Pnkj Kumr , Avirl Mdhup , Prsd R. Klvl , Stym Suws

a Chemical and Materials Engineering, University of Nevada, Reno, NV, 89557, USA

b Materials Engineering, Indian Institute of Science, Bangalore, 560012, India

Keywords:Accumulative roll bonding Defence application AZ31 alloy Texture Composite Laminates

ABSTRACT This article presents the texture development of magnesium AZ31 alloy in the accumulative roll bonded(ARB) AZ31/AZ31 multilayer and AZ31/AA5086 laminate composite. The comparative study demonstrates that the texture evolution in AZ31 in a multilayer system is strongly influenced by the interfaces.A typical basal texture of AZ31 has been observed in AZ31/AZ31 multilayer with texture intensity increased with the rolling deformation. Presence of AZ31/AA5086 interface in the laminate composite leads to a tilted basal texture along the rolling direction (RD) in AZ31 alloy. The texture intensity of composite increased initially with rolling reduction and weakened at the higher rolling strain. Weakening of texture in AZ31 during the laminate processing at higher strain has been attributed to the development of wavy interfaces in AZ31/AA5086 laminate.

1. Introduction

With increased logistic burden and fuel price,there is a need to develop and implement the lightweight strategies for military/defence platforms (air, sea, and land) without compromising the functionalities.The use of ultra-lightweight structural components can significantly reduce the logistic burden and improve fuel efficiency. Magnesium (Mg) alloys due to extraordinary strength-toweight ratio have the capability to replace other structural alloys,i.e., aluminum and titanium [1]. Since 1940's the Mg alloys have been actively considered as alternative structural materials for defence platforms[2].Post World War II,research on Mg alloys has been extensively pursued to use in military aircraft with a focus to develop speed and range.The usage of AZ31(Mg alloy)in a military aerial platform has tremendously increased in the late 1940's [2].These alloys were frequently used in sheet form in military aircraft fuselage and wings, and gearbox housing [3]. Unfortunately, the widespread use of Mg alloy was restricted by many factors. Poor mechanical performance and formability are one of the major barriers for extensive usage of Mg in defence [2]. Significant research has been pursued over the years with an aim to enhance the mechanical performance of the Mg alloy [4-6]. Fabrication of Mg alloy,however,is still a bottleneck for its extensive application[7]. Mg due to hexagonal crystal lattice shows plastic anisotropic that result in poor formability [8,9]. The crystallographic orientations of grains defined in terms of “texture” in polycrystalline materials play a significant role in plastic anisotropy, thereby formability [9-13]. Therefore, it is critical to develop texture condition for favorable manufacturing of Mg alloy.Such understanding may result in widespread application of the magnesium alloys in general and defence/military applications in particular.

Wrought AZ31 Magnesium (Mg) alloy has been significant interest for structural application in military applications. Hot processing of AZ31 has been sought as only means to fabricate the AZ31 components. While limited-slip system of Mg leads to poor room temperature ductility, the typical basal texture developed during the hot/cold rolling results in plastic anisotropy and poor formability [14,15]. Significant efforts have been made in past to improve the room temperature ductility and formability of AZ31.The grain refinement and controlled texture have proven to be promising approaches [16-20]. The weakening of basal texture component significantly reduced in-plane anisotropy, thus improving the formability [21,22]. Variety of processing strategies have been developed over the years to obtain ultrafine grains with a controlled bulk texture of Mg alloy [9,22-27]. Amongst all, severe plastic deformation (SPD) techniques are the most efficient methods to produce bulk materials with ultrafine grains without the addition of any grain refiner.In various SPD techniques such as high pressure torsion (HPT) [28], multi-axial forging (MAF), equal channel angular press (ECAP) [29-31] and accumulative roll bonding (ARB); ECAP method has been widely investigated to produce ultrafine grain structure of bulk AZ31 [32-34]. A significant increase in ductility and formability has been shown by reducing grain size and controlling the texture of bulk specimen using ECAP process[35-38].However,ECAP being a batch process is limited only to lab scale. Unlike EACP, ARB is a continuous SPD process which has significant potential to adapt at industrial scale to fabricate ultrafine grains of the bulk materials. Majority of the ARB investigation of AZ31 in literature has been directed towards the microstructure refinement [39-42]. Refinement in the microstructure has been attributed to shear strain produced, along with the compressive strain during rolling. Therefore, ARB process differs to the conventional rolling in a way that it is related to the shear strain superimposed with compressive strain produced in ARB. In general,conventional rolling develops{0002}type basal texture in the AZ31 sheet[43-45].This texture behavior in the rolled AZ31 is attributed to the twining activity of type {} during compressive deformation. Since ARB involves complicated deformation mechanism including shear deformation, texture evolution is expected to differ from the conventional rolling. While texture dictates ductility and formability parameters, texture evolution studies of ARB processed AZ31 multilayer are rare [46]. The constituent layers co-deform homogeneously during ARB processing,given same phases (material), strengthen of texture is observed with increasing strain [41,46,47].

