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Original Article
DOI: 10.1016/j.jmrt.2019.11.048
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Available online 1 December 2019
Improvement in mechanical properties of Al-Zn-Mg alloy by applying electric pulse during hot extrusion
Visits
113
Shuoshuo Lia, Liang Chena,
Corresponding author
chenliang@sdu.edu.cn

Corresponding author.
, Xingrong Chub, Jianwei Tanga, Guoqun Zhaoa, Cunsheng Zhanga
a Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong 250061, PR China
b Associated Engineering Research Center of Mechanics and Mechatronic Equipment, Shandong University, Weihai 264209, PR China
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Table 1. Detailed parameters of the extrusion experiments (Jp - peak current density, Jr - root mean square (RMS) current density).
Table 2. Fractions of the main textural components in the extruded samples.
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Abstract

For the first time, an electric pulse was applied during the hot extrusion of an Al-Zn-Mg alloy. Extrusion experiments were conducted with and without the presence of the electric pulse to compare the effects of the electric pulse on the microstructure and mechanical properties of the extruded alloy. The grain growth behavior was promoted by the thermal effects, and dynamic recrystallization was promoted by the athermal effects. Therefore, the width of the elongated grains slightly increased; further, both the fraction and the grain size of recrystallization also increased. It was observed that the types of textures were not affected; however, the fractions of the textural components changed and their intensity was enhanced. The electric pulse significantly enhanced the elongation of the extruded alloy and slightly decreased its ultimate tensile strength. The elongation of the sample extruded with an 80-V/250-Hz electric pulse was highest, at 11.9%, which was 19% higher than that of the sample that received no electric pulse. Moreover, both the hardness and resistance to intergranular corrosion were enhanced by the electric pulse.

Keywords:
Electric pulse
Extrusion
Al-Zn-Mg
Microstructure
Mechanical properties
Full Text
1Introduction

As they are lightweight, exhibit excellent mechanical properties, are easily recyclable, and have high corrosion resistance, Al alloys have been widely applied in high-speed trains, aerospace, and the automotive industry [1–3]. The microstructure and mechanical properties of these alloys can be improved by hot extrusion, which is a primary plastic-forming process [4–6]. The extruded microstructure is sensitive to the process parameters, such as the extrusion speed, billet temperature, and die temperature [7–9]. If the process parameters cannot be controlled well, coarsening and inhomogeneous microstructures might appear, which would adversely affect the mechanical properties.

Recently, researchers have attempted to apply electric pulses (EPs) to deformation or heat treatment processes. Some theories regarding the influence of EP on various materials have been proposed, such as the electrically induced migration, electro-plastic, and electro-superplastic effects [10–12], which propose that EPs could promote the diffusion of atoms and enhance the plasticity of materials. Moreover, EPs can affect materials through a variety of processes, such as Joule heating, electron winds, and electrostatic fields [13–15]. The thermal effect of Joule heating could slightly promote grain growth, while the athermal effects could improve the microstructure by increasing the nucleation rate and promoting atomic diffusion [16–18]. Xu et al. [15] found that the second phases rapidly dissolved into the matrix of a 7075 Al alloy during solution treatment with the assistance of EP, and the combined effects of the fine precipitates and grains created by the EP could improve the mechanical properties. Roh et al. [19] proposed that the formability of Al alloy was improved by applying an EP to a sample during quasi-static uniaxial tensile loading. Xu et al. [20] studied the effects of EP on a hot-rolled 2024 Al alloy, and demonstrated that the microstructure was refined and mechanical properties were enhanced. Tang et al. [21] studied the punching of 2024T4 Al sheets, and found that the ultimate tensile load was increased with the assistance of the EP. Chen et al. [22] applied an EP in the hot extrusion of AZ91 alloys, and a pulse of 70 V/250 Hz was considered optimal for achieving excellent performance.

The hot extrusion of Al alloys has also been widely studied. During hot extrusion, the evolution of the microstructure is complex due to the occurrence of dynamic recovery (DRV), dynamic recrystallization (DRX), and grain growth. Fan et al. [23] studied a novel co-extrusion process using dissimilar 1060 and 6063 Al alloys, and their results showed that complete DRX occurred, and the height of the welding chamber significantly affected the micro-texture of the extruded profiles. Güzel et al. [24] proposed a new method for analyzing microstructure evolution during hot extrusion, and found that the microstructure varied greatly at different positions in extruded Al-Mg-Si alloys. Fan et al. [25] reported that grain elongation and geometrical dynamic recrystallization (GDRX) occurred during the extrusion of a 1100 Al alloy in a porthole die. Zhao et al. [26] analyzed the continuous extrusion of 6063 Al, and their results revealed that the grain size was significantly affected by the wheel velocity. Li et al. [27] studied the effects of asymmetric feeding on the extrusion of high-strength Al-Zn-Mg alloys, and found that the microstructure of all extruded plates consisted of elongated grains with a small number of fine grains.

