EFFECT OF MODIFIER AND GRAIN REFINER ON CAST AL-7SI

ALUMINUM ALLOY: A REVIEW N. R. Rathod

a, J.V. Manghani

b

a PG Student, Department of Mechanical Engineering, National Institute of Technology, Surat, India

b Assistant Professor, Department of Mechanical Engineering, National Institute of Technology, Surat, India.

Abstract: The wide popularity of Al–7Si alloys in the automobile industry stems from their high strength to

weight ratio, excellent casting characteristics, and good mechanical properties including wear resistance, low

coefficient of thermal expansion, high thermal conductivity and high corrosion resistance. Al–7Si alloys are

mainly used in cast form in critical components like pistons, valve lifters, cylinder liners, engine blocks, etc.

These applications demand the study of techniques to improve the wear properties of these alloys. The present

seminar attempts to achieve this at least partially by grain refinement and modification of these alloys.

In the recent years the usage of cast Al–7Si alloy components in automotive and marine industries has increased

significantly. Such alloys are invariably treated for modification prior to casting to achieve improved properties

and performance. Modification define as it converts eutectic silicon acicular structure in fine fibrous/globular

eutectic silicon, which otherwise exist in the form of large plate/needle like morphology. It is well established

that modification of eutectic silicon can be achieved by several methods like faster solidification, mould

vibration, melt agitation in mushy state and melt inoculation by using some elements like Na, Sr, Sb etc. Among

the modifiers used, Sr and Sb are known to result in good modification in hypoeutectic Al–Si alloys. However,

it has been reported that modification effect of Sr diminishes on longer melt treatment time (melt holding time),

while Sb results in good modification on longer melt treatment time.

Grain refinement has also drawn the attention of many researchers during last few years. It has been reported

that grain refinement leads to fine equiaxed grain structure, which in turn results in improved mechanical and

wear properties. Modifier and grain refiners are to be added to the molten Al–7Si alloys to get combined grain

refinement and modification effects. Many have reported the grain refinement and modification by addition of

grain refiner and modifier simultaneously. However, it is a difficult task to control the addition levels of

individual grain refiner and modifier in actual industrial practice. It has been reported that the combined addition

of the Al–Ti–B grain refiner and Al–Sr master alloy results in mutual poisoning of the grain refinement and

modification effects.

Recently developed a new Al-Ti-C-Sr master alloy “Modifiner” which plays a dual role both as grain refiner as

well as modifier at a time.

Key words: Fading, Melt holding time, Rare Earth, MM, Grain refiner, Modifiner.

1. Introduction

Mechanical properties of Al-Si alloys greatly influenced by morphology of Si phase. Si is polyhedral in shape

and eutectic Si is coarse acicular in shape. Presence of coarse acicular shape is highly detrimental as needles act

as stress raises and leads to premature failure. Transformation of acicular Si morphology to fibrous one resulting

in noticeable improvement in mechanical properties by chemical treatment of the molten metal is called

Modification [1]. It is well established that modification of eutectic silicon can be achieved by several method s

like Faster solidification, Mould vibration and Melt agitation [2, 3]. There are three main non-destructive testing

methods to control modification: a. Thermal analysis, b. Electrical conductivity and c. Ultrasound. The

measurement of ultrasonic attenuation allows an estimate of the modification rating in Al-Si alloys. There is a

tendency towards a linear correlation between attenuation and the modification rating, where attenuation

decreases as the modification rating increases [5]. Sr & Sb are known to result in good modification in

hypoeutectic Al-Si alloys [2, 3]. Modification effect diminishes on longer melt holding [4]. Grain refinement,

also known as inoculation is the set of techniques used to implement grain boundary strengthening in

metallurgy. The specific techniques and corresponding mechanisms will vary based on what materials are being

considered [6]. One method for controlling grain size in aluminum alloys is by introducing particles to serve as

nucleants, such as Al-5%Ti. Grains will grow via heterogeneous nucleation; that is, for a given degree of under

cooling beneath the melting temperature, aluminum particles in the melt will nucleate on the surface of the

added particles. Grains will grow in the form of dendrites growing radially away from the surface of the

nucleants. Solute particles can then be added (called grain refiners) which limit the growth of dendrites, leading

to grain refinement. TiB2 is a common grain refiner for Al alloys; however, novel refiners such as Al3Sc have

been suggested [7].

