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Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded Al–Zn–Mg–Cu alloy bars

Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded... International Journal of Minerals, Metallurgy and Materials Volume 24, Number 11, November 2017, Page 1284 DOI: 10.1007/s12613-017-1521-3 Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded Al–Zn–Mg–Cu alloy bars 1) 2) 1) 1) Zhi-hao Zhang , Jie Xue , Yan-bin Jiang , and Feng Jin 1) Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China 2) Beijing Hangxing Machine Manufacture Co., Ltd, Beijing 100083, China (Received: 10 May 2017; revised: 3 July 2017; accepted: 7 July 2017) Abstract: Taking extruded Al–Zn–Mg–Cu alloy (7A04 alloy) bars as the research object, the effect and mechanism of pre-annealing treatments on the microstructure and mechanical properties of the aged alloy bars were investigated. The results show that a pre-annealing treatment at 350°C for 15 h before a T6 treatment substantially reduced the sensitivity of the microstructure and mechanical properties of the extruded 7A04 aluminum alloy specimens toward the extrusion temperature. The average grain sizes of the specimens extruded at 390 and 430°C after T6 treatment were 3.4 and 8.1 μm, respectively, and their elongations to failure were 7.0% and 9.2%, respectively. However, after pre-annealing + T6 treatment, the differences in both the grain sizes and the elongations of the specimens became small, i.e., their average grain sizes were 3.2 and 3.8 μm and their elongations were 12.0% and 13.3%, respectively. For the specimens extruded at the same temperature, pre-annealing treatment obviously improved the plasticity of the alloy, which is attributed to an increase in soft texture or to grain refinement in the specimens as a result of the pre-annealing + T6 treatment. Keywords: aluminum alloy; hot extrusion; pre-annealing treatment; microstructure; mechanical properties 1. Introduction truded products and reduce the overall service performance of equipment. The applications of aluminum alloy products in the fields Isothermal extrusion, which can be achieved via of rail transportation and aerospace have led to more strin- closed-loop speed control [6] or billet gradient temperature gent requirements with respect to mechanical properties. In heating (cooling) [7], can effectively overcome the problem addition to the factors such as the alloy material, shape and of nonuniform temperature distribution along the length di- size precision, and the surface quality, the homogeneity and rection of the extruded product. For the nonuniform temper- consistency of alloys’ microstructure and properties are also ature distribution in the cross section, the current main solu- keys to ensuring high performance of formed products. tion is to adjust the heat flow during extrusion (e.g., plastic Extrusion is an important forming method for wrought deformation heat, friction heat, and heat transfer between aluminum alloys. Because of the characteristics of extrusion the billet and mold) [1]. technology, an uneven temperature distribution inevitably Some studies [8−11] have indicated that, when a de- exists in the extruded products along the length direction formed aluminum alloy was annealed at low temperatures and in the cross section. For example, the maximum tem- (i.e., pre-annealing treatment), the stored deformation ener- perature difference in the cross section of a large profile can gy was partly consumed and the driving force of recrystalli- reach 60°C [1], leading to inhomogeneities in both the mi- zation nucleation and growth was reduced during the sub- crostructure and the mechanical properties of the products sequent solution treatment, which affected the grain size of along the length direction and in the cross section [2−5]; the alloy. Therefore, controlling the microstructure and me- these inhomogeneities seriously affect the service life of ex- chanical properties of an aluminum alloy profile via a Corresponding author: Yan-bin Jiang Email: [email protected] © The Author(s) 2017. This article is published with open access at link.springer.com Z.H. Zhang et al., Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded … 1285 pre-annealing treatment is theoretically feasible. was obtained by the linear intercept method. The degree of In this paper, taking 7A04 aluminum alloy bars extruded recrystallization and the grain orientation were detected by at different temperatures as the research object, we investi- the electron backscattering diffraction (EBSD) technique. gated the effects of pre-annealing on the microstructure and EBSD specimens were prepared by electrolytic polishing at mechanical properties of the extruded bars after solid solu- a voltage of 25 V in 5wt% perchloric acid at −25°C. The tion and aging treatments (i.e., T6). The proper size and distribution of precipitates were observed by trans- pre-annealing conditions that result in microstructure and mission electron microscopy (TEM). mechanical properties as similar as possible for bars ex- The {111}, {200}, {220}, and {113} incomplete pole truded at different temperatures can be obtained to reduce figures for each specimen were constructed from the results the sensitivity of the microstructure and mechanical proper- obtained using a Brucker D8 Discovery texture tester. The ties of the alloy to the extrusion temperature. Finally, this orientation distribution function (ODF) of the expansion work provided a reference for improving homogeneities in coefficient l = 22 was obtained using the method of both the microstructure and the mechanical properties of the spherical harmonic series expansion; the “true ODF” (l = 23) extruded profiles. was obtained by texture analysis to eliminate the influence of a ghost peak [13−14]. 