Determination Of Formability Of Bimetallic Sheet Using Forming Limit Diagram

Abstract

Abstract The Formability of a sheet plays an important role in all sheet metal forming operations. It is required to determine the formability of sheet to know the behavior of the sheet under various loading conditions. By determining the formability, it will be easier to predict the failure point of a sheet metal. Further it suggests us the maximum permissible value of load to be applied on the sheet material so that it will not fail. Forming Limit Diagram is a graphical way of depicting the forming behaviour of a sheet metal. FLD estimates various regions of sheet metal of plastic deformation and suggests a permissible value of load to be applied. However FLD of Aluminium, MS, Copper etc are known from the record books but FLD for bimetallic sheet is not yet determined. Bimetal refers to an object that is composed of two separate metals joined together. Instead of being a mixture of two or more metals, like alloys, bi-metallic objects consist of layers of different metals. Generally, these bimetals are used for thermal applications like the thermostat, thermocouple and other electrical devices which need circuit breakers. But these bi-metallic sheets have better strength, thermal resistance and ductility than the singular sheet metal. Hence, the aim of this work is to determine whether these bi-metallic sheets are suitable for deep drawing operation or not. This can be determined by finding out the formability of these sheets. The main objective of this work is to draw the Forming Limit Diagram of bimetallic sheet made of Aluminium and Copper and to find the formability of the sheet. To achieve this objective the strength of the bimetallic sheet is determined first and then Chemical Etching of the bimetallic sheet is done which creates a grid pattern of circles on the surface of the sheet. The sheet is then cut into different widths by keeping the height of the specimen constant. Erichsen cupping test is performed for different widths of bimetallic strips. During the experimental process the sheet is initially fixed between die and punch. Then the punch is made to advance through specimens until a fine hair line fracture appears. The circles on the surface of the sheet will be deformed into ellipse. Deep drawing is carried out for the same bimetallic sheet where the blank of the bimetallic sheet is drawn into cup. The major and minor axes of the ellipse which corresponds to major and minor strains are measured. The strains are measured along different portions of the cup namely flange portion, side walls and the base of the cup and this is included in the Forming Limit Diagram of Bimetallic sheet. By taking major strain on Y-axis and minor strain on X-axis, a curve is obtained which is called as Forming Limit curve or Forming Limit Diagram.