Experimental and Numerical Investigations of Formability of Two-Ply Clad Sheet of Stainless Steel and Aluminium Alloy

Abstract

In pursuit of newer lightweight components, clad sheets emerge as a solution to meet the demand for automotive and aerospace applications. The present work focuses on the effect of warm forming temperature on the tensile properties, microstructural characteristics, formability, and residual stress of a two-ply clad sheet composed of SS430 and AA1050 layers. Tensile properties of the clad sheet and individual components are determined through testing of laser cut specimens as per standard at ambient and elevated temperatures (220 °C). These tensile properties are used in the material model for prediction of failure in Erichsen cupping experiments through simulations. At 220 °C, the yield and ultimate tensile strengths of clad sheets are found to decrease by approximately 10% and 18%, respectively, but ductility is observed to increase by almost 5%. Electron backscatter diffraction analysis is done to measure and evaluate the microstructural characteristics of the clad material after the tensile deformation at both temperatures. The kernel average misorientations measured from electron backscatter diffraction show that the transition layer retained its integrity during the plastic deformation, whereas the number fraction at peak misorientation values has risen substantially on either side of the transition layer, i.e., on the steel and aluminium layers. The pole figure maps indicate a weak texture of the AA1050 layer, but a stronger texture of the SS430 layer after warm forming than after forming at ambient temperature. The cupping experiments are performed to assess the formability of the clad and individual layers, at two different temperatures. To study the effect of each layer during forming, two possible cases of sheet placements are investigated; i.e., in the first case, the SS430 is on the outer side, and AA1050 is in contact with the punch, whereas in the second case, it is the opposite. The clad sheet shows better formability when the steel layer is on the outer side of the dome. The formability of the clad sheet is observed to increase by 15% at the warm forming temperature. The predicted simulation results are in good agreement with the experimental results. In order to investigate the effect of temperature on the residual stress, the variation of stresses was also determined and mapped on the tested samples across the dome after forming at ambient and warm temperatures. The residual stress across the dome of the tested samples after the warm forming is significantly reduced compared to that of the samples tested at room temperature. The favourable tensile properties, microstructural characteristics, and reduced residual stress at elevated temperature indicate better formability of the clad sheet with good bond integrity at warm forming temperature.