The plastic deformation of forgings follows the following basic rules:
1. The Law of Shear Stress
The plastic deformation of a forging only occurs when its internal shear stress reaches a certain critical value. This law is called the law of shear stress.
The critical value of shear stress depends on the forging material itself, the forging deformation temperature, the deformation speed and the degree of deformation. Generally speaking, the higher the carbon content of the steel, or the higher the alloy content, the higher the critical value of the shear stress, and the less likely to be plastic deformation; increasing the deformation temperature, reducing the deformation speed, or reducing the degree of deformation will reduce the shear The critical value of stress is more likely to occur plastic deformation.
2. Forgings have elastic deformation during plastic deformation
Since the metal has undergone elastic deformation before plastic deformation during forging, the plastic deformation of the metal must be accompanied by the occurrence of elastic deformation.
Since the metal is elastically deformed during plastic deformation, the plastic deformation of the workpiece remains after the external force that deforms the metal is removed, but the elastic deformation part needs to be restored, which will cause a certain change in the size and shape of the workpiece. The phenomenon of changes in the size and shape of the workpiece caused by deformation is called springback. In production, it is necessary to pay attention to the impact of springback on the size and shape of the workpiece.
3. The assumption of constant volume
In the process of plastic deformation, the volume of the metal before the deformation is equal to the volume after the deformation. This law is called the law of volume invariance.
In the forging production, according to the law of constant volume, it is possible to carry out technological calculations such as the size of the forging, the size of the process parts and the size of the mold.
4. Law of least resistance
If the particles of an object can move in different directions, each particle on the object will move in the direction of least resistance. This law is called the law of least resistance. The resistance mentioned here includes frictional force and the restriction of tool shape on metal flow. The law of minimum resistance explains the possibility and direction of metal particle flow.
The law of minimum resistance is a very important law for forging. Both free forging and die forging use the law of minimum resistance to improve the quality of parts, reduce the tonnage of equipment and increase productivity.
(1) Upsetting of round section blanks
When a round billet is upset, the internal particles must move along the radius in the horizontal direction. This is because the path of the particles moving in the radial direction is the shortest and the resistance is also the smallest. Therefore, the blank of the circular section is upset It is still round.
(2) Upsetting of rectangular section blank
The flow direction of metal particles with rectangular cross-section can be divided into four flow regions. The initial stage of upsetting is the same as that of square upsetting, and the resistance on the diagonal is the largest, forming an ellipse.
According to the law of minimum resistance, the closer the distance from the periphery, the smaller the resistance, and the forgings must flow in this direction, so more metal flows out of the trapezoidal area than the triangular area. The convex belly appearing on the long side is large, and the convex belly appearing on the short side is small, that is to say, there is more metal flowing to the long-side normal and less metal flowing to the short-side normal, and the ellipse will gradually approach the circle. If the aspect ratio of the rectangle is not large, or the degree of deformation is large, the rectangular blank will be upset to a circle.
(3) Upsetting of square section blank
A square section blank is thick on the flat anvil, and a round pie phenomenon can be obtained: the square blank gradually becomes round on the flat station with the increase of the degree of deformation, as shown in Figure 7-8c. This is because there is friction on the contact surface between the flat anvil and the metal, and the distance of the bisector of the square blank corner is the longest and the friction is the largest. Therefore, the metal particles flow along the shortest normal direction, and the bulges appear on each side , And gradually tend to round.
(4) Billet length
The blank is drawn on a flat anvil, and different feeding amounts can be selected to obtain workpieces with different lengths of elongation. The elongation is greater than the elongation of the width: the amount of feed is less than the width of the material, the cross section forms a number of rectangles, and the length of the The flow is fast, and the axially long workpiece is obtained, and the effect of pulling the length is the best.
The elongation is equal to the drawing length of the widening amount: the feeding amount is equal to the width of the blank, the cross section forms a number of squares, the metal flow in the length direction and the width direction is the same, and the workpieces with equal deformation in the axial direction and the width direction are obtained.
The elongation is less than the drawing length of the widening amount: the feeding amount is greater than the blank width, the cross section forms a number of rectangles, the flow in the width direction is fast, and the workpiece with large width extension is obtained.
(5) Die forging
The forgings should be qualitatively analyzed according to the law of minimum resistance, and the forging die can be designed reasonably by adjusting the resistance in a certain direction.
The metal will have two directions of flow (A and flash tank). In order to ensure that the metal fills the die cavity, the amount of metal flow at A should be increased. The following two measures can be taken: the rough surface of the flash groove is used to increase the resistance of the die forging to the flash groove; Reduce the resistance of metal to A.
