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Al sheet fabricated by combination of porthole die co-extrusion and subsequent hot rolling
Journal of Alloys and Compounds 784 (2019) 727e738
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Study on Al/Mg/Al sheet fabricated by combination of porthole die coextrusion and subsequent hot rolling Jianwei Tang, Liang Chen*, Guoqun Zhao, Cunsheng Zhang, Junquan Yu 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 4 September 2018 Received in revised form 16 December 2018 Accepted 1 January 2019 Available online 4 January 2019
Al/Mg/Al sheet was fabricated by the proposed porthole die co-extrusion and subsequent hot rolling (PCE-R) method. The intermetallic compounds of b-Al3Mg2 and g-Mg17Al12 were formed in transition layer after PCE-R process, and the thicknesses of b and g layers became larger at higher rolling temperature or higher reduction ratio. Partial dynamic recrystallization (DRX) occurred in Al layer, and Al layer mainly consisted of shear-typed {111} fiber textures. However, if the rolling reduction is as high as 75%, Al layer exhibited quite different texture components. Mg layer consisted of fine equiaxed grains and several elongated grains, implying the occurrence of near complete DRX. With the increase of rolling temperature or reduction ratio, the number of Mg17Al12 particles in Mg layer was reduced, resulting into larger grain size. Mg layer had strong basal plane texture with c-axis parallel to transverse direction. Al/ Mg/Al sheet rolled at 300 C with 65% reduction exhibited two stress-drop fracture mechanism during tensile test, and it showed excellent tensile strength of 149 MPa, and elongation of 0.13. However, Al/Mg/ Al sheet rolled with 75% reduction had inferior tensile properties due to the existence of some cracks after rolling. © 2019 Elsevier B.V. All rights reserved.
Keywords: Al/Mg/Al Microstructure Dynamic recrystallization Texture
1. Introduction Mg alloys have been used in automobile and high-speed train because of the distinct merits of high specific strength and low density [1,2]. However, the practical application of Mg alloys is still restricted by their intrinsic properties, such as poor corrosion resistance and low formability at ambient temperature. Contrarily, Al alloys as the light-weight materials have excellent corrosion resistance, since the thin and dense oxide layer can be formed on Al surface. According to recent studies [3e5], Al/Mg/Al laminates exhibit more excellent corrosion resistance and mechanical properties than those of Mg alloys, and it will not cause obvious weight increment. Al/Mg laminates can be fabricated by explosive welding [6], vacuum diffusion bonding [7], laser cladding [8], hot rolling [9e12], and hot extrusion [13e15]. Among these methods, hot rolling and hot extrusion own the advantages of high efficiency and low cost. For hot rolling, the parameters of rolling temperature, reduction ratio and subsequent annealing have significant effects on the
* Corresponding author. E-mail address: [email protected] (L. Chen). https://doi.org/10.1016/j.jallcom.2019.01.005 0925-8388/© 2019 Elsevier B.V. All rights reserved.
quality of rolled Al/Mg/Al laminate. Luo et al. [9] proposed that 5052 Al/AZ31B Mg/5052 Al laminate can be fabricated by a twopass hot rolling, where the first pass at low temperature can reduce the surface oxidation on Al/Mg interface. Matsumoto et al. [10] bonded Al/Mg-Li clad plate by cold rolling, and investigated the effects of annealing on the plate quality. Zhang et al. [11] fabricated Al/Mg/Al laminate through hot rolling, and studied the microstructure evolution, bonding strength and thickness ratio. Macwan et al. [12] combined hot and cold rolling to produce Al/Mg/Al composite plate, and studied the interface, microstructure and tensile properties of the plate after annealing. These above open literature proved the feasibility of producing Al/Mg laminate by rolling process. However, the preparation of Mg plate is usually difficult and costly. Moreover, strong basal texture can be formed during rolling process of Mg Alloy, and it has negative effects on the formability [13]. Finally, the impurity and oxidation existed on Al/ Mg interface are difficult to be removed, and it is harmful for the bonding quality. Hot extrusion is another effective method to produce Al/Mg composites, since both Al and Mg alloys have excellent extrudability. Negendank et al. [14] carried out co-extrusion using Al sleeve and Mg core, and Wu et al. [15] also obtained Al/Mg
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laminates with sound interface by extruding 7050 Al core and AZ31 Mg sleeve. Besides, Mahmoodkhani and Wells [1] proposed that the fabrication of Al/Mg composite can be achieved through coextrusion using Al plate and cylindrical Mg billet. Tokunaga et al. [16] put Al plate between Mg billet and an extrusion die, and obtained a Mg bar with uniform Al coating. However, it is difficult and wasteful to prepare the hybrid billet (Al sleeve and Mg core), and adverse effects of both impurity and oxidation are hard to be avoided. In our recent research [17], Al/Mg/Al laminate with sound Al/Mg interface was fabricated based on porthole die co-extrusion (PCE) process. It was found that PCE temperature had close relationship with the morphology of Al/Mg interface, microstructure, and mechanical properties of the extruded laminate. During PCE process, the contaminated surfaces of initial billets were remained in container, which is beneficial for avoiding the adverse effects resulting from the impurity and oxidation. Importantly, the homogenized Al and Mg billets with cylindrical shape are easy to be prepared. However, PCE process is usually difficult to produce thin Al/Mg/Al laminate sheet due to the capacity limitation of extruder. The combination of PCE and subsequent hot rolling (PCE-R) processes was proposed to fabricate the Al/Mg/Al sheet, and it is aimed to obtain Al/Mg/Al sheet with excellent bonding interface, microstructure and mechanical properties. The hot rolling experiments were conducted at various rolling temperature and reduction ratio. The formation and evolution of transition layer in Al/Mg interface were investigated, and the microstructure of the extruded and hot rolled Al/Mg/Al sheet were well examined. The tensile tests were carried out to study the effects of rolling temperature and reduction ratio on the strength and elongation of Al/Mg/Al sheet. Through this study, a novel and effective method based on PCE-R process is developed to fabricate Al/Mg/Al sheet.
2. Experimental procedures The chemical compositions of as-received AA6063 and AZ91 Mg alloy are listed in Table 1. The homogenization treatment of AA6063 and AZ91 ingots were performed at 480 C for 12 h, and 420 C for 10 h followed by the air cooling, respectively. After that, these
Table 1 Chemical compositions (wt.%) of the as-received AA6063 and AZ91 Mg alloys. Alloy
Chemical compositions (wt.%) Si
Fe
Cu
Mn
Zn
Al
Mg
AA6063 AZ91
0.45 0.031
0.35 e
0.10 e
0.12 0.33
0.12 0.64
Bal. 9.31
0.80 Bal.
homogenized alloys were machined to cylindrical billets for PCE-R process. The designed PCE-R process is schematically shown in Fig. 1, where the length, thickness, and width directions of the rolled Al/ Mg/Al sheet indicate the extrusion direction (ED), transverse direction (TD), and normal direction (ND), respectively. As shown in Fig. 1, PCE-R consists of PCE and subsequent hot rolling processes. The detailed experimental setup and procedures of PCE process have been illustrated in our recent study [17], and a brief introduction is given as below. The billets were placed inside container, and they were heated to 370 C and held for 15min. Then, PCE process was performed with a constant velocity. Finally, the plate shaped Al/Mg/Al laminate with a square cross-section of 12 12 mm was extruded out. After that, the subsequent hot rolling was conducted using a 2-high reversible laboratory rolling mill. The extruded Al/Mg/Al laminate was firstly preheated to experimental temperature in the furnace, and held for 15 min. Then, the laminate was hot rolled to the required thickness after multiple passes, during which the intermediate annealing was performed at 300 C for 30 min to reduce the strain hardening. After that, the rolled Al/Mg/Al sheet was cooled down in the air. In this study, the effects of rolling temperature and reduction ratio were studied, since both of them significantly influence the microstructure characteristics of hot rolled Al/Mg/Al sheet. The detailed process parameters are shown in Table 2. According to Refs. [18,19], the pass reduction has significant effect on the microstructure, formability and mechanical properties of rolled sheets. Thus, the amount of rolling pass was varied to achieve different reduction ratio. The extruded Al/Mg/Al laminate without hot rolling is designated as PCE-R0. Besides, the Al/Mg/Al sheets rolled at 300 C with a reduction ratio of 65%, 350 C with a reduction ratio of 65%, and 300 C with a reduction ratio of 75% are designated as PCE-R1, PCE-R2, and PCE-R3, respectively. After rolling process, the thickness of PCE-R1, PCE-R2 and PCE-R3 are approximate 4.2 mm, 4.2 mm and 3 mm, respectively. For PCE-R1 and PCE-R2, the thickness of Al layer and Mg layer are around 1.3 mm and 1.6 mm, while that of PCE-R3 are around 0.9 mm and 1.2 mm. The morphology of Al/Mg interface and the distribution of secondary phase were observed by means of scanning electron microscopy (SEM). The element distribution and interface component across Al/Mg interface were examined by energy dispersive spectroscopy (EDS) and electron probe micro-analyzer (EPMA). The grain morphology and micro-texture of Al and Mg layers near Al/ Mg interface were analyzed by electron back-scattered diffraction (EBSD). For EBSD measurements, Al specimen was electro-polished in the solution of 10 ml perchloric acid and 90 ml methanol at 30 V for 10 s, and Mg specimen was electro-polished using the mixture
Fig. 1. Schematic diagram of the designed PCE-R process. (Unit: mm).
