Effect of tool plunge depth on reinforcement particles distribution in surface composite fabrication via friction stir processing

Sndeep Rthee,Schin Mheshwri,Arshd Noor Siddiquee,Mnu Srivstv

aDivision of Manufacturing Processes and Automation Engineering(MPAE),Netaji Subhas Institute of Technology,New Delhi,India

bDepartment of Mechanical Engineering,Jamia Millia Islamia,A Central University,New Delhi,India

Effect of tool plunge depth on reinforcement particles distribution in surface composite fabrication via friction stir processing

Sandeep Ratheea,*,Sachin Maheshwaria,Arshad Noor Siddiqueeb,Manu Srivastavaa

aDivision of Manufacturing Processes and Automation Engineering(MPAE),Netaji Subhas Institute of Technology,New Delhi,India

bDepartment of Mechanical Engineering,Jamia Millia Islamia,A Central University,New Delhi,India

A R T I C L E I N F O

Article history:

Received 31 August 2016

Received in revised form

20 October 2016

Accepted 15 November 2016

Available online 24 December 2016

Metal matrix composites

Aluminium matrix surface composites are gaining alluring role especially in aerospace,defence,and marine industries.Friction stir processing(FSP)is a promising novel solid state technique for surface composites fabrication.In this study,AA6061/SiC surface composites were fabricated and the effect of tool plunge depth on pattern of reinforcement particles dispersion in metal matrix was investigated.Six varying tool plunge depths were chosen at constant levels of shoulder diameter and tool tilt angle to observe the exclusive effect of plunge variation.Process parameters chosen for the experimentation are speed of rotation,travel speed and tool tilt angle which were taken as 1400 rpm,40 mm/min,and 2.5°respectively.Macro and the microstructural study were performed using stereo zoom and optical microscope respectively.Results re flected that lower plunge depth levels lead to insuf ficient heat generation and cavity formation towards the stir zone center.On the other hand,higher levels of plunge depth result in ejection of reinforcement particles and even sticking of material to tool shoulder.Thus,an optimal plunge depth is needed in developing defect free surface composites.

?2016 The Authors.Published by Elsevier Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

It is common knowledge that aluminium alloys are materials of choice for various structural applications in aerospace,defence, automobile,and marine industries owing to their lower weight density,higher strength to weight ratio and higher corrosion resistance[1].However,stiffness and strength of some of these alloys is not adequate for some structural purposes thereby necessitating requirement of suitable reinforcement.Aluminium metal matrix composites(AMMCs)exhibit improved metallurgical, mechanical,and tribological characteristics[1-3].Metal matrix composites(MMCs)can be synthesized using various techniques like laser technique[4],electron beam irradiation[5],plasma spraying[6],casting[7],mechanical alloying[8],etc.Most of these techniques are based on the principle of liquid phase processing which leads to formation of intermetallic reactions and undesirable phases between base metal(BM)and reinforcement[9,10].In view of the these shortcomings,employment of a process for composite fabrication which can be conducted below melting points of the matrix material can go a long way to improve and consequently optimize the MMCs design and fabrication issue.Friction stir processing(FSP)offers an excellent choice for development of surface composites(SCs)of metal alloys[11].

FSP is a newly developed solid state processing technique which is a variant of friction stir welding(FSW)process initiated at The Welding Institute(TWI)in 1991[12].In its simple operation,a nonconsumable rotating tool with an exclusively designed pin and shoulder plummets into a BM plate and is made to traverse in prede fined direction to cover up the desired realm.Softening and plasticization of BM occurs owing to frictional heat generation between rotating tool and the workpiece[10,13].As the tool traverses,the material is forged beneath the shoulder resulting in the processed region.The work on composite fabrication using FSP,was started with the maiden work done by Mishra et al.[11].In this work,composites with Al 5083 alloy as BM and SiC as reinforcement were fabricated.A maximum microhardness of 173 Hv was achieved using 27 vol%of SiC particles which is almost double of the microhardness of BM(85 Hv).Numerous research studies on composite fabrication are reported since the initial work by Mishra et al.and a number of research projects are still in progress.Initially, FSP was used to modify aluminium alloys but with the passage of time FSP has gained a shining role in developing composites ofother alloys like magnesium[14],copper[15],titanium[16]and even steel[17].

The addition of hard phase reinforcement particles make MMCs brittle[18].In many engineering applications,the service life of materials mainly depends on the surface properties of materials. Therefore,SCs are mostly prepared by combining a ductile metallic matrix with hard ceramic reinforcement up to a desired depth.The soundness of SCs apart from other aspects mainly depends on optimal selection of process parameters.Several authors[19-22] investigated the effect of various process parameters in developing SCs.Dolatkhah et al.[19]evaluated the effects of rotational and travel speeds,FSP pass count and size of reinforcement particles in fabrication of Al 5052/SiC composites.They reported that speed of rotation and FSP pass counts have major effect on uniform dispersion of reinforcement particles.Similarly,Zohoor et al.[21] investigated the effects of speed of rotation,FSP pass count and size of reinforcement particles in the fabrication of AA5083/Cu composites.They reported that best powder dispersion was achieved with four FSP pass count.Devaraju et al.[20]reported that speed of rotation and type of reinforcement particles have a strong impact on wear,microhardness and tensile strength of fabricated AA 6061/SiC+Al2O3hybrid SCs.Reddy et al.[23]investigated the effect of reinforcement particles(B4C and SiC)on the wear and mechanical properties of fabricated SCs.

