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Effects of solution treatment on the microstructure and mechanical properties of naturally aged EN AW 2024 Al alloy sheet
Journal of Alloys and Compounds 824 (2020) 153943
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of solution treatment on the microstructure and mechanical properties of naturally aged EN AW 2024 Al alloy sheet Mengchao Liang , Liang Chen *, Guoqun Zhao , Yunyue Guo Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong, 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2019 Received in revised form 14 January 2020 Accepted 19 January 2020 Available online 23 January 2020
The solution experiments on cold rolled 2024 Al sheet with a 1070 Al coating were carried out at various temperatures and holding time. The effects of solution parameters on the microstructure evolution and mechanical properties were studied. The results showed that the alloying elements diffused from the 2024 Al matrix to the 1070 Al coating, following the peak-valley pattern. The cold rolled 2024 Al sheet mainly have the second phases of Al2Cu, S (Al2CuMg), Si, Mg2Si, and the impurities of AlFeMnSi, AlCuFeMnSi, AlCuFeMn, and the amount of them was reduced with increasing solution temperature or holding time. T (Al20Cu2Mn3) had precipitated before aging, and only the CueMg clusters were formed during the natural aging, which should be the main strengthening mechanism. The grain structure was elongated during cold rolling, while the static recrystallization and grain growth occurred during the solution treatment. The ultimate tensile strength, yield strength and elongation were gradually enhanced with increasing the solution temperature to 510 C. When the solution temperature is further increased to 530 C, the elongation was obviously decreased due to the over-burning. Moreover, it was found that the solution temperature had much stronger effects on the mechanical properties than that of the holding time. © 2020 Published by Elsevier B.V.
Keywords: 2024 Al Solution Natural aging Microstructure Mechanical properties
1. Introduction EN AW 2024 alloy is a typical 2xxx series (AleCueMg) wrought Al alloy, and it has the advantages of low density, high specific strength, excellent fatigue resistance and good machinability [1e3]. 2024 Al sheet fabricated through hot and cold rolling has been widely applied in the field of aerospace, such as the aircraft skin. Since 2024 Al is heat treatable, the deformed microstructure can be modified during the subsequent heat treatment processes to further improve the mechanical properties. The solution and aging are the most common heat treatments for Al alloys. The soluble phases can dissolve into the Al matrix during the solution stage, and the supersaturated solid solution is formed due to the rapid quenching. Then, various and large amount of strengthening phases precipitate during the artificial or natural aging. The parameters of both solution and aging treatments play important roles on the final mechanical properties of the Al alloys, and thus it has attracted much attention from the researchers [4e6].
* Corresponding author. E-mail address: [email protected] (L. Chen). https://doi.org/10.1016/j.jallcom.2020.153943 0925-8388/© 2020 Published by Elsevier B.V.
The research works have been carried out on solution and aging treatments of Al alloys. The microstructure characteristics and the response of mechanical properties were reported in details. Gubicza [7] performed artificial aging at various temperatures using the AleZneMgeZr alloy processed by equal channel angular pressing (ECAP), and the results indicated that 170 C aging led to a lower hardness than 120 C aging. Li et al. [8] investigated the effects of solution and aging on hot and cold rolled AleZneMgeCu alloy, and it was concluded that the strength was obviously enhanced due to the combined effects of dislocations, textures, grain size and precipitations. Sun et al. [9] conducted the solution plus double aging on 7075 Al alloys fabricated by selective laser melting, and some typical S (Al2CuMg) and q (Al2Cu) phases precipitated at the grain boundary, resulting in an obvious increase of hardness. Guo et al. [10] studied the solute clustering in AleMgeSieCueZn alloys during aging process, and it was found that the addition of Zn facilitated the formation of fine co-clusters containing Mg, Si, Cu and Zn atoms, which caused the enhanced hardening response. Liu et al. [11] found that the individual onedimensional GPB1 units, GPII zones and q’ precipitations were formed during two-stage hardening peaks of an Al-5.0Cu-0.4 Mg
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Table 1 Chemical compositions of the as-received 2024 Al alloy. Element
Cu
Mg
Mn
Fe
Si
Zn
Ti
Al
wt.%
4.1
1.48
0.5
0.1
0.09
0.03
0.04
Bal.
(wt.%) alloy. Zuiko et al. [12] proposed that the pre-straining could promote the precipitation during aging of an AleCueMg alloy, and the yield stress could be obviously increased by 30%. Meyruey et al. [13] investigated the over-ageing behavior of an AleMgeSi alloy within the temperature range between 100 C and 350 C, and it was claimed that the over-ageing was accompanied by the progressive replacement of b00 precipitate by a succession of different types of semi-coherent precipitates. Till now, the precipitation behavior during aging process and the effects of aging parameters on mechanical properties of various Al alloys have been studied. However, as the prior step, the solution treatment was rarely reported. 2024 Al alloy has high content of alloying elements, such as 3.8e4.9 wt% Cu and 1.2e1.8 wt% Mg. Large amount of intermetallic compounds such as S, q, AlFeSiMnCu, Al7Cu2Fe, and Al3(Cu, Fe, Mn) can be found [14e16], and the complex microstructure evolution should occur during the solution treatment. Firstly, the supersaturated degree of solid solution affects the distribution, morphology and size of the precipitations during the aging treatment [17,18]. Secondly, the static recrystallization and grain growth easily occur during solution treatment [19]. Both of these two factors greatly affect the final mechanical properties of Al alloys. On the other hand, the commercial 2024 Al alloy sheet usually follows the production route of hot rolling, cold rolling, solution and natural aging. Since only the natural aging is performed at room temperature, the control of solution condition becomes much more important for 2024 Al alloy sheet. Hence, the cold rolled 2024 Al sheet with a 1070 Al coating was solution treated at various temperatures and holding time. The microstructure features such as the diffusion layer, second phase, precipitate and grain structure were well studied. Moreover, the tensile properties and the hardness of the naturally aged specimens were tested. The effects of solution parameters on the microstructure evolution and the response of mechanical properties were discussed. The main purpose of this study is to clarify the influence of solution temperature and holding time on the final mechanical properties of the naturally aged specimens, which is important for improving the quality of cold rolled 2024 Al sheet. 2. Experimental procedures The cold rolled 2024 Al sheet with a thickness of 1.5 mm was received from Nanshan Aluminum Co., Ltd. The chemical compositions of the 2024 Al sheet are listed in Table 1. In order to further enhance the corrosion resistance, a 1070 Al (99.7 wt.% Al) layer with the thickness of around 65 mm was coated on the surfaces of the 2024 Al sheet. For simplification, the as-received sheet is called 2024 Al sheet in this study. The fabrication route of the 2024 Al sheet is described as follows. Firstly, both surfaces of the as-cast 2024 Al slab was milled to a thickness of 420 mm. Two 1070 Al
sheets with a thickness of 20 mm were put on the upper and lower sides of the 2024 Al slab to make a sandwich structure, and the spot welding was used to fix them. Then, the sandwich billet was preheated to 450 C, and it was hot rolled to a thickness of 4 mm after 20 passes. Finally, the sheet was cold rolled to a thickness of 1.5 mm after 3 passes. The solution treatment was performed in a tube furnace with an argon atmosphere. When the preset temperature was reached, the specimen was put inside the furnace to initiate the solution. Since the main objective of the present study is to clarify the effects of solution parameters on the microstructure and mechanical properties of naturally aged 2024 Al sheet, the solution was performed within the temperature range of 460e530 C and the holding time range of 10e120 min. The detailed parameters of the experiments are listed in Table 2. After solution, the specimen was immediately quenched into cooled water. The mechanical properties of the 2024 Al sheet varied significantly during the initial stage of natural aging, while a stable state could be achieved after 96 h. For example, the variation of hardness after solution treated at 495 C for 60 min is shown in Fig. 1. As is seen, the hardness varies slightly after 96 h aging time, which indicates a stable state. Hence, all the mechanical tests were performed after 96 h natural aging. The location and dimension of the specimens are schematically shown in Fig. 2, where the length, width and thickness directions of the sheet were designated as rolling direction (RD), transverse direction (TD) and normal direction (ND), respectively. The specimens for tensile tests were designed to have a gauge length of 25 mm. The tensile tests were performed at room temperature with a stretching speed of 1.5 mm/ min. The Vickers hardness of the specimens was measured by using a load of 500 g and dwelling for 15 s. The microstructure was observed by means of optical microscope (OM), scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS), electron back-scattered diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM). The OM specimens were grounded, polished and etched by using the Keller’s reagent of 1 ml HF, 1.5 ml HCl, 2.5 ml HNO3 and 95 ml H2O. The EBSD specimens were electropolished at 26 V for 10 s in a solution of 10 ml nitric acid and 90 ml methanol. The HRTEM specimens were firstly grounded to a thickness of 50 mm, and then the electro-polishing was performed with an electrolyte of 30% HNO3 in methanol. In order to identify the phases before and after solution, the X-ray diffraction (XRD) with Cu-Ka radiation was conducted using the parameters of 50 kV and 150 mA. The differential thermal analysis reference (DSC) was performed with the heating speed of 10 C/min, and the pure Al was used as the reference. 3. Results and discussion 3.1. Formation of the diffusion layer It has been mentioned that the investigated 2024 Al sheet has a 1070 Al coating with the thickness of around 65 mm for the purpose of improving corrosion resistance. During solution treatment, holding at high temperature should facilitate the element diffusion. Hence, the diffusion behavior was firstly studied and the results are
Table 2 The temperature and holding time of the solution experiments. No.
