November 2017, Volume 4, Issue 11 JETIR (ISSN A ...November 2017, Volume 4, Issue 11 JETIR...

November 2017, Volume 4, Issue 11 JETIR (ISSN-2349-5162)

JETIR1711076 Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org 415

A REVIEW ON ADVANCED CHALLENGES FOR

IMPROVEMENT THE MECHANICAL PROPERTIES

OF AL-SI ALLOY AUTOMOTIVE PARTS 1A. M. Omran,

2H. S. Wasly,

3Moatasem M. Kh

Mining and Metallurgical Engineering Department, Faculty of Engineering, Al-Azhar University, Qena, Egypt

Abstract: The reduction of fuel consumption is the main target for the automotive industry. The use of lightweight materials can help to

reduce vehicle weight and improve fuel economy. The development of Al-Si alloy and its based material for engine application is

investigated in this article, focusing on improving the material’s properties, specially fatigue limit and wear resistance, which are

important properties for engine block materials. The article introduces a description of the microstructure of Al-Si alloy and its effects on

this material’s behavior. Some parameters to enhance these properties are discussed such as modification, grain refinement, casting

methods, alloying elements, and composite production. It was found that aluminum is the ideal light-weighting material for automotive

industry. Higher silicon content usually increases the wear resistance of Al-Si alloy as it increases the alloy’s hardness. Also, grain

refiners and modifiers can form fine precipitates, refine grain size, modify silicon phase morphology, and reduce the effects of defects

and thus can usually increase mechanical properties. Composite is another way to improve Al-Si alloy’s properties.

Key Words: Al-Si alloys; Automotive parts; Heat treatment; Modification; grain refinement, Composites

1. INTRODUCTION

The selection of the suitable materials to reduce energy consumption and air pollution is a challenge for the automotive industry.

Aluminum is one of the most important materials for using in automotive industry. The spread use of aluminum and its alloys compared with

the other types of materials refer to their good physical and mechanical properties, such as: high castability, low specific gravity, low

shrinkage, good weldability and good resistance to wear and corrosion. [1,2,3]. The weight reduction has a direct effect on fuel efficiency.

For example, weight reduction enables the manufacture to develop the same vehicle performance with a smaller engine, which enables the

use of a smaller transmission and a smaller fuel tank. With this, it is estimated that 10% of vehicle weight reduction results in 8–10% of fuel

economy improvement. So, the automotive materials can have an important impact on the environment [3,4,5]. The aim for weight reduction

has driven a gradual decrease in the amount of steel and cast iron used in vehicles and the corresponding increase in the amount of

alternative materials, especially aluminum and plastics. [6,7,8]. For economic and environmental requirements, Al-Si alloys have been

commercially used to produce an engine block due to their high strength over weight ratio [3, 9,10]. In addition to high fatigue strength and

wear resistance, engine block material is also supposed to possess good castability and machinability. This is because engine block has a

very complex structure. It is initially cast and thereafter subject to mechanical machining [11,12].

Aluminum usage in automotive applications has grown more than 80% in the last years. A total of about 110 kg of aluminum /vehicle in

1996 is predicted to rise to 250 or 340 kg, with or without taking body panel or structure applications into account, by last few years. There

are strong predictions for aluminum applications in hoods, trunk lids and doors hanging on a steel frame. Aluminum castings have been

applied to various automobile parts for a long period. As a key trend, the material for engine blocks, which is one of the heavier parts, is

being switched from cast iron to aluminum resulting in significant weight reduction. In automotive power train, aluminum castings have been

used for almost 100% of pistons, about 75% of cylinder heads, 85% of intake manifolds and transmission. For chassis applications,

aluminum castings are used for about 40% of wheels, and for brackets, brake components, suspension (control arms, supports), steering

components (air bag supports, steering shafts, knuckles, housings, wheels) and instrument panels [13-15].

This Work aims to investigate the development of Al-Si alloy and its based material for automotive application, especially in improving

the material‟s properties, as fatigue and wear resistance, which are important properties for engine block materials. Also, an attempt to

present the factors affecting the improving of mechanical properties of Al-Si automotive parts and to show the relation between the

microstructure of Al-Si alloy and its effects on this material‟s behavior. Some parameters to enhance these properties are discussed such as

modification, grain refinement, alloying elements

2. ALUMINUM SILICON ALLOYS

Among aluminum casting alloys, (Al–Si) alloys containing silicon as the major alloying element comprises more than 90 % of the total

aluminum castings produced and have wide spread applications, especially in the transportation industry. The properties of Al–Si alloys are

strongly dependent on casting process, composition and melt treatment given to the alloy [16,17]. The casting of Al-Si alloys is primarily

concerned with the solidification process. This is essentially a phase transformation liquid state to solid state. Phase diagrams (Fig. 1) gives a

great deal about how this transformation occurs, what phases form; at what temperatures the phases form; the relative amounts of each

phase; the composition of phases, and how solute elements are distributed between the phases and how difficult or easy it will be to place a

specific alloying element into aluminum. The content of silicon in the Al–Si alloys varies from 4 to 25 % Si. According to the binary phase

diagram (Figure 1), Al–Si alloys are further classified as hypoeutectic (<12% Si), eutectic (11.6-12.6 % Si) and hypereutectic (14–25% Si)

depending on the silicon concentration in the alloy. Hypereutectic Al-Si alloy with low density, high specific stiffness, high-temperature

resistance, wear resistance, and low coefficient of thermal expansion is of great interest in transportation industry due to its ability to replace

cast iron in automobile engine parts. Some of the major applications of hypereutectic Al–Si alloy include high-performance automobile

engine parts such as connecting rods, rocker arms, cylinder, pistons, and valve retainers [18].



