An Investigation of Hardness and Microstructure Evolution of Heat Treatable Aluminum Alloys during and after Equal-Channel Angular

Pressing

Emanuela Cerri1, a 1Dept. of Innovation Engineering, University of Salento, via per Arnesano, 73100 Lecce (ITALY)

aemanuela.cerri@unisalento.it

Keywords: severe plastic deformation, Al-Mg-Si, mechanical properties

Abstract. The influence of severe plastic deformation induced by ECAP on microstructure modification and aging effect was studied in two modified Al-Mg-Si aluminium alloys. The microstructure of both alloys in different heat treated and deformed state was characterised by X-Rays diffraction and polarised light microscopy. The effect of artificial aging was investigated after ECAP performed on samples in the as extruded condition. The aging effect was followed by hardness and electrical conductivity measurements. At higher aging temperature (170°C) the alloys showed an increasing softening with time due to recovery or/and grain coarsening effect. At the lower aging temperature, the hardness remains almost constant due to enhanced precipitation hardening effect.

Introduction

Equal channel angular pressing (ECAP) is a very interesting method for modifying microstructure in producing ultra fine grained materials. It consists of pressing test samples through a die containing two channels, equal in cross-section and intersecting at an angle Φ. The sample deforms by simple shear [1,2] and retains the same cross-sectional area so that it is possible to repeat the pressing for several cycles. The equivalent shear strain ε� generated in the work piece is given by the following relation [3]: ε� = N/√3 (2 cot(Φ/2 + Ψ/2) + Ψ cosec (Φ/2 +Ψ/2)) (1) where N is the number of passes through the die, Ψ is the outer corner angle of ECAP die (Fig. 1). The equivalent strain ε� depends on both Φ and Ψ angles and ε∼1 (N=1) if Φ=90° and Ψ approaches zero [4]. Ultra Fine Grained materials are characterized by higher strength, according to the Hall-Petch relationship, and a higher toughness at ambient temperature. If the UFG microstructure is retained to elevated temperature too, there is also a potential for achieving superplastic formability [5]. To improve the quality of processed material it is essential to understand the deformation behavior in the workpiece and the effect of materials properties on it [6]. In fact the microstructure and mechanical properties of the deformed materials are directly related to the degree of plastic deformation and to its homogeneity [7]. The plastic deformation behaviour is controlled by both die design parameters [2,8] and parameters related to material properties, strain rate sensitivity and hardening exponent [9]. In this paper two modified 6082 aluminium alloys, one containing Zr and the other containing Zr and Sc have been severely strained by ECAP. The choice of 6000 series is related to the extensive use in automotive and aerospace industries as a result of their good physical and chemical properties such as corrosion

Fig. 1. Schematic representation of ECAP showing angles and planes.

Materials Science Forum Vols. 633-634 (2010) pp 333-340© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.633-634.333

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of thepublisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 79.35.68.174-10/08/09,09:54:23)

resistance, formability, weldability [10] and because of their age hardenability [11]. The additions of Zr and Sc also provide these alloys for particles which eventually impede grain growth at elevated temperatures [12–19]. The aim of the present study is to understand the influence of severe plastic deformation on aging treatments performed on the two extruded aluminium alloys. The addition of Zr let Al3Zr form during solidification, while Zr and Sc make the alloy able to form Al3(Sc,Zr) particles from the melt too. The effect in both cases is to obtain a very refined cast structure because these particles act as crystallisation nuclei. In the second case, the grain refining effect is higher, because it is reduced the necessary Sc content for getting critical size of Al3Sc as a crystallisation nuclei [14]. Moreover, finite element analysis have considered the effect of strain hardening rate on mechanical properties of processed alloys and deformation behavior has been investigated too.

Experimental procedures

Two modified 6082 aluminum alloys were processed by ECAP after extrusion. Their chemical compositions is reported in Table 1, while their mechanical properties in Table 2. ECAP experiments were conducted using a die with an internal angle φ= 90° between the vertical and horizontal cylindrical channel and a curvature angle ψ=35°.

