Abstract
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.
1. 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. The number of ways to construct FLCs and their significance has been represented by many researchers; a few of them are explained inTable 1.
A different concept has been introduced on double-walled porous functionally graded magneto-electro-elastic sandwich plates by Saffari et al. [24]. In this study, we investigated the effect of uniform and nonuniform temperature distributions on sound transmission loss on the sandwich plates with subsonic external flow.
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. Moreover, the impact of sheet rolling direction in detail has not been reported yet. The 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 effects (0°–0°, 0°–45°, 45°–45°, 0°–90°, and 90°–90°).
2. Experimental Methodology
2.1. Base Materials and Specimen Preparation
The base metal AA5052 alloy sheet of 1 mm thickness and ultraclear PVC sheet of thickness 0.5 mm were selected as sandwich sheets. The 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. The PVC sheet was also cut with the same dimensions. The 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. The 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 different 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 (effective) area (Figure 3).
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The obtained limit strains were used for developing the FLC for each case. For the initial limit strain of FLC, FLC0 was calculated by Keeler and Brazier [25], and equation (1) resulted in a relationship between the FLC0 and sheet thickness (t in mm) and the strain hardening exponent (n).
The details of forming a limit diagram with various zones are described in [4]. The calculated major strains and minor strains, i.e., limit strains, from the various formability tests were plotted to identify the forming limit curve (FLC).
2.2. 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. The 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 different rolling directions, namely, 0°, 45°, and 90°, whereas sandwich sheets were made with a combination of different 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).
3. 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.
3.1. Mechanical Properties of AA5052 Sheet and Sandwich Sheets
The strain hardening exponents of the AA5052 alloy sheet and Al-PVC-Al sandwich sheets were evaluated using uniaxial tensile test. The 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.
3.2. 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. The test specimens were marked with circular grids of 5 mm diameter in the effective zone. The 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).
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3.3. Limit Strain Calculations and Developing Forming Limit Diagram
From the tested AA5052 alloy sheet, the deformed zone was identified 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. The initial limit strain for forming limit curve (FLC0) was evaluated using equation (1).
3.3.1. 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), FLC0 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 0- and 90-degree AA5052 sheet are the same, which have better formability compared to the 45-degree sheet. This difference is accounted for by variation in the strain hardening values (Table 2).
3.3.2. FLDs of AA5052-PVC-AA5052 (Al-P-Al) Sandwich Sheets
The 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 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 first the metal layer and then the polymer. This is being noted in all the sheets.
The 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 FLC0 identification, equation (1) does not show the suitability for limit strain evaluations. For this, necking /failure zone strains have been identified 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.
Figures 9–13 show FLDs of 0-P-0, 0-P-45, 0-P-90, 45-P-45, and 90-P-90 sandwich sheet rolling direction combinations.
The 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. The impact yield strength (σ), strength coefficient (K), and strain hardening exponent (n) along with plastic anisotropy (R) influence the deformation phenomenon in the sandwich sheets. These properties irrespective of parent materials are prompting the best formability for materials. In comparison with all these FLDs, the best formability is depicted in the 90-P-90 sandwich sheet, followed by the 0-P-0, 0-P-90, 45-P-45, and 0-P-45 sandwich sheets, respectively. This research can lead the industry sector to use the sandwich sheets with a combination of different rolling directions for superior formability.
The variation or improvement in the formability in terms of percentage is tabulated in Table 6 with limit strain comparison.
4. Conclusions
The 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) effect on formability analysis to base materials and sandwich sheets, is seen clearly. This work has brought the conclusion that 0-degree and 90-degree rolling have more or less similar forming behaviour in AA5052 alloy 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. The 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].
Data Availability
The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.
Ethical Approval
All procedures performed in the studies were in accordance with the ethical standards of the institutional and/or national research committee and with the comparable ethical standards.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Authors’ Contributions
P. Praveen Kumar Reddy investigated and executed the entire objective of the work, using proper research methodology. Chinmaya Prasad Padhy selected the topic and supervised and executed the research as per the scientific principle. Perumalla Janaki Ramulu performed verifications, executed the literature, and compiled the data. Every author has a significant contribution towards the successful completion of research work associated with the manuscript. The authors give consent for the publication of the Submitted Research article in the Scientific World Journal.