Fig. 1 Part of Al-Si phase diagram [4]

2.1 Some important readings in Al-Si phase diagram

The deep looking at the Al-Si phase diagram; first part provides a better understanding about what the parameters affecting the quality

and mechanical properties of Al-Si alloys products. During the solidification of Al-Si alloys; the silicon concentration in the liquid portion of

the casting increases. Silicon segregates and accumulates in the liquid phase. This segregation (k) during solidification is described by a

distribution coefficient: [19]

……………………. (1)

Another important term which can be determined from the phase diagram is the slope of the liquidus curve (msl) which determine by this

equation for the Al-Si system;

[

]

……………………….. (2)

For silicon in aluminum, m is equal to 6.6 °C/% wt. Si. The last important factor is the solubility of the element in liquid aluminum at

typical furnace temperatures. For silicon this maximum concentration is equal to the eutectic composition, 12.6 weight percent Si [19]. Table

1 shows alloy constants for several elements calculated from phase diagrams. Several important things can be concluded from table 1:

• Ni, Fe, Si and Cu segregate very strongly during solidification, k closed to 0.

• Zn and Mg segregate only moderately, k closed to 0.5.

• Mn hardly segregates at all. The concentration of Mn in solid aluminum is 94% of the liquid k closed to 1. This is an important

factor in the improved performance of die casting alloys, where Mn replaces Fe to prevent die soldering [19]

• The elements below Mn have a value of k greater than one. This means there is a „negative‟ segregation the equilibrium

concentration in the solid is greater than that in the liquid. As a result, the melting point of aluminum increases.

Table 1 Alloy constants for several elements calculated from phase diagrams [19]

c m k Element

6 -3.3 0.007 Ni

1.8 -3 0.02 Fe

12.5 -6.6 0.13 Si

33.2 -3.4 0.17 Cu

50 -1.6 0.4 Zn

34 -6.2 0.51 Mg

1.9 -1.6 0.94 Mn

0.15 13.3 1.5 Nb

0.4 3.5 2 Cr

0.5 8 2.4 Hf

0.1 70 2.5 Ta

0.1 5 2.5 Mo

0.11 4.5 2.5 Zr

0.1 10 4 V

0.15 30 9 Ti

3. THE FACTORS AFFECTING THE MECHANICAL PROPERTIES OF AL-SI ALLOYS

The mechanical properties of the Al-Si alloy are dependent on the size, shape and distribution of eutectic and primary silicon particles.

Small, spherical, uniformly distributed silicon particles enhance the strength properties of Al-Si alloys. There are many factors affecting the

mechanical properties of Al-Si alloys automotive parts, these factors are: Composition (alloying elements), modification, grain refinements,

casting methods, heat treatments and composite

3.1 Alloying elements

The alloying element developed Al-Si alloys can be classified into:

Elements enhanced mechanical properties such as Si, Cu, Mg and Ni.

Elements reduce die soldering and improve cast properties such as Mn and Fe.

Elements improve silicon morphology (modifiers) such as Na, Sr, and P.



Elements decrease particles size (grain refiners) such as Ti and Boron

3.1.1 Elements enhanced mechanical properties (Cu, Mg and Ni)

Copper

Cu is partially soluble in Al solid solution with maximum solid solubility (5.65 wt.%). The main phase is θ Al2Cu, it solidifies in two

forms one blocky or massive shape (Fig 2.a) and the other is fine eutectic form (Fig 2.b, c). The other Cu containing phase is Al5Cu2Mg8Si6

called Q phase although this usually appears in much smaller amounts than θ Al2Cu. Both phases form after the main Al-Si eutectic reaction.

Increasing Cu content increases the porosity in casting (Fig 2.d) [19].

(a) Blocky like Cu Al2 b) fine eutectic CuAl2

c) Complex shape Al5Cu2Si6Mg8 d) Effect of Cu with porosity

Fig. 2 Microstructure of the different phases of Cu present in Al-Si alloys

Magnesium

Magnesium in small additions, up to 0.7%, causes increases in tensile and fatigue strength, and large decreases in ductility and impact

strength involves the formation of Mg2Si precipitates in the bulk metal. Some magnesium will precipitate as Mg2Si forming AlMg2Si ternary

eutectic phase diagram as shown in Fig. (3. a). The Mg2Si phase forms in Chinese script morphology and upon solution treatment radially

dissolved in solid solution as shown in Fig. (3. b) [20].

a b

Fig 3 a) Al-Mg2Si ternary system phase diagram b) Microstructure of Al-Mg-Si alloy [20].

The properties of alloys containing Mg will be affected by how the Mg is present in the matrix, be as coarse phases after solidification, as

atoms in solid solution, as GP zones formed at room temperature or as precipitates during artificial ageing [21].

