Forming limit diagrams of fine-grained Al 5083 produced by equal channel angular rolling process

The objective of this research is to study the strain forming limits of Al–Mg alloy (5083) sheet, fabricated by equal channel angular rolling process at room temperature. For this purpose, the equal channel angular rolling process was executed at room temperature in three passes. Mechanical properties, microhardness, and microstructure were investigated after the equal channel angular rolling process. Uniaxial tensile tests of the equal channel angular rolling process produced samples and showed that yield and ultimate stresses increase, while the uniform elongation to fracture decreases in comparison with the annealed state. There was a continuous hardness enhancement by increasing the number of the equal channel angular rolling passes. After the third pass, the amount of hardness raised by 73% in comparison with the annealed sample. In the fourth pass, the hardness reduced slightly, that was attributed to the strain saturation in room temperature and was followed by high surface cracks. In the annealed condition, the average grain size was 45 µm, and after the third equal channel angular rolling pass, this amount was reduced to 10 µm. Furthermore, the forming limit diagrams were determined experimentally, using the Nakazima test. The obtained results show that after the third pass, the forming limit diagrams’ level move downward, meaning that a reduction occurred in the forming limits of equal channel angular rolled samples.


Introduction
In recent years, fabricating nanostructure sheet metals and alloys via severe plastic deformation (SPD) of metallic materials has been considered to a great extent, as it develops fine grains up to sub-micrometer or even nanometer order in polycrystalline materials [1,2]. Ultrafine-grained (UFG) material has attracted significant research interest, because it reveals high strength besides good ductility and toughness. Recently, as one of the outstanding procedures to generate UFG sheet metallic materials, equal channel angular rolling (ECAR) has progressed [3][4]. ECAR is a type of SPD method by which large strains can be exerted to the material [5]. This method is based on equal channel angular pressing (ECAP) and has been lately used in few sheet and strip metals to produce UFG structure with favorable properties [6,7]. ECAR can be considered as an affordable process in order to produce high-quality aluminum alloy sheets wherewith the material is passed across the channels of a die set, without variation in strip cross-section zone [8]. Therefore, it can be used in a continuous procedure and with various passes to produce ultrafine grains and favorable mechanical properties [7,9]. The high-quality ECARed alloy sheets can be utilized in automotive and airplane industries. When the metal sample passes through the intersection area of ECAR die channels, the shaping zone is found to be nonuniform. Therefore, the material undergoes simultaneous plastic deformations in different strain levels and locations [10]. In the literature, there is no investigation carried out on forming limit diagrams of ECARed sheets, especially for Al-Mg alloy (5083). Understanding the forming behavior of ECARed sheets plays an important role in its practical efficacy along with a thorough production cost analysis. The forming limit diagram (FLD) is widely accepted as an efficient tool to analyze the formability of sheet metals [11][12][13][14][15][16][17][18][19][20][21][22][23][24].
In this paper, the forming limit diagrams of fine-grained Al-Mg alloy (5083) sheets, fabricated by equal channel angular rolling process at room temperature were determined experimentally for the first time. For this purpose, the ECAR process was performed at room temperature in three passes. Mechanical properties, micro-hardness, and microstructure were investigated after the ECAR process. Finally, the FLDs were determined experimentally, using the Nakazima test.

Material
In the present study, an aluminum alloy 5083 sheet with 2 mm thickness was cut in order to make specimens ready for the test. Then, the samples were annealed at 450 ° C for one hour and were air-cooled according to American Society for Testing and Materials (ASTM) B918-01 to relieve all the internal residual stresses.
Non-heat-treatable aluminum-magnesium alloys (5XXX series) are used in different industries, e.g. marine, TV towers and cryogenics. They have primarily potential for lightweight structural application in automotive and aerospace industries. It is because of their properties such as corrosion resistance as well as reasonable strength, ductility and weld ability. It is believed that Post-print of "HR Rahimi, M Sedighi, R Hashemi, (2018) Forming limit diagrams of fine-grained Al 5083 produced by equal channel angular rolling process, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications, 232(11), 922-930. https://doi.org/10.1177/1464420716655560 4 recognition of forming capability in these alloys will enhance the potential for such applications [25].
The material chemical composition (in wt. %) was specified by quantometery analysis and is reported in Table 1. The quantometery analysis is a way by which we can measure the chemical composition of different elements in metal alloys. X-ray fluorescence (XRF) analysis is one of the most common non-destructive methods for qualitative as well as quantitative determination of elemental composition of materials. It is suitable for solids, liquids as well as powders. When high energy photons (x-rays or gamma-rays) are absorbed by atoms, inner shell electrons are ejected from the atom, becoming "photoelectrons". This leaves the atom in an excited state, with a vacancy in the inner shell. Outer shell electrons then fall into the vacancy, emitting photons with energy equal to the energy difference between the two states. Since each element has a unique set of energy levels, each element emits a pattern of X-rays characteristic of the element, termed "characteristic Xrays". The intensity of the X-rays increases with the concentration of the corresponding element [26].