Further, it is shown that the presence of bi-phase interface can significantly affect texture evolution in the laminate composite of small layer thickness[48-50].Nevertheless,reports on the effect of interface on texture evolution in bi-phase Mg/Al laminate composite are very limited. Very few scattered studies of texture evolution in dissimilar crystal structure Mg/Al during the ARB processing have been reported [51-53].

Multilayer and Mg alloy layer composites are promising materials for use in the military vehicular application. In this present investigation, we have successfully fabricated the AZ31/AZ31 multilayer, and AZ31/AA5086 laminate composite materials using ARB process and the respective bulk texture evolution during the process has been studied. The comparative study is performed to explore the effect of bi-phase interface on the texture evolution in AZ31 during the ARB deformation process.

2. Experimental procedures

2.1. ARB processing of materials

AZ31 alloy of composition is given in Table 1. The starting material was in the form of 1 mm thick sheet. The starting material was rolled to a 50% reduction. Two such sheets were cleaned to remove oxide layers and stacked,and subsequently rolled further to 50% reduction at 300°C. This process continues up to four passes which result in 16 layers in 1 mm thick AZ31 multilayer.The strainimparted in each pass was 0.8 and the total accumulative strain in the multilayer was 3.2.The same ARB processing was conducted at 300°C with the stack of AZ31 and AA5086 (see Table 1 for composition) for five passes to develop AZ31/AA5086 laminar composite.During the ARB processing a total accumulative strain of 4.0 achieved with a 32 alternate AZ31 and AA5086 layers.

Table 1 Chemical composition (wt%) of AZ31 Mg alloy and AA5086 aluminum alloy.

2.2. Characterization of as processed materials

The as-processed materials were subjected to the microstructural and texture characterizations. Microstructural characterization was conducted using scanning electron microscopy (SEM).Standard polishing practice was followed to examine the crosssection of the processed strip. The specimens were extracted from the ARB processed sheets.A FEI Quanta 200 scanning electron microscope was used for this purpose.Bulk texture measurements were carried out on the ARB processed sheets using the X-ray diffraction method.A Bruker DB Discover X-ray texture goniometer with Co Kα radiation was used for this purpose.Six incomplete pole figures (0<α<75°) (10.0), (00.2), (10.1), (10.2), (10.3), and (11.2)were measured for AZ31 in both AZ31/AZ31 multilayers and AA5086/AZ31 laminates.

3. Result and discussion

The bulk texture of ARB processed materials was measured for constituent AZ31 layers by X-ray diffraction method.At low strain,a tendency of symmetrical spreading of {0002} basal texture along RD in AZ31 alloy in both multilayer and the laminate composite was observed as shown in Figs.1 and 2, respectively. The (0002) pole figures reveal the distinction in texture evaluation at higher strain.The as-rolled AZ31 (starting) exhibits tilted basal texture towards RD. With increasing strain, the reduction in tilting along the RD is observed in multilayer.The texture evolution during 1st pass to 3rd pass is quite similar in nature. However, in the 4th pass, texture tilting is reduced, and the contour forms nearly at the basal pole(Fig.1(d)) indicating the formation of strong basal texture.

Fig.1. (0002) pole figure of AZ31 (a) starting materials (b) after 1st pass (c) after 3rd pass (d) after 4th pass in AZ31-AZ31 multilayer.

