Tensile properties of 2024 Al alloy processed by enhanced solid-solution and equal-channel angular pressing

XU Xiao-jing(许晓静)¹, CAO Jin-qi(曹进琪)¹, CHENG Xiao-nong(程晓农)¹, MO Ji-ping(莫纪平)¹

School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Equal-channel angular pressing(ECAP) of an enhanced solid-solution treated 2024 Al alloy was successfully performed at room temperature, with an imposed equivalent normal strain of about 0.5. A very high hardness about HV191 and yield strength about 610 MPa (30% higher than those of the unECAPed 2024 Al alloy) in terms of commercial aluminum alloys were observed for the ECAPed 2024 Al alloy. In addition to the strengthening, this process allows the ECAPed 2024 Al alloy have a moderate level of tensile ductility (about 12.7%) and a significant strain hardening capability up to tensile failure. After aged at 373 K for 48 h, the ECAPed alloy increases its hardness (about HV201) and tensile ductility (about 14 %) further. The TEM results show that the ECAPed 2024 Al alloy presents a plate structure (about 50-100 nm) with high density of dislocation and additional thin plate (approximately ＜10 nm＝ inside. The XRD results show that the ECAP processing decreases the texture and increases the dislocation density of the alloy considerably. The theoretical calculations show that the increase of dislocation density resulting from ECAP processing makes a considerable contribution about 55.2 % for the improvement of yield strength.

Key words: aluminum alloy; enhanced solid-solution treatment; equal-channel angular pressing; mechanical properties

1 Introduction

Enhanced solid-solution treatment can accelerate the dissolution of coarse second phase in aluminum alloy, leading to a precipitation of second phase in finer size and greater quantity in subsequent processing, so it can improve the mechanical properties of aluminum alloy considerably[1-2]. Large deformation processing (LDP), such as equal channel angular pressing(ECAP) techniques, has also been proved to be very effective in improving strength of metal and alloy due to the introduction of structure defects, such as dislocation and grain boundary[3-5]. It is of great interest to add these two strengthening effects together to significantly improve the strength of materials, make them more useful.

The objectives of this study are to examine the tensile properties of the 2024 Al subjected to the processing composed of an enhanced solid-solution treatment and subsequent equal channel angular pressing, and to analyze the main mechanisms of the improve- ment of strength.

2 Experimental

Commercial rods of 2024 Al alloy with a diameter of 10 mm were used. The Al alloy has the nominal chemical composition (mass fraction, %) of 4.26 Cu, 1.45 Mg, 0.71 Mn, 0.24 Si, 0.28 Fe, 0.067 Zn, 0.039 Ti, and balance Al. Prior to ECAP processing, the alloy was solution treated at 773 K for a long time of 14 h and then was quenched into room-temperature water to form an enhanced solid-solution microstructure. The grain size was measured to be approximately 40 μm. After the solid-solution treatment, the ECAP processing was immediately carried out for only single pass at room temperature, using a die shown in Fig.1. This die configuration imposed an equivalent normal strain of approximately 0.5 per ECAP pass.

The unECAPed 2024 Al (i.e. only enhanced solid-solution treated 2024 Al) and the ECAPed 2024 Al were naturally aged at room temperature for at least three days. Among them, some was further artificially aged at 373 K for 48 h.

Microhardness was measured with a load of 2 N and  a loading time of 20 s using a Vickers diamond pyramid indenter. The tensile properties was tested with the sample size of 3.0 mm × 1.5 mm cross-section and 5.0 mm gauge length, at an initial strain rate of 1 × 10^-3 s^-1 and room temperature, using a Shimadzu Universal Tester. The X-ray diffraction(XRD) analysis was performed using a Rigaku diffractometer (model D/max- 2500PC) with Cu K_α radiation (λ=0.154 056 nm).
 

Fig.1 Schematic diagram for configuration of used ECAP die (mm)

3 Results and discussion

3.1 Hardness

Table 1 gives the Hv microhardness of the unECAPed and ECAPed 2024 Al alloys. A significant increase in hardness from about 147 Hv to about 191.5 Hv is observed. After aged at 373 K for 48 h, the unECAPed and ECAPed 2024 Al alloys both increase their microhardness further to about 157 Hv and about 201.5 Hv, respectively.

Table 1 Microhardness of unECAPed and ECAPed 2024 Al alloys

3.2 Tensile properties

The stress—elongation curves of the unECAPed and ECAPed 2024 Al alloy are shown in Fig.2. The strength of the ECAPed 2024 Al is significantly higher than that of the unECAPed 2024 Al. The yield strength(YS) of the ECAPed 2024 Al is as high as 610 MPa, which is 30% higher than that of the unECAPed 2024 Al alloy (460 MPa). In addition to the strengthening, this ECAP processing allows the ECAPed 2024 Al alloy has a moderate level of tensile ductility (12.7%). Though this tensile ductility is smaller than that of the unECAPed alloy (20%), it is still a very high value in terms of the commercial aluminum alloys with a YS of 610 MPa. It is also worth noting that the ECAPed 2024 Al alloy exhibits a remarkable strain hardening capability, which rarely can be observed in ECAP-processed materials.