Magnesium and Copper

The addition of Mg and Cu together as alloying elements has been studied by various authors. The sequence of solidification in Al-Si-

Cu-Mg casting alloys during solidification can be described as follows [22]:

i. Formation of a primary α-aluminum dendritic network at temperatures below 610°C, leading to an increase in the concentration of

Si, Cu and Mg in the remaining liquid.

ii. At about 560°C, the aluminum-silicon eutectic temperature, the eutectic mixture of Si and Al forms, leading to further increase in

Cu and Mg content in the remaining liquid.

iii. At about 540°C, Mg2Si and Al8Mg3FeSi6 form.

iv. At about 525°C, the interdendritic, sometimes called “massive” or “blocky” Al2Cu phase forms together with Al5FeSi platelets.

v. At about 507°C eutectic of Al2Cu with interspersed Al forms. The phase Al5Mg8Si6Cu2 also forms at this temperature, usually with

an ultrafine eutectic structure.

In as cast condition, in a study carried by Ouellet and Samuel [23] no significant difference in hardness behavior was found for Mg

additions over 0.3 wt. % to Al-8Si-3Cu-Fe type alloy. Additions up to 0.33 wt. % were found to have a hardening effect up to 52%. A

marginal hardening effect (6.5%) was found with an increase of Mg up to 0.5 wt. %. The Mg addition was found to lower the formation



temperature of Mg2Si and Al2Cu+Al5Mg8Si6Cu2, increasing this way the melting temperature range for this alloy. It was also found that the

amount of Al5Mg8Si6Cu2 increased with Mg content.

Nickel

When Ni is added to the Al–Si system, the eutectic transformation is characterized by the simultaneous formation of eutectic Si and

Al3Ni. Consequently, eutectic Si and Al3Ni form a geometrically entangled system, as can be clearly seen in Fig. 4[24].

Fig. 4 Eutectic Si and Al3Ni form geometrically entangled system of AlSi7 Ni1.5(Mg)

a) Si–Ni without EDX elemental mapping b) Si–Ni EDX elemental mapping [24]

Nickel can enhance Al-Si alloy‟s strength and hardness at elevated temperature according to the content of silicon in the Al-Si alloys as

show in Fig. 5 [24].

Fig 5 Effect of Ni on Mechanical properties of near eutectic alloys and hypoeutectic alloys tested at 250oC [24]

Nickel can enhance the high temperature of performance of Al-Si foundry alloys by stabilizing the contiguity of the eutectic network.

This happens by increasing the volume fraction of rigid phases in the eutectic phase. Exceeding a certain limit of volume fraction doesn't

result in a further improvement of interconnectivity and strengthening stagnates [24].

JE Hanafee et al [25] Investigate the effect of nickel on hot hardness of aluminum alloys. It is shown that nickel can be utilized to improve

the hot hardness (up to 315 oC) of aluminum-silicon (10 to 16 % silicon) casting and forging alloys. The maximum benefits are realized by

developing a large volume and favorable distribution of nickel aluminide. The addition of more than the eutectic amount of silicon was not

particularly helpful in improving hot hardness. While the addition of more than the eutectic amount of nickel improve hot hardness.

3.1.2 Elements reduce die soldering and the casting properties (Fe and Mn)

Iron

Iron increases hot tear resistance and reduces die soldering but decreases flowability and feeding characteristics. Iron is usually present

in aluminum alloys as an impurity. Its presence decreases the mechanical properties of the alloy above a concentration of 0.6%, due to

formation of phases such as FeAl3 and βAlFeSi as shown in Fig.6 [24].

a) Al5(FeSib) b)Al

15(FeMn)

3Si

2

Fig. 6 Morphology of Fe-rich phases [24]

The dominant Fe-rich phase is plate-shaped Al5FeSi. Long Al5FeSi platelets (more than 500 μm) can adversely affect mechanical

properties, especially ductility, and also lead to the formation of excessive shrinkage porosity defects in castings. John A. Taylor [26]

suggested that the formation of large Al5FeSi platelets at high Fe-contents facilitates the nucleation of eutectic Si, therefore leading to a rapid

deterioration of the interdendritic permeability. The deleterious effect of Al5FeSi can be reduced by increasing the cooling rate, superheating

the molten metal, or by the addition of a suitable “neutralizer” like Mn, Co, Cr, Ni, V, Mo and Be. The most common addition has been Mn.

Excess Mn may reduce Al5FeSi phase (Fig. 6a) and promote formation Fe-rich phases Al15(FeMn)3Si2 in skeleton-like form (or Chinese

script form) (Fig. 6b). This compact morphology “Chinese script” (skeleton-like) does not initiate cracks in the cast material to the same

extent as Al5FeSi does. [27]



Manganese

Manganese, by itself, is reported to have little to no effect on the mechanical properties of cast aluminium silicon. In the presence of

significant iron (> 0.5%) manganese is added to create Al(Mn,Fe)Si "Chinese script" precipitates instead of more embrittling β-AlFeSi

plates. The presence of α -phase particles instead of platelets β-phase improves the mechanical properties, particularly ductility [24].

Additions of Mn to neutralize the effects of iron are common, at Mn: Fe ratio of ~ 0.5, however, the benefits of this treatment are not always

apparent. Excess Mn may reduce β-phase and promote α-phase formation, and this may improve ductility but it can lead to hard spots and

difficulties in machining. Mn additions do not always improve castability and reduce porosity in high Fe alloys. Its affect is sensitive to alloy

composition. The addition of Mn to melts with high iron levels can also promote the formation of sludge, if the sludge factor (derived

by[%Fe] +2[%Mn] +3[%Cr]) exceeds a particular value for a given alloy and melt holding temperature. This is a serious problem for die-

casters who use low melt temperatures and high impurity secondary alloys [28].

