking (adhesive)was also observed,as shown in Fig.7m-p.

Fig.6.Tool profles after trial runs.(a)SF+60%WC hardfaced deposit along with the HAZ and substrate interface.(b)SF+60%Cr3C2 hardfaced deposit. (c)SF+60%TiC hardfaced deposit.(d)SF+60%B4C hardfaced deposit.(e)SF+60%B4N hardfaced deposit.

Fig.7.Surface and bead profles of FSWed AISI 304 plates joined using PTA hardfaced tool(after trail runs).

3.4.Evaluation of tool wear rate

The tool wear rate was evaluated by calculating the tool cross-sectional area(Fig.8a)before and after FSW trials.The tool cross section consists of the entire pin profle and 1.5 mm below the shoulder surface to include the wear on the shoulder surface in addition to the wear on the pin.In almost all the tools used in this investigation,the majority of wear occurred in the pin region and the amount of wear that occurred at the shoulder of the tool was very less.Fig.8b shows the reduction in the cross-sectional area with respect to the life(length of the weld)of the tools developed in this investigation.Fig.8c shows the linear ft of the normalised wear data.It can be observed that the material losses from the original crosssectional areas of the PTA hardfaced WC,CrC,TiC,B4C and B4N tools were 31.2%,29%,22.3%,14.9%,and 3.6%, respectively,per unit metre length.Based on the normalised wear data,the reduction in cross-sectional area of B4N tool material was observed to be 88.5%,87.6%,83.9%and 75.8% under a considered weld length compared to WC,CrC,TiC and B4C tools.

3.5.Tool debris classifcation by image analysis

The tools which passed the plunge and shear tests were used to fabricate few joints,and the stir zone of the FSW joints were analysed using optical microscopy to fnd out theinclusion of tool debris.Fig.9 shows the analysis results of tool debris in stir zone(pin infuenced)produced using SF+60%WC hardfaced tool.

Fig.8.Tool wear analysis.a)Schematic showing cross sectional area calculation.b)Loss of material over entire tool life.c)Normalized tool wear plot.

Similar analysis was carried out for other hardfaced combinations and the consolidated results obtained from the tool debris image analysis are presented in Table 1.It could be inferred that the hardfaced tool containing SF+WC combination ends up with producing more Vol.%of tool debris followed by SF mixture containing CrC,TiC and B4C.The hardfaced tool containing SF+B4C did not produce any debris in the weld nugget/stir zone region of welded joints. From the above-mentioned tests,it was inferred that the SF+B4C combination survived the plunge and traverse shear tests,and there was no micro level tool debris inclusion in the weld nugget for a weld length of 900 mm.This tool was further used to make few joints.The resultant properties are discussed in the following sections.

3.6.Tensile,impact toughness and bend properties

The effects of rotational speed and welding speed on the transverse tensile strength and impact toughness are shown in Fig.10.The joint fabricated at a rotational speed of 600 rpm yielded superior tensile strength compared to other joints.It is governed by the base metal yield and ultimate tensile strength since the fracture takes place in the base metal.Other joints were found to fail in the advancing side(banded structure)of the weld metal region.

The tensile strength decreases with the increase in rotational speed and the decrease in the welding speed due to the coarsening of banded structure.The failure occurs in the weld metal region due to the presence of banded structure.All the joints exhibit lower impact toughness compared to the base metal,which is attributed to the presence of high density of intermetallic phases in the bottom of the stir zone.Among the joints,less reduction in toughness values was observed in case of the joints fabricated at rotational and welding speeds of 400 rpm and 110 mm/min respectively.Fracture elongation was also evaluated.It was observed that the fracture elongation was almost the same as that of the base metal for the specimens which failed in the base metal region.But when the fracture elongation was insuffcient,the fracture was initiated from the weld metal region.However,all the specimens have passed the bend test under 2T condition,which indicates that the joints exhibit good ductility(Fig.11).

