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9/21/2010

MME 6203; Lecture 13

The Treatment of Liquid q AluminiumSilicon Alloys

1. Introduction 2. Aluminium ­ Silicon Foundry Alloys 3. Eutectic Modification

AKMB Rashid Department of MME BUET, Dhaka

Topics to discuss

Commercially important aluminium alloys, C i ll i l i i ll current market and future potential Common AlSi foundry alloys, their characteristics, and typical applications Eutectic modification

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1. Commercial Importance of Aluminium Foundry Alloys

Use f l i i U of aluminium casting alloys constitute i second only to th ti ll tit t is d l t the ferrous castings.

Worldwide, approximately 20% of total aluminium production is, on average, converted into cast parts.

A wide range of alloying elements including Zn, Mg, Cu, Si, Fe, Li, Mn, Ni, Ag, Sn, and Ti can be added to aluminium aluminium.

Because of their varying solid solubilities, some are used as solid solution strengtheners, while others are added to form various desirable intermetallic compounds.

Casting Alloy Designations

Unalloyed composition (Al 99.0% or greater) Copper C Silicon with magnesium and/or copper Silicon Magnesium Unused Zinc Tin Unused

1xx.x 2xx.x 2 3xx.x 4xx.x 5xx.x 6xx.x 7xx.x 7xx x 8xx.x 9xx.x

190.x indicates commercially pure aluminium with 99.90 % purity. Digit on the right of decimal point indicates the product form: 0 ­ castings 1 ­ ingots

*Adopted by the Aluminium Association, USA.

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Aluminium Cast Alloy Properties

Low specific gravity Relatively low melting Negligible gas solubility (with the exception of hydrogen) Excellent castability, especially near the eutectic composition Good machinability and surface finish Good corrosion resistance Good electrical and thermal conductivity 3.5 to 8.5 % volumetric shrinkage during solidification Mechanical properties inferior to those of wrought products

Selection of a particular alloy based upon:

castability (fluidity, hot tearing, shrinkage characteristics) mechanical properties usage properties

Selection of casting alloys for general uses:

machinability corrosion resistance hardness mechanical properties

Selection of alloys for special purpose applications (for their unique properties):

high temperature resistance low thermal expansion coefficient (390.0) bearing properties (high Sn alloys)

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Casting Processes

Can be produced by more than one process. Quality requirements, technical limitations and economic considerations dictate the choice of a casting process. p Three main casting processes:

sand casting ­ large casting (up to several tons), one to several thousands in quantity permanent mould casting (gravity and low pressure) ­ medium sized (up to 100 kg), 1000 to 100,000 units high pressure die casting ­ small castings (up to 50 kg), large quantities (10,000 to 100 000) (10 000 100,000)

Other casting processes include:

investment casting plaster moulding ceramic moulding squeeze casting lost foam casting ceramic moulding centrifugal casting semi-solid casting

Current Market for Al Castings

High pressure die casting accounted for the largest share (58 %).

permanent mould casting and sand castings accounts for 30 % and 8% (1996)

These three major processes represents about 90% of all aluminium cast parts.

Aluminium Castings in Automotive Industry

60 and 70% of all aluminium castings produced are destined for the transportation industry.

engine blocks, cylinder heads, intake manifolds, pistons, wheels, compressor parts, brake callipers, t t b k lli transmission and steering systems i i d t i t cost effective replacement for ferrous parts generally produced by permanent mould and sand casting processes (because of the large numbers involved), although the permanent mould process (gravity and low pressure) are often used (when good mechanical properties are required)

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The Future Potential for Al Castings

The future demand for aluminium casting to be determined by improvements in quality, advances in materials, and development in processes. The single most important market opportunity:

transportation sector (especially in automobile and commercial vehicles) Cylinder heads Aluminium wheels Engine blocks Aluminium pistons

Possible Competitors:

(to manufacture engine block and cylinder heads) Magnesium alloys Thin-walled cast irons

2. Al-Si Foundry Alloys

The Aluminium Association identified and classified 7 alloy series with about 240 alloy compositions.

