Effect Of Precipitation Processes In Aluminum Alloy 6101 T1Used In The Electric Transmission Lines

Effect Of Precipitation Processes In Aluminum Alloy 6101 T1Used In The Electric Transmission

Lines

M.D. Hadid¹, M. Zidani¹, T. Djimaoui¹,S. Messaoudi¹, L.Bessais¹and D. Miroud²

1Laboratoire de Génie Energétique et Matériaux (LGEM) Université de Biskra, B.P: 145

07000 Biskra, Algérie

2Laboratoire des Sciences et Génie des Matériaux(LSGM), USTHB, Alger, 16000 Algérie

hadidmohamed@outlook.com

M. H .Mathon³and T.Baudin⁴

3Laboratoire Léon Brillouin, CEA (DSM- DRECAM) - CNRS, CEA Saclay

91191 Gif sur Yvette, France

4Laboratoire Synthèse, Propriétés et Modélisation, SP2M, ICMMO-UMR CNRS8182

Université Paris-Sud Paris, France

Abstract—Many experiments are performed on aluminum alloy 6101 T1 to characterize the processes of precipitate overcoming by the dislocations. IT has been well known that the nuclei of metastable phases and composition fluctuation are found in an Al- Mg2Si alloy during the early stage of aging. The combined effects of high temperature and the cold drawing of this alloy causeda highest microhardness of the material accompanied by a development of a fibrous texture. The electrical resistivity of the alloy decrease with the increasing aging time.

Keywords—Aging; dislocations; Aluminum alloy;

Microhardness; Electrical resistivity

I. INTRODUCTION

The direct industrial importance of conductivity in aluminum alloys is best emphasized by the extensive use of aluminum alloys as electrical conductors in power lines. The common disadvantage of the aluminum alloy wires made from 6101 is ‘‘the higher tensile strength, the lower electrical conductivity’’. The reality is that there is no single ‘‘wonder material’’. As such, the vast majority of overhead line conductors are nonhomogeneous (made up of more than one material). Typically, this involves a high strength core material surrounded by a high conductivity material. To satisfy minimum engineering requirements from the material of AAAC conductor, it must have high tensile stress above 295 N/mm2 to carry dead load of conductor, wind and ice loads. At the same time it must have high conductivity to increase current carrying capacity of the conductor thus decreasing power loss in transmission lines. Previous studies on the aging sequence of this Al-Mg-Si alloys system before the equilibrium phase appears are summarized.The specific heat and the electrical resistivity measurements have been performed over the entire range of aging temperatures, and the existence of clusters and Guinier–Preston (GP) zones have been proposed[1–2]. However, the structures of the zones

formed during aging were identified only in a limited range of aging temperatures higher than 423 K using X-ray and electron diffraction techniques[3]. The mechanical properties are significantly influenced by process parameters such as natural aging presaging, heating rate to the final aging temperature and artificial aging conditions.

In this study we focus on Al-Mg-Si alloys (6101 series), which have advantageous mechanical and electrical properties.

These alloys are used by the national company of electric cables of Biskra (ENICAB). The aim of our research is the study of aging effect (low temperature) on the development of mechanical and electrical properties of this alloy wires cold drawn.

II. MATERIALSTUDIED

Aluminum alloy 6101 series (Al-Mg-Si) of MIDAL provider (BAHRAIN) in the form of wire rod of 9.5 mm diameter was used in this investigation. The chemical composition of the alloy is listed trading in“Tab. 1,”.

The rod were cold drawn to 3.45 mm in diameter without intermediate annealing, finally, the wires were artificially aged at170°C for different times. To carry out this study, more strain rate is measured. The deformation rate is calculated from the following equation:

[(S − s) ]/S × 100 (1)

S: section before deformation and s: section after deformation .The different rates of deformation used in this work are the following: (ε0= 0%, ε1= 53% and ε2= 92%).