2. Experimental The tensile specimens with a diameter of 3 mm and a gauge length of 15 mm were machined with their axes pa- The chemical composition of the 7A04 aluminum alloy rallel to the extrusion direction, consistent with the method specified in standard GB/T228.1—2010. Tensile tests were billets used in the present work is shown in Table 1. The billets with a size of φ40 mm × 60 mm were subjected to a carried out at room temperature on an MTS-810 testing −3 −1 machine at a strain rate of 1 × 10 homogenization treatment at 460°C for 24 h [12] in an s ; tensile deformation SGM-M30/12 box-type resistance furnace and were then was measured using an extensometer. Three samples cor- immediately quenched in water at room temperature. The responding to each condition were tested, and the average grain size of the billets after the homogenization treatment values are reported as the test results. was approximately 28.2 μm. The homogenized billets were then extruded into round bars with a diameter of 8.4 mm; 3. Results the extrusion ratio was 23. Fig. 1 shows the average grain sizes of the specimens ex- Table 1. Chemical composition of the 7A04 aluminum alloy truded at different extrusion temperatures after the T6 treat- wt% ment. It reveals that the average grain size of the specimens Zn Mg Cu Cr Fe Si Mn Al in the cross section first increases and then decreases with 5.15 2.30 1.70 0.21 0.30 0.09 0.23 Bal. increasing extrusion temperature; i.e., the sensitivity of the aged microstructure to the extrusion temperature differs. In To reduce the influence of the difference in microstruc- addition, when the temperature of the extruded 7A04 alu- ture between the front and end of the bars on the experi- minum alloy along the extrusion direction changes from 390 mental results, the specimens were collected from the to 500°C, the microstructure of the extruded product after middle section of the bars (i.e., the stable extrusion stage). T6 treatment is inhomogeneous. In particular, when the ex- The specimens were divided into two groups. One group trusion temperature is 390 and 430°C, the difference in the was subjected to the T6 process (475°C for 1 h + 120°C for average grain size of the two specimens (3.4 and 8.1 μm, 24 h) [12]. The other group was treated by being respectively) is largest. Therefore, the specimens extruded at pre-annealed (the annealing temperature ranged from 100 to 390 and 430°C were used as the main research objects in the 410°C, and the holding time ranged from 0.5 to 20 h) and following experiments in this work. then subjected to the T6 process. The extruded specimens were pre-annealed at 100–410°C The optical micrographs of the aged specimens (the ob- for 15 h and then subjected to T6 treatment; the average servation position was the center of the specimens) were grain size of the T6-treated specimens is shown in Fig. 2, observed using a Zeiss Axiovert 200 MAT optical micro- which reveals that the average grain size of the specimen scope. The specimens for optical microscopy were polished extruded at 390°C changes little with the pre-annealing and etched in Graff Sargent agent (1 mL HF + 16 mL HNO + temperature, ranging from 3.2 to 3.4 μm. In the case of the 3 g CrO + 83 mL water). The grain size of the specimens specimen extruded at 430°C, when the pre-annealing tem- 1286 Int. J. Miner. Metall. Mater., Vol. 24, No. 11, Nov. 2017 perature was lower than 290°C, the average grain size was in pre-annealing time, the grain size increases conversely. approximately 8.0 μm. When the pre-annealing temperature Therefore, when the pre-annealing time is 15 h, the differ- was increased from 290 to 350°C, the average grain size de- ence in the average grain size of the specimens extruded at creased sharply, reaching a minimum of 3.8 μm at 350°C. 390 and 430°C is least, and the average grain sizes are 3.3 When the pre-annealing temperature was increased further, and 3.8 μm, respectively, indicating that the suitable the average grain size increased. When the pre-annealing pre-annealing time is 15 h. temperature exceeded 410°C, the average grain size was approximately the same as the initial grain size. Therefore, the suitable pre-annealing temperature is 350°C, which in- duces the least difference in the average grain size of the aged specimens extruded at 390 and 430°C. Fig. 3. Influence of pre-annealing time on the average grain size of aged specimens (the pre-annealing temperature is 350°C). In summary, the microstructure of the extruded specimen after T6 treatment is sensitive to the extrusion temperature, especially in the case of specimens extruded at 390 or 430°C. Fig. 1. Influence of extrusion temperature on the average However, the pre-annealing step (the suitable pre-annealing grain size of the specimens after T6 treatment. process is 350°C for 15 h) before the T6 treatment can sub- stantially reduce the microstructure sensitivity of the speci- men to the extrusion temperature. To further analyze the microstructure difference of the alloy after T6 treatment and after the pre-annealing (350°C for 15 h) + T6 treatment, we characterized the correspond- ing specimens by EBSD, as shown in Fig. 4. The proportion of low-angle grain boundaries was increased by the pre-annealing treatment such that the grain of the specimens extruded at different temperatures was refined to different extents. The recrystallization fraction was analyzed using the EBSD post-processing software HKL Channel5. The results show that the recrystallization fractions of the speci- Fig. 2. Influence of pre-annealing temperature on the average mens extruded at 390 and 430°C after T6 treatment are grain size of the aged specimens (the pre-annealing time is 15 h). 38.1% and 43.8%, respectively, whereas those after pre-annealing + T6 treatment are dramatically lower: 13.8% The average grain size of the extruded specimens treated and 13.0%, respectively. by pre-annealing for different times and then by T6 treat- ment is shown in Fig. 3. The average grain size of the spe- Table 2 shows the room-temperature mechanical properties of the specimens extruded at different temperatures after T6 cimen extruded at 390°C is independent of the pre-annealing time. For the specimen extruded at 430°C, treatment or after pre-annealing + T6 treatment; the repre- sentative engineering stress–engineering strain curves of the when the pre-annealing time is approximately 0.5–10 h, the average grain size also remains unchanged. However, when aged bars are given in Fig. 5. The tensile strengths (R ) and the pre-annealing time is greater than 10 h, the average grain yield strengths (R ) of the specimens extruded at 390 and p0.2 size decreases gradually and is approximately 3.8 μm in the 430°C after T6 treatment are all respectively similar; however, case of a pre-annealing time of 15 h. With a further increase the elongations (A) are 7.0% and 9.2%, respectively (i.e., the Z.H. Zhang et al., Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded … 1287 difference in elongation is large). In the case of the specimens