Int. J. Materials Engineering Innovation, Vol. 4, No. 1, 2013 57

Copyright © 2013 Inderscience Enterprises Ltd.

The effect of magnesium concentration and deformation on the ageing behaviour of Al-Mg alloys

Swami Naidu Gurugubelli*, A.V.S.S.K.S. Gupta and N.R.M.R. Bhargava Department of Metallurgical Engineering, JNTUK College of Engineering, Vizianagaram, Andhra Pradesh, 535003, India E-mail: gsnaidujntuk@gmail.com E-mail: avs_gupta@rediffmail.com E-mail: nrmrbhargava@rediffmail.com *Corresponding author

Abstract: Cylindrical samples of Al-Mg alloys with Mg content 2 to 8 wt% were solution treated at 450°C for two hours and aged at 200°C. The effect of Magnesium concentration and deformation on the ageing behaviour of Al-Mg alloys was studied. Dilute Al-Mg alloys were not responded for age hardening due to the insufficient amount of magnesium to form critical fractions of GP zones and coherent precipitates. For higher deformations large amounts of coherent precipitates were not formed during ageing due to the increased activation energy, and hence the peak hardness values were reduced with increased deformation. Mathematical equations were developed for the paths followed by solution treated hardness and peak hardness curves during ageing of the cold worked alloys and extrapolated using MATLAB. Solution treated Al-8Mg alloys deformed to more than 30% resulted in no improvement of the hardness during ageing and it is identified that solution treated Al-8Mg alloys subjected to 30% deformation possess optimum hardness.

Keywords: ageing; hardness; alloy; deformation.

Reference to this paper should be made as follows: Gurugubelli, S.N., Gupta, A.V.S.S.K.S. and Bhargava, N.R.M.R. (2013) ‘The effect of magnesium concentration and deformation on the ageing behaviour of Al-Mg alloys’, Int. J. Materials Engineering Innovation, Vol. 4, No. 1, pp.57–64.

Biographical notes: Swami Naidu Gurugubelli is working as an Associate Professor in the Department of Metallurgical Engineering at JNTUH Hyderabad. He has published several papers in international journals and international conferences. He did his PhD in the Department of Metallurgical Engineering at JNTUH Hyderabad. His areas of interest include metal forming, alloys, deformation behaviour, nano characterisation.

A.V.S.S.K.S. Gupta is working as a Professor in the Department of Mechanical Engineering at JNTUH Hyderabad. He has published several papers in international journals and international conferences. He has guided many students for the doctoral degrees.

N.R.M.R. Bhargava is working as a Professor of Metallurgical Engineering at AU, Visakhapatnam. He held several administrative positions during the last ten years. He has published several papers in international journals and international conferences. He has guided many students for the doctoral degrees.

58 S.N. Gurugubelli et al.

1 Introduction

The need of the automotive and aircraft industries to produce cost efficiently integral components of complex geometry led to an increased exploitation of the Al-Mg cast alloys (Sharke and Staley, 1996). Al-Mg alloys possess excellent castability in addition to good tensile fatigue properties and corrosion resistance (Brungs, 1997). The trend for increased use of castings for structural applications has been supported by recent advancements such as the tighter specification of composition, the higher rates of solidification and improved heat treatment conditions (Anon, 2000). The tight controls, currently applied during the production of premium quality castings, as well as the gain in understanding the background physical metallurgy of the age hardened aluminium alloys allow for improving both casting processes and metal quality (Tirayakilogou and Campbell, 2000). Precipitation behaviour and changes in mechanical properties of Al-Mg alloys have been extensively studied by previous workers and it has been confirmed that the changes were caused mainly by the formation of Guinier-Preston (GP) zones (Van Rooyen et al., 1988). Sato et al. (1982) have reported that the Al-Mg supersaturated solid solution decomposition to be a four stage process below 50°C. i.e., solid solution → GP zones → β″ → β′ → β. The GP zones are thin plates elongated along [100] directions consisting of solute-rich clusters which may be only one or two atom planes in thickness (Sato et al., 1982). They could be observed at the beginning of aging at relatively low temperature, e.g., room temperature, in an Al-Mg alloy homogenised at an elevated temperature and subsequently quenched (Sato et al., 1982). These zones change, with enhanced aging, into spherical particles β″ (Al3Mg) (8–10 nm in diameter) having an Li2 structure in which Al and Mg atoms are alternately aligned in three dimensional periodicity along the [100] directions (Kojima et al., 1981). The change in mechanical properties during ageing is related to the formation of GP zones, β″, β′ or β phases.

Availability of dislocations (through reinforcements/cold work) may have different roles to play on the formation and transformation of metastable precipitates depending on coherency (i.e., if the precipitates are coherent or semicoherent) (Porter and Easterling, 1989). There are three possible roles of dislocations on the formation/ transformation of coherent and semicoherent precipitates:

1 dislocations from thermal mismatch may provide easier nucleation sites for coherent and semicoherent metastable precipitates than non-coherent equilibrium precipitates (Dutta et al., 1994).

