AA5052-PVC-AA5052 (Al-PVC-Al) Sandwich Sheets Forming Analysis through In-Plane Plane Stretching Tests

Sheet metal forming is one of the key processes for the automotive sector to be considered. Sheet metal formability is being tested as received, joining them with different welding/joining processes (i.e., tailored blanks) and making them as sandwich forms to reduce the total weight of the body. These sandwich formations of sheets are an advanced method by incorporating PVC/polymer sheets in between metal sheets with a suitable binder. The present work has investigated the formability of AA5052-PVC-AA5052 (Al-PVC-Al) sandwich sheets by considering the sheet rolling direction as a parameter. The mechanical properties of base metal and sandwich sheets were evaluated by conducting the uniaxial tensile tests. For forming behaviour of Al-PVC-Al sandwich sheets, in-plane plane stretching tests were performed on the universal tensile testing machine. From the results, it has been observed that 0-degree and 90-degree rolling direction of AA5052 sheets provided almost similar forming behaviour where the 45-degree rolling direction showed less formability. The limit strains (by which the forming limit curve has been developed and the safe and failure zones are separated) are 0.043, 0.038, and 0.043 of 0°, 45°, and 90°, respectively. Considering 0°-P-90°, 90°-P-90, 0°-P-45°, 0°-P-90°, and 45°-P-45° sandwich sheets with their corresponding limit strains of 0.060, 0.058,0.057, 0.052, and 0.050, a better formability is seen in 0°-P-90° sandwich, followed by 90°-P-90, 0°-P-45°, 0°-P-90°, and 45°-P-45°. The improvement in the formability is calculated as 28.33%, 25.86%, and 24.0% in comparison with the base metal in 0-degree, 90-degree, and 45-degree rolling directions and 0°-P-90°, 90°-P-90, and 45°-P-45° sandwich sheets.


Introduction
Forming behaviour analysis of any sheet metal leads to the proper application in an industry sector.Identifying the forming limit strains and constructing forming limit curve/ diagram (FLC/D) are major concerns for sheet metals.Te number of ways to construct FLCs and their signifcance has been represented by many researchers; a few of them are explained inTable 1.
A diferent concept has been introduced on doublewalled porous functionally graded magneto-electro-elastic sandwich plates by Safari et al. [24].In this study, we investigated the efect of uniform and nonuniform temperature distributions on sound transmission loss on the sandwich plates with subsonic external fow.
Fabrication and the formability of the various sandwich sheets made of Al alloy-based, steel-based, Cu-based, and Al and steel combination along with polymers have been observed.From the above-stated forming studies, it has been noted that deep drawing, stretching, and in-plane plane stretching tests are used for formability analysis by considering sandwich thickness, test parameters, machine capabilities, metal-polymer binder capabilities, sheet rolling directions, and sheet dimensions, etc., as main parameters.Paul [4] Reviewed and indicated the various parameters such as limit strain evaluation method, punch profle, microstructure, prestraining path, strain rate, and temperature infuencing the FLCs Te tensile properties and their relationship with FLCs.Demonstrated the microstructure characteristics of FLCs Sandwich sheets made of metal-polymer-metal formability evaluation and construction of FLD have been discussed Moreover, the impact of sheet rolling direction in detail has not been reported yet.Te present work aimed to establish the FLCs of Al-polymer-Al sandwich sheets with respect to three rolling directions (0 °, 45 °, and 90 °) and their combined efects (0 °-0 °, 0 °-45 °, 45 °-45 °, 0 °-90 °, and 90 °-90 °).

Experimental Methodology
2.1.Base Materials and Specimen Preparation.Te base metal AA5052 alloy sheet of 1 mm thickness and ultraclear PVC sheet of thickness 0.5 mm were selected as sandwich sheets.Te AA5052 sheet was cut in 0 °, 45 °, and 90 °rolling directions with the dimensions as shown in Figure 1.Based on these dimensions, AA5052 sheet specimens were cut using a CNC milling machine in three rolling directions as shown in Figure 2. Te PVC sheet was also cut with the same dimensions.Te surfaces of the AA5052 sheets were cleaned before the binder was applied.Equal quantities (by volume) of resin and hardener of Araldite Standard were taken and mixed until it forms a uniform colour.A thin coat of the mixture was applied on both the surfaces of AA5052 specimen sheets and PVC sheets inserted in between them.
Te pressure was applied using c-clamps such that excess resin came out.For proper adhesive bond formation, Al-P-Al sandwich sheets were kept up to 6-8 hours under uniform clamping conditions.Afterwards, c-clamps were removed, and sandwich sheets were kept idle for 24 hours; later, all the sandwich sheets were cleaned and taken for further tests.Sandwich sheets were made with a combination of diferent rolling directions of AA5052 such as 0 °-P-0 °, 45 °-P-45 °, 90 °-P-90 °, 0 °-P-45 °, and 0 °-P-90 °(P refers PVC).On base metal and sandwich sheets, a circular (5 mm diameter) grid pattern was stamped near the grooved (efective) area (Figure 3).
Te obtained limit strains were used for developing the FLC for each case.For the initial limit strain of FLC, FLC 0 was calculated by Keeler and Brazier [25], and equation (1) resulted in a relationship between the FLC 0 and sheet thickness (t in mm) and the strain hardening exponent (n).
Te details of forming a limit diagram with various zones are described in [4].Te calculated major strains and minor strains, i.e., limit strains, from the various formability tests were plotted to identify the forming limit curve (FLC).