This large number of alloy systems have been developed to serve individual processes and needs. Some alloys differ only in impurities or alloying elements. (identified by a different prefix letter, for example 356.0 and A356.0) Many big foundries in North America, Europe and Japan developed in-house casting alloys (having proprietary compositions).

Aluminium alloy are either heat treatable or non-heat treatable.

As-cast alloys are identified by a suffix F following the alloy number. Heat treatment of castings results iin iimproved mechanicall properties. H tt t t f ti lt d h i ti Temper designations (e.g., 0, T4, T6, and T7) used to identify heat treatment cycle.

High pressure alloys are not normally heat treated.

because of the occurrence of blistering at solution temperatures.

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Table: Composition of Common Aluminium ­ Silicon Casting Alloys

AA No. 319.0 332.0 355.0 356.0 A356.0 B356.0 357.0 A357.0 B357.0 380.0 383.0 384.0 390.0 393.0 413.0 443.0 Products Si S, P* P S, P S, P S, P S, P S, P S, P S, P D D D D S, P, D D S, P 5.5-6.5 8.5-10.5 4.5-5.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 6.5-7.5 7.5-9.5 9.5-11.5 10.5-12.0 16.0-18.0 21.0-23.0 11.0-13.0 4.5-6.0 Fe 1.0 1.2 0.6 0.6 0.20 0.09 0.15 0.20 0.09 2.0 1.3 1.3 1.3 1.3 2.0 0.8 Cu 3.0-4.0 2.0-4.0 1.0-1.5 0.25 0.20 0.05 0.05 0.20 0.05 3.0-4.0 2.0-3.0 3.0-4.5 4.0-5.0 0.7-1.1 1.0 0.6 Mn 0.50 0.50 0.50 0.35 0.10 0.05 0.03 0.10 0.05 0.50 0.50 0.50 0.10 0.10 0.35 0.50 Mg 0.10 0.50-1.5 0.40-0.6 0.20-0.45 0.25-0.45 0.25-0.45 0.45-0.6 0.40-0.7 0.40-0.6 0.10 0.10 0.10 0.45-0.65 0.7-1.3 0.10 0.05 Cr ... ... 0.25 ... ... ... ... ... ... ... ... ... ... ... ... 0.25 Ni 0.35 0.50 ... ... ... ... ... ... ... 0.50 0.30 0.50 ... 2.0-2.5 0.50 ... Zn 1.0 1.0 0.35 0.35 0.10 0.05 0.05 0.10 0.05 3.0 3.0 3.0 0.10 0.10 0.50 0.50 Ti 0.25 0.25 0.25 0.25 0.20 0.04-0.20 0.20 0.04-0.20 0.04-0.20 ... ... ... 0.20 0.10-0.20 ... 0.25 Sn ... ... ... ... ... ... ... ... ... 0.35 0.15 0.35 ... ... 0.15 ...

* S ­ Sand casting, P ­ Permanent mould casting, D ­ High pressure die casting

Table: Characteristics of Aluminium ­ Silicon Casting Alloys

AA No. 319.0 332.0 355.0 A356.0 A357.0 380.0 390.0 413.0 443.0 Casting g Method S, P* P S, P S, P S, P D D D S, P Resistance to Tearing 2 1 1 1 1 2 2 1 1 Pressure Tightness 2 2 1 1 1 1 2 2 1 y Fluidity 2 1 1 1 1 2 2 1 2 Shrinkage g Tendency 2 2 1 1 1 1 Corrosion Resistance 3 3 3 2 2 5 2 2 2 Machinability 3 4 3 3 3 3 4 4 5 Weldability 2 2 2 2 2 4 2 4 1