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Table 2 Experimental conditions of PCE-R process. Specimen
Extrusion temperature
Extrusion ratio
Rolling Temperature
Reduction ratio
Rolling pass
PCE-R0 PCE-R1 PCE-R2 PCE-R3
370 C 370 C 370 C 370 C
5.2 5.2 5.2 5.2
e 300 C 350 C 300 C
e 65% 65% 75%
e 5 5 6
of 800 ml ethanol, 100 ml propanol, 18.5 ml distilled water, 75 g citric acid and 10 g hydroxyquinoline (ACII electrolyte) with the cooling of liquid nitrogen at 30 V for 40 s. The tensile specimen along ED with dog-boned geometry was prepared, and the tensile test was conducted with a tensile speed of 0.6 mm/s at ambient temperature. Moreover, the Al/Mg interfacial morphology of the fractured tensile specimens were observed by SEM. 3. Results and discussion 3.1. Al/Mg interface and transition layer The morphology of Al/Mg interface and the corresponding element distribution are shown in Fig. 2, where the Al and Mg elements are represented by green and red points, respectively. It is obvious that the transition layer was formed in Al/Mg interface after PCE and PCE-R processes, and two sub-layers can be obviously identified. According to EDS line analysis, the concentration of Al
element increases from Mg layer to Al layer across the transition layer, while the opposite tendency exists in Mg element. For PCER0, the continuous variation of the element concentration indicates that the materials were miscible and the intermetallic compounds (IMCs) was not formed. However, two distinct reaction layers can be observed in the transition layers of hot rolled PCE-R1, PCE-R2, and PCE-R3. Thus, EPMA analysis was conducted to investigate the phase on transition layer. The locations of spot concentration analysis were marked in Fig. 3, and the results are listed in Table 3. It proves that only the Al and Mg solid solution exist in the transition layer of PCE-R0. However, in other specimens of PCE-R1, PCE-R2 and PCE-R3, the reaction layer adjacent Al should be Al3Mg2, while that adjacent Mg should be Mg17Al12. Based on the Al-Mg phase diagram and the previous works [20,21], the IMCs of Al3Mg2 and Mg17Al12 can also be identified. For convenience, the IMCs of Al3Mg2 and Mg17Al12 are named as b and g phases, and they are marked in Fig. 2. At the beginning of diffusion process, Mg concentration in Al layer and Al concentration in Mg layer are much
Fig. 2. SEM images, EDS mapping, and EDS line scanning across Al/Mg interface of (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and (d)PCE-R3.
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Fig. 3. Spot concentration analysis by EPMA, (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and (d) PCE-R3.
Table 3 Results of spot concentration analysis corresponding to the points indicated in Fig. 3. Specimen
PCE-R0
PCE-R1
PCE-R2
PCE-R3
Point
1 2 3 4 1 2 3 4 5 1 2 3 4 5 6 1 2 3 4
Element compositions (at.o/o) Al
Mg
85.65 73.15 51.98 27.36 88.88 63.5 57.58 39.46 18.52 93.42 58.08 58.15 57.96 40.19 28.01 92.89 62.33 40.67 19.77
14.35 26.85 48.02 72.64 11.12 36.5 42.42 60.54 81.48 6.58 41.92 41.85 42.04 59.81 71.99 7.11 37.67 59.33 80.23
Molar ratio
Interface component
5.97 2.72 1.08 0.38 7.99 1.74 1.34 0.65 0.23 14.20 1.39 1.39 1.38 0.67 0.39 13.06 1.65 0.69 0.25
solid solution solid solution solid solution solid solution IMC-b þ Al IMC-b IMC-b IMC-g IMC-g þ Mg IMC-b þ Al IMC-b IMC-b IMC-b IMC-g IMC-g þ Mg IMC-b þ Al IMC-b IMC-g IMC-g þ Mg
(Al) (Al) (Al) (Mg)
lower than their solubility limits, which provides driving force for inter-diffusion. Hence, Al-based solid solution and Mg-based solid solution are formed in transition layer adjacent Al layer and Mg layer, respectively. With the proceeding of inter-diffusion process, the unstable supersaturated solid solution is formed, and the nucleation of intermediate phase (b and g) initiates. Finally, the IMCs grow transversely and form a whole body [7]. In this study,
the degree of inter-diffusion during PCE process was insufficient to reach the solubility limit. Thus, the ICMs were not formed, and two sub-layers are Al-based and Mg-based solid solutions. However, the solubility limits of Al and Mg were exceeded after PCE-R process, since the inter-diffusion can be enhanced at high rolling temperature and reduction ratio. Consequently, the IMCs were formed in transition layer of PCE-R1, PCE-R2, and PCE-R3. The bonding quality of Al/Mg interface has close relationship with the transition layer. The formation of IMCs is a symbol of achieving metallurgical bonding. However, the existence of hard and brittle IMCs is negative for the bonding quality, if the thickness of IMCs is too large. The average thickness of transition layer and IMCs (b and g) layers were measured by selecting 10 positions along ED, and the results are plotted in Fig. 4. It is noted that PCER1, PCE-R2, and PCE-R3 have thicker transition layer compared with PCE-R0, which is caused by the subsequent hot rolling process. With the increase of rolling temperature or reduction ratio, the thicknesses of transition layer and IMCs layers increase. Moreover, the variation of b layer is more noticeable than that of g layer. The above phenomena are explained as follows. The diffusion in solid materials is difficult and there are three basic mechanisms to promote the inter-diffusion of elements, which are (i) mechanically induced atomic displacements, (ii) pipe diffusion along dislocations, and (iii) plastic deformation induced vacancies. During PCE-R process, lots of dislocations and vacancies were induced by severe plastic deformation. However, in this study, the dislocations are difficult to carry atoms when they move because of the high strain rate generated in rolling process [22]. Thus, the inter-diffusion of elements should be closely related to the concentration of vacancy, and the diffusion coefficient D is expressed as Eq. (1) proposed by Sauvage et al. [23].
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coefficient and concentration of vacancy increase based on Eqs. (1) and (2), and the mobility of vacancies can also be enhanced. As a result, more activated vacancies can be induced at higher temperature, which accelerates the elements diffusion. During subsequent hot rolling process, a temperature rise was induced by the severe plastic deformation, and the temperature rise became more obvious with high reduction ratio, which can enhance the element diffusion. In addition, plastic deformation caused the significant reduction of the energies of Em and Ef [24], resulting into higher diffusion coefficient based on Eqs. (1) and (2). Besides, combined Figs. 2 and 4, b layer has much larger thickness than g layer, which is explained as follows. Firstly, b-phase has lower activation energy for inter-diffusion than g-phase, which leads to that b-phase has higher inter-diffusion coefficient than g-phase [25]. Then, the similar crystal structure between Al matrix (FCC) and b phase (FCC) promotes the growth of b phase, while the different crystal structure between Mg matrix (HCP) and g phase (BCC) inhibits the growth of g. Fig. 4. Thickness of the transition layer and IMCs (b and g) layers.
3.2. Microstructure characteristics of Al layer
D ¼ Ava2 Cv expð Em =KB TÞ
(1)
where, A is crystallographic parameter related to crystal structure, v is vibrational frequencies of the atoms, a is lattice constant, Cv is vacancy concentration, Em is vacancy migration energy, KB is Boltzmann constant, and T is temperature. The vacancy concentration Cv can be expressed as,
. . Cv ¼ exp Sf KB exp Ef KB T
(2)
where, Sf is vacancy formation entropy, and Ef is vacancy formation energy. With the increase of temperature, both the diffusion
EBSD analysis was conducted to investigate the microstructure of Al layer near the Al/Mg interface, and the grain morphology and the distribution of high angle boundaries (HABs) and low angle boundaries (LABs) are shown in Fig. 5, where thick-black lines and thin-grey lines indicate HABs (misorientation angle >15 ) and LABs (misorientation angle between 2 and 15 ), respectively. As is seen, the microstructure of all sheets is composed of coarse elongated grains and small amount of fine equiaxed grains, which distributed along the boundaries of coarse elongated grains. Furthermore, lots of LABs were formed inside coarse grains. Based on the above phenomena, it is confirmed that the partial continuous dynamic recrystallization (CDRX) occurred [26]. During PCE process, the strip shaped coarse grains along ED were firstly formed by the
Fig. 5. Inverse pole figure maps of the grain morphology in Al layer near the Al/Mg interface of (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and PCE-R3.
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elongation of initial coarse grains. Then, lots of dislocations and LABs were induced under the effects of strain hardening and dynamic recovery (DRV). Finally, these dislocations accumulated in some LABs, which led to the transformation from LABs to HABs and the occurrence of CDRX. When the subsequent hot rolling process was conducted, the grains were further elongated along ED again, and more fine equiaxed grains were formed by means of CDRX and grain fragmentation due to the strong shear strain in the Al/Mg interface. As a result, a typical rolling structure containing large amount of coarse elongated grains along ED and some fine equiaxed grains was obtained. Fig. 6 shows the average grain size (AGS) and kernel average misorientation (KAM) value of Al layer near Al/Mg interface. The grain size is obtained from EBSD analysis, and it is calculated based on the equivalent circle diameter. The AGS is calculated as 15.9 mm, 5.1 mm, 11.8 mm, and 4.8 mm for PCE-R0, PCE-R1, PCE-R2, and PCE-
Fig. 6. AGS and KAM values of Al layer near the Al/Mg interface.
R3, respectively. Thus, it is known that the temperature and reduction ratio of the subsequent hot rolling significantly affect AGS in Al layer. During hot rolling process, severe shear deformation occurred near the Al/Mg interface, which can promote the grain refinement by partial CDRX and grain fragmentation. Hence, the AGS of PCE-R1, PCE-R2, and PCE-R3 is smaller than that of PCE-R0. Compared PCE-R1 and PCE-R2 in Fig. 6, it is found that AGS of Al layer near Al/Mg interface is much larger at higher rolling temperature. The reason of this phenomenon can firstly attribute to the high mobility of grain growth at high temperature [27]. Besides, KAM value is the average misorientation of a pixel with its neighbouring pixels, which provides a direct estimation of dislocation activity. In other words, smaller KAM value indicates lower dislocations density [28]. It is obvious from Fig. 6 that KAM value decreases with increasing temperature, and that means the decrease of dislocations density. Thus, more dislocations in PCE-R1 rolled at low temperature promotes the grain refinement by fragmentation. Compared PCE-R1 and PCE-R3 in Fig. 5, it is noted that AGS slightly decreases with the rise of reduction ratio. The reason is that both the strain and temperature rise became higher with increasing reduction ratio. Based on the KAM value shown in Fig. 6, high strain facilitates the occurrence of CDRX and also increases the dislocation density. Hence, due to the high reduction ratio of specimen PCE-R3, higher degree of CDRX and grain fragmentation were achieved during the hot rolling process. Besides, the static recrystallization during the intermediate annealing should be more obvious. These two reasons lead to the grain refinement of PCE-R3. However, the obvious temperature rise is adverse for the grain refinement. Thus, due to these contrary effects, the AGS slightly decreases with increasing reduction ratio. Fig. 7 shows the relative frequency of misorientation angle, where the fraction of HABs is defined as fHABs. It is clearly that the fHABs of PCE-R1, PCE-R2, and PCE-R3 is larger than that of PCE-R0. This is because CDRX process further proceeds during the subsequent hot rolling, which is favorable for the transformation from LABs to HABs. Compared PCE-R1 to PCE-R2, fHABs increases at
Fig. 7. Relative frequency of misorientation angles in Al layer near the Al/Mg interface of (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and (d) PCE-R3.
