Influence of the initial grain size in single point incremental forming process for thin sheets metal and microparts: Experimental investigations R. Ben Hmida, S. Thibaud ⇑ , A. Gilbin, F. Richard Université de Franche-Comté, 25000 Besançon Cedex, France FEMTO-ST Institute, Department of Applied Mechanics, UMR CNRS 6174, University of Franche-Comté, 24 rue de l’Epitaphe, 25000 Besançon, France article info Article history: Received 19 July 2012 Accepted 31 August 2012 Available online 10 September 2012 Keywords: Single point incremental forming Size effect Microparts Thin sheet metal Tensile test Copper alloys abstract 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 repre- sent 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. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Incremental sheet forming (ISF) is a strategic forming approach for the manufacturing of thin sheets metal components. The principle consists in deforming locally a blank using a hemi- spherical forming tool with a small diameter in comparison with the dimensions of the sheet metal. The advantage of this process is to ensure complex shapes only by controlling the forming tool paths by using a CNC milling machine. The use of ISF is interest- ing for the small batch production and rapid prototyping [1,2]. Incremental forming process has been developed, for the first time, by Powell and Andrew [3]. Several studies have been carried out to investigate the influence of process parameters on surface quality [4], geometric accuracy [5], forming forces [6–8], thinning [9] and sustainability [10]. Ambrogio et al. [5] have discussed the dimensional accuracy of the incremental forming operations by using experimental and numerical techniques. A good agreement between the numerical prediction and the obtained geometry has been pointed out. Jeswiet et al. [6], Duflou et al. [7] and Petek et al. [8] have inves- tigated the formability by considering the influence of several process parameters on the magnitude of forming forces. Hussain and Gao [9] have presented a method to test the thin- ning limits of sheet metals in negative incremental forming. The proposed strategy allows testing the thinning limits of sheet met- als at reduced processing time and cost. Giuseppe et al. [10] have investigated the efficient use of mate- rials and process energy saving for the stamping and incremental forming processes. They found that the investigated processes are quite different to each other as their environmental impact is concerned since both the required process energy and the possible material savings are rather different for the two operations. Formability in incremental sheet forming was also analyzed by several authors. Filice et al. [11] have constructed the forming limit diagram from various strain conditions. After that, Park and Kim [12] and Fratini et al. [13] compared the formability of traditional process and SPIF process. The results show that the process allows higher formability than those observed in the traditional process. Generally, the most commonly used materials in incremental forming are aluminum and steel alloys, although many studies have investigated the forming of other families of materials. Frent- zen et al. [14] and Martins et al. [15] have developed studies on the incremental forming of PVC plates and Jackson et al. [16] on sand- wich panels composed of propylene with mild steel and aluminum 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.08.077 ⇑ Corresponding author at: FEMTO-ST Institute, Department of Applied Mechan- ics, 24 Chemin de l’Epitaphe, 25000 Besançon, France. Tel.: +33 (0)3 81 66 66 11; fax: +33 (0)3 81 66 67 00. E-mail address: sebastien.thibaud@univ-fcomte.fr (S. Thibaud). Materials and Design 45 (2013) 155–165 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes