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Al 7050 laminate composite fabricated by extrusion
Author’s Accepted Manuscript Microstructure and mechanical behavior of a Mg AZ31/Al 7050 laminate composite fabricated by extrusion Yang Wu, Bo Feng, Yunchang Xin, Rui Hong, Huihui Yu, Qing Liu www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(15)30039-3 http://dx.doi.org/10.1016/j.msea.2015.05.094 MSA32422
To appear in: Materials Science & Engineering A Received date: 7 May 2015 Revised date: 23 May 2015 Accepted date: 26 May 2015 Cite this article as: Yang Wu, Bo Feng, Yunchang Xin, Rui Hong, Huihui Yu and Qing Liu, Microstructure and mechanical behavior of a Mg AZ31/Al 7050 laminate composite fabricated by extrusion, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.05.094 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and Mechanical Behavior of a Mg AZ31/Al 7050 Laminate Composite Fabricated by Extrusion Yang Wu, Bo Feng, Yunchang Xin*, Rui Hong, Huihui Yu, Qing Liu School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
Abstract: In the present study, a well bonded Mg/Al laminate containing a soft Mg AZ31 sleeve and an ultra hard Al 7050 core was successfully fabricated by extrusion directly from the as-cast alloys, with the aim to harden Mg alloys. Microstructure and mechanical behavior of the composite were systematically studied. Our results show that the Mg layer contains a bimodal grain structure. The typical texture of the monolithic Mg alloy extruded plate develop in the Mg layer in Mg/Al laminate with most basal poles parallel to the normal direction (ND) and a small fraction of them close to the transverse direction (TD). The Al layer has high fractions of S {123}<634> and cube {001}<100> texture components. The ultra hard Al layer effectively enhances the yield strength. An Al fraction of 40.2% can enhance the yield strength from 155 MPa to 300 MPa, while only increase the weight by 20%. The relationship between the strength of the Mg/Al composite and the strengths and fractions of the Mg and Al layers were studied. The experimental yield strength of the composite is slightly higher than that predicted by the rule of mixtures, while the predicted ultimate strength greatly differs from the experimental one. The main reason lies in that, during tension of the Mg/Al composite, the Mg and the Al layers fracture before they reach their ultimate strength.
Keywords:
magnesium
alloys;
aluminum
alloys;
laminate
composite;
microstructure; mechanical properties; *
Corresponding author: Dr. Yunchang Xin Tel.:+86-23-65106407. Fax: +86-23-65106407. Electronic mail: [email protected]
1. Introduction Magnesium alloys are being increasingly concerned in the automotive and aerospace industries for weight saving [1]. However, their applications have been greatly limited by their low strength or poor formability [2]. Hybrid metal composites that combine the advantages of different metals are effective to tailor mechanical properties such as strength, ductility, impacting, fatigue and fracture [3-5]. A typical example is the dissimilar laminates that are fabricated by roll bonding or accumulative roll bonding (ARB) [6]. Aluminum alloys also have a low density and excellent mechanical properties, e.g. good ductility or high strength [7]. Yield strengths of the commercial high strength Mg alloys (e.g. AZ80 or ZK60) are mainly in a range of 250-350 MPa [8-10], while that of some commercial high strength Al alloys (e.g. 7050 or 2095) are up to 600-700 MPa [7, 11]. Therefore, a Mg/Al bimetal structure has the potential to improve mechanical properties of Mg alloys without an obvious increase of the weight of constructions. Mg/Al multilayer laminates have spawn much interest in the field of light-weight constructions in quest of excellent impacting or damping capacities [2, 12]. The commonly used method to fabricate Mg/Al laminates is roll bonding.
Various Mg/Al laminate composites (e.g. Al 1060/Mg AZ31 [13], Al 5052/Mg AZ31 [14, 15], Al 5052/pure Mg [16], Al 7075/Mg AZ31 [17]) are prepared using roll bonding or ARB. However, the poor rolling ability of Mg alloys leads to a high cost to prepare thin Mg plates for subsequent roll bonding [18]. The hot-rolled or cold-rolled plates often have a strong basal texture which greatly reduces the deformation ability. Therefore, the strong basal texture of thin Mg plate also greatly deteriorate the roll bonding performance of Mg alloys [8, 11, 19]. Extrusion is also a process to prepare thin plates in a mass production manner. Extrusion process is suitable for both Mg alloys and Al alloys. Therefore, Mg/Al laminates can be fabricated from as-cast Mg and Al alloys. From this point of view, extrusion process is a highly efficient method. However, the usage of extrusion to fabricate Mg/Al laminates is rarely reported. Up to now, only one publication has reported the application of extrusion to fabricate an Al 6082/ Mg AZ31 plate [3]. However, in that publication, a rolled thin AZ31 plate in halved aluminum billet was used for extrusion. The usage of thin Mg plate for extrusion processing greatly reduces the efficiency of this method. Al 7050 is a typical high strength Al alloy with yield strength up to 600 MPa [7], hence, it is a desirable candidate to harden a soft Mg alloy. However, the quite different mechanical properties between the Mg alloys and the 7050 plates lead to difficulties in roll bonding of the two materials. In the present study, a Mg AZ31/Al 7050 laminate with excellent layer/layer bonding was successfully fabricated directly from as-cast Mg and Al alloy alloys. Microstructure and mechanical properties of the as-extruded laminate were systematically studied.
For a composite containing hybrid metals, the relationship between the strength of the composite and the strengths and fractions of each constituent is an important issue. A deep understanding about this topic contributes to predicting mechanical properties of composites. Therefore, the relationship between strength of the as-prepared Mg/Al plate and the strengths and fractions of Mg and Al layers were evaluated by accurate measurements of the mechanical properties of each component cut directly from the Mg/Al composite.