cimens after pre-annealing + T6 treatment are obviously im- extruded at 390 and 430°C after pre-annealing + T6 treatment, proved and the difference between the two specimens is also although their strengths are basically unchanged, their elonga- smaller compared with that between the specimens after T6 tions increase substantially to 12.0% and 13.3%, respectively, treatment. Therefore, the pre-annealing treatment can, to some and the difference in elongations decreases. Meanwhile, as extent, reduce the sensitivity of the mechanical properties of evident from Fig. 5, the elongations of the two kinds of spe- the specimen to the extrusion temperature. Fig. 4. EBSD images of the specimens extruded at different temperatures after different heat treatments: (a) 390°C extrusion + T6 treatment; (b) 430°C extrusion + T6 treatment; (c) 390°C extrusion + pre-annealing + T6 treatment; (d) 430°C extrusion + pre-annealing + T6 treatment. Table 2. Mechanical properties of the specimens extruded at different temperatures after different heat treatments Extrusion temperature / °C Heat treatment R / MPa R / MPa A / % m p0.2 +8.2 +7.8 +0.5 666.4 618.0 7.0 390 T6 −7.7 −6.2 −0.4 +5.6 +10.4 +0.6 678.1 618.3 9.2 430 T6 −7.8 −9.4 −0.5 +4.5 +6.2 +0.3 670.7 625.3 12.0 390 Pre-annealing + T6 −3.1 −4.3 −0.1 +6.3 +3.8 +0.2 682.7 623.7 13.3 430 Pre-annealing + T6 −5.1 −4.6 −0.3 4. Discussion 4.1. Influence mechanism of pre-annealing on the grain size of the alloy after solid-solution treatment and aging When the 7A04 aluminum alloy billet was extruded at 390°C, high deformation stored energy was produced, which provided a large driving force for static recrystalliza- tion during subsequent heat treatment and resulted in grain refinement. When the alloy billet was extruded at 430°C, the deformation stored energy was diminished but was suffi- cient to induce static recrystallization during subsequent so- lution treatment. In addition, our preliminary experiment Fig. 5. Representative engineering stress–engineering strain curves of the aged bars. results indicated dynamic recrystallization grains with 1288 Int. J. Miner. Metall. Mater., Vol. 24, No. 11, Nov. 2017 k is the effect of grain boundaries on strength, and d is the high-angle grain boundaries exhibited a high grain-boundary grain diameter. Parameters σ and k are constant. According 0 y migration rate after extrusion at 430°C of 7A04 aluminum to Eq. (1), a smaller average grain size results in an alloy. Therefore, these dynamic recrystallization grains are increase in yield strength. However, the average grain size prone to consuming small recrystallization grains generated was observed to have little effect on yield strength in the during the subsequent heat treatment, which induces an ap- present work. For example, the average grain sizes of the parent increase in average grain size. Such behaviors result samples extruded at 390 and 430°C after T6 treatment are in large difference in the average grain size of the specimens 3.4 and 8.1 μm, respectively, whereas their yield strengths extruded at 390 and 430°C after T6 treatment. are approximately the same at about 618 MPa. Pre-annealing with different annealing temperatures and Ma et al. [16] found that the main strengthening times before T6 treatment influenced the microstructures of mechanism of Al–Zn–Mg–Cu alloy was precipitation the two specimens differently. For the specimen extruded at strengthening, which is mainly related to the size and 430°C, when the pre-annealing temperature was less than distribution of precipitates in the aged alloy. Fig. 6 shows 290°C or the pre-annealing time was less than 10 h, the re- the TEM images of the samples extruded at different lease of deformation stored energy was limited because of temperatures after the T6 treatment. In Fig. 6, the sizes of the low annealing temperature and short annealing time, the matrix precipitations (Mpt) distributed dispersively in which had little effect on the nucleation and growth of the specimens extruded at 390 and 430°C are approximately grains during the subsequent solution treatment. When the the same, about 5 nm, and the widths of the precipitate-free pre-annealing temperature was 290–380°C or the zone (PFZ) are also approximately the same, about 55 nm. pre-annealing time was approximately 10–20 h, the release Precipitate particles formed at grain boundaries of the of deformation stored energy increased; i.e., the driving specimen extruded at 390°C are smaller than those formed force for static recrystallization was reduced, which influ- at grain boundaries of the specimen extruded at 430°C. enced the nucleation and growth of recrystallization grains. Although the morphology of precipitate particles formed at Such behavior can suppress static recrystallization and retain grain boundaries differs, it strongly influences the corrosion a large number of small-angle grain boundaries, leading to resistance of the alloy and does not markedly affect the different grain refinements. When the pre-annealing temper- alloy’s strength. Therefore, the change in average grain size ature exceeded 380°C or the pre-annealing time was longer has no effect on the strength of the aged specimens extruded than 20 h, recrystallization occurred during pre-annealing at 390 and 430°C, i.e., their yield strength and tensile and grains apparently grew after the solid-solution treatment, strength are basically the same. resulting in a large grain size. From Table 2, the elongations to failure of the specimens For the specimen extruded at 390°C, its deformation by different heat treatments differ and many factors influence stored energy was higher than that of the specimen extruded the elongation. The main factors are the grain size, texture, at 430°C because of the low deformation temperature. Al- and Mpt of the material. However, according to Fig. 6, when though the deformation stored energy was reduced after the specimens extruded at 390 and 430°C were treated by the pre-annealing treatment, subgrain formation by polygo- T6, the size and distribution of Mpt and the width of PFZ nization was substantially carried out and the subgrains con- were approximately the same, indicating that these parameters tinued to grow under the remaining deformation stored have little effect on the plasticity of the specimens. energy after the pre-annealing treatment. Therefore, the av- Macroscopic texture analysis of the specimens was carried erage