4.Discussions

Fig.9.Image analysis of SF+60%WC PTA hardfaced tool debris in the weld metal.a)Optical micrograph of pin infuenced region.b)Binary coded image. c)Volume fraction analysis(ASTM E 562).

Of the two types of FSW tools developed,APS deposited tools failed during the plunging stage of friction stir welding. In atmospheric plasma spraying,the base material rarely gets heated above 300°C.Thus the kind of bonding,which takes place between the substrate and coating,is a mechanical bonding.Further,the cooling rates of the droplets are too high, so the possibility of metallurgical bonding is less,though it has been reported by Hiemann[9]that some amount of diffusion between the substrate and deposits takes place,which is restricted to metallic and MMC based coatings.

The fracture surface of the SF+60%WC APS coating is shown in Fig.12.The fracture surface displays brittle type fracture with large cleavage facets,inter-and intra-lamellar voids and intermittently embedded solid WC particulates.Due to the intrinsic nature of APS coatings(mechanical bonding and low cohesivity between the lamellae),the lap shear and tensile bond strength values of the investigated coatings were 22 MPa and 45 MPa,respectively.The lower shear and bond strength of the APS coatings led to the premature failure of coated tool during the plunging stage of the FSW process.

On the other hand,from the plunge and shear test results,it was found that the PTA hardfaced SF+60%WC tool underwent severe wear damage(Fig.6a-d).The reason for severe wear during plunge and shear tests might be due to the degradation of WC during PTA deposition.The application of tungsten carbide SF based alloy composites by PTAW involves application of the powder feedstock into a transferred(from cathode to workpiece)plasma arc.The temperature at the core of the arc plasma may reach 18,000°C.The powder is fed into the vicinity of the arc,but is not fed through the arc as in FCAW or GMAW,for example[10].Marimuthu et al.[11] reported that the powder consumable can be easily altered togive optimum characteristics.For example,carbide loading, carbide type,matrix alloy chemical composition,and carbide morphology can easily be changed to give different microstructures and the resulting wear resistance that is required for a particular application.For these reasons,PTAW application of Ceramic/SF alloy composites is the preferred method.

Table 1Consolidated results of measured inclusion using Image Analysis based on ASTM Standards.

Fig.10.Effect of process parameters on tensile strength and impact toughness.

In our investigation,a dilution of 7%was achieved under optimised process parameters condition.Even under such idealised condition,the results indirectly indicate that the carbides get dissolved,particularly towards the deposit surface.Dissolution of the W2C-WC structure results in tungsten and carbon going into solution in the nickel alloy matrix, resulting in reprecipitation of various undesirable complex carbides upon the cooling of weld pool.A parallel effect results in overall hardening of the matrix alloy due to super saturation of tungsten and carbon as reported by Berger et al. [12].This hardening can be benefcial in regards to wear resistance assuming that the impact and shear forces are mild. The impact force encountered by the tool during the plunging stage of FSW of stainless steels is quite high.However,a hardened matrix may begin to fail from mechanisms other than wear under such impact conditions.The impact and wear resistances of SF+60%B4N hardfaced tool were much superior compared to other hardfaced tools,which resulted in debris free weld nugget.The mechanical properties of the PTAW deposited composites are extremely controlled by the structure and properties of the reinforcement/matrix interface. These controlling factors are affected by fabrication process, including operation time,temperature and the type of matrix or reinforcement.The reasons for high hot shear wear resistance of SF+60%B4N hardfaced composite tool may be due to the high structural stability,hardness,fracture toughness and oxidation resistance.The role of SF matrix and the advantage of B4N particles are discussed in the following sections.

Fig.11.Bent test specimens.

Fig.12.Fracture surface of SF+60 WC APS coating.

4.1.The advantages of using B4N particle in SF matrix

From this investigation,it is clear that a dominant factor of wetting seems to be the dissociation of B4N into boron and nitrogen dissolved in the melt(Fig.13).

Fig.13.XRD of boron nitride deposit.