2. Experiments and methods 2.1 Extrusion and mechanical tests Mg alloy AZ31 and Al alloy 7050 in the as-cast and homogenized condition were used for the fabrication of the Mg/Al composite plate. As explained in Fig. 1a, the Mg alloy billet was machined into a hollow cup with an outer diameter of 80 mm and an inner diameter of 40 mm. An as-cast Al alloy cylinder with a diameter of 40 mm was cut, polished and filled into the Mg hollow cup. This bimetal billet was kept in furnace at 470 °C for 2 h to complete the solid solution of Al 7050, and immediately extruded at 470 °C using an extrusion rate of 4.5 mm/min. The extruded Mg/Al laminate was immediately quenched into water after exiting the die followed by a subsequent two-stage aging at 120 °C for 8 h and at 150 °C for 9 h. The final extruded plate has a width of 60 mm and a thickness of 3 mm. If extrusion of Mg/Al bimetal billet was performed at a temperature below 470 °C, dynamic precipitation would take place during extrusion, which will greatly reduce aging hardening response of Al 7050 after extrusion. As Mg/Al intermetallics generally have a melting
temperature of about 430 °C, a further solid solution treatment of 7050 cannot be performed after extrusion. The as-used extrusion process can acquire a fully solid solution treated Al 7050 layer after extrusion and the subsequent aging can effectively improve strength of Al 7050. So the as-used extrusion process was desirable to acquire better strength. The preparation of specimens for tensile tests is shown in Fig. 1b. Specimens for tensile test of the Mg AZ31/Al 7050 laminate (the designated Mg/Al), the Al 7050 layer (the designated Al), and the Mg AZ31 layer (the designated Mg) were all cut directly from the as-extruded plate. Tensile tests along the extrusion direction (ED) at room temperature were performed on a Shimadzu mechanical testing system using a strain rate of 10-3 s-1. Each mechanical test was repeated three times. 2.2 Characterization of microstructure and texture Microstructure of the Mg/Al laminate was examined using an optical microscopy (ZEISS, Axiovert 40MAT) and a scanning electron microscopy (SEM, TESCAN Vega TS 5136XM). Element distribution in the transition layer between Mg and Al layers was analyzed by energy dispersive X-ray spectroscopy (EDS). Electron backscattered diffraction (EBSD) analyses were conducted on a FEI Nova 400 SEM equipped with a HKL Chanel 5 system using a step size of 1 μm. Samples for EBSD mapping were mechanically ground followed by electrochemical polishing in the AC2 electrolyte. The EBSD data were analyzed using the Channel 5 software. The texture components in Al alloy layers were subsequently analyzed by the orientation distribution
function
(ODF).
Five
texture
components ， cube{001}<100>,
Goss{011}<100>, brass{011}<211>, S{123}<634> and copper{112}<111>, were analyzed. They were calculated by integrating the ODF within 15° of the ideal texture component.
3. Results 3.1. Microstructure Fig. 2 presents optical microstructure of the Mg layer in the Mg/Al laminate. As regions a and d in Fig. 2f experienced different stress states, microstructures in the two regions were both examined. As seen in Fig. 2a and d, an obvious gradient microstructure from the surface to the center appears in region labeled ‘a’, while is absent in region ‘d’. The surface of region ‘a’ contains much finer structure than the center. The high magnification views in Fig. 2b, c and e indicate that the Mg layer contains a bimodal grain structure containing both large grains up to 50 μm and fine grains less than 5 μm. Fibrous and unrecrystallized structure is also discerned in region ‘a’. SEM cross-sectional micrographs of the Mg/Al laminate are shown in Fig. 3. In the Mg/Al sandwich, the Mg layer is well bonded to the Al layer. A transition layer with a thickness of about 10 μm exists between the Al and the Mg layers. This transition layer contains two distinct sub-layers. EDS mapping clearly shows that the sub-layer adjacent to the Mg matrix contains a higher Mg content and a lower Al content, while a higher Al content and a low Mg content exist in the sub-layer next to the Al matrix. This double-layered transition layer often appears in Mg/Al composites
[20, 21]. It is reported that the sub-layer adjacent to Mg is mainly composed of Mg17Al12 and that next to Al contains much Mg2Al3 [21].
3.2. Texture EBSD analyses of microstructure and crystallographic orientation of the Mg layer in the Mg/Al laminate are given in Fig. 4. Two types of texture component, a high fraction (85% in area fraction) of (0002) poles largely parallel to ND and a small fraction (15% in area fraction) of basal poles perpendicular to TD is noticed. The two subsets were picked out and further analyzed in Fig. 4b and c. A preferred distribution of the prismatic planes exist in the texture component <0002>//TD, but not in <0002>//ND. Due to the low fraction of the texture component with <0002>//TD, the __
preferred distribution of prismatic planes is not obvious in the {1010} pole figure in Fig. 4a. The appearance of preferred distribution of prismatic planes indicates a high activity of prismatic slip during extrusion. Microstructure and crystallographic orientation map of the Al layer in Mg/Al laminate are shown in Fig. 5. A large fraction of fibrous and elongated grains and a small fraction of equaxied grains appear in the examined region. Many low angle boundaries with misorientation below 10° are discerned too. Texture of the extruded Al alloys may contain several different components, e.g. copper, S, and brass [22] and cube and Goss orientations are also important for the texture analysis of deformed Al alloys [16]. The fractions of five typical texture components，cube{001}<100>, Goss{011}<100>, brass{011}<211>, S{123}<634> and copper{112}<111>, were
analyzed and are shown in Fig. 5b and Table 1. The Al layer contains high fractions of S and cube components. The components Goss, brass and copper are nearly absent.
3.3. Mechanical behavior True stress-strain curves of the Mg/Al, Mg and Al under tension along the ED are shown in Fig. 6. The yield strengths, ultimate strengths and elongations to fracture derived from those stress-strain curves are listed in Table 2. The Mg/Al composite has a yield strength up to 300 MPa, but a quite low elongation of 1.6%. The Mg layer in Mg/Al laminate has a much lower yield strength (155 MPa) and a poor plasticity (4.7%) in comparison to the Al 7050 layer with a yield strength up to 498 MPa and a better elongation. It can be seen that the hard Al 7050 layer effectively improves the yield strength. Compared to the Mg alloy, the Mg/Al laminate has a higher elastic modulus, too. The as-fabricated Mg/Al laminate has a quite low elongation during tension along the ED. During ED tension, cracking of Mg/Al interface might take place. In order to examine how this interface cracking affect plasticity, microstructure of Mg/Al interface was examined using SEM. As shown in Fig. 7, severe interface cracking appears just after yielding during ED tension. The quick cracking during at interface during ED tension is suspected to an important reason for the low ductility of Mg/Al composite.