2 faster pipe diffusion through the thermal mismatch dislocations may accelerate precipitate coarsening and this, in turn, may quicken the transformation from the coherent to semicoherent phase (Dutta et al., 1994)

3 the supply of matrix dislocations may influence the transformation of coherent and semicoherent precipitates was reported by Porter and Easterling (1989).

A fourth possibility, of a decrease of low-temperature solute diffusivity due to the adsorption of vacancies at dislocations and interfaces, was also reported by Dutta et al. (1994). Which may reduce (and possibly retard) metastable precipitation. In the present investigations an attempt is made to determine the age hardening response of dilute and concentrated Al-Mg alloys. The effect of deformation on age hardening cycle is also studied. Mathematical model is developed to determine the solutionised hardness and peak hardness to be obtained during the ageing of deformed Al-Mg alloys. MATLAB

The effect of magnesium concentration and deformation 59

simulations are made to determine the optimum deformation which can be given for the alloys to get the optimum hardness just by solutionising but not ageing.

2 Experiment

Pencil ingots of 100 mm × 10 mm ф of Al-2% Mg, Al-4% Mg, Al-6% Mg and Al-8% Mg are produced by melting pure aluminium and magnesium in an induction heating furnace and cast in cast iron moulds at 690°C. Chemical composition of the alloys using optical emission spectrometer Q8 Magellan is given in Table 1. Table 1 Chemical composition of the alloys

Alloy composition wt %

Al Mg Fe Si Mn Ni Cu Zn P Al-2Mg 97.823 1.98 0.124 0.050 0.018 0.0029 0.0012 <0.000010 <0.00010 Al-4Mg 95.928 3.875 0.124 0.050 0.018 0.0029 0.0012 <0.000010 <0.00010 Al-6Mg 93.911 5.892 0.124 0.050 0.018 0.0029 0.0012 <0.000010 <0.00010

Cylindrical specimens of 1:1 aspect ratio are prepared from the ingots homogenised at 100°C for 24 hours. The samples are solution treated for two hours at 450°C and quenched in water. All the alloys are aged at 200°C in a muffle furnace. Samples of each composition are collected from the furnace at fixed intervals and Hardness measurements are obtained using Vickers hardness tester with a load of 5 Kg. Five hardness readings are taken for each sample. The average hardness measurement is used to evaluate the optimum hardness of the sample at a particular ageing time. The solution treated alloys responded for age hardening (Al-8%Mg) are given 10%, 15% and 20% deformation in the hydraulic press at a strain rate of 1 mm/min and aged at 200°C and hardness measurements are obtained.

3 Results and discussion

The Vickers hardness measurements obtained during ageing at 200°C for Al-Mg alloy samples of different compositions are shown in Figure 1. The alloys with magnesium 2–6% have shown no significant increase in hardness due to ageing at 200°C. In these alloys the amount of magnesium will not be sufficient enough to form critical fractions of GP zones and coherent precipitates. Hence these alloys are not responded for age hardening. In Al-8Mg alloys an increase in hardness is observed with ageing. The vickers hardness is increased from the solution treated value of 60 VHN to a maximum of 109 VHN after 16 hours of ageing, about 82% increase in hardness is observed. The increase in hardness could be explained by hindrance of dislocations by impurity atoms, i.e., foreign particles of second phase, since the material after quenching from 450°C (solution heat treatment) will have excess vacancy concentration (Esmaeli et al., 2003). Esmaeli et al. (2003) have reported that as the ageing time increases, the density of the initial meta-stable precipitates or GP zones increases. Hence, the degree of irregularity in the lattices causes an increase in the mechanical properties of the alloy. The strengthening effect of the alloy could also be explained as a result of interference with the motion of

60 S.N. Gurugubelli et al.

dislocation due to the presence of foreign particle of any other phase. Further heat treatment at higher temperatures and time decreases the hardness of the alloy. This reduction may be attributed to the coalescence of the precipitates into larger ones. Therefore, there will be fewer obstacles to the movement of dislocations.

Figure 1 Age-hardening curves for al-mg alloys aged at 200°C (see online version for colours)

It is clear that zone formation must involve a diffusion process since solute atoms have to be transferred from the matrix to determine solute rich zones. Due to the quenched-in excess vacancies, the diffusion is much enhanced at the beginning of ageing which leads to a large amount of GP zones formed just after quenching (Yamamoto et al., 1998). Then the vacancy concentration decreases with ageing, because they migrate inside the crystal and disappear at various sinks. Despite the importance of GP zones as the first precipitates formed in the decomposition of these alloys, there have been relatively few studies that provide clear data on their shape, structure and composition (Ortner et al., 1988). The present investigations are made at an ageing temperature of 200°C, and hence probably the maximum hardening is obtained when β particles are formed in a big enough quantity.

