Bi-directional dieless incremental flanging of sheet metals using a bar tool with tapered shoulders

Abstract

Flanging of sheet metals in a rapid and cost-efficient way is strongly needed in practice. This paper describes a flexible and versatile flanging method based on the single-point incremental sheet forming (ISF) technology utilizing simple bar tools with tapered shoulders in a two-stage procedure. The experiment and numerical simulation used Al 6061 sheets with a thickness of 1 mm. The results confirmed that forward/backward flanging of various open edges and hole rims of plates can be performed by this method without dedicated die. This gives a more complete flexible forming chain based on ISF. In contrast to conventional ISF, however, dieless incremental flanging gives particular force and deformation modes and might form different defects including plate warpage and buckling at flange onset. A larger inclination angle θ of the tool shoulder decreases the tendency of buckling but might incur larger warpage. The recommended θ-value for round-hole-flanging is 20–30°. Curved plates have less warpage versus planar ones because the curvature enhances structural stiffness. Moreover, control strategies of typical defects were presented mainly through the path optimization and geometry modification of tools.

Introduction

Flanging is a basic sheet metal forming process widely used in the production of thin-walled components. It involves bending sheet peripheries over a curved line using dedicated tooling on press. According to the state of circumferential strain or thickness variation of flange, there are two major classes of flanging—the stretch flanging with concave bending lines, and shrink flanging with convex bending lines (Fig. 1). Round hole-flanging can be regarded as stretch flanging with closed concave bending line. Up to now, extensive studies have been executed on various flanging processes. For example, Wang et al. (1995) studied stretch/shrink flanging on open edges of planar plates, Kacem et al. (2013) studied hole-flanging, Thipprakmas et al. (2007) investigated fine hole-flanging with wall thinning during forming, while Xu et al. (2004) examined curved flanging on non-planar sheets, with respect to the deformation features, formability, forming load, etc.

To continue meeting the diverse requirements of modern productions, especially those in small-batches, flexible manufacturing processes based on NC (Numerical Control) technologies such as incremental sheet forming (ISF) have attracted huge attention in the domain of sheet metal forming, as summarized by Allwood and Utsunomiya (2006). To date, ISF has mainly been applied to the formation of metallic sheets with a bi-stretching deformation mode. To fully take advantage of ISF, attempts have also been made to extend the technology into varying formations of sheet metals including flanging. For example, Cui and Gao (2010) studied the incremental forming process for producing prototype parts with round hole-flanges. They used a forming strategy that increased the part diameter in small steps until it reaches the final optimum part geometry. It can then produce a relatively higher neck height, uniform wall thickness, and maximum limiting formingratio LFR (defined as D/D₀, where D is the finished part diameter, and D₀ denotes the pre-cut hole diameter of blanks). Other researchers (Centeno et al., 2012) investigated the fabrication of conical and cylindrical hole-flanges by ISF. Aiming to reduce limitations in process time and geometric accuracy, Bambach et al. (2014) proposed a process design for performing hole-flanging operations by incremental sheet forming. Recently, Voswinckel et al. (2013) examined the incremental flanging of open plate edge. Generally, these studies were focused on a so-called “forward” flanging (Fig. 2a and b) in which the flanged wall is downward bent under the axial pressure of a bar tool. To attain higher accuracy, a partial die is usually used to support the blank from the opposite side (Fig. 2b), which belongs to a two-point ISF process.

Flanging is usually an auxiliary operation after deep-drawing and trimming (Maout et al., 2010). For some semi-finished products with a closed or half closed cross-section like a U shape, it is difficult to make an outward flanging/hole-flanging on the wall by conventional stamping because there is insufficient space for the tools to operate inside the workpiece. In light of this, Petek and Kuzman (2012) proposed a “backward” incremental round-hole-flanging approach as shown in Fig. 2c. Here, a bar tool with a ball head caused upward bending of the hole rim. Because the contact area between the tool and the blank is small and keeps changing during the course, it is hard to precisely control the deformation; therefore, a backup die is indispensable. Some groups have also studied the production of tubular parts by incremental forming. For example, Teramae et al. (2007) developed an incremental hole-flanging process for fabricating branched tubing without welding. However, the overall knowledge on incremental flanging is lacking and further studies are urgently needed.

The current study seeks to develop an agile, cost-efficient and versatile flanging approach based on the single-point ISF technology, with which various flanging and hole-flanging processes on planar or contoured plates along forward/backward direction can be performed, omitting either the dedicated die or punch. The method is extended from the former work on hole flanging of tubes conducted by the present author and coworkers’ (Yang et al., 2014). Deformation characteristics and key points are reported. The goal is to validate the method and reveal fundamental rules of the process that are of great importance in industrial applications.