* Ratings: 1 ­ best, 5 ­ worst

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Aluminium ­ Silicon Alloys

Fig. 2.1: Equilibrium binary AlSi phase diagram

Fig. 2.2: Pseudo binary AlMg2Si phase diagram

Binary eutectic and hypoeutectic Al-Si alloys

Characterised by good castability and corrosion resistance. The near-eutectic 413.0 alloy (~12% Si) contains a predominant eutectic phase, and therefore must be modified with either strontium or sodium to ensure adequate tensile strength and ductility. For high-pressure die casting, 413.0 alloy has better castability than 443.0 alloy (~5.25% Si). The alloy 443.0 can be used for all p y processes and when ductility, corrosion resistance and pressure tightness are more important.

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Strengthening of Al-Si alloys

Achieved by small amounts of Cu, Mg or Ni. In the hypoeutectic ranges, Si provides good casting properties and Cu improves tensile strength, machinability and thermal conductivity at the expense of a reduction in ductility and corrosion resistance. Alloy 319.0 is extensively used for sand and permanent mould y y p casting. Alloy 380.0 has been used as the principal high pressure die casting alloys. Composition of 380.0 alloys vary widely in different countries in both alloying elements (Si and Cu) and impurities (Fe, Zn, Mn). High Fe is preferred to avoid die adherence but it promotes the formation of brittle plates of -AlFeSi or other complex p p intermetallics in presence of Mn. Mg level is specified as below 0.3% to avoid the formation of coarse Mg2Si which deteriorates tensile strength. Generally 319.0 and 380.0 both alloys are supplied in as-cast conditions, but strength and machinability 319.0 alloy can be improved by T6 or T5 heat treatment.

Age hardenable Al-Si alloys containing Mg

Very important alloy group. After solution treatment (T4) and quenching, ageing results in a uniform precipitation of Mg2Si precipitates throughout the aluminium dendrites. Can be sand cast or permanent mould cast with excellent castability, pressure tightness and corrosion resistance. Structure control through eutectic modification and heat treatment provides a wide range of properties. Increased Fe level and slower solidification rates have negative effect on the mechanical properties. Porosity is detrimental to tensile strength and elongation. Alloy 357.0 contains higher Mg level ( %) than alloy 356.0 (0.6%) (0.3% Mg). Consequently, heat treated 357.0 alloys have higher tensile strength than 356.0 alloys. A356.0 and A357.0 alloys are higher purity versions of 356.0 and 357.0 alloys (low Fe levels). B356.0 and B357.0 are even more purer. Addition of Be also improves properties.

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Al-Si alloys containing Cu and Mg

Alloys 355.0 and 332.0 have higher strength but lower ductility and corrosion resistance. Alloy 332.0 has higher Si and Cu levels, and used in internal combustion engines because of their thermal stability and lower coefficient of thermal expansions. Alloys such as 390.0 and 393.0 contains 15 to 25% Si and exhibit excellent wear resistance and low thermal expansion. Phosphorous addition improves machinability as it makes the primary silicon particles finer and more evenly dispersed.

Hypereutectic alloys

Aluminium ­ Copper Alloys

Use of Al-Cu alloys has recently been declined due to their replacement by Al-Si alloys which exhibit better casting properties. Generally, Al-Cu alloys contain less than 5.7% Cu and typical solid solution alloys. After solid solution treatment, CuAl2 particles are precipitated out from the quenched alloys. Generally, Al-Cu alloys have a greater tendency to hot tearing and microshrinkage f hi k formation. Al fl idi iis llow i Also, fluidity and good feeding and gating is required to ensure casting soundness. Recently, an new alloy Al-4.5%Cu0.25%Mg-0.7%Ag is developed which is heat treatable and exhibits higher tensile strength than Al-Si-Mg casting alloys.

