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					           JOURNAL OF MECHANICAL ENGINEERING AND
                     TECHNOLOGY (JMET)
 Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3924 (Print) Volume 1, Issue 1, July -December (2013)
 ISSN 2347-3932 (Online),
ISSN 2347-3932 (Online)
                                                                              JMET
Volume 1, Issue 1, July-December (2013), pp. 66-72                      ©IAEME
© IAEME: http://www.iaeme.com/JMET.asp




 THE INFLUENCES OF THE FRICTION STIR PROCESSING ON THE
  MICROSTRUCTURE AND HARDNESS OF AA6061 ALUMINIUM
                     SHEET METAL
                               *
                                   T. Prakash1 and P. Sasikumar2
       1
           Asst. Prof (Sr. Grade) and 2Professor Department of Mechanical Engineering
              2
                KPR Institute of Engineering and Technology, Coimbatore – 641407


ABSTRACT

        Friction stir processing (FSP) is an emerging surface engineering technology that can
eliminate casting defects locally by refining microstructures, thereby improving the
mechanical properties of the material. In this study, the influence of the Friction Stir
Processing (FSP) on the microstructure and mechanical properties in terms of hardness for
commercially available AA6061 Al sheet metals was studied and investigated. Samples were
subjected to FSP by varying the number of passes using cylindrical geometry type high speed
steel tool fixed in the Vertical milling machine. Micro structural observations were carried
out by employing optical microscopy on the modified surfaces. From the microstructural
evaluation, it was observed that the grain size of the processed area was around 70%
decreased as compared to unprocessed parent metal. This is expected due to grain refinement
by the FSP tool in the materials during processing. The hardness results showed that by
increasing the number of passes the hardness of the produced composite surfaces increases
steadily. It was noted here that the strength of stirred surface area was around 1.75 times
higher than the unstirred surface area.

Key words: Friction stirs processing, microstructure, mechanical properties

1. INTRODUCTION

   Recently friction stir processing (FSP) was developed by Mishra et al. [1,2] as a generic
tool for microstructural modification based on the basic principles of FSW (Fig.1). In this
work, a rotating pinned tool is fixed in the vertical milling machine (fig 2). The tool is
inserted in a monolithic work piece for localized microstructural modification for specific
property enhancement. For example, high-strain rate superplasticity was obtained in
commercial 7075Al alloy by FSP [1–3]. Furthermore, FSP technique has been used to
produce surface composite on aluminum substrate [4], homogenization of powder metallurgy
aluminum alloy [5], microstructural modification of metal matrix composites [6] and property



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Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)

enhancement in cast aluminum alloys [7]. FSW/FSP is emerging as a very effective solid-
state joining/processing technique.




                     Fig. 1 Schematic drawing of friction stir processing

        Dispersion of nanosized reinforcements on metallic substrate to produce surface metal
matrix nanocomposite (SMMNC) and the control of its distribution are difficult to achieve by
conventional surface treatments [9]. The processing of SMMNC at temperatures below
melting point of substrate, is beneficial as unwanted interfacial reaction between
reinforcement and matrix can be avoided [8, 10]. Much attention has been paid to friction stir
processing (FSP) as a surface modification technique based on friction stir welding (FSW)
[11]. FSP is also one of a solid-state processing technique to refine the microstructure. This is
carried out using the same approach as friction stir welding (FSW), in which a non-
consumable rotating tool with a specially designed pin and shoulder is plunged into the plate
to be processed and traversed along the line of the plate. Localized heating is produced by the
friction between the rotating tool and the workpiece which increases the local temperature of
the material to the range where it can be plastically deformed. As the rotating tool traverses
along the plate, metal is essentially extruded around the tool before being forged by the large
down pressure. During this process, the material undergoes intense plastic deformation
resulting in significant grain refinement [8, 12–16]. The local heating due to friction and
forging action of the tool deform and process the material at elevated temperature. Since the
material flows at elevated temperatures, the process also offers the possibility of
redistributing the particles in metal matrix composites [17-20].The main objective of the
present study is to investigate the effect of FSP on the microstructural and strength
improvement of AA 6061 Al sheet metal.

2. EXPERIMENTAL PROCEDURE

2.1 Friction stir processing (FSP)
       The starting materials were monolithic cold-rolled plates of AA 6061aluminium alloy
with a nominal composition as shown in the table 1. The surface of plates was cleaned with
grinding paper before processing. The dimensions of the workpiece were 50 mm×100 mm×3
mm. The plate was subjected to a FSP as shown in the macrograph in Fig.2; the tool used to
carry out the FSP is also shown as a inset (Fig.2) which is composed of a plain cylindrical
shoulder with pin. The tool was made from high speed steel with the dimensions of 12 mm
shoulder diameter and 4 mm pin diameter with length of 2 mm. The designed FSP tool was

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Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)

mounted in vertical milling machine and the workpiece was rigidly fixed on the machine
table. First the pin is plunged (0.3 mm depth) into the workpiece surface to be processed and
then the tool was allowed to rotate in forward direction with rotation speed (ω) of 550 rpm.
The traverse speed of 60mm/min was used during FSP. These parameters were optimised by
carrying out several trial and error experiments. All FSP studies were carried out at room
temperature.