Temperature (oC)
Holding time (min)
No.
Temperature (oC)
Holding time (min)
No.
Temperature (oC)
Holding time (min)
1# 2# 3# 4# 5#
460 480 495 510 530
10 10 10 10 10
6# 7# 8# 9# 10#
460 480 495 510 530
60 60 60 60 60
11# 12# 13# 14# 15#
460 480 495 510 530
120 120 120 120 120
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that of the lattice. After long time diffusion, the grain boundary diffusion is far ahead of the lattice, and the peak is observed. Contrarily, the low kinetics of lattice diffusion leads to the formation of valley. 3.2. Secondary phases
Fig. 1. Variation of hardness with aging time after solution treated at 495 C for 60 min.
Fig. 2. Location and dimension of the specimens for microstructure observation and mechanical tests. (Unit: mm).
shown in Fig. 3. It is obvious that a diffusion layer was formed, and its depth was increased with the increase of solution temperature or holding time. When the temperature is as low as 460 C, only the narrow diffusion layer appeared, even with long holding time of 120 min. However, when the temperature is 530 C and holding time is 120 min, the diffusion layer had covered the whole 1070 Al coating. The diffusion depth of all specimens is summarized in Fig. 4. As is seen, both the temperature or holding time significantly affect the diffusion depth. Generally, the diffusion from 2024 Al matrix to pure Al coating is harmful. For example, the diffusion of Cu can reduce the difference of electric potential between the matrix and the coating, which results in a decline of the corrosion resistance. Although the diffusion behavior cannot be avoided, there should be a limit depth of the diffusion layer, according to the application fields of the 2024 Al sheet. In the present study, the 1/2 thickness of the pure Al coating is defined as the diffusion limit, as indicated in Fig. 4, from which some un-qualified parameters of solution treatment should be avoided. On the other hand, it is seen from Fig. 3 that the frontier of the diffusion layer is not flat, which indicates an non-uniform diffusion. As previously reported by Dorward et al. [20], this kind of phenomenon is an evidence of peak-valley pattern diffusion, which is schematically drawn in Fig. 5. The peaks and valleys are representative of the diffusion of grain boundary and lattice, respectively. The diffusion kinetics of grain boundary is much higher than
The 2024 Al alloy has high content of alloying elements, and various second phases usually exist. Since the chemical compositions of the second phases cannot be identified only based on the particle morphology, the EDS was performed on the cold rolled and solution treated 2024 Al sheet. The EDS mapping results of the cold rolled specimen is shown in Fig. 6. As is seen, the main strengthening second phase is AlCuMg, which is a mixture of S (Al2CuMg) and CuAl2. Moreover, the phases of Si, MgSi and AlFeMnSi can also be observed. After carefully check the whole specimen, some impurities are found, as is seen from Fig. 7. The AlCuFeMnSi shown in Fig. 7(a) is a composite phase with a smooth edge, and it consists of bright and dark areas. According to the EDS mapping results, the chemical compositions of the bright and dark areas are obviously different. The Cu element mainly concentrates in the bright area, and the Mn and Si concentrate in the dark area, while the Fe element distribute uniformly in the whole AlCuFeMnSi particle. The EDS spot results of points 1 and 2 listed in Table 3 also confirm this kind of phenomenon. Fig. 7(b) shows the composite phase of AlCuFeMn, which also consists of the bright and dark areas. However, the difference is that the dark areas distribute dispersedly inside the bright matrix. Moreover, the edge of AlCuFeMn is rough, and some cracks appear inside the particle, which indicate that the particle is brittle. Based on the EDS results in Fig. 7(b) and Table 3, Cu, Fe and Si have the uniform distribution, while the Mn concentrates in the dark area. Fig. 7(c) shows a particle of AlFeMnSi phase with flat edge, and all of the elements distribute uniformly. It is noted that some cracks appear inside or nearby the particle. This kind of coarse AlFeMnSi particle should be brittle, and it is harmful on the mechanical properties of the Al sheet [21,22]. The distribution of second phase in the solution treated specimens is shown in Fig. 8. It can be seen that the amount of the dispersed granular and rod-like particles had been significantly reduced, while the particles with large size were still remained. Additionally, it is obvious that the overall amount of second phase decreased with the increase of temperature or holding time. However, as shown in Fig. 8(e) and (g), when the temperature or holding time was further increased to 530 C or 120 min, the decreasing of second phase was not obvious. The chemical compositions of the remained second phases are indicated by different colors in the right side images of Fig. 8. It is observed from Fig. 8(b) that the dispersed granular AlCuMg almost disappeared, and some rod-like and coarse AlCuMg particles were remained in 6# specimen solution treated at 460 C for 60 min. With increasing temperature to 495 C or holding time to 120 min, the number of rodlike AlCuMg was greatly reduced, and the size of coarse AlCuMg became smaller, as shown in Fig. 8(d) and (h). With the further increase of temperature to 530 C, almost all of the AlCuMg had dissolved into the matrix, as shown in Fig. 8(f). The evolution of the other phases during solution is explained as follows. The fine MgSi phase only existed in specimen 6# with low holding temperature and short holding time, while it disappeared in other cases. The AlCu phase always co-existed with the AlCuMg phase, and it might be separated from the AlCuMg. The Fe containing phases such as AlFeMnSi and AlCuFeMn were difficult to be dissolved even if the temperature is as high as 530 C. Fig. 9 shows the XRD results of the cold rolled and solution treated 8# specimens. The main second phases of cold rolled specimen are S (Al2CuMg), Al2Cu and T (Al20Cu2Mn3). After holding
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Fig. 3. Optical micrographs of the diffusion layer under various solution temperatures and holding time.
at 495 C for 60 min, only the T (Al20Cu2Mn3) phase was remained. These facts indicate that the S (Al2CuMg) and Al2Cu phases have lower melting point, and most of them had dissolved into the matrix during solution. According to Refs. [23e25], as an important
dispersed phase in AleCueMg alloys, T (Al20Cu2Mn3) precipitated during the homogenization process, and then its size and morphology keep unchanged during the subsequent deformation and heat treatment. Finally, it should be mentioned that T (Al20Cu2Mn3) could not be observed in Figs. 7 and 8 due to its small size,
Fig. 4. The variation of diffusion depth with solution temperature and holding time.
Fig. 5. Schematic diagram of the peak-valley pattern diffusion.
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Fig. 6. Second phases in cold rolled 2024 Al sheet: (a) SEM image, (b) phases indicated by colors, and the element mapping of (c) Al, (d) Cu, (e) Mg, (f) Si, (g) Mn and (h) Fe. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 7. SEM images and element mapping of (a) AlCuFeMnSi, (b) AlCuFeMn and (c) AlFeMnSi in cold rolled 2024 Al sheet.