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higher rolling temperature, which indicates higher fraction of CDRX. The growth of sub-grains and nucleation of DRXed grains can be promoted at higher deformation temperature, since more energy is provided. In addition, high temperature can promote the slip and climb of dislocation, which results into merging of subgrains, and transformation from LABs to HABs. Compared PCE-R1 to PCE-R3, fHABs also increases with the increase of reduction ratio. As mentioned above, large reduction ratio induces high strain and temperature rise. Both of them can enhance the occurrence of CDRX, which resulting the increase of fHABs when reduction ratio becomes higher. The texture evolution of Al layer near the Al/Mg interface should be complicated, since it suffered both hot extrusion and rolling processes. The orientation distribution function (ODF) sections are plotted in Fig. 8 to examine the micro-texture of Al layer. As is seen, PCE-R0 and PCE-R2 have shear-typed {111} fiber texture. PCE-R1 consists of {111} fiber texture and component with 10 shift of 41 from Copper texture. PCE-R3 has R-goss and some other textures. According to Ref. [17], the shear-typed {111} fiber texture was formed for PCE-R0 due to the shearing stress generated on Al/Mg interface during extrusion process, and the mutual transformation between those shear-typed components can be achieved under different deformation condition. During the subsequent hot rolling process, the plane strain promoted the transformation from sheartyped texture to rolling texture [29]. Thus, for PCE-R1, both sheartyped and rolling texture are obtained. However, in case of PCER2 rolled at higher temperature, the shear-typed texture occupies dominant position again. It reflects that these grains with rolling orientation become unstable at high temperature. As discussed above, PCE-R3 has high fHABs and DRX degree. Moreover, DRXed grains have more random orientation [30]. Thus, PCE-R3 exhibits quite different texture components. 3.3. Microstructure characteristics of Mg layer The grain structure and distribution of second particles in Mg layer near the Al/Mg interface are shown in Fig. 9. It is obvious that PCE-R0 and PCE-R1 consist of fine equiaxed grains and small amount of coarse elongated grains, while PCE-R2 and PCE-R3 only have relative coarse equiaxed grains. Thus, it indicates that near complete DRX and grain growth occurred during the PCE-R process.
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Compared with PCE-R0, the AGS of PCE-R1 is smaller due to the reoccurrence of DRX and the formation of more Mg17Al12 particles during hot rolling process. Moreover, in case of PCE-R2 and PCE-R3, the AGS increases with increasing temperature or reduction ratio. As is shown in Fig. 9, large amount of Mg17Al12 particles distribute along grain boundaries in PCE-R1, while some Mg17Al12 particles obviously becomes coarse and the amount becomes small in PCER2 and PCE-R3. Since Al segregation existed in homogenized AZ91 billet, the Mg17Al12 particles were dynamically precipitated in the following hot extrusion and rolling processes. Based on Ref. [31], precipitation of Mg17Al12 particles can be promoted by strain, which is denominated as strain-induced dynamic precipitation (SIDP). Thus, PCR-R1 have more Mg17Al12 particles compared with PCE-R0 caused by high strain experienced during the subsequent hot rolling process. The increasing rolling temperature enhanced the solid solubility of Al in Mg matrix, and the number of Mg17Al12 particles is reduced in PCR-R2. Similarly, larger reduction ratio induced temperature rise, which also reduces the amount of Mg17Al12 particles in PCR-R3. As is known, the amount and distribution of Mg17Al12 particles strongly affect the grain size of Mg alloys due to particle stimulated nucleation (PSN) and Zener pinning effects. Based upon the above reasons, the grain can be refined after the subsequent hot rolling process. However, the grains become coarse due to the less and coarse Mg17Al12 particles, with increasing rolling temperature or reduction ratio. Fig. 10 shows the grain morphology and grain size distributions in Mg layer near the Al/Mg interface of the specimens PCE-R0 and PCE-R1. The upper areas of Fig. 10 (a) and (b) correspond to the locations near Al/Mg interface, and the lower areas are the locations far away from Al/Mg interface. Both PCE-R0 and PCE-R1 consist of large amount of fine equiaxed grains and several coarse elongated grains. Moreover, more LABs are observed inside both fine and coarse grains in PCE-R1 than those in PCE-R0, which proves that DRX took place again during the subsequent hot rolling process. The AGS is reduced from 2.38 mm to 2.04 mm after hot rolling process due to the occurrence of DRX. These above EBSD results are in good accordance with the results shown in Fig. 9. On the other hand, it is noted that the grains near the Al/Mg interface are relatively finer than those far away from the Al/Mg interface in both specimens PCE-R0 and PCE-R1. During extrusion or hot rolling processes, a much severer strain was achieved in the area near Al/
Fig. 8. ODF sections in Al layer near the Al/Mg interface of (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and (d) PCE-R3.
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Fig. 9. SEM images of the grain morphology and precipitate distribution in Mg layer near the Al/Mg interface of (a) (b) PCE-R0, (c) (d) PCE-R1, (e) (f) PCE-R2, and (g) (h) PCE-R3.
Mg interface [32]. The severe strain can promote the nucleation rate of recrystallization, and thus it is beneficial for the formation of finer grain structure. The pole figures of PCE-R0 and PCE-R1 are plotted in Fig. 11. The typical strong basal plane texture with c-axis parallel to TD was formed in both PCE-R0 and PCE-R1. Besides, the slight preferred distribution of prismatic planes can be observed in PCE-R0, and it should be a weak {0001}<11e20> texture. However, in case of PCER1, the typical basal plane texture was formed due to the uniform distribution of prismatic planes in {10-10} and {11e20} pole figures. According to Ref. [33], during initial stage of porthole die extrusion, the shearing texture was induced by strong shearing stress between material and die wall. Then, during the solid bonding stage, c-axis tends to be parallel with TD due to the severe compressing stress along TD. Finally, during the extruding stage, the basal plane
gradually rotates around ED to be perpendicular with ND, which makes c-axis have a relatively small deviation along TD. Besides, the tensile stress along ED was imposed on the materials during extrusion process due to the shear friction between the billets and bridge. This fact should be the reason of the formation of {0001} <11e20> texture [34]. However, the tensile stress becomes weak at end of extrusion process, and it is responsible for weaken of {0001} <11e20> texture. As a result, PCE-R0 exhibits strong basal plane texture with c-axis parallel with TD, and weak {0001}<11e20> texture. During the subsequent rolling process, strong compressing stress was induced along TD, and it is beneficial for the rotating of caxis to be parallel to TD. Therefore, {0001}<11e20> texture disappeared and the intensity of basal plane texture was enhanced to 16.68.
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Fig. 10. Inverse pole figure maps and grain size distributions in Mg layer near the Al/Mg interface of (a) (c) PCE-R0 and (b) (d) PCE-R1.
Fig. 11. Pole figures in Mg layer near the Al/Mg interface of (a) PCE-R0 and (b) PCE-R1.
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3.4. Tensile property Fig. 12 shows the stress-strain data obtained from tensile tests. As is seen, in PCE-R1 and PCE-R2, the stress experiences two drops with the gradual increase of strain, while only one stress-drop is observed in PCE-R3. It means that the fracture mechanism is not affected by rolling temperature, while it varies with different reduction ratio. On the other hand, with the increase of rolling temperature from PCE-R1 (300 C) to PCE-R2 (350 C), the tensile strength decreases from 149 MPa to 98 MPa, and the elongation decreases from 0.13 to 0.11. In case of PCE-R3 hot rolled with large reduction ratio, its tensile strength and elongation are significantly inferior. During the tensile tests, it was observed that the Mg alloy firstly fractured, resulting in the first sudden drop of the stress.
Then, the Al alloy fractured, and it caused the second drop of the stress. Fig. 13 shows the SEM images in Al/Mg interface of the fractured tensile specimens. After tensile tests, the cracks appearing and propagating with a zig-zag mode along the Al/Mg interface are observed. This is because Mg alloys have relatively lower plastic deformation capacity than Al alloys, and such mismatch induced a localized high shear stress along the interface during tensile test. The similar phenomenon was also reported by Ref. [12]. As mentioned above, the first stress-drop is caused by the fracture of Mg layer, which indicates that Mg layer strongly affects the tensile properties of Al/Mg/Al sheet. Hence, the variation of tensile properties of different specimens is attempted to be explained based on the grain size and precipitation in Mg layer, and the thickness of IMCs layers. The specimen of PCE-R1 has finer grain structure and precipitates in Mg layer, which are beneficial for improving the tensile properties of strength and elongation [35]. Moreover, PCE-R2 owns thicker IMCs layer, where the cracks are easily formed during tensile tests. These facts result in the better tensile properties of PCE-R1 than those of PCE-R2. PCE-R3 presents quite low strength and elongation, and there is no sudden drop on its stress-strain curve. The main reasons should attribute to the facts that some cracks had been formed in PCE-R3. Fig. 14 shows the enlarged images of Al layer, Mg layer and Al/Mg interface of PCE-R3 just after hot rolling with high reduction. As is seen, there is no crack in Al layer due to the good plasticity of AA6063. However, a crack with length of about 370 mm was located along the Al/Mg interface, and a crack penetrated the whole thickness of Mg layer. During the subsequent tensile test, the Mg layer should easily fracture at low stress level due to the existed crack, and there is no sudden stress-drop was observed. This is also the reason that the tensile properties of PCE-R3 are inferior.
4. Conclusions
Fig. 12. Engineering stress-strain curves obtained from tensile tests.
The Al/Mg/Al sheet was fabricated based on the proposed PCE-R process. Moreover, the effects of rolling temperature and reduction
Fig. 13. SEM images of the morphology of Al/Mg interface after tensile tests of (a) PCE-R1, (b) PCE-R2, and (c) PCE-R3.