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2. Mechanism of Modification:

In the recent years the usage of cast Al–7Si alloy components in automotive and marine industries has increased

significantly. Such alloys are invariably treated for modification prior to casting to achieve improved properties

and performance. It is well established that modification of eutectic silicon can be achieved by several methods

like faster solidification, mould vibration, melt agitation in mushy state and melt inoculation by using some

elements like Na, Sr, Sb etc. Among the modifiers used, Sr and Sb are known to result in good modification in

hypoeutectic Al–Si alloys. However, it has been reported that modification effect of Sr diminishes on longer

melt treatment time (melt holding time), while Sb results in good modification on longer melt treatment time

[1].

Before Modification After Modification

Figure 1: Effect of Modification [1]

Shu Zu Lu suggested the idea “Impurity Induced Twinning” to explain the morphological changes in the

presence of modifiers. Modifiers have absorbed up on the silicon growth front and these sufficiently large

impurity atoms promote multiple twinning. The multiple twinning is effected by displacing a monolayer growth

step to be alternative staking sequence; geometrically the ideal radius ratio r Modifier: r si Silicon; would be

1.646 [8]. While atomic size seems to be a strong factor for impurity induced twinning, it still appears as if

further essential factors might have an influence on the modifying abilities such as melting point, vapour

pressure or free energy of oxide formation [9].

Elements Atomic radius Ratio r/rSi Reported modifying effect

Ba 2.18 1.85 +

Sr 2.16 1.84 +

Eu 2.02 1.72 +

Ca 1.97 1.67 +

Yb 1.93 1.65 ?

La 1.87 1.9 +

Na 1.87 1.58 ++

Ce 1.86 1.56 +

Si 1.75 1.00 -

Table 1: r/rSi ratio for various elements used as modifiers [9]

2.1 Effect of minor alloying element on modification of Al-Si alloys:

There are several variables, which affect the evolution of the microstructure as like type and quality of modifier,

impurities present in the melt, freezing rate, processing parameters, silicon content of alloy, etc. Higher cooling

rate is beneficial for modification and hence, lesser quantity of modifier would be sufficient at high cooling rates

[10].

2.1.1 Sodium

For many years modification of alloys based on the Al–Si system was achieved only by the addition of sodium

salts or small quantities (0.005–0.015%) of metallic sodium to the melt although the actual amount of sodium

needed may be as little as 0.001%. Sodium may depress the eutectic temperature by as much as 12 °C and a

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finer microstructure is therefore to be expected because the rate of nucleation will be greater in the under-cooled

condition. Sodium reduces the potency of nucleating sites for the eutectic phases, notably silicon. Sodium is that

it neutralizes the effect of phosphorus, probably by the preferential formation of the compound NaP. Major

problems with the Na modification is, it reduced fluidity and rapid loss of elements by evaporation/ oxidati on

[1]. Minor addition of modifying elements like (Na) transfer the eutectic Si from a coarse plate like network in a

finer, more fibrous morphology [11].

2.1.2 Strontium

Strontium is added as an Al–Sr or Al–Si–Sr master alloy which also refines the Al–Si eutectic and results in

castings having tensile properties comparable with those obtained when using sodium. Loss of strontium

through volatilization during melting is much less and the modified microstructure can be retained if alloys are

re melted. Strontium additions is that they suppress formation of primary silicon in hypereutectic compositions

which may improve their ductility and toughness [1].

Figure 2. Fracture toughness of alloy A357 (Al–7Si–0.5 Mg). Castings solidified over a range of cooling

rates, with and without strontium modification [1]

Sr modifiers are environment friendly and lead to reduced scrap levels, increases profitability and long tool life.