Table 1. Chemical compositions (wt. %) of the modified 6082 aluminium alloys

For this design, the effective strain that occurred on a single pass through the die is close to 1.

Molybdenum bisulfide (MoS2) was used as lubricant. Rods with diameter of 10mm and length of 100mm were cut from the extruded billets. Four repetitive pressing were conducted at room temperature on each rod according to route Bc. After ECAP, specimens cut from the rods were aged at 110 and 170°C. Static aging was performed up to 190°C on the as received samples to verify the aging potential of the as-extruded material. Static aging was also performed at 170°C and 190°C after solution treatment at 530°C for 2 h in order to maximize hardness according to precipitation hardening. Microhardness (HV0.5) and electrical conductivity (120 kHz) measurements were carried out on sample cross sections to evaluate the effect of heat treatment on both ECAP processed and aged samples. The microstructure of the alloy before and after ECAP was observed by polarized light and SEM. For observations, samples were ground according to standard method, electropolished (80 ml perchloric acid, 120 ml distilled water, 800 ml ethanol, 20 V) and anodized (5% HBF4 in distilled water, 20 V). Grain size measurements were carried out on the cross section of the extruded samples. X-ray diffraction measurements were also performed on aged samples and processed samples to complete the microstructural investigations (Cu Kα radiation, 45 kV, 40 mA).

Results and discussion

Electron backscattered channelling contrast (EBSC) micrographs of the 6082Zr alloy are illustrated in Fig.2. Fig. 2(a) shows the microstructure of the as-extruded sample (N=0) showing equiaxed grains of an average grain size of (50 ±10) µm. After four passes, the grains have been severely deformed and the initial microstructure is no more distinguishable. Fig. 2b shows the grain size to be less than 0.5 µm and not equiaxed as in the starting sample. The microstructure of the as-extruded 6082ZrSc alloy (fig. 3a) shows a finer grain size with an average of (5±1) µm. Elongated rod-shaped intermetallics of 2-3 µm in length also are present in the extruded samples identified as Mg2Si [12]. After ECAP (fig. 3b and 3c) the grain size is refined to values comparable to 6082Zr alloy (< 0.5µm). The final grain size at N=4 results indipendent from the starting extrusion grain dimension.

Fe Si Mg Mn Zr Sc Al 6082Zr 0.16 0.51 0.34 0.014 0.1 - bal.

6082ZrSc 0.17 0.52 0.33 0.014 0.1 0.1 bal

334 Ductility of Bulk Nanostructured Materials

a) Fig. 2. Microstructure of 6082Zr a) as-extruded (N=0) and b) after N=4 ECAP passes, route Bc.

Fig. 3. Microstructure of 6082ZrSc a) in the as received condition (N=0) and b), c) after four ECAP passes (N=4).

Fig. 4 shows the X-Rays diffractometry of as-extruded and severely deformed samples. The 6082Zr sample (Fig. 4a) shows the presence of Al-Zr type particles, AlMnSi and and AlFeSi based intermetallics in the as-extruded. The 6082ZrSc alloy (fig. 4b) contains also the AlSc phases. These plots indicates that ECAP process causes the disappearance, maybe through dissolution, of Mg2Si, Al3Zr4 and AlZr3 peaks. The hypothesis of particle dissolution and the consequent solute redistribution in the matrix, together with severe plastic deformation effect (high density of dislocations and point defects) could explain the increment of lattice parameter of Al occurring in the deformed spectrum [20].