4. Discussion 4.1 Microstructure and texture of Mg/Al laminates The present study shows that Mg AZ31/Al 7050 laminate with excellent layer/layer bonding can be successfully fabricated by extrusion directly from the as-cast Mg and Al billets. Therefore, fabrication of Mg/Al laminate by extrusion provides a highly efficient method for mass productions. It should be pointed out that this extrusion process to fabricate metal laminates is attractive for Mg/Al or Mg/Mg laminates, but not suitable for bimetal laminates containing steel or titanium. In this case, roll bonding is the suitable and predominant method. During extrusion of hybrid metal billets, a strong friction stress and shear exist at the hybrid metal interface [23]. This may contribute to refine the grains in both the Mg and Al layers. In the Mg/Al laminate, Mg AZ31 (77 Wm-1K-1) possesses a much lower thermal conductivity than Al 7050 (157 Wm-1K-1) [7, 24]. It is expected that the adiabatic heating localized near the core-sleeve interface during extrusion is mainly conducted by the Al alloy core. This will benefit grain refinement in the Mg layer. This is confirmed in the present study, the high extrusion temperature of 470 °C does not generate obvious grain coarsening in the Mg layer (Fig. 2). As seen in Fig. 4, the Mg layer in Mg/Al laminate contains a high fraction of the texture component with basal poles largely parallel to the ND and a small fraction of that with (0002) poles close to the TD. This type of texture is the typical texture of monolithic Mg alloy extruded plates [25]. This indicates that the different stress state during extrusion of Mg/Al bimetal billet from that during extrusion of monolithic Mg
billet hardly affects the texture type of Mg layer. In another recent publication [26], it is also found that the Mg alloy constituent in a Mg/Al extruded rod also shows a texture similar to that in a monolithic Mg alloy extruded rod. It seems that the texture of the Mg layer in a Mg/Al extruded composite is insensitive to the variations of the extrusion stress state. It is reported that only a strong basal texture with (0002) poles parallel to the ND exists in the Mg layer of Mg/Al laminates fabricated by roll bonding [14, 15]. Therefore, the Mg layer in an extruded Mg/Al laminate has a weakened texture compared with that in a roll-bonded one, due to the generation of the texture component <0002>//TD. This will benefit the subsequent deformation capability of Mg/Al composites. It is well established that a strong β-fiber texture consisting of the copper, S, and bass orientations often develops in deformed Al or Al alloys [11]. The Al layer in the as-extruded Mg/Al laminate also has a strong β-fiber texture with a high fraction of S component up to 58.3%. The high fraction of β-fiber texture in Al layer is mainly due to the high extrusion strain. The Al layer also contains 18.9% of cube component. Generally, the cube component is a recrystallization texture and often appears in most hot-worked Al alloys with characteristic shifts or scatterings of the orientation peak [11]. As the fractions of different texture components in hot worked Al alloys are highly dependent on the processing parameters, it is difficult to judge whether the stress-state during extrusion of Mg/Al bimetal billet will pose a great influence on the texture of the Al 7050 layer in the present study.
4.2 Mechanical behavior of metal composites The inclusion of Al alloy core in Mg alloy sleeve leads to an increase in the weight. The average density of Mg/Al laminate can be calculated by the rule of mixtures. The alloy core has a volume fraction of 40.2% in the gage part of the specimens for tensile tests. If an Al alloy density of 2.7 g/cm3 and a Mg alloy density of 1.8 g/cm3 are used in this calculation, the density of Mg/Al laminate is about 2.16 g/cm3. Compared to pure Mg, the weight of the Mg/Al laminate only increases by 20%, while yield strength rises by about 90%. The specific strengths of each plate were calculated and listed in Table 3. The Al 7050 layer has the highest specific strength. However, the Mg/Al laminate has a much higher specific strength than the Mg layer. Therefore, an ultra-hard Al alloy core can effectively improve strength of Mg alloy plate without obvious compromise in the weight for structure design. The relationship between the strength of a composite and the strengths and fractions of each constituent is extensively studied. It is often claimed that this relationship can be described by the rule of mixture as follows:
c 1 V1 2 V2
(1)
Where i and Vi are respectively the strength and volume fraction of the constituent ‘i’. However, the accuracy of the rule of mixtures needs to be further verified, as the mechanical properties of each constituent used in equation (1) in many studies are not directly measured from the specimens cut from the composites, but, the predicted or deduced values. In the present study, the mechanical properties of the
Mg layer and Al layer are tested from specimens cut directly in the composite, which allows a precise evaluation about the efficiency of the rule of mixtures. The experimentally measured yield strengths and ultimate strengths and the ones predicted by the rule of mixtures are listed in Table 4. The predicted yield strength (289 MPa) only shows a slight deviation from the experimental one (300 MPa). However, the experimental ultimate strength (330 MPa) greatly differs from the predicted one (359 MPa). It seems that yield strength of Mg/Al laminate can be effectively predicted by the rule of mixtures, but the rule of mixtures does not work for predicting the ultimate strength. However, as seen in Fig. 6, the Mg/Al laminate reaches the peak stress at a strain of about 2.4%, while peak stresses of the Mg layer and Al layer in the Mg/Al laminate generally appear at strains 5.7% and 7.5%, respectively. That is, during tension of the Mg/Al laminate, the Mg and the Al layers fracture before they reach their maximum strength. As the Mg/Al laminate fractures at a strain of 2.4%, the flow stresses of Mg and Al layer at strain 2.4% (as denoted by the dash line in Fig. 6) were used to predict the peak stress of Mg/Al laminate by the rule of mixtures. As shown in Table 4, the experimental measured peak stress of Mg/Al laminate (330 MPa) is in good accordance with the one calculated by the rule of mixture (324 MPa). Therefore, whether the rule of mixtures works in predicting the ultimate strength of Mg/Al composite is closely related to the flow behavior of the composite. Lesuer et al. [27] and Chen et al. [28] reported that the average role worked for the tensile flow curve or overall hardening behavior of bimetal composite.