Fig. 2. (0002) pole figure of AZ31 in (a) starting materials (b) after 1st pass (c) after 3rd pass (d) After 4th pass (e) After 5th pass in AA5086-AZ31 laminate composite.

The spread in texture could be attributed to non-basal slip activity and twinning. It has been reported that the {} twin formation persist through initial stage of deformation and overall course of deformation [54]. In addition to the dislocation slip,twinning has been known to play an important role in the deformation of Mg alloy. The strain accommodated by twin during deformation is proportional to the twin fraction. Schmid and Boas[55], have reported that the maximum strain accommodated by twin is 0.065. Double twin may accommodate further strain however, the texture tilting is observed at strain as high as 0.6. This implies that along with twining,non-basal slip is active during the ARB processing.The activation of non-basal slip activity in pure Mg at the stress concentration region has been earlier shown by Hauser et al.[56].Several studies have reported the activation of non-basal slip to accommodate slip during the deformation [57-59]. Koike et al.[17]have shown the activity of non-basal slip based on result of 40% dislocation activity contribution during the deformation of AZ31 alloy.The straining of the sample starts from interface during ARB [60] and acts as the strain concentration sites in AZ31/AZ31 multilayer. The dislocations accumulate at interfaces and give rise to a large strain concentration near the interfaces which activates non-basal slip. Although deformation of AZ31 is initiated with the combined activity of basal slip and twinning, the activity of basal slip and twinning decreased due to hardening of the systems which promotes the activity of slip. The slip allow c-axis compression which facilitates the basal pole to rotate away from the compression axis. Therefore, it can be argued that tilting of basal texture in multilayer at high strain of 0.6 is due to the activation ofslip during ARB.As strain is increased to 1.2,a basal texture form with negligible tilting.This characteristic is similar to the texture observed in hot deformation of AZ31 [39,61,62]. Small recrystallized grains form at distorted region in the vicinity of interface at strain>0.6.Sub-grains form in the distorted region and ultimately high angle grain boundaries appear by sub-boundary migration and coalescence. These new recrystallized grains tend to form cluster and a ductile shear zone[40].Orientation of ductile shear zone is favorable for the basal slip. Basal slip becomes the main deformation mechanism, and large strain can be accommodated by the material. With increasing strain, the rate of recrystallization increase thus helping basal slip activity.Jeong et al.[63]have sowed the perfect basal texture during hot rolling of AZ31 specimen. Roostaei et al. [64] have reported that annealing at 350°C of ARB processed AZ31 increases the intensity of basal texture. These results clearly indicate that there is a significant effect of temperature on texture evolution during the ARB processing of AZ31. Therefore, in the present case, increase in basal texture intensity in AZ31/AZ31 multilayer at higher strain is attributed to the dynamic recrystallization assisted deformation.

Fig. 3. Variation of texture intensity during ARB processing in AZ31-AZ31 multilayer and AZ31-AA5086 laminate composite.

Fig. 4. SEM micrographs showing the cross-sectional view of AA5086-AZ31 laminate composite after 5th passes.

The dissimilar AA5086/AZ31 laminate composite, however,shows distinct textural behavior with the deformation. In the 1st pass, tilted basal texture towards RD has been observed. The reduction in tilting is observed with increasing strain up to 3rd pass. However, increasing ARB passes result in spreading of basal pole and weakening of texture.Fig.3 shows the variation of texture intensity with deformation in both the multilayer and laminate composite systems. The texture intensity is defined as the ratio of volume fraction of preferred orientation to the random orientation-higher the ratio,stronger the texture in materials and vice-versa.It can be clearly observed that the texture intensity of AZ31/AZ31 system increases with deformation, while in the AA5086/AZ31 lamellar composite, the intensity increase initially with deformation and decreased towards random at higher strain. The results presented clearly indicate that the texture evolution is largely controlled by operating deformation mechanism irrespective of the initial texture.Thus,it is speculated that the interface is responsible for the distinct deformation mechanism and hence the textural behavior during the ARB process.