Table 3 EDS results of the chemical compositions (at.%) of the points indicated in Fig. 7. Point
Al
Cu
Fe
Mn
Si
1 2 3 4
69.56 73.09 68.29 71.31
19.32 5.8 21.86 16.18
7.57 8.79 7.25 3.68
2.52 5.97 2.6 8.41
1.03 6.36 e e
and the TEM was conducted to examine this phase in the following section. The above analysis about XRD results agrees well with the discussion on the SEM images shown in Figs. 7 and 8. 3.3. Precipitates Fig. 10 shows the TEM images of the naturally aged specimens experienced various solution parameters. As is seen, lots of rod-like precipitates of T(Al20Cu2Mn3) with a certain preferred orientation distributed dispersedly. The width of these precipitates is around 50e100 nm, and the length is around 300e800 nm. As reported in Refs. [23,24], T(Al20Cu2Mn3) precipitated during the homogenization process, and its length direction of [010]T is preferred to be parallel with the [010]Al. After solution treated under various
parameters, there is no difference in the size, distribution and morphology of T(Al20Cu2Mn3), which indicates the good thermal stability. Moreover, the S and S0 precipitates are not observed from Fig. 10. It means that the strengthening effect of the 2024 Al sheet after solution and natural aging is not caused by S and S’. In order to further clarify the strengthening mechanism, the cold rolled and some naturally aged specimens were selected for DSC analysis, and the results are shown in Fig. 11. It is obvious that the endothermic peaks (indicated by B, C, D, and E) around 200 C appeared in all naturally aged specimens, while such endothermic peak cannot be found in the cold rolled specimen. Based on the previous study [26], this peak corresponds to the dissolving of CueMg clusters. Hence, it is an evidence that lots of CueMg clusters were formed during the natural aging treatment, and the amount of the CueMg clusters increases with higher solution temperature. Combined the TEM and DSC results, it is concluded that the formation of CueMg clusters should be one of the main factors that caused the strengthening effects on the 2024 Al sheet. 3.4. Grain structure During cold rolling process, the grains were usually elongated along RD direction due to the severe plastic deformation, and a lot
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Fig. 8. SEM images of the second phases in solution treated specimens (a) (b) 6# (460 C þ 60 min), (c) (d) 8# (495 C þ 60 min), (e) (f) 10# (530 C þ 60 min), and (g) (h) 13# (495 C þ 120 min). The right side images correspond to the enlarged areas of the dotted box marked in the left side images, and various phases were distinguished by different colors. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 9. XRD results of the (a) cold rolled and solution treated 8# specimens (495 C þ 60 min).
of strain energy was stored. Then, during the solution treatment, the static recrystallization might occur due to the high temperature, which caused the significant variation of grain structure. The
inverse pole figure (IPF) maps obtained from EBSD analysis of the specimens are shown in Fig. 12. The obvious fiber structure is observed from Fig. 12(a). It means that the low temperature of 460 C and short time of 10 min is insufficient to bring high degree of static recrystallization, and the cold rolled structure was remained. However, few fine grains can be seen in the grain boundary area, and these grains locate at the junctions or the bulge parts of the zigzagged grain boundaries. These facts indicate that partial recrystallization occurred. In the other specimens solution treated by higher temperature or longer holding time, the complete recrystallized structures were formed, as shown from Fig. 8(b)-(e). In addition, the grain size difference is most obvious in Fig. 8(b) of 8# specimen, while the grain size distribution becomes relatively uniform in the other specimens. One possible reason is that large amount of second phases were remained in 8# specimen during the solution of holding at 460 C for 60 min, as previously discussed on Fig. 8(a). These second phases have strong pinning effect on the grain growth. The grain growth could only occur in the area with rare particles, while the grain growth was strongly inhibited in the area with abundant particles. Hence, the obvious difference in grain size was observed. With increasing solution temperature or holding time, the overall amount of second phases was significantly reduced, which results in the weakening of pinning effect. Then, the
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Fig. 10. TEM bright field images of the naturally aged specimens (a) 6# (460 C þ 60 min), (b) 8# (495 C þ 60 min), (c) 10# (530 C þ 60 min), and (d) 13# (495 C þ 120 min).
Fig. 11. DSC curves of the cold rolled and naturally aged specimens after different solution treatment.
grain growth occurred in the whole area, and the grain size difference was reduced. However, since the precipitate of T(Al20Cu2Mn3) existed in all specimens, the kinetics of grain growth is not fast due to its pinning effect. 3.5. Mechanical properties Based on above analysis, it is known that the solution parameters greatly affect the microstructure features, which should cause the variation of mechanical properties. Fig. 13 shows the engineering stress-strain curves obtained from tensile tests of all naturally aged specimens, and Table 4 summarizes the results of ultimate tensile strength (UTS), yield strength (YS) and elongation. For all holding time, both UTS and YS increase with the increase of
temperature, and they reach the peak values at 510 C. When the temperature is 510 C and holding time are 10 min, 60 min and 120 min, the UTS are 465 MPa, 471 MPa, 469 MPa, and the YS are 289 MPa, 294 MPa, 291 MPa, respectively. It means that the holding time has slight effect on the UTS and YS, when the temperature is 510 C. Then, both the UTS and YS slightly decrease in case of 530 C. The elongation exhibits the similar tendency with that of the strength. The peak values of elongation are achieved at 510 C, and then the elongation was reduced significantly if the temperature was further increased to 530 C. When the temperature is 510 C and holding time are 10 min, 60 min and 120 min, the values of elongation are 22.3%, 22.4% and 22.8%, respectively. It indicates that the effects of holding time on elongation are also quite slight, when the solution temperature is 510 C. Fig. 14 shows the variation of Vickers hardness of all naturally aged specimens. As is shown, when the holding time is 10 min, the hardness gradually increases with increasing temperature. However, when the holding time is 60 min or 120 min, the hardness firstly increases and then decreases with increasing temperature, and the peak hardness appears at the temperature of 510 C. As discussed on Fig. 8, if the solution temperature is low, lots of second phases were remained in the matrix, which reduces the supersaturated degree of the solid solution and weakens the solution strengthening effects. On the contrary, more phases dissolved with the increase of solution temperature, which brought stronger solution strengthening effects. If the temperature is not high enough, the dissolving of second phase is not obvious only by prolonging holding time. Therefore, the effects of holding time on the tensile properties are slight. The formation of CueMg clusters during natural aging process is also an important factor affecting the tensile properties. The high degree of supersaturated solid solution would facilitate the formation of CueMg clusters. As discussed on Fig. 12, the static recrystallization occurred during solution treatment. Although the grain growth became obvious
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Fig. 12. IPF maps of the naturally aged specimens (a) 1# (460 C þ 10 min), (b) 6# (460 C þ 60 min), (c) 8# (495 C þ 60 min), (d) 10# (530 C þ 60 min), and (e) 13# (495 C þ 120 min).
especially at high temperature and with long holding time, the difference of grain size was also reduced. This kind of structure with uniform grain size might be one of the possible reasons for improving the mechanical properties. Another important phenomena observed from Fig. 14 is that the mechanical properties after solution treated at 530 C became inferior, especially for the elongation. It is known that the over-burning temperature of 2024 Al alloy is around 510 C. The high solution temperature of 530 C could result in the over-burning and the partial melting of grain boundaries. Then, the oxygen entered inside the Al sheet through the channel of liquid grain boundaries. After cooling to room temperature, the grain boundaries with oxidations became brittle, and it caused the obvious reduction of elongation. The morphology of the center of the fracture surfaces was observed by SEM, and the results are shown in Fig. 15. The dimples can be observed in all specimens, which indicate a ductile fracture mode [27e29]. As shown in Fig. 15(a), the dimples are small and shallow for 6# specimen held at 460 C for 60 min, and the tear ridge and the transgranular flat facet also exist. As marked by the arrows in Fig. 15(a), the coarse second phases or impurities were observed inside the open voids. It means that the bonding between these coarse particles and the Al matrix is not good, which easily results in the appearance of crack or big voids. In 8# specimen held at 495 C for 60 min, more second phases with low temperature
melting point were dissolved. Hence, the number of the voids caused by coarse particles was reduced, and only the dimples and flat facets were observed, as shown in Fig. 15(b). All of these facts indicate a better plasticity. Fig. 15(c) and (d) show the different positions of 13# specimen held at 530 C for 60 min. As is seen, it is more close to the intergranular fracture mode with inferior plasticity, and it is also an evidence of the occurrence of over-burning at 530 C. The above analysis on the fracture morphology agrees well with the variation of elongation shown in Fig. 14. Overall, the CueMg clusters were formed during the natural aging process, while the precipitates were not formed. Hence, the number and size of the second phases play important roles on the mechanical properties. The coarse and brittle second phases are harmful on the ductility of the 2024 Al sheet. Since the temperature range of solution for 2024 Al sheet is narrow, it should be well controlled to avoid the over-burning.
