Forming Principle and Typical Forming Mode

Working Principle

In the combined stamping and forging process, various kinds of thin-walled 2D/3D components with multi thicknesses and ribs/webs or pins are formed in near net shape (Merklein et al. 2011a, b, 2012; Yang et al. 2011). They are formed from sheet metal workpiece with uniform thickness by combining some bulk forming modes and stamping (sheet metal forming) modes. Major forming modes in this process are:

These forming modes are combined properly to form the whole shape, change thickness locally, and form web, ribs, or pins on the workpiece. In the upsetting and die forging of bulk forming mode, the workpiece is compressed partially in its thickness direction to generate vertical and lateral metal flow. In many cases, the vertical metal flow is effective to form ribs/webs or pins on the workpiece. However, the lateral metal flow along the tool surface needs to be adequately controlled to prevent defects such as wrinkling/bucking at the thinner portion and folding/shear of ribs as shown in Fig. 30.

It is widely known that the forming pressure in the compression of disk or plate increases with the increasing radius or width to thickness ratio of the workpiece, as shown in Fig. 31 (Thomsen et al. 1965). This is caused by the friction between workpiece and tools. Therefore, the lubricating condition for workpiece and tools is significantly important to reduce the forming pressure and control the metal flow.

The frictional condition also affects the lateral and vertical (extrusion) metal flow. That is, lower friction makes the lateral flow easier, and higher friction increases the extrusion of rib or pin in vertical direction.

The advantages of combined stamping and forging process are:

(a) Combining major benefits of conventional stamping and forging (bulk forming) into one process, i.e., compressing workpiece in its thickness direction by bulk forming mode and forming whole shape by stamping mode
(b) Capability of near-net-shape forming of thin-walled components with multiple wall thicknesses and complex shape from sheet workpiece under lower forming load as compared with conventional cold forging
(c) Improvement of material formability due to bulk forming mode under compressive stress and optimization of wall thickness
(d) Milder frictional condition between tool and workpiece as compared with conventional cold forging

On the other hand, this process has the following disadvantages:

(a) Useful forming mode and sequence for forming (e.g., decrease of forming pressure and better control of lateral metal flow) have not been developed enough for higher strength materials.
(b) Design rule for forming tool and products have not been clearly established.
(c) Press machine and tooling system specialized for combined stamping and forging process are still under development.
(d) Unsatisfactory accumulation of technical know-how for wider commercialization.

Forming Process and Tooling Design

The important technical issues in combined stamping and forging process are high forming pressure and potential defects caused by lateral material flow in the bulk forming modes as mentioned above (Merklein et al. 2011a, b, 2012; Nakano 2001; Nakano et al. 2006). Therefore, the optimization in forming sequence in multistep forming and tooling design is essential to reduce the forming pressure and control the metal flow for successful forming.

Figures 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, and 42 show potential forming modes and process designs for the combined stamping and forging process.

1. In the deep drawing from the workpiece with a uniform thickness as shown in the left in Fig. 32, the workpiece thickness is thickened at the flange portion by the circumferential shrinking deformation during the deep drawing. Therefore, the deep drawing of the cup from tapered blank (shown in the center and left in Fig. 32) is effective to improve the cup thickness distribution and formability in deep drawing. The tapered blank can be made by upsetting from an initial workpiece with uniform thickness.
2. The combination of deep drawing and upsetting/extrusion (Fig. 33) is useful for forming cups with boss at its center (Merklein et al. 2012; Nakano 2001; Nakano et al. 2006). The follow or solid boss can be extruded by compressing workpiece and reduce its thickness around boss.
3. Local upsetting/forging/pin extrusion (Fig. 34) can be used for forming chamfer, holes, and pins (Merklein et al. 2012; Nakano 2001).
4. The cup with sharp bottom corner and thin wall can be formed by combination of upsetting, deep drawing, and ironing (Fig. 35) (Merklein et al. 2012; Nakano et al. 2006). In this method, the local thinning of workpiece at cup corner is prevented successfully.
5. Rib/web can be formed in rectangular components by forging mode (Fig. 36). In this method, the rib and web is extruded vertically by compressing the workpiece in its thickness direction.
6. Multistep upsetting process was proposed to move the material in lateral direction and increase the thickness at the end portion of workpiece (Fig. 37) (Merklein et al. 2011a; Oyachi et al. 2011).
7. Incremental forging process in multiforming stages (Figs. 38, 39, and 40): In this process, the workpiece is compressed incrementally to cause the metal flows from the central region toward the free end of the workpiece. A cushion mechanism is used to prevent the backward metal flow and buckling of the thin portion of workpiece which is compressed in the previous step. This process has a potential for forming multi thickness components with the rib and web under reduced forming load. However, some further R&D works are required to make clear the design rule for forming process and tooling as well as the forming limitation/capability of this process.
8. Forming of cup-shaped and gear-teethed component was tried by deep drawing and extrusion/forging process (Fig. 41) (Merklein et al. 2011b). The thin material around gear teeth is removed by a subsequent trimming operation.
9. Coining/forging can be applied to form grooves at cup corner after deep drawing (Fig. 42).

In these forming modes, the elastic deflection of forming tool components cannot be neglected because the workpiece thickness is small and forming pressure is high. The elastic deflection of forming tool components should be compensated by modifying the component dimensions (height, etc.) for high precision forming.

Therefore, FE simulation is essential for forming process design and optimization of process parameter as well as tool design.

As for press machine, the conventional press machines for stamping and cold forging can be used for the combined stamping and forging process. However, higher rigidity of bolster/slide and press frame is recommended for stamping press because the forming pressure/load in the combined stamping and forging process is higher as compared with conventional stamping. Servo press (mechanical) is suitable to the combined stamping and forging process because of its high capability in high-accuracy setting of ram stroke.

A superior lubricity is required for the lubricants used in combined stamping and forging process to prevent galling on tool surface and to reduce the friction. The chemical conversion coating such as zinc phosphate + metallic soap (for steels) and aluminum fluoride + metallic soap (for aluminum alloys) is preferable in severe frictional conditions. The emulsion-type lubricant (lubricant dispersed in water) with high performance is applicable to the forming of aluminum alloys and low carbon steels under moderate frictional condition.