Effects of Thermomechanical Treatment on the Microstructures and Mechanical Properties of Al-Mg-Si Alloy

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3^ème Conférence Internationale sur

le Soudage, le CND et l’Industrie des Matériaux et Alliages (IC-WNDT-MI’12) Oran du 26 au 28 Novembre 2012,

http://www.csc.dz/ic-wndt-mi12/index.php 179

Effects of Thermomechanical Treatment on the Microstructures and Mechanical Properties of Al-Mg-Si Alloy

Hichem FARH, Khaled CHAIBAINOU

Centre de recherche Scientifique et Technique en soudage et Contrôle CSC/Unité de Recherche en Technologie Industriel URTI/, BP1037. Site Université Badji Mokhtar, Chaiba, 23000. Annaba,

e.mail : farhichem@gmail.com

Abstract :

In this paper we studied the effect of deformation in the thermomechanical treatment on the microstructure and age hardening behavior in an Al-Mg-Si aluminum alloy. Transmission electron microscopy is used in order to follow the distribution and the morphology of the hardening precipitates. The maximum of hardness increases and is shifted to low time with an increase of the deformation. The coarsening and the density of the precipitates are more significant when the alloy is more deformed.

Key words: Al-Mg-Si alloys, thermomechanical treatment, artificial ageing, precipitation.

1 Introduction

Al-Mg-Si alloys provide good dent resistance, which is of extra benefit to automotive body panel applications [1]. Automotive components often require complex shaped designs with high strength. At present the required material properties are achieved by thermally treating the material [1]. This thermal treatment consists of three main processes: 1- SHT(Solution Heat Treatment): this is where the material is held at an elevated temperature for a sufficient time, so that all the constituents are taken into solid solution, giving one single phase. 2- Quenching: this is when the material is rapidly cooled from the SHT temperature to room temperature so as to `freeze' this super-saturated state within the material at room temperature, giving a microstructure condition known as `Super Saturated Solid Solution' (SSSS). 3- Ageing: age-hardening is the final stage in the development of the properties of heat treatable alloys, it is the controlled decomposition of the SSSS to form finely dispersed precipitates [2-3]. Some alloys undergo ageing at room temperature (natural ageing), but most require heating for a time interval at one or more elevated temperatures (artificial ageing) [4]. The precipitation sequence in quenched Al-Mg-Si alloys is usually given as follows [5-8]: Supersaturated solid solution  GP zones (spherical clusters / unknown structure)  β′′ (needle/ monoclinic structure)  β′ (rod / hexagonal crystal structure)  β (Mg2Si) platelets / fcc CaF2).

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http://www.csc.dz/ic-wndt-mi12/index.php 180 2. Experimental Procedure

The Al-Mg-Si alloy was provided by Alcan International Limited of Canada. It was prepared by direct chill casting process (DC) in a mould of a 178 mm of diameter. The chemical composition of the investigated alloy is given in table 1.

The thermomechanical treatment procedures for all specimens were divided into two types.

The as cast specimens were annealed for 2 hours at 550 °C using a heating rate of 50 °C min-1. They were then cold deformed (0%, 1%, 5%, 10%, 25% and 50 % -reduction in thickness) followed by artificial ageing at 180 °C for different times.

The specimens were annealed for 2 hours at 550 °C using a heating rate of 50°C min-1, water quenched at room temperature, cold deformed (0%, 1%, 5%, 10%, 25% and 50 % -reduction in thickness-) then preageing at 150 °C for 10h followed by artificial ageing at 180 °C for different times.

Tableau 1: Chemical composition of the alloy.

Cu Mg Si Mn Cr Fe Zr Al

0.42 1.01 0.62 0.006 0.002 0.21 0.13 bal

3. Results and Discussion

Figure 1 presents the variation of microhardness as function of the ageing time at 180◦C. It can be seen the effect of the deformation ratio during single artificial ageing of the studied alloy immediately after solutionizing and quenching at room temperature. The curves showed that the HV increases with an increase of the deformation ratio. The age hardening curves of the alloy, when subjected to all testing conditions, have the same shape, that is the hardness increases gradually and approaches a maximum value (HV max) and then decreases gradually. In fact, the change of HV max with deformation ratio has resulted into the following groups: G1 (0%–10%) and G2 (25%–50%). The HV max in the second group (G2) is higher than it is in the first group (G1). The earlier maximum of HV present at about 65 minutes of ageing is observed in the second group (25%–50%). We notice also that the maximums of hardness are shifted to low ageing time when the alloy is more deformed. The HV max of the alloy increases by about 15% when deformation ratio is 25%, compared to no deformed alloy. In fact, the deformations accelerate the precipitation of the hardening phase β^//

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10¹ 10² 10³ 10⁴ 10⁵ 10⁶

40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

V H ard ness, kg .mm

-2

Aging time/s

% 0 % 1 % 5 % 10 % 25 % 50

Figure 1: Effect of the deformation ratio in the thermo mechanical treatment on single age hardening of the alloy at 180°

Figure 2 shows the effect of the deformation ratio. The alloy is solution treated for 2 hours at 550 ◦ C, water quenched at room temperature, and then pre-ageing at 150 ◦C for 10 hours followed by artificial ageing at 180 ◦C for different times. We notice similar shape of the curves as observed in figure 1. However, it can be seen that the HVmax of two-step artificial aged alloy is higher than that of single artificial aged alloy for the different cold rolling deformation ratio. Furthermore, we observe similar changes of HVmax with increasing deformation ratio that observed in single aged alloy. The effect of the deformation in the single ageing alloy is more significant than that in the two-step ageing alloy.

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10¹ 10² 10³ 10⁴ 10⁵ 10⁶

40 50 60 70 80 90 100 110 120 130 140 150 160 170 180

V H ard ness, kg .mm

-2

Aging time/s

% 0 % 1 % 5 % 10 % 25

Figure 2: Effect of the deformation ratio in the thermomechanical treatment on age hardening of the alloy aged at 150°C/10 h+180°C

Figure 3 shows the transmission electron microscopy micrographs of the alloy at different ageing condition.

The bright-field image reveals needle-shaped precipitates with the long axis along <100> of the matrix direction in the alloy aged at 180◦C for 5 hours (Figure 4(a)). According to Dutta and Allen[9], these precipitates are designated as β^// . A noticeable modification in the precipitate structure is found after deformation as shown in figure 3(b) and figure 3(c). The needle-shape precipitates become larger in size and less homogeneous.

Figure 3: Transmission electron microscopy micrographs of the alloy under Different conditions: (a) 180°C/5h, (b) 30% deformation+180°C/5h, (c) 30% deformation+150°C/10 h＋_{180°C/5 h}

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http://www.csc.dz/ic-wndt-mi12/index.php 183 Conclusion

The effects of the deformation and ageing treatments on the microstructure and the mechanical properties of Al MgSi alloy have been investigated by transmission electron microscopy and hardness measurements.

The main experimental results can be summarized as follows

The combination of the thermomechanical treatment and the two-step artificial ageing causes an increase in hardness and strength due to the acceleration of the precipitation of the hardening phases. The peak- aged hardness has increased by about 15% when the alloy is deformed 25% reduction in thickness.

The maximum of hardness in the case of the two-step artificial aged alloy is higher than that of single artificial aged alloy for the different cold rolling deformation ratio.

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