The present study also confirms the flow curve and strain hardening of the extruded Mg/Al laminate also follows the average role. As shown in Fig. 7, the interface of Mg and Al layers cracks just after yielding. In fact, brittle intermetallics such as Mg17Al12 and Mg2Al3 often form at the interface of hot-worked Mg/Al composite. Those brittle intermetallics easily crack during loading, leading to the early cracking of interface. This interface cracking will affect stress conduction between Mg and Al layers, generating a stress concentration or a localized deformation. This is probably an important reason for the quite low elongation of Al/Mg laminate during ED tension.
5. Conclusion In the present study, a well bonded Mg/Al laminate with a soft Mg AZ31 sleeve and an ultra hard Al 7050 core was successfully fabricated by extrusion directly from the as cast Mg and Al alloys, with the aim to harden the Mg alloys. Microstructure and mechanical behavior of the composite laminate were systematically studied. The relationship between the strength of Mg/Al plate and the strengths and fractions of Mg layer and Al layer were addressed, too. Several conclusions are reached as follows: (1) The ultra hard Al layer effectively enhances the yield strength. An Al fraction of 40.2% can enhance the yield strength from 155 MPa to 300 Mpa while only increases the weight by 20%. (2) The experimental yield strength of composite shows a small difference from that predicted by the rule of mixtures, while the ultimate strength calculated by the
rule of mixtures greatly differs from the experimental one. This is mainly due to the fact that the Mg and the Al layers during tension of Mg/Al composite fracture before they reach their ultimate strength. (3) The Mg layer in Mg/Al laminate has the typical texture of monolithic Mg alloy extruded plate with most basal poles parallel to the ND and a small fraction of them close to the TD. The Al layer contains high fractions of S component up to 58. 3% and cube component of 18.9%. Goss, brass and copper textures are nearly absent.
Acknowledgement This project is co-supported by the Natural Science Foundation of China (51421001), the National High Technology Research and Development Program of China (2013AA031304) and the Fundamental Research Funds for the Central Universities (CDJZR12130073, 1061120115CDJXY130015).
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[7] J.C. Williams, E.A. Starke Jr, Acta Mater. 51 (2003) 5775-5799. [8] S.R. Agnew, J.F. Nie, Scr. Mater. 63 (2010) 671-673. [9] K. Hono, C.L. Mendis, T.T. Sasaki, K. Oh-ishi, Scr. Mater. 63 (2010) 710-715. [10] J.F. Nie, Metall. Mater. Trans. A 43 (2012) 3891-3939. [11] J. Hirsch, T. Al-Samman, Acta Mater. 61 (2013) 818-843. [12] X.P. Zhang, S. Castagne, T.H. Yang, C.F. Gu, J.T. Wang, Mater. Des. 32 (2011) 1152-1158. [13] C.Y. Liu, R. Jing, Q. Wang, B. Zhang, Y.Z. Jia, M.Z. Ma, R.P. Liu, Mater. Sci. Eng. A 558 (2012) 510-516. [14] C. Luo, W. Liang, Z. Chen, J. Zhang, C. Chi, F. Yang, Mater. Charact. 84 (2013) 34-40. [15] K. Wu, H. Chang, E. Maawad, W.M. Gan, H.G. Brokmeier, M.Y. Zheng, Mater. Sci. Eng. A 527 (2010) 3073-3078. [16] H. Chang, M.Y. Zheng, W.M. Gan, K. Wu, E. Maawad, H.G. Brokmeier, Scr. Mater. 61 (2009) 717-720. [17] X.P. Zhang, T.H. Yang, S. Castagne, J.T. Wang, Mater. Sci. Eng. A 528 (2011) 1954-1960. [18] C.Y. Liu, R. Jing, Q. Wang, B. Zhang, Y.Z. Jia, M.Z. Ma, R.P. Liu, Mater. Sci. Eng. A 558 (2012) 510-516. [19] D.C. Foley, M. Al-Maharbi, K.T. Hartwig, I. Karaman, L.J. Kecskes, S.N. Mathaudhu, Scr. Mater. 64 (2011) 193-196. [20] M. Paramsothy, M. Gupta, N. Srikanth, J. Comp. Mater. 42 (2008) 1297-1307.
[21] M. Negendank, S. Mueller, W. Reimers, J. Mater. Process. Technol. 212 (2012) 1954-1962. [22] F. Pérocheau, J.H. Driver, Int. J. Plast. 16 (2000) 73-89. [23] K. Osakada, M. Limb, P.B. Mellor, Int. J. Mech. Sci. 15 (1973) 291-&. [24] D. Yang, P. Cizek, P. Hodgson, C.e. Wen, Scr. Mater. 62 (2010) 321-324. [25] S. Gall, R.S. Coelho, S. Müller, W. Reimers, Mater. Sci. Eng. A 579 (2013) 180-187. [26] D.L. Yin, J.T. Wang, J.Q. Liu, X. Zhao, J. Alloys Compd. 478 (2009) 789-795. [27] D.R. Lesuer, C.K. Syn, O.D. Sherby, J. Wadsworth, J.J. Lewandowski, W.H. Hunt, Int. Materi. Rev. 41 (1996) 169-197. [28] X.X. Chen, P.D. Wu, J.D. Embury, Y. Huang, Model. Simul. Mater. Sci. Eng. 18 (2010).
Table 1. The fractions of different texture components in the Al layer of Mg/Al laminate. Goss{011}<10
S{123}<634
cube{001}<10
brass{011}<21
copper{112}<11
0>
>
0>
1>
1>
0%
58.3%
18.9%
0%
1.6%
Table 2. The yield strengths, ultimate strengths and elongations to fracture under tension along the ED. Yield strength
Ultimate strength
(MPa)
(MPa)
Mg/Al-Mg
155
217
4.7
Mg/Al-Al
489
572
6.6
Mg/Al
300
330
1.6
Samples
Elongation (%)
Table 3. The specific strengths of Mg layer, Al layer and the Mg/Al laminate. Samples
Mg
Al
Mg/Al
Specific
0.086
0.18
0.139
strength(N·m/kg)
Table 4. The yield strengths and ultimate strengths of Mg/Al laminate measured by tensile tests and calculated by the rule of mixtures. The experimental stress of the Mg/Al laminate and the stress calculated by the rule of mixtures at the 2.4% strain is also given.