As mentioned above, EPs are effective for improving the mechanical properties of various materials, and hot extrusion is the most important method of Al alloy processing. However, EPs have not yet been applied to the hot extrusion of Al alloys. Therefore, in this study, a setup was designed to conduct hot extrusion with the assistance of an EP. For comparison, extrusion experiments were conducted with and without the assistance of an EP. The grain structure, second phase, and texture of the extruded Al-Zn-Mg alloys were examined, and Vickers hardness and tensile tests were also conducted. The effects of the EP on the microstructure and mechanical properties of the extruded 7075 Al alloy were also discussed.

2Experimental procedures

An as-cast 7075 Al billet with the chemical compositions of Al-5.50Zn-2.35Mg-1.36Cu-0.40Si-0.40Fe (wt.%) was received from Shandong Yancon Light Alloy Co., Ltd., China. The billet was homogenized at 530 °C for 20 h, and then cooled to room temperature of 25 °C in air. The homogenized billet was then machined to a diameter of 20 mm for the extrusion experiments. Fig. 1(a) shows the extrusion setup consisting of a container, die, extrusion ram, and pulsed DC power supply. A pressing machine (MTS CMT5305, China) was used to apply the extrusion load. The ram velocity was maintained at 0.1 mm/s throughout the extrusion process. Conventional extrusion without an EP was conducted at billet temperatures of 420 and 470 °C, and the samples were named CE420 and CE470, respectively. Extrusion was also conducted with EP parameters of 70 V/250 Hz and 80 V/250 Hz was also carried out, and the samples were named EP70 and EP80, respectively. The billet temperature for both EP70 and EP80 was set at 420 °C. The parameters of the extrusion experiments are detailed in Table 1. A bar with a diameter of 5 mm was extruded, and the extrusion ratio was calculated to be 16.

Fig. 1.

Schematic of the (a) extrusion setup and (b) location of the samples in the microstructure observation and mechanical properties tests.

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Table 1.

Detailed parameters of the extrusion experiments (Jp - peak current density, Jr - root mean square (RMS) current density).

No.  Ram velocity (mm/s)  Temperature (°C)  Voltage (V)  Frequency (Hz)  Jp (A/mm2Jr (A/mm2
CE420  0.1  420  –  –  –  – 
CE470  0.1  470  –  –  –  – 
EP70  0.1  420  70  250  70.89  9.33 
EP80  0.1  420  80  250  81.89  10.76 

The grain structure of the as-homogenized billet was examined using an optical microscope (OM, Olympus GX51, Japan) and a scanning electron microscope (SEM, JEOL JSM-7800 F, Japan). For OM observation, the samples were first ground and polished, and then etched in a solution containing 1.0 mL hydrofluoric acid, 1.5 mL hydrochloric acid, 2.5 mL nitric acid, and 95.0 mL distilled water. The SEM samples were ground using silicon carbide paper (1200#) and polished using a diamond polishing agent (1.5 μm). The microstructure and texture of the extruded samples were analyzed by an electron backscatter diffraction (EBSD, Oxford NordlysMax3, Britain), for which the samples were electro-polished in a solution containing 10 mL perchloric acid and 90 mL methanol at 30 V for 7 s. The tensile samples were machined parallel with the extrusion direction (ED), and the main dimension of the tensile samples is shown in Fig. 1(b). The tensile tests were conducted with a stretching rate of 0.2 mm/min on a testing machine of MTS CMT5504, China. The fractured surface was observed by SEM. The Vickers micro-hardness was measured with a load of 300 g and dwelling time of 10 s. The hardness tests were conducted at 18 locations on each sample, and the average values were calculated. Finally, an intergranular corrosion (IGC) test was conducted by soaking the samples in a solution containing 57 g sodium chloride, 1 L distilled water, and 10 mL hydrogen peroxide at 35 °C for 6 h.