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Fig. 14. SEM images of PCE-R3 at (a) low magnification and the enlarged areas of (b) Z1, (c) Z2, and (d) Z3.
ratio on transition layer, microstructure, and tensile properties of Al/Mg/Al sheet were well investigated. Some important conclusions are drawn as follows. (1) The transition layer with solid solution was formed in Al/Mg interface after PCE process. However, after PCE-R process, two types of IMCs layers (b-Al3Mg2 and g-Mg17Al12) were formed in transition layer. The thicknesses of transition layer, b layer, and g layer increase with the increase of rolling temperature or reduction ratio. (2) The elongated grains and small amount of fine equiaxed grains were observed in Al layer, which implies the occurrence of partial DRX. The microstructure in Al layer was refined after subsequent hot rolling, and the grain size becomes larger at higher rolling temperature or smaller reduction ratio. Al layer of PCE-R0, PCE-R1, and PCE-R2 mainly consists of shear-typed {111} fiber textures, while PCE-R3 hot rolled with large reduction ratio exhibits quite different texture components. (3) Near complete DRX occurs in Mg layer, resulting into the microstructure with fine equiaxed grains and several elongated coarse grains. The grain size in Mg layer increases with increasing rolling temperature and reduction ratio because of the decreasing amount of Mg17Al12 particles. Mg layer exhibits strong basal plane texture with c-axis parallel to TD, and its intensity becomes higher after rolling process. (4) Two stress-drop fracture mechanism was observed for PCER1 and PCE-R2. Moreover, PCE-R1 after extrusion and rolling at 300 C with 65% reduction owns best tensile strength of 149 MPa, and elongation of 0.13. PCE-R3 rolled with 75% reduction has inferior tensile properties due to the appearance of some cracks.
Acknowledgements The authors would like to acknowledge the financial support
from National Natural Science Foundation of China (51875317), Young Scholars Program of Shandong University (2018WLJH26), and China Postdoctoral Science Foundation (2018T110686). References [1] Y. Mahmoodkhani, M.A. Wells, Co-extrusion process to produce AleMg eutectic clad magnesium products at elevated temperatures, J. Mater. Process. Technol. 232 (2016) 175e183. [2] Y. Meng, S. Fukushima, S. Sugiyama, J. Yanagimoto, Cold formability of AZ31 wrought magnesium alloy undergoing semisolid spheroidization treatment, Mater. Sci. Eng. A 624 (2015) 148e156. chet, Designing hybrid materials, Acta Mater. 51 (2003) [3] M.F. Ashby, Y.J.M. Bre 5801e5821. [4] B. Feng, Y. Xin, R. Hong, H. Yu, Y. Wu, Q. Liu, The effect of architecture on the mechanical properties of Mg-3Al-1Zn rods containing hard Al alloy cores, Scr. Mater. 98 (2015) 56e59. [5] T. Tokunaga, K. Matsuura, M. Ohno, Superplastic behavior of Al-coated Mg alloy sheet, J. Alloys Compd. 601 (2014) 179e185. [6] Y.B. Yan, Z.W. Zhang, W. Shen, J.H. Wang, L.K. Zhang, B.A. Chin, Microstructure and properties of magnesium AZ31B-aluminum 7075 explosively welded composite plate, Mater. Sci. Eng. A 527 (2010) 2241e2245. [7] W. Liu, L. Long, Y. Ma, L. Wu, Microstructure evolution and mechanical properties of Mg/Al diffusion bonded joints, J. Alloys Compd. 643 (2015) 34e39. [8] C. Tan, H. Zhu, T. Kuang, J. Shi, H. Liu, Z. Liu, Laser cladding Al-based amorphous-nanocrystalline composite coatings on AZ80 magnesium alloy under water cooling condition, J. Alloys Compd. 690 (2017) 108e115. [9] C. Luo, W. Liang, Z. Chen, J. Zhang, C. Chi, F. Yang, Effect of high temperature annealing and subsequent hot rolling on microstructural evolution at the bond-interface of Al/Mg/Al alloy laminated composites, Mater. Char. 84 (2013) 34e40. [10] H. Matsumoto, S. Watanabe, S. Hanada, Fabrication of pure Al/Mg-Li alloy clad plate and its mechanical properties, J. Mater. Process. Technol. 169 (2005) 9e15. [11] X.P. Zhang, T.H. Yang, S. Castagne, J.T. Wang, Microstructure; bonding strength and thickness ratio of Al/Mg/Al alloy laminated composites prepared by hot rolling, Mater. Sci. Eng. A 528 (2011) 1954e1960. [12] A. Macwan, X.Q. Jiang, C. Li, D.L. Chen, Effect of annealing on interface microstructures and tensile properties of rolled Al/Mg/Al tri-layer clad sheets, Mater. Sci. Eng. A 587 (2013) 344e351. [13] D.C. Foley, M. Al-Maharbi, K.T. Hartwig, I. Karaman, L.J. Kecskes, S.N. Mathaudhu, Grain refinement vs. crystallographic texture: mechanical anisotropy in a magnesium alloy, Scr. Mater. 64 (2011) 193e196. [14] M. Negendank, S. Mueller, W. Reimers, Coextrusion of Mg-Al macro composites, J. Mater. Process. Technol. 212 (2012) 1954e1962.
738
J. Tang et al. / Journal of Alloys and Compounds 784 (2019) 727e738
[15] Y. Wu, B. Feng, Y. Xin, R. Hong, H. Yu, Q. Liu, Microstructure and mechanical behavior of a Mg AZ31/Al 7050 laminate composite fabricated by extrusion, Mater. Sci. Eng. A 640 (2015) 454e459. [16] T. Tokunaga, K. Matsuura, M. Ohno, Aluminum coating on magnesium-based alloy by hot extrusion and its characteristics, Mater. Trans. 53 (2012) 1034e1041. [17] L. Chen, J. Tang, G. Zhao, C. Zhang, X. Chu, Fabrication of Al/Mg/Al laminate by a porthole die co-extrusion process, J. Mater. Process. Technol. 258 (2018) 165e173. [18] J. Jiang, Y. Liu, G. Xiao, Y. Wang, Y. Ju, Effect of pass reduction on microstructure, mechanical properties and texture of hot-rolled 7075 alloy, Mater. Char. 147 (2019) 324e339. [19] X. Huang, K. Suzuki, A. Watazu, I. Shigematsu, N. Saito, Effects of thickness reduction per pass on microstructure and texture of Mg-3Al-1Zn alloy sheet processed by differential speed rolling, Scr. Mater. 60 (2009) 964e967. [20] J.S. Kim, K.S. Lee, Y.N. Kwon, B.J. Lee, Y.W. Chang, S. Lee, Improvement of interfacial bonding strength in roll-bonded Mg/Al clad sheets through annealing and secondary rolling process, Mater. Sci. Eng. A 628 (2015) 1e10. [21] Z. Liang, G. Qin, L. Wang, X. Meng, F. Li, Microstructural characterization and mechanical properties of dissimilar friction welding of 1060 aluminum to AZ31B magnesium alloy, Mater. Sci. Eng. A 645 (2015) 170e180. [22] M. Ma, P. Huo, W.C. Liu, G.J. Wang, D.M. Wang, Microstructure and mechanical properties of Al/Ti/Al laminated composites prepared by roll bonding, Mater. Sci. Eng. A 636 (2015) 301e310. [23] X. Sauvage, F. Wetscher, P. Pareige, Mechanical alloying of Cu and Fe induced by severe plastic deformation of a Cu-Fe composite, Acta Mater. 53 (2005) 2127e2135. [24] C. Wang, Y. Jiang, J. Xie, D. Zhou, X. Zhang, Interface formation and bonding mechanism of embedded aluminum-steel composite sheet during cold roll bonding, Mater. Sci. Eng. A 708 (2017) 50e59. [25] S. Brennan, K. Bermudez, N.S. Kulkarni, Y. Sohn, Interdiffusion in the Mg-Al system and intrinsic diffusion in b-MgAl, Metall. Mater. Trans. A 43 (2012)
4043e4052. [26] S. Gourdet, F. Montheillet, A model of continuous dynamic recrystallization, Acta Mater. 51 (2003) 2685e2699. [27] J. Tang, L. Chen, X. Fan, G. Zhao, C. Zhang, Co-extrusion of dissimilar AA6063/ AA7075 by porthole die at various temperatures, J. Alloys Compd. 764 (2018) 162e169. [28] N. Nayan, N.P. Gurao, S.V.S. Narayana Murty, A.K. Jha, B. Pant, S.C. Sharma, K.M. George, Microstructure and micro-texture evolution during large strain deformation of an aluminium-copper-lithium alloy AA 2195, Mater. Des. 65 (2015) 862e868. [29] H.W. Kim, S.B. Kang, N. Tsuji, Y.M. Amino, Deformation textures of AA8011 aluminum alloy sheets severely deformed by accumulative roll bonding, Metall. Mater. Trans. A 36 (2005) 3151e3163. [30] H. Borkar, R. Gauvin, M. Pekguleryuz, Effect of extrusion temperature on texture evolution and recrystallization in extruded Mg-1o/oMn and Mg1o/oMn-1.6o/oSr alloys, J. Alloys Compd. 555 (2013) 219e224. [31] F. Guo, D. Zhang, X. Yang, L. Jiang, F. Pan, Strain-induced dynamic precipitation of Mg17Al12 phases in Mg-8Al alloys sheets rolled at 748K, Mater. Sci. Eng. A 636 (2015) 516e521. [32] G. Chen, L. Chen, G. Zhao, C. Zhang, W. Cui, Microstructure analysis of an AlZn-Mg alloy during porthole die extrusion based on modeling of constitutive equation and dynamic recrystallization, J. Alloys Compd. 710 (2017) 80e91. [33] L. Chen, J. Zhang, J. Tang, G. Chen, G. Zhao, C. Zhang, Microstructure and texture evolution during porthole die extrusion of Mg-Al-Zn alloy, J. Mater. Process. Technol. 259 (2018) 346e352. €upel-Herold, R.K. Mishra, Mechanical response [34] A.S. Khan, A. Pandey, T. Gna and texture evolution of AZ31 alloy at large strains for different strain rates and temperatures, Int. J. Plast. 27 (2011) 688e706. [35] T. Han, G. Huang, Q. Deng, G. Wang, B. Jiang, A. Tang, Y. Zhu, F. Pan, Grain refining and mechanical properties of AZ31 alloy processed by accumulated extrusion bonding, J. Alloys Compd. 745 (2018) 599e608.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Study on Al/Mg/Al sheet fabricated by combination of porthole die coextrusion and subsequent hot rolling Jianwei Tang, Liang Chen*, Guoqun Zhao, Cunsheng Zhang, Junquan Yu 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 4 September 2018 Received in revised form 16 December 2018 Accepted 1 January 2019 Available online 4 January 2019
Al/Mg/Al sheet was fabricated by the proposed porthole die co-extrusion and subsequent hot rolling (PCE-R) method. The intermetallic compounds of b-Al3Mg2 and g-Mg17Al12 were formed in transition layer after PCE-R process, and the thicknesses of b and g layers became larger at higher rolling temperature or higher reduction ratio. Partial dynamic recrystallization (DRX) occurred in Al layer, and Al layer mainly consisted of shear-typed {111} fiber textures. However, if the rolling reduction is as high as 75%, Al layer exhibited quite different texture components. Mg layer consisted of fine equiaxed grains and several elongated grains, implying the occurrence of near complete DRX. With the increase of rolling temperature or reduction ratio, the number of Mg17Al12 particles in Mg layer was reduced, resulting into larger grain size. Mg layer had strong basal plane texture with c-axis parallel to transverse direction. Al/ Mg/Al sheet rolled at 300 C with 65% reduction exhibited two stress-drop fracture mechanism during tensile test, and it showed excellent tensile strength of 149 MPa, and elongation of 0.13. However, Al/Mg/ Al sheet rolled with 75% reduction had inferior tensile properties due to the existence of some cracks after rolling. © 2019 Elsevier B.V. All rights reserved.
