*2.2.1. Upsetting force Fu*

The axial upsetting is a closed-die forging process. Although the metal flow is different from that of tubing upsetting at beginning, the final upsetting force is similar to tubing upsetting force. So, according to calculation method of tubing upsetting force, the upsetting force can be expressed as an empirical formula:

$$F\_u = 1.3pA$$

where *p* is the average upsetting stress (MPa), *A* is the area of wall section (mm2).

## *2.2.2. Methods for reducing forming load*

Because the axial upsetting is a closed-die forging process, the upsetting force increases rapidly at the end of metal forming. Relief cavity can be set in dies or blank to increase free flow surface and avoid full closing realizing reduction of upsetting force. The design of relief cavity can adopt the following three styles: a) center hole relief at the bottom of cup, b) relief cavity at the dies corresponding to outer corner between wall and bottom, c) combined relief style.

1. Hole relief

If there is a center hole at the bottom of cup, hole relief method can be used to reduce upsetting force. In this method, piercing process must be carried out before axial upsetting. Then, while the outer wall is upset axially, the metal is enforced to flow to the center of the bottom, at where there is a free flow surface. The metal forming is no longer a closed die forging. So, the upsetting force can be decreased. Certainly, as a result of axial upsetting, the diameter of hole is decreased during the wall thickening. It is suggested that the center hole of part can be obtained by means of designing a proper relief hole which will be shrunk to specified dimension. This is able to reduce the upsetting force as well as avoid piercing again.

2. Relief cavity

From the metal flow of upsetting process mentioned in above section, outer corner between wall and bottom is the last formed position of part. During forming this position, the upsetting force increases rapidly. It can be considered to adding a relief 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 unnecessary metal away.

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 manufacture circular cavities in bearing plate and cylinder die respectively.

1-Bearing plate, 2-Cylinder die

34 Metal Forming – Process, Tools, Design

**2.2. Calculation of forming load** 

be expressed as an empirical formula:

*2.2.2. Methods for reducing forming load* 

upsetting force as well as avoid piercing again.

*2.2.1. Upsetting force Fu*

relief style.

1. Hole relief

2. Relief cavity

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

The axial upsetting is a closed-die forging process. Although the metal flow is different from that of tubing upsetting at beginning, the final upsetting force is similar to tubing upsetting force. So, according to calculation method of tubing upsetting force, the upsetting force can

1.3 *uF pA* =

Because the axial upsetting is a closed-die forging process, the upsetting force increases rapidly at the end of metal forming. Relief cavity can be set in dies or blank to increase free flow surface and avoid full closing realizing reduction of upsetting force. The design of relief cavity can adopt the following three styles: a) center hole relief at the bottom of cup, b) relief cavity at the dies corresponding to outer corner between wall and bottom, c) combined

If there is a center hole at the bottom of cup, hole relief method can be used to reduce upsetting force. In this method, piercing process must be carried out before axial upsetting. Then, while the outer wall is upset axially, the metal is enforced to flow to the center of the bottom, at where there is a free flow surface. The metal forming is no longer a closed die forging. So, the upsetting force can be decreased. Certainly, as a result of axial upsetting, the diameter of hole is decreased during the wall thickening. It is suggested that the center hole of part can be obtained by means of designing a proper relief hole which will be shrunk to specified dimension. This is able to reduce the

From the metal flow of upsetting process mentioned in above section, outer corner between wall and bottom is the last formed position of part. During forming this position, the upsetting force increases rapidly. It can be considered to adding a relief

where *p* is the average upsetting stress (MPa), *A* is the area of wall section (mm2).

thickening ratio decreases under the condition of any inner corner radius.

**Figure 6.** Mode of relief cavity.

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 free surface resulting in a decrease in upsetting force.

3. Combined relief method

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

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 method.