TABLE I. CHEMICAL COMPOSITION OF THE AS RECEIVED WIRE. ALL VALUES IN WT%

Mg Si Fe Cu Al

0.687 0.589 0.211 0.020 bal

III. MICROSTRUCTURAL CHARACTERIZATION A. Caracterisation of initial and cold drawn material

“Fig. 1,”shows the optical microstructures of the alloy cold drawn wire ε2= 92%. The microstructure shows the existence of certain phases in the grains. This structure indicates the high strain received by the metal during fabrication. The wire acquires a fibrous textured microstructure[4, 5]. The cavities located at grain boundary are also observed, whether pre-existing or nucleate, can develop during the deformation processes of diffusion and / or plastic deformation of the surrounding matrix. The aluminum alloys deformed by biaxial compression and uniaxial tension were attributed to the germination of the cavity of the surfaces of intersection of seals sliding beads [6].

Fig. 1. Optical microstructures of the aluminum alloy cold drawn wire (ε2=

92%) (a) cross section and (b) longitudinal section [7].

B. Caracterisation of Agedmaterial

The monitoring of the evolution of the microstructure of the alloy wires cold drawn and aging is devoted to the study of strain rate effect, ie, we took three wires in the same alloy studied differently distorted. The first without deformation reduction rate (ε0 = 0%), the second medium deformed (ε1 = 53%) and the third had a strong deformation (ε2 = 92%).

The microstructure changes that occur during artificial of

6000 series alloys have been well studied. The process starts with the formation of Guiner–Preston (GP) zones of Si and Mg2Si phases from the supersaturated solid solution. This transform to coherent needle like GP zones of monoclinic Mg2Si (β″). Further aging treatment yields the formation of a semi coherent, hexagonal Mg2Si rod-shape (β') phase. Finally, non coherent equilibrium cubic Mg2Si plates (β) form from the β' particles [8].

The intermetallic phases were observed in the 6101 alloy after aging at 170°C for 4 hours. As can be seen in Figure 3a, the precipitates in this alloy with an average size of about 2 to 4 m are heterogeneously distributed. The analysis by EDAX (Energy Dispersive X-ray analysis) has been used to determine the chemical composition of these precipitates, According to the chemical microanalysis precipitates performed on these samples, the precipitate is Mg2Si.

Fig. 2. MEB micrography of the cold drawn wire (ε2= 92%) and aged at170°C for 4hours showing the precipitate Mg2Si (Chimical analysed by EDAX)[7].

Guinier proposedthat the zone in this alloy had a chemical compositionnearly equal to Mg2Si stoichiometry and a part of the crystalstructure of the anti-CaF2 type, and that its shape was aneedle. On the other hand, Thomas[9] proposed that zoneswere made by alternately stacking two Mg layers and oneSi layer on the basis of the (011) lattice plane of the matrix,and that the zones grew along the [100] direction of thematrix.Recently, Huppert and Hornbogen[10] proposed theformation mechanism of the GP zone in this alloy as follows.First, the clusters composed of Al, Mg, Si, and vacancieswere produced during the early stage of aging.Second, Al atoms diffused from the clusters to the matrix,and then GP zones comprised of Mg and Si atoms werefinally formed. However, they had no direct evidence anddid not describe the structure of the GP zones.

20µm Cavités (a)

20µm Cavités (b)

Mg2Si EDS 1

EDAX III. MICROSTRUCTURAL CHARACTERIZATION

A. Caracterisation of initial and cold drawn material

“Fig. 1,”shows the optical microstructures of the alloy cold drawn wire ε2= 92%. The microstructure shows the existence of certain phases in the grains. This structure indicates the high strain received by the metal during fabrication. The wire acquires a fibrous textured microstructure[4, 5]. The cavities located at grain boundary are also observed, whether pre-existing or nucleate, can develop during the deformation processes of diffusion and / or plastic deformation of the surrounding matrix. The aluminum alloys deformed by biaxial compression and uniaxial tension were attributed to the germination of the cavity of the surfaces of intersection of seals sliding beads [6].

Fig. 1. Optical microstructures of the aluminum alloy cold drawn wire (ε2=

92%) (a) cross section and (b) longitudinal section [7].

B. Caracterisation of Agedmaterial

The monitoring of the evolution of the microstructure of the alloy wires cold drawn and aging is devoted to the study of strain rate effect, ie, we took three wires in the same alloy studied differently distorted. The first without deformation reduction rate (ε0 = 0%), the second medium deformed (ε1 = 53%) and the third had a strong deformation (ε2 = 92%).