Fig.2 Stress—elongation curves of unECAPed (a) and ECAP (b) of processed 2024 Al alloys

3.3 TEM observation

Fig.3 shows the TEM image for the ECAPed 2024 Al alloy. It can be seen that the ECAPed 2024 Al alloy presents a plates structure (50-100 nm) with high density of dislocation and additional thin plate (approximately＜10 nm) inside.

3.4 XRD analysis

Fig.4 shows the X-ray diffraction(XRD) pattern and the full-width at half-maximum(FWHM) diffraction peaks for the unECAPed and ECAPed 2024 Al alloys. Compared Fig.4(a) with (c), it can be seen that the ECAP processing can decrease the texture considerably, as indicated by its peak height ratio, which is close to the value for Al with randomly oriented grains. With the comparison of Figs.4(b) and (d), it can be easily found that the ECAP processing causes the XRD peaks to broaden considerably.

Fig.3 TEM image for ECAPed 2024 Al alloy

3.5 Contribution of dislocation increase to improve- ment of strength

The average coherent diffraction domain size (d) and lattice microstrain (e) can be calculated from the XRD line broadening by using the integral breadth analysis based on the following equation [6]:

                 (1)

where  δ2θ is the integral breadth (equal to FWHM/0.9 [7-8]), θ is the position of peak maximum, and λ is the wavelength of Cu K_α radiation. Fig.5 shows a least squares fit of the (δ2θ)²/tan²θ against (δ2θ)/(tan θ sin θ) for all measured peaks of the unECAPed and ECAPed 2024 Al. The calculated average coherent diffraction domain size(d) and the root mean square lattice microstrain () are listed in Table 2.

For the materials subjected to severe plastic deformation, the dislocations density(ρ) can be represented in terms of d and  by[9]

                       (2)

where  b is the Burgers vector and equals to 0.286 nm for FCC Al. The calculated dislocation density is also listed in Table 2.

The dislocation strengthening (σ_ρ) can be estimated by the Taylor relationship.

σ_ρ=MαGbρ^1/2                                 (3)

where  α is a constant; M is the average Taylor factor; b is the magnitude of the Burger’s vector; G is the shear modulus; and ρ is the dislocation density. Substituting the typical values of α, M, G, and b for Al[10] (i.e., α=0.24, M=3.06, G=26 GPa, and b=0.286 nm) into Eqn.(3), the strengthening contribution (Δσ_ρ) due to the increase of dislocation density was calculated to be about 82.9 MPa as listed in Table 2. This value divided by the improvement in yield strength (610-460) MPa produces a value of about 55.2%, illustrating that the increase of dislocations density resulting from ECAP processing is primarily responsible for the high strength of the ECAPed 2024 Al alloy. Besides the strengthening resulting from the increase of dislocation density, ECAP-induced second-phase precipitation and grain refinement is considered to be another two main strengthening contributors since ECAP strain can lead to a finer size of precipitation phase and grain boundary.
 

Fig.4 XRD spectra ((a), (c)) and FWHM ((b), (d)) of unECAPed and ECAPed 2024 Al alloy: (a), (b) UnECAPed; (c), (d) ECAPed

 

Fig.5 Integral breadth analysis to calculate coherent domain size and lattice strain

Table 2 Microstructural and mechanical features calculated from XRD data

4 Conclusions

1) Enhanced solid-solution treated 2024 Al alloy can be successfully equal-channel angular pressed at room temperature, with an imposed equivalent normal strain of 0.5. The mechanical properties of the ECAP processed 2024 Al alloy were characterized not only by a very high hardness of HV91 and yield strength of 610 MPa, but also by a moderate level of tensile ductility of 12.7 %. A lower temperature artificially ageing treatment can increase the hardness and tensile ductility of the ECAPed 2024 Al alloy further.

2) The ECAPed 2024 Al alloy presents a plates structure (50-100 nm) with high density of dislocation and additional thin plate (about ＜10 nm) inside.

3) The ECAP processing decreases the texture and increases the dislocation of the alloy considerably. This increase is primarily responsible for the high strength of the ECAPed 2024 Al alloy.

4) The combination of pre-ECAP enhanced solid-solution treatment, ECAP processing at low temperature and post-ECAP low-temperature aging treatment has the potential to render Al alloys become very strong.

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(Edited by LI Xiang-qun)

Foundation item: Project(02KJD460004) supported by the Natural Science Foundation of Jiangsu Province, China

Corresponding author: XU Xiao-jing; Tel: +86-511-8791295; E-mail: xjxu67@ujs.edu.cn