4. Conclusions The effects of solution parameters on the microstructure and mechanical properties of the cold rolled 2024 Al sheet were investigated by experimental methods. The main conclusions are drawn as follows.
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Fig. 13. Engineering stress-strain curves of the naturally aged specimens after solution treated for (a) 10 min, (b) 60 min and (c) 120 min.
Table 4 Summary of the values of UTS, YS and elongation. No.
Solution condition
UTS (MPa)
YS (MPa)
Elongation (%)
1# 2# 3# 4# 5#
460 480 495 510 530
C C C C C
þ þ þ þ þ
10 10 10 10 10
min min min min min
393 422 440 465 444
232 255 271 289 287
19.2 19.7 20.3 22.3 12.5
6# 7# 8# 9# 10#
460 480 495 510 530
C C C C C
þ þ þ þ þ
60 60 60 60 60
min min min min min
408 438 460 471 460
244 268 285 294 282
20.3 21.5 22.9 22.4 18.2
11# 12# 13# 14# 15#
460 480 495 510 530
þ þ þ þ þ
120 120 120 120 120
403 431 451 469 463
237 262 278 291 281
21.1 20.8 21.1 22.8 18.5
C C C C C
min min min min min
(1) The diffusion from 2024 Al matrix to 1070 Al coating occurred during solution treatment, and it followed the peak-valley pattern. The diffusion depth increased with the increase of solution temperature or holding time. In order to maintain a good corrosion resistance, the diffusion depth should be controlled. (2) The cold rolled 2024 Al sheet mainly had the second phases of Al2Cu, S (Al2CuMg), Si, Mg2Si, and the impurities of
Fig. 14. Vickers hardness of the naturally aged specimens after solution treated at different conditions.
AlFeMnSi, AlCuFeMnSi, AlCuFeMn. With increasing solution temperature or holding time, the overall amount of second phases was significantly reduced, while the coarse Fe containing phases were difficult to be dissolved even if the temperature is as high as 530 C.
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Fig. 15. Fracture morphology of the naturally aged specimens (a) 6# (460 C þ 60 min), (b) 8# (495 C þ 60 min), (c) (d) 10# (530 C þ 60 min).
(3) T (Al20Cu2Mn3) were remained during the rolling, solution and naturally aging processes. The size and morphology of T (Al20Cu2Mn3) were not affected by the solution parameters. High solution degree facilitated the formation of CueMg clusters during natural aging, which should be the main reason for the strengthening effects. (4) After holding at 465 C for 10 min, the elongated grain structure was remained, and only the partial static recrystallization occurred. With increasing temperature or holding time, the complete recrystallized structure was achieved, and the grain growth took place. (5) The UTS, YS and elongation increased with increasing solution temperature, and reached the peak values at 510 C. When the solution temperature is 530 C, the UTS and YS were slightly decreased, while the elongation was obviously decreased due to the over-burning. The solution temperature exhibited much stronger effects on the mechanical properties than that of the holding time.
CRediT authorship contribution statement Mengchao Liang: Investigation, Methodology, Data curation, Writing - original draft. Liang Chen: Conceptualization, Investigation, Funding acquisition, Project administration, Writing - original draft. Guoqun Zhao: Supervision, Funding acquisition, Resources, Writing - review & editing. Yunyue Guo: Methodology, Validation, Data curation. Acknowledgements The authors would like to acknowledge the financial supports from the National Key Research and Development Program of China (No. 2017YFB0306402), National Natural Science Foundation of
China (No. U1708251), and Key Research and Development Program of Shandong Province (No. 2019JZZY010360). References [1] G. Chen, J. Liu, X. Shu, H. Gu, B. Zhang, J. Feng, Beam scanning effect on properties optimization of thick-plate 2A12 aluminum alloy electron-beam welding joints, Mater. Sci. Eng. A 744 (2019) 583e592. [2] S. Zhang, T. Zhang, Y. He, D. Liu, J. Wang, X. Du, B. Ma, Long-term atmospheric corrosion of aluminum alloy 2024-T4 in coastal environment: surface and sectional corrosion behavior, J. Alloys Compd. 789 (2019) 460e471. [3] M. de Bonfils-Lahovary, C. Josse, L. Laffont, C. Blanc, Influence of hydrogen on the propagation of intergranular corrosion defects in 2024 aluminium alloy, Corrosion Sci. 148 (2019) 198e205. [4] A. De Luca, D.N. Seidman, D.C. Dunand, Effects of Mo and Mn microadditions on strengthening and over-aging resistance of nanoprecipitationstrengthened Al-Zr-Sc-Er-Si alloys, Acta Mater. 165 (2019) 1e14. [5] M. Vlach, J. Cí zek, B. Smola, O. Melikhova, Ma Vl cek, V. Kodetov a, H. Kudrnov a, P. Hruska, Heat treatment and age hardening of Al-Si-Mg-Mn commercial alloy with addition of Sc and Zr, Mater. Char. 129 (2017) 1e8. [6] R. Ivanov, A. Deschamps, F. De Geuser, High throughput evaluation of the effect of Mg concentration on natural ageing of Al-Cu-Li-(Mg) alloys, Scripta Mater. 150 (2018) 156e159. ba r, J. Lendvai, N.Q. Chinh, The influence of artificial aging on [7] J. Gubicza, J.L. La the microstructure and hardness of an Al-Zn-Mg-Zr alloy processed by equalchannel angular pressing, J. Mater. Sci. 54 (2019) 10918e10928. [8] H. Li, P. Chen, Z. Wang, F. Zhu, R. Song, Z. Zheng, Tensile properties, microstructures and fracture behaviors of an Al-Zn-Mg-Cu alloy during ageing after solution treating and cold-rolling, Mater. Sci. Eng. A 742 (2019) 798e812. [9] S. Sun, P. Liu, J. Hu, C. Hong, X. Qiao, S. Liu, R. Zhang, C. Wu, Effect of solid solution plus double aging on microstructural characterization of 7075 Al alloys fabricated by selective laser melting (SLM), Optic Laser. Technol. 114 (2019) 158e163. [10] M.X. Guo, Y.D. Zhang, G.J. Li, S.B. Jin, G. Sha, J.S. Zhang, L.Z. Zhuang, E.J. Laverni, Solute clustering in Al-Mg-Si-Cu-(Zn) alloys during aging, J. Alloys Compd. 774 (2019) 347e363. [11] Y. Liu, F. Teng, F.H. Cao, Z.X. Yin, Y. Jiang, S.B. Wang, P.K. Shen, Defective GPzones and their evolution in an Al-Cu-Mg alloy during high-temperature aging, J. Alloys Compd. 774 (2019) 988e996. [12] I. Zuiko, R. Kaibyshev, Effect of plastic deformation on the ageing behaviour of an Al-Cu-Mg alloy with a high Cu/Mg ratio, Mater. Sci. Eng. A 737 (2018) 401e412. [13] G. Meyruey, V. Massardier, W. Lefebvre, M. Perez, Over-ageing of an Al-Mg-Si alloy with silicon excess, Mater. Sci. Eng. A 730 (2018) 92e105.