grain sizes of the aged specimens extruded at 390°C out; the ODF of different specimens is shown in Fig. 7. The with or without pre-annealing treatment were basically un- maximum intensities of the main texture components of the changed; however, only the number of small-angle grain four specimens are shown in Table 3. A-, copper-, and S-type boundaries increased. textures have been reported to be hard orientations [17−18], 4.2. Influence mechanism of pre-annealing on the me- whereas cube and Gauss textures have been reported to be chanical properties of the alloy after solid-solution soft orientations. Hard orientation hinders the dislocation treatment and aging slip, which is detrimental to the ductility of the alloy. By contrast, the hindering effect of soft orientation toward dis- In general, the yield strength of Al–Zn–Mg–Cu alloy location slip is weaker than that of hard orientation; there- should satisfy the Hall–Pech equation [15]: fore, increasing the number of soft orientations is conducive −1 σσ=+ kd (1) 0y to improving the plasticity of the alloy. where σ is the yield strength, σ is the deformation resistance, 0 When the extrusion temperature was 390°C, the average Z.H. Zhang et al., Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded … 1289 grain size of the specimens treated by pre-annealing + T6 or where G is the shear modulus and γ is the interfacial by T6 alone is approximately the same; thus, the average surface energy. According to Eq. (2), the effective shear grain size has little effect on the plasticity of the specimens. stress τ increases with decreasing grain size d; that is, grain However, a texture difference is clearly observed between refinement can improve crack propogation resistance, which the specimens subjected to different heat treatments. Al- enhances the plasticity of the alloy. Therefore, for the though the texture components of the specimens are similar, specimen extruded at 430°C, the elongation of the specimen the largest intensity of the main textures differs. The inten- treated by pre-annealing + T6 treatment is higher than that sity of all of the textures in the specimens treated by of the specimen treated by T6. pre-annealing + T6 was higher than that of the specimens In the case of the specimens extruded at 390 and 430°C treated by T6 alone; the soft orientation number (cube tex- after T6 treatment, their average grain sizes are 3.4 and 8.1 ture), in particular, increased substantially. For the sample μm, respectively. From the viewpoint of the effect of grain extruded at 390°C following by T6 treatment, the substantial size on the mechanical properties, the elongation of the improvement of elongation to failure of the specimen after specimen extruded at 430°C should be smaller than that of pre-annealing + T6 treatment is mainly ascribed to the in- the specimen extruded at 390°C. However, the soft orien- crease of soft orientation. tation (cube texture) in the specimen extruded at 430°C is For the specimen extruded at 430°C, both the texture much more than that in the specimen extruded at 390°C. component and maximum texture intensity of the specimens From the viewpoint of the effect of texture on the mechan- treated by pre-annealing + T6 are similar to those of the ical property, the elongation of the specimen extruded at specimens treated by T6. However, the grains of the speci- 430°C is higher. A quantitative comparison of the contri- men treated by pre-annealing are remarkably refined, and butions of grain size and grain orientation to plasticity of the average grain size decreases from 8.1 to 3.8 μm. Plastic the alloy is difficult. However, on the basis of the afore- deformation and crack formation of the alloy are related to mentioned experimental results and analyses, in the grain the dislocation motion. According to dislocation piling-up size range studied in this work, the contribution of grain theory [19], effective shear stress (τ ) which induces crack orientation to plasticity is larger than that of the grain size, propagation can be described as resulting in the specimen extruded at 430°C, which has a 2Gγ  larger size grain and more soft orientation textures, exhi- τ = (2) e biting better plasticity.  Fig. 6. TEM images of the specimens extruded at various temperatures after T6 treatment (GBP: grain boundary precipitation; Mpt: the matrix precipitation; PFZ: precipitate-free zone): (a, b) extruded at 390°C; (c, d) extruded at 430°C. 1290 Int. J. Miner. Metall. Mater., Vol. 24, No. 11, Nov. 2017 (a) (b) (c) (d) (e) Fig. 7. Orientation distribution function (ODF) of the specimens extruded at different temperatures after different heat treatments (Φ, ϕ , and ϕ : Euler angles; Rt-C: rotational cubic texture; C: cubic texture; A: A-type texture; B: brass-type texture; G: gauss tex- 1 2 ture; Cu: copper-type texture; Rt-Cu: rotational copper-type texture; G/B: transition texture of gauss texture to brass-type texture; Rt-G: rotational gauss texture; S: S-type texture; numbers in the figures: intensity of texture; curves: texture contour): (a) 390°C ex- trusion + T6 treatment; (b) 430°C extrusion + T6 treatment; (c) 390°C extrusion + pre-annealing + T6 treatment; (d) 430°C extru- sion + pre-annealing + T6 treatment; (e) typical components in an ODF. Z.H. Zhang et al., Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded … 1291 Table 3. Maximum intensity of the main texture component of the specimens extruded at different temperatures after different heat treatments Main texture component type Extrusion temperature / °C Heat treatment A Copper S Cube Gauss 390 T6 22 17 10 9 18 430 T6 29 18 17 26 21 390 Pre-annealing + T6 21 18 16 27 21 430 Pre-annealing + T6 20 17 16 23 19 The specimens extruded at 390 and 430°C were treated author(s) and the source, provide a link to the Creative by pre-annealing + T6, and their elongations are basically Commons license, and indicate if changes were made. the same because their grain sizes, texture component, and maximum intensity of the textures are basically the same. References 5. Conclusions [1] W.R. Hou, Z.H. Zhang, J.X. Xie, Q.M. Ma, and H.T. Gai, Temperature inhomogeneity on cross section of Al alloy hol- low profile based on reverse point tracking method, Chin. J. 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Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded Al–Zn–Mg–Cu alloy bars