Hardness and electrical conductivity measurements are reported in Fig. 5. Fig. 5(a) shows the aging curves of the 6082Zr obtained at 170° and 190°C for the as-extruded samples (N=0) after solution treatment and post ECAP aging at 170°C for N=1 and N=4. A significant increase in hardness occurs after 4 ECAP passes respect to the as-extruded state (54±2HV) up to 106HV. This value is also higher than the peaks measured for aging at 170 and 190°C when N=0. The reason to compare post-ECAP aged samples with the material solution treated and aged is connected to the high values that are usually obtained during the latter heat treatment in the static case and not only to microstructure mechanisms. In fact, the large increase in hardness during ECAP of the extruded material (almost doubled after 4 passes respect to N=0) can be attributed to the considerable substructure refinement which occurs during a severe plastic deformation [21-23]. During post ECAP aging, the pressed materials exhibits a decrease in hardness with time. The hardness of the post ECAP aged sample at 170°C for 2 hours is comparable with the static peak-aging value for the 6082Zr alloy. The decrease of hardness with time depends on the recovery process and/or recrystallisation that may have occurred during annealing of the severely strained microstructure at

1 µµµµm

N=0 N=4

�=0 �=4 �=4

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b) a) c)

Materials Science Forum Vols. 633-634 335

this temperature. Precipitation has also occurred during post ECAP aging treatments. In fact, it is reasonable to suppose that precipitation occurs in samples with a high density of dislocations as ECA pressed specimens. Moreover, electrical conductivity (Fig. 5b) increases very fast during post-ECAP aging and the kinetics is enhanced by the number of ECAP passes. If the aging is performed at a relatively high temperature like 170°C, the effect of recovery overwhelms the hardening associated to precipitation, leading to a decreasing stage in the hardness curves. In fact, the increased diffusion and the strong stress field induced by the significant amount of dislocation density during ECAP pressing, may influence the kinetics of precipitation and morphology of particles, leading to the development of incoherent interface and to negative contribution to hardness. In order to reduce the effect of recovery process during post ECAP treatments, aging were performed also at lower temperature, 90°C (Fig. 5c) for the 6082ZrSc. The hardness remains almost constant during time. This observation confirms that softening phenomena are reduced at this temperature. A significant increase in hardness occurs after 4 passes respect to the as-extruded state (65±2HV) to 108HV. Even for the

Fig. 4. X-Ray diffractometry of as-extruded and severely strained a) 6082Zr and b) 6082ZrSc.

2,6 2,4 2,2 2,0 1,8 1,6 1,4 1,20

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Fig. 5. Post-ECAP aging showing a) hardness and b) electrical conductivity for 6082Zr and c) hardness for 6082ZrSc. d) hardness vs. number of ECAP passes for both alloys. 6082ZrSc, this value is much higher than the peak ones obtained for static aging at 170 and 190°C. Moreover, the post-ECAP aging value at 170°C for 12 hours corresponds to the T6 peak-aging. Fig.5d illustrates the hardness variation with the number of ECAP passes for the two investigated alloys. The hardness increases more than 50% just at N=1 for both the alloys and at N=4 the values are coincident, according to the grain size shown in Fig. 2 and 3. Fig. 6. Tem micrographs a) of 6082ZrSc after four ECAP passes, b) after ECAP+ ageing at 170°C for 2h, and c) of 6082 Zr after ECAP and ageing at 170°C for 2h.