Fig. 2.3: Binary AlCu Phase Diagram

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Aluminium ­ Magnesium Alloys

Constitutes 5xx.x series alloys and are characterised by high corrosion resistance, g , good machinability and y excellent appearance when anodised. These alloys are very sensitive to Si content and are generally difficult to cast (in fact, they are one of the most difficult cast foundry alloys !!) due to their high solidification ranges and high dross formation They formation. require good gating and feeding and greater chilling to ensure casting soundness. Often a small amount of Be is added to control drossage.

Fig. 2.4: Binary AlMg Phase Diagram

Common Al Foundry Applications

Binary Al-Si alloys Sand/die cast alloys (413.0, 443.0) are the materials of choice for many automotive, domestic food and pump castings. Other applications include food processing equipment, castings exposed to marine atmosphere. Alloys 319.0 All 319 0 iis obtained f b i d from recycled materials. P l d i l Parts made with these d ih h alloys include cylinder heads and intake manifolds. Hypereutectic 390.0 alloys are used to pressure die cast engine blocks without iron liners. Low pressure die cast engine block are also produced. 380.0 alloys are used to pressure die cast engine blocks with cast-in iron liners. Engine casings, transmission parts and various other automotive parts are also produced. Al-Si-Mg ll Al Si M alloys Properties of A356 0 and A357 0 alloys are very attractive f many P ti f A356.0 d A357.0 ll tt ti for automotive and aircraft part applications. Heat treated C355.0 alloys are cast to produce tank engines, pump parts, high speed rotating parts and impellers. Al-Si-Cu-Mg alloys Automobile gas and diesel pistons and cylinders are made of die cast 332.0 alloys. Addition of Ni improves elevated temperature properties.

Al-Si-Cu ll Al Si C alloys

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Cast Microstructures

Microstructure of Al-Si alloys depends strongly both on composition and the casting process. The rapid cooling in pressure die casting causes fine eutectic structure, small primary aluminium dendritic cells and arm spacing, and reduced p y p g, grain size. Slower rates encountered in permanent mould and sand casting necessitate the use of eutectic modifiers such as strontium or sodium to obtain a finely dispersed eutectic silicon phase. In hypereutectic alloys, phosphorous is added to control the primary chunky silicon particles. Grain refiners are added to produce a fine equiaxed grain structure. Chemical modification dramatically alters the morphology of the eutectic silicon. Even at the rapid cooling rates encountered with high pressure die casting, the eutectic is changed from acicular or lamellar to fibrous with the addition of strontium or sodium.

Grain refinement improves the resistance to hot tearing, decreases porosity and increases mass feeding. As a result, a grain refined part is more homogeneous with better casting soundness and increased mechanical properties properties. Al-Si alloys show better resistance to hot cracking index of 1, compared to Al-Cu and Al-Mg alloys which have a hot cracking resistance index of 4. Therefore, grain refining is paramount for AlCu and Al-Mg alloys to obtain casting soundness. In case of Al-Si alloys, grain refining becomes somewhat ineffective in permanent mould casting due to faster cooling rate and presence of high liquid volume fraction.

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(a) Ascast (unmodified)

(b) Partially modified with 0.01% Sr

(c) Fully modified with 0.02% Sr

Fig. 2.5: Microstructure of 413 alloy (x100).

(a) Ascast (unmodified)

(b) Modified with 0.008% Sr

Fig. 2.6: Microstructure of A356 alloy (x100).

Fig. 2.7: Microstructure of A357 alloy (x100).

Fig. 2.8: Microstructure of A357 alloy containing 0.65% Mg (x100) 2Al8FeMg3Si6

Fig. 2.9: Microstructure of A357 alloy containing 0.60% Mg and 0.35% Fe (x100) 2Al8FeMg3Si6 3Al5FeSi

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Fig. 2.10: Microstructure of 319 alloy (x100).

Fig. 2.11: Microstructure of 380 alloy cast at a low cooling rate (1 deg/s) (x100).

Fig. 2.12: Microstructure of 380 alloy cast by high pressure die casting (x100).

