EXPERIMENTAL AND NUMERICAL STUDIES ON SPRINGBACK OF 2-PLY AND 3-PLY LAMINATE COMPOSITES OF ALUMINIUM AND STAINLESS STEEL

Abstract

To meet the requirement of materials having higher tensile strength to weight ratio along with corrosion resistance, the 2-ply and 3-ply clad sheets of aluminium and stainless steel are being utilized in domestic applications, manufacturing of automotive sheet metal parts, shipbuilding, and other industries. Metal clad sheet composites are a different kind of composite material where alternating metal layers are bonded together with inter-metallic bond. The sheet metal components manufactured by forming processes may undergo some geometrical changes due to the phenomenon of springback. The behaviour of springback during bending of 2-ply and 3-ply clad sheet metals is very complex due to the combination of sheets of different mechanical properties and thicknesses. Accurate prediction of springback in clad sheets in a bending operation will help in improving the die design by involving springback compensation. It would facilitate the optimum selection of materials, blank design, and other design variables so as to manufacture parts with minimum springback. In the present work, an analytical method has been developed that predicts the amount of springback in bending of clad sheet using input parameters from stress-strain diagrams of sheet materials like tensile properties, strain hardening exponent and anisotropy of parent sheets. The total bending moment in the analytical model at a cross sectional plane is calculated from the bending stresses for different layers. Springback is calculated by superimposing the effect of a negative bending moment of the same magnitude so as to simulate the unloading phenomenon. Numerical simulations have also been done for the simulation of V bending of clad sheet using Abaqus software based on FEM (finite element method). Experimental work has also been carried out and compared with the simulation and vii analytical results. The data from stress-strain plots for the parent and clad sheets was generated using computerized UTM. The parent and clad sheets are characterized using tensile properties, strain hardening exponent and anisotropy. The typical yield strengths (0.2% offset) of clad sheets for the tensile specimens oriented at 0°, 45° and 90° are observed to be 115MPa, 117MPa and 114MPa for a 2-ply and 153.5MPa, 174MPa and 176 MPa for a 3-ply sheet, respectively. A set of three punch profile radii was used to conduct V-bending experiments for the measurement of springback on a computerized UTM. The results from the analytical model and FEM simulation are seen found to be in close agreement with the experimental results. It is observed that an increase in punch profile radius causes a significant increase in springback for both 2-ply and 3-ply clad sheets. For a 2-ply clad sheet, the bend samples (with AA1050 as inner layer ) which are oriented at 45° to the rolling direction, the experimental values of springback, higher than the other two orientations, are observed to be 4.12 and 4.88 with punch profile radii of 15mm and 20mm, respectively. In case of a 3-ply clad sheet, the bend samples (with SS304 as inner layer) oriented at 90° to the rolling direction, the experimental values of springback higher than the other two orientations, are found to be 5.15° and 6.10° with punch radii of 15mm and 20mm, respectively. Similar trend of results is obtained with analytical and simulation techniques. FE simulation results are closer to the values of experimental results than the analytical model results. The analytical model assumes a plane strain condition and neglects the neutral axis shift. These assumptions are not made for the simulation. Also, a more robust material model is used in FE simulations. The effect of sheet setting on the die is also investigated in this work. Effect of the position of aluminium sheet at both inner and outer layers is studied. Sheet setting viii conditions affect the values of springback due to the tensile strength variations and different bending radii during V-bending operations. For both the sheet settings, the residual stresses as predicted Abaqus simulations are in good agreement with the experimental results except for a few cases.