The microstructure changes that occur during artificial of

6000 series alloys have been well studied. The process starts with the formation of Guiner–Preston (GP) zones of Si and Mg2Si phases from the supersaturated solid solution. This transform to coherent needle like GP zones of monoclinic Mg2Si (β″). Further aging treatment yields the formation of a semi coherent, hexagonal Mg2Si rod-shape (β') phase. Finally, non coherent equilibrium cubic Mg2Si plates (β) form from the β' particles [8].

The intermetallic phases were observed in the 6101 alloy after aging at 170°C for 4 hours. As can be seen in Figure 3a, the precipitates in this alloy with an average size of about 2 to 4 m are heterogeneously distributed. The analysis by EDAX (Energy Dispersive X-ray analysis) has been used to determine the chemical composition of these precipitates, According to the chemical microanalysis precipitates performed on these samples, the precipitate is Mg2Si.

Fig. 2. MEB micrography of the cold drawn wire (ε2= 92%) and aged at170°C for 4hours showing the precipitate Mg2Si (Chimical analysed by EDAX)[7].

Guinier proposedthat the zone in this alloy had a chemical compositionnearly equal to Mg2Si stoichiometry and a part of the crystalstructure of the anti-CaF2 type, and that its shape was aneedle. On the other hand, Thomas[9] proposed that zoneswere made by alternately stacking two Mg layers and oneSi layer on the basis of the (011) lattice plane of the matrix,and that the zones grew along the [100] direction of thematrix.Recently, Huppert and Hornbogen[10] proposed theformation mechanism of the GP zone in this alloy as follows.First, the clusters composed of Al, Mg, Si, and vacancieswere produced during the early stage of aging.Second, Al atoms diffused from the clusters to the matrix,and then GP zones comprised of Mg and Si atoms werefinally formed. However, they had no direct evidence anddid not describe the structure of the GP zones.

20µm Cavités (a)

20µm Cavités (b)

Mg2Si EDS 1

EDAX III. MICROSTRUCTURAL CHARACTERIZATION

A. Caracterisation of initial and cold drawn material

“Fig. 1,”shows the optical microstructures of the alloy cold drawn wire ε2= 92%. The microstructure shows the existence of certain phases in the grains. This structure indicates the high strain received by the metal during fabrication. The wire acquires a fibrous textured microstructure[4, 5]. The cavities located at grain boundary are also observed, whether pre-existing or nucleate, can develop during the deformation processes of diffusion and / or plastic deformation of the surrounding matrix. The aluminum alloys deformed by biaxial compression and uniaxial tension were attributed to the germination of the cavity of the surfaces of intersection of seals sliding beads [6].

Fig. 1. Optical microstructures of the aluminum alloy cold drawn wire (ε2=

92%) (a) cross section and (b) longitudinal section [7].

B. Caracterisation of Agedmaterial

The monitoring of the evolution of the microstructure of the alloy wires cold drawn and aging is devoted to the study of strain rate effect, ie, we took three wires in the same alloy studied differently distorted. The first without deformation reduction rate (ε0 = 0%), the second medium deformed (ε1 = 53%) and the third had a strong deformation (ε2 = 92%).

The microstructure changes that occur during artificial of

6000 series alloys have been well studied. The process starts with the formation of Guiner–Preston (GP) zones of Si and Mg2Si phases from the supersaturated solid solution. This transform to coherent needle like GP zones of monoclinic Mg2Si (β″). Further aging treatment yields the formation of a semi coherent, hexagonal Mg2Si rod-shape (β') phase. Finally, non coherent equilibrium cubic Mg2Si plates (β) form from the β' particles [8].

The intermetallic phases were observed in the 6101 alloy after aging at 170°C for 4 hours. As can be seen in Figure 3a, the precipitates in this alloy with an average size of about 2 to 4 m are heterogeneously distributed. The analysis by EDAX (Energy Dispersive X-ray analysis) has been used to determine the chemical composition of these precipitates, According to the chemical microanalysis precipitates performed on these samples, the precipitate is Mg2Si.

Fig. 2. MEB micrography of the cold drawn wire (ε2= 92%) and aged at170°C for 4hours showing the precipitate Mg2Si (Chimical analysed by EDAX)[7].