Keywords: Al/Mg/Al Microstructure Dynamic recrystallization Texture
1. Introduction Mg alloys have been used in automobile and high-speed train because of the distinct merits of high specific strength and low density [1,2]. However, the practical application of Mg alloys is still restricted by their intrinsic properties, such as poor corrosion resistance and low formability at ambient temperature. Contrarily, Al alloys as the light-weight materials have excellent corrosion resistance, since the thin and dense oxide layer can be formed on Al surface. According to recent studies [3e5], Al/Mg/Al laminates exhibit more excellent corrosion resistance and mechanical properties than those of Mg alloys, and it will not cause obvious weight increment. Al/Mg laminates can be fabricated by explosive welding [6], vacuum diffusion bonding [7], laser cladding [8], hot rolling [9e12], and hot extrusion [13e15]. Among these methods, hot rolling and hot extrusion own the advantages of high efficiency and low cost. For hot rolling, the parameters of rolling temperature, reduction ratio and subsequent annealing have significant effects on the
* Corresponding author. E-mail address: [email protected] (L. Chen). https://doi.org/10.1016/j.jallcom.2019.01.005 0925-8388/© 2019 Elsevier B.V. All rights reserved.
quality of rolled Al/Mg/Al laminate. Luo et al. [9] proposed that 5052 Al/AZ31B Mg/5052 Al laminate can be fabricated by a twopass hot rolling, where the first pass at low temperature can reduce the surface oxidation on Al/Mg interface. Matsumoto et al. [10] bonded Al/Mg-Li clad plate by cold rolling, and investigated the effects of annealing on the plate quality. Zhang et al. [11] fabricated Al/Mg/Al laminate through hot rolling, and studied the microstructure evolution, bonding strength and thickness ratio. Macwan et al. [12] combined hot and cold rolling to produce Al/Mg/Al composite plate, and studied the interface, microstructure and tensile properties of the plate after annealing. These above open literature proved the feasibility of producing Al/Mg laminate by rolling process. However, the preparation of Mg plate is usually difficult and costly. Moreover, strong basal texture can be formed during rolling process of Mg Alloy, and it has negative effects on the formability [13]. Finally, the impurity and oxidation existed on Al/ Mg interface are difficult to be removed, and it is harmful for the bonding quality. Hot extrusion is another effective method to produce Al/Mg composites, since both Al and Mg alloys have excellent extrudability. Negendank et al. [14] carried out co-extrusion using Al sleeve and Mg core, and Wu et al. [15] also obtained Al/Mg
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laminates with sound interface by extruding 7050 Al core and AZ31 Mg sleeve. Besides, Mahmoodkhani and Wells [1] proposed that the fabrication of Al/Mg composite can be achieved through coextrusion using Al plate and cylindrical Mg billet. Tokunaga et al. [16] put Al plate between Mg billet and an extrusion die, and obtained a Mg bar with uniform Al coating. However, it is difficult and wasteful to prepare the hybrid billet (Al sleeve and Mg core), and adverse effects of both impurity and oxidation are hard to be avoided. In our recent research [17], Al/Mg/Al laminate with sound Al/Mg interface was fabricated based on porthole die co-extrusion (PCE) process. It was found that PCE temperature had close relationship with the morphology of Al/Mg interface, microstructure, and mechanical properties of the extruded laminate. During PCE process, the contaminated surfaces of initial billets were remained in container, which is beneficial for avoiding the adverse effects resulting from the impurity and oxidation. Importantly, the homogenized Al and Mg billets with cylindrical shape are easy to be prepared. However, PCE process is usually difficult to produce thin Al/Mg/Al laminate sheet due to the capacity limitation of extruder. The combination of PCE and subsequent hot rolling (PCE-R) processes was proposed to fabricate the Al/Mg/Al sheet, and it is aimed to obtain Al/Mg/Al sheet with excellent bonding interface, microstructure and mechanical properties. The hot rolling experiments were conducted at various rolling temperature and reduction ratio. The formation and evolution of transition layer in Al/Mg interface were investigated, and the microstructure of the extruded and hot rolled Al/Mg/Al sheet were well examined. The tensile tests were carried out to study the effects of rolling temperature and reduction ratio on the strength and elongation of Al/Mg/Al sheet. Through this study, a novel and effective method based on PCE-R process is developed to fabricate Al/Mg/Al sheet.
2. Experimental procedures The chemical compositions of as-received AA6063 and AZ91 Mg alloy are listed in Table 1. The homogenization treatment of AA6063 and AZ91 ingots were performed at 480 C for 12 h, and 420 C for 10 h followed by the air cooling, respectively. After that, these
Table 1 Chemical compositions (wt.%) of the as-received AA6063 and AZ91 Mg alloys. Alloy
Chemical compositions (wt.%) Si
Fe
Cu
Mn
Zn
Al
Mg
AA6063 AZ91
0.45 0.031
0.35 e
0.10 e
0.12 0.33
0.12 0.64
Bal. 9.31
0.80 Bal.
homogenized alloys were machined to cylindrical billets for PCE-R process. The designed PCE-R process is schematically shown in Fig. 1, where the length, thickness, and width directions of the rolled Al/ Mg/Al sheet indicate the extrusion direction (ED), transverse direction (TD), and normal direction (ND), respectively. As shown in Fig. 1, PCE-R consists of PCE and subsequent hot rolling processes. The detailed experimental setup and procedures of PCE process have been illustrated in our recent study [17], and a brief introduction is given as below. The billets were placed inside container, and they were heated to 370 C and held for 15min. Then, PCE process was performed with a constant velocity. Finally, the plate shaped Al/Mg/Al laminate with a square cross-section of 12 12 mm was extruded out. After that, the subsequent hot rolling was conducted using a 2-high reversible laboratory rolling mill. The extruded Al/Mg/Al laminate was firstly preheated to experimental temperature in the furnace, and held for 15 min. Then, the laminate was hot rolled to the required thickness after multiple passes, during which the intermediate annealing was performed at 300 C for 30 min to reduce the strain hardening. After that, the rolled Al/Mg/Al sheet was cooled down in the air. In this study, the effects of rolling temperature and reduction ratio were studied, since both of them significantly influence the microstructure characteristics of hot rolled Al/Mg/Al sheet. The detailed process parameters are shown in Table 2. According to Refs. [18,19], the pass reduction has significant effect on the microstructure, formability and mechanical properties of rolled sheets. Thus, the amount of rolling pass was varied to achieve different reduction ratio. The extruded Al/Mg/Al laminate without hot rolling is designated as PCE-R0. Besides, the Al/Mg/Al sheets rolled at 300 C with a reduction ratio of 65%, 350 C with a reduction ratio of 65%, and 300 C with a reduction ratio of 75% are designated as PCE-R1, PCE-R2, and PCE-R3, respectively. After rolling process, the thickness of PCE-R1, PCE-R2 and PCE-R3 are approximate 4.2 mm, 4.2 mm and 3 mm, respectively. For PCE-R1 and PCE-R2, the thickness of Al layer and Mg layer are around 1.3 mm and 1.6 mm, while that of PCE-R3 are around 0.9 mm and 1.2 mm. The morphology of Al/Mg interface and the distribution of secondary phase were observed by means of scanning electron microscopy (SEM). The element distribution and interface component across Al/Mg interface were examined by energy dispersive spectroscopy (EDS) and electron probe micro-analyzer (EPMA). The grain morphology and micro-texture of Al and Mg layers near Al/ Mg interface were analyzed by electron back-scattered diffraction (EBSD). For EBSD measurements, Al specimen was electro-polished in the solution of 10 ml perchloric acid and 90 ml methanol at 30 V for 10 s, and Mg specimen was electro-polished using the mixture
Fig. 1. Schematic diagram of the designed PCE-R process. (Unit: mm).