The corresponding ageing curves are shown in Figure 2. Hardness peaks occur at a reducing ageing time (i.e., accelerated ageing behaviour) with cold work. The cold work increases the activation energy and hence the ageing time is reduced with increase in cold work. The peak hardness values are also reduced with cold work. Because of the accelerated ageing behaviour of cold worked alloys, large amounts of coherent precipitates will not be formed during ageing and hence the peak hardness values are reduced with increased cold work. The peak hardness values obtained for 10%, 15% and 20% deformed Al-8%Mg alloys are 102, 100 and 95 VHN respectively and their corresponding ageing times are nine, four and three hours. For 20% deformed Al-8Mg alloy the peak hardness of 95VHN is reached very early, i.e., in three hours only. Considerable difference in the peak hardness values of undeformed and 20% deformed Al-8Mg alloy is observed. Hence 20% cold work of the alloy before ageing may be beneficial in distributing the precipitation throughout the grain and producing higher strength with a reduction in susceptibility to intergranular corrosion.

The effect of magnesium concentration and deformation 61

Figure 2 Age-hardening curves for cold worked Al-8Mg alloys aged at 200°C (see online version for colours)

Table 2 Solution treated and peak hardness values of cold worked Al-8mg alloys aged at 200°C

% deformation

0% 10% 15% 20% Solution treated hardness (VHN) 60.4 67 72 81 Peak hardness (VHN) 108.6 102.27 100.22 95.2

Figure 3 Variation of solution treated and peak hardness with deformation during aging at 200°C (see online version for colours)

The solution treated and peak hardness values during ageing of deformed Al-8Mg alloys are shown in Table 2. The corresponding ageing curves are shown in Figure 3. With increase in cold work, solution treated hardness is increased but the peak hardness is decreased. The deformation of these alloys leads to the decomposition of Mg containing Al solid solution. Impurities in the solid solution are arranged along dislocations formed

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during deformation, and the redistribution of dislocations leads to the formation of new grain boundaries. Hence the hardness is increased with increase in cold work.

Mathematical equations are developed for the paths followed by solution treated hardness and peak hardness of the cold worked alloys aged at 200°C with amount of cold work. Assuming both the hardness values are varying exponentially with deformation: The general equation of exponential curve is

bxy ae= (1)

where

y = solutionised hardness

x = %deformation.

Taking logarithm both sides

log logLogy a bx e= +

i.e.

z A Bx= +

where

z = logy

A = loga

B = bloge

The normal equations are 4 4

1 14i ii I

Z A B X= =

= +∑ ∑ (2)

4 4 42

1 1 1i i i i

i i I

x z A x B x= = =

= +∑ ∑ ∑ (3)

The values 4 4 4 4 2

4 4 4 47.3728, 45, 84.288, 725i i i i ii i i i

z x z x x= = = =

= = = =∑ ∑ ∑ ∑ are obtain

ed from the experimental results. Substituting the above obtained values in equations (2) and (3) we get:

7.3728 4 45A B= + (4)

84.288 45 725A B= + (5)

Solving the equations (4) and (5) we get the values of _A’ and _B’. 31.77408, 6.144 10A B −= = ×

Therefore

59.44016414a =

and

The effect of magnesium concentration and deformation 63

0.014146903b =

Substituting the values of _a’ and _b’ in equation (1), we get the equation for variation of solutionised hardness with deformation

(0.014146903)59.44016414 Xy e= (6)

Using similar procedure the following equation is obtained for the variation of peak hardness of the cold worked alloys during ageing with amount of cold work.

( 0.006307687246)108.9102039 Xy e −= (7)

The solution treated hardness vs. deformation and peak hardness vs. deformation curves are extrapolated using MATLAB and the intersecting point is predicted. The extrapolated curves intersecting at a point are shown in Figure 4. The intersecting point shows same hardness values for both solution treated sample and peak aged sample. The corresponding deformation of the samples is identified as 30%. It shows that, in solution treated Al-8Mg alloys, if the alloys are deformed to more than 30%, ageing treatment does not improve the hardness of the alloy. Up to 30% deformation, ageing treatment resulted in an increase in hardness.

Figure 4 Extrapolation of solution treated hardness and peak hardness curves using MATLAB (see online version for colours)

4 Conclusions

The following conclusions are made from the present investigations:

1 Al-2Mg, Al-4Mg and Al-6Mg alloys have shown no response to age hardening.

2 In Al-8Mg alloys the solution treated hardness is found to be 60VHN and the peak hardness was 109 VHN. About 82% increase in hardness is observed due to ageing of Al8Mg alloys at 200°C.

64 S.N. Gurugubelli et al.

3 Peak hardness of 109 VHN is observed in Al-8Mg alloys after 16 hours of aging at 200°C.

4 In the solution treated and cold worked Al-8Mg alloys the solution treated hardness is increased and peak hardness is decreased with deformation.

5 The peak ageing time is decreased with deformation.

6 Mathematical equations are developed to determine the peak hardness and solution treated hardness values in solution treated and cold worked Al-Mg alloys.

7 The solution treated hardness vs. deformation and peak hardness vs. deformation curves are extrapolated using MATLAB and the intersecting point is predicted. The corresponding deformation of the samples is predicted as 30% where both the hardness values are the same.

8 Solution treated Al-8Mg alloys deformed to more than 30% resulted in no improvement of hardness and hence it is concluded that solution treated Al-8Mg alloys subjected to 30% deformation possess optimum hardness.

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