Effect of Severe Plastic Deformation on Structure and Properties of Al-Mg-Si Alloy of 6060 Type

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Specimens of three series of the alloy, corresponding to different initial treatments were prepared to investigate the effect of structure of the initial state of the alloy on the SPD and the final properties of the alloy. The three series were as follows:

1) Series A – Homogenization at 560ºС for 12 hours;

2) Series B – Homogenization at 560ºС for 12 hours + Air Quenching from 480ºС + Aging at 185ºС for 6 hours + Natural Aging;

3) Series C – Re-quenching of the specimens of the B series from 480ºС + Aging at 185ºС for 6 hours.

A combined method of conventional hydrostatic extrusion (HE) and equal-channel angular hydroextrusion (ECAH) was used as the SPD technique. The general deformation route of the specimens was the following: HE + n-ECAH (where n is the number of passes) + HE. The number of passes during the ECAH was n = 1, 2, 4. The sequence of deformation, the values of the strain e after each stage and total accumulated strain are presented in Table 2.

Table 2. The sequence of deformation of the specimens of the A, B, C alloy series

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The method of equal-channel angular hydroextrusion (ECAH), first developed at the Donetsk Institute for Physics and Engineering, NASU, is a modified method of equal-channel angular pressing. ECAH technique (Figure 1) was proposed by Spuskanyuk et al. for the processing of lengthy billets (l/d≥10) in ^[6]. Cylindrical billets were extruded from the container by a high-pressure fluid through an angular die with an angle Φ=90º (Figure 1). Deformation was realized at room temperature under the pressure of 150MPa for HE and 700MPa for ECAH.

Figure 1. Schematic of the ECAH technique.

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The effect of SPD on the aging of the 6060 alloy has been studied.

The structure of the alloys in the initial state, after SPD and aging was studied using transmission electron microscopy (JEM-2000FXII), volumetry, and Vickers Hardness measurements. The temperature intervals of the phase transformations were determined by measuring of the temperature coefficient of resistivity α_t=1/ρ₀ dρ/dT.

3. Results and Discussion

3.1. The Structure and Properties of the 6060 Alloy in the Initial State

Specimens in the homogenized state (series A) and after aging (series B) were studied. Mechanical properties of the specimens after the initial treatment are presented in Table 3.

Table 3. Mechanical properties of the A and B series specimens after the initial treatment

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A TEM study of the alloys structure was carried out. Representative examples are shown in Figure 2. The rod-like coarse particles of the equilibrium β phase, ∼200-600nm long and ∼50-60nm thick, were present in the alloy in the homogenized state (Figure 2a). Precipitation density of the particles is small; they are incoherent with the matrix and associated with dislocations. Fine needle-like particles of the metastable β′ phase, ∼20nm long and ∼5-6nm thick, were formed in the matrix after the decomposition of the supersaturated solid solution (Figure 2b). The particles had a high precipitation density, and were coherently bound with the matrix. These strengthening precipitates allowed improving to nearly double the strength of the alloy.

Figure 2. The structure of the 6060 alloy in the state before SPD: а) A series; b) B series

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Figure 3. Correlation of changes in the Vickers Hardness and density of alloy specimens depending on the accumulated strain for (a) A, (b) B, and (c) C series

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3.2. The Structure and Properties of the 6060 Alloy in the Initial State

Density and hardness were measured for all the states of the alloy with the finite value of ∑e (Table 2). The data obtained are presented in Figure 3. It should be pointed out that the correlation between changes in density and hardness is observed during the increase of cumulative strain. For homogenized specimens (A series) the curves reached saturation, but for aged samples (B and C series) the density and hardness decreased after the accumulation of strain to ∑e=5. Mechanical properties of these specimens are presented in Table 4.

The maximum value of ultimate tensile strength (UTS) was observed for aged specimens. For example, for specimens of B-2 series (accumulated strain ∑e=5.1), UTS increased from 207MPa in the initial aged state to 391MPa, i.e., by almost 90%. However, the elongation to failure δ decreased from 11% to 7.6%. For homogenized specimens of the A-4 series (accumulated strain ∑e=7.4), UTS increased from 116MPa in the initial state to 351MPa, whereas the elongation decreased from 22% to 6.1%.

Table 4. Mechanical properties of specimens of the A, B, C series after SPD

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Figure 4. The effect of SPD on the structure of the A series specimens: (а, b) A-0; (c) A-1; (d) A-2; (e) A-4

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The structures shown in Figure. 4 and Figure 5 are heterogeneous, and exhibit recrystallized and nonrecrystallized grains. Some regions of incomplete dynamic recrystallization may also be observed.