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Table 2 Experimental conditions of PCE-R process. Specimen
Extrusion temperature
Extrusion ratio
Rolling Temperature
Reduction ratio
Rolling pass
PCE-R0 PCE-R1 PCE-R2 PCE-R3
370 C 370 C 370 C 370 C
5.2 5.2 5.2 5.2
e 300 C 350 C 300 C
e 65% 65% 75%
e 5 5 6
of 800 ml ethanol, 100 ml propanol, 18.5 ml distilled water, 75 g citric acid and 10 g hydroxyquinoline (ACII electrolyte) with the cooling of liquid nitrogen at 30 V for 40 s. The tensile specimen along ED with dog-boned geometry was prepared, and the tensile test was conducted with a tensile speed of 0.6 mm/s at ambient temperature. Moreover, the Al/Mg interfacial morphology of the fractured tensile specimens were observed by SEM. 3. Results and discussion 3.1. Al/Mg interface and transition layer The morphology of Al/Mg interface and the corresponding element distribution are shown in Fig. 2, where the Al and Mg elements are represented by green and red points, respectively. It is obvious that the transition layer was formed in Al/Mg interface after PCE and PCE-R processes, and two sub-layers can be obviously identified. According to EDS line analysis, the concentration of Al
element increases from Mg layer to Al layer across the transition layer, while the opposite tendency exists in Mg element. For PCER0, the continuous variation of the element concentration indicates that the materials were miscible and the intermetallic compounds (IMCs) was not formed. However, two distinct reaction layers can be observed in the transition layers of hot rolled PCE-R1, PCE-R2, and PCE-R3. Thus, EPMA analysis was conducted to investigate the phase on transition layer. The locations of spot concentration analysis were marked in Fig. 3, and the results are listed in Table 3. It proves that only the Al and Mg solid solution exist in the transition layer of PCE-R0. However, in other specimens of PCE-R1, PCE-R2 and PCE-R3, the reaction layer adjacent Al should be Al3Mg2, while that adjacent Mg should be Mg17Al12. Based on the Al-Mg phase diagram and the previous works [20,21], the IMCs of Al3Mg2 and Mg17Al12 can also be identified. For convenience, the IMCs of Al3Mg2 and Mg17Al12 are named as b and g phases, and they are marked in Fig. 2. At the beginning of diffusion process, Mg concentration in Al layer and Al concentration in Mg layer are much
Fig. 2. SEM images, EDS mapping, and EDS line scanning across Al/Mg interface of (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and (d)PCE-R3.
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Fig. 3. Spot concentration analysis by EPMA, (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and (d) PCE-R3.
Table 3 Results of spot concentration analysis corresponding to the points indicated in Fig. 3. Specimen
PCE-R0
PCE-R1
PCE-R2
PCE-R3
Point
1 2 3 4 1 2 3 4 5 1 2 3 4 5 6 1 2 3 4
Element compositions (at.o/o) Al
Mg
85.65 73.15 51.98 27.36 88.88 63.5 57.58 39.46 18.52 93.42 58.08 58.15 57.96 40.19 28.01 92.89 62.33 40.67 19.77
14.35 26.85 48.02 72.64 11.12 36.5 42.42 60.54 81.48 6.58 41.92 41.85 42.04 59.81 71.99 7.11 37.67 59.33 80.23
Molar ratio
Interface component
5.97 2.72 1.08 0.38 7.99 1.74 1.34 0.65 0.23 14.20 1.39 1.39 1.38 0.67 0.39 13.06 1.65 0.69 0.25
solid solution solid solution solid solution solid solution IMC-b þ Al IMC-b IMC-b IMC-g IMC-g þ Mg IMC-b þ Al IMC-b IMC-b IMC-b IMC-g IMC-g þ Mg IMC-b þ Al IMC-b IMC-g IMC-g þ Mg
(Al) (Al) (Al) (Mg)
lower than their solubility limits, which provides driving force for inter-diffusion. Hence, Al-based solid solution and Mg-based solid solution are formed in transition layer adjacent Al layer and Mg layer, respectively. With the proceeding of inter-diffusion process, the unstable supersaturated solid solution is formed, and the nucleation of intermediate phase (b and g) initiates. Finally, the IMCs grow transversely and form a whole body [7]. In this study,
the degree of inter-diffusion during PCE process was insufficient to reach the solubility limit. Thus, the ICMs were not formed, and two sub-layers are Al-based and Mg-based solid solutions. However, the solubility limits of Al and Mg were exceeded after PCE-R process, since the inter-diffusion can be enhanced at high rolling temperature and reduction ratio. Consequently, the IMCs were formed in transition layer of PCE-R1, PCE-R2, and PCE-R3. The bonding quality of Al/Mg interface has close relationship with the transition layer. The formation of IMCs is a symbol of achieving metallurgical bonding. However, the existence of hard and brittle IMCs is negative for the bonding quality, if the thickness of IMCs is too large. The average thickness of transition layer and IMCs (b and g) layers were measured by selecting 10 positions along ED, and the results are plotted in Fig. 4. It is noted that PCER1, PCE-R2, and PCE-R3 have thicker transition layer compared with PCE-R0, which is caused by the subsequent hot rolling process. With the increase of rolling temperature or reduction ratio, the thicknesses of transition layer and IMCs layers increase. Moreover, the variation of b layer is more noticeable than that of g layer. The above phenomena are explained as follows. The diffusion in solid materials is difficult and there are three basic mechanisms to promote the inter-diffusion of elements, which are (i) mechanically induced atomic displacements, (ii) pipe diffusion along dislocations, and (iii) plastic deformation induced vacancies. During PCE-R process, lots of dislocations and vacancies were induced by severe plastic deformation. However, in this study, the dislocations are difficult to carry atoms when they move because of the high strain rate generated in rolling process [22]. Thus, the inter-diffusion of elements should be closely related to the concentration of vacancy, and the diffusion coefficient D is expressed as Eq. (1) proposed by Sauvage et al. [23].
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coefficient and concentration of vacancy increase based on Eqs. (1) and (2), and the mobility of vacancies can also be enhanced. As a result, more activated vacancies can be induced at higher temperature, which accelerates the elements diffusion. During subsequent hot rolling process, a temperature rise was induced by the severe plastic deformation, and the temperature rise became more obvious with high reduction ratio, which can enhance the element diffusion. In addition, plastic deformation caused the significant reduction of the energies of Em and Ef [24], resulting into higher diffusion coefficient based on Eqs. (1) and (2). Besides, combined Figs. 2 and 4, b layer has much larger thickness than g layer, which is explained as follows. Firstly, b-phase has lower activation energy for inter-diffusion than g-phase, which leads to that b-phase has higher inter-diffusion coefficient than g-phase [25]. Then, the similar crystal structure between Al matrix (FCC) and b phase (FCC) promotes the growth of b phase, while the different crystal structure between Mg matrix (HCP) and g phase (BCC) inhibits the growth of g. Fig. 4. Thickness of the transition layer and IMCs (b and g) layers.
3.2. Microstructure characteristics of Al layer
D ¼ Ava2 Cv expð Em =KB TÞ
(1)
where, A is crystallographic parameter related to crystal structure, v is vibrational frequencies of the atoms, a is lattice constant, Cv is vacancy concentration, Em is vacancy migration energy, KB is Boltzmann constant, and T is temperature. The vacancy concentration Cv can be expressed as,
. . Cv ¼ exp Sf KB exp Ef KB T
(2)
where, Sf is vacancy formation entropy, and Ef is vacancy formation energy. With the increase of temperature, both the diffusion
EBSD analysis was conducted to investigate the microstructure of Al layer near the Al/Mg interface, and the grain morphology and the distribution of high angle boundaries (HABs) and low angle boundaries (LABs) are shown in Fig. 5, where thick-black lines and thin-grey lines indicate HABs (misorientation angle >15 ) and LABs (misorientation angle between 2 and 15 ), respectively. As is seen, the microstructure of all sheets is composed of coarse elongated grains and small amount of fine equiaxed grains, which distributed along the boundaries of coarse elongated grains. Furthermore, lots of LABs were formed inside coarse grains. Based on the above phenomena, it is confirmed that the partial continuous dynamic recrystallization (CDRX) occurred [26]. During PCE process, the strip shaped coarse grains along ED were firstly formed by the
Fig. 5. Inverse pole figure maps of the grain morphology in Al layer near the Al/Mg interface of (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and PCE-R3.
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elongation of initial coarse grains. Then, lots of dislocations and LABs were induced under the effects of strain hardening and dynamic recovery (DRV). Finally, these dislocations accumulated in some LABs, which led to the transformation from LABs to HABs and the occurrence of CDRX. When the subsequent hot rolling process was conducted, the grains were further elongated along ED again, and more fine equiaxed grains were formed by means of CDRX and grain fragmentation due to the strong shear strain in the Al/Mg interface. As a result, a typical rolling structure containing large amount of coarse elongated grains along ED and some fine equiaxed grains was obtained. Fig. 6 shows the average grain size (AGS) and kernel average misorientation (KAM) value of Al layer near Al/Mg interface. The grain size is obtained from EBSD analysis, and it is calculated based on the equivalent circle diameter. The AGS is calculated as 15.9 mm, 5.1 mm, 11.8 mm, and 4.8 mm for PCE-R0, PCE-R1, PCE-R2, and PCE-
Fig. 6. AGS and KAM values of Al layer near the Al/Mg interface.
R3, respectively. Thus, it is known that the temperature and reduction ratio of the subsequent hot rolling significantly affect AGS in Al layer. During hot rolling process, severe shear deformation occurred near the Al/Mg interface, which can promote the grain refinement by partial CDRX and grain fragmentation. Hence, the AGS of PCE-R1, PCE-R2, and PCE-R3 is smaller than that of PCE-R0. Compared PCE-R1 and PCE-R2 in Fig. 6, it is found that AGS of Al layer near Al/Mg interface is much larger at higher rolling temperature. The reason of this phenomenon can firstly attribute to the high mobility of grain growth at high temperature [27]. Besides, KAM value is the average misorientation of a pixel with its neighbouring pixels, which provides a direct estimation of dislocation activity. In other words, smaller KAM value indicates lower dislocations density [28]. It is obvious from Fig. 6 that KAM value decreases with increasing temperature, and that means the decrease of dislocations density. Thus, more dislocations in PCE-R1 rolled at low temperature promotes the grain refinement by fragmentation. Compared PCE-R1 and PCE-R3 in Fig. 5, it is noted that AGS slightly decreases with the rise of reduction ratio. The reason is that both the strain and temperature rise became higher with increasing reduction ratio. Based on the KAM value shown in Fig. 6, high strain facilitates the occurrence of CDRX and also increases the dislocation density. Hence, due to the high reduction ratio of specimen PCE-R3, higher degree of CDRX and grain fragmentation were achieved during the hot rolling process. Besides, the static recrystallization during the intermediate annealing should be more obvious. These two reasons lead to the grain refinement of PCE-R3. However, the obvious temperature rise is adverse for the grain refinement. Thus, due to these contrary effects, the AGS slightly decreases with increasing reduction ratio. Fig. 7 shows the relative frequency of misorientation angle, where the fraction of HABs is defined as fHABs. It is clearly that the fHABs of PCE-R1, PCE-R2, and PCE-R3 is larger than that of PCE-R0. This is because CDRX process further proceeds during the subsequent hot rolling, which is favorable for the transformation from LABs to HABs. Compared PCE-R1 to PCE-R2, fHABs increases at
Fig. 7. Relative frequency of misorientation angles in Al layer near the Al/Mg interface of (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and (d) PCE-R3.