Fig. 2.13: Microstructure of 390 alloy, chill cast and treated with phosphorous (x100).

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3. Eutectic Modification

The transformation of silicon phase from acicular (large plates with sharp sides and ends) to fibrous (with a fine, apparently globular morphology) structure is termed as modification.

Ascast, unmodified structure of A356

Sodium modified structure of A356

The discovery modification process was one of the most significant related to aluminium foundry alloys.

The Fundamentals of Modification

Any workable explanation regarding eutectic modification must deal the following facts: [1] Several elements are known to cause modification.

These include some groups IA, IIA, and rare earth elements Of all of these, sodium is the most effective in producing a fine, uniform, fibrous structure;

[2] Modifiers are effective at very low concentration levels

typically 0.01 ­ 0 02%; 0 01 0.02%;

[3] A modified structure can be produced without any addition by very rapid solidification (quench modification).

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The Solidification of Silicon

During freezing of an Al-Si eutectic, it is the silicon phase which plays the critical role in modification.

aluminium solid solution exerts only a very minor influence. y y

Silicon is nonmetal, and as such it freezes in faceted manner.

it forms crystals along (111) plane and grows along <112> directions twin crystals are easily formed across the (111) plane does not favour branching of silicon crystals

An unmodificed silicon occurs in essentially an unbranched, flat-type morphology.

Fig. 14.2: Schematic representation of the growth of an acicular silicon crystal from the melt.

Fig. 14.3: Twinning in a crystal. Note the continuity of the atom planes across the twin plane.

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Modified fibrous structure of silicon forms due to a very different type of growth, which allows a free and easy branching.

This difference appears to lie in the number of twins found in modified and unmodified silicon. The modified silicon fibres contain orders of magnitude more twins than do unmodified silicon plates, and the surface of the fibres is microfaceted and rough as a consequence of the intersections of myriads of twin planes with it.

Silicon fibres are crystallographically imperfect, and each surface imperfection is a potential site for branching to occur should it be required by the solidification conditions conditions.

As a result, fibres in the chemically modified eutectic are able to bend, curve and split to create a fine microstructure. The plates of the unmodified structure are inhibited by their relative crystallographic perfection and can do little but form in a coarse acicular fashion.

This remarkable difference in twin density is caused by the addition of only a fraction of one weight-percent of modifier.

Atoms of modifier absorb onto the growth steps of the silicon solid-liquid interface, and if the modifier atom radius has the correct size with respect to the atomic radius of silicon (rmodifier : rsilicon = 1.646), a growth of twin will be caused at the interface, Fig. 14.4. interface Fig 14 4 This phenomenon is called impurity induced twining twining.

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Modifying Efficiency

Table 14.1: Measured twin spacings in several modified systems. Structure Acicular Fibrous Fibrous Fibrous Fibrous Fibrous Modifier None Na Sr Ba Yb Ca Twin spacing at constant freezing rate (nm) 400 5 30 30 50 100

Quench Modification

A fibrous eutectic structure can be obtained, in the absence of chemical modifiers, by rapid solidification in the growth rate range of 400 ­ 1000 mm/s.

Quench modified structures have silicon similar to that produced in an unmodified structures. This structure is simply an exceedingly fine form of unmodified eutectic occasioned by very rapid solidification.

Chemical modifiers are more effective at higher freezing rates. g g

For example, it is much easier to modify a chill casting than a heavy section sand casting. In the presence of a chemical modifier, both the twinning frequency and the angle of branching increase with freezing rate. Both of these promote modification and lead to finer structure.

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Chemical Modification by Na, Sr, and Sb

Several elements have been found to produce the fibrous modified structure.

sodium, potassium, rubidium, cerium, calcium, strontium, barium, lanthanum, ytterbium

A few elements act to produce a finer version of the coarse

acicular/lamellar structure.

arsenic, antimony, selenium, calcium , y, ,

For the moment, only sodium, strontium and antimony

find any significant industrial use.

