INFLUENCE OF Ti_ B AND Sr ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A356 ALLOY

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INFLUENCE OF Ti_ B AND Sr ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF A356 ALLOY Powered By Docstoc
					International Journal of Mechanical Engineering (IJMET), ISSN 0976 – 6340(Print),
 International Journal of Mechanical Engineering and Technology
and Technology (IJMET), ISSN 09761, July - Aug (2010), © IAEME
 ISSN 0976 – 6359(Online) Volume 1, Number – 6340(Print)              IJMET
ISSN 0976 – 6359(Online) Volume 1
Number 1, July - Aug (2010), pp. 49-59                          ©IAEME
© IAEME, http://www.iaeme.com/ijmet.html

                INFLUENCE OF Ti, B AND Sr ON THE
  MICROSTRUCTURE AND MECHANICAL PROPERTIES
                                 OF A356 ALLOY
                                    D. G. Mallapur
                  Department of Metallurgical and Materials Engineering
                       National Institute of Technology, Surathkal
                       Karnataka, Email: drmallapur@yahoo.com

                                  K. Rajendra Udupa
                  Department of Metallurgical and Materials Engineering
                       National Institute of Technology, Karnataka

                                       S. A. Kori
                          Department of Mechanical Engineering
                            Basaveshwar Engineering College
                               Bagalkot, Karnataka, India.

 ABSTRACT
 In the present investigation, the microstructural and mechanical properties study of A356
 alloy, have been discussed. The microstructural aspect of cast A356 alloy employed in
 the present study is strongly dependent on the grain refinement (Ti and B) and
 modification (Sr). The mechanical properties such as PS, UTS, %E, %R, Young’s
 modulus and VHN have been investigated. This paper deals with the combined effect of
 grain refinement and modification, which improves the overall mechanical properties of
 the alloy. The quality of castings and their properties can be achieved by refining of α-Al
 dendrites in A356 alloy by means of the addition of elements such as Ti and B which
 reduces the size of α-Al dendrites, which otherwise solidifies with coarse columnar α-Al
 dendritic structure. In addition modification is normally adopted to achieve improved
 mechanical properties. Metallographic studies reveal that the structure changes from
 coarse columnar dendrites to fine equiaxed ones on the addition of grain refiner and
 further, plate like eutectic silicon to fine particles on addition of 0.20% of Al-10Sr
 modifier. The present result shows that a reduction in the size of α-Al dendrites,



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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online) Volume 1, Number 1, July - Aug (2010), © IAEME


modification of eutectic Si and improvement in the mechanical properties were observed
with the addition of grain refiner Al-3Ti, Al-3B and modifier Al-10Sr either individual
addition or in combination.
Keywords: A356 alloy, grain refinement, modification, mechanical properties,
microstructure.
1. INTRODUCTION
        Aluminium silicon casting alloys are essential to the automotive, aerospace and
engineering sectors. Al-Si alloys allow complex shapes to be cast; however the silicon
forms brittle needle-like particles that reduce impact strength in cast structures. As an
additive to Al-Si casting alloys, strontium improves strength, enhances mechanical
properties and disperses porosity as it modifies the eutectic structure. The modified alloy
displays a finer, less needle-like microstructure. Al–Si alloys, which comprise 85% to
90% of the total aluminum-cast parts produced, exhibit excellent cast ability, mechanical
and physical properties Ejiofor J.U et al, 1997. The microstructure and alloy constituents
are necessitated to achieve optimum mechanical properties. A356 aluminum alloys are
mostly used for cast hypoeutectic Al-Si alloys to improve flow ability of the melt and
interfacial properties Caceres C.H et al, 1996 and Jeng S.C et al, 1997. However, eutectic
Si particles present along solidification cells of the A356 aluminum alloy deteriorate
strength, ductility, and fracture toughness, and thus researches to develop processes for
enhanced distribution of eutectic Si particles have been actively pursued Benzerga A.A et
al, 2001 and Gokhale A.M., 2005. A356 alloy belongs to group of hypoeutectic Al-Si
alloys and has a wide field of application in the automotive and avionics industries. It is
used in the heat-treated condition in which an optimal ratio of physical and mechanical
properties is obtained Conley J. G et al, 2000. The alloy solidifies in a broad temperature
interval (430C) and is amenable to treatment in semi solid state as well as castings Pacs M
et al, 1963. For this reason it is the subject of rheological investigations Yang X et al,
2002, as well as methods of treatment in the semi solid state Pacs M et al, 1963 and
Freitas de E.D et al, 2004. By these methods it is possible to obtain castings with reduced
porosity of a non-dendritic structure and with good mechanical properties. Besides this
A356 alloy is used as a matrix for obtaining composites Natarajan N et al, 2006, which
have an enhanced wear resistance, favourable mechanical properties at room temperature


