Third Day Poster Session, 17 June 2010 - NanoTR-VI

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Poster Session, Thursday, June 17
Theme F686 - N1123
Fabrication of Surface Nano-composite Layer on Mild Steel Using Friction Stir Processing Technique
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A. AbediP
US.F.Kashani-BozorgUP P*
1
PSchool of Metallurgy and Materials Engineering, University College of Engineering, University of Tehran, Tehran, Iran
Abstract-Friction stir processing technique was employed for the fabrication of surface nano-composite layers on a mild steel substrate. The SiC
powder was inserted into the groove which was passed by the hardened rotating tool. Optimizing the rate of tool rotation/advancing speed and
depth of the groove resulted in surface nano-composite layer with uniform dispersion of nano-size SiC particulates. The fabricated surface nanocomposite
layer showed to have a maximum micro hardness value of ~480HV compared to ~136HV of the untreated substrate.
Mild steel is used as a structural material in the industry and
construction. However, the wear resistance property of mild
steel is considered to be poor in certain applications.
Dispersion of hard ceramic particles in the metallic matrix has
received considerable interest due to improvement of strength,
stiffness and wears resistance as compared to the monolithic
counterparts [1]. Friction stir processing (FSP) is a solid state
technique for the fabrication of surface composite layer. In
FSP, a rotating tool consisting of a shoulder and a probe is
plunged into a work piece and then travels in the expected
direction. The tool serves two primary functions: heating and
deforming the material. After extreme levels of plastic
deformation and thermal exposure, the processed zone
normally exhibits significant microstructural refinement [2]. In
comparison with other surface modification techniques (high
energy laser treatment, plasma spraying, etc), FSP is carried
out at the temperatures below melting point of substrate [3]. In
this work, FSP was carried out by high power conventional
miller machine. Mild steel plate with a thickness of 10mm and
nano-size SiC powder with an average size of ~70nm were
used as substrate and reinforcement particulates, respectively.
The tool material has smooth frustum shape; it was made of
WC that inserted to mild steel body. This was applied due to
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reducing the risk of brittle fracture of WC. A 3P angle was
applied to the tool. For lying SiC nano-particles, a groove with
depth and width of 1.5 and 1 mm was machined thorough the
work pieces, respectively. A “technological hole” was drilled
to mild steel plate in initial of a groove. This hole can ease
process and decrease wear of tool in plunging phases. To
avoid surface oxidation of the FSP zone, argon shielding was
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employed around the tools at a flow rate of 10P PmmP P/sec. In
order to achieve the favorite result, several rotational and
transverse speeds were employed; uniform dispersion nanoparticles
was obtained using 1000 rpm and 55 mm/min as
rotational and advancing speeds, respectively. Microstructural
observations of cross-section of the friction stir processed
zone were performed by scanning electron microscopy.
Samples were prepared by wire cut in 3×1.5×1 cm pieces.
These samples were mechanically ground with abrasive paper
and polished with 3m diamond, and then etched in a solution
consisting of 5ml nitric acid and 95ml ethanol solution. Also
micro hardness was measured by 200gram load for 12s, in 3
mm under surface, transferring the entire stirred zone into base
metal.
Experimental results revealed that a defect-free friction stir
processed zone was obtained at the applied parameters. The
upper surface showed very smooth quality and there are
almost no prominences or dispersion. The friction stir
processed sample displayed several microstructurally distinct
regions including the stir zone along the processed centerline,
heat affected zone (HAZ) surrounding the stir zone and base
metal. Using suitable depth of groove, the SiC particles were
well dispersed within the stir zone as shown in Figure 1. No
discernible defect and porosities were observed.
Figure 1. Secondary electron image of the fabricated nano-composite
surface layer exhibiting uniform dispersion of SiC nano-particles.
If the groove is superficial, a composite layer with low SiC
content is acquired. On the other hand, if a deep groove is
used, clustering of particles is occurred. So depth of the
groove should be selected optimal. Non-uniform distribution
was resulted using relatively high transverse speed. Also
scattering of nano-particles was observed out of the groove
due to high rotational speed. Increase in FSP passes can result
in more dispersion of nano-particles [3]. The microstructure of
stir zone was characterized by the presence of acicular ferrite.
The chaotic arrangement of the plates represents fine grained
interlocking morphologies. Acicular ferrite is formed in the
same temperature range as bainite (approximately 400 to
o
600P PC) by the same type of transformation mechanism [4].
According to the observed microstructure, the major
contributions to the hardness of the surface composite layers
fabricated by FSP are (1) the fine grain size of the Fe-based
matrix due to severe plastic deformation and (2) Orowan
strengthening due to fine dispersion of nano-size SiC particles.
A maximum hardness value of ~480 HV was achieved, while
that of the as-received base metal was ~136 HV.
*Corresponding author: HTfkashani@ut.ac.irT
[1] Clyne T.W., Whithers P.J. Cambridge University Press,
Cambridge, United Kingdom, (1993).
[2] Fujii H., Cui L., Tsuji N., Maeda M., Nakata K. and Nogi K.,
Material science and Engineering A 429,(2006).
[3] Shafiei-Zarghani A, Kashani-Bozorg S. F., and Zarei-Hanzaki A,
Material Science and Engineering A 500, (2009).
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[4] Bhadeshia H.K.D.H, “ Bainite in steels”, 2P , Institute of
the materials, London, (2001).
6th Nanoscience and Nanotechnology Conference, zmir, 2010 748