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Materials Science; Materials Science, general; Metallic Materials; Characterization and Evaluation of Materials; Ceramics, Glass, Composites, Natural Materials; Surfaces and Interfaces, Thin Films; Tribology, Corrosion and Coatings
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Abstract

International Journal of Minerals, Metallurgy and Materials Volume 24, Number 11, November 2017, Page 1284 DOI: 10.1007/s12613-017-1521-3 Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded Al–Zn–Mg–Cu alloy bars 1) 2) 1) 1) Zhi-hao Zhang , Jie Xue , Yan-bin Jiang , and Feng Jin 1) Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China 2) Beijing Hangxing Machine Manufacture Co., Ltd, Beijing 100083, China (Received: 10 May 2017; revised: 3 July 2017; accepted: 7 July 2017) Abstract: Taking extruded Al–Zn–Mg–Cu alloy (7A04 alloy) bars as the research object, the effect and mechanism of pre-annealing treatments on the microstructure and mechanical properties of the aged alloy bars were investigated. The results show that a pre-annealing treatment at 350°C for 15 h before a T6 treatment substantially reduced the sensitivity of the microstructure and mechanical properties of the extruded 7A04 aluminum alloy specimens toward the extrusion temperature. The average grain sizes of the specimens extruded at 390 and 430°C after T6 treatment were 3.4 and 8.1 μm, respectively, and their elongations to failure were 7.0% and 9.2%, respectively. However, after pre-annealing + T6 treatment, the differences in both the grain sizes and the elongations of the specimens became small, i.e., their average grain sizes were 3.2 and 3.8 μm and their elongations were 12.0% and 13.3%, respectively. For the specimens extruded at the same temperature, pre-annealing treatment obviously improved the plasticity of the alloy, which is attributed to an increase in soft texture or to grain refinement in the specimens as a result of the pre-annealing + T6 treatment. Keywords: aluminum alloy; hot extrusion; pre-annealing treatment; microstructure; mechanical properties 1. Introduction truded products and reduce the overall service performance of equipment. The applications of aluminum alloy products in the fields Isothermal extrusion, which can be achieved via of rail transportation and aerospace have led to more strin- closed-loop speed control [6] or billet gradient temperature gent requirements with respect to mechanical properties. In heating (cooling) [7], can effectively overcome the problem addition to the factors such as the alloy material, shape and of nonuniform temperature distribution along the length di- size precision, and the surface quality, the homogeneity and rection of the extruded product. For the nonuniform temper- consistency of alloys’ microstructure and properties are also ature distribution in the cross section, the current main solu- keys to ensuring high performance of formed products. tion is to adjust the heat flow during extrusion (e.g., plastic Extrusion is an important forming method for wrought deformation heat, friction heat, and heat transfer between aluminum alloys. Because of the characteristics of extrusion the billet and mold) [1]. technology, an uneven temperature distribution inevitably Some studies [8−11] have indicated that, when a de- exists in the extruded products along the length direction formed aluminum alloy was annealed at low temperatures and in the cross section. For example, the maximum tem- (i.e., pre-annealing treatment), the stored deformation ener- perature difference in the cross section of a large profile can gy was partly consumed and the driving force of recrystalli- reach 60°C [1], leading to inhomogeneities in both the mi- zation nucleation and growth was reduced during the sub- crostructure and the mechanical properties of the products sequent solution treatment, which affected the grain size of along the length direction and in the cross section [2−5]; the alloy. Therefore, controlling the microstructure and me- these inhomogeneities seriously affect the service life of ex- chanical properties of an aluminum alloy profile via a Corresponding author: Yan-bin Jiang Email: [email protected] © The Author(s) 2017. This article is published with open access at link.springer.com Z.H. Zhang et al., Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded … 1285 pre-annealing treatment is theoretically feasible. was obtained by the linear intercept method. The degree of In this paper, taking 7A04 aluminum alloy bars extruded recrystallization and the grain orientation were detected by at different temperatures as the research object, we investi- the electron backscattering diffraction (EBSD) technique. gated the effects of pre-annealing on the microstructure and EBSD specimens were prepared by electrolytic polishing at mechanical properties of the extruded bars after solid solu- a voltage of 25 V in 5wt% perchloric acid at −25°C. The tion and aging treatments (i.e., T6). The proper size and distribution of precipitates were observed by trans- pre-annealing conditions that result in microstructure and mission electron microscopy (TEM). mechanical properties as similar as possible for bars ex- The {111}, {200}, {220}, and {113} incomplete pole truded at different temperatures can be obtained to reduce figures for each specimen were constructed from the results the sensitivity of the microstructure and mechanical proper- obtained using a Brucker D8 Discovery texture tester. The ties of the alloy to the extrusion temperature. Finally, this orientation distribution function (ODF) of the expansion work provided a reference for improving homogeneities in coefficient l = 22 was obtained using the method of both the microstructure and the mechanical properties of the spherical harmonic series expansion; the “true ODF” (l = 23) extruded profiles. was obtained by texture analysis to eliminate the influence of a ghost peak [13−14]. 