Fig. 4 shows the SEM micrographs of the AA5086/AZ31composite up to five passes of ARB processing. It can be clearly seen that inhomogeneous deformation occurred in AA5086 layer,while AZ31 layer deforms more uniformly. It has been earlier shown that the harder phase is more prone to local instabilities during the codeformation of two dissimilar metals [65,66]. Thus, deformation cause localized necking in AA5086 layers and it finally ruptures.The thicknesses of both Mg layer and Al layer decreased gradually with the increase in ARB cycle,but magnitude of thickness reduction in AZ31 layer is greater compared to the AA5086 layer (Fig. 4). This observation clearly indicates the localized deformation in AA5086 layer to accommodate strain(see Fig.5).

Fig. 5. SEM micrographs showing the cross-sectional view of AA5086/AZ31 laminate composite after (a) after 1st pass (b) after 3rd pass (c) after 4th pass (d) after 5th pass.

The texture of AZ31 in AA5086/AZ31 ARB processed sample shows (Fig. 2(a-e)) typical rolling texture of AZ31 i.e., basal pole tilted towards RD throughout the 5 cycles of ARB processing. This clearly indicates the role of AA5086 (Al alloy) in the texture evolution of AZ31 in AA5086/AZ31 composite.The increased intensity of basal texture up to 3rd cycle of ARB could be attributed to the basal slip activity as explained above. However, decrease in intensity with the further reduction can be explained based on rotational dynamic recrystallization(RDRX).During the RDRX,new grains form having orientations favorable to basal slip which lead to the randomization of texture during the deformation[21,67,68].In addition,shear bands may also play an important role in the texture evolution.The formation of a shear band in the hard layer has been observed in dissimilar metal ARB due to local in-plane shear stress[51,69]. With increasing strain during the ARB processing, harder phase deforms locally generating local shear band. To make deformation compatible, softer phase deforms along the shear band which simply cause the wavy interface as shown in Fig. 4. It can also be observed from the figure that,with the increase in ARB cycle,magnitude of waviness in the layers has been increased.If we correlate the micrographs with the respective texture during the ARB cycle, we observed that increasing wavy layers reduces the texture intensity. The processing up to 3 cycle of ARB does not shown necking of AA5086 later.In the 4th pass,local necking can be observed. The respective texture clearly indicates the reduction in texture intensity.In the 5th cycle of ARB,necking and the resulting rupture has been observed. The corresponding texture shows a large reduction in the intensity lower than the initial texture strength. Microstructure and texture correlation clearly indicates the role of the harder phase in texture evolution of Mg.Therefore,reduction in the textural intensity of AZ31 during ARB of AA5086/AZ31 is attributed to the localized deformation of the harder AA5086 layer.

4. Conclusions

This study demonstrates the conditions in which texture can be randomized,which can provides guidelines for processing of AZ31 alloy. These results can be applied to overcome current challenges in the processing of Mg alloy to fabricate critical structural for aerial defence platforms. The study demonstrates, fabrication of AZ31-AZ31 multilayer and AZ31/AA5086 laminate composite using ARB. In the present investigation, a systematic study of texture development in AZ31/AZ31 multilayer and AZ31/AA5086 laminate composite has been carried out. The comparative study has led to the following imported conclusions:

(i) Accumulative roll bonding (ARB) of AZ31/AZ31 up to four passes and multilayer of AA5086/AZ31 up to five passes were successfully produced.

(ii) The strong basal texture in AZ31 was observed in case of AZ31/AZ31 at larger strain which is attributed to the dynamic recrystallization (DRX). A significant waviness is noticed at the interface after third pass of ARB of AZ31/AA5086 which was attributed to shear band formation. The weakening of characteristic basal texture of AZ31 was also noticed after the third pass of ARB in AA5086/AZ31 which is also ascribed to shear bands formation.

(iii) Texture development in constituent AZ31 layers is independent of the second phase (AA 5086) at lower rolling strain. The effect of hard second phase is evident at the higher rolling strain. A tilted basal texture in AZ31 in both multilayer and laminate composite is observed at a low rolling reduction.Higher rolling reduction in multilayer leads to a strong basal texture while the texture randomizes in two-phase laminates.

Acknowledgment

The authors are thankful to Dr. Mano Misra and Dr. Pradeep Menezes both at University of Nevada,Reno for valuable discussion.