Tensile Testing of Base Metal Sheet and Sandwich Sheets.
Uniaxial tensile tests were conducted using the MCS computerised universal testing machine of 1-ton capacity.Te specimens of base metal sheet AA5052 and AA5052-PVC-AA5052 were made based on the width constraint method.All the tests had been conducted with a strain rate of 1 mm/min, and reputation was also maintained for each sample.From the base sheet of AA5052, samples were cut in three diferent rolling directions, namely, 0 °, 45 °, and 90 °, whereas sandwich sheets were made with a combination of diferent rolling directions of AA5052, such as 0 °-P-0 °, 45 °-P-45 °, 90 °-P-90 °, 0 °-P-45 °, and 0 °-P-90 °(P refers PVC).

Results and Discussion
As per the previous section explanation, as received based on AA5052 alloy sheet of 1 mm thickness along with 0.5 mm thickness, PVC sandwich sheets have been fabricated and tested for mechanical properties and in-plane plane stretching tests.

Mechanical Properties of AA5052 Sheet and Sandwich
Sheets.Te strain hardening exponents of the AA5052 alloy sheet and Al-PVC-Al sandwich sheets were evaluated using uniaxial tensile test.Te obtained results of yield strength and strain hardening exponent are tabulated in Tables 2  and 3.
From the results of tensile tests of the base sheet and sandwich sheets, 0 °and 90 °showed similar trends in the base sheet, whereas the 90 °-P-90 °sandwich sheet has more strain hardening value compared to other sandwich sheets.

In-Plane Plane Stretching Tests.
In-plane plane stretching tests were performed on a universal testing machine under a constant strain rate of 1 mm/min which made sure that none of the samples failed in the gripping zone.Te test specimens were marked with circular grids of  Te Scientifc World Journal 5 mm diameter in the efective zone.Te failure phenomenon has been observed similarly in the rolling directions.For illustration, AA5052 alloy sheet specimens of 0 °, 45 °, and 90 °failed in in-plane stretching test are shown in Figure 4, respectively.In a similar manner, the failure phenomenon is also seen in Al-P-Al sandwich sheets (Figure 5).

Limit Strain Calculations and Developing Forming Limit
Diagram.From the tested AA5052 alloy sheet, the deformed zone was identifed and the major and minor strains were measured using equation (1) as mentioned in Section 2.
From the obtained major and minor strains, FLDs were developed for base AA5052 alloy sheets and Al-P-Al sandwich sheets corresponding to their rolling directions.Te initial limit strain for forming limit curve (FLC 0 ) was evaluated using equation (1).

FLDs of AA5052 Alloy Sheets. Figures 6-8 show the
FLDs of the AA5052 alloy in-plane plane stretching testing specimen with sheet rolling direction as a parameter.Figure 6 depicts the FLD of the 0-degree rolling direction sheet with the division of safe limit strains and failure zone.Based on equation ( 1), FLC 0 is calculated by which FLC has been drawn.Table 4 shows the limit strain values which are evaluated by using equation ( 1) for base metals.For this case, 0 °and 90 °base sheets show similar FL strains.In a similar manner, for sheet 45-and 90-degree rolling direction sheets, FLDs have been developed (Figures 7 and 8) with the same approach.It has been observed that FLCs of 0and 90-degree AA5052 sheet are the same, which have better formability compared to the 45-degree sheet.Tis diference is accounted for by variation in the strain hardening values (Table 2).