The fragmentation of the grain structure, from 200–500μm to 0.3–0.5μm, and the appearance of elongated recrystallized and nonrecrystallized grains are observed in the A-0 specimen after hydrostatic extrusion (cf. Figure. 4a and Figure 4b). A netlike dislocation structure is present inside the nonrecrystallized grains. Deformation-induced dissolution of the particles of the equilibrium -phase takes place. The particle size is reduced compared to that of the initial state: the length to 100nm, and the thickness to 20nm. The particles are mainly located on the grain boundaries (Figure 4b). Electron-diffraction analysis (Figure 6a-b) revealed that after this treatment the reciprocal lattice reflections are characterized by an azimuthal extension, which indicates the formation of the substructure.

Figure 5. The effect of SPD on the structure of the C series specimens: (а, b) C-0; (c, d) C-1; (e) C-2; (f) C-4

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Figure 6. (a) the quenched alloy in the initial state; (b) A-0; (c) A-2; (d) C-2; (e) B-2 after aging at 140ºС for 1 hour; (f) B-2 after aging at 200ºС for 3 hours. ED patterns showing evolution of the alloy structure after SPD

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Areas surrounded by the shell, which contains a netlike dislocation structure, appear in the matrix after ECAH when n=1 (A-1) (Figure 4c). Particles of the equilibrium β-phase are observed. The increase in the number of passes to n=4 (A-4) resulted in the formation of an alternating chain structure of recrystallized and nonrecrystallized grains (Figure 4e). β-phase particles are not observed. A well-developed substructure is formed after hydrostatic extrusion of aged specimens (C-0), as may be seen from Figure 5a-b. β-phase particles in the C-0, C-1, C-2 series specimens are not observed. β-phase particles appear after increasing the accumulated strain to ∑e=7.4 (Figure 5 f).

The comparison of the structural changes after SPD for homogenized specimens (A series) and aged specimens (C series) has shown that in the aged samples the processes of structure formation are accelerated (Figure 4, Figure 5 and Figure 6 c-d).

3.3. The Effect of SPD on the Aging of the 6060 Alloy

To determine the ability for dispersion hardening of the alloy after SPD, the study of the temperature coefficient of resistivity, α_Т, during continuous heating at the rate of 3º/min in the temperature range 20–400ºС was carried out, besides hardness change isochrones in the temperature range of aging 20–200ºC for all deformed specimens and isotherms HV=f(τ) at temperatures of 120ºС, 140ºС and 200ºС, respectively, for B-2, B-1, B-4 specimens were plotted.

Figure 7. The change of αТ = f(T) for (a) specimens without SPD, and specimens corresponding to (b) A series, (c) B series

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Figure 8. The change of hardness during isochronal annealing for 30 minutes after SPD of: A (а), B (b), C (c) series specimens; and the effect of preliminary heat treatment on the change of hardness (d)

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The changes in α_Т for homogenized, quenched and aged samples without SPD are shown in Figure 7a. The change of α_Т=f(T) implies that its minimum is observed in the temperature range ∼175ºС–325ºС for the quenched state (maximum supersaturation of solid solution). No minimum is observed for the aged state. For all states after SPD the minima of curves α_Т=f(T) were observed (Figure 7b-c), therefore, the solid solution was supersaturated. The maximum supersaturation was observed for homogenized specimens.

For homogenized specimens, Figure 8, the maximum hardness is observed at the maximum accumulated strain in the A-4 specimen, the minimum hardness after HE in the A-0 specimen. A plateau or a slight increase in the curve HV=f(T) is seen in the temperature range 100ºС–140ºС.

For the artificially aged specimens of the C series, as well as for the A series, the minimum hardness is observed after hydrostatic extrusion and the maximum - for the C-2 specimen (Figure 8c).

For specimens of B series, the minimum hardness is observed in the B-4 specimen which has the maximum accumulated strain, and the maximum hardness - in the B-2 specimen (Figure 8b). This series demonstrates the most hardening.

Figure 9. Hardness change during the isothermal aging of B series specimens

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Figure 10. The structure of B series specimens after (a-d) aging at 140ºС for 1 hour: (а) B-0; (b) B-1; (c) B-2; (d) B-4; and (e-f) aging at 200ºС for 3 hours; (e) B-2; (f) B-4

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