Whereas Sr is easy to add to the melt compared to Na, Less evaporation of Sr when melt is kept for

homogenization, Sr gives permanent effect compared to Na and Sr based alloy cannot be used in chlorine

atmosphere. Figure 2 shows the casting solidification over range of cooling rates, with and without modification

[1]. Electron Probe Microanalysis (EPMA) results showed that 0.02wt% added Sr was distributed unevenly in

modified aluminum silicon hypoeutectic alloy, and its effect was interfered with adding 0.1wt% Ti as grain

refiner. Filtering the melt removed some of Sr content from the melt, resulting in decrease in Sr modification

producing coarse Si Phase; however this helped the grain refinement of the aluminium phase possibly by

reducing the positioning effect of Sr on Ti content [12]. Improvement in mechanical properties by adding Sr as

modifier in the A356 (A356+ 0.2% of Al-10Si) such as %elongation, %reduction, ultimate tensile strength, etc

[13]. Sr & Sb are known to result in good modification in hypoeutectic Al-Si alloys. Modification effect of Sr

diminishes on longer melt treatment time (melt holding time) [14,15]. Sr did not affect the solidification of Al

dendrites but modified the kinematics of the Al-Si eutectic [16]. An Al-Ti-C-Sr master alloy “Modifier”; best

results are obtained below 30min of the melt holding of Modifier treated Al-7Si alloy. Modification effect

diminishes on longer melt holding time. It has also reported that mutual poisoning effect of B and Sr results in

the formation of SrB6 compounds which has adverse effect on the grain refinement & modification [17]. Recent

studies have confirmed that Sr also changes significantly the nucleation behaviour of the eutectic phases. It was

reported that Sr segregated exclusively within the eutectic Si and the distribution of Sr within this approximately

homogenous. To achieve a well modified structure in hypoeutectic alloys the Sr levels is generally in the range

of about 80-120 ppm Sr (low melt holding time). The longer holding time leads to reduction of the Sr levels

within the alloy to 1ppm and thus to the formation of unmodified eutectic microstructure displaying coarse

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eutectic Si. It is suspected that Sr is lost by oxidation during the prolonged melt holding (60min) after the

addition of modifier [11].

2.1.3 Rare Earth

Rare earth elements such as La(Lanthanum), Ce(Cerium), Y(Yttrium), and MM(Misch metal) were reported to

act as an effective modifier. Rare earth will modify Si phase from acicular needles to fibrous one is carried out

to enhance the tensile properties [18].The misch metal contained 50-52% Cerium, 20-22% Lanthanum, 15-17%

neodymium and 10-12% of other rare earth elements. The chief metals presents in misch metals satisfy the

requirement of modifying elements [19]. Based on experimental results, it can be found that minor MM (≤

0.2wt%) results in partial modification while more MM additive (0.3-0.1 wt% ) produce full modification. The

atom ratio of La:Si and Ce:Si is 1.59 and 1.56 respectively. It was confirmed that the formation of Re-containing

intermetallics compounds decrease the tensile properties as well as %elongation in the MM-modified A356

alloys [13]. Rare earth effects increases with the amount of rare earth addition and cooling rate. The twin density

of eutectic silicon remains same regardless of the addition of rare earth. Cerium did not refine the primary

silicon as phosphor, but it was moderately effective in modifying the eutectic silicon [20]. These elements have

the tendency to react with the solvent phase to form intermetallics compounds. A higher percentage of misch

metal than sodium is needed for modification possibly because of the tendency of the modifying addition to

react with the solvent phase thus inhabiting formation of stable silicon compounds. The nucleation temperature

of silicon is depressed to the temperature of globular growth with larger addition of misch metal [19]. Misch

metal addition increase under cooling up to 25K with 0.2% addition giving rise to modified precipitate shapes. It

forms intermetallics compounds Al4Ce, Al4LaAl2Ce, SiCe, SiCe2 etc. That suppresses growth of Si. Fading

effect seems much less though amount to be added is high [21]. It is effective in scavenging interstitial

impurities such as oxygen, nitrogen, carbon etc. contributing in strength and toughness [22]. Rare earth addition

to Al-Si alloy is found to modify eutectic Si and improve a mechanical property that is an additional advantage,

when compared to other element added as neutralizer on ill effect of iron. Gas porosity in LM25 can also be

reduced by addition of rare earth metals [23].