            Table 1. Chemical Composition (wt %) of aluminium alloy AA6061

        Chemical
                           Si      Fe       Cu       Mn    Mg      Zn       Ti     Al
       Composistion
          6061            0.4      0.7     0.15     0.15   0.8    0.25     0.15    Bal.




              Fig. 2. Macrograph showing the FSP tracks on the AA6061 plate

2.2 Characterization
        The FSP samples for microstructural characterization were cut from the unstirred,
shoulder stirred and pin-stirred zone of the plate. The samples were polished with emery
paper up to 1200 grit followed by polishing with alumina suspension and diamond paste on a
velvet cloth. Further, samples were etched with Keller reagent to distinguish the grain
boundaries, identify precipitates and difference in composition. Finally, the microstructure
investigation was carried out using optical microscopy.

2.3 Mechanical property evaluation
        The mechanical property in terms of hardness tests were carried out to assess the
effect of FSP on the AA6061 Al alloy sheet metal. Vickers micro hardness tests were carried
out using a Wolpert Wilson Micro hardness tester. A load of 50 g and a dwell time of 20 s
were used. The average micro hardness measurements were made on the FSP treated
surfaces.




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   Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
   ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)

3. RESULTS AND DISCUSSION
          The effect of friction stir processing on the microstructure and strength in terms of
   hardness for AA 6061 aluminium alloy is discussed here below:

   3.1. Microstructural evaluation




                           Fig. 3. Microstructure of parent metal AA 6061



               (a                                  (




              Fig. 4. Microstructure of AA 6061 after FSP: (a) Two pass (b) Four pass

          The microstructure of AA 6061 Al alloy in as-received condition (parent metal) is
   shown in Fig. 3. From the Fig. 3, it can be clearly seen that the average grain sizes of AA
   6061 is around 85 µm for unprocessed samples (i.e. parent metal). After friction stir
   processing, the microstructures of AA 6061 for two pass and four pass is shown in Fig.4. It
   can be noted clearly here that the material grain sizes are completely refined after FSP due to
   sever plastic deformation engendered by the FSP tool in the parent metal during processing.
   The grain size of AA 6061 after FSP is around 35 µm which was decreased by 2.4 times as
   compared to unprocessed parent metal for four passes. Further, the grain size of the alloy was
   decreased markedly with respect to number of passes. This is to the fact that the rotation
   speed of FSP tool and its travelling speed were expected to cause more heat input which
   might affects the thickness of the surface layer and as a result that refines the grain size of the
   material.

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Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)

3.2. Micro hardness evaluation
        The variation of Vicker’s hardness with different areas of AA6061 Al alloy processed
by FSP is shown in Table 2. From the result it can be observed that in the case of AA6061 Al
alloy the hardness of stirred zone is around 35 %, 37 %, 45 % & 49 % greater than unstirred
zone with respect to number of passes for single, double, three & four passes respectively.
This is expected due to grain refinement in the structure obtained by severe plastic
deformation. Further it can be noted in the sample that as the number of passes increases the
hardness of stirred zone increases rapidly first (up to three passes) and then increases slightly.
This is due to the fact that the strengthening of the material reaches saturated condition up to
three passes and beyond which there is no significant improvement in the strength. Hence in
this work the samples are processed via FSP up to four passes. The hardness variation with
number of passes for AA 6061 Al alloy is also shown in Fig. 5. For instance, the variation of
hardness with distance measured from parent metal (left)-to-centre of stirred zone-to-parent
metal (right) of AA 6061 al alloy for double passes is also shown in Fig. 6.