3.1.3 Elements improve silicon morphology (modifiers, as Na, and Sr)

If one looks at phase diagrams for the Al-Si system proposed in the literature, it will be found that there is disagreement as to the

exact eutectic composition and to a lesser extent, the eutectic temperature. This is because the formation of the Al-Si eutectic is sensitive to

small amounts of impurities, especially P, Na and other alkaline earth elements [19].

Sodium is used in aluminum silicon alloys as a modifier of the eutectic phase. By refining the coarse plates of eutectic silicon into a

finely dispersed fibrous structure, sodium additions, in small quantities, can greatly improve mechanical properties of cast aluminum silicon

alloys. Excess sodium modification results in over-modification.

Strontium is used as a modifier, similar to sodium. It has been noted to be a less effective modifier than sodium, but is less sensitive

to over-modification. Excess strontium does not lead to a largely over modified structure and the resultant drop in mechanical properties is

smaller. Small precipitates of Al2SrSi2 have been noted when excess strontium is added and could be associated with a noted drop in tensile

strength and ductility when excess strontium is added. Strontium is also retained longer in the melt than sodium, for up to a total of

approximately 4 hours. This allows strontium modified alloy to be re-melted without the requirement for further modification alloy

additions.

The Microstructure of binary Al-Si alloys, in the unmodified state, near to the eutectic composition exhibit acicular or lamellar

eutectic silicon which is in the form of large plates with sharp sides and edges. Al-Si alloys containing more than about 12% Si exhibit a

hypereutectic microstructure normally containing primary silicon phase in a eutectic matrix. Cast eutectic alloys with coarse acicular silicon

show low strength and ductility because of the coarse plate-like nature of the Si phase that leads to premature crack initiation and fracture in

tension. Similarly, the primary silicon in normal hypereutectic alloys is usually very coarse and imparts poor properties to these alloys.

Therefore, alloys with a predominantly eutectic structure must be modified to ensure adequate mechanical strength and ductility. It is widely

recognized that the elements (Na, Mg, Ca, Sr) are effective modifiers of Al-Si eutectic; only sodium and strontium, however, have been used

extensively in the commercial production of these alloys. Refinement of primary silicon is usually achieved by the addition of phosphor to

the melt. It is also reported that rare earth metals are also capable of modifying the eutectic structure of cast Al-Si alloys [29-33].

Figure 7 shows microstructures of Al-Si alloys containing 13 wt. % Si. Adding a small quantity of a ternary element, here Sodium

causes modification of the microstructure. This addition effectively moves the eutectic point to a higher silicon concentration and lower

temperature. This modifies the growth of the eutectic silicon to produce an irregular fibrous form (Fig.7 a, b). The eutectic point has moved

far enough to make the alloy, at this composition, hypo-eutectic instead of hyper-eutectic Figure 8. So, by adding a very small impurity of

0.01% Na the microstructure of the alloy is changed and its properties are greatly improved. We can more easily predict the effects of

compositional changes from Fig. (8).

Figure 7: Light Micrographs of Al- Si alloys containing 13% Si (a) unmodified (b) Modified by 0.01 Na

Adding a small quantity of a ternary element causes modification of the microstructure. This addition effectively moves the eutectic point

to a higher silicon concentration and lower temperature. This modifies the growth of the eutectic silicon to produce an irregular fibrous form

rather than the usual flakes. The eutectic point has moved far enough to make the alloy, at this composition, hypo-eutectic instead of hyper-

eutectic, so now primary alpha forms, rather than primary Si. This can be seen on its micrograph.

Fig 8 Effect of modification in the Al-Si phase diagram



3.1.4 Elements decrease grain size (grain refiners, as Ti and B)

Titanium is added to aluminum-silicon casting alloys to achieve grain refinement. Small particles of titanium compounds act as

nucleants for primary aluminum solidification. Boron is used as a nucleant to promote grain refinement and to remove titanium, vanadium,

chromium and zirconium from high purity aluminum. It has been noted that boron has a detrimental effect on strontium modification.

Titanium (Ti) and boron (B) are used to refine primary aluminum grains. Titanium, added in aluminum alloy, forms TiAl3, which serves to

nucleate primary aluminum dendrites. More frequent nucleation of dendrites means a large number of smaller grains. Grain refining 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. Titanium with boron form TiB2 and TiAl3, which together are more effective grain refiners than TiAl3 alone [34].

Grain refinement in aluminum casting has been studied for many decades. A fine grain structure is well known to benefit both the

mechanical and technological properties of aluminum alloys. Several factors affect the as-cast microstructure through influencing either

nucleation or growth during solidification. The effect of solute elements on grain refinement, which can be quantified by the growth

restriction factor (GRF) Q, is of great importance for controlling the as-cast grain size in aluminum alloy [35]. Growth restriction factor

(GRF) can be calculated by the following equation:

Q= Σ (Ki-1) mi Ci ………………………..(3)

Where;

Ci: the concentration of each individual element presents in the melt.

Ki: represents the distribution coefficient for each element in the binary Al-Si system.

Mi: is the slope of the liquidus line.

Titanium has higher growth restriction effect than any other element. Most grain refining agents therefore contain an excess of Ti, which

goes into solution in the melt as shown in table (2).