M. Liang et al. / Journal of Alloys and Compounds 824 (2020) 153943 [14] S.C. Wang, M.J. Starink, Precipitates and intermetallic phases in precipitation hardening Al-Cu-Mg-(Li) based alloys, Int. Mater. Rev. 50 (2005) 93e215. [15] M. Schneider, O. Yezerska, M.M. Lohrengel, Anodic oxide formation on AA2024: electrochemical and microstructure investigation, Corrosion Eng. Sci. Technol. 43 (2008) 304e312. [16] A. Boag, A.E. Hughes, N.C. Wilson, A. Torpy, C.M. MacRae, A.M. Glenn, T.H. Muster, How complex is the microstructure of AA2024-T3? Corrosion Sci. 51 (2009) 1565e1568. [17] Z. Zhu, M.J. Starink, Solution strengthening and age hardening capability of AlMg-Mn alloys with small additions of Cu, Mater. Sci. Eng. A 488 (2008) 125e133. [18] M. Nakai, T. Eto, New aspect of development of high strength aluminum alloys for aerospace applications, Mater. Sci. Eng. A 285 (2000) 62e68. [19] X.Y. Liu, Q.L. Pan, Z.L. Lu, S.F. Cao, Y.B. He, W.B. Li, Effects of solution treatment on the microstructure and mechanical properties of Al-Cu-Mg-Ag alloy, Mater. Des. 31 (2010) 4392e4397. [20] R.C. Dorward, Lattice and grain boundary diffusion of copper into cladding on Al clad AI-Cu-Mg alloy sheet, Mater. Sci. Technol. 14 (1998) 187e192. [21] J. Wang, Z. Liu, S. Bai, J. Cao, J. Zhao, D. Zeng, Microstructure and three-point bending fatigue behavior of Al-Cu-Mg-Ag alloys with various Mg contents, J. Mater. Eng. Perform. 28 (2019) 6614e6625. [22] Y. Jin, P. Cai, W. Wen, H. Nagaumi, B. Xu, Y. Zhang, T. Zhai, The anisotropy of
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fatigue crack nucleation in an AA7075 T651 Al alloy plate, Mater. Sci. Eng. A 622 (2015) 7e15. Z.Q. Feng, Y.Q. Yang, B. Huang, M.H. Li, Y.X. Chen, J.G. Ru, Crystal substructures of the rotation-twinned T (Al20Cu2Mn3) phase in 2024 aluminum alloy, J. Alloys Compd. 583 (2014) 445e451. Z. Shen, C. Liu, Q. Ding, S. Wang, X. Wei, L. Chen, J. Li, Z. Zhang, The structure determination of Al20Cu2Mn3 by near atomic resolution chemical mapping, J. Alloys Compd. 601 (2014) 25e30. S.C. Liu, Microstructure evolution of 2024 aluminum alloys subjected to thermal cycling in simulated LEO space environment, Adv. Mater. Res. 787 (2013) 35e39. S.C. Wang, M.J. Starink, Two types of S phase precipitates in Al-Cu-Mg alloys, Acta Mater. 55 (2007) 933e941. G. Chen, L. Chen, G. Zhao, C. Zhang, Microstructure evolution during solution treatment of extruded Al-Zn-Mg profile containing a longitudinal weld seam, J. Alloys Compd. 729 (2017) 210e221. S. Yuan, L. Chen, J. Tang, G. Zhao, C. Zhang, J. Yu, Correlation between homogenization treatment and subsequent hot extrusion of Al-Mg-Si alloy, J. Mater. Sci. 54 (2019) 9843e9856. J. Yu, G. Zhao, L. Chen, Investigation of interface evolution, microstructure and mechanical properties of solid-state bonding seams in hot extrusion process of aluminum alloy profiles, J. Mater. Process. Technol. 230 (2016) 153e166.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of solution treatment on the microstructure and mechanical properties of naturally aged EN AW 2024 Al alloy sheet Mengchao Liang , Liang Chen *, Guoqun Zhao , Yunyue Guo Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan, Shandong, 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 July 2019 Received in revised form 14 January 2020 Accepted 19 January 2020 Available online 23 January 2020
The solution experiments on cold rolled 2024 Al sheet with a 1070 Al coating were carried out at various temperatures and holding time. The effects of solution parameters on the microstructure evolution and mechanical properties were studied. The results showed that the alloying elements diffused from the 2024 Al matrix to the 1070 Al coating, following the peak-valley pattern. The cold rolled 2024 Al sheet mainly have the second phases of Al2Cu, S (Al2CuMg), Si, Mg2Si, and the impurities of AlFeMnSi, AlCuFeMnSi, AlCuFeMn, and the amount of them was reduced with increasing solution temperature or holding time. T (Al20Cu2Mn3) had precipitated before aging, and only the CueMg clusters were formed during the natural aging, which should be the main strengthening mechanism. The grain structure was elongated during cold rolling, while the static recrystallization and grain growth occurred during the solution treatment. The ultimate tensile strength, yield strength and elongation were gradually enhanced with increasing the solution temperature to 510 C. When the solution temperature is further increased to 530 C, the elongation was obviously decreased due to the over-burning. Moreover, it was found that the solution temperature had much stronger effects on the mechanical properties than that of the holding time. © 2020 Published by Elsevier B.V.
Keywords: 2024 Al Solution Natural aging Microstructure Mechanical properties
1. Introduction EN AW 2024 alloy is a typical 2xxx series (AleCueMg) wrought Al alloy, and it has the advantages of low density, high specific strength, excellent fatigue resistance and good machinability [1e3]. 2024 Al sheet fabricated through hot and cold rolling has been widely applied in the field of aerospace, such as the aircraft skin. Since 2024 Al is heat treatable, the deformed microstructure can be modified during the subsequent heat treatment processes to further improve the mechanical properties. The solution and aging are the most common heat treatments for Al alloys. The soluble phases can dissolve into the Al matrix during the solution stage, and the supersaturated solid solution is formed due to the rapid quenching. Then, various and large amount of strengthening phases precipitate during the artificial or natural aging. The parameters of both solution and aging treatments play important roles on the final mechanical properties of the Al alloys, and thus it has attracted much attention from the researchers [4e6].
* Corresponding author. E-mail address: [email protected] (L. Chen). https://doi.org/10.1016/j.jallcom.2020.153943 0925-8388/© 2020 Published by Elsevier B.V.
The research works have been carried out on solution and aging treatments of Al alloys. The microstructure characteristics and the response of mechanical properties were reported in details. Gubicza [7] performed artificial aging at various temperatures using the AleZneMgeZr alloy processed by equal channel angular pressing (ECAP), and the results indicated that 170 C aging led to a lower hardness than 120 C aging. Li et al. [8] investigated the effects of solution and aging on hot and cold rolled AleZneMgeCu alloy, and it was concluded that the strength was obviously enhanced due to the combined effects of dislocations, textures, grain size and precipitations. Sun et al. [9] conducted the solution plus double aging on 7075 Al alloys fabricated by selective laser melting, and some typical S (Al2CuMg) and q (Al2Cu) phases precipitated at the grain boundary, resulting in an obvious increase of hardness. Guo et al. [10] studied the solute clustering in AleMgeSieCueZn alloys during aging process, and it was found that the addition of Zn facilitated the formation of fine co-clusters containing Mg, Si, Cu and Zn atoms, which caused the enhanced hardening response. Liu et al. [11] found that the individual onedimensional GPB1 units, GPII zones and q’ precipitations were formed during two-stage hardening peaks of an Al-5.0Cu-0.4 Mg
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Table 1 Chemical compositions of the as-received 2024 Al alloy. Element
Cu
Mg
Mn
Fe
Si
Zn
Ti
Al
wt.%
4.1
1.48
0.5
0.1
0.09
0.03
0.04
Bal.
