EFFECT OF AGEING ON THE MECHANICAL AND ELECTRICAL PROPERTIES OF Al-Zn-Mg-Cu ALLOY

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1 Available online at Proceedings of the 1 st International Conference on Engineering Materials and Metallurgical Engineering December, 2016 Bangladesh Council of Scientific and Industrial Research (BCSIR) Dhaka, Bangladesh EFFECT OF AGEING ON THE MECHANICAL AND ELECTRICAL PROPERTIES OF Al-Zn-Mg-Cu ALLOY Ayesha Akter *, K.M. Shorowardi, H.M.M.A. Rashed, Department of Materials and Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka-1000,Bangladesh ABSTRACT The microstructure, hardness and electrical conductivity of a new Al-9.6%Zn-3.1%Mg-1.84%Cu alloy after one step aging treatment were thoroughly investigated in this experiment.after the homogenizing heat treatment, a series of test samples taken from a large plate were continuously aged at various precipitation hardening conditions to develop hardness and electrical conductivity variation relations with aging time and temperatures. The results show that electrical conductivity of the alloy continuously increased with the increase of the aging time and temperature. At the initial stage of aging, hardness also continued to increase, reached the peak value and finally decreased with the increase of aging time and temperature. When aged at 180 C up to 6 hours, the main precipitated phases are GP zones and metastable η phase causing increased hardness but when aged at 200 C, precipitated phases are mainly η phases which cause to decrease the hardness rapidly. The precipitation process is also notably accelerated compared with that aged at 180 C. Keywords: Al Zn Mg Cu alloy; precipitate; ageing; hardness; microstructure; electrical conductivity 1. INTRODUCTION 7XXX series Aluminum alloys are widely being used for the aerospace, transport and structural industry due to their unique combination of properties such as high strength, high ductility, excellent stress corrosion cracking resistance and fracture toughness [1-3]. This group of alloy usually contains a good amount of Zinc, Magnesium and Copper as the main alloying elements and some minor additive elements such as Zirconium, Manganese, Iron, Silicon etc. These elements help in the formation of small and rigid phases such as η', η (MgZn 2 ), T (AlCuMgZn), S (Al 2 CuMg) dispersed in the matrix phase [4]. They create obstacles in the dislocation movement, hinder their motion and improve the mechanical properties. Thus the mechanical property can be altered via heat treatment and composition variation if its heat treatable. During casting, coarse intermetallic particles, residual stress development and micro segregation occurs upon solidification through fast quenching. They can significantly hamper mechanical properties. So homogenization treatment after solidification is a must before subsequent processing. But, some coarse constituents including large eutectic structures may still be present in the homogenized structure. So some more heat treatments sometimes become essential to have desired property. During aging, various phase change and precipitate formation occurs. It is necessary to understand the available phases and their forming mechanisms to have complete knowledge about the alloy. Due to the precision and fast response of non-destructive measurement, age hardening mechanism and strength can be predicted by co relating it with the measured indentation hardness and electrical conductivity (measured by eddy current method) values in the heat treatable sample [5]. Rosen et al used electrical conductivity to study the changes in the mechanical properties during the age hardening process in AA 2024 and evaluated quantitatively the extent of removal of the minor constituents Cu and Mg from the solid solution [6]. Starink et al. reported a model for predicting the yield strength based mainly on microstructural data, which could predict proof strength with accuracy of 14 MPa [7]. Salazar et al. investigated the relation between strength, hardness and electrical conductivity of Al 7010 alloy and established a relationship that can be used to predict UTS value with 95% confidence. The 0.2% PS can also be estimated with a similar accuracy from UTS and PS/UTS ratio can be determined from the combination of hardness and electrical conductivity [5]. Due to legislation on fuel consumption, industries are focusing on the replacement of heavy steel in structural and automotive application by lighter alloys. As being lighter, Al and its alloys are specially being used in the transport industry to reduce fuel consumption and greenhouse gas emission. Every day,

