Influence of Thermomecaniques Treatments on the Proprietes of Al-5.8%Zn-2.7%Mg Alloy

doi:10.4236/msa.2012.38071

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Materials Sciences and Applications, 2012, 3, 509-512

http://dx.doi.org/10.4236/msa.2012.38071 Published Online August 2012 (http://www.SciRP.org/journal/msa)

509

Influence of Thermomecaniques Treatments on the

Proprietes of Al-5.8%Zn-2.7%Mg Alloy

Mazouz Hamoudi, Bensaada Said, Mohamed Tewfik Bouziane

Laboratoire LARHYSS, Université de Biskra, Biskra, Algeria.

Email: [email protected]

Received February 19th, 2012; revised April 15th, 2012; accepted June 11th, 2012

ABSTRACT

The aim of this work is the demonstration of the effect of thermomechanical treatments on the properties of the alloy

Al-5.8%Zn-2.7%Mg by thermal analysis techniques and calorimetry, which are methods of test widely used for

research purposes and quality control. The effects of thermomechanical treatments on the two variables namely the

coefficient of thermal expansion and heat capacity, we can provide further information for better understanding of the

phenomena responsible for the thermodynamic behavior of the alloy. The results showed the one hand, there is great

similarity between the linear coefficient of thermal expansion and heat capacity, and secondly the effect of plastic

deformation is evidenced by changes in the shape of the curves from the rough. Similarly the kinetics of precipitation of

η phase is accelerated in the case of samples homogenized and homogenized + distorted and accompanied by a shift in

the temperature range to lower temperatures than those recorded in the case of the material state Gross.

Keywords: Aluminum Alloys; Thermomechanical Treatments; Linear Coefficient of Thermal Dilation; Heat Capacity;

Precipitation

1. Introduction

Despite the importance of work in this area, aluminum

alloys continue to be the center of interest of several

research projects in materials science. Their use in the

automotive and aerospace industries depends heavily on

their mechanical and thermal characteristics. By using

different types of heat treatment and thermomechanical

can get a wide spectrum of mechanical properties. For

conditions of employment data, the thermomechanical

treatment of alloys requires a perfect knowledge of

changes in thermodynamic properties it entails.

The continuing development of modern techniques,

the complication and expansion of requirements to be

met by metal alloys have different properties and

qualities that are even today the study of thermome-

chanical processing is experiencing continued growth.

New modes of heat treatment, thermomechanical and

thermochemical alloys should be designed to meet the

requirements of modern industry. In this work, we pro-

pose to study the behavior dilatometry and calorimetry

(DSC) of the alloy Al-5.8%Zn-2.7%Mg in different

thermomechanical states.

2. Experimental Methods

The alloy Al-5.8%Zn-2.7%Mg was prepared by the con-

ventional method of casting ingots by melting in an

induction furnace from Leybold. The fusion is performed

in refractory crucibles (Al2O3). Its chemical composition

is shown in Table 1.

The temperatures and durations of heat treatments are

shown in Table 2.

The analytical techniques used in this regard are the

dilatometer and the DCS. It should be noted that all heat

treatments were performed in a furnace summers type

Adamel or the temperature gradient is about +/− 2˚C.

 The first sample is in the rough cast of [B] (Figure

1(a)).

 The second underwent a homogenization annealing

[H] at T = 465˚C and held for 20 hours, then quen-

ched in water (20˚C) (Figure 1(b)).

 The third sample, in addition to the homogenization

treatment, the alloy was cold rolled (Ε = 20%) [H + D]

(Figure 1(c)).

3. Results and Discussion

3.1. Dilatometric Study of the Sample in the

Rough

The Figure 2 shows the variation of the linear coefficient

of thermal expansion α of the alloy Al-5.8%Zn-2.7%Mg

in the raw state as a funcperature in the tion of tem

Influence of Thermomecaniques Treatments on the Proprietes of Al-5.8%Zn-2.7%Mg Alloy

510

Table 1. Chemical composition of the alloy Al-5.8%Zn-2.7%Mg.

Elément Al Zn Mg Cu Zr Fe Si

%en poids 91.45 5.8 2.7 0.01 0.01 0.01 0.01

Table 2. Temperature and duration of heat tr eatme nts.

Alliage Type des traitements thermiques Température (˚C) Temps de maintien (h)

Homogénéisation 465 20

Trempe 20

Al-5.8%Zn-2.7%Mg

Revenu 25 - 425 Variable

(a) (b) (c)

Figure 1. Microstructure of samples of the alloy Al-5.8%Zn-2.7%Mg: (a) Gross State (B); (b) State Homogenized (H) and (c)

Deformed State + Homogenized (H + D).

Transformations Domaine de température ˚C

Expansion 90 - 125

Contraction 175 - 225

Expansion 270 - 300

Expansion 350 - 400

Figure 2. Variation of α as a function of T ˚C of the alloy

Al-5.8%Zn-2.7%Mg in the rough.

interval [50˚C - 400˚C]. It is observed that occurs four

(04) transformations corresponding to different tempera-

ture ranges and is characterized by the peaks.

3.2. Calorimetric Study of the Sample in the

Rough

The Figure 3 shows the change in the DSC curve of the

alloy Al-5.8%Zn-2.7%Mg in the raw state as a function

of temperature and in the interval [50˚C - 400˚C].