Addition of Sodium

1. flux 2. elemental sodium (vacuum packed in small aluminium cans or kerosene packed) due to its higher reactivity and to minimise its oxidation and hydrogenation 3. Al-Na master alloy not practical due to very low solubility in aluminium (about 0.01%)

Addition of Antimony

antimony is a toxic material. it can also react with hydrogen dissolved in liquid aluminium to form deadly stibine gas : Sb + H = SbH3. for these reasons, antimony is not added to melts in the foundry. antimony treated alloy is purchased as pre-modified ingot from primary aluminium suppliers and is simply remelted and cast. as antimony is very stable in the melt, losses are virtually nil, and no extra additions are required.

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Addition of strontium

1. elemental strontium not preferred reactive with air and water vapour, and within minutes becomes covered with a mixture of SrO, SrO2, Sr(OH)2, and (CaSr)NO3, which prevents the 2. Al-Sr master alloy Al3.5%Sr Al10%Sr Al10%Sr14%Si 90%Sr10%Al mildly reactive

Both sodium and strontium premodified ingot is available from alloy producers which can simply be remelted and cast.

Fig. 14.5: The AlSr phase diagram.

significant losses of modifiers, particularly sodium, occur on remelting. precise control of modifier amounts is often difficult, usually necessary to top-up the sodium or strontium concentration after remelting.

The melt treatment process

Addition of modifiers is simple and can be done by plunging the additive held in a perforated cup or bell below the surface of the melt.

a gentle stirring action improves the dissolution rate but this should not be so severe as to unduly agitate the bath surface, otherwise hydrogen gas pickup will be facilitated.

The addition of sodium is accompanied by a violent reaction which in itself usually causes severe agitation and can result in increased hydrogen levels levels. Strontium treatment is quiescent and hydrogen pickup is not a problem.

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Dissolution of Sodium

Sodium melts at 98 C and thus will enter into the melt which is normally treated in the range 775-800 C and the dissolution of p y sodium is practically instantaneous.

sodium has a very high vapour pressure (about 0.2 atm at 730 C) a large amounts of sodium which are added boil of almost immediately. poor recovery (20-30% of the addition)

A lowering of melt temperature is of little use as the dissolution process slows significantly below 700 C. Thus sodium is characterised by an easy dissolution above 700 C but a poor and somewhat unpredictable recovery.

Dissolution of Strontium

High (above 90%) and very reproducible recovery. More complex dissolution characteristics.

high strontium containing master alloys (e.g., 90%Sr-10%Al) have elemental Sr and the compound AlSr low Sr containing master alloys (e.g., Al-10%Sr) consist of almost pure Al and the compound Al4Sr these different structures of the master alloys impart very different dissolution characteristics.

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High Sr containing master alloys Dissolves by a process known as reactive dissolution.

elemental Sr (and perhaps the compound AlSr as well) reacts with the liquid Al-Si alloys to produce new intermetallic compounds the reaction is highly exothermic and can raise the liquid bath temperature by 100 C within a very short time time. the intensity of this reaction diminishes as the melt treatment temperature is increased.

Fig. 14.6: Temperature at the centre of a 90%Sr10%Al master alloy dissolving in an A356 bath.

The presence of exothermic reaction seems to be necessary in order to achieve a high recovery of strontium when using these alloys. The best recovery is usually achieved under conditions which promote the greatest exothermic reaction, i.e., at low temperatures.

In the absence of an exothermic reaction, strontium does dissolve into the melt but at a much slower rate rate. High Sr containing master alloys therefore dissolve best at low rather than high temperatures and should be added at the lowest practical temperature.

Fig. 14.7: The recoveries of strontium added as a 90%Sr10%Al master alloy to A356 melts held at various temperatures.