(wt.%) alloy. Zuiko et al. [12] proposed that the pre-straining could promote the precipitation during aging of an AleCueMg alloy, and the yield stress could be obviously increased by 30%. Meyruey et al. [13] investigated the over-ageing behavior of an AleMgeSi alloy within the temperature range between 100 C and 350 C, and it was claimed that the over-ageing was accompanied by the progressive replacement of b00 precipitate by a succession of different types of semi-coherent precipitates. Till now, the precipitation behavior during aging process and the effects of aging parameters on mechanical properties of various Al alloys have been studied. However, as the prior step, the solution treatment was rarely reported. 2024 Al alloy has high content of alloying elements, such as 3.8e4.9 wt% Cu and 1.2e1.8 wt% Mg. Large amount of intermetallic compounds such as S, q, AlFeSiMnCu, Al7Cu2Fe, and Al3(Cu, Fe, Mn) can be found [14e16], and the complex microstructure evolution should occur during the solution treatment. Firstly, the supersaturated degree of solid solution affects the distribution, morphology and size of the precipitations during the aging treatment [17,18]. Secondly, the static recrystallization and grain growth easily occur during solution treatment [19]. Both of these two factors greatly affect the final mechanical properties of Al alloys. On the other hand, the commercial 2024 Al alloy sheet usually follows the production route of hot rolling, cold rolling, solution and natural aging. Since only the natural aging is performed at room temperature, the control of solution condition becomes much more important for 2024 Al alloy sheet. Hence, the cold rolled 2024 Al sheet with a 1070 Al coating was solution treated at various temperatures and holding time. The microstructure features such as the diffusion layer, second phase, precipitate and grain structure were well studied. Moreover, the tensile properties and the hardness of the naturally aged specimens were tested. The effects of solution parameters on the microstructure evolution and the response of mechanical properties were discussed. The main purpose of this study is to clarify the influence of solution temperature and holding time on the final mechanical properties of the naturally aged specimens, which is important for improving the quality of cold rolled 2024 Al sheet. 2. Experimental procedures The cold rolled 2024 Al sheet with a thickness of 1.5 mm was received from Nanshan Aluminum Co., Ltd. The chemical compositions of the 2024 Al sheet are listed in Table 1. In order to further enhance the corrosion resistance, a 1070 Al (99.7 wt.% Al) layer with the thickness of around 65 mm was coated on the surfaces of the 2024 Al sheet. For simplification, the as-received sheet is called 2024 Al sheet in this study. The fabrication route of the 2024 Al sheet is described as follows. Firstly, both surfaces of the as-cast 2024 Al slab was milled to a thickness of 420 mm. Two 1070 Al
sheets with a thickness of 20 mm were put on the upper and lower sides of the 2024 Al slab to make a sandwich structure, and the spot welding was used to fix them. Then, the sandwich billet was preheated to 450 C, and it was hot rolled to a thickness of 4 mm after 20 passes. Finally, the sheet was cold rolled to a thickness of 1.5 mm after 3 passes. The solution treatment was performed in a tube furnace with an argon atmosphere. When the preset temperature was reached, the specimen was put inside the furnace to initiate the solution. Since the main objective of the present study is to clarify the effects of solution parameters on the microstructure and mechanical properties of naturally aged 2024 Al sheet, the solution was performed within the temperature range of 460e530 C and the holding time range of 10e120 min. The detailed parameters of the experiments are listed in Table 2. After solution, the specimen was immediately quenched into cooled water. The mechanical properties of the 2024 Al sheet varied significantly during the initial stage of natural aging, while a stable state could be achieved after 96 h. For example, the variation of hardness after solution treated at 495 C for 60 min is shown in Fig. 1. As is seen, the hardness varies slightly after 96 h aging time, which indicates a stable state. Hence, all the mechanical tests were performed after 96 h natural aging. The location and dimension of the specimens are schematically shown in Fig. 2, where the length, width and thickness directions of the sheet were designated as rolling direction (RD), transverse direction (TD) and normal direction (ND), respectively. The specimens for tensile tests were designed to have a gauge length of 25 mm. The tensile tests were performed at room temperature with a stretching speed of 1.5 mm/ min. The Vickers hardness of the specimens was measured by using a load of 500 g and dwelling for 15 s. The microstructure was observed by means of optical microscope (OM), scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS), electron back-scattered diffraction (EBSD) and high resolution transmission electron microscopy (HRTEM). The OM specimens were grounded, polished and etched by using the Keller’s reagent of 1 ml HF, 1.5 ml HCl, 2.5 ml HNO3 and 95 ml H2O. The EBSD specimens were electropolished at 26 V for 10 s in a solution of 10 ml nitric acid and 90 ml methanol. The HRTEM specimens were firstly grounded to a thickness of 50 mm, and then the electro-polishing was performed with an electrolyte of 30% HNO3 in methanol. In order to identify the phases before and after solution, the X-ray diffraction (XRD) with Cu-Ka radiation was conducted using the parameters of 50 kV and 150 mA. The differential thermal analysis reference (DSC) was performed with the heating speed of 10 C/min, and the pure Al was used as the reference. 3. Results and discussion 3.1. Formation of the diffusion layer It has been mentioned that the investigated 2024 Al sheet has a 1070 Al coating with the thickness of around 65 mm for the purpose of improving corrosion resistance. During solution treatment, holding at high temperature should facilitate the element diffusion. Hence, the diffusion behavior was firstly studied and the results are
Table 2 The temperature and holding time of the solution experiments. No.
Temperature (oC)
Holding time (min)
No.
Temperature (oC)
Holding time (min)
No.
Temperature (oC)
Holding time (min)
1# 2# 3# 4# 5#
460 480 495 510 530
10 10 10 10 10
6# 7# 8# 9# 10#
460 480 495 510 530
60 60 60 60 60
11# 12# 13# 14# 15#
460 480 495 510 530
120 120 120 120 120
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3
that of the lattice. After long time diffusion, the grain boundary diffusion is far ahead of the lattice, and the peak is observed. Contrarily, the low kinetics of lattice diffusion leads to the formation of valley. 3.2. Secondary phases
Fig. 1. Variation of hardness with aging time after solution treated at 495 C for 60 min.
Fig. 2. Location and dimension of the specimens for microstructure observation and mechanical tests. (Unit: mm).