2.1.4 Phosphorous

Phosphorous is uses as the modifying/refining agent. The addition is normally in the form of chemicals

containing phosphorous compounds [24]. It is moderately effective in modifying the eutectic silicon [20].

Refinement by P is more or less permanent and can withstand re melts. The only precaution is P reacts with Na

and Sr and these elements tends to nullify each other. Thus care must be taken whenever basic secondary

materials containing Na/Sr is used for producing a hypereutectic alloy, the effects of metals should be removed

by using sulphur powder or by degassing with chlorine before phosphorous modification is carried out [24].

2.2 Modification of Al-7Si:

1. The addition of 0.5%Sr to A356-10%B4C cast composites contributes to a higher wear resistance due to

silicon particle modification [25].

2. Addition of 50ppm Sr, the eutectic Si was well modified but any size of primary Al phase increased with

the amount of Sr [26].

3. Modification for the electromagnetic stirred condition has wider effect in the melt. Thus it is presumed that

the morphology of eutectic Si becomes finer fibrous and globular [26].

4. Sr & Sb are known to result in good modification in hypoeutectic Al-Si alloys [14, 15].

5. Modification effect of Sr diminishes on longer melt treatment time (melt holding time) [14, 15].

6. Sb results in good modification on longer melt treatment time [14, 15].

7. Ba modifier with 1.0 wt% exhibited the best results for mechanical property. It also improves the wear

properties of alloy [27].

8. It has also reported that mutual poisoning effect of B and Sr results in the formation of SrB6 compounds

which has an adverse effect on the grain refinement & modification [4].

9. Improvement in mechanical properties by adding Sr as modifier in the A356 (A356+0.20%of Al-10Sr).

139, 195, 4.78%, 7.15%, 92, 281 for 0.2% PS (MPa), UTS (MPa), %E, %R, YM and VHN respectively

[13].

2.3 Variables affecting Modification [10]:

There are several variables, which affect the evolution of microstructure. There are several variables, which

affect the evolution of the microstructure. These include:

1. Type and quantity of modifier

2. Impurity present in the melt

3. Freezing rate

4. Processing parameters

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5. Silicon content of alloy

3. Effect of Grain Refiner:

Grain refiners modify the form of inter granular shrinkage cavities in cast Al alloys and thereby improve tensile

properties. Normally reduction in grain size to 1/10 of its value gives two-fold increase in UTS and fourfold

increase in elongation [28].

1. Fine grain ensures high toughness & high yield strength.

2. Uniform distribution of 2nd

phase and micro porosity of fine scale resulting in improved machinability.

3. Good surface finish.

Grain refinement achieved by the different methods such as: a.) Rapid solidification of molten metal, b.)

Mechanical vibration/ electromagnetic vibration, c.) Inoculation/ heterogeneous nucleation [29].

The requirements of inoculants for it to act as an effective nucleating site are the following [29]:

1. It should have Melting point higher than alloy being solidified.

2. Sufficient number of nucleating particles.

3. Should initiate freezing at very small under cooling.

4. Should be uniform distributed.

5. The size of nucleating particles should be larger than critical size, which depends on the under cooling of

melt.

The most common grain refiners used are Al-Ti, Al-Ti-C, Al-Ti-B to melt before castings to promote

refinement. Al-Ti-B is most widely used alloy with Ti:B ratios varying from 3:1 to 50:1 [1].

3.1Behaviour of Grain Refiners:

3.1.1 Titanium and Boron [30]:

Titanium (Ti) and boron (B) are used to refine primary aluminum grains. Titanium alone, added as a titanium

aluminum master alloy, forms TiAl3, which serves to nucleate primary aluminum dendrites. More frequent

nucleation or initiation of dendrites means a larger number of smaller grains. Grain refining efficiency is better

when titanium and boron are used in combination. Master alloys of aluminum with 5% titanium and 1% boron

are commonly used additives for this purpose. They form TiB2 and TiAl3, which together are more effective

grain refiners than TiAl3 alone. The most efficient grain refiner for Al-Si alloys has a Ti- B ratio closer to 1.5:1.