Guinier proposedthat the zone in this alloy had a chemical compositionnearly equal to Mg2Si stoichiometry and a part of the crystalstructure of the anti-CaF2 type, and that its shape was aneedle. On the other hand, Thomas[9] proposed that zoneswere made by alternately stacking two Mg layers and oneSi layer on the basis of the (011) lattice plane of the matrix,and that the zones grew along the [100] direction of thematrix.Recently, Huppert and Hornbogen[10] proposed theformation mechanism of the GP zone in this alloy as follows.First, the clusters composed of Al, Mg, Si, and vacancieswere produced during the early stage of aging.Second, Al atoms diffused from the clusters to the matrix,and then GP zones comprised of Mg and Si atoms werefinally formed. However, they had no direct evidence anddid not describe the structure of the GP zones.

20µm Cavités (a)

20µm Cavités (b)

Mg2Si EDS 1

EDAX

Fig. 3. Schematic illustration of the growth process of the GP zone [9].

IV. EVOLUTIONPROPERTIES AFTER COLD DRAWN AND AGING

A. Microhardness

The results of microhardness of wires after cold drawn and heat treatment at 170 ° C are shown in “Fig. 4,”.The curves of microhardness of the alloy for both states (drawn and aged) showed that the microhardness increases with an increase of deformation level [11]. In fact, the deformations accelerate the precipitation of the hardening phase β". The age hardening curves of the alloy, when subjected to all testing conditions, have the same shape that is the hardness increases gradually with an increase of deformation level. The greatest increases in hardness are attained in the alloy after 4h of aging at 170°C.

Fig. 4. Effect of deformation level and aging time at 170°C on the microhardness of wires.

The cold drawing performed before the heat treatment produces a high hardening of the material. The level of

deformation by drawing exceeds 92%. This cold deformation produces a microstructure with a high dislocation density [12].

According to [3] in wire drawing through the die, it is the plastic deformation that is the origin of a dislocation movement. This deformation causes a general modification of its mechanical properties, because the combined effect of the tensile force applied to the thread and the lateral compression which occurs along the walls of the die as a reaction force.

This phenomenon is called structural strain hardening, which leads to an increase of the mechanical properties of the drawn wire.

The presentation of the variation in microhardness as a function of aging time at 170 ° C, after each stage of deformation by drawing“Fig. 5,”gives us another insight into the evolution of the microhardness.

There were a decrease in microhardness after 1h of aging, and then this latter was increased to a maximum after 4 hours of aging.The decrease in microhardness at the beginning of treatment is probably due to rearrangement of dislocations or a decrease in dislocation density as during restoration of a hardened metal. By against the increase of resistance of the alloy during the present heat treatment is due to the precipitation of the alloying elements. Moreover, this variation of hardness as a function of aging time shows that the microhardness values of the deformed wires are higher than the rod wire values, in particular for strongly deformed wire.

Fig. 5. Evolution of microhardness of wires after aging at 170 ° C.

B. Electrical Properties

The electrical resistivity is a consequence of disturbances in the atomic periodicity in a crystal structure, e.g., atomic vibrations owing to thermal agitation, defects in the crystal structure such as vacancies, dislocations or grain boundaries, other substitution of impurity atoms in pure metal lattice sites.

It has been known for a longtime that the resistivity of alloys, which is the inverse of the electrical conductivity, increases almost linearly with amount of elements in solid solution.

Only a part of the alloying elements in usually found in solution, the rest is found in dispersoids or precipitates. To calculate the resistivity, the amounts of elements in solid solution (the solubility) have to be determined. The 60

70 80 90 100 110 120 130

0 53 92

Microhardness Hv/0,1Kg

Deformation levelƐ (%)

not aged 1h 2h 4h

60 70 80 90 100 110 120 130

0 1 2 4

Microhardness Hv/0,1Kg

Aging time (h)

ɛ0=0%

ɛ1=53%

ɛ2=92%

equilibrium solubility can be determined from equilibrium phase diagrams, but commercial alloys are generally not in an equilibrium state. During aging, some of an alloying element precipitates from the super saturated solution into dispersoids or precipitates. The degree of precipitation depends mainly on aging temperature and aging type [13].