Calculated Value
yield
ultimate
stress at the strain of
strength/MPa
strength/MPa
2.4%
289
359
324
300
330
330
Experimental Value
Fig. 1. (a) Schematic diagrams showing (a) the fabrication of Mg/Al laminate composite by extrusion and (b) the preparation of specimens for tension tests along the extrusion direction.
Fig. 2. Optical microstructure of the Mg layer at different regions of Mg/Al laminate: (a) and (d) microstructures corresponding to the regions denoted by a and d in (f), respectively, (b) high magnification view near the surface in (a), (c) high magnification view near the center in (a), (e) high magnification view of (d).
Fig. 3. (a) and (b) cross sectional SEM micrographs between the Mg and the Al layers and (c) EDS mapping of (b). Here, the points in red and green represents the elements of Mg and Al, respectively.
Fig. 4. (a) Inverse pole figure map and pole figures of the Mg layer in Mg/Al laminate, (b) and (c) two subsets with basal poles close to the normal direction (ND) and the transverse direction (TD), respectively.
Fig. 5. (a) Inverse pole figure map and pole figures of the Al layer in Mg/Al laminate and (b) the distribution of different texture components: Goss(G), S, Cube, brass(B) and copper (C).
Fig. 6. True stress-strain curves under tension along the ED of the Mg/Al laminate, the Mg layer and the Al layer.
Fig. 7 SEM micrographs showing the cracking of Mg/Al interface after tension along the ED to 300 MPa: (a) a low magnification view and (b) a high magnification view.
PII: DOI: Reference:
S0921-5093(15)30039-3 http://dx.doi.org/10.1016/j.msea.2015.05.094 MSA32422
To appear in: Materials Science & Engineering A Received date: 7 May 2015 Revised date: 23 May 2015 Accepted date: 26 May 2015 Cite this article as: Yang Wu, Bo Feng, Yunchang Xin, Rui Hong, Huihui Yu and Qing Liu, Microstructure and mechanical behavior of a Mg AZ31/Al 7050 laminate composite fabricated by extrusion, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.05.094 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and Mechanical Behavior of a Mg AZ31/Al 7050 Laminate Composite Fabricated by Extrusion Yang Wu, Bo Feng, Yunchang Xin*, Rui Hong, Huihui Yu, Qing Liu School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
Abstract: In the present study, a well bonded Mg/Al laminate containing a soft Mg AZ31 sleeve and an ultra hard Al 7050 core was successfully fabricated by extrusion directly from the as-cast alloys, with the aim to harden Mg alloys. Microstructure and mechanical behavior of the composite were systematically studied. Our results show that the Mg layer contains a bimodal grain structure. The typical texture of the monolithic Mg alloy extruded plate develop in the Mg layer in Mg/Al laminate with most basal poles parallel to the normal direction (ND) and a small fraction of them close to the transverse direction (TD). The Al layer has high fractions of S {123}<634> and cube {001}<100> texture components. The ultra hard Al layer effectively enhances the yield strength. An Al fraction of 40.2% can enhance the yield strength from 155 MPa to 300 MPa, while only increase the weight by 20%. The relationship between the strength of the Mg/Al composite and the strengths and fractions of the Mg and Al layers were studied. The experimental yield strength of the composite is slightly higher than that predicted by the rule of mixtures, while the predicted ultimate strength greatly differs from the experimental one. The main reason lies in that, during tension of the Mg/Al composite, the Mg and the Al layers fracture before they reach their ultimate strength.
Keywords:
magnesium
alloys;
aluminum
alloys;
laminate
composite;
microstructure; mechanical properties; *
Corresponding author: Dr. Yunchang Xin Tel.:+86-23-65106407. Fax: +86-23-65106407. Electronic mail: [email protected]
1. Introduction Magnesium alloys are being increasingly concerned in the automotive and aerospace industries for weight saving [1]. However, their applications have been greatly limited by their low strength or poor formability [2]. Hybrid metal composites that combine the advantages of different metals are effective to tailor mechanical properties such as strength, ductility, impacting, fatigue and fracture [3-5]. A typical example is the dissimilar laminates that are fabricated by roll bonding or accumulative roll bonding (ARB) [6]. Aluminum alloys also have a low density and excellent mechanical properties, e.g. good ductility or high strength [7]. Yield strengths of the commercial high strength Mg alloys (e.g. AZ80 or ZK60) are mainly in a range of 250-350 MPa [8-10], while that of some commercial high strength Al alloys (e.g. 7050 or 2095) are up to 600-700 MPa [7, 11]. Therefore, a Mg/Al bimetal structure has the potential to improve mechanical properties of Mg alloys without an obvious increase of the weight of constructions. Mg/Al multilayer laminates have spawn much interest in the field of light-weight constructions in quest of excellent impacting or damping capacities [2, 12]. The commonly used method to fabricate Mg/Al laminates is roll bonding.
Various Mg/Al laminate composites (e.g. Al 1060/Mg AZ31 [13], Al 5052/Mg AZ31 [14, 15], Al 5052/pure Mg [16], Al 7075/Mg AZ31 [17]) are prepared using roll bonding or ARB. However, the poor rolling ability of Mg alloys leads to a high cost to prepare thin Mg plates for subsequent roll bonding [18]. The hot-rolled or cold-rolled plates often have a strong basal texture which greatly reduces the deformation ability. Therefore, the strong basal texture of thin Mg plate also greatly deteriorate the roll bonding performance of Mg alloys [8, 11, 19]. Extrusion is also a process to prepare thin plates in a mass production manner. Extrusion process is suitable for both Mg alloys and Al alloys. Therefore, Mg/Al laminates can be fabricated from as-cast Mg and Al alloys. From this point of view, extrusion process is a highly efficient method. However, the usage of extrusion to fabricate Mg/Al laminates is rarely reported. Up to now, only one publication has reported the application of extrusion to fabricate an Al 6082/ Mg AZ31 plate [3]. However, in that publication, a rolled thin AZ31 plate in halved aluminum billet was used for extrusion. The usage of thin Mg plate for extrusion processing greatly reduces the efficiency of this method. Al 7050 is a typical high strength Al alloy with yield strength up to 600 MPa [7], hence, it is a desirable candidate to harden a soft Mg alloy. However, the quite different mechanical properties between the Mg alloys and the 7050 plates lead to difficulties in roll bonding of the two materials. In the present study, a Mg AZ31/Al 7050 laminate with excellent layer/layer bonding was successfully fabricated directly from as-cast Mg and Al alloy alloys. Microstructure and mechanical properties of the as-extruded laminate were systematically studied.