3Results and discussion3.1Homogenized microstructure

Fig. 2 shows the microstructure and energy dispersive spectroscopy (EDS) results of the as-homogenized 7075 Al alloy. As shown in Fig. 2(a), the billet consisted of coarse and equiaxed grains with an average grain size of approximately 120 μm. The SEM images and EDS results of the as-homogenized billets without etching are shown in Fig. 2(b–d). There are two main types of second phases. The first is the coarse and continuous phase located on the grain boundaries containing Al, Fe, and Cu, and the second is the fine dispersed particles distributed inside the grains containing Al, Mg, Cu, and Zn. According to previous studies [28,29], the former is likely Al23CuFe4, and the latter could be MgZn2 or Al2CuMg.

Fig. 2.

(a) OM and (b) SEM images of the homogenized billet, and the EDS results of points (c) 1 and (d) 2.

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3.2Extruded microstructure

The distribution of the second phases in the extruded samples is shown in Fig. 3. Some large, chainlike second phases were observed in all cases. Based on the EDS results, these coarse phases were identified as Al23CuFe4. The chainlike Al23CuFe4 phases were first distributed along the grain boundaries of the homogenized billets, and were then elongated by hot extrusion. Additionally, the amount and size of coarse Al23CuFe4 were very similar in each sample because it was difficult for the coarse Al23CuFe4 phase to dissolve into the Al-matrix at billet temperatures of 420 and 470 °C [29]. Some fine MgZn2 phases were also dispersed in the samples; however, the amount and size of them are different. As shown in Fig. 3(a), there was a high amount of MgZn2 in CE420, which was extruded at 420 °C. As the billet temperature increased to 470 °C, MgZn2 almost disappeared, as shown in Fig. 3(b). This indicates that 470 °C is sufficient for the dissolution of MgZn2. As shown in Fig. 3(c) and (d), MgZn2 was also present in EP70 and EP80; therefore, the ideal temperature for EP extrusion is below 470 °C. Compared with CE420, the number of MgZn2 in EP70 and EP80 contained less and smaller MgZn2 than CE420. This could be caused by the coupling of the athermal and thermal effects of EP. The athermal effect can promote the diffusion of atoms and dissolution of the second phases [30], as the application of EP reduces the diffusion activation energy of atoms, while increasing their mobility [31]. The higher temperature due to the thermal effect also accelerated dissolution. Stronger EP effects can be generated at a higher voltage of 80 V. Therefore, EP80 contained less and smaller MgZn2 than EP70. Therefore, the temperature used in extrusion with assistance from EP assistance should exceed 420 °C, and be below 470 °C.

Fig. 3.

SEM images of the distribution of second phases in samples (a) CE420, (b) CE470, (c) EP70, and (d) EP80.

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The EBSD maps of the extruded samples are shown in Fig. 4, where the grey lines indicate the low-angle grain boundaries (LABs) with misorientation angles between 2° and 15°, and the black lines indicate the high-angle grain boundaries (HABs) with a misorientation angle of over 15°. As shown, the microstructure consisted of elongated and fine equiaxed grains, which indicates the occurrence of partial DRX. The average widths of the elongated grains in CE420, CE470, EP70, and EP80 were 11.2, 17.0, 14.6, and 16.9 μm, respectively. By comparing Fig. 4(a) and (b), it can be seen that the width of the elongated grains was larger for CE470 due to the higher billet temperature of 470 °C [32,33]. Moreover, the grain width of EP70 was slightly larger than that of CE420, and much smaller than that of CE470. This indicates that the thermal effects of EPs, such as Joule heating, played an important role in the processing of EP70 [34]. However, the number of DRX grains in sample EP80 greatly increased, as shown in Fig. 4(d). This could be because the athermal effect of EP promoted DRX and was predominant in the processing of EP80. Moreover, the <001> and <111> orientations of most grains in all samples were parallel with the direction of ED, suggesting that strong <001> and <111> fiber textures formed during hot extrusion.

Fig. 4.

EBSD maps of samples (a) CE420, (b) CE470, (c) EP70, and (d) EP80.

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Fig. 5 shows the distribution of DRX grains. The fractions of DRX grains in CE420, CE470, EP70, and EP80 were 2.72, 3.11, 1.12, and 3.51%, and they were 1.75, 2.06, 1.93, and 2.39 μm in size, respectively. CE470 exhibited a larger fraction of larger DRX grains than those in CE420, as the higher temperature promoted both DRX and grain growth [35]. Both the thermal and athermal effects of the EP in EP80, which was treated with a voltage of 80 V, became stronger. As reported in Refs. [36,37], DRX could be promoted by the coupling of the thermal and athermal effects of EPs. The thermal effects refer to violent collisions between the fast-moving electrons and atoms inside the material with EP processing, resulting in Joule heating, the intensification of the random thermal motion of atoms, and an increase in the mobility of atoms. All of these facts contribute to the promotion of DRX. Moreover, with the assistance of the pulse current, the free electrons exhibited directional movement, which can influence dislocation. Consequently, both the mobility and recovery of dislocations were enhanced, which improve nucleation for recrystallization [38]. Additionally, the speed of the growth of DRX grains increased under high temperatures. Therefore, EP80 exhibited a larger fraction of larger DRX grains.

Fig. 5.

Distribution of DRX grains in samples (a) CE420, (b) CE470, (c) EP70, and (d) EP80.

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Fig. 6 shows the inverse pole figures (IPFs) of the extruded samples. The main textures in all samples were <111> and <001> fiber textures, which are common in extruded Al alloys. The <111> fiber texture was strongest in CE420, EP70, and EP80, while the <001> fiber texture was strongest in CE470. As reported in Ref. [33], strong textures could form during extrusion due to severe plastic deformation, and the <001> or <111> orientation of the grains gradually became parallel with the ED, which resulted in the formation of a strong, fibrous texture. The intensities of the textures in EP70 and EP80 were higher than those in CE420 and CE470, because EP can prevent dislocation tangling and promote dislocation gliding [39], thereby allowing the grains to enter the preferred orientation more easily and enhancing the formation of micro-textures. Additionally, the intensity of the textures in EP70 was higher than that of EP80, as the large number of DRX grains in EP80 weakened the texture intensity [30]. Therefore, the EP strongly affected the texture intensity by promoting dislocation slip and DRX.

Fig. 6.

IPFs of the extruded (a) CE420, (b) CE470, (c) EP70, and (d) EP80 samples.

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To accurately identify the textural components, the φ2 = 45, 65, and 90° orientation distribution function (ODF) sections of the extruded specimens were drawn, as shown in Fig. 7. The main textural components of all samples were {110}<111 > Y, {112}<111> Copper, {110}<100> Goss, {001}<100> Cube, and {123}<634 > S. Therefore, the texture type was not affected by the billet temperature or application of EP. However, the distribution and fraction of the different textural components varied significantly between the different samples. The distribution of the main components is shown in Fig. 8, and their fractions are listed in Table 2. The colors in Fig. 8 represent the different components, and the maximum angle deviation was set to 20°. As shown, the main components were distributed along ED in a “banded” shape. Overall, the fractions of Y and Copper were much larger fractions than those of the other components. The fraction of Copper was larger than that of Y in samples CE420 and CE470, while the fraction of Copper was smaller than that of Y in samples EP70 and EP80, which received the EP. The variation of the textures agrees well with the IPF results shown in Fig. 6.

Fig. 7.

ODF sections of samples (a) CE420, (b) CE470, (c) EP70, and (d) EP80.

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Fig. 8.

Distribution of the main textural components in samples (a) CE420, (b) CE470, (c) EP70, and (d) EP80.

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Table 2.

Fractions of the main textural components in the extruded samples.

No.Textural component (%)
Y {110} <111>  Copper {112} <111>  Goss {110} <100>  Cube {001} <100>  S {123} <634> 
CE420  27.5  33.2  10.4  15.6  3.09 
CE470  20.6  27.6  21.9  20.3  3.96 
EP70  36.9  29.7  16.8  12.6  0.79 
EP80  40.5  30.2  10.2  2.71  1.78 
3.3Vickers hardness

Fig. 9 shows the Vickers hardness results of the homogenized and extruded 7075 Al alloy samples. The hardness of the extruded alloys was much higher than that of the homogenized billet. Moreover, the hardness increased from 78.5 to 81.5 HV as the temperature increased from 420 (CE420) to 470 °C (CE470). The hardness of an alloy is affected by both the grain size and second phases. The grain size of CE420 was smaller than that of CE470. However, most of the MgZn2 particles dissolved in the Al matrix in CE470, resulting in the high solution strengthening. The hardness of EP70 and EP80 was further enhanced by the application of the EP. The athermal effect of EPs promotes the diffusion of atoms [22]; therefore, the smaller second phases were distributed more uniformly, which enhanced the dispersion and further improved the hardness. Moreover, although the width of the elongated grains in EP80 exceeded that of the grains in EP70, the higher fraction of DRX grains in EP80 contributed to the increase in hardness.

Fig. 9.

Vickers hardness of the homogenized and extruded samples.