Majority of published research mainly focuses on the evaluation of effects of process parameters,namely tool rotation speed,travel speed,FSP pass count and tool dimensions on the surface and mechanical properties of fabricated SCs.Also,reinforcement particle type and its size remains center of research focus.Literature also report that the SCs imperfections can be reduced by accurate prediction of these process parameters.In addition to these process parameters,correct decision on suitable tool plunge depth(TPD)is also essential to achieve defect free and uniformly distributed SCs. Interestingly,TPD is not changed in-situ after the process has started and investigations on effect of TPD over distribution of reinforcement particles in SCs fabrication are very few.Present work investigates the effect of varying TPD on SCs fabrication by keeping all other parameters constant at optimized level.Six levels of TPD from 0.10 mm to 0.35 mm in steps of 0.05 mm were used to investigate and determine the role of plunge depth on material flow,uniformity of powder dispersion and tendency of defect formation.Additionally,proper care has also been taken to minimize the adverse effects originating from factors like machine vibrations, non-uniformity of BM and backing plate thickness which is normally not paid enough attention.

Fig.1.Showing images of SiC powder and tools used;(a)SEM image of SiC powder;(b)Tools used during FSP.

Fig.2.Steps in SCs fabrication using groove technique.

2.Materials and methods

In this study,AA 6061-T6 alloy sheet of 5 mm thickness was used as base material.AA 6061-T6 is commonly used in aerospace, defence and marine sectors due to its light weight,good strength to weight ratio and good corrosion resistance[24,25].The composition(weight%)of BM is 0.85%Mg,0.68%Si,0.22%Cu,0.07%Zn, 0.05%Ti,0.032%Mn,0.06%Cr and remaining aluminium.FSP samples of size 60 mmwide and 200 mm long were machined from the sheet.SiC powder having average particle size of～10μm was used as reinforcement(SEM image is shown in Fig.1(a).Six such FSP samples were prepared by cleaning them with acetone and machining square grooves of 2 mm width and 2 mm depth along the length.Subsequently,SiC powder was filled and compressed in the groove and upper surface(open)of the groove was closed by means of a tool(15 mm shoulder diameter,see Fig.1(b)without a pin in order to prevent the sputtering of powder during FSP.Finally, FSP was performed on a retro fitted vertical milling machine as shown in Fig.3 using a tool with a threaded cylindrical pin.The tools utilized for FSP were made of H-13 tool steel(see Fig.1(b)). Steps involved in SCs formation using groove technique are schematically illustrated in Fig.2.The values of process parameters such as speed of rotation,traverse speed and tilt angle of tool(see Table 1)were chosen by trial experiments performed on AA6061with TPD of 0.20 mm.Six experiments were performed by varying TPD from 0.10 mm to 0.35 mm in steps of 0.05 mm and keeping all other parameters(as shown in Table 1)as constant.

After single pass FSP on all six samples,coupons for macro and microstructural study were machined using wire-EDM from the middle of each SC sample in a direction perpendicular to the processing route.These samples were polished following standard metallographic procedures and then etched with Keller's reagent (175 ml water,20 ml HNO3,3 ml HCL and 2 ml HF)for 30 s.Macrographic images were taken by stereo zoom microscope while micrographic images were taken by metallurgical optical microscope and scanning electron microscope.

Fig.3.FSP setup.

Table 1Showing constant process parameters used in SCs fabrication.

3.Results

3.1.Macroscopic observations

Table 2 shows macroscopic images of SCs(S1-S6)along with their visual description.It is evident from the macrographic images that the pattern of reinforcement particles distribution in of all SCs is different.The width of processed region was found to be approximately same as diameter of tool shoulder.However,width/ area of stir zone and SC layer depth varies in all SCs.

Table 2Macroscopic images of all samples(S1-S6)along with their description.

Fig.4.Showing microstructural images of SCs taken at 100 magnifications,(a)corresponds to S1;(b)S2;(c)S3;(d)S4;(e)S5;(f)S6.

3.2.Microstructural characterization

Fig.4(a)-(f)reveals the microstructural images of SCs(S1-S6respectively).A large cavity is present in centre of the SZ(see Fig.4(a)),which is also visible in macrograph corresponds to S1. Fig.4(b)shows that SiC particles dispersion slightly increases and cavity size decreases(with increase in TPD from 0.10 to 0.15 mm) which indicate the improvement in material flow and particles dispersion.With further increase in TPD from 0.15 to 0.20 mm,the improvement in material flow can be further seen in Fig.4(c)in which more particles were distributed in the aluminium matrix as compared to S1and S2.Finally,uniform dispersion of SiC particles was achieved(at TPD of 0.25 mm)without any defect formation as shown in Fig.4(d)whose SEM images are shown in Fig.5.Fig.4(e) and(f)shows that the uniformity of powder dispersion decreases (at TPD of 0.30 and 0.35 respectively).Also,the tendency of defect formation increases.

