Up to this point, the processes described were achieved through static loading, as in the forging of a disk between two platens of a press (Fig. 1). We now examine how the same result (upsetting) can be achieved by a high-energy rate-forming
(HERF) process. (The process is also called high-velocity or HVF).
If, hypothetically, the disk is thrown with a high speed at the bottom platen, the entire kinetic energy of the rushing disk will be absorbed at the moment of impact with the platen. If the projectile achieves bullet speeds, it may, on impact, either penetrate the platen, like an armor-piercing bullet, weld to the platen, deform, or undergo two of the above simultaneously.
Figure 21 represents the most common design for the use of explosives in a HERF process. A blank made of a plate or sheet metal is placed over a die cavity of the desired shape. A vacuum must be formed in the cavity below the blank by evacuating the air. The tank above the bank is filled with water. An explosive charge is placed just below the surface of the water, directly above the center of the blank.
When the explosive charge is detonated, a shock wave moves through the water. Water is a very effective shock-wave-transmitting medium through which the impact of the explosion is transmitted from the source to the workpiece target.
The effectiveness of the energy transfer is demonstrated by observing uses in other fields. For example, sonar under water is most efficient and sensitive. The destructive force of the shock wave has been used for centuries (now illegally) by fishermen to destroy (or to stun) all life in a vast sea or pond space. Submarine warfare demonstrates the sharpness by which the shock wave from a bomb hits the submarine, as if it had been hit directly by a hammer.
On reaching the blank, the shock wave hits it so hard that the blank rushes downward and conforms to the cavity. Once the shock wave has hit the blank and set it in motion, the rest of the operation is performed by the inertia of the moving blank. The blank moves downward as a plane during forming. Halfway through the operation the part would look like a flat-bottomed bowl with sides conforming to the cavity. This intermediate shape is shown in Fig. 22. This shape would also result if the explosive charge were insufficient to complete the operation. For smaller parts the surge of energy can be provided through other chemicals or by an abrupt electrical discharge of energy from a battery of capacitors.
Friction and Lubrication
One of the last frontiers in the understanding of metal forming is the friction phenomena between the tool and the workpiece. No matter how much care is taken to form a smooth tool surface, the surfaces of both tool and workpiece are irregular surfaces with peaks and valleys. Opposite peaks clash with each other, resulting in damage to both surfaces. Temperature rises due to the rubbing action. A thin layer under both surfaces undergoes severe plastic deformation.
Disciplines affecting friction and
Models of the typical behavior of the asperities of the surfaces of two solids interfacing one another under pressure, and sliding with respect to each other, are described in Fig. 23. Many more possible outcomes of the clashing of the asperities may occur. One specific behavior, described in part (b) of Fig. 23, is the steady state flow of the asperity, identified as the wave motion. In the model of this motion, the “wave model” (Fig. 24), wedges of the harder surface indent into the softer surface because of the applied pressure, thus producing opposing ridges on the surface of the softer component (Leslie, 1804). According to Leslie, the ridges are supressed down under the sliding wedges, only to rise again in front of the moving wedges. This perpetual supression and uprising of the ridges are motions similar to the motion of ocean waves.