2 compositional variation and heat treatment schedules are being varied to develop exciting alternatives of the existing material. In this research work, effects of homogenizing, solutionizing and age hardening heat treatment on the hardness and conductivity have been investigated. Al-Zn-Mg-Cu alloy was casted and homogenized at 420 C for six hours and solutionized at 450 C for 2 hours. Solutionized products were age hardened at 180 and 200 C up to 10 hours. Hardness, conductivity and microstructural studies of the homogenized and age hardened product gives idea about the precipitation mechanism and precipitated phases. 2. EXPERIMENTAL Alloy was prepared by taking pure Al (99.8%), pure Mg (99.9%), pure zinc (99%) ingots and electrolytic Cu (99.9), and melting them in a pit furnace. The cast product was obtained by pouring the liquid into a metal mould (pre-heated to 200 C) followed by air cooling. Composition of the cast product was examined using X- ray fluorescence (XRF) technique. The product is then homogenized in the PLF 110/30 Protherm furnace at 420 C temperature for 6 hours. Microstructures of the as cast and homogenized product were observed in Optica B-600 MET trinocular upright metallurgical microscope, and images of same resolution were acquired using OpticaTM Vision Pro software package. To inspect the phases by obtaining chemical composition, EDS analysis was performed during acquiring images in Scanning electron microscope. Calculation of phase diagram (CALPHAD) method was also used for thermodynamic and compositional analysis. Homogenized products were solutionized at 450 C for 2 hours and age hardened at two different temperatures (180 C and 200 C). Age hardening period varied from 2 hours to 10 hours. Micrographic images of the age hardened samples were again taken using the same microscopes. Hardness and conductivity of the age hardened samples were measured on the surface polished condition. Hardness was measured by Rockwell hardness testing machine using 100 kg load and 1/16 diamond indenter. Conductivity was measured using 979 type conductivity meter which works on the principle of eddy current.for every test, at least five readings for each specimen at different locations were taken to avoid the probable effect of any alloying element segregation and the average value was considered.. 3. RESULTS AND DISCUSSION Chemical composition of the cast product was obtained using XRF technique. Table 1 shows the chemical composition of the alloy that is being investigated in this experiment Table 1: Chemical composition of the cast product Analyte Al Zn Mg Cu Si Fe % amount Microstructure of the as cast and homogenized products are shown in figure 1. It is seen that, cast structure contains a lot of segregation and inhomogeneities. When it is heat treated at 420 C, grain becomes more uniform and segregation is minimized. Still the homogenized structure contains eutectic grain boundary phases and dispersed phases. (a) (b) FIG. 1: MICRO STRUCTURE OF (A) AS CAST (B) HOMOGENIZED Al-Zn-Mg-Cu ALLOY SHOWING SEGREGATIONS SEM and EDS analysis of the homogenized sample was done in the backscattered mode (figure 2). Identification of the specific phases has been done by using EDS for compositional analysis (table 2). 127

3 FIG. 2: SEM IMAGE AND EDS GRAPHS OF THE SAMPLE Table 2: Results of EDS analysis Location Al Mg Zn Cu Fe Si identification Al 2 CuMg Al-Cu-Mg-Zn MgZn 2 Figure 3 shows the hardness of as cast and homogenized product. It is seen that there is a little decrease in the hardness of the homogenized product. This can be due to the dissolution of some phases such as MgZn 2, Al 2 CuMg in the matrix due to the homogenization treatment at 420 C obtained from EDS results and CALPHAD analysis (figure 4).

4 HRB As Cast Homogenized FIG. 3: EFFECT OF HOMOGENIZING TREATMENT ON THE HARDNESS OF THE Al-Zn-Mg-Cu ALLOY FIG. 4: CHANGE IN THE WT % FRACTION OF THE SOLID OF THE Al-Zn-Mg-Cu ALLOY To understand the aging effect on hardness and conductivity and to find and empirical relation between them, homogenized samples were solutionized at 450 C for 2 hours and then age hardened at various temperatures and times. Microstructures of the age hardened samples are shown in figure 5 below. a b c d e