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online) Volume 1, Number 1, July - Aug (2010), © IAEME


and enhanced mechanical properties at elevated temperatures. Cast aluminium A356
alloy is one of the most well developed aluminium alloys due to its outstanding
properties. It is widely employed in numerous automotive and industrial weight sensitive
applications, such as aeronautics and space flight. A356 alloy contains about 50 vol. %
eutectic phases and finds wide application in the marine, electrical, automobile and
aircraft industries. It is well known that on solidification of hypoeutectic Al-Si alloys the
primary α-Al solidifies with coarse columnar or twinned columnar Murty B.S, et al,
2002. Actually, in most cases high-level mechanical properties are needed for industrial
applications, so the performance of this alloy has been the subject of many
micromechanical investigations Gokhale A.M., 2005, L ´opez, V.H et al, 2003, Yu Y.B et
al 1999 and Yang Z et al, 2005. Since the strength and hardness of alloys mainly depend
on their microstructure, a lot of efforts have been made to refine the microstructure of the
castings in order to enhance the mechanical properties of aluminium-A356 alloy. Adding
modifier and refiner Wang J et al, 2003 and Liao, H.C et al 2002 to the melt is a common
way of doing this, and has been adopted by many researchers. The microstructure of alloy
A356 comprises of an aluminium matrix, which is strengthened by MgSi precipitates and,
to a far lesser extent, by Si precipitates, and a dispersion of eutectic silicon particles and
Fe-rich intermetallics. The variables affecting the microstructure mainly include
composition, solidification conditions, and heat treatment. The physics governing the
formation of aluminium dendrites and the eutectic structure are reasonably well
understood Flemings M.C et al 2004 and Spear R.E et al 1963. Mechanical properties of
Al-Si alloys are related to the morphology of silicon particles (size, shape and
distribution), grain size, shape and dendrite parameters, “Aluminum Foundry Products,”
Metals Handbook, 9th ed., Vol. 2, 145, Ganger D.A et al “Solidification of Eutectic
Alloys, Metals Handbook,” 9th ed., Vol. 15, 159, Closset B et al 1982 and Liao H. et at
2002. Modification changes silicon morphology and is achieved by rapid solidification,
chemical modification and thermal modification in the solid state. The refinement of
grain structure is achieved by controlling casting process parameters and / or melt
chemistry (i.e. grain refinement, eutectic modification). Each control methodology
refines a certain aspect of the microstructure.



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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online) Volume 1, Number 1, July - Aug (2010), © IAEME


        Thus, keeping in view, an attempt has been made to study the effect of minor
additions of Ti, B and Sr in the form of master alloys on the microstructure and
mechanical properties of A356 alloy either individually or in combinations.
2. EXPERIMENTAL DETAILS
        Specimens for microstructure and mechanical properties were prepared by
melting A356 alloy in a resistance furnace (Kanthal heating elements, M/s Cera Therm
International, India) under a cover flux (45%NaCl+45%KCl+10%NaF) and the melt was
held at 720°C. After degassing with solid hexachloroethane (C2Cl6), master alloy chips
(Al-3Ti, Al-3B and Al-10Sr as the case may be) duly packed in an aluminium foil were
added to the melt. The melt was stirred for 30 seconds with zircon coated steel rod after
the addition of master alloys, after which no further stirring was carried out. Melt were
poured at ‘0’ min. and ‘5’ min. into cylindrical graphite mould (25mm φ and 100mm
height) surrounded by fire clay brick with it’s top open for pouring and also the melt was
poured into split type graphite mould (12.5mm φ and 125mm height) for preparing tensile
(10mm φ x 50mm length) specimens.
        The ‘0’ min. refers to the melt without the addition master alloys. The cast alloys
are characterized by microscopy, macroscopy and SEM microanalysis. Figure. 1a shows
the cylindrical graphite mould surrounded by fireclay bricks and Figure.1b shows
castings obtained from cylindrical graphite mould showing the sections selected for
characterization.




                Figure 1(a)                             Figure 1(b)
        Figure 1a Cylindrical graphite mould surrounded by fireclay bricks and 1b
Castings obtained from cylindrical graphite mould showing the section selected for
characterization.