For a composite containing hybrid metals, the relationship between the strength of the composite and the strengths and fractions of each constituent is an important issue. A deep understanding about this topic contributes to predicting mechanical properties of composites. Therefore, the relationship between strength of the as-prepared Mg/Al plate and the strengths and fractions of Mg and Al layers were evaluated by accurate measurements of the mechanical properties of each component cut directly from the Mg/Al composite.
2. Experiments and methods 2.1 Extrusion and mechanical tests Mg alloy AZ31 and Al alloy 7050 in the as-cast and homogenized condition were used for the fabrication of the Mg/Al composite plate. As explained in Fig. 1a, the Mg alloy billet was machined into a hollow cup with an outer diameter of 80 mm and an inner diameter of 40 mm. An as-cast Al alloy cylinder with a diameter of 40 mm was cut, polished and filled into the Mg hollow cup. This bimetal billet was kept in furnace at 470 °C for 2 h to complete the solid solution of Al 7050, and immediately extruded at 470 °C using an extrusion rate of 4.5 mm/min. The extruded Mg/Al laminate was immediately quenched into water after exiting the die followed by a subsequent two-stage aging at 120 °C for 8 h and at 150 °C for 9 h. The final extruded plate has a width of 60 mm and a thickness of 3 mm. If extrusion of Mg/Al bimetal billet was performed at a temperature below 470 °C, dynamic precipitation would take place during extrusion, which will greatly reduce aging hardening response of Al 7050 after extrusion. As Mg/Al intermetallics generally have a melting
temperature of about 430 °C, a further solid solution treatment of 7050 cannot be performed after extrusion. The as-used extrusion process can acquire a fully solid solution treated Al 7050 layer after extrusion and the subsequent aging can effectively improve strength of Al 7050. So the as-used extrusion process was desirable to acquire better strength. The preparation of specimens for tensile tests is shown in Fig. 1b. Specimens for tensile test of the Mg AZ31/Al 7050 laminate (the designated Mg/Al), the Al 7050 layer (the designated Al), and the Mg AZ31 layer (the designated Mg) were all cut directly from the as-extruded plate. Tensile tests along the extrusion direction (ED) at room temperature were performed on a Shimadzu mechanical testing system using a strain rate of 10-3 s-1. Each mechanical test was repeated three times. 2.2 Characterization of microstructure and texture Microstructure of the Mg/Al laminate was examined using an optical microscopy (ZEISS, Axiovert 40MAT) and a scanning electron microscopy (SEM, TESCAN Vega TS 5136XM). Element distribution in the transition layer between Mg and Al layers was analyzed by energy dispersive X-ray spectroscopy (EDS). Electron backscattered diffraction (EBSD) analyses were conducted on a FEI Nova 400 SEM equipped with a HKL Chanel 5 system using a step size of 1 μm. Samples for EBSD mapping were mechanically ground followed by electrochemical polishing in the AC2 electrolyte. The EBSD data were analyzed using the Channel 5 software. The texture components in Al alloy layers were subsequently analyzed by the orientation distribution
function
(ODF).
Five
texture
components ， cube{001}<100>,
Goss{011}<100>, brass{011}<211>, S{123}<634> and copper{112}<111>, were analyzed. They were calculated by integrating the ODF within 15° of the ideal texture component.
3. Results 3.1. Microstructure Fig. 2 presents optical microstructure of the Mg layer in the Mg/Al laminate. As regions a and d in Fig. 2f experienced different stress states, microstructures in the two regions were both examined. As seen in Fig. 2a and d, an obvious gradient microstructure from the surface to the center appears in region labeled ‘a’, while is absent in region ‘d’. The surface of region ‘a’ contains much finer structure than the center. The high magnification views in Fig. 2b, c and e indicate that the Mg layer contains a bimodal grain structure containing both large grains up to 50 μm and fine grains less than 5 μm. Fibrous and unrecrystallized structure is also discerned in region ‘a’. SEM cross-sectional micrographs of the Mg/Al laminate are shown in Fig. 3. In the Mg/Al sandwich, the Mg layer is well bonded to the Al layer. A transition layer with a thickness of about 10 μm exists between the Al and the Mg layers. This transition layer contains two distinct sub-layers. EDS mapping clearly shows that the sub-layer adjacent to the Mg matrix contains a higher Mg content and a lower Al content, while a higher Al content and a low Mg content exist in the sub-layer next to the Al matrix. This double-layered transition layer often appears in Mg/Al composites
[20, 21]. It is reported that the sub-layer adjacent to Mg is mainly composed of Mg17Al12 and that next to Al contains much Mg2Al3 [21].
3.2. Texture EBSD analyses of microstructure and crystallographic orientation of the Mg layer in the Mg/Al laminate are given in Fig. 4. Two types of texture component, a high fraction (85% in area fraction) of (0002) poles largely parallel to ND and a small fraction (15% in area fraction) of basal poles perpendicular to TD is noticed. The two subsets were picked out and further analyzed in Fig. 4b and c. A preferred distribution of the prismatic planes exist in the texture component <0002>//TD, but not in <0002>//ND. Due to the low fraction of the texture component with <0002>//TD, the __
preferred distribution of prismatic planes is not obvious in the {1010} pole figure in Fig. 4a. The appearance of preferred distribution of prismatic planes indicates a high activity of prismatic slip during extrusion. Microstructure and crystallographic orientation map of the Al layer in Mg/Al laminate are shown in Fig. 5. A large fraction of fibrous and elongated grains and a small fraction of equaxied grains appear in the examined region. Many low angle boundaries with misorientation below 10° are discerned too. Texture of the extruded Al alloys may contain several different components, e.g. copper, S, and brass [22] and cube and Goss orientations are also important for the texture analysis of deformed Al alloys [16]. The fractions of five typical texture components，cube{001}<100>, Goss{011}<100>, brass{011}<211>, S{123}<634> and copper{112}<111>, were
analyzed and are shown in Fig. 5b and Table 1. The Al layer contains high fractions of S and cube components. The components Goss, brass and copper are nearly absent.