Equal Channel Angular Rolling Process
A schematic illustration of an ECAR set, utilized to introduce shear deformation into sheet metal samples is shown in Fig. 1a. The process was conducted with a rolling machine (two feeding rolls) and dies. The outlet and inlet channels' thickness is 2 mm. In Fig. 1a, the curvature angle (ψ) is 0º, and the oblique angle (φ) that is the intersection angle between the inlet and the outlet channels is 130º. The initial thickness of Al strip is 2 mm, is fed through the feeding rolls, and is reduced to 1.95 mm thickness strip. After passing through the forming zone, the sample preserves its primary thickness (2 mm).
Three overall paths are employed to pass the material through dies in the equal channel angular pressing (ECAP) process [27]. In path A, the specimen passes repeatedly without rotation. And in B and C paths, it is trundled by 90º and 180º after any passing, respectively. In the ECAR process, as opposed to ECAP process, there are only two overall feasible paths as material passage through dies, where A and C paths which are characterized in the ECAP process can be used for sheet metal samples. Fig. 2 is a schematic representation of A and C passing paths, feeding the rolls and dies assembled in ECAR equipment. In this study, the ECAR process was done for three passes with 3 m/min feeding speed and processing path A. It is assumed that the bottom and top surfaces of the specimen are in contact with the lower and upper dies, respectively. Using Equation (1) [28], the effective shear strain inflicted to the sample after third pass will be 1.54: In Equation (1), N represents the number of passes, ɸ is the angle of inclination, and K is the thickness ratio and is equal to 0.975. This equation is derived from the modified Segal model for shear deformation computation [28]. According to the experimental results, it can be considered that dimensional changes of length, thickness, and width of the sample are insignificant. The Al 5083 specimen, fabricated by the ECAR process at ambient temperature is shown in Fig. 3.

Experimental Setup for Sheet Metal Forming Limits Determination
The Nakazima deep drawing test is used to carry out the biaxial stretch-forming tests to obtain the FLDs [29,30]. Rectangular samples of different widths were cut from the sheet metals with 200 mm length, perpendicular to the rolling direction. In order to mark the circular grid with 2.5 mm diameter on the surface of the sheet specimen, the electro-chemical method is used. For stretching sheet samples, a 50-ton constant speed hydraulic press was employed. An abrupt change in load-displacement diagram is utilized as the stopping criterion in the test (Fig. 4). The specimens with different geometries are used to obtain the forming limit curve in this research.  Fig. 6. The circular grids are deformed to elliptic shapes during the tests. After conducting the out-of-plane stretching test for each samples, the limited strains are determined from the major and minor axes of the ellipse that is located in the nearest distance from the localized necking zone. Therefore, Mylar tape is used (Fig. 7). Mylar tape is a graded transparent tape calibrated to the circular grid etched on the undeformed sheet.
The tape is placed over an ellipse on the distorted sheet to match its major or minor diameter, enabling the strain to be read directly as a percentage [31]. The major and minor engineering Where a, b, and c denote the ellipse's the major and the minor diameters and the initial circle diameter, respectively.

Tensile Behavior
An STM-50 (SANTAM Company) electronic tensile machine was employed to accomplish the tensile tests. The mechanical and material properties were determined by standard tests, using specimens which were prepared pursuant to ASTM-E8 characteristics at a constant crosshead speed of 2 mm/min [32]. The mechanical and material properties of each sample are presented in Table 2. after the first pass [35]. At this stage, the dislocation density in the inside and boundary of primary coarse grains, increases, and a high fraction of low angle grain boundaries is formed [35, 36, and 37]. Although the enhancement in the strength persists until 3-pass, the rate of increase was less than that of the first pass. The ECAR process leads to the substantial decrease in the elongation at break, about 50%, until 3-pass. As shown in the stress-strain diagrams (Fig. 8), the yield and the tensile strength had almost the same values in upper passes; however, by increasing the number of ECAR passes the strain hardening was reduced.