Through GRF Q, a simple relation between grain size (d) and the alloy composition for a constant set of casting conditions is suggested by

Easton and St. John [36] as

d = a + b /Q ………………………..(4)

The constant (a) is related to the maximum number of particles which are active nucleants and the gradient (b) determines the nucleant

potency of the particles. Generally, GRF represents the segregating power of all elements during solidification in the alloy. These

segregating elements accumulate at the solid/liquid interface, preventing its further growth and, at the same time, lead to the formation of a

constitutionally undercooled zone in front of the interface, which might activate the potent nucleants present there [37].

Table (2) The segregating power of some elements in aluminum [37]

Figure 9 shows the role of peritectic reaction in grain refinement in cast Al. From which it can be seen that the addition of eutectic-

forming alloying elements is less effect on grain refining than the peritectic-forming alloying elements.

Fig. (9) Peritectic reaction role in grain refinement in cast Al (a) addition of eutectic-forming alloying elements

(b) addition of peritectic-forming alloying elements

3.2 Grain refinement Grain Refinement is a process used for grain boundary strengthening of casting material. Grain refinement of Al and its alloys improve

the mechanical properties of casting along with surface finish. Grain refiners in aluminum alloy are important for successful castings. For

Reaction

type

Maximum

concentration

(wt%)

(ki-1) m mi ki Element

Peritectic 0.15 245.6 30.7 9.0~ Ti

Peritectic 0.10 ~ 105.0 70.0 2.5 Ta

Peritectic 0.10 ~ 30.0 10.0 4.0 V

Peritectic 0.50 ~ 11.2 8.0 2.4 Hf

Peritectic 0.10 7.5 5.0 2.5 Mo

Peritectic 0.11 ~ 6.6 4.5 2.5 Zr

Peritectic 0.15 ~ 6.65 13.3 1.5 Nb

Eutectic 12.6 ~ 5.9 -6.6 0.11 Si

Eutectic 0.4 ~ 3.5 3.5 2.0 Cr

Eutectic 6.00 ~ 3.5 -3.3 0.007 Ni

Eutectic 3.40 ~ 3.0 -6.2 0.51 Mg

Eutectic 1.80 ~ 2.9 -3.0 0.02 Fe

Eutectic 33.20 2.8 -3.4 0.17 Cu

Eutectic 1.90 0.1 -1.6 0.94 Mn



example, mechanical properties can be improved, susceptibility to hot tearing is reduced and fluidity is improved. There are several ways to

introduce grain refiner particles into the melt. The most widely accepted method is to add commercial Al-Ti-B master alloys into the melt in

the form of rod coil or waffle plate [38-41].

Hypo-eutectic aluminum-silicon alloys have a huge portion of primary α-Al in their microstructure. An unmodified Al–Si alloy has

large, brittle flakes of silicon, which result in poor ductility to the casting. Modifiers are added to eutectic and hypo-eutectic Al–Si alloys to

refine the eutectic Si phase from angular platelets to fine fibers. This change in microstructure results in an additional development in the

mechanical properties [42-44].

A fine equiaxed structure has many advantages like improved mechanical properties, better feeding during solidification, reduced and

more evenly distributed shrinkage porosity, better dispersion of second phase particles, better surface finish and other desired properties. Al-

Ti master alloys such as Al-3Ti-3B and Al-Ti-1B alloys can be used for grain refinement of aluminum-silicon alloys [40]

Grain refinement plays a vital role in cast and wrought aluminum alloys. There are number of reasons why the control of grain size is

important in semi continuously cast alloys. Firstly, reduced mechanical properties have been noted in plate products for structural application

when a uniform as cast grain size is not achieved. Secondly, a coarse-grained structure may result in a variety of surface defects in alloys

used in rolled or extruded form for architectural applications. Thirdly, hot cracking in the shell zone of cast ingot is more severe if the grain

structure is not equiaxed. An equiaxed structure allows a higher casting rate to be achieved before hot cracking is produced. [45].

Lu et al. [46] reported the effect of different master alloys on the grain size of small castings made from Al-5%Si alloys. Figure 10, show the

interesting fact that the Al-B master alloy is a more efficient grain refiner than both Al-Ti and Al-Ti-B alloys. As is evident, the addition of

boron tripled grain refinement. The procedure of introducing boron into the alloy, however, is of significant importance from the point of

view of the size and shape of the boron particles.

Figure 10 effect of addition of different grain refiners on the grain refinement of Al-Si alloys [46].

Al-Ti, and Al-5Ti-1B are less effective than Al-B alloys for refining Al-Si alloys as indicated in Figure (10) due to the consideration of

the binary Si-Ti system. Sigworth et al. [19] suggested that possibly of forming the titanium silicide coats the surface of TiAl3 and thus

poisons the effectiveness of the nuclei present in Al-Ti master alloy. On the other hand, when Al-B is the grain refiner, it is the AIB2 phase

which acts as the nucleant and the presence of Si enhances its nucleation potential.