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Low Sr containing master alloys Behave quite differently and exhibit classical dissolution behaviour.

dissolution improves as the temperature increases most of the strontium is locked up in intermetallic compounds and the addition of strontium into the bath takes place when the gradual dissolution of these compounds takes place. thus, thus strontium recoveries are higher at higher melt temperatures, and are very poor if the temperatures temperature is too low.

(a) Al10%Sr (b) Al10%Sr14%Si Fig. 14.8: Recovery of lowstrontium, highaluminium master alloy in A356.

Effect of Modification on Microstructure

The microstructural change from acicular to fibrous silicon is not a sharp one.

castings with inadequate amount of either sodium or strontium will exhibit a mixed microstructure ­ one containing regions of fibrous silicon, lamellar silicon and acicular silicon.

Modification with strontium is often less uniform than with sodium. Use of antimony can produce only a lamellar silicon and never a fibrous silicon.

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Fig. 14.9: Rating system for modified microstructures. Class 1: Unmodified Class 2-4: Undermodified Class 5: Modified Class 6: Supermodified

Class 1: Acicular Class 2: Lamellar Class 5: Fibrous

The modification rating (M.R.)

If a given sample contains roughly 20% class 3, 50% class 4 and 30% class 5 structure, then its modification rating would be M.R. = (0.20)3 + (0.50)4 + (0.30)5 = 4.10 and the sample could be said to be reasonably modified not modified, perfectly modified (for which the modification rating would be 5).

Variables controlling modification

1. 2. 2 3. 4. 5. Type of modifier used, Impurities present in the melt, I ii i h l Amount of modifier used, Freezing rate, and Silicon content of the alloy.

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Type of modifier

Both Na and Sr are capable of producing the full range of structures.

sodium is the more powerful modifier in that it produces more uniformly modified structures at lower concentrations than does strontium

Antimony will only yield lamellar structures (class 2)

Impurities present in the melt

All alloying/impurity elements can interact with the modifying elements.

Phosphorous, in particular, makes the modification difficult, and that alloy which are easy to modify contain low phosphorous content. Antimony interacts with both sodium and strontium in a negative fashion, and antimony containing melt requires an exceptionally high level of either modifier to produce structures of class 2 or higher.

Amount of modifier used

For a given set of casting conditions and alloy composition, there is a critical modifier level required to produce a given microstructure.

A higher concentration of modifier will produce a higher microstructure class, at least up to class 5 Too high a level is undesirable, as overmodification can occur.

Figure shows the development of a class 5 microstructure from class 1 of A356 alloy by the use of an increased strontium level.

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Freezing rate

Higher solidification rates assist the modification process.

Lower modifier levels are required in permanent mould castings than in heavy-section sand castings. Modification of die castings are also found to be benefitted even though the freezing rate is very fast for the dissolution process

The lamellar structure produced by antimony addition is particularly sensitive to freezing rate.

Antimony treatment is not recommended for sand castings, as these solidify too slowly to ensure a uniform lamellar structure. Antimony treatment is therefore usually restricted to permanent mould applications.

Silicon content

Higher silicon concentrations require larger amounts of modifier to produce complete modification.

An increase of up to 50% in the amount of strontium is needed when silicon level is changed from 7 to 11%.

Overmodification

Addition of sodium or strontium levels higher than that needed to produce a class 5 structure exerts a deleterious effect on the properties of the alloy.

Overmodification with sodium

Overmodification takes place if sodium content exceeds 0.018 to 0.020%. A coarsening of silicon occurs associated with bands of primary aluminaium.

The rejection of sodium in front of solidifying interface takes place and the compound place, AlSiNa forms, which serves to nucleate coarse silicon particles After some growth of silicon particles, the adjacent liquid becomes aluminium rich, and a sudden nucleation and growth of aluminium around the silicon particle takes place.