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online) Volume 1, Number 1, July - Aug (2010), © IAEME


2.1.     SPECIMEN             PREPARATION                 FOR        MICROSTRUCTURE
STUDIES
        Specimens of 5mm height from the section that was left after macroscopic study
were taken for optical microscopic studies. The specimens so obtained were initially
polished using belt grinder and then on a series of SiC water proof emery papers with
increasing fineness, to remove any of the scratches present. Then the samples were
polished on a disc polisher using 75µm Al2O3 powder, until the mirror finish and scratch
free surface is obtained. Final polishing was carried out using elctropolishing machine
(Model: Electropol, METATECH, Pune) with electrolyte having composition of 72.4%
Methanol, 7.8% perchloric acid, 9.8% Butylcellosolve and 10% distilled water by
volume. Polished samples were cleaned with soap solution, distilled water and ethyl
alcohol followed with drying. The polished samples were etched using Keller’s reagent
(2.5%HNO3+1.5%HCl+1%HF+95%H2O by volume) for about 75-90s in order to
develop microstructure with grain boundaries. The samples so prepared and polished
were taken for optical microscopy, SEM/EDX analysis.
2.2. MECHANICAL PROPERTY STUDIES
        For the mechanical property studies, the tensile specimen shown in Figure 2
(10mm dia. x 50mm length) were prepared and properties like (0.2% PS, UTS, % E, % R,
YM and VHN) of A356 alloy were evaluated before and after grain refinement and
modification. The tensile tests were carried out using the Precision Controlled
Computerized Tensile Testing Machine (UNITEK 9450PC, Blue Star India). A part of
split type graphite mould (12.5 mm diameter and 125 mm height) for preparing tensile
specimens (12.5 mm diameter x 50 mm length) is shown in the Figure 2b.




              Figure 2(a)                                                 Figure 2(b)


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online) Volume 1, Number 1, July - Aug (2010), © IAEME


        Figure 2a Tensile specimen (10mm x 50mm Gauge length) and 2b Part of split
type graphite mould (12.5 mm diameter and 125 mm height).

3. RESULTS AND DISCUSSIONS

3.1. MICROSTRUCTURAL STUDIES
        Figure 3a shows the SEM photomicrograph of A356 alloy in the absence of grain
refiner. From figure it is clear that in the absence of Al-3Ti master alloy, A356 alloy
shows coarse columnar α-Al dendritic structure and unmodified needle/plate like eutectic
silicon. With the addition of 0.65% of Al-3Ti the master alloy, A356 alloy shows
response towards grain refinement with structural transition from coarse columnar
dendritic structure to fine equiaxed structure as shown in figure 3b. With the addition of
0.60% of Al-3B master alloy, the structure of A356 alloy changes from columnar to finer
equiaxed α-Al dendrites compared to the addition of Al-3Ti grain refiner as clearly
observed in figure 3c, while eutectic silicon remains unmodified as expected. This could
be due to the presence of AlB2 particles present in the Al-3B master alloy and these
particles are act as heterogeneous nucleating sites during solidification of α-Al. While
addition of A356 alloy to 0.20% of Al-10Sr master alloy, the plate like eutectic Si is
converted in to fine particles and α-Al dendrites remain as columnar dendritic structure
only as clearly seen in figure3d. However, figure 3e shows the simultaneous refinement
(α-Al dendrites) and modification (eutectic Si) of Al356 alloy due to the combined action
of AlB2 and Al4Sr particles present in Al-3B grain refiner and Al-10Sr modifier
respectively.




                Figure 3(a)                                               Figure 3(b)


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online) Volume 1, Number 1, July - Aug (2010), © IAEME




              Figure 3(c)                                                 Figure 3(d)




                                   Figure 3(e)
        Figure 3 SEM photomicrographs of A356 alloy (3a) as cast alloy; (3b) with
0.65% of Al-3Ti grain refiner; (3c) with 0.60% of Al-3B grain refiner; (3d) with 0.20%
of Al-10Sr modifier and (3e) combined addition of 0.65% Al-3Ti, 0.60 % of Al-3B grain
refiner and 0.20% of Al-10Sr modifier.
3.2. MECHANICAL PROPERTIES
        Figure 4 shows the influence of the Ti, B and Sr on the mechanical properties of
A356 alloy. From the figure, it is clearly observed that the improvement in the
mechanical properties such as PS, UTS), %E, %R, YM and VHN increases with the
addition of master alloys containing Ti, B and Sr due to change in the microstructure. It is
also clear that the combined addition of grain refiner and modifier to A356 alloy has
resulted in maximum improvement in mechanical properties as compared to the
individual addition of grain refiners, modifier and in an untreated as cast condition.
Addition of the grain refiners to A356 alloy predominanantly converts columnar
grain/dendritic structure to fine equiaxed grain/dendritic structure thereby by enhances
the mechanical properties. The effect of silicon on the mechanical properties of Al-Si