3.2.1 Mechanism of grain refinement of Al alloys.

Al–Ti and, Al–5%Ti–1%B and Al–Ti-C master alloy are used for refining of Al and Al wrought alloys. Several theories are used to

explain the role of grain refiners for grain refinement of Al alloys like peritectic, boride, carbide. Hyper nucleation theory, Duplex nucleation

theory, and peritectic hulk theory. Major accepted theories; Peritectic Hulk Theory [47]. Backerud et al. [48] proposed the peritectic hulk

theory in the early 1990s. This theory recognized that Al3Ti is a more potent nucleant than the TiB2 and attempted to explain how the borides

increase the stability of aluminides. It was suggested that the borides form a shell around the aluminides, and slow down dissolution of the

aluminides. The aluminides eventually dissolve and leave a cell of liquid with approximately the peritectic composition. The peritectic

reaction can then take place to form the α-Al. Schematic Sequence of Peritectic Hulk theory is as the following (shown in Fig. 11) [47].

a) Master alloys when addition the matrix liquefies leaves TiAl3 and boride crystals

b) Dissolution of free borides in the bulk Liquid and re-precipitation at the surface of dissolving TiAl3 crystals, thus reinforcing the

boride shell originally present in the duplex crystal.

c) Simultaneous dissolution of duplex type aluminide gives rise to many nuclei with protective boride shell.

d) Counter diffusion of Ti and Al occur leading to the formation of Liquid hulk enriched in Ti.

e) On cooling, a-Al nucleates from the liquid hulk by peritectic reaction and grows by passing through the boride shell.

f) On prolonged holding, fading occurs due to equilibration of Ti with the bulk Liquid, leaving behind boride clusters.

The most common use for grain refinement of Al-Si alloys are Al–3Ti–3B and Al–3B (Fig 12). From Fig.12 two groups from Al-7Si

alloy were refined using Al–3Ti–3B and Al–3B respectively. The upper group was refined with Al–3Ti–3B at different holding times, 2,5,15

and 30 min. The smallest grain size belongs to 2 min holding time and the grain sizes increased with more holding time. The 2nd group

refined with Al–3B also at different holding times, also the grain size was grown as holding time increase. The 2nd group refined with Al–

3B also at different holding times, also the grain size was grown as holding time increase. But the grain sizes of the 1st group were smaller

than the gran sizes of the 2nd group at the same holding times. This is due to the grain refining of the 1st group using

Figure (11) Schematic Sequence of Peritectic Hulk theory [46]



Al–3Ti–3B grain refiner was done by TiB2 and AlB2 nucleants which led to more refining as indicated in Table 3. But the refining in the

2nd one was done by only AlB2. In general, cast alloys are more difficult to grain refine than commercial aluminium. The reason for this is

thought to be the high level of alloying elements particularly silicon. TiAl3 crystals are poor nuclei, considering the binary Si-Ti system,

Sigworth et al. suggested that possibly the titanium silicide coats the surface of TiAl3 and thus poisons the effectiveness of the nuclei present

in Al-Ti master alloy. TiB2 Particles are Excellent nuclei and are mostly used because when Al-B is added the grain refiner, it forms AIB2

phase which acts as the nucleant and the presence of Si enhances its nucleation potential, while AlB2 is the best nucleus but not suitable for

industry because the boride agglomerate with time forming slag and fading the grain refining efficiency.

Fig. 12 Performance tests of the Al–3Ti–3B and Al–3B grain refiners used with the Al–7Si alloy at an addition level of 0.005 wt% B.

Table (3) indicates summary of grain refinement effect in Al and Al-Si cast alloys. From this table it can be concluded that the efficiency

of grain refining is depending on the type of nucleant uses. TiAl3, TiB2 and TiC nucleant are more efficient for refining of commercial

aluminium according to Halk pretectic, boride and carbide theories and because the titanium silicide coats the surface of TiAl3 and thus

poisons the effectiveness of the nuclei present in Al-Ti master alloy [45-47]. While AlB2, TiB2 and TiC nucleant are more efficient for

refining of Al-Si alloys due to the presence of Si enhances its nucleation potential.

Table (3) Grain refinement effect in Al and Al-Si cast alloys

Effect in Al-Si Effect in Al Nucleant Master alloy

Poor Good TiAl3 Al-Ti

Acceptable Very good TiAl3, TiB2 2.2)< Al-5Ti-1B (Ti/B

Very good Acceptable AlB2, TiB2 2.2)>Al-Ti-B (Ti/B

Acceptable Very good TiAl3, TiC 4)< Al-Ti-C (Ti/C

Not tested Excellent TiC Al-TiC

Excellent Poor AlB2 Al-4B

Excellent Not tested TiB2, TiC Al-3Ti-1B-0.2C

3.3 Heat treatment

The Si particles have a plate-like morphology in unmodified aluminum alloys, which act as crack initiators and have a negative influence

of ductility. The alloy ductility can be improved by changing the morphology of the silicon particles to more fibrous form [48]. This can be

done by using a high cooling rate, by addition of a chemical modifier, by exposing the casting to a high temperature for long periods, or by a

combination of these processes [49].

Heat-treatment is of major importance since it is commonly used to alter the mechanical properties of cast aluminum alloys. Heat-

treatment improves the strength of aluminum alloys through a process known as precipitation-hardening which occurs during the heating and

cooling of an aluminum alloy and in which precipitates are formed in the aluminum matrix [50]. The improvement in the mechanical

properties of Al alloys as a result of heat treatment depends upon the change in solubility of the alloying constituents with temperature

[27,51]. Figure 13 shows the major steps of the heat treatments which are normally used to improve the mechanical properties of aluminum.

The alloy should first be solution treated at a temperature just below the eutectic temperature for long enough to allow solutionizing of the

second phase. Then it should be quenched to room temperature. Finally, it should be heated to a lower temperature to allow precipitation.

[52].

Figure 13 The three steps for precipitation hardening [53].