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Overmodification with strontium

Two distinct phenomena prevails:

1. coarsening of silicon and reversion of silicon structure from the fibrous to an interconnected plate form, and 2. the appearance of strontium containing intermetallics (e.g., Al4SrSi2) in the microstructure.

Fig. 14.13: Coarse silicon caused by overmodification with 0.09% Sr in an A356 alloy (x400).

Fig. 14.14: Al4SrSi2 phase caused by overmodification of 356 alloy (x270).

Both of these effects need not necessarily occur simultaneously but reduce the properties of overmodified alloys causing them to revert to values more typical of untreated material.

Modifier Fading

Of the three commercially important modifers, Na, Sr, and Sb, only the first two suffer fading (i.e., a reduction in composition) with ti ith time.

antimony is very stable chemically; not known to undergo any natural fading effect.

Two types of phenomena cause modifier fading:

1. Vaporisation due to a high vapour pressure at melt temperatures, 2. Oxidation due to an excessive chemical affinity for oxygen. Modifying element remains in the melt, but in a chemically combined form Such forms are ineffective as modifier only a free atomic form in the liquid alloy can cause modification

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Sodium Fading

Worst affected ­ one of the major disadvantages of sodium usages. Precise control of sodium concentration is very difficult to achieve. Evaporation is the main problem (0.2 atm vapour pressure). But once dissolved, it does not oxidise readily.

Sand castings are cooled very slowly than permanent mould castings, so they will require a higher melt sodium. fading process for sodium flux is different than that of pure sodium. Large melts are less prone to fading than small ones (due a lesser A/V ratio). Stirring of melt will increase fading sharply. Hence, degassing (even with an inert gas) is not recommended after sodium treatment.

Fig.14.15: Some typical sodium variation with time. Reduction of sodium from liquid bath by chlorination by bubbling a nitrogen-freon (CCl2F2) mixture, by chlorineinert gas fluxing, or by using hexachloroethane tablets.

Strontium Fading

Strontium fades considerably more slowly than sodium Regarded as a semi-permanent modifier. Loss of strontium occurs primarily by oxidation.

vapour pressure of strontium is very low (only about 0.001 atm at 730 C), and the oxide of strontium is more stable than that of aluminium or silicon.

Thin sections (e.g., cylinder liner) solidify more quickly, requiring less Sr for modification than ticker sections (e.g., (e g cylinder heads). heads)

Like sodium, removal of strontium from the bath can be accomplished easily by adding chlorine.

Fig. 14.16: Some typical strontium losses during furnace holding.

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Effect of Phosphorous

Phosphorous is present in aluminium alloys either as trace element or as addition to improve wear properties.

In hypoeutectic alloys, phosphorous addition results in coarsening of silicon particles. In hypereutectic alloys phosphorous reacts with aluminium to form fine AlP particles which help nucleating primary silicon.

Phosphorous interferes with modification by either Na, Sr, or Sb, and alloys with higher phosphorous levels require larger retained modifier concentrations in order to produce an acceptable cast structure.

If the phosphorous concentration can be reduced to less than 1 ppm, an Al-Si casting alloy freezes with a lamellar (class 2) structure without the addition of any modifying agent. A high phosphorous content will cause the eutectic solidification to solidify in an acicular form and necessitate modification.

If the melt contains more phosphorous, more modifier is required, and the holding time will be less due to shorter available fade time.

Fig. 14.17: Sodiumphosphorous interactions. (F ­ Fibrous, L ­ Lamellar, A ­ Acicular).

Fig. 14.18: Strontiumphosphorous interaction in A357 alloy; solidification time = 60 sec. (F ­ Fibrous, L ­ Lamellar, A ­ Acicular).

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Fig. 14.19: Antimonyphosphorous interaction.

Fig.14.20: The effect phosphorous level on the allowable melt holding time with sodium modification. If there is a high phosphorous concentration, only 15 minutes of holding time is possible; at a lower concentration, the melt can be held for 30 minutes.

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