**1.3. State-of-the-art of SFP**

With the increasing demand of lightweight and high properties of parts, SFP has become a research hotspot in the field of metal forming [1]. More and more parts were made by SFP instead of conventional method [2]. Tan et al. developed a two-stage forming process of tailor blanks having local thickening for controlling the distribution of wall thickness of

stamping parts. In the first stage, the target portion of the sheet for the local thickening was drawn into the die cavity, and then the bulging ring was compressed with the flat die under the clamping of the flange portion in the second stage [3]. Mori et al. developed a two-stage cold stamping process for forming magnesium alloy cups having a small corner radius from commercial magnesium alloy sheet. In the first stage, a cup having large corner radius was formed by deep drawing using a punch having large corner radius, and then the corner radius of the cup was decreased by compressing the side wall in the second stage. In the deep drawing of the first stage, fracture was prevented by decreasing the concentration of deformation with the punch having large corner radius. The radii of the bottom and side corners of the square cup were reduced by a rubber punch for applying pressure at these corners in the second stage [4]. Mori et al. also developed a plate forging process of tailored blanks having local thickening for the deep drawing of square cups to improve the drawability. A sheet having uniform thickness was bent into a hat shape of two inclined portions, and then was compressed with a flat die under restraint of both edges to thicken the two inclined portions. The bending and compression were repeated after a right-angled rotation of the sheet for thickening in the perpendicular direction. The thickness of the rectangular ring portion equivalent to the bottom corner of the square cup was increased, particularly the thickening at the four corners of the rectangular ring undergoing large decrease in wall thickness in the deep drawing of square cups became double [5]. Wang et al. prompted a drawing-thickening technology with axial force for double-cup shape workpieces by combining the characteristics of cold extrusion with drawing process [6]. An axial thrust was exerted to the sidewall during backward drawing to achieve the purpose of drawing and thickening [7]. Wang et al. also adopted SFP to form a flywheel plate and a sleeve with thickened wall instead of a traditional process, such as cutting and weld assembling [8,9].

Stamping-Forging Processing of Sheet Metal Parts 33

unchanged. In this section we mainly talk about the sheet metal upsetting thickening

For the cup part with thickened outer wall, the axial upsetting can be used to thicken the wall after drawing. The schematic of outer wall thickening process is shown in Fig. 4. At first stage, the sheet metal with uniform thickness is drawn into the die cavity by large round corner punch for preventing the occurrence of fracture. At second stage, the formed cup is ironed firstly by small round corner punch to make bottom to specified dimension. Then, a circular upsetting punch compress the outer wall to a thickened dimension and

Due to the wrinkling is easy to occur during axial upsetting, it's important to determine the limitation of thickening. In this section, thickening ratio of upset thickness to initial thickness is presented to define the formability. There are several geometry parameters play important roles in thickening ratio, such as wall height and inner corner radius, etc. The allowable thickening ratio under different conditions is shown in Fig. 5. The digits in the

processing that inner diameter decreased and outer diameter remained.

make the outer round corner to a specified radius.

**Figure 4.** Schematic of outer wall thickening process.

**Figure 5.** Allowable thickening ratio under different conditions.

Compared with traditional metal forming methods joining parts of different thickness by welding, the SFP method mentioned above can not only decrease the cost, but also can produce high quality sheet metal parts with shorten supply chains. With the development of industry, especially automotive industry, large quantities of parts with different wall thickness are needed. Thus, it is important to research SFP technology to manufacture such kind of sheet metal parts.
