Evaluation Method of Bend Formability of Copper Alloy Strips for Spring Applications

Abstract

With the miniaturization of switches, relays, connectors and so on, more severe bend formability is required for copper alloy strips, which are main comprising materials of these electronic components. Therefore, it is inevitably necessary to find a quantitative evaluation method for the bend formability of copper alloy strips besides development of these materials.In this research, the authors have deveroped a newmethod to estimate the bend formability of copper alloys using their three basic mechanical propertiesobtained from tensile test, and made an attempt to improve the bend formability of copper alloys for spring applications with the help of thismethod.The results are summarized as follows:1) To evaluate the bend formability of copper alloy strips, strain distribution coefficient (f), which is a new representation of basic mechanical property, is proposed in addition to uniform strain (eu) and maximum strain due to necking (em). Derivation method of f from tensile test results is shown.Furthermore we present quantitative relation between the bend formability and the three basic mechanical properties.2) To improve the bend formability, it is necessary to control the three basic mechanical properties, especially, increasing eu and f, which suppress wrinkle occurrence and development during bend process, is effective.3) For improving the bend formability of NB-109 alloy for spring use, it is effective to maintain large eu and f through decreasing cold rolling ratio after recrystalization treatment. In the case of high cold rolling ratio, it is effective to increase eu and f through rising final annealing temperature.