3.3.1 Heat treatment of Al-Si alloys

3.3.1 Solution heat-treating Heat treatment at relatively high temperature (495-540

oC) is required to activate diffusion mechanisms [54]. Heat treatment was used to

Dissolve Mg-rich phases formed during solidification, and homogenize the alloying elements, such as Mg and Si. To achieve an elevated



yield stress subsequent, to change the morphology of eutectic Si from polyhedral, or fibrous morphology in the modified alloys, to globular

structure and to increase strength or improve the ductility of the alloy. The heat treatment of the Al-Si alloy normally involves the

precipitation of the hard-secondary particles from the matrix to produce precipitate strengthening of the matrix [55]. A typical heat treatment

applied to sand and gravity die-cast Al-Si alloys is the T6 heat treatment system. The T6 heat treatment system involves the following stages:

[55]

1. Solution treatment at a relatively high temperature to dissolve Cu- and Mg-rich particles formed during solidification to achieve a high

and homogeneous concentration of the alloying elements in solid solution.

2. Quenching, usually to room temperature, to obtain a supersaturated solid solution of solute atoms and vacancies.

3. Age-hardening, to cause precipitation from the supersaturated solid solution, either at room temperature (natural ageing) or at, an

elevated temperature (artificial ageing).

Studies by Gauthier et al. [56] on the solution heat treatment of 319 alloy over a temperature range of (480- 540 oC), for solution times

of up to 24 hours, showed that the best combination of tensile strength and ductility was obtained when the as-cast material was solution

heat-treated at 515oC for (8 -16 hours), followed by quenching in warm water at 60

oC. A higher solution temperature was seen to result in the

partial melting of the copper phase, the formation of a structureless form of the phase and related porosity upon quenching, with a

consequent deterioration of the tensile properties.

3.3.2- Quenching

Quenching is the next important step in the heat treatment cycle after solution heat treatment. The objectives of quenching are to

suppress precipitation during quenching; to retain the maximum amount of the precipitation hardening elements in solution to form a

supersaturated solid solution at low temperatures; and to trap as many vacancies as possible within the atomic lattice [57,58]. If the quench

rate is sufficiently high, a high concentration of solute in solid solution and vacancies is retained. While if the cooling is too slow, particles

precipitate heterogeneously at grain boundaries or dislocation which result in a reduction in supper saturation of solute and concomitantly a

lower maximum yield strength after ageing. In Al-Si casting alloys; Si may diffuse from the matrix to eutectic Si particles and Mg2Si phases

may form on the eutectic Si particles or in the matrix, reducing the supersaturation of Mg and Si in the matrix [59].

Solution heat treatment at 540°C for few hours, the Si particles become coarser and the interparticle distance increases. Rayleigh

instability occurs; silicon particles undergo necking and are broken down into fragments. Due to the instability of the interfaces between the

two different phases and a reduction in the total interface energy, spheroidization and coarsening processes occur (as shown in Fig. 14). A

prolonged solution treatment leads to extensive coarsening of the particles, with a small effect on the spheroidization level. The interparticle

spacing increases too. Because the coarsening and spheroidization are diffusion-controlled processes, [60] they are directly proportional to

the solution temperature and time. These findings are in agreement with others [61].

Fig. 14 Rayleigh instability during Solution heat treatment at 540°C for few hours [65]

3.3.3. Artificial ageing

After solution treatment and quenching, the matrix has a high supersaturation of solute atoms and vacancies. Clusters enriched in Si

and Mg atoms form rapidly from the supersaturated matrix and evolve into GP (Guinier–Preston) zones. Metastable coherent or semi-

coherent precipitates form either from the GP zones or from the supersaturated matrix when the GP zones have dissolved. The precipitates

grow by diffusion of atoms from the supersaturated solid solution to the precipitates. The precipitates continue to grow in accordance with

Ostwald ripening when the supersaturation is lost. The length of each step in the sequence depends on the thermal history, the alloy

composition, and the artificial ageing temperature [62]. In Al-Si-Mg alloys separate clusters of Mg and Si atoms form initially, which

develop into co-clusters [63].

GP zones form from the co-clusters, which elongate and transform into the β''-Mg5Si6 phase [63], which is the phase having the

greatest strength contribution. Upon over ageing some of the β'' phases transform into the rod-like β‟ phase. The Mg: Si ratio increases

through the precipitation sequence [63,64], which make the supersaturation of Si an important parameter as it influences the fraction of

precipitates formed during initial ageing. The precipitation sequence for Al-Si-Cu-Mg alloys is similar, but more complex, as the Q'' phase

and the θ' phase may also form Cu can increase the fraction of the β'' phase formed, but it can also form the Q'' phase [65,66], which has a

lower strength contribution compared to the β'' phase. The β'' phase is therefore preferred, rather than the Q'' phase. It is however not clearly

stated when the Q'' phase forms at the expense of the β'' phase in cast alloys. For wrought alloys it has been shown that the fraction of the Q''

phase increases with natural ageing and artificial ageing time and temperature. [67,68].

