**2.1. Design of thickening process and thickening ratio in single upsetting**

In the SFP of cup parts with thickened wall, the axial upsetting of the wall is similar to tube upsetting. There are four situations of the forged piece formed from tube stock by upsetting processing: inner diameter remained and outer diameter enhanced, inner diameter decreased and outer diameter remained, inner diameter decreased and outer diameter enhanced, both inner and outer diameter enhanced and the thickness of the wall of tube is unchanged simultaneously. For the sheet metal upsetting thickening processing, there is no deformation mode that both inner and outer diameter enhanced and thickness is basically unchanged. In this section we mainly talk about the sheet metal upsetting thickening processing that inner diameter decreased and outer diameter remained.

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 make the outer round corner to a specified radius.

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

32 Metal Forming – Process, Tools, Design

kind of sheet metal parts.

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

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

traditional process, such as cutting and weld assembling [8,9].

**2. Thickening of outer wall of cup parts with axial upsetting** 

**2.1. Design of thickening process and thickening ratio in single upsetting** 

In the SFP of cup parts with thickened wall, the axial upsetting of the wall is similar to tube upsetting. There are four situations of the forged piece formed from tube stock by upsetting processing: inner diameter remained and outer diameter enhanced, inner diameter decreased and outer diameter remained, inner diameter decreased and outer diameter enhanced, both inner and outer diameter enhanced and the thickness of the wall of tube is unchanged simultaneously. For the sheet metal upsetting thickening processing, there is no deformation mode that both inner and outer diameter enhanced and thickness is basically 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

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

figure are the thickening ratios obtained from simulation results in which the outer diameter of part is 120 mm. The number 1 represents the part occurred folding under corresponding conditions. It can be seen that the zone enclosed by lines is suitable for thickening the outer wall. When the ratio of inner corner radius to wall thickness is about 0.5, the thickening ratio has the largest value, which achieves to 1.4. With the increasing of wall height, the thickening ratio decreases under the condition of any inner corner radius.

Stamping-Forging Processing of Sheet Metal Parts 35

cavity in the die corresponding outer corner, which can increase free flow surface and decrease the upsetting force. After upsetting, machining should be carried out to clear

The position and shape of relief cavity can be designed as two modes as shown in Fig. 6: a) at the bearing plate under the bottom of the cup, which needs to manufacture a circular cavity in bearing plate; b) at the die and bearing plate, which needs to

a) At the bearing plate b) At the die and bearing plate

As using relief cavity method, the metal flow is similar to that of without relief cavity before the die fully filled. There is just a few of material enforced to flow into relief cavity after the wall formed. Finally, the cavity is not fully filled, which remain a little of

The two relief methods mentioned above can be used together. A center hole is pierced before upsetting, as well as a relief cavity is designed in the dies. In this way, the

The center hole relief method is suitable for single wall part having center hole at the bottom. After axial upsetting, the formed part does not need more machining and can keep complete streamline. However, the effect of this method on reducing force is less than that of relief cavity. This is because the material has to flow to the center of the bottom, nevertheless, in the relief cavity method, the unnecessary material flow to relief cavity directly; the flow distance of the former method is longer than that of the later method. But the part manufactured by relief cavity method has to be machined to clear the unnecessary material away, which will break the streamline. In brief, it is necessary to take into account part structure and performance requirements when choose relief

In this section, an application of axial upsetting is presented. The application object is flywheel plate used in self-changing gearbox. The dimensional drawing and three dimension model of

upsetting force relief can be more than that of single method, just not significant.

free surface resulting in a decrease in upsetting force.

manufacture circular cavities in bearing plate and cylinder die respectively.

the unnecessary metal away.

1-Bearing plate, 2-Cylinder die **Figure 6.** Mode of relief cavity.

3. Combined relief method

method.

**2.3. Application** 