10-1 100 101 102 103 104

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TEM micrographs of ECAP (N=4) and post ECAP aged samples are illustrated in Fig. 6a and 6b for the 6082ZrSc while in Fig. 6c for the 6082Zr. The effect of ageing at 170°C after ECAP is to decrease hardness with time, as shown in Fig. 5. The microstructure shows the presence of very small spherical precipitates after N=4 in the 6082ZrSc (fig.6a) that, of course, remain also after ageing at 170°C for 2h in Fig. 6b, but they do not seem to substantially affect hardness. In the 6082Zr (fig. 6c) the density of precipitates looks the same and the morphology of precipitates remains spherical as in Fig. 6a and 6b. No significant differences arise in precipitate morphology or density in the two modified alloys, except that the macroscopic results (hardness) is 10 point lower for the 6082Zr respect to the 6082ZrSc after ageing ECAP sample at 170°C for 24h (fig. 5a and 5c). This light difference has to be attributed to restoration processes occurring during post ECAP aging rather than precipitation. The presence of Zr and Sc do not affect the spherical morphology of precipitates occurring during ECAP and post ECAP ageing in this type of alloys [22]. In aged commercial AlMgSi alloys, spherical (or needle shaped) G.P. zones, needle shaped β”, rod-shaped β’ and disc-shaped β particles have been observed. The difference in precipitate morphology and size may be associated with an increased diffusion and strong stress field induced by significant increase of dislocation density and other defects. A detailed discussion is beyond the scope of this study. Surely, the presence of solute atoms and/or point defects just after N=1 or N=4 is not much different because the electrical conductivity values (fig. 5b) are almost the same. The defect and/or solute atoms density changes when post-ECAP ageing starts, and it is lower for the higher number of passes because the electrical conductivity is higher for N=4 post-ECAP aged samples respect to N=1 ones (fig. 5b).

Fig. 7. Average microhardness for the external, medium and central circular zones for a) the 6082ZrSc and b) 6082Zr after N=4 passes. c) Scheme of microhardness measurements. In order to test the homogeneity of microstructure after N=4 ECAP passes, a cross section of the billet after deformation has been ideally divided in ten sectors and the microhardness measured on the radius according to Fig. 7 every 300 µm. The data were then organized in average. The more external microhardness values (up to 1.5 mm from the surface) were averaged and plotted as a function of the radius number. For the middle (from 1.5 up to 3 mm from the surface) and central indentations (form 3 mm up to the centre of the disk) the same procedure was performed too. The results for the 6082ZrSc and 6082Zr alloys are shown in Fig. 7a and 7b respectively, while Fig. 7c represents the radius positions and their number. For both alloys, the external microhardness average values are lower than the others. In particular, radius number 10 always shows the lowest value. It seems that the outer side (the bottom of the billet) remains less deformed after four passes too. These results have been supported and compared by FEM simulation [24]. Fig. 8 illustrates results from 3D-FEM simulation (N=1) and the corresponding microstructure for the 6082Zr alloy.

0 1 2 3 4 5 6 7 8 9 10 1180

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338 Ductility of Bulk Nanostructured Materials

For details concerning Finite Elements Analysis refers to [24]. A light inhomogeneity in deformation can be observed in the section from the top to the bottom of the horizontal simulated bar just at N=1. Moreover, the corresponding microstructure of the sample of three important zones is reported, i.e. just before the shear zone (undeformed), in the shear zone and in the longitudinal plane parallel to material flow direction to illustrate material deformation path.

Fig. 8. 3D-FEM results of plastic equivalent deformation and microstructure evolution during one ECAP pass in a) longitudinal plane before the shear zone, b) in the shear zone and c) along the billet after deformation. Conclusions

In the present study, the effect of post ECAP aging on hardness has been verified for two modified 6082 extruded aluminium alloys.In both alloys, the post ECAP aging curves shows a decreasing values of hardness with time at the higher temperature, while a reduction of aging temperature to

a)

b) c)

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100 °C or less, enhances the precipitation hardening contribution over the recovery and/or grain coarsening effect. In any case, the ECAP process (four pass, route Bc) substantially increases the hardness of the alloys respect to their peak values obtained without any previous deformation (almost doubled). The presence of precipitates during post ECAP aging has been confirmed by X-ray analysis.

Three dimensional FEM analysis of ECAP shows lower equivalent plastic strain on the outer side of both cross and longitudinal sections of workpieces. This behavior remains after four ECAP passes, as confirmed by hardness measurements on cross sections of 6082 alloys. Thus the specimen is less hard in the area where the deformation is lower (bottom part). The grain size of the two alloys, that in the as-extruded were quite different, become comparable after four ECAP passes. This microstructure feature finds confirmation in mean hardness values after ECAP. In fact there is a progressive increases in hardness with the number of passes for the alloys up to the same values when N=4.

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