3.3. Mechanical behavior True stress-strain curves of the Mg/Al, Mg and Al under tension along the ED are shown in Fig. 6. The yield strengths, ultimate strengths and elongations to fracture derived from those stress-strain curves are listed in Table 2. The Mg/Al composite has a yield strength up to 300 MPa, but a quite low elongation of 1.6%. The Mg layer in Mg/Al laminate has a much lower yield strength (155 MPa) and a poor plasticity (4.7%) in comparison to the Al 7050 layer with a yield strength up to 498 MPa and a better elongation. It can be seen that the hard Al 7050 layer effectively improves the yield strength. Compared to the Mg alloy, the Mg/Al laminate has a higher elastic modulus, too. The as-fabricated Mg/Al laminate has a quite low elongation during tension along the ED. During ED tension, cracking of Mg/Al interface might take place. In order to examine how this interface cracking affect plasticity, microstructure of Mg/Al interface was examined using SEM. As shown in Fig. 7, severe interface cracking appears just after yielding during ED tension. The quick cracking during at interface during ED tension is suspected to an important reason for the low ductility of Mg/Al composite.
4. Discussion 4.1 Microstructure and texture of Mg/Al laminates The present study shows that Mg AZ31/Al 7050 laminate with excellent layer/layer bonding can be successfully fabricated by extrusion directly from the as-cast Mg and Al billets. Therefore, fabrication of Mg/Al laminate by extrusion provides a highly efficient method for mass productions. It should be pointed out that this extrusion process to fabricate metal laminates is attractive for Mg/Al or Mg/Mg laminates, but not suitable for bimetal laminates containing steel or titanium. In this case, roll bonding is the suitable and predominant method. During extrusion of hybrid metal billets, a strong friction stress and shear exist at the hybrid metal interface [23]. This may contribute to refine the grains in both the Mg and Al layers. In the Mg/Al laminate, Mg AZ31 (77 Wm-1K-1) possesses a much lower thermal conductivity than Al 7050 (157 Wm-1K-1) [7, 24]. It is expected that the adiabatic heating localized near the core-sleeve interface during extrusion is mainly conducted by the Al alloy core. This will benefit grain refinement in the Mg layer. This is confirmed in the present study, the high extrusion temperature of 470 °C does not generate obvious grain coarsening in the Mg layer (Fig. 2). As seen in Fig. 4, the Mg layer in Mg/Al laminate contains a high fraction of the texture component with basal poles largely parallel to the ND and a small fraction of that with (0002) poles close to the TD. This type of texture is the typical texture of monolithic Mg alloy extruded plates [25]. This indicates that the different stress state during extrusion of Mg/Al bimetal billet from that during extrusion of monolithic Mg
billet hardly affects the texture type of Mg layer. In another recent publication [26], it is also found that the Mg alloy constituent in a Mg/Al extruded rod also shows a texture similar to that in a monolithic Mg alloy extruded rod. It seems that the texture of the Mg layer in a Mg/Al extruded composite is insensitive to the variations of the extrusion stress state. It is reported that only a strong basal texture with (0002) poles parallel to the ND exists in the Mg layer of Mg/Al laminates fabricated by roll bonding [14, 15]. Therefore, the Mg layer in an extruded Mg/Al laminate has a weakened texture compared with that in a roll-bonded one, due to the generation of the texture component <0002>//TD. This will benefit the subsequent deformation capability of Mg/Al composites. It is well established that a strong β-fiber texture consisting of the copper, S, and bass orientations often develops in deformed Al or Al alloys [11]. The Al layer in the as-extruded Mg/Al laminate also has a strong β-fiber texture with a high fraction of S component up to 58.3%. The high fraction of β-fiber texture in Al layer is mainly due to the high extrusion strain. The Al layer also contains 18.9% of cube component. Generally, the cube component is a recrystallization texture and often appears in most hot-worked Al alloys with characteristic shifts or scatterings of the orientation peak [11]. As the fractions of different texture components in hot worked Al alloys are highly dependent on the processing parameters, it is difficult to judge whether the stress-state during extrusion of Mg/Al bimetal billet will pose a great influence on the texture of the Al 7050 layer in the present study.