That is a special case, applicable to 3XX and 4XX alloys and not to the other alloy systems.

3.2 Grain Refinement of Al-7Si:

1. The composition of the Al-7Si alloy also has an effect on the performance of grain refiners (Ti/B ratios

5.0/1.0 and 6.0/0.0) [31].

2. Refined A356 alloy (T6 treated) was found to exhibit much higher damping capacity than that of A356 base

alloy, by using grain refiner as AlTiB master alloy [32].

3. Its damping capacity value at room temp is 12.3*10-3

increased by 8*10-3

compared to A356 base alloy [32,

33].

4. The improvement in damping capacity of refined A356 alloy is ascribed to the change of Si size and grain

refinement [32].

5. After reinforcement α-Al dendrite decreases from 22µm to 23µm in size and mechanical properties

increased by 30MPa, 24MPa, 4.1% in tensile strength, yield strength and elongation respectively [33, 34].

6. Damping capacity of A356 alloy increases with increasing the temperature and decreases with increases the

frequency [34].

7. The optimum percentage of Ti and B for alloy studied is approximately between 0.06%Ti, 0.01%B and

0.08%Ti, 0.02%B. Higher percentage of Ti and B re melt in the formation of Ti based intermetallics

compounds within eutectic region [35].

8. Al5Ti1B is effective in refining the structure. However there is still some degree of under cooling which

may suggest there might be some room for improving Al5Ti1B grain refining effectiveness [35].

9. Application of mechanical vibration led to increase grain refinement and density of A356 Al alloy [36].

10. Increasing vibration time up to 15min led to decrease in the size of α-Al phase for all vibration frequency

(10-50Hz) [36].

11. Increasing vibration frequency up to 50Hz led to decrease in the size of α-Al phase for all vibration time (5-

15min) [36].

12. Max. achieved grain refinement was 53% in 50Hz and 15min size of α-Al phase reached to 173µm [36].

13. Increasing vibration time and frequency led to degassing because of applying mechanical vibration have

strong effect on the density of alloy. Density of alloy was improved from 2.3gm/cm3 to 2.66gm/cm3 at 50

Hz and 15min Vibration condition [36].

14. The wear rate of composites improves significantly with the TiB2 content [37].

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15. It is suggest that the wear rate per unit amount of reinforcement should be taken in to consideration to

explain the load carrying capacity of metal matrix composites [37].

16. The morphology and size of wear debris is dictated by the loading condition and also the amount of TiB2 in

the composites [37].

17. Addition of 2.5wt% TiB2 lowers coefficient of Friction, followed by an increase in coefficient of friction

for higher TiB2 content [37].

18. A difference in the effect of grain refiner on primary aluminium nucleation temperature was observed based

on the cooling rate. At slower rates, adding 0.1 wt.% Ti (as Al-3Ti-B) resulted in a significant change of 20

°C in whereas at higher rates same amount of grain refiner altered by around 5 °C [38].

19. Wear rate of Al–7Si alloy was found to be dependent on the DAS (Dendritic Arm spacing) and the size of

the silicon particles irrespective of the master alloy used for grain refinement [39].

20. Grain refinement improves the load bearing capacity of Al–7Si alloy [39].

4. Combined effect of Grain Refiner and Modifier on Al-7Si Al alloy:

Modifier and grain refiners are to be added to the molten Al–7Si alloys to get combined grain refinement and

modification effects. Many researchers have reported the grain refinement and modification by addition of gain

refiner and modifier simultaneously. However, it is a difficult task to control the addition levels of individual

grain refiner and modifier in actual industrial practice. It has been reported that the combined addition of the Al –

Ti–B grain refiner and Al–Sr master alloy results in mutual poisoning of the grain refinement and modification

effects.