shown in Fig. 3. It is obvious that a diffusion layer was formed, and its depth was increased with the increase of solution temperature or holding time. When the temperature is as low as 460 C, only the narrow diffusion layer appeared, even with long holding time of 120 min. However, when the temperature is 530 C and holding time is 120 min, the diffusion layer had covered the whole 1070 Al coating. The diffusion depth of all specimens is summarized in Fig. 4. As is seen, both the temperature or holding time significantly affect the diffusion depth. Generally, the diffusion from 2024 Al matrix to pure Al coating is harmful. For example, the diffusion of Cu can reduce the difference of electric potential between the matrix and the coating, which results in a decline of the corrosion resistance. Although the diffusion behavior cannot be avoided, there should be a limit depth of the diffusion layer, according to the application fields of the 2024 Al sheet. In the present study, the 1/2 thickness of the pure Al coating is defined as the diffusion limit, as indicated in Fig. 4, from which some un-qualified parameters of solution treatment should be avoided. On the other hand, it is seen from Fig. 3 that the frontier of the diffusion layer is not flat, which indicates an non-uniform diffusion. As previously reported by Dorward et al. [20], this kind of phenomenon is an evidence of peak-valley pattern diffusion, which is schematically drawn in Fig. 5. The peaks and valleys are representative of the diffusion of grain boundary and lattice, respectively. The diffusion kinetics of grain boundary is much higher than
The 2024 Al alloy has high content of alloying elements, and various second phases usually exist. Since the chemical compositions of the second phases cannot be identified only based on the particle morphology, the EDS was performed on the cold rolled and solution treated 2024 Al sheet. The EDS mapping results of the cold rolled specimen is shown in Fig. 6. As is seen, the main strengthening second phase is AlCuMg, which is a mixture of S (Al2CuMg) and CuAl2. Moreover, the phases of Si, MgSi and AlFeMnSi can also be observed. After carefully check the whole specimen, some impurities are found, as is seen from Fig. 7. The AlCuFeMnSi shown in Fig. 7(a) is a composite phase with a smooth edge, and it consists of bright and dark areas. According to the EDS mapping results, the chemical compositions of the bright and dark areas are obviously different. The Cu element mainly concentrates in the bright area, and the Mn and Si concentrate in the dark area, while the Fe element distribute uniformly in the whole AlCuFeMnSi particle. The EDS spot results of points 1 and 2 listed in Table 3 also confirm this kind of phenomenon. Fig. 7(b) shows the composite phase of AlCuFeMn, which also consists of the bright and dark areas. However, the difference is that the dark areas distribute dispersedly inside the bright matrix. Moreover, the edge of AlCuFeMn is rough, and some cracks appear inside the particle, which indicate that the particle is brittle. Based on the EDS results in Fig. 7(b) and Table 3, Cu, Fe and Si have the uniform distribution, while the Mn concentrates in the dark area. Fig. 7(c) shows a particle of AlFeMnSi phase with flat edge, and all of the elements distribute uniformly. It is noted that some cracks appear inside or nearby the particle. This kind of coarse AlFeMnSi particle should be brittle, and it is harmful on the mechanical properties of the Al sheet [21,22]. The distribution of second phase in the solution treated specimens is shown in Fig. 8. It can be seen that the amount of the dispersed granular and rod-like particles had been significantly reduced, while the particles with large size were still remained. Additionally, it is obvious that the overall amount of second phase decreased with the increase of temperature or holding time. However, as shown in Fig. 8(e) and (g), when the temperature or holding time was further increased to 530 C or 120 min, the decreasing of second phase was not obvious. The chemical compositions of the remained second phases are indicated by different colors in the right side images of Fig. 8. It is observed from Fig. 8(b) that the dispersed granular AlCuMg almost disappeared, and some rod-like and coarse AlCuMg particles were remained in 6# specimen solution treated at 460 C for 60 min. With increasing temperature to 495 C or holding time to 120 min, the number of rodlike AlCuMg was greatly reduced, and the size of coarse AlCuMg became smaller, as shown in Fig. 8(d) and (h). With the further increase of temperature to 530 C, almost all of the AlCuMg had dissolved into the matrix, as shown in Fig. 8(f). The evolution of the other phases during solution is explained as follows. The fine MgSi phase only existed in specimen 6# with low holding temperature and short holding time, while it disappeared in other cases. The AlCu phase always co-existed with the AlCuMg phase, and it might be separated from the AlCuMg. The Fe containing phases such as AlFeMnSi and AlCuFeMn were difficult to be dissolved even if the temperature is as high as 530 C. Fig. 9 shows the XRD results of the cold rolled and solution treated 8# specimens. The main second phases of cold rolled specimen are S (Al2CuMg), Al2Cu and T (Al20Cu2Mn3). After holding
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Fig. 3. Optical micrographs of the diffusion layer under various solution temperatures and holding time.
at 495 C for 60 min, only the T (Al20Cu2Mn3) phase was remained. These facts indicate that the S (Al2CuMg) and Al2Cu phases have lower melting point, and most of them had dissolved into the matrix during solution. According to Refs. [23e25], as an important
dispersed phase in AleCueMg alloys, T (Al20Cu2Mn3) precipitated during the homogenization process, and then its size and morphology keep unchanged during the subsequent deformation and heat treatment. Finally, it should be mentioned that T (Al20Cu2Mn3) could not be observed in Figs. 7 and 8 due to its small size,
Fig. 4. The variation of diffusion depth with solution temperature and holding time.
Fig. 5. Schematic diagram of the peak-valley pattern diffusion.
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Fig. 6. Second phases in cold rolled 2024 Al sheet: (a) SEM image, (b) phases indicated by colors, and the element mapping of (c) Al, (d) Cu, (e) Mg, (f) Si, (g) Mn and (h) Fe. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 7. SEM images and element mapping of (a) AlCuFeMnSi, (b) AlCuFeMn and (c) AlFeMnSi in cold rolled 2024 Al sheet.
Table 3 EDS results of the chemical compositions (at.%) of the points indicated in Fig. 7. Point
Al
Cu
Fe
Mn
Si
1 2 3 4
69.56 73.09 68.29 71.31
19.32 5.8 21.86 16.18
7.57 8.79 7.25 3.68
2.52 5.97 2.6 8.41
1.03 6.36 e e
and the TEM was conducted to examine this phase in the following section. The above analysis about XRD results agrees well with the discussion on the SEM images shown in Figs. 7 and 8. 3.3. Precipitates Fig. 10 shows the TEM images of the naturally aged specimens experienced various solution parameters. As is seen, lots of rod-like precipitates of T(Al20Cu2Mn3) with a certain preferred orientation distributed dispersedly. The width of these precipitates is around 50e100 nm, and the length is around 300e800 nm. As reported in Refs. [23,24], T(Al20Cu2Mn3) precipitated during the homogenization process, and its length direction of [010]T is preferred to be parallel with the [010]Al. After solution treated under various
parameters, there is no difference in the size, distribution and morphology of T(Al20Cu2Mn3), which indicates the good thermal stability. Moreover, the S and S0 precipitates are not observed from Fig. 10. It means that the strengthening effect of the 2024 Al sheet after solution and natural aging is not caused by S and S’. In order to further clarify the strengthening mechanism, the cold rolled and some naturally aged specimens were selected for DSC analysis, and the results are shown in Fig. 11. It is obvious that the endothermic peaks (indicated by B, C, D, and E) around 200 C appeared in all naturally aged specimens, while such endothermic peak cannot be found in the cold rolled specimen. Based on the previous study [26], this peak corresponds to the dissolving of CueMg clusters. Hence, it is an evidence that lots of CueMg clusters were formed during the natural aging treatment, and the amount of the CueMg clusters increases with higher solution temperature. Combined the TEM and DSC results, it is concluded that the formation of CueMg clusters should be one of the main factors that caused the strengthening effects on the 2024 Al sheet. 3.4. Grain structure During cold rolling process, the grains were usually elongated along RD direction due to the severe plastic deformation, and a lot
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Fig. 8. SEM images of the second phases in solution treated specimens (a) (b) 6# (460 C þ 60 min), (c) (d) 8# (495 C þ 60 min), (e) (f) 10# (530 C þ 60 min), and (g) (h) 13# (495 C þ 120 min). The right side images correspond to the enlarged areas of the dotted box marked in the left side images, and various phases were distinguished by different colors. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 9. XRD results of the (a) cold rolled and solution treated 8# specimens (495 C þ 60 min).
of strain energy was stored. Then, during the solution treatment, the static recrystallization might occur due to the high temperature, which caused the significant variation of grain structure. The
inverse pole figure (IPF) maps obtained from EBSD analysis of the specimens are shown in Fig. 12. The obvious fiber structure is observed from Fig. 12(a). It means that the low temperature of 460 C and short time of 10 min is insufficient to bring high degree of static recrystallization, and the cold rolled structure was remained. However, few fine grains can be seen in the grain boundary area, and these grains locate at the junctions or the bulge parts of the zigzagged grain boundaries. These facts indicate that partial recrystallization occurred. In the other specimens solution treated by higher temperature or longer holding time, the complete recrystallized structures were formed, as shown from Fig. 8(b)-(e). In addition, the grain size difference is most obvious in Fig. 8(b) of 8# specimen, while the grain size distribution becomes relatively uniform in the other specimens. One possible reason is that large amount of second phases were remained in 8# specimen during the solution of holding at 460 C for 60 min, as previously discussed on Fig. 8(a). These second phases have strong pinning effect on the grain growth. The grain growth could only occur in the area with rare particles, while the grain growth was strongly inhibited in the area with abundant particles. Hence, the obvious difference in grain size was observed. With increasing solution temperature or holding time, the overall amount of second phases was significantly reduced, which results in the weakening of pinning effect. Then, the
M. Liang et al. / Journal of Alloys and Compounds 824 (2020) 153943
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Fig. 10. TEM bright field images of the naturally aged specimens (a) 6# (460 C þ 60 min), (b) 8# (495 C þ 60 min), (c) 10# (530 C þ 60 min), and (d) 13# (495 C þ 120 min).