            Fig.5. Variation of hardness with number of passes processed via FSP
                                      AA 6061 Al alloy




  Fig.6. Variation of hardness with distance measured from parent metal (left)-to-centre of
   zone-to-parent metal (right) for AA 6061 Al alloy processed via FSP (No. of pass = 2)




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Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)

    Table 2. Variation of Hardness with different areas of AA6061 processed by FSP

                                             Hardness, HV0.5 (MPa)
               No. of                                             Shoulder
    Samples              Left end      Shoulder     Center of                   Right end
               Passes                                              stirred
                          (parent       stirred      stirred                     (parent
                                                                    area
                           metal)     area (left)     zone                        metal)
                                                                   (right)
               1 pass       865          1129          1141         1104           845
               2 pass       967          1177          1333          1085          907
    AA6061
               3 pass       975          1227          1405          1154          984
               4 pass       982          1273          1472          1193          875


4. CONCLUSIONS

       In the present study, the plain AA 6061 Al-alloy surface was successfully modified by
FSP. The microstructure and micro hardness were evaluated and investigated. The following
conclusions were made based on present investigation:
   1. It was demonstrated that FSP was an appropriate method to modify the microstructure
       and mechanical properties of AA 6061 Al-alloy. This is to fact that FSP decreased the
       grain size and increased the hardness of processed material.
   2. The rotation speed of FSP tool and its travelling speed were expected to cause more
       heat input which affects the thickness of the surface layer which refines the grain size
       of the material.
   3. The micro hardness of the stirred surface of AA 6061 Al–alloy was increased
       significantly compared with the unprocessed surface. Further, it was noted that the
       hardness was increased from around 160 to 185 % higher than the unprocessed
       surface which was due to grain refinement.
   4. With further research efforts and increased understanding, FSP could be conducted
       for improving the mechanical behaviour, like fatigue and creep response and new tool
       design for light weight based materials and metal matrix composites.

5. REFERENCES

[1] R.S. Mishra, M.W. Mahoney, S.X. McFadden, N.A. Mara, A.K. Mukherjee, High
Strain Rate Superplasticity In A Friction Star Processed 7075 Al Alloy, 42 (2000).
[2] R.S. Mishra, M.W. Mahoney, Friction Stir Processing: A New Grain Refinement
Technique to Achieve High Strain Rate Superplasticity in Commercial Alloys, Mater. Sci.
Forum 357–359 (2001).
[3] Z.Y. Ma, R.S. Mishra, M.W. Mahoney, Superplastic deformation behaviour of      friction
stir processed 7075Al alloy, Acta Mater. 50 (2002).
[4] R.S. Mishra, Z.Y. Ma, I. Charit, Friction Stir Processing: A Novel Technique for
Fabrication of Surface Composite, Mater. Sci. Eng. A 341 (2002) 307.
[5] P.B. Berbon, W.H. Bingel, R.S. Mishra, C.C. Bampton, M.W. Mahoney, Friction         stir

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Journal of Mechanical Engineering and Technology (JMET) ISSN 2347-3924 (Print),
ISSN 2347-3932 (Online), Volume 1, Issue 1, July -December (2013)

processing: a tool to homogenize nanocomposite aluminum alloys, 44 (2001).
 [6] J.E. Spowart, Z.Y. Ma, R.S. Mishra, in: K.V. Jata, M.W. Mahoney, R.S. Mishra,          S.L.
Semiatin, T. Lienert (Eds.), Friction Stir Welding and Processing II, TMS,       2003, pp. 243–
252.
[7] Z.Y. Ma, S.R. Sharma, R.S. Mishra, M.W. Manohey, Mater. Sci. Forum 426–432
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[8] R.S. Mishra, Z.Y. Ma, Friction Stir Welding and Processing, Materials Science           and
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[10] W. Wang, Q. Shi, P. Liu, H. Li, T. Li, A novel way to produce bulk SiCp         reinforced
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[11] F. Nascimento, T. Santos, P. Vilaca, R.M.Miranda, L. Quintino, “Microstructural
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[12] C.J. Hsu, C.Y. Chang, P.W. Kao, N.J. Ho, C.P. Chang, “Al-Al3Ti nano composites
produced in situ by friction stir processing”Acta Materialia 54 (2006) 5241–5249.
[13] C.J. Hsu, P.W. Kao, N.J. Ho, “Intermetallic-reinforced aluminum matrix composites
produced in situ by friction stir processing”, Materials Letters 61 (2007) 1315–1318.
[14] Y. Morisada, H. Fujii, T. Nagaoka, M. Fukusumi, Materials and Design, Scripta
Materialia 55 (2006) 1067–1070.
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[16] S.R. Sharma, Z.Y. Ma, R.S. Mishra, Materials and Design, Scripta Materialia 51 (2004)
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[17] R.S. Mishra, Z.Y. Ma, Friction stir welding and processing, Mater. Sci. Eng. R 50
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[18] J.Q. Su, T.W. Nelson, C.J. Sterling, Grain refinement of aluminum alloys by friction stir
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[20] Z.Y. Ma, S.R. Sharma, R.S. Mishra, Effect of friction stir processing on the
microstructure of cast A356 aluminum, Mater. Sci. Eng. A 433 (2006) 269–278.




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