TITLE OF PROJECT

HEAT TREATMENT OF Al-Si-Cu CAST ALLOY

TECHNICAL REPORT

Submitted by

Name Student ID

Sachin Rana 104117857

Krishna Baktarwala 104340057

Vivek Modi 104337941

Sarat chandra Nedunuri 104334656

Under Guidance of Dr. Jerry H. Sokolowski

Subject: Metal Casting Technology (06-89-512-01)

University of Windsor Windsor, Canada.

April, 2015

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Summary

This report details the heat treatment of Al-Si-Cu alloy and various process parameters.

Emphasis has been given to T6 heat treatment technique which is particularly used for

aluminum cast alloys of sand and gravity die casting. Various steps involved in solution

heat treatment are discussed concisely. A brief discussion has been made about

alternate heat treatment sources and optimization of solution heat treatment. Paper

review has been done on addition of Mg and Sr and their subsequent effect on

mechanical properties of the A319 alloy after subject to artificial ageing. These changes

in properties have been concisely reported and suitable observations have been made.

Second paper review has been done on changes in microstructure and mechanical

properties of Al-Si 319 alloy after addition of S and Sb.

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Introduction

Today, the automotive and aerospace industries rely strongly on the aluminum products

and want them to be stronger, reliable, economical, comfortable and ecological. This

trend has led to the sound casting techniques of aluminum alloys and their properties

are observed to be modified due to their heat treatability by addition of Si, Mg and other

elements. 3xx series of aluminum is found to be most widely heat treat and a lot of

publications have been made on the heat treatment of Al-Si-Cu/Mg alloy. Subsequent

changes in mechanical properties such as tensile strength and hardness have been

recorded well in many journals and papers.

Though heat treatment has its own unique advantages, many challenges are being

faced today by manufacturers for higher performance due to increased competition,

higher energy costs and increasing ecological issues. Age hardening depends upon

various factors fraction size, coherency of precipitates formed and proper distribution of

particles. Mechanical properties enhanced after the heat treatment depend on

solidification rate, microstructure, temperature, quenching rate, impurities added and

other controlling parameters. Al-Si-Cu-Mg alloys and Al-Si-Mg alloys generally have a

high age hardening response, while Al-Si-Cu alloys have a slow and low age hardening

response [1].

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Aim

Main objective of this paper is to understand heat treatment process employed for Al-Si-

Cu cast alloys with prime emphasis on T6 heat treatment and various controlling

parameters. Changes in microstructure and mechanical properties of the cast alloys

after heat treating and addition of impurities have been studied.

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T6 heat treatment technique:

The T6 heat treatment technique is normally applied to sand and gravity die casting to

increase the strength of cast Al-Si components containing Cu and/or Mg. It involves

following stages [2]:

1. Solution heat treatment: Solution treatment is carried out at a high temperature

and main objective is to dissolve Cu- and Mg- rich particles formed during the

solidification of casting to form a homogenous solid solution of the particles

present in alloy.

2. Quenching: It is carried out at room temperature to obtain the supersaturated

solid solution of vacancies and solute atoms.

3. Age hardening: This process is carried out to obtain the precipitates from the

supersaturated solution. It may be carried out at a controlled elevated

temperature (Artificial ageing) or at room temperature (Natural ageing). The T6

heat treatment is illustrated in figure 1.

Figure 1. The T6 heat treatment process [3]

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As-cast condition:

This is the condition of the metal just after the casting process and no further treatment

has been done to modify physical/mechanical properties of the element. This condition

has been studied extensively by Samuel [4], Djurdjevic [5] and Li [6]. Al2Cu is normally

present in different form in the as –cast condition as shown in figure 2.

Figure 2 (a) Eutectic Al2Cu (b) blocky Al2cu [7]

Solution heat treatment:

It is carried out at a high temperature, close to the eutectic temperature for a sufficient

length of time and main objectives [8] of this process are as given:

1. Dissolve particles formed during solidification containing Cu and Mg.

2. Homogenize the alloying elements in the matrix

3. Change of the morphology the eutectic Si particles.

As the solution treatment temperature increases, the rate of above mentioned process

and strength increases. Strength increases because the solubility of the solute atoms in

the solution increases. However, very high temperature causes the localized melting of

Cu; incipient melting of phases which leads to distortion and gradual loss in mechanical

properties [9]. Cu containing phases start to melt at 519 ◦C in an A319 alloy with low-Mg

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concentration [10]. Optimization of solution treatment is necessary because too long

solution treatment will result in excess of energy loss and too short treatment will result

in improper dissolution of solute particles.

Alternate Heat treatment source:

Fluidized bed (FBs) heat treatment [11] rate is found to be much greater than

conventional source of heat treatment such as electrical resistance furnaces (CFs), salt

baths, air chamber furnaces, induction heaters, and infrared heating. This high rate

results in an increased Si fragmentation and spheroidisation during solution heat

treatment as well as high precipitation rate of phases like Al5Cu2Mg8Si6 and Al2Cu

during ageing. The total time taken for heat treatment for T6 temper is found be less

than 2 hours employing FB heat treatment technique as well as much lesser

consumption of energy [11]. Thereby, it reduces heat cycle time and is found to be

environmentally friendly [12]. Figure 3 shows the microstructure of the Al-Si-Mg-Cu (319

alloy) before and after T6 heat temper using fluidized bed.