The precipitation sequence in Al-Si-Cu alloys is influenced by the high density of dislocations formed during quenching due to the

difference in thermal expansion between the Si particles and the α-Al matrix [69]. Fine and evenly dispersed θ‟‟ phases formed in the center

of the dendrites, while coarse θ‟ phases formed on the dislocations close to the Si particles. The coarse θ‟ phases have a negligible strength

contribution and can be seen as a loss of Cu atoms [61]. Table 3 shows the precipitation sequence of some Al alloy systems. The effect of

heat treatment (age-hardening-T6) on morphology of eutectic Si is showed in Fig.15. In which the morphology changes of eutectic Si



observed after age hardening are displayed just for holding time 16 hours. Eutectic Si without heat treatment (untreated as-cast state) occurs

in platelets form (Fig. 15, a). After heat treatment by all temperature of artificial aging were noticed that the Si platelets were spheroidized to

rounded shape (Fig. 15, b–d). [27]

Table (4) The precipitation sequence of some Al alloy systems [62]

Equilibrium

precipitate Sequence of precipitate Alloy

θ (Al2Cu) Thin plate of GP zones Al ii) θ'' iii) θ' Al-Cu

β-FCC (Mg2Si), a=

0.639 GP Zone as needles (10 nm) long - β' as rods Al-Si-Mg

S(Al2CuMg) GP (Cu, Mg) Zones as rods ii) S'

(orthorhombic) Al-Cu-Mg

a) untreated state b) T6 – 150 °C, 16 h

c) T6 – 170 °C, 16 h d) T6 – 190 °C, 16 h

Fig 15 Effect of heat treatment on morphology of eutectic Si [27]

Al-Al2Cu-Si phase without heat treatment (as-cast state) occurs in form compact oval troops (Fig. 16, a). After age-hardening compact

Al-Al2Cu-Si phase dissolved and disintegrated to separated Al2Cu particles (Fig. 16, b-d). The amount of these phases was not visible on

optical microscope. On SEM microscope we observed these phases in form of very small particles for every temperature of artificial aging

(Fig. 16, b – d). By observation we had to use a big extension, because we did not see these elements. Small precipitates (Al2Cu) incipient

formed by age-hardening were invisible in the optical microscope and electron microscope so it is necessary for observation using TEM

microscopy. [27].

a) as-cast b) T6 -150 °C, 16 hours

c) T6 – 170 °C, 16 hours d) T6 – 190 °C, 16 hours

Fig. 16 Changes of morphology Al-Al2Cu-Si phase [27]

Fe-phases in experimental secondary AlSi9Cu3 cast alloy precipitate first of all as Al15(FeMn)3Si2 phase. Al15(MnFe)3Si2 changes during

experimental heat treatment are demonstrated in Fig. 17. In as-cast samples, Al15(MnFe)3Si2 phase has a compact skeleton-like morphology

(Fig. 17, a). During age hardening compact phase dissolved and fragmented to smaller skeleton particles. Figure 17 b, shows that

temperature of artificial aging markedly affected the Fe rich phase's morphology [27].



Fig. 17 Morphology changes of skeleton-like Fe-rich phase Al15(FeMn)3Si2 [27]:

a) as-cast Al15(FeMn)3Si2, b) T6 -170 °C, 16 hours

3.4 Composites

Composites is another way to improve Al-Si alloy‟s properties. Aluminum alloys are used in advanced applications because their

combination of high strength, low density, durability, machinability, availability and cost is very attractive compared to competing materials

[39].

Aluminum and its alloys have attracted most attention as base metal in metal matrix composites [1]. Aluminum MMCS are widely used

in aircraft, aerospace, automobiles and various other fields [40]. The reinforcements should be stable in the given working temperature and

non- reactive too. The most commonly used reinforcement Particulates are; silicon carbide (SiC) and aluminium oxide (Al2O3). SiC

reinforcement increases the tensile strength, hardness, density and wear resistance of Al and its alloys [41,42].

With the increase in reinforcement ratio, tensile strength, hardness and density of Al MMC material increased, but impact toughness

decreased. The particle distribution plays a very vital role in the properties of the Al-MMC and is improved by intensive shearing. Al2O3

reinforcement has good compressive strength and wear resistance. Boron Carbide is one of the hardest known elements. It has high elastic

modulus and fracture toughness. The addition of Boron Carbide (B4C) in Al matrix increases the hardness, but does not improve the wear

resistance significantly [40] Zircon is usually used as a hybrid reinforcement. It increases the wear resistance significantly [6]. The use of fly

ash reinforcements has been increased due to their low cost and availability as waste by-product in thermal power plants. The major

constituents of fly ash are SiO2, Al2O3, Fe2O3, and CaO. Fly ash increases the wear resistance and the electromagnetic shielding effect of the

Al MMC [40].

4. CONCLUSION

The choice of material for automotive applications such as chassis, autobody and many structural components is due to its low weight,

good formability and corrosion resistance. Aluminum is the ideal light-weight material as it allows a weight saving of up to 50% over

competing materials in most applications without compromising safety. Aluminum usage in automotive applications has increased from 60

kg of aluminum uses in vehicle before 1996 reached to 340 kg with taking body panel into account, recently. The silicon phase is important,

coarse silicon usually reduces fatigue life due to microcrack initiation. Higher silicon content usually increases the wear resistance of Al-Si

alloy as it increases the alloy‟s hardness. Selection of the optimum casting method for decreasing casting defects such as porosity and

inclusion, usually reduce the alloy‟s fatigue and wear resistance due to microcrack initiation. Also, improve Al-Si alloy‟s microstructure and

increase its properties. Grain refiners and modifiers can form fine precipitates, refine grain size, modify silicon phase morphology, and

reduce the effects of defects and thus can usually increase mechanical properties specially, fatigue and wear resistance. Heat treatment is

required to activate diffusion mechanisms, homogenize the alloying elements, change the morphology and to increase strength or improve

the ductility of the alloy. Composites are another way to improve Al-Si alloy‟s properties.

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