After having completed the drawing of the alloy 6101 to required diameters, as can be seen in“Fig. 6,”the resistivity of the alloy increases. We see that the samples not aged have the higher resistivity and the samples aged at 4h have the lower resistivity. After the cold drawn, wires exposed to precipitation hardening treatment at 170°C with different times. As is presentedin “Fig. 7,” the resistivity of all wire samples deformed and not deformed decrease after aging.

According to the graphic drawn from the test results, the lower resistivity was detected after 4h of aging with the not deformed wire.

Fig. 6. Effect of deformation level on electrical resistivity of wires.

Fig. 7. Effect of aging time on electrical resistivity of wire rod and deformed wires.

Conclusion

The effects of cold drawing and heat treatment conditions on the formation of intermetallic compounds and its influences on mechanical and electrical properties of the 6101 Al–Mg–Si alloys have been examined in this research project. The following conclusions are drawn.

• This work has shown the effect of hardening by cold drawing combined by a thermal aging treatment on the mechanical and electrical behavior of the material.

• The precipitate that may form during thermal processing is: Mg2Si.

• The combination between hardening and precipitation of the secondary phase, has given good mechanical properties to the material.

• The mechanical characteristics of this alloy (microhardness) gradually increase as a function of aging time. At the end of 4 hours of aging at 170°C, the microhardnessincreases significantly in all deformed wires, however, the wire deformed at ε2= 92%still having the superior value.

• Electrical resistivity decreases continuously as and as aging time is increasing.

References

[l] H. Suzuki, M. Kanno, and Y. Shiraishi: J. J. Inst. Light Met., 1978, vol.

28, pp. 233-40.

[2] I. Dutta and S.M. Allen: J. Mater. Sci. Lett., 1991, vol. 10, pp. 323- 26.

[3] J.P. Lynch, L.M. Brown, and M.H. Jacobs: Acta Metall., 1982, vol.30, pp.

1389-95.

[4] M. Kanno, H. Suzuki and Y. Shiraishi, Japan. J. Inst. Light. Met, 1978, pp.

553-557.

[5] M. Zidani, S., Messaoudi, F., Dendouga, T., Baudin, C., Derfouf, A.

Boulagroun and M.H.,Mathon,Matec web of conference, vol. 5, 2013, pp. 1-3.

[6]M.G. Zelin, H.S.Yang, R.Z. Valiev, and A.K. Mukherjee, Metall. Trans., 1993, pp. 417-424.

[7] M.D. Hadid, magister memory, Ecole Nationale Polytechnique, Alger, 2012.

[8] H. Farh,R. Guemini, F. Serradj, K. Djemmal, Turkish Journal of Physics, vol. 34, 2010, pp. 117-122.

[9] G. Thomas, J. Inst. Met., 1961–62, vol. 90, pp. 57-63.

[10]G.Huppert and E. Hornbogen, Proc. 4th Int. Conf. on AluminumAlloys, Atlanta, GA, 1994, part 1, Georgia Institute Tech., tlanta,GA, pp. 628- 35, 1994.

[11] M. Zidani, F. Dendouga, S. Messaoudi, L. Bessais, M.D. Hadid. C.

Derfouf, A.L. Helbert and T.Baudin, “Characteristics of texture evolution of copper wire drawn distended for electrical cabling by electron back scattering diffraction”, publishing in Thomson Reuters,2014.

[12]M. Zidani, M.D. Hadid, S. Messaoudi, F. Dendouga, L. Bessais, F. Baira, M.Bayarassou, A.L. Helbert, M.H. Mathon, and T.Baudin, “The drawing process of the wires of copper and aluminium evolution of the microstructure and (mechanical/electrical) properties”,Publishing in Thomson Reuters, 2015,in press.

[13] P.Olafsson, and R.Sandstrm, Mater SciTechnol, vol. 17, 2001, pp.655.

0,0311 0,0316 0,0321 0,0326 0,0331 0,0336 0,0341 0,0346 0,0351 0,0356

0 53 92

Electrical resistivity

Deformation levelƐ (%)

not aged 1h 2h 4h

0,031 0,0315 0,032 0,0325 0,033 0,0335 0,034 0,0345 0,035 0,0355

0 1 2 3 4

Electrical resistivity

Aging time (h)

ɛ0=0%

ɛ1=53%

ɛ2=92%