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3.4Tensile properties

Tensile tests were conducted on the extruded samples to evaluate their tensile properties. The engineering stress-strain curves are shown in Fig. 10(a), and the ultimate tensile strength (UTS), yield strength (YS), and elongation values are summarized in Fig. 10(b). As shown, in the extruded specimens without EP, the UTS decreased from 489.3 to 473.0 MPa as the temperature increased from 420 (CE420) to 470 °C (CE470), and YS decreased from 332.8 to 318.1 MPa. Meanwhile, the elongation increased from 10.0 to 11.0%. The UTS and YS slightly decreased by approximately 3.3 and 4.4%, respectively, while the elongation increased by approximately 10%. The effects of microstructure on the mechanical properties of alloys are complex, and many factors, such as the grain size, DRX fraction, second phase, and texture, should be considered. The plastic deformation of Al alloys is accompanied by dislocation slip. If there are higher amounts of finer grains and second phases, dislocation slip is strongly hindered. Therefore, although the strengthening of the solution could have increased the strength of the alloy, the coarse grains and weakened fiber texture of CE470 resulted in lower UTS and YS. Weakened fiber texture, a large DRX fraction, and small amount of second phases are favorable for increasing elongation. The UTS and YS of EP70 decreased to 481.7 and 325.7 MPa, respectively, and elongation increased to 10.4%. Although the strong <111> fibers in EP70 might have improved its strength, the coarse grain size decreased the strength of the sample and the dispersed second phases were beneficial for improving its ductility. Finally, EP80 exhibited the lowest UTS of 459.3 MPa and YS of 301.6 MPa, and highest elongation rate of 11.9%. The UTS and YS of EP80 were 6.1 and 9.3% lower than those of CE420, respectively, while the elongation rate was 19.0% higher. EP80 contained a larger fraction of DRX grains, and the tensile stress distributed evenly to each grain during the tensile test, which resulted in higher elongation. Furthermore, EP80 contained fine, dispersed MgZn2 particles, which may have also improved elongation.

Fig. 10.

(a) Engineering stress-strain curves and (b) tensile properties of the extruded samples.

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3.5IGC

The morphology of the IGC-tested samples examined, and the results are shown in Fig. 11. The maximum corrosion depths of CE420, CE470, EP70, and EP80 were 124.2, 87.9, 113.4, and 97.7 μm, respectively, indicating that, at the same billet temperature, the application of an EP can improve the IGC resistance of extruded Al alloys. IGC is a type of local corrosion caused by electrochemical heterogeneity in a material that will spread along the grain boundary [40], and the second phases typically initiate this corrosion. Therefore, the IGC resistance of Al alloys is closely related to the grain size and second phase particles. CE420 exhibited coarser and a higher amount of second phases. Therefore, larger corrosion pits were created in the early stage of the IGC test. Consequently, corrosion could develop deeper along the grain boundaries. However, CE470 exhibited a lower amount of second phases, and the IGC resistance was enhanced. The EP specimens also exhibited moderate corrosion resistance as they contained a moderate number and of moderately sized second phases.

Fig. 11.

Morphologies of IGC samples (a) CE420, (b) CE470, (c) EP70, and (d) EP80.

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4Conclusion

Hot extrusion experiments without and with the assistance of electric pulse (EP) were conducted, and the effects of the EP on the microstructure and mechanical properties of extruded 7075 Al alloys were analyzed. The main conclusions are as follows.

  • 1)

    The coarse Al23CuFe4 phases were not changed by the application of the EP, while the number and size of MgZn2 phases were reduced. This is because the athermal effect of the EP promoted the diffusion of atoms and dissolution of MgZn2 phases.

  • 2)

    The width of the elongated grains slightly increased under the effect of the EP due to the thermal effect. When an 80-V/250-Hz EP was applied, both the fraction and size of DRX grains increased, as the nucleation activation energy was provided by the EP.

  • 3)

    Strong <001> and <111> fiber textures appeared in all extruded specimens. The type of texture was not affected by the application of the EP, while the fraction of different textural components changed and their intensity was enhanced.

  • 4)

    The EP greatly increased the elongation and slightly decreased strength of the extruded 7075 Al alloy. EP80 exhibited the highest elongation rate of 11.9%, which was 19% higher than that of CE420 (extruded at the same billet temperature). Moreover, the hardness was also increased by the application of EP. The IGC resistance of samples EP70 and EP80 was better than that of CE420.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors would like to acknowledge the financial support from National Natural Science Foundation of China (U1708251, 51735008), Fundamental Research Funds of Shandong University (2017JC005), and Young Scholars Program of Shandong University (2018WLJH26).

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