Figure 3. Typical microstructure of Al-Si-Mg-Cu (319

alloy); as-cast (a) and solution heat treated for 15

minutes at 493°C using fluidized bed (b) [12].

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Post solution treatment

Dissolution of the blocky Al2Cu and the eutectic Al2Cu has been studied [13, 7, 14] and

takes place in different manner in the solution treatment. Dissolution of the eutectic

Al2Cu is somewhat simple as compared to the blocky Al2Cu. The eutectic Al2cu first

undergoes fragmentation and it then undergoes spheroidisation. Finally it undergoes

radial diffusion of Cu in the matrix as shown in fig 4. Whereas, the blocky Al2Cu does

not undergo fragmentation and is dissolved by spheroidisation and diffusion; thus taking

a longer time.

Figure 4. Dissolution process of (a) eutectic Al2Cu (b) Blocky Al2Cu particles [15]

Two-step solution treatment:

This is an innovative process also known as conventional solution treatment by

Sokolowski [16, 17] for optimization of the solution treatment process. In this process

alloy is first heated at a low temperature of 495 degree C for 8h to dissolve Cu

containing phases and then it is treated at a high temperature of 520 degree C for 2h to

obtain homogenous solution. This resulted in improved mechanical properties by the

reduced amount of copper rich phase in 319 alloy giving increased strength and

ductility. Holding time for the first process and subsequently second process play a vital

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role in overall success of this treatment. Observations showed this treatment gave

optimum results as compared to the conventional method of treating at 495 degree

C/8h.

Quenching:

It is the next important step after Solution heat treatment and its prime objective is to get

the maximum precipitation hardening elements to form a supersaturated solution and to

trap maximum vacancies with lattice atoms [18, 19]. Controlling the temperature is too

critical for success of the Quenching process because very high temperature leads to

faster diffusion and lower super-saturation and vice-versa for lower temperature. The

temperature range between 450 degree C and 200 degree C is found to be the most

critical for Al-Si cast alloys. Hence the time spent between these intervals should be as

low as possible to avoid the precipitation. Rapid quenching results in the best

combination of strength and ductility and the criticality of the process depends upon the

quench media and quench interval.

Ageing:

Main objective of this process is to obtain the uniform distribution of the precipitates in

order to gain maximum strength of the elements. Depending upon the application,

ageing can be of two types: (a) Natural ageing (room temperature) and (b) Artificial

ageing (Elevated temperature of around 150-210 degree C).

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Artificial ageing:

Precipitation sequence of Al-Si-Cu alloy (319 alloys) is generally described by the

formation of Al2Cu precipitates. The precipitation sequence is describes as follows [20-

22]: Αss → GP Zones → θ′′ → θ′ → θ (Al2Cu).

Figure 5 Mechanical properties of the Al–Si–Cu alloy. Yield strength (open symbols)

and UTS (solid symbols) at a) 170 ◦C and b) 210 ◦C [9].

Figure 5 shows the ageing curves for Al-Si-Cu alloy for coarseness of three different

microstructures which are found to be more or less same but the yield strength of the

coarsest microstructure is least. Complete dissolution and homogenization for the

coarsest microstructure takes place at heat treatment of 6h and 495 degree C. Lower

yield strength after artificial ageing is found to be due to lower content of Cu in solid

solution after solution treatment [9].

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PAPER REVIEW 1

Effect of Mg and Sr- modification on the mechanical Properties of Al 319

subjected to artificial aging [23]

Aluminum-based 319-type cast alloys are commonly used in the automotive industry

to manufacture cylinder heads and engine blocks. These applications require good

mechanical properties and in order to achieve them through precipitation hardening,

artificial aging treatments are applied to the products.

Interestingly the Strontium ( Sr) modified Al 319 alloy containing 0.4 wt% Magnesium

(Mg) (i.e alloy 319 + Mg + Sr) when undergone aging heat treatments to different

temperatures and for different hours of time, it was found that mechanical properties

like micro-hardness, ultimate tensile strength, yield strength and impact properties

have given their best combination at a particular temperature and time.

The different chemical compositions of Al 319 alloy used for performing artificial

aging heat treatment at different temperatures for different hours of time were as in

the below table 2:

Alloy Code Element (wt % )

Si Cu Fe Mg Mn Zn Ti Sr

319 6.15 3.53 0.09 0.05 0.001 0.008 0.15 0.0001

319+Sr 6.21 3.55 0.11 0.06 0.001 0.007 0.13 0.020

319+Mg 6.18 3.58 0.1 0.41 0.001 0.008 0.16 0.0001

319+Mg+Sr 6.17 3.57 0.09 0.43 0.001 0.008 0.16 0.020

Table 2. Chemical compositions of Al 319 alloy

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In the case of Sr added Al 319 alloy, it was observed that properties like elongation

(El) and total absorbed impact energy (Et) were improved after artificial aging. This

phenomenon is to be expected, since these properties depend to a great extent on

the morphology and distribution of brittle phases, which are responsible for the

fracture in this type of material. Therefore, modifying the eutectic silicon phase will

improve the ductility and impact toughness of the alloy.