2. Experimental The tensile specimens with a diameter of 3 mm and a gauge length of 15 mm were machined with their axes pa- The chemical composition of the 7A04 aluminum alloy rallel to the extrusion direction, consistent with the method specified in standard GB/T228.1—2010. Tensile tests were billets used in the present work is shown in Table 1. The billets with a size of φ40 mm × 60 mm were subjected to a carried out at room temperature on an MTS-810 testing −3 −1 machine at a strain rate of 1 × 10 homogenization treatment at 460°C for 24 h [12] in an s ; tensile deformation SGM-M30/12 box-type resistance furnace and were then was measured using an extensometer. Three samples cor- immediately quenched in water at room temperature. The responding to each condition were tested, and the average grain size of the billets after the homogenization treatment values are reported as the test results. was approximately 28.2 μm. The homogenized billets were then extruded into round bars with a diameter of 8.4 mm; 3. Results the extrusion ratio was 23. Fig. 1 shows the average grain sizes of the specimens ex- Table 1. Chemical composition of the 7A04 aluminum alloy truded at different extrusion temperatures after the T6 treat- wt% ment. It reveals that the average grain size of the specimens Zn Mg Cu Cr Fe Si Mn Al in the cross section first increases and then decreases with 5.15 2.30 1.70 0.21 0.30 0.09 0.23 Bal. increasing extrusion temperature; i.e., the sensitivity of the aged microstructure to the extrusion temperature differs. In To reduce the influence of the difference in microstruc- addition, when the temperature of the extruded 7A04 alu- ture between the front and end of the bars on the experi- minum alloy along the extrusion direction changes from 390 mental results, the specimens were collected from the to 500°C, the microstructure of the extruded product after middle section of the bars (i.e., the stable extrusion stage). T6 treatment is inhomogeneous. In particular, when the ex- The specimens were divided into two groups. One group trusion temperature is 390 and 430°C, the difference in the was subjected to the T6 process (475°C for 1 h + 120°C for average grain size of the two specimens (3.4 and 8.1 μm, 24 h) [12]. The other group was treated by being respectively) is largest. Therefore, the specimens extruded at pre-annealed (the annealing temperature ranged from 100 to 390 and 430°C were used as the main research objects in the 410°C, and the holding time ranged from 0.5 to 20 h) and following experiments in this work. then subjected to the T6 process. The extruded specimens were pre-annealed at 100–410°C The optical micrographs of the aged specimens (the ob- for 15 h and then subjected to T6 treatment; the average servation position was the center of the specimens) were grain size of the T6-treated specimens is shown in Fig. 2, observed using a Zeiss Axiovert 200 MAT optical micro- which reveals that the average grain size of the specimen scope. The specimens for optical microscopy were polished extruded at 390°C changes little with the pre-annealing and etched in Graff Sargent agent (1 mL HF + 16 mL HNO + temperature, ranging from 3.2 to 3.4 μm. In the case of the 3 g CrO + 83 mL water). The grain size of the specimens specimen extruded at 430°C, when the pre-annealing tem- 1286 Int. J. Miner. Metall. Mater., Vol. 24, No. 11, Nov. 2017 perature was lower than 290°C, the average grain size was in pre-annealing time, the grain size increases conversely. approximately 8.0 μm. When the pre-annealing temperature Therefore, when the pre-annealing time is 15 h, the differ- was increased from 290 to 350°C, the average grain size de- ence in the average grain size of the specimens extruded at creased sharply, reaching a minimum of 3.8 μm at 350°C. 390 and 430°C is least, and the average grain sizes are 3.3 When the pre-annealing temperature was increased further, and 3.8 μm, respectively, indicating that the suitable the average grain size increased. When the pre-annealing pre-annealing time is 15 h. temperature exceeded 410°C, the average grain size was approximately the same as the initial grain size. Therefore, the suitable pre-annealing temperature is 350°C, which in- duces the least difference in the average grain size of the aged specimens extruded at 390 and 430°C. Fig. 3. Influence of pre-annealing time on the average grain size of aged specimens (the pre-annealing temperature is 350°C). In summary, the microstructure of the extruded specimen after T6 treatment is sensitive to the extrusion temperature, especially in the case of specimens extruded at 390 or 430°C. Fig. 1. Influence of extrusion temperature on the average However, the pre-annealing step (the suitable pre-annealing grain size of the specimens after T6 treatment. process is 350°C for 15 h) before the T6 treatment can sub- stantially reduce the microstructure sensitivity of the speci- men to the extrusion temperature. To further analyze the microstructure difference of the alloy after T6 treatment and after the pre-annealing (350°C for 15 h) + T6 treatment, we characterized the correspond- ing specimens by EBSD, as shown in Fig. 4. The proportion of low-angle grain boundaries was increased by the pre-annealing treatment such that the grain of the specimens extruded at different temperatures was refined to different extents. The recrystallization fraction was analyzed using the EBSD post-processing software HKL Channel5. The results show that the recrystallization fractions of the speci- Fig. 2. Influence of pre-annealing temperature on the average mens extruded at 390 and 430°C after T6 treatment are grain size of the aged specimens (the pre-annealing time is 15 h). 38.1% and 43.8%, respectively, whereas those after pre-annealing + T6 treatment are dramatically lower: 13.8% The average grain size of the extruded specimens treated and 13.0%, respectively. by pre-annealing for different times and then by T6 treat- ment is shown in Fig. 3. The average grain size of the spe- Table 2 shows the room-temperature mechanical properties of the specimens extruded at different temperatures after T6 cimen extruded at 390°C is independent of the pre-annealing time. For the specimen extruded at 430°C, treatment or after pre-annealing + T6 treatment; the repre- sentative engineering stress–engineering strain curves of the when the pre-annealing time is approximately 0.5–10 h, the average grain size also remains unchanged. However, when aged bars are given in Fig. 5. The tensile strengths (R ) and the pre-annealing time is greater than 10 h, the average grain yield strengths (R ) of the specimens extruded at 390 and p0.2 size decreases gradually and is approximately 3.8 μm in the 430°C after T6 treatment are all respectively similar; however, case of a pre-annealing time of 15 h. With a further increase the elongations (A) are 7.0% and 9.2%, respectively (i.e., the Z.H. Zhang et al., Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded … 1287 difference in elongation is large). In the case of the specimens cimens after pre-annealing + T6 treatment are obviously im- extruded at 390 and 430°C after pre-annealing + T6 treatment, proved and the difference between the two specimens is also although their strengths are basically unchanged, their elonga- smaller compared with that between the specimens after T6 tions increase substantially to 12.0% and 13.3%, respectively, treatment. Therefore, the pre-annealing treatment can, to some and the difference in elongations decreases. Meanwhile, as extent, reduce the sensitivity of the mechanical properties of evident from Fig. 5, the elongations of the two kinds of spe- the specimen to the extrusion temperature. Fig. 4. EBSD images of the specimens extruded at different temperatures after different heat treatments: (a) 390°C extrusion + T6 treatment; (b) 430°C extrusion + T6 treatment; (c) 390°C extrusion + pre-annealing + T6 treatment; (d) 430°C extrusion + pre-annealing + T6 treatment. Table 2. Mechanical properties of the specimens extruded at different temperatures after different heat treatments Extrusion temperature / °C Heat treatment R / MPa R / MPa A / % m p0.2 +8.2 +7.8 +0.5 666.4 618.0 7.0 390 T6 −7.7 −6.2 −0.4 +5.6 +10.4 +0.6 678.1 618.3 9.2 430 T6 −7.8 −9.4 −0.5 +4.5 +6.2 +0.3 670.7 625.3 12.0 390 Pre-annealing + T6 −3.1 −4.3 −0.1 +6.3 +3.8 +0.2 682.7 623.7 13.3 430 Pre-annealing + T6 −5.1 −4.6 −0.3 4. Discussion 4.1. Influence mechanism of pre-annealing on the grain size of the alloy after solid-solution treatment and aging When the 7A04 aluminum alloy billet was extruded at 390°C, high deformation stored energy was produced, which provided a large driving force for static recrystalliza- tion during subsequent heat treatment and resulted in grain refinement. When the alloy billet was extruded at 430°C, the deformation stored energy was diminished but was suffi- cient to induce static recrystallization during subsequent so- lution treatment. In addition, our preliminary experiment Fig. 5. Representative engineering stress–engineering strain curves of the aged bars. results indicated dynamic recrystallization grains with 1288 Int. J. Miner. Metall. Mater., Vol. 24, No. 11, Nov. 2017 k is the effect of grain boundaries on strength, and d is the high-angle grain boundaries exhibited a high grain-boundary grain diameter. Parameters σ and k are constant. According 0 y migration rate after extrusion at 430°C of 7A04 aluminum to Eq. (1), a smaller average grain size results in an alloy. Therefore, these dynamic recrystallization grains are increase in yield strength. However, the average grain size prone to consuming small recrystallization grains generated was observed to have little effect on yield strength in the during the subsequent heat treatment, which induces an ap- present work. For example, the average grain sizes of the parent increase in average grain size. Such behaviors result samples extruded at 390 and 430°C after T6 treatment are in large difference in the average grain size of the specimens 3.4 and 8.1 μm, respectively, whereas their yield strengths extruded at 390 and 430°C after T6 treatment. are approximately the same at about 618 MPa. Pre-annealing with different annealing temperatures and Ma et al. [16] found that the main strengthening times before T6 treatment influenced the microstructures of mechanism of Al–Zn–Mg–Cu alloy was precipitation the two specimens differently. For the specimen extruded at strengthening, which is mainly related to the size and 430°C, when the pre-annealing temperature was less than distribution of precipitates in the aged alloy. Fig. 6 shows 290°C or the pre-annealing time was less than 10 h, the re- the TEM images of the samples extruded at different lease of deformation stored energy was limited because of temperatures after the T6 treatment. In Fig. 6, the sizes of the low annealing temperature and short annealing time, the matrix precipitations (Mpt) distributed dispersively in which had little effect on the nucleation and growth of the specimens extruded at 390 and 430°C are approximately grains during the subsequent solution treatment. When the the same, about 5 nm, and the widths of the precipitate-free pre-annealing temperature was 290–380°C or the zone (PFZ) are also approximately the same, about 55 nm. pre-annealing time was approximately 10–20 h, the release Precipitate particles formed at grain boundaries of the of deformation stored energy increased; i.e., the driving specimen extruded at 390°C are smaller than those formed force for static recrystallization was reduced, which influ- at grain boundaries of the specimen extruded at 430°C. enced the nucleation and growth of recrystallization grains. Although the morphology of precipitate particles formed at Such behavior can suppress static recrystallization and retain grain boundaries differs, it strongly influences the corrosion a large number of small-angle grain boundaries, leading to resistance of the alloy and does not markedly affect the different grain refinements. When the pre-annealing temper- alloy’s strength. Therefore, the change in average grain size ature exceeded 380°C or the pre-annealing time was longer has no effect on the strength of the aged specimens extruded than 20 h, recrystallization occurred during pre-annealing at 390 and 430°C, i.e., their yield strength and tensile and grains apparently grew after the solid-solution treatment, strength are basically the same. resulting in a large grain size. From Table 2, the elongations to failure of the specimens For the specimen extruded at 390°C, its deformation by different heat treatments differ and many factors influence stored energy was higher than that of the specimen extruded the elongation. The main factors are the grain size, texture, at 430°C because of the low deformation temperature. Al- and Mpt of the material. However, according to Fig. 6, when though the deformation stored energy was reduced after the specimens extruded at 390 and 430°C were treated by the pre-annealing treatment, subgrain formation by polygo- T6, the size and distribution of Mpt and the width of PFZ nization was substantially carried out and the subgrains con- were approximately the same, indicating that these parameters tinued to grow under the remaining deformation stored have little effect on the plasticity of the specimens. energy after the