5 f g h i j FIG. 5: MICROSTRUCTURES OF THE SAMPLES AGED UNDER VARIOUS CONDITIONS: (a) 180 C, 2 HOURS (b) 180 C, 4 HOURS (c) 180 C, 6 HOURS (d) 180 C, 8 HOURS (e) 180 C, 10 HOURS (f) 200 C, 2 HOURS (g) 200 C, 4 HOURS (h) 200 C, 6 HOURS (i) 200 C, 8 HOURS (j) 200 C, 10 HOURS From the microstructures (fig. 5), it is seen that, there is very little change in the microstructures up to 6 hours when aged at 180 C. Gang et al. showed that very small clusters form in the matrix immediately after quenching [8]. When other clusters nucleate and grow, these previously formed clusters grow into blocky GP zones. For reducing surface energy, smaller GP zones convert to elongated clusters and finally lead to the formation of η phase. At high temperature, precipitation process accelerates due to the increase of driving force [9]. In this experiment, similar things may have happened. Initially GP zones and metastable phases may have formed which are not visible in the optical microscopes due to their nano range sizes. With increasing time, precipitates grow in size and become visible in the optical micrograph. At 200 C, with increasing aging time, it is seen from the microstructures that, non-dendritic polygonal grains are produced (5i and 5j). These grain boundary precipitates are stable η phase and they are much larger than the matrix η phase. After 10 hours aging, grain boundary precipitates becomes very large due to faster diffusion of solute atoms from the matrix into the grain boundary. Grain boundary precipitates drain solute atoms from the adjacent regions and there are insufficient solute atoms to form matrix precipitates. As a result, areas adjacent to the grain boundary are purified forming precipitate free zone (PFZ). HRB a) 180 C 200 C Conductivity (vs IACS)% b) 180 C 200 C Time, hr Time, hr FIG. 6: EFFECT OF AGEING TEMPERATURE ON (a) HARDNESS AND (b) CONDUCTIVITY OF STUDIED ALLOY Figure 6 shows the electrical conductivity and hardness variation with increasing aging time period.it is seen that, with aging time, hardness continues to increase up to 6 hours at 180 C and then it began to decrease. At 200 C, hardness values decrease further with time and finally reach to the lowest value. The initial increase in

6 hardness values can be described by the fact that may be there was initially very fine and coherent GP zonephases and other intermediate precipitate formation reaction. So hardness value reaches to in HRB scale. They behave like foreign atoms or inclusion in the solid solution of the host matrix. Due to this, lattice strain around the inclusions develops and causes lattice distortion. This lattice distortion makes the dislocation movement harder causing alloy deformation harder. Zhao at el. showed that substitutional atoms of Mg and Zn in the aluminium matrix cause elastic distortion which by turn produces hard alloy due to the solid solution strengthening effect [10]. Hence the present alloy may have become hard by the structural precipitates of GP zones and small MgZn 2 precipitates. Chemical hardening in the aged aluminium alloy also can contribute to the hardness increase though the effect is very less prominent. With the increase of aging time, over aging condition prevails. Precipitates began to increase in size, become less numerous and lose their coherency. This can be said because after 6 hours, hardness values continue to drop. Formed precipitates became large and incoherent with the matrix; lattice strain which was developed earlier was lost. At 200 C, there is a gradual drop of hardness with time. After 10 hours of aging treatment, hardness became the lowest which is 47.5 HRB. Microstructure at this stage shows the presence of coarse grain boundary polygonal η phase which is responsible for decreasing hardness to a greater extent. Hardness variation indicates the presence of GP zones and gradual change in microstructure causing hardness values to change after each period of ageing. Increasing the value of electrical conductivity is a great way of improving the stress corrosion cracking resistance of an alloy. SCC resistance has been proved to be increased with the increase of the electrical conductivity of the Al-Zn-Mg-Cu alloy [11]. In this study, it is seen that, electrical conductivity is increasing with the increase of aging time and temperature. This raise in electrical conductivity is again the result of the amount, type and size of the precipitates formed in the matrix. Generally, all the alloy addition in aluminium matrix tends to suppress the electrical conductivity. GP zone is very effective in scattering electrons. As a result they decrease the conductivity. But, when the diffusion of solute atoms from the solid solution into the grain boundary occurs, matrix is purified and so the conductivity increases. However, overall increase is predominant here due to the purification action of the matrix. As such there is continuous precipitation of GP zones, η' and η precipitates. Straink et al gave an equation (1) of electrical conductivity which is, 1/σ M(t) = ρ M(t) = ρ 0 +r Zn x Zn (t)+ r Mg x Mg (t)(1) Where σ M(t) is the conductivity of the matrix phase; ρ M(t) is the resistivity of the matrix phase; ρ 0 is the resistivity of the alloy after precipitation of the precipitation-hardening elements (Zn, Mg and Cu) has been completed; x Zn (t) and x Mg (t) are the concentrations of the Zn and Mg elements in the matrix phase (which is time dependent, due to precipitation that can occur); r Zn (t) and r Mg (t) are constant. The main composition of the precipitates in the alloy is MgZn 2, Al 2 CuMg etc. So with the increase of the aging time, precipitates become larger. As a result, concentrations of the alloying element such as x Zn (t) and x Mg (t) decrease which in turn increases the conductivity of the alloy. Again, according to literature review, it is found that the driving force required for precipitation increases with the increase of the aging temperature. So at higher aging temperature, precipitates grow faster than the lower aging temperature. Therefore conductivity at 200 C temperature is higher than the conductivity at 180 C temperature at the same aging time. Aging time also plays the same role in increasing conductivity because precipitates grow larger with time. So, conductivity value after 10 hours aging at 200 C reaches to 38.2 from 33.5 (conductivity after 2 hours aging at 180 C). 5. CONCLUSION Precipitation process strongly influences the electrical conductivity and hardness of the studied alloy. With the increase of aging time and temperature, hardness is decreased after increasing up to 6 hours but conductivity is continuously increased. When aged at 180 C for 6 hours, hardness reached to the peak value and when aged at 200 C for 10 hours, conductivity reached to the peak value. Low temperature precipitates are may be GP zones which are responsible for hardness increase. Identification of GP zones are not possible by optical microscopy as they are very fine. At high temperature, coarse polygonal η phase becomes the main precipitates causing conductivity increase.temperature increase promotes the precipitation process. Precipitate size and distribution is under study for the better understanding of the correlation between structure and property. 131