FLDs of AA5052-PVC-AA5052 (Al-P-Al) Sandwich
Sheets.Te sandwich sheets have been fabricated with 0-P-0, 0-P-45, 45-P-45, 0-P-90, and 90-P-90 sheet rolling direction combinations to see the forming behaviour.Circular   Sandwich sheets YS (MPa) n 0 °-P-0 °129.25 0.39 45 °-P-45 °125.3 0.29 90 °-P-90 °129. 15 0.44 0 °-P-45 °120.1650.30 0 °-P-90 °136.350.28 6 Te Scientifc World Journal grid sampling was carried out on the sandwich sheets for limit strain calculations.During the test, none of the sheets failed in the grip zone.All sheets failed as per the expectation like frst the metal layer and then the polymer.Tis is being noted in all the sheets.Te FLDs have been developed for all cases such as 0-P-0, 0-P-45, 0-P-90, 45-P-45, and 90-P-90 sandwich sheet rolling direction combinations.Limit strain calculation has been performed based on the necking phenomenon which indicates the limit strains near to necking or failure of the sheet.For FLC 0 identifcation, equation (1) does not show the suitability for limit strain evaluations.For this, necking /failure zone strains have been identifed and separated from the safe zone and failure zone with manual FLC.Table 5 shows the limit strain values of 0-P-0, 0-P-45, 0-P-90, 45-P-45, and 90-P-90 sandwich sheets by which safe and failure zones are separated.From the limit strain values (Table 5), the maximum limit strain has been noted for the 0-P-0 sandwich sheet, followed by 90-P-90, 0-P-45, 0-P-90, and 45-P-45 sandwich sheets.From these values, better formability is seen for the 0-P-0 sandwich sheet with a slight variation to the 90-P-90 sandwich sheet.
Te maximum limit strain is observed in the 0-P-0 sandwich sheet as 0.06 because the strain hardening value of the 0-P-0 sandwich sheet is 0.39 which is relatively higher than that of the other sandwich sheets except 90-P-90.Te impact yield strength (σ), strength coefcient (K), and strain hardening exponent (n) along with plastic anisotropy (R) infuence the deformation phenomenon in the sandwich
Te variation or improvement in the formability in terms of percentage is tabulated in Table 6 with limit strain comparison.

Conclusions
Te present work addressed the formability analysis of AA5052 alloy sheets of 1 mm thickness and AA5052-PVC-AA5052 sandwich sheets of 2.5 mm thickness by considering rolling direction as a parameter.From the obtained results of this work, the importance of plastic anisotropy, i.e., rolling directions (0, 45, and 90) efect on formability analysis to base materials and sandwich sheets, is seen clearly.Tis work has brought the conclusion that 0-degree and 90-degree rolling have more or less similar forming behaviour in

Table 1 : 1 ]
List of related literature studies and their important outputs.Marciniak biaxial stretch test and produced the in-plane FLCs by using a single punch/die on low-carbon drawing quality sheet steel (0.76 to 1.5 mm thickness) Obtained in-plane FLCs and compared with the conventional out-of-plane dome tests Results confrmed that (a) sheet thickness has an intrinsic infuence on forming limits and (b) plastic anisotropy and (c) in-plane FLCs were somewhat lesser than out-of-plane FLCs near plane strain Levy and Van Tyne [2] Stress-based FLCs were developed using the uniaxial tensile test data of IF RePhos, HSLA 350, HS440W, DP 500, DP600, DP800, DP980, and TRIP600 with 0.7, 1.4, and 2.1 mm Keeler-Brazier equation for developing the FLCs FLCs were well correlated with strain-based FLCs Pundan et al. (2017) FLDs developed by Nakajima testing on DP600GA (1.2 mm thickness), DQ (0.7 mm thickness), HIF (0.8 mm thickness), and EDD (0.8 mm thickness) Proposed a method based on a combination of experiments and the use of CrachLab Te forming limit diagram generated using the proposed methodology was compared with the one obtained using the standard procedure, and a good correlation was obtained Lumelskyj et al. [3] Evaluated the FLCs using fnite element simulations with Nakazima formability tests on a DC04 steel sheet of 1 mm thick Nakazima formability tests through experiments and simulations Te FLCs strains were calculated by using the strain localization criteria

Figure 1 :
Figure 1: Schematic representation of an in-plane plane-strain tensile test specimen.

Figure 3 :
Figure 3: In-plane plane stretching specimen of the base metal sheet and Al-PVC-Al sandwich sheet.(a) AA5052 alloy sheet.(b) AA5052-PVC-AA5052 sandwich sheet of 45 °rolling direction (top) and thickness direction of sandwich (bottom).

Table 2 :
Mechanical properties of the base metal AA5052 sheet in diferent rolling directions.

Table 4 :
Limit strain value of the AA5052 base metal.

Table 6 :
Percent variation of limit strains in base sheet and sandwich sheets., whereas 90-degree combinations dominated in sandwich sheets.From the mechanical properties such as yield strength (YS) and strain hardening exponent (n), one can estimate the sheet metal formability prior to the formability test.Te formability of any sheet metal will not depend on a single parameter; however, it includes material properties, manufacturing process, thinning behaviour, strain rate, and formability parameters in a synergetic manner[26, 27].