Recently developed, A new Al-Ti-C-Sr master alloy “Modifiner” which plays a dual role both as grain refiner as

well as modifier at a time.

1. EMPA (Electron probe Microanalysis technique) results showed that 0.02 wt% added Sr was distributed

unevenly in modified samples, and its effect was interfered with adding 0.1 wt% Ti as grain refiner [40].

2. Filtering the melt removed some of Sr content from the melt, resulting in a decrease in Sr modification,

producing coarser silicon phase; however, this helped the grain refinement of the aluminium phase (shown

a decrease in grain size) possibly by reducing the positioning effect of Sr on Ti content [40].

3. The Al–Si alloys investigated exhibit plastic deformation and work hardening during wear testing; the grain

refined and modified Al–7Si–2.5Cu cast alloys work-hardened to a greater extent than the un-treated alloys

[17].

4. The wear resistance decreases with increasing load in case of unmodified alloys. In the grain refined and

modified alloys, it increases with increasing load due to higher work hardening rates [17].

5. Coefficient of friction and temperature rise sharply at high wear load [17].

6. Al–1Ti–3B master alloy is found to be more efficient grain refiner for hypoeutectic Al–Si cast alloy when

compared to Al–5Ti–1B master alloy [17].

7. Combined grain refinement and modification minimizes oxidation of the matrix. The best-combined grain

refined and modified (Al–1Ti–3B + Sr) alloys recorded minimum coefficient of friction and temperature at

high-wear load [17].

8. Addition of grain refiner (Al-3Ti and Al-3B) and modifier Al-10Sr master alloys to A356 alloys leads to

change in microstructure [41].

9. Addition of grain refiners to A356 alloy converts predominantly columnar dendritic structure into fine

equiaxed dendritic structure and addition of modifier changes plate like eutectic Si into fine particles, which

leads to the improvements in mechanical properties [41].

10. Combined grain refinement and modification can be obtained by the addition of new Al–Ti–C–Sr master

alloy [3, 4, 14].

11. The grain refining and modifying efficiency of the master alloy developed is effective at shorter holding

time (5 min), and its efficiency drops on longer holding (120 min) of the melt due to fading (for 0.2 and 0.5

wt% addition of the master alloy) [3, 4, 14].

12. Best results are obtained below 30min of the melt holding of a Modifiner treated Al-7Si alloy [3, 4, 14].

13. Grain refinement can be retained even for long holding at higher addition level of 1.0 wt%, although there

is some fading of modification [3, 4, 14].

14. Modification effect diminishes on longer melting holding [3, 4, 14].

5. Summery

1. Higher cooling rate is beneficial for modification and hence, lesser quantity of modifier would be sufficient

at high cooling rates.

2. Modification effect diminishes on longer melt holding.

3. Degassing with Cl2 will remove some/entire modifier if Na/Sr are used.

4. Sb results in good modification on longer melt treatment time.

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5. It has also reported that mutual poisoning effect of B and Sr results in the formation of SrB 6 compounds

which has an adverse effect on the grain refinement & modification.

6. The improvement in damping capacity of refined A356 alloy is ascribed to the change of Si size and grain

refinement.

7. Damping capacity of A356 alloy increases with increasing the temperature and decreases with increases the

frequency.

8. The wear rate of composites improves significantly with the TiB2 content.

9. Wear rate of Al–7Si alloy was found to be dependent on the DAS (Dendritic Arm spacing) and the size of

the silicon particles irrespective of the master alloy used for grain refinement.

10. Fading phenomenon affects the wear rate of the grain refined Al–7Si alloy.

11. Grain refinement improves the load bearing capacity of Al–7Si alloy.

12. Combined grain refinement and modification can be obtained by the addition of new Al–Ti–C–Sr master

alloy.

13. Best results are obtained below 30min of the melt holding of a Modifiner (Al–Ti–C–Sr master alloy) treated

Al-7Si alloy.

14. The wear resistance decreases with increasing load in case of unmodified alloys. In the grain refined and

modified alloys, it increases with increasing load due to higher work hardening rates.

15. Coefficient of friction and temperature rise sharply at high wear load.

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