7 6. ACKOWLEDGEMENT The financial support from Higher Education and Quality Enhancement Project (HEQEP) under the sub project CP3117 is greatly acknowledged. Authors are also grateful for the technical support provided by Bangladesh University of Engineering and Technology (BUET). 7. REFERENCES 1. P. Sepehrband and S. Esmaeili, Application of Recently Developed Approaches to Microstructural Characterization and Yield Strength Modeling of Aluminum Alloy AA7030, Mater. Sci. Eng. A 487 (2008): X.M. Li and M.J. Starink, The Effect of Compositional Variations on Characteristics of Coarse Intermetallic Particles in Overaged 7000 Aluminium Alloys, Mater. Sci. Technol. 171 (2001): T. Marlaud, A. Deschamps, F. Bley, W. Lefebvre, and B.Baroux, Evolution of Precipitate Microstructures During the Retrogression and Re-Ageing Heat Treatment of an Al-Zn-Mg-Cu Alloy, Acta Mater. 58 (2010): J.Yu and X. Li, Modelling of the Precipitated Phases and Properties of Al-Zn-Mg-Cu Alloys, Journal of phase equilibria and diffusion 32 (2011): M. A. Salazar-Guapuriche, Y.Y. Zhao, A. Pitman, A. Greene,"Correlation of Strength with Hardness and Electrical Conductivity for Aluminium Alloy 7010", Mater. Sci. Forum (2006): M. Rosen, E. Horowits, L. Swartzendruber, S. Fick and R. Mehrabian, The Aging Process in Aluminium 2024 Studied by Means of Eddy Currents., Mater. Sci. and Engi. 53 (1982): M. J. Starink and S. C. Wang, A Model for the Yield Strength of Overaged Al Zn Mg Cu Alloys, Acta Mater. 51 (2003): G. Sha and A. Cerezo, Early-stage Precipitation in Al Zn Mg Cu Alloy (7050), Acta Mater. 52(2004): R. Shimizu and H. Tanaka, New Coarsening Mechanisms for Spinodal Decomposition Having Droplet Pattern in Binary Fluid Mixture: Collision- Induced Collisions, Phys. Rev. Lett.72 (1994): Y. H. Zhao, X. Z. Liao, Z. Jin, R. Z. Valiev and Y. T. Zhu, Microstructures and mechanical properties of ultrafine grained 7075 Al alloy processed by ECAP and their evolutions during annealing, Acta Mater. 52 (2004): J. Zang, K. Zhang, S. Dai, Precipitation Behavior and Properties of a New High Strength Al Zn Mg Cu Alloy, Trans. Nonferrous Met. Soc. China 22(2012):