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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
ISSN 0976 – 6359(Online) Volume 1, Number 1, July - Aug (2010), © IAEME


alloys is a well-known fact. The mechanical properties depend on the shape, size and
distribution of eutectic silicon and α-Al grains/dendrites in case of Al-Si alloys. It is also
clear from the experimental results that the combined addition of 0.65% Al-3Ti, 0.60 %
of Al-3B grain refiner and 0.20% of Al-10Sr modifier to A356 alloy resulted in
maximum UTS, when compared to the individual addition of grain refiner and modifier
in an untreated conditions. The effect of grain refinement and modification on the
mechanical properties is shown in Table 1. In the absence of grain refiner and modifier,
A356 alloy shows 131MPa 0.2% PS, 185MPa UTS, 3.25 %E, 3.32%R, 87YM and
245VHN, while with the combined addition of grain refiner and modifier, 142MPa 0.2%
PS, 202MPa UTS, 4.92 %E, 7.55%R, 100YM and 298VHN were obtained.
Alloy           Alloy Composition              0.2% PS        UTS         %      %      YM VHN
 No.                                            (MPa)        (MPa)        E       R
  1       A356                                    131          185       3.25 3.32       87        245

   2      A356 + 0.65% of Al-3Ti                  132          188       4.05 5.85       88        252

   3      A356 + 0.60% of Al-3B                   135          192       4.52 6.74       89        260

   4      A356 + 0.20% of Al-10Sr                 139          195       4.78 7.15       92        281

   5      A356 + 0.65% of Al-3Ti +                142          202       4.92 7.55      100        298
          0.60% of Al-3B + 0.20% of
          Al-10Sr
                     Table 1 Mechanical property studies on A356 alloy




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ISSN 0976 – 6359(Online) Volume 1, Number 1, July - Aug (2010), © IAEME




                                  320
                                  300          0 .2 % P .S ;     U TS;     % E;
                                               % R;          YM;       VHN
                                  280
                                  260
                                  240
                                  220
          Mechanical Properties

                                  200
                                  180
                                  160
                                  140
                                  120
                                  100
                                   80
                                   60
                                   40
                                   20
                                    0
                                        A llo y 1      A llo y 2           A llo y 3           A llo y 4   A llo y 5
                                                                   A llo y C o m p o sitio n



        Figure 4 Mechanical properties of A356 alloy before and after the addition of
grain refiner and or modifier

4. CONCLUSIONS
        Addition of grain refiner (Al-3Ti and Al-3B) and modifier Al-10Sr master alloys
to A356 alloys leads to change in microstructure.
        Improved mechanical properties were obtained due to the change in the
microstructure of A356 alloy due to the addition of grain refiner/modifier.
5. REFERENCES
[1]   “Aluminum Foundry Products,” Metals Handbook, 9th ed., Vol. 2, 145.
[2]   Benzerga A.A, Hong S.S., Kim K.S., Needleman A and Van E. der Giessen.,
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      Materialia, Vol.49, No.15, pp. 3071-3083.
[3]   Caceres C.H. and Selling B.I., (1996), Casting defects and the tensile properties of
      an Al-Si-Mg alloy” Materials Science Engineering: A, Vol. 220, No.1, December
      15, pp.109 - 116(8).
[4]   Closset B.M. and Gruzleski J., (1982), AFS Trans., Vol.90, pp.453.


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International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),
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[5]   Conley J. G., Huang J., Asada J. and Akiba K., (2000), “Modelling the effects of
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[6]   Ejiofor J.U. and Reddy R.G., (1997), “Developments in the Processing and
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[7]   Flemings M.C., (1974), “Solidification Processing”, McGraw-Hill, New York.
[8]   Freitas de E. D., Ferracini Junior E. G, Piffer V. P. and Ferrante M., (2004),
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[9]   Ganger D.A. and Elliot R., “Solidification of Eutectic Alloys, Metals Handbook,”
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[10] Gokhale A.M. and Patel G.R., (2005), “Analysis of variability in tensile ductility of
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[11] Jeng S.C. and Chen S.W., (1997), “The Solidification characteristics of 6061 and
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[13] Liao, H.C., Sun Y., and Sun G.X., (2002), “Correlation between mechanical
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[15] Murty B.S., Kori S.A. and Chakraborty M., (2002), “Grain refinement of
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[16] Natarajan N., Vijayarangan S. and Rajendran I., (2006), “Wear behavior of
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[17] Pacs M. and Zoqui E. J., 92005), “Semi-solid behavior of new Al-Si-Mg alloys for
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[18] Spear R.E. and Gardner G.R., (1963), AFS Trans., Vol. 71, pp. 209-215.
[19] Wang J., He S.X., Sun B.D., and Nishio M., (2003), “Evolution of the rheocasting
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[20] Yang Z, Kang C.G and Seo P.K, (2005), “Evolution of the rheocasting structure of
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[21] Yang X., Jing Y. and Liu J., (2002), “The rheological behavior for thixocasting of
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      extremely high temperature”, Scripta Materialia, Vol. 47, pp. 767-771.




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