Fig. 11. DSC curves of the cold rolled and naturally aged specimens after different solution treatment.
grain growth occurred in the whole area, and the grain size difference was reduced. However, since the precipitate of T(Al20Cu2Mn3) existed in all specimens, the kinetics of grain growth is not fast due to its pinning effect. 3.5. Mechanical properties Based on above analysis, it is known that the solution parameters greatly affect the microstructure features, which should cause the variation of mechanical properties. Fig. 13 shows the engineering stress-strain curves obtained from tensile tests of all naturally aged specimens, and Table 4 summarizes the results of ultimate tensile strength (UTS), yield strength (YS) and elongation. For all holding time, both UTS and YS increase with the increase of
temperature, and they reach the peak values at 510 C. When the temperature is 510 C and holding time are 10 min, 60 min and 120 min, the UTS are 465 MPa, 471 MPa, 469 MPa, and the YS are 289 MPa, 294 MPa, 291 MPa, respectively. It means that the holding time has slight effect on the UTS and YS, when the temperature is 510 C. Then, both the UTS and YS slightly decrease in case of 530 C. The elongation exhibits the similar tendency with that of the strength. The peak values of elongation are achieved at 510 C, and then the elongation was reduced significantly if the temperature was further increased to 530 C. When the temperature is 510 C and holding time are 10 min, 60 min and 120 min, the values of elongation are 22.3%, 22.4% and 22.8%, respectively. It indicates that the effects of holding time on elongation are also quite slight, when the solution temperature is 510 C. Fig. 14 shows the variation of Vickers hardness of all naturally aged specimens. As is shown, when the holding time is 10 min, the hardness gradually increases with increasing temperature. However, when the holding time is 60 min or 120 min, the hardness firstly increases and then decreases with increasing temperature, and the peak hardness appears at the temperature of 510 C. As discussed on Fig. 8, if the solution temperature is low, lots of second phases were remained in the matrix, which reduces the supersaturated degree of the solid solution and weakens the solution strengthening effects. On the contrary, more phases dissolved with the increase of solution temperature, which brought stronger solution strengthening effects. If the temperature is not high enough, the dissolving of second phase is not obvious only by prolonging holding time. Therefore, the effects of holding time on the tensile properties are slight. The formation of CueMg clusters during natural aging process is also an important factor affecting the tensile properties. The high degree of supersaturated solid solution would facilitate the formation of CueMg clusters. As discussed on Fig. 12, the static recrystallization occurred during solution treatment. Although the grain growth became obvious
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M. Liang et al. / Journal of Alloys and Compounds 824 (2020) 153943
Fig. 12. IPF maps of the naturally aged specimens (a) 1# (460 C þ 10 min), (b) 6# (460 C þ 60 min), (c) 8# (495 C þ 60 min), (d) 10# (530 C þ 60 min), and (e) 13# (495 C þ 120 min).
especially at high temperature and with long holding time, the difference of grain size was also reduced. This kind of structure with uniform grain size might be one of the possible reasons for improving the mechanical properties. Another important phenomena observed from Fig. 14 is that the mechanical properties after solution treated at 530 C became inferior, especially for the elongation. It is known that the over-burning temperature of 2024 Al alloy is around 510 C. The high solution temperature of 530 C could result in the over-burning and the partial melting of grain boundaries. Then, the oxygen entered inside the Al sheet through the channel of liquid grain boundaries. After cooling to room temperature, the grain boundaries with oxidations became brittle, and it caused the obvious reduction of elongation. The morphology of the center of the fracture surfaces was observed by SEM, and the results are shown in Fig. 15. The dimples can be observed in all specimens, which indicate a ductile fracture mode [27e29]. As shown in Fig. 15(a), the dimples are small and shallow for 6# specimen held at 460 C for 60 min, and the tear ridge and the transgranular flat facet also exist. As marked by the arrows in Fig. 15(a), the coarse second phases or impurities were observed inside the open voids. It means that the bonding between these coarse particles and the Al matrix is not good, which easily results in the appearance of crack or big voids. In 8# specimen held at 495 C for 60 min, more second phases with low temperature
melting point were dissolved. Hence, the number of the voids caused by coarse particles was reduced, and only the dimples and flat facets were observed, as shown in Fig. 15(b). All of these facts indicate a better plasticity. Fig. 15(c) and (d) show the different positions of 13# specimen held at 530 C for 60 min. As is seen, it is more close to the intergranular fracture mode with inferior plasticity, and it is also an evidence of the occurrence of over-burning at 530 C. The above analysis on the fracture morphology agrees well with the variation of elongation shown in Fig. 14. Overall, the CueMg clusters were formed during the natural aging process, while the precipitates were not formed. Hence, the number and size of the second phases play important roles on the mechanical properties. The coarse and brittle second phases are harmful on the ductility of the 2024 Al sheet. Since the temperature range of solution for 2024 Al sheet is narrow, it should be well controlled to avoid the over-burning.
4. Conclusions The effects of solution parameters on the microstructure and mechanical properties of the cold rolled 2024 Al sheet were investigated by experimental methods. The main conclusions are drawn as follows.
M. Liang et al. / Journal of Alloys and Compounds 824 (2020) 153943
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Fig. 13. Engineering stress-strain curves of the naturally aged specimens after solution treated for (a) 10 min, (b) 60 min and (c) 120 min.
Table 4 Summary of the values of UTS, YS and elongation. No.
Solution condition
UTS (MPa)
YS (MPa)
Elongation (%)
1# 2# 3# 4# 5#
460 480 495 510 530
C C C C C
þ þ þ þ þ
10 10 10 10 10
min min min min min
393 422 440 465 444
232 255 271 289 287
19.2 19.7 20.3 22.3 12.5
6# 7# 8# 9# 10#
460 480 495 510 530
C C C C C
þ þ þ þ þ
60 60 60 60 60
min min min min min
408 438 460 471 460
244 268 285 294 282
20.3 21.5 22.9 22.4 18.2
11# 12# 13# 14# 15#
460 480 495 510 530
þ þ þ þ þ
120 120 120 120 120
403 431 451 469 463
237 262 278 291 281
21.1 20.8 21.1 22.8 18.5
C C C C C
min min min min min
(1) The diffusion from 2024 Al matrix to 1070 Al coating occurred during solution treatment, and it followed the peak-valley pattern. The diffusion depth increased with the increase of solution temperature or holding time. In order to maintain a good corrosion resistance, the diffusion depth should be controlled. (2) The cold rolled 2024 Al sheet mainly had the second phases of Al2Cu, S (Al2CuMg), Si, Mg2Si, and the impurities of
Fig. 14. Vickers hardness of the naturally aged specimens after solution treated at different conditions.
AlFeMnSi, AlCuFeMnSi, AlCuFeMn. With increasing solution temperature or holding time, the overall amount of second phases was significantly reduced, while the coarse Fe containing phases were difficult to be dissolved even if the temperature is as high as 530 C.
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M. Liang et al. / Journal of Alloys and Compounds 824 (2020) 153943
Fig. 15. Fracture morphology of the naturally aged specimens (a) 6# (460 C þ 60 min), (b) 8# (495 C þ 60 min), (c) (d) 10# (530 C þ 60 min).
(3) T (Al20Cu2Mn3) were remained during the rolling, solution and naturally aging processes. The size and morphology of T (Al20Cu2Mn3) were not affected by the solution parameters. High solution degree facilitated the formation of CueMg clusters during natural aging, which should be the main reason for the strengthening effects. (4) After holding at 465 C for 10 min, the elongated grain structure was remained, and only the partial static recrystallization occurred. With increasing temperature or holding time, the complete recrystallized structure was achieved, and the grain growth took place. (5) The UTS, YS and elongation increased with increasing solution temperature, and reached the peak values at 510 C. When the solution temperature is 530 C, the UTS and YS were slightly decreased, while the elongation was obviously decreased due to the over-burning. The solution temperature exhibited much stronger effects on the mechanical properties than that of the holding time.
CRediT authorship contribution statement Mengchao Liang: Investigation, Methodology, Data curation, Writing - original draft. Liang Chen: Conceptualization, Investigation, Funding acquisition, Project administration, Writing - original draft. Guoqun Zhao: Supervision, Funding acquisition, Resources, Writing - review & editing. Yunyue Guo: Methodology, Validation, Data curation. Acknowledgements The authors would like to acknowledge the financial supports from the National Key Research and Development Program of China (No. 2017YFB0306402), National Natural Science Foundation of
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