This dissociation is promoted by the increase in solubility of boron or nitrogen and by consumption of these elements. Owing to these,the interfacial reaction through the dissociation of B4N particle may provide a good wettability,resulting in strengthening of the deposit(Fig.6e).In general,the wettability is improved by a decrease in the surface energy of liquid surface.So,it is interesting to know the effect of a surface active element on the matching incorporation time,as reported by Kennedy[13].Then,such elements as chromium and silicon which reduces the surface energy of liquid matrix were already present in the melt.Furthermore,Sarikaya,et al. [14]pointed out that these alloying elements are considered to adsorb at the interface between the B4N particle and the melt as well as on the melt surface.One of the advantages of the B4N based composite is its high strength at elevated temperatures and its characteristic close lattice matching with the SF matrix[15].The aforementioned observation led to the outstanding performance of the B4N based tool.It is not clear why SF readily wets B4N,but it is possible that the stoichiometry of the B4N powder plays an important role.The B4N reinforcement in the PTAW deposited MMCs is more fracture resistant compared to the others.The B4N particles are refractory in nature[16]and are able to withstand the thermal shock during FSW,and the composite is hard and also more fracture resistant than other considered reinforcements.The SF+60%B4N PTA hardfaced tool effectively withstood the temperature and high axial and shear stresses produced during the motion of the FSW tool during welding of stainless steels, proving its capability to be used as candidate tool material to weld high melting temperature materials.

5.Conclusions

1)In this investigation,an attempt was made to modify the surface of the FSW tool material by providing refractory composite coatings deposited using APS and PTA hardfacing process.

2)The carbide powders,namely tungsten carbide,chromium carbide,boron carbide,titanium carbide and boron nitride, were chosen for coating and hardfacing purpose.The selffuxing(SF)alloy powder NiCrFeBSiC was chosen as the binder.

3)Due to the intrinsic nature(mechanical bonding and low cohesivity between the lamellae)of APS coatings,the lap shear and tensile bond strength values of the investigated coatings were 22 MPa and 45 MPa,respectively.The low values of shear and bond strength of the APS coatings led to the premature failure of coated tool during the plunging stage of the FSW process.

4)The normalised wears of the PTA hardfaced SF+B4N tool material were reduced by 88.5%,87.6%,83.9%and 75.8%,respectively,over a given length compared to WC, CrC,TiC and B4C tools.

5)Among the friction stir welded austenitic stainless steel joints made using PTA hardfaced SF+B4N tool,less reduction in toughness values was observed in case of the joints fabricated at rotational and welding speeds of 400 rpm and 110 mm/min respectively.

6)The reasons for high hot shear wear resistance of SF+60%B4N hardfaced composite tool may be due to its high structural stability,hardness,fracture toughness and oxidation resistance.

Acknowledgements

The authors wish to place their sincere thanks to the Department of Science and Technology(DST),Government of India for the fnancial support through Fast Track Scheme for Young Scientists R&D project(SR/FTP/ETA043/2009)to carry out this investigation.The authors are grateful to Dr.G. Madhusudhan Reddy,Scientist-‘G’and Head,Metal Joining Group,Defence Metallurgical Research Laboratory(DMRL), Hyderabad,for his support and guidance to this investigation.

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Received 5 May 2014;revised 23 June 2014;accepted 7 July 2014 Available online 27 July 2014

*Corresponding author.Tel.:+91 44 27469700x236;fax:+91 44 27469772.

E-mail addresses:lakshminarayananak@ssn.edu.in,akln2k2@yahoo.com (A.K. LAKSHMINARAYANAN), csrcn@rediffmail.com (C.S. RAMACHANDRAN),visvabalu@yahoo.com(V.BALASUBRAMANIAN).

Peer review under responsibility of China Ordnance Society.

http://dx.doi.org/10.1016/j.dt.2014.07.003

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Copyright?2014,China Ordnance Society.Production and hosting by Elsevier B.V.All rights reserved.