What is closed die forging?
Closed die forging, also known as impression die forging, is a forging process in which the metal flow and filling take place entirely within a closed die. This means that the deformed metal cannot exit the die, and the volume of the metal remains constant within the cavity. As a result, the metal is forced to flow and fill the cavity, which only produces small amount of flash without any burrs. Therefore, closed die forging is also known as flashless forging.
During closed die forging, no flash groove is needed, which reduces the amount of flash material and eliminates the need for flash trimming equipment. Closed die forging can achieve high precision and is suitable for forging disc-shaped or axisymmetric short shaft type parts. However, it requires high precision in cutting the raw material, which limits its applications.
What is the process of metal flow in closed die forging?
The process of metal flow in closed die forging can be divided into three stages:
The first stage is the basic forming stage. During this stage, the workpiece begins to deform and fill the cavity, and the increase in deformation force is relatively slow. Depending on the nature of the forging, this stage can involve upsetting, pressing, punching, or extruding.
The second stage is the filling stage. During this stage, the deformation force increases by 2-3 times compared to the first stage, but the deformation amount ΔH2 is relatively small.
The third stage is the formation of vertical flash. At this stage, the workpiece has already become a rigid body and further forging pressure is needed. The pressure can fracture the cast structure of dendrites, compact the porous structure, and produce vertical flash at the end of the workpiece. The thinner the flash, the higher the height, and the greater the compressive stress on the sidewall of the cavity. In severe cases, it can lead to the damage of the cavity.
The characteristics of closed die forging are as follows:
(1) It eliminates the use of flash material and the flash trimming process.
(2) Materials deform within the closed cavity with significant three-dimensional compressive stress, which enhances plasticity and is advantageous for low ductility materials.
(3) The flow lines are more reasonable, which improves mechanical properties of forgings.
(4) The size of the raw material and the positioning within the cavity must be accurate, it is difficult to remove the forging from the die, and a knockout mechanism needs to be designed to facilitate the removal of the forging from the die.
(5) Its application is limited and is mostly suitable for disc-shaped or axisymmetric short shaft forgings.