4.Discussion

Image corresponds to S1(Table 2)is the macroscopic image of SC fabricated at plunge depth of 0.10 mm.A large cavity defect appears at the center of SZ and powder distribution is very less.Shoulder driven flow causes the powder distribution.SC layer depth is quite less.This may be attributed to less material flow due to low heat generation at lower plunge depth of 0.10 mm.At low plunge depth, the contact area between tool shoulder and base metal is less.With the increase of plunge depth from 0.10 mm to 0.15 mm,the contact area between shoulder and workpiece increases which results in more heat generation.Also,the vertical pressure on the base metal increases resulting in better forging and improved material flow and particles dispersion[26]as shown in S2(Table 2).Depth of SC layer also increases.However,a cavity remains un filled in this sample that implies that heat generation was still not adequate.As plunge depth further increases from 0.15 to 0.20 mm,cavity sizedecreases to its minimum and uniformity in powder dispersion increases as shown in S3(Table 2)and Fig.4(c).Also,the SC layer achieves a depth which is approximately equal to tool pin height in bulk matrix.

Fig.5.shows the SEM images of SCs fabricated at 0.25 mm plunge depth,(a)distribution of SiC particles;(b)Maximum size of SiC particle after FSP.

The cavity finally disappears in S4(Table 2)fabricated at plunge depth of 0.25 mm owing to suf ficient heat generation due to the adequate contact area of tool shoulder and workpiece.Distribution of reinforcement particles also becomes visibly uniform(see Fig.4(d)).And SC layer depth becomes equal to tool pin height.

Any further increase of TPD from 0.25 to 0.30 and 0.35 mm results in a reduction of powder dispersion and decrease of SC depth as shown in S5and S6(Table 2)and Fig.4(e)and(f)respectively.It may be assumed that after an optimumvalue of TPD,the increase of plunge depth causes excess heat generation which results in more softening of material near tool matrix interface.This excess softening of material leads to more flash generation.The high tool pressure(at higher TPD),the shoulder expels the reinforcement particles with or in form of flash resulting in less dispersion of SiC in the metal matrix.Same results can be seen from S6,in which SiC distribution further decreases and defect appears in the SZ.Also, the frictional mode between shoulder and material(to be stirred) changes from sliding to sticking due to high pressure exerted by tool shoulder.This high pressure can results in thinning and may even damage of specimen.Thus,low penetration depths causes less material flow while high penetration depths results in excessive flash and damage of the specimen.

Fig.5(a)-(b)shows SEM images of SC fabricated at 0.25 mm plunge depth.It is evident from the figure that SiC particles are well distributed in aluminium matrix and their particle size viz-a-viz original size has reduced drastically.This reduction may occur due to the fragmentation of SiC particles owing to high strain induced by FSP and vigorous stirring action of tool.The vigorous stirring of tool might also have ground the sharp edges of SiC particles(see Fig.5(b)).

Thus,reduction in the size of SiC particles can be attributed to severe stresses and shear effects caused by the tool rotation.Similar results were reported by other researchers[27,28].Moreover,the distributed particles have large size variations and maximium size of SiC particles after FSP was found as 7.28 μm(see Fig.5(b)).The average SiC particle size was found as～2.1μm which is a huge reduction as compared to as-received 10μm average particle size.

This investigation has demonstrated that the size of SiC particles was subdivided during FSP causing an increase in particle density and reduction in grain size and interparticle spacing.Moreover,the scope of FSP for SCs fabrication may be further investigated by using smaller particles size of order of nanometer which may results in further improvement in properties of nano SCs as compared to micro SCs.

5.Conclusions

The practical bene fit of this work is to provide adequate TPD to fabricate sound AA 6061/SiC surface composites.Following conclusions are drawn from the present study:

1)Low plunge depth results in less material flow and cavity formation at centre of SZ owing to less heat generation at low contact area between tool shoulder and base metal.

2)The optimum plunge depth obtained from current work is 0.25 mm with 20 mm shoulder diameter and 2.5°tilt angle.The particles were found well distributed in the aluminium matrix.

3)During FSP,fragmentation of SiC particles takes place owing to high plastic strains and stirring action of FSP tool.

4)The penetration depths above the optimum value may results in sticking of workpiece to the shoulder,flashing out of reinforcement particles,thinning of processed specimen and even damage of the specimens.

Acknowledgments

The authors thank the University Grants Commission(UGC)for its financial assistance(vide sanction order No.F.3-40/2012(SAP-II) under its SAP(DRS-I)sanctioned to the Department of Mechanical Engineering for the project entitled Friction Stir Welding,Ultrasonic Machining.

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*Corresponding author.TRF,MPAE Division,NSIT,New Delhi,110078,India.

E-mail address:rathee8@gmail.com(S.Rathee).

Peer review under responsibility of China Ordnance Society.

Friction stir processing

Tool plunge depth

Microstructural characterization