4.2 Mechanical behavior of metal composites The inclusion of Al alloy core in Mg alloy sleeve leads to an increase in the weight. The average density of Mg/Al laminate can be calculated by the rule of mixtures. The alloy core has a volume fraction of 40.2% in the gage part of the specimens for tensile tests. If an Al alloy density of 2.7 g/cm3 and a Mg alloy density of 1.8 g/cm3 are used in this calculation, the density of Mg/Al laminate is about 2.16 g/cm3. Compared to pure Mg, the weight of the Mg/Al laminate only increases by 20%, while yield strength rises by about 90%. The specific strengths of each plate were calculated and listed in Table 3. The Al 7050 layer has the highest specific strength. However, the Mg/Al laminate has a much higher specific strength than the Mg layer. Therefore, an ultra-hard Al alloy core can effectively improve strength of Mg alloy plate without obvious compromise in the weight for structure design. The relationship between the strength of a composite and the strengths and fractions of each constituent is extensively studied. It is often claimed that this relationship can be described by the rule of mixture as follows:
c 1 V1 2 V2
(1)
Where i and Vi are respectively the strength and volume fraction of the constituent ‘i’. However, the accuracy of the rule of mixtures needs to be further verified, as the mechanical properties of each constituent used in equation (1) in many studies are not directly measured from the specimens cut from the composites, but, the predicted or deduced values. In the present study, the mechanical properties of the
Mg layer and Al layer are tested from specimens cut directly in the composite, which allows a precise evaluation about the efficiency of the rule of mixtures. The experimentally measured yield strengths and ultimate strengths and the ones predicted by the rule of mixtures are listed in Table 4. The predicted yield strength (289 MPa) only shows a slight deviation from the experimental one (300 MPa). However, the experimental ultimate strength (330 MPa) greatly differs from the predicted one (359 MPa). It seems that yield strength of Mg/Al laminate can be effectively predicted by the rule of mixtures, but the rule of mixtures does not work for predicting the ultimate strength. However, as seen in Fig. 6, the Mg/Al laminate reaches the peak stress at a strain of about 2.4%, while peak stresses of the Mg layer and Al layer in the Mg/Al laminate generally appear at strains 5.7% and 7.5%, respectively. That is, during tension of the Mg/Al laminate, the Mg and the Al layers fracture before they reach their maximum strength. As the Mg/Al laminate fractures at a strain of 2.4%, the flow stresses of Mg and Al layer at strain 2.4% (as denoted by the dash line in Fig. 6) were used to predict the peak stress of Mg/Al laminate by the rule of mixtures. As shown in Table 4, the experimental measured peak stress of Mg/Al laminate (330 MPa) is in good accordance with the one calculated by the rule of mixture (324 MPa). Therefore, whether the rule of mixtures works in predicting the ultimate strength of Mg/Al composite is closely related to the flow behavior of the composite. Lesuer et al. [27] and Chen et al. [28] reported that the average role worked for the tensile flow curve or overall hardening behavior of bimetal composite.
The present study also confirms the flow curve and strain hardening of the extruded Mg/Al laminate also follows the average role. As shown in Fig. 7, the interface of Mg and Al layers cracks just after yielding. In fact, brittle intermetallics such as Mg17Al12 and Mg2Al3 often form at the interface of hot-worked Mg/Al composite. Those brittle intermetallics easily crack during loading, leading to the early cracking of interface. This interface cracking will affect stress conduction between Mg and Al layers, generating a stress concentration or a localized deformation. This is probably an important reason for the quite low elongation of Al/Mg laminate during ED tension.
5. Conclusion In the present study, a well bonded Mg/Al laminate with a soft Mg AZ31 sleeve and an ultra hard Al 7050 core was successfully fabricated by extrusion directly from the as cast Mg and Al alloys, with the aim to harden the Mg alloys. Microstructure and mechanical behavior of the composite laminate were systematically studied. The relationship between the strength of Mg/Al plate and the strengths and fractions of Mg layer and Al layer were addressed, too. Several conclusions are reached as follows: (1) The ultra hard Al layer effectively enhances the yield strength. An Al fraction of 40.2% can enhance the yield strength from 155 MPa to 300 Mpa while only increases the weight by 20%. (2) The experimental yield strength of composite shows a small difference from that predicted by the rule of mixtures, while the ultimate strength calculated by the
rule of mixtures greatly differs from the experimental one. This is mainly due to the fact that the Mg and the Al layers during tension of Mg/Al composite fracture before they reach their ultimate strength. (3) The Mg layer in Mg/Al laminate has the typical texture of monolithic Mg alloy extruded plate with most basal poles parallel to the ND and a small fraction of them close to the TD. The Al layer contains high fractions of S component up to 58. 3% and cube component of 18.9%. Goss, brass and copper textures are nearly absent.
Acknowledgement This project is co-supported by the Natural Science Foundation of China (51421001), the National High Technology Research and Development Program of China (2013AA031304) and the Fundamental Research Funds for the Central Universities (CDJZR12130073, 1061120115CDJXY130015).
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Table 1. The fractions of different texture components in the Al layer of Mg/Al laminate. Goss{011}<10
S{123}<634
cube{001}<10
brass{011}<21
copper{112}<11
0>
>
0>
1>
1>
0%
58.3%
18.9%
0%
1.6%
Table 2. The yield strengths, ultimate strengths and elongations to fracture under tension along the ED. Yield strength
Ultimate strength
(MPa)
(MPa)
Mg/Al-Mg
155
217
4.7
Mg/Al-Al
489
572
6.6
Mg/Al
300
330
1.6
Samples
Elongation (%)
Table 3. The specific strengths of Mg layer, Al layer and the Mg/Al laminate. Samples
Mg
Al
Mg/Al
Specific
0.086
0.18
0.139
strength(N·m/kg)
Table 4. The yield strengths and ultimate strengths of Mg/Al laminate measured by tensile tests and calculated by the rule of mixtures. The experimental stress of the Mg/Al laminate and the stress calculated by the rule of mixtures at the 2.4% strain is also given.
Calculated Value
yield
ultimate
stress at the strain of
strength/MPa
strength/MPa
2.4%
289
359
324
300
330
330
Experimental Value
Fig. 1. (a) Schematic diagrams showing (a) the fabrication of Mg/Al laminate composite by extrusion and (b) the preparation of specimens for tension tests along the extrusion direction.
Fig. 2. Optical microstructure of the Mg layer at different regions of Mg/Al laminate: (a) and (d) microstructures corresponding to the regions denoted by a and d in (f), respectively, (b) high magnification view near the surface in (a), (c) high magnification view near the center in (a), (e) high magnification view of (d).
Fig. 3. (a) and (b) cross sectional SEM micrographs between the Mg and the Al layers and (c) EDS mapping of (b). Here, the points in red and green represents the elements of Mg and Al, respectively.
Fig. 4. (a) Inverse pole figure map and pole figures of the Mg layer in Mg/Al laminate, (b) and (c) two subsets with basal poles close to the normal direction (ND) and the transverse direction (TD), respectively.
Fig. 5. (a) Inverse pole figure map and pole figures of the Al layer in Mg/Al laminate and (b) the distribution of different texture components: Goss(G), S, Cube, brass(B) and copper (C).
Fig. 6. True stress-strain curves under tension along the ED of the Mg/Al laminate, the Mg layer and the Al layer.
Fig. 7 SEM micrographs showing the cracking of Mg/Al interface after tension along the ED to 300 MPa: (a) a low magnification view and (b) a high magnification view.