Influence of the initial grain size in single point incremental forming process for thin sheets metal and microparts: Experimental investigations

Single point incremental forming (SPIF) is a manufacturing process which allows complex shape by the CNC controlled movement of a hemispherical forming tool. The development of thin sheet metal forming processes is limited by the influence of miniaturization at the micro-scale due to size effects. The initial grain size plays a major role on the material behavior at micro-scale. In this paper, the influence of the initial grain size in micro-SPIF is proposed for thin sheet metals. Tensile tests with different grain sizes have been performed on copper foils. A set of experimental tests of single point incremental sheet forming were conducted on blanks with several grain sizes using two forming strategies. A dedicated material behavior model is proposed and identified. The effects on the forming force evolutions are demonstrated and discussed. An analytical model is also proposed to represent these forces during process in order to set out the tendencies. Tensile test results revealed that the yield stress, tensile strength and ductility decrease when the ratio of sheet thickness to grain size decreases. Also, the forming forces show an agreement with the results of tensile tests. Remarkably, the decrease in the level of forming forces with respect to the grain size has been demonstrated in relation with the Hall-Petch effect. (C) 2012 Elsevier Ltd. All rights reserved.