Micro-Hardness
The micro-hardness values measured across the sheet thickness of ECARed samples are illustrated in Fig. 10. It is evident that after the first pass of the ECAR process, the micro- 13 hardness values increase significantly, and an approximate 37% increase is seen in comparison with the annealed samples. The rapid rise in micro-hardness in the first pass seems to be attributed to the strain hardening as a result of sub-grain boundary formations rather than grain refinements [38]. The subsequent ECAR passes do not lead to such considerable variations of micro-hardness. However, the rate of micro-hardness enhancement is saturated in next passes of ECAR. This phenomenon might also take place in other SPD processes (e.g., see [39, 40, and 41]). The strengthening in the next passes could be related to the rapid enhancement of dislocation density [39], followed by dislocations rearrangement to sub-grains (cells) and grain refinement [43][44][45][46][47]. The maximum amount of micro-hardness was obtained in the third pass, having increased by 73% in comparison with the annealed sample. Due to the large cracks on sheet surface after the fourth pass, the values declined slightly.

Optical Micrographs
The optical micrographs of the annealed and 3-pass ECARed samples are shown in Fig. 11. The grain size of the samples was measured via the photo tool software. The average grain size of the annealed sample was about 45µm. It is heterogeneous and illustrates a typical recrystallized structure with angular grains (Fig. 13a). Grains of the third pass ECARed sample were reduced to ∼10±5 µm (Fig. 13b).

Forming Limit Diagrams
As mentioned in Section 3 (Table 2), the mechanical properties and forming behaviors of different ECAR passes, obtained from tensile tests indicate that the yield strength of ECARed samples are somewhat higher than those of their base metals (annealed samples); besides, their strain hardening exponents are less than those of their base metals. In addition, the uniform elongations of ECARed samples are 62 to 100% less than those of their base metals. Fig. 12 shows the forming limit diagrams, obtained from experimental tests for the annealed samples and the first to third pass ECARed Al 5083. By comparing the achieved FLDs, it can be concluded that the formability of ECARed samples will become less than that of related base metals through increasing the number of ECAR passes (Fig. 12). Moreover, it can be seen that after increasing the number of ECAR passes, the forming limit diagram slope increases on the left side and decreases on the right side. This phenomenon can be accounted for by the influences of the ECAR process on the microstructure and anisotropy of sheets.  Table 3 shows the changes of micro-hardness, mechanical properties, and formability of aluminum (5083-O) ECARed in comparison with those of the annealed samples. The values in Table 3 were obtained, using Equation 4: Where is defined as the value of the parameter in particular pass of the ECAR process, and is the same parameter in the annealed sample.

Conclusions
In this research, the effects of the ECAR process on formability, mechanical properties, and the micro-structural evolution of Al (5083-O) were investigated at room temperature. The following results can be exposed: (1). By means of the equal angle channel rolling process, an evident increase in the yield and ultimate strength may be achieved along with a decrease in elongation at break of samples, compared to the annealed samples. The yield strength value was 156 MPa in the annealed state which rose up to 273 MPa after the ECAR process third pass; and, in comparison with the annealed samples, a double increase in strength value was obtained. In addition, the effective strain, applied to the material after three passes of the ECAR process was 1.54. This effective strain quantity could be achieved in rolling process with about 75% reduction in thickness.
(2). The results showed that after the first pass of the ECAR process, the micro-hardness values increased significantly, and in comparison with the annealed sample, an approximate 37% increase was observed. This magnitude remained almost invariable during the process up to the third pass. Eventually, in the next passing due to created surface cracks, the micro-hardness was reduced slightly. The maximum available hardness after all the 3-passes of the ECAR process was 120 Vickers. That is a 73% increase, compared to the annealed condition.
(3). The grain size of Al (5083-O) sheets was reduced by increasing the number of the ECAR passes. The maximum rate of grain refinement occurred during the first pass. The rate reduced in subsequent passes. The average grain size was 45 µm in the annealed condition, while after the third pass of the ECAR at room temperature, the average grain size was reduced to 10 µm.
(4). After three passes of the ECAR, the yield strength increased, whereas the elongation and formability decreased. However, the rates of the increase in yield strength and the decrease in the strain forming limits were not the same.