pre-annealing treatment. Therefore, the av- Macroscopic texture analysis of the specimens was carried erage grain sizes of the aged specimens extruded at 390°C out; the ODF of different specimens is shown in Fig. 7. The with or without pre-annealing treatment were basically un- maximum intensities of the main texture components of the changed; however, only the number of small-angle grain four specimens are shown in Table 3. A-, copper-, and S-type boundaries increased. textures have been reported to be hard orientations [17−18], 4.2. Influence mechanism of pre-annealing on the me- whereas cube and Gauss textures have been reported to be chanical properties of the alloy after solid-solution soft orientations. Hard orientation hinders the dislocation treatment and aging slip, which is detrimental to the ductility of the alloy. By contrast, the hindering effect of soft orientation toward dis- In general, the yield strength of Al–Zn–Mg–Cu alloy location slip is weaker than that of hard orientation; there- should satisfy the Hall–Pech equation [15]: fore, increasing the number of soft orientations is conducive −1 σσ=+ kd (1) 0y to improving the plasticity of the alloy. where σ is the yield strength, σ is the deformation resistance, 0 When the extrusion temperature was 390°C, the average Z.H. Zhang et al., Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded … 1289 grain size of the specimens treated by pre-annealing + T6 or where G is the shear modulus and γ is the interfacial by T6 alone is approximately the same; thus, the average surface energy. According to Eq. (2), the effective shear grain size has little effect on the plasticity of the specimens. stress τ increases with decreasing grain size d; that is, grain However, a texture difference is clearly observed between refinement can improve crack propogation resistance, which the specimens subjected to different heat treatments. Al- enhances the plasticity of the alloy. Therefore, for the though the texture components of the specimens are similar, specimen extruded at 430°C, the elongation of the specimen the largest intensity of the main textures differs. The inten- treated by pre-annealing + T6 treatment is higher than that sity of all of the textures in the specimens treated by of the specimen treated by T6. pre-annealing + T6 was higher than that of the specimens In the case of the specimens extruded at 390 and 430°C treated by T6 alone; the soft orientation number (cube tex- after T6 treatment, their average grain sizes are 3.4 and 8.1 ture), in particular, increased substantially. For the sample μm, respectively. From the viewpoint of the effect of grain extruded at 390°C following by T6 treatment, the substantial size on the mechanical properties, the elongation of the improvement of elongation to failure of the specimen after specimen extruded at 430°C should be smaller than that of pre-annealing + T6 treatment is mainly ascribed to the in- the specimen extruded at 390°C. However, the soft orien- crease of soft orientation. tation (cube texture) in the specimen extruded at 430°C is For the specimen extruded at 430°C, both the texture much more than that in the specimen extruded at 390°C. component and maximum texture intensity of the specimens From the viewpoint of the effect of texture on the mechan- treated by pre-annealing + T6 are similar to those of the ical property, the elongation of the specimen extruded at specimens treated by T6. However, the grains of the speci- 430°C is higher. A quantitative comparison of the contri- men treated by pre-annealing are remarkably refined, and butions of grain size and grain orientation to plasticity of the average grain size decreases from 8.1 to 3.8 μm. Plastic the alloy is difficult. However, on the basis of the afore- deformation and crack formation of the alloy are related to mentioned experimental results and analyses, in the grain the dislocation motion. According to dislocation piling-up size range studied in this work, the contribution of grain theory [19], effective shear stress (τ ) which induces crack orientation to plasticity is larger than that of the grain size, propagation can be described as resulting in the specimen extruded at 430°C, which has a 2Gγ  larger size grain and more soft orientation textures, exhi- τ = (2) e biting better plasticity.  Fig. 6. TEM images of the specimens extruded at various temperatures after T6 treatment (GBP: grain boundary precipitation; Mpt: the matrix precipitation; PFZ: precipitate-free zone): (a, b) extruded at 390°C; (c, d) extruded at 430°C. 1290 Int. J. Miner. Metall. Mater., Vol. 24, No. 11, Nov. 2017 (a) (b) (c) (d) (e) Fig. 7. Orientation distribution function (ODF) of the specimens extruded at different temperatures after different heat treatments (Φ, ϕ , and ϕ : Euler angles; Rt-C: rotational cubic texture; C: cubic texture; A: A-type texture; B: brass-type texture; G: gauss tex- 1 2 ture; Cu: copper-type texture; Rt-Cu: rotational copper-type texture; G/B: transition texture of gauss texture to brass-type texture; Rt-G: rotational gauss texture; S: S-type texture; numbers in the figures: intensity of texture; curves: texture contour): (a) 390°C ex- trusion + T6 treatment; (b) 430°C extrusion + T6 treatment; (c) 390°C extrusion + pre-annealing + T6 treatment; (d) 430°C extru- sion + pre-annealing + T6 treatment; (e) typical components in an ODF. Z.H. Zhang et al., Effect of pre-annealing treatment on the microstructure and mechanical properties of extruded … 1291 Table 3. Maximum intensity of the main texture component of the specimens extruded at different temperatures after different heat treatments Main texture component type Extrusion temperature / °C Heat treatment A Copper S Cube Gauss 390 T6 22 17 10 9 18 430 T6 29 18 17 26 21 390 Pre-annealing + T6 21 18 16 27 21 430 Pre-annealing + T6 20 17 16 23 19 The specimens extruded at 390 and 430°C were treated author(s) and the source, provide a link to the Creative by pre-annealing + T6, and their elongations are basically Commons license, and indicate if changes were made. the same because their grain sizes, texture component, and maximum intensity of the textures are basically the same. References 5. Conclusions [1] W.R. Hou, Z.H. Zhang, J.X. Xie, Q.M. Ma, and H.T. Gai, Temperature inhomogeneity on cross section of Al alloy hol- low profile based on reverse point tracking method, Chin. J. 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