Sheet metals are frequently used in products produced by deep drawing method such as kitchen utensils, medical storage boxes and automobile bodies. In places where willing high strength and lightness, multi layer sheets are preferred instead of single sheets. Layered sheets are divided into 3 main groups as metal / metal, metal / polymer / metal and metal / composite / metal. Bimetallic sheets are a layered sheet obtained by combining two different sheet metals with methods such as welding, bonding, rolling or hot pressing. With using different metals, a composite sheet has different properties and thicknesses can be obtained with superior properties of each sheets. In this way, lighter, corrosion-resistant, high-strength materials can be obtained. It is especially preferred in sectors where material lightness is at the forefront for energy saving such as automotive industry.
Deep drawing is basically the process of drawing sheet metal into the die with the help of a punch. This manufacturing technique is a frequently used method in productions made with bimetallic sheets. However, problems such as separation between layers, wrinkling and tearing are encountered in products produced by this method. There are different studies in the literature investigating these problems. For example, Mori and Kurimoto  examined the formability and wrinkling behavior of the sheets with the press-forming tests of pressure welded hot rolled stainless steel - aluminum sheets. As a result, they observed that the formability of the bimetallic sheet in stretching and deep drawing processes is better when the steel layer comes into contact with the punch. They also stated that the wrinkling state during deep drawing is controlled by the harder stainless steel. Chen et al.  investigated the earing behavior of Al / Cu bimetallic sheet during cylindrical deep drawing. They bonded aluminum and copper sheets by cold rolling. Afterwards, they determined the level and change of earing by subjecting the sheets they annealed at different temperatures to deep drawing process. Morovvati et al.  investigated the required blank holder force (BHF) to prevent wrinkling in deep drawing of an adhesive bonded aluminum / steel bimetallic sheet. As a result, they showed that the BHF required to prevent wrinkling in the layered sheets consisting of high and low strength sheets is at a value between the BHFs required for each layer. In another study, Aghchai et al.  examined the effect of the mechanical properties of materials on the formability of Al3004 / St12 bimetallic sheet. The proposed theoretical model showed that the forming limit curve (FLC) of laminated sheet is between the FLCs of the constituent materials. Afshin and Kadkhodayan  investigated the effects of different factors such as temperature, grain size, BHF, layer alignment and friction on the deep drawing process of Al 1050 / St 304 and Al 5052 / St 304 bimetallic sheets. As a result, they determined that the damage occurred in the areas of the punch radius and cup walls. In addition, they found that the punch force was less if the steel layer was on the upper side than when the aluminum layer was on the upper side.
Finite element (FE) analysis is using for predict the problems encountered in deep drawing processes. Experiments in many studies have been supported by FE analysis. In the analysis of bimetallic sheets, it is very important to use the model that gives the most realistic result in the shortest time. Bimetallic sheets have been modeled with many different assumptions in the literature. Takuda et al.  simulated the deep drawing process of the St / Al bimetallic sheet. They neglected the resin layer in the FE model of the sheets and bonded with polyurethane resin. Therefore, the resin layer was not modeled and it was accepted that there was no slippage between the layers. Parsa et al.  investigated numerically and experimentally the effect of thickness ratio and layer sequence on the drawing ratio that can be obtained in deep drawing of aluminum / stainless steel bimetallic sheets. They accepted that there is no slippage between layers in the models created axisymmetrically. Bagherzadeh et al.  modeled the hydro-mechanical deep drawing (HMDD) process of St/Al bimetallic sheets using finite element method (FEM). St-Al bimetallic sheet were laminated by a polyurethane base adhesive. Similar to previous studies, they modeled bimetallic sheet as two different layers, but made the assumption that there was no slippage between layers.
Marandi et al.  investigated bulge test of Al-Cu bimetallic sheets using FEM and confirmed it by experimental test. Al-Cu bimetallic sheets were manufactured by explosive welding method. They assumed that layers in common interface were sticking firmly to each other and any slip or separation did not occur between layers while subjecting stresses and deformation. In the light of this assumption, each layer was modelled separately and then layers were bonded to each other with tie constraint. Simulation results of their study showed similarity with the experimental results. Sakhtemanian et al.  carried out the influences of some process prameters on bimetallic sheets’ incremental forming, such as layers’ arrangement, forming load, surface finish. Titanium and low-carbon steel sheet were connected to each other by explosive welding method. They validated the FE results using experimental data. In their study, the shell element was used for meshing the blank and they assumed that the sheets were perfectly bonded. To simulate the bimetallic sheet, one section was defined for each layer and the material properties of each sheet were attributed to each section. Then both sections were attached to each other using tie constraint. Thereby, no slippage permitted between layers. Karajibani et al.  also used same modelling method for AL-Cu bimetallic sheet. As in previous studies, they fabricated the bimetallic sheets using explosive welding.
Tseng et al.  experimentally investigated the formability of aluminum / copper bimetallic sheets manufactured with roll bonding in different thickness ratios with FE simulation. In the models they created, they modeled bimetallic sheet as a single sheet. They determined the material properties with the tensile tests made on the laminated sheets. In another study, Morovvati et al.  investigated the BHF required to prevent wrinkling in deep drawing of Al/St bimetallic sheet bonded with polyurethane adhesive. By modeling the polyurethane interlayer in the FE model of the sheets, they accepted that this layer adheres tightly to the aluminum and steel layers and that there is no slippage and separation between the layers.
Nejad et al.  modeled the study, which was previously performed experimentally by Atrian and Fereshteh-Saniee , with the Abaqus finite element program. After determining that the models were compatible with the experimental results, they analyzed the effects of punch radius, die radius, friction coefficients of steel and brass layers on surfaces, BHF and sheet diameter on punch force and thickness reduction statistically with response surface method. While modeling bimetallic sheets, they accepted the presence of Columb friction between layers. Therefore, they created a model that allows sliding and separation between layers. They determined the average friction coefficient of steel and aluminum sheets as the friction coefficient between these two layers.
Liu and Xue  experimentally examined the formability of AA5052 / polyethylene / AA5052 sandwich plates in their study. They prepared three kinds of AA5052 / polyethylene / AA5052 sandwich samples with different thicknesses of core material by hot pressing bonding method. They compared the FLC determined experimentally with the results of FE analysis. In the model created in their study, all metal and polymer layers are modeled. The inter-layer contact state is defined by Cohesive Zone Model (CZM). In their later work, they examined the deep drawing behavior of the same sheets. In their studies, which they supported with the FEM, they continued to use the CZM .
In this study, St / Al bimetallic sheets are modeled with the assumptions used in the above studies and different assumptions in addition to these, and the most suitable model type for bimetallic sheets is tried to be determined. For this purpose, T-peel test and Single Lap Joint test were performed to determine the peel resistance and shear strength of the samples. These tests are modeled in the ABAQUS FE program and the CZM parameters are determined. Then, by using 5 different models with the determined parameters, the behavior of the bimetallic sheets in the deep drawing process was simulated. The simulation results are compared with the experimental results and the optimum model type that gives the most realistic result in the shortest time is determined.