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higher rolling temperature, which indicates higher fraction of CDRX. The growth of sub-grains and nucleation of DRXed grains can be promoted at higher deformation temperature, since more energy is provided. In addition, high temperature can promote the slip and climb of dislocation, which results into merging of subgrains, and transformation from LABs to HABs. Compared PCE-R1 to PCE-R3, fHABs also increases with the increase of reduction ratio. As mentioned above, large reduction ratio induces high strain and temperature rise. Both of them can enhance the occurrence of CDRX, which resulting the increase of fHABs when reduction ratio becomes higher. The texture evolution of Al layer near the Al/Mg interface should be complicated, since it suffered both hot extrusion and rolling processes. The orientation distribution function (ODF) sections are plotted in Fig. 8 to examine the micro-texture of Al layer. As is seen, PCE-R0 and PCE-R2 have shear-typed {111} fiber texture. PCE-R1 consists of {111} fiber texture and component with 10 shift of 41 from Copper texture. PCE-R3 has R-goss and some other textures. According to Ref. [17], the shear-typed {111} fiber texture was formed for PCE-R0 due to the shearing stress generated on Al/Mg interface during extrusion process, and the mutual transformation between those shear-typed components can be achieved under different deformation condition. During the subsequent hot rolling process, the plane strain promoted the transformation from sheartyped texture to rolling texture [29]. Thus, for PCE-R1, both sheartyped and rolling texture are obtained. However, in case of PCER2 rolled at higher temperature, the shear-typed texture occupies dominant position again. It reflects that these grains with rolling orientation become unstable at high temperature. As discussed above, PCE-R3 has high fHABs and DRX degree. Moreover, DRXed grains have more random orientation [30]. Thus, PCE-R3 exhibits quite different texture components. 3.3. Microstructure characteristics of Mg layer The grain structure and distribution of second particles in Mg layer near the Al/Mg interface are shown in Fig. 9. It is obvious that PCE-R0 and PCE-R1 consist of fine equiaxed grains and small amount of coarse elongated grains, while PCE-R2 and PCE-R3 only have relative coarse equiaxed grains. Thus, it indicates that near complete DRX and grain growth occurred during the PCE-R process.
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Compared with PCE-R0, the AGS of PCE-R1 is smaller due to the reoccurrence of DRX and the formation of more Mg17Al12 particles during hot rolling process. Moreover, in case of PCE-R2 and PCE-R3, the AGS increases with increasing temperature or reduction ratio. As is shown in Fig. 9, large amount of Mg17Al12 particles distribute along grain boundaries in PCE-R1, while some Mg17Al12 particles obviously becomes coarse and the amount becomes small in PCER2 and PCE-R3. Since Al segregation existed in homogenized AZ91 billet, the Mg17Al12 particles were dynamically precipitated in the following hot extrusion and rolling processes. Based on Ref. [31], precipitation of Mg17Al12 particles can be promoted by strain, which is denominated as strain-induced dynamic precipitation (SIDP). Thus, PCR-R1 have more Mg17Al12 particles compared with PCE-R0 caused by high strain experienced during the subsequent hot rolling process. The increasing rolling temperature enhanced the solid solubility of Al in Mg matrix, and the number of Mg17Al12 particles is reduced in PCR-R2. Similarly, larger reduction ratio induced temperature rise, which also reduces the amount of Mg17Al12 particles in PCR-R3. As is known, the amount and distribution of Mg17Al12 particles strongly affect the grain size of Mg alloys due to particle stimulated nucleation (PSN) and Zener pinning effects. Based upon the above reasons, the grain can be refined after the subsequent hot rolling process. However, the grains become coarse due to the less and coarse Mg17Al12 particles, with increasing rolling temperature or reduction ratio. Fig. 10 shows the grain morphology and grain size distributions in Mg layer near the Al/Mg interface of the specimens PCE-R0 and PCE-R1. The upper areas of Fig. 10 (a) and (b) correspond to the locations near Al/Mg interface, and the lower areas are the locations far away from Al/Mg interface. Both PCE-R0 and PCE-R1 consist of large amount of fine equiaxed grains and several coarse elongated grains. Moreover, more LABs are observed inside both fine and coarse grains in PCE-R1 than those in PCE-R0, which proves that DRX took place again during the subsequent hot rolling process. The AGS is reduced from 2.38 mm to 2.04 mm after hot rolling process due to the occurrence of DRX. These above EBSD results are in good accordance with the results shown in Fig. 9. On the other hand, it is noted that the grains near the Al/Mg interface are relatively finer than those far away from the Al/Mg interface in both specimens PCE-R0 and PCE-R1. During extrusion or hot rolling processes, a much severer strain was achieved in the area near Al/
Fig. 8. ODF sections in Al layer near the Al/Mg interface of (a) PCE-R0, (b) PCE-R1, (c) PCE-R2, and (d) PCE-R3.
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Fig. 9. SEM images of the grain morphology and precipitate distribution in Mg layer near the Al/Mg interface of (a) (b) PCE-R0, (c) (d) PCE-R1, (e) (f) PCE-R2, and (g) (h) PCE-R3.
Mg interface [32]. The severe strain can promote the nucleation rate of recrystallization, and thus it is beneficial for the formation of finer grain structure. The pole figures of PCE-R0 and PCE-R1 are plotted in Fig. 11. The typical strong basal plane texture with c-axis parallel to TD was formed in both PCE-R0 and PCE-R1. Besides, the slight preferred distribution of prismatic planes can be observed in PCE-R0, and it should be a weak {0001}<11e20> texture. However, in case of PCER1, the typical basal plane texture was formed due to the uniform distribution of prismatic planes in {10-10} and {11e20} pole figures. According to Ref. [33], during initial stage of porthole die extrusion, the shearing texture was induced by strong shearing stress between material and die wall. Then, during the solid bonding stage, c-axis tends to be parallel with TD due to the severe compressing stress along TD. Finally, during the extruding stage, the basal plane
gradually rotates around ED to be perpendicular with ND, which makes c-axis have a relatively small deviation along TD. Besides, the tensile stress along ED was imposed on the materials during extrusion process due to the shear friction between the billets and bridge. This fact should be the reason of the formation of {0001} <11e20> texture [34]. However, the tensile stress becomes weak at end of extrusion process, and it is responsible for weaken of {0001} <11e20> texture. As a result, PCE-R0 exhibits strong basal plane texture with c-axis parallel with TD, and weak {0001}<11e20> texture. During the subsequent rolling process, strong compressing stress was induced along TD, and it is beneficial for the rotating of caxis to be parallel to TD. Therefore, {0001}<11e20> texture disappeared and the intensity of basal plane texture was enhanced to 16.68.
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Fig. 10. Inverse pole figure maps and grain size distributions in Mg layer near the Al/Mg interface of (a) (c) PCE-R0 and (b) (d) PCE-R1.
Fig. 11. Pole figures in Mg layer near the Al/Mg interface of (a) PCE-R0 and (b) PCE-R1.
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3.4. Tensile property Fig. 12 shows the stress-strain data obtained from tensile tests. As is seen, in PCE-R1 and PCE-R2, the stress experiences two drops with the gradual increase of strain, while only one stress-drop is observed in PCE-R3. It means that the fracture mechanism is not affected by rolling temperature, while it varies with different reduction ratio. On the other hand, with the increase of rolling temperature from PCE-R1 (300 C) to PCE-R2 (350 C), the tensile strength decreases from 149 MPa to 98 MPa, and the elongation decreases from 0.13 to 0.11. In case of PCE-R3 hot rolled with large reduction ratio, its tensile strength and elongation are significantly inferior. During the tensile tests, it was observed that the Mg alloy firstly fractured, resulting in the first sudden drop of the stress.
Then, the Al alloy fractured, and it caused the second drop of the stress. Fig. 13 shows the SEM images in Al/Mg interface of the fractured tensile specimens. After tensile tests, the cracks appearing and propagating with a zig-zag mode along the Al/Mg interface are observed. This is because Mg alloys have relatively lower plastic deformation capacity than Al alloys, and such mismatch induced a localized high shear stress along the interface during tensile test. The similar phenomenon was also reported by Ref. [12]. As mentioned above, the first stress-drop is caused by the fracture of Mg layer, which indicates that Mg layer strongly affects the tensile properties of Al/Mg/Al sheet. Hence, the variation of tensile properties of different specimens is attempted to be explained based on the grain size and precipitation in Mg layer, and the thickness of IMCs layers. The specimen of PCE-R1 has finer grain structure and precipitates in Mg layer, which are beneficial for improving the tensile properties of strength and elongation [35]. Moreover, PCE-R2 owns thicker IMCs layer, where the cracks are easily formed during tensile tests. These facts result in the better tensile properties of PCE-R1 than those of PCE-R2. PCE-R3 presents quite low strength and elongation, and there is no sudden drop on its stress-strain curve. The main reasons should attribute to the facts that some cracks had been formed in PCE-R3. Fig. 14 shows the enlarged images of Al layer, Mg layer and Al/Mg interface of PCE-R3 just after hot rolling with high reduction. As is seen, there is no crack in Al layer due to the good plasticity of AA6063. However, a crack with length of about 370 mm was located along the Al/Mg interface, and a crack penetrated the whole thickness of Mg layer. During the subsequent tensile test, the Mg layer should easily fracture at low stress level due to the existed crack, and there is no sudden stress-drop was observed. This is also the reason that the tensile properties of PCE-R3 are inferior.
4. Conclusions
Fig. 12. Engineering stress-strain curves obtained from tensile tests.
The Al/Mg/Al sheet was fabricated based on the proposed PCE-R process. Moreover, the effects of rolling temperature and reduction
Fig. 13. SEM images of the morphology of Al/Mg interface after tensile tests of (a) PCE-R1, (b) PCE-R2, and (c) PCE-R3.