It is observed that occurs three (03) transformations

corresponding to peaks:

One might think at first approach as the first three

transformations, there is a similarity between the results

found by both experimental techniques. There is a gap of

about twenty degrees C for there to be a perfect overlap

of the results found by the techniques of dilatometry and

calorimetry. The work of Deschamps et al. [1] and Ben-

abdoun [2] suggest that this discrepancy is certainly due

to poor calibration of the DSC. By cons for the fourth

processing corresponding to the third expansion observed

in the temperature range 350˚C - 400˚C, we find no

confirmation by a peak in this temperature range.

 The first peak, endothermic, is the first expansion ob-

served on the dilatometric curves and can be attri-

buted to the dissolution of Guinier and Preston zones

Influence of Thermomecaniques Treatments on the Proprietes of Al-5.8%Zn-2.7%Mg Alloy 511

Pics Domaine de température ˚C

Endothermique 100 - 140

Exothermique 200 - 240

Exothermique 260 - 280

Figure 3. Variation of the DCS as a function of T ˚C of the

alloy Al-5.8%Zn-2.7%Mg in the rough.

[3-7].

 The second peak, exothermic, one corresponding to

the contraction during the heating of the alloy in

question, in the field [50˚C - 400˚C], is probably due

to the appearance of the metastable phase η’ quoted

by Mondolfo et al. [8].

 The third exothermic peak recorded on the DSC curve

during heating of the alloy, corresponds perfectly to

the second expansion observed by dilatometry. This is

certainly related to the formation of the stable phase η

(MgZn) [9-11].

3.3. Influence of Thermomechanical Treatments

on the Expansion Curves Heat of the Alloy

Al-5.8%Zn-2.7%Mg

The Figure 4 shows the variation of the linear coefficient

of thermal expansion α of the alloy Al-5.8%Zn-2.7%Mg

as a function of temperature in the interval [50˚C - 400˚C]

for the three structural states studied (Gross, homo-

genized and Homogenized + Garbled).

It also notes that there is a slight shift in the tempe-

rature ranges corresponding to these phase transforma-

tions. The latter depend on the history of the material

before income and this confirms the work of Deschamps

[5] and Benabdoun [6].

3.4. Influence of Thermomechanical Treatments

on the DSC Curves Alloy

Al-5.8%Zn-2.7%Mg

Examination of the DSC thermograms (Figure 5) con-

Figure 4. Variation of α as a function of T ˚C of the alloy

Al-5.8%Zn-2.7%Mg for the three structural states.

Figure 5. Variation of the DCS as a function of T ˚C of the

Al-5.8%Zn-2.7%Mg alloy for the three structural states.

firms, quite clearly, the existence of at least three (03) of

these changes occurred during the tempering, the appear-

ance of three different peaks, while giving us additional

information on their intensity and nature of precipitation

or dissolution. It is found that these treatments have no

significant effect on the dissolution of GP zones (endo-

thermic peak corresponding to the first expansion) and

the precipitation of equilibrium phase η (MgZn) corres-

ponding to the second exothermic peak corresponding to

the second expansion recorded dilatometric curves. For

cons, the effects of heat treatment and homogenization in

particular those associated with plastic deformation are

visible by a shift to higher temperatures low precipitation

of the metastable phase η’. This is mainly due to defects

introduced during plastic deformation that significantly

enhance decomposition of the supersaturated solid solu-

tion. The increase in the number of gaps facilitates the

diffusion of atoms of magnesium and zinc, and the high

density of dislocations stimulates diffusion along the

channels of dislocations. Thus the activation energy of

Influence of Thermomecaniques Treatments on the Proprietes of Al-5.8%Zn-2.7%Mg Alloy

512

the reaction of precipitation of deformed samples is

much lower than that of samples with little or no dis-

tortion. Therefore the process of germination and growth

of new phases are favored.

On the other hand, the plastic deformation introduced

a supersaturated solid solution already containing an ex-

cess of defects due to the rapid quenching, further

increasing their density, and therefore encourages a

greater degree than the different phase transformations

may occur during heating of the material. This explains

why the phenomena related to the dissolution of the

zones, and precipitation of two phases η’ and η occur

earlier, at lower temperatures than for the case of the raw

material and/homogenized or that generally observed in

various aluminum alloys 7000 series.

4. Conclusions

The great diversity of phases present in alloys Al-Zn-Mg

makes this system very complex. They are likely to

improve physical properties by hardening or otherwise

alter significantly.

 Both technical analysis of thermodynamic properties

namely dilatometry and calorimetry may give us

additional information on the nature and sequence of

the various phenomena of precipitation or dissolution

of phases present during the tempering of the alloy

studied in the temperature range [50˚C - 400˚C].

 There is a great similarity between the curves of

thermal expansion and calorimetric study of the alloy.

 The first three transformations observed by the dila-

tometer technique namely the first two expansions

and the only contraction are directly related to the

appearance of different peaks recorded on the DSC

curves.

 The first expansion is directly related to one endo-

thermic peak appears on the DSC curves, it is cer-

tainly due to the dissolution of GP zones.

 The only contraction experienced by the material dur-

ing tempering is represented by the first exothermic

peak, the latter is due to the appearance of the

metastable phase η’.

 The second expansion corresponding to the second

exothermic peak may be due to the appearance of the

stable phase η (MgZn).

 The effects of heat treatment and homogenization in

particular those associated with plastic deformation

are highlighted by a shift toward lower temperatures

of the precipitation of metastable phase η’.

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