5 influence of ti, b - iaeme of ti, b.pdf · international journal of mechanical engineering and...

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print),

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

49

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: [email protected]

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,

International Journal of Mechanical Engineering

and Technology (IJMET), ISSN 0976 – 6340(Print)

ISSN 0976 – 6359(Online) Volume 1

Number 1, July - Aug (2010), pp. 49-59

© IAEME, http://www.iaeme.com/ijmet.html

IJMET

© I A E M E



50

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



51

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.



52

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.



53

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.




54

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.




55

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



56

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

No.

Alloy Composition 0.2% PS

(MPa)

UTS

(MPa)

%

E

%

R

YM VHN

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 +

0.60% of Al-3B + 0.20% of

Al-10Sr

142 202 4.92 7.55 100 298

Table 1 Mechanical property studies on A356 alloy



57

A lloy 1 A lloy 2 A lloy 3 A lloy 4 A lloy 5

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

Mec

han

ical

Pro

per

ties

A llo y C om p o sition

0 .2 % P .S ; U T S ; % E ;

% R ; Y M ; V H 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.,

(2001), “Smaller is Softer: An inverse size effect in a cast aluminum alloy, Acta

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.



58

[5] Conley J. G., Huang J., Asada J. and Akiba K., (2000), “Modelling the effects of

cooling rate, hydrogen content, grain refiner and modifier on microscopy formation

in Al A356 alloys”, Materials Science and Engineering: A, Vol. 285, No.1-2, pp.49-

55.

[6] Ejiofor J.U. and Reddy R.G., (1997), “Developments in the Processing and

Properties of Particulate Al-Si Alloy Composites”, Journal of Metals, Vol. 49, pp.

31-37.

[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),

“Microstructure, material flow and tensile properties of A356 alloy thixoformed

parts”, Material research, Vol. 7, No. 4, pp.595-603.

[9] Ganger D.A. and Elliot R., “Solidification of Eutectic Alloys, Metals Handbook,”

9th ed., Vol. 15, pp.159.

[10] Gokhale A.M. and Patel G.R., (2005), “Analysis of variability in tensile ductility of

a semi-solid metal cast A356 Al-alloy”, Materials Science and Engineering: A, Vol.

392, No.-2, pp.184-190.

[11] Jeng S.C. and Chen S.W., (1997), “The Solidification characteristics of 6061 and

A356 Aluminum alloys and their ceramic particle-reinforced composites”, Acta

Materialia, Vol.45, No. 12, pp. 4887-4899.

[12] L ´opez, V.H., Scoles A., and Kennedy, A.R., (2003), “The thermal stability of TiC

particles in an Al-7 wt % Si alloy”, Materials Science and Engineering: A, Vol.356,

pp. 316-325.

[13] Liao, H.C., Sun Y., and Sun G.X., (2002), “Correlation between mechanical

properties and amount of dendritic α-Al phase in as-cast near-eutectic Al–11.6%Si

alloys modified with strontium”, Materials Science and Engineering,: A, Vol. 335,

pp. 62-66.

[14] Liao H., Sun Y. and Sun G., (2002), “Correlation between mechanical properties

and amount of dendritic α-Al phase in as-cast near-eutectic Al-11.6% Si alloys

modified with strontium”, Materials Science and Engineering: A, Vol.335, No.1,

September 25, pp.62-66(5).



59

[15] Murty B.S., Kori S.A. and Chakraborty M., (2002), “Grain refinement of

aluminium and its alloys by heterogeneous nucleation and alloying”, International

Materials Reviews, Vol. 47, No.1, pp 3-29.

[16] Natarajan N., Vijayarangan S. and Rajendran I., (2006), “Wear behavior of

A356/25SiCp aluminium matrix composites sliding against automobile friction

material”, Wear, Vol. 261, No. 7-8, pp. 812-822.

[17] Pacs M. and Zoqui E. J., 92005), “Semi-solid behavior of new Al-Si-Mg alloys for

thixoforming”, Materials Science and Engineering: A, Vol. 406, No.1-2, pp. 63-73.

[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

structure of A356 alloy investigated by large-scale crystal orientation observation”,

Journal of Material Processing Technology, Vol. 141, pp. 29-34.

[20] Yang Z, Kang C.G and Seo P.K, (2005), “Evolution of the rheocasting structure of

A356 alloy investigated by large-scale crystal orientation observation”, Scripta

Materialia, Vol. 52, pp.283–288.

[21] Yang X., Jing Y. and Liu J., (2002), “The rheological behavior for thixocasting of

semi-solid Aluminium alloy (A356)”, Journal of Materials Processing Technology,

130-131, pp. 569-573.

[22] Yu Y.B., Song P.Y., Kim S.S., and Jee J.H., (1999), “Possibility of improving

tensile strength of semi-solid processed A356 alloy by a post heat treatment at an

extremely high temperature”, Scripta Materialia, Vol. 47, pp. 767-771.

5 influence of ti, b - iaeme of ti, b.pdf · international journal of mechanical engineering and...

Documents