In the case of Mg added Al 319 alloy, the strength properties and micro hardness

were improved drastically but the elongation and impact energy properties were

decreased. It is noted that these values tend to be lower than those obtained with

the base alloy since additions of Mg will increase strength and hardness at the

expense of reduced ductility.

The summarized table for all the four cases with its highest properties after the

specified artificial aging treatment is as below table 3 :

Alloy 319 319 + Sr 319 + Mg 319 + Mg+ Sr

UTS (MPa) 376

8h@170°C

376

4h@150°C

428

8h@170°C

423

8h@170°C YS (MPa) 258 230 403 398

VHN 110 86.5 134 133

% El 8.4 4h@150°C 8.9 4h@170°C 2.8 4h@150°C 4.43 2h@150°C

Et(J) 20 24 4h@150°C 7.9 2h@150°C 13.4 4h@150°C

Table 3. Summarized table for all the four cases

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Industrially, 319-type alloys are used in the manufacture of automotive engine blocks

and cylinder heads where the demand for mechanical properties state that desirable

values range from 250MPa to 300MPa for yield and ultimate tensile strength, 1% for

elongation, 10 J for impact toughness and 100 VHN for micro hardness. It is

prominent from table 2 that alloy 319 +Mg + Sr can afford high tensile strength and

micro hardness values through heat treatment and the process of Sr-modification

enhances its ductility and impact toughness, this is what it makes for having the best

combination of mechanical properties.

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PAPER REVIEW 2

Effects of S & Sb additions on the Microstructure and Mechanical Properties of

the Al-Si 319 alloy [24]

The simultaneous addition of Sulphur and antimony shows there effect on the

microstructure and mechanical properties of the Al-Si 319 alloy, containing high

levels of iron and manganese. It was noted that Sulphur and antimony act

independently to increase the mechanical properties of Al 319 alloy.

The Sulphur influences the shape and distribution of the Al 15(Fe,Mn) 3Si2 phase,

promoting its occurrence as dendritic Chinese script-like morphology. On the other

hand, antimony additions help in the formation of a lamellar eutectic structure in Al-

Si eutectic phase. As a result of these changes in microstructure, an improvement in

the values of ultimate tensile strength, elongation to failure and hardness was

observed with respect to those shown by the untreated 319 alloy.

Figure – 6 Microstructure of a 319 Al-Si alloy in its reference untreated condition

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The addition of Sulphur and antimony with different proportions was made to the

liquid metal of 319 Al- Si alloy. It was found that at 0.04 wt % of Sulphur dendritic

Chinese script-like morphology was achieved and at 0.30 wt% of Sb, 0.01 wt % of S

the desired lamellar eutectic structure was achieved as shown in the below figures

respectively.

Figure 7. Microstructure of sulphur treated 319 Al-Si alloy with 0.04 wt% S

concentration

Figure 8. Microstructure 319 Al-Si alloy with the addition of 0.30 wt% Sb, 0.01 wt% S.

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The addition of Sulphur and antimony promotes microstructural changes in AI-Si 319

alloys, and the two elements act independent of one another.

The effect of Sulphur on microstructure may be due to its strong surface active

nature. On the other hand, the effect of antimony on the modification of the Si

morphology may be related to a change in the Si growth mode.

The microstructural modification of the iron-rich intermetallic and the Si eutectic

brings about improvements in the mechanical properties of the alloy, especially in

regard to its ductility.

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Conclusion

T6 heat treatment process is the most popular and widely used technique for

enhancement of the mechanical and physical properties of the cast aluminum alloys.

Several aspects have to be kept in mind and overall heat treatment process parameters

are to be looked for overall success of the cast product and subsequent enhancement

in properties instead of focusing on only solution treatment or age hardening. Prior

modeling of the microstructure using commercial software can be beneficial in deciding

subsequent processes.

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References

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Cu/Mg Casting Alloys”.

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USA): ASM International; 1984. p. 136–7.

[3] Sundman B., Jansson B., Andersson J.O. Calphad 9, 1985; 153-190.

[4] Samuel, E.H., Samuel, A.M., Doty, H.W., 1996b. Factors controlling the type and

morphology of Cu-containing phases in 319 Al alloy. AFS Trans. 30, 893–901.

[5] Djurdjevic, M., Stockwell, T., Sokolowski, J., 1999. The effect of strontium on the

microstructure of the aluminium–silicon and aluminium–copper eutectics in

the 319 aluminium alloy. Int. J. Cast Metal. Res. 12, 67–73

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