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Fig. 14. SEM images of PCE-R3 at (a) low magnification and the enlarged areas of (b) Z1, (c) Z2, and (d) Z3.
ratio on transition layer, microstructure, and tensile properties of Al/Mg/Al sheet were well investigated. Some important conclusions are drawn as follows. (1) The transition layer with solid solution was formed in Al/Mg interface after PCE process. However, after PCE-R process, two types of IMCs layers (b-Al3Mg2 and g-Mg17Al12) were formed in transition layer. The thicknesses of transition layer, b layer, and g layer increase with the increase of rolling temperature or reduction ratio. (2) The elongated grains and small amount of fine equiaxed grains were observed in Al layer, which implies the occurrence of partial DRX. The microstructure in Al layer was refined after subsequent hot rolling, and the grain size becomes larger at higher rolling temperature or smaller reduction ratio. Al layer of PCE-R0, PCE-R1, and PCE-R2 mainly consists of shear-typed {111} fiber textures, while PCE-R3 hot rolled with large reduction ratio exhibits quite different texture components. (3) Near complete DRX occurs in Mg layer, resulting into the microstructure with fine equiaxed grains and several elongated coarse grains. The grain size in Mg layer increases with increasing rolling temperature and reduction ratio because of the decreasing amount of Mg17Al12 particles. Mg layer exhibits strong basal plane texture with c-axis parallel to TD, and its intensity becomes higher after rolling process. (4) Two stress-drop fracture mechanism was observed for PCER1 and PCE-R2. Moreover, PCE-R1 after extrusion and rolling at 300 C with 65% reduction owns best tensile strength of 149 MPa, and elongation of 0.13. PCE-R3 rolled with 75% reduction has inferior tensile properties due to the appearance of some cracks.
Acknowledgements The authors would like to acknowledge the financial support
from National Natural Science Foundation of China (51875317), Young Scholars Program of Shandong University (2018WLJH26), and China Postdoctoral Science Foundation (2018T110686). References [1] Y. Mahmoodkhani, M.A. Wells, Co-extrusion process to produce AleMg eutectic clad magnesium products at elevated temperatures, J. Mater. Process. Technol. 232 (2016) 175e183. [2] Y. Meng, S. Fukushima, S. Sugiyama, J. Yanagimoto, Cold formability of AZ31 wrought magnesium alloy undergoing semisolid spheroidization treatment, Mater. Sci. Eng. A 624 (2015) 148e156. chet, Designing hybrid materials, Acta Mater. 51 (2003) [3] M.F. Ashby, Y.J.M. Bre 5801e5821. [4] B. Feng, Y. Xin, R. Hong, H. Yu, Y. Wu, Q. Liu, The effect of architecture on the mechanical properties of Mg-3Al-1Zn rods containing hard Al alloy cores, Scr. Mater. 98 (2015) 56e59. [5] T. Tokunaga, K. Matsuura, M. Ohno, Superplastic behavior of Al-coated Mg alloy sheet, J. Alloys Compd. 601 (2014) 179e185. [6] Y.B. Yan, Z.W. Zhang, W. Shen, J.H. Wang, L.K. Zhang, B.A. Chin, Microstructure and properties of magnesium AZ31B-aluminum 7075 explosively welded composite plate, Mater. Sci. Eng. A 527 (2010) 2241e2245. [7] W. Liu, L. Long, Y. Ma, L. Wu, Microstructure evolution and mechanical properties of Mg/Al diffusion bonded joints, J. Alloys Compd. 643 (2015) 34e39. [8] C. Tan, H. Zhu, T. Kuang, J. Shi, H. Liu, Z. Liu, Laser cladding Al-based amorphous-nanocrystalline composite coatings on AZ80 magnesium alloy under water cooling condition, J. Alloys Compd. 690 (2017) 108e115. [9] C. Luo, W. Liang, Z. Chen, J. Zhang, C. Chi, F. Yang, Effect of high temperature annealing and subsequent hot rolling on microstructural evolution at the bond-interface of Al/Mg/Al alloy laminated composites, Mater. Char. 84 (2013) 34e40. [10] H. Matsumoto, S. Watanabe, S. Hanada, Fabrication of pure Al/Mg-Li alloy clad plate and its mechanical properties, J. Mater. Process. Technol. 169 (2005) 9e15. [11] X.P. Zhang, T.H. Yang, S. Castagne, J.T. Wang, Microstructure; bonding strength and thickness ratio of Al/Mg/Al alloy laminated composites prepared by hot rolling, Mater. Sci. Eng. A 528 (2011) 1954e1960. [12] A. Macwan, X.Q. Jiang, C. Li, D.L. Chen, Effect of annealing on interface microstructures and tensile properties of rolled Al/Mg/Al tri-layer clad sheets, Mater. Sci. Eng. A 587 (2013) 344e351. [13] D.C. Foley, M. Al-Maharbi, K.T. Hartwig, I. Karaman, L.J. Kecskes, S.N. Mathaudhu, Grain refinement vs. crystallographic texture: mechanical anisotropy in a magnesium alloy, Scr. Mater. 64 (2011) 193e196. [14] M. Negendank, S. Mueller, W. Reimers, Coextrusion of Mg-Al macro composites, J. Mater. Process. Technol. 212 (2012) 1954e1962.
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J. Tang et al. / Journal of Alloys and Compounds 784 (2019) 727e738
[15] Y. Wu, B. Feng, Y. Xin, R. Hong, H. Yu, Q. Liu, Microstructure and mechanical behavior of a Mg AZ31/Al 7050 laminate composite fabricated by extrusion, Mater. Sci. Eng. A 640 (2015) 454e459. [16] T. Tokunaga, K. Matsuura, M. Ohno, Aluminum coating on magnesium-based alloy by hot extrusion and its characteristics, Mater. Trans. 53 (2012) 1034e1041. [17] L. Chen, J. Tang, G. Zhao, C. Zhang, X. Chu, Fabrication of Al/Mg/Al laminate by a porthole die co-extrusion process, J. Mater. Process. Technol. 258 (2018) 165e173. [18] J. Jiang, Y. Liu, G. Xiao, Y. Wang, Y. Ju, Effect of pass reduction on microstructure, mechanical properties and texture of hot-rolled 7075 alloy, Mater. Char. 147 (2019) 324e339. [19] X. Huang, K. Suzuki, A. Watazu, I. Shigematsu, N. Saito, Effects of thickness reduction per pass on microstructure and texture of Mg-3Al-1Zn alloy sheet processed by differential speed rolling, Scr. Mater. 60 (2009) 964e967. [20] J.S. Kim, K.S. Lee, Y.N. Kwon, B.J. Lee, Y.W. Chang, S. Lee, Improvement of interfacial bonding strength in roll-bonded Mg/Al clad sheets through annealing and secondary rolling process, Mater. Sci. Eng. A 628 (2015) 1e10. [21] Z. Liang, G. Qin, L. Wang, X. Meng, F. Li, Microstructural characterization and mechanical properties of dissimilar friction welding of 1060 aluminum to AZ31B magnesium alloy, Mater. Sci. Eng. A 645 (2015) 170e180. [22] M. Ma, P. Huo, W.C. Liu, G.J. Wang, D.M. Wang, Microstructure and mechanical properties of Al/Ti/Al laminated composites prepared by roll bonding, Mater. Sci. Eng. A 636 (2015) 301e310. [23] X. Sauvage, F. Wetscher, P. Pareige, Mechanical alloying of Cu and Fe induced by severe plastic deformation of a Cu-Fe composite, Acta Mater. 53 (2005) 2127e2135. [24] C. Wang, Y. Jiang, J. Xie, D. Zhou, X. Zhang, Interface formation and bonding mechanism of embedded aluminum-steel composite sheet during cold roll bonding, Mater. Sci. Eng. A 708 (2017) 50e59. [25] S. Brennan, K. Bermudez, N.S. Kulkarni, Y. Sohn, Interdiffusion in the Mg-Al system and intrinsic diffusion in b-MgAl, Metall. Mater. Trans. A 43 (2012)
4043e4052. [26] S. Gourdet, F. Montheillet, A model of continuous dynamic recrystallization, Acta Mater. 51 (2003) 2685e2699. [27] J. Tang, L. Chen, X. Fan, G. Zhao, C. Zhang, Co-extrusion of dissimilar AA6063/ AA7075 by porthole die at various temperatures, J. Alloys Compd. 764 (2018) 162e169. [28] N. Nayan, N.P. Gurao, S.V.S. Narayana Murty, A.K. Jha, B. Pant, S.C. Sharma, K.M. George, Microstructure and micro-texture evolution during large strain deformation of an aluminium-copper-lithium alloy AA 2195, Mater. Des. 65 (2015) 862e868. [29] H.W. Kim, S.B. Kang, N. Tsuji, Y.M. Amino, Deformation textures of AA8011 aluminum alloy sheets severely deformed by accumulative roll bonding, Metall. Mater. Trans. A 36 (2005) 3151e3163. [30] H. Borkar, R. Gauvin, M. Pekguleryuz, Effect of extrusion temperature on texture evolution and recrystallization in extruded Mg-1o/oMn and Mg1o/oMn-1.6o/oSr alloys, J. Alloys Compd. 555 (2013) 219e224. [31] F. Guo, D. Zhang, X. Yang, L. Jiang, F. Pan, Strain-induced dynamic precipitation of Mg17Al12 phases in Mg-8Al alloys sheets rolled at 748K, Mater. Sci. Eng. A 636 (2015) 516e521. [32] G. Chen, L. Chen, G. Zhao, C. Zhang, W. Cui, Microstructure analysis of an AlZn-Mg alloy during porthole die extrusion based on modeling of constitutive equation and dynamic recrystallization, J. Alloys Compd. 710 (2017) 80e91. [33] L. Chen, J. Zhang, J. Tang, G. Chen, G. Zhao, C. Zhang, Microstructure and texture evolution during porthole die extrusion of Mg-Al-Zn alloy, J. Mater. Process. Technol. 259 (2018) 346e352. €upel-Herold, R.K. Mishra, Mechanical response [34] A.S. Khan, A. Pandey, T. Gna and texture evolution of AZ31 alloy at large strains for different strain rates and temperatures, Int. J. Plast. 27 (2011) 688e706. [35] T. Han, G. Huang, Q. Deng, G. Wang, B. Jiang, A. Tang, Y. Zhu, F. Pan, Grain refining and mechanical properties of AZ31 alloy processed by accumulated extrusion bonding, J. Alloys Compd. 745 (2018) 599e608.