Folding Defect of Forging (Forging), Cause of Formation and
作者：admin 来源：本站 发布时间：2019-02-18 14:48:25 浏览量：
Die forgings have many closed sections, the distance between the two ribs is short, the ribs are thin, the distance between the two ribs and the web thickness are large, and many parts of the surface are not machined. Because the ribs of die forgings are much thinner, folding problems often occur at the intersection of ribs and webs, and at the joints of ribs and edges in production. Folding on die forgings destroys its continuity. Fatigue cracks occur because it weakens the section or concentrates stress in use, which greatly reduces the load-carrying capacity of forgings. Ribs are generally used to provide rigidity or to provide installation or linking surfaces for other parts, so folding defects should be prevented.
1. The location and cause analysis of folding defect in aluminium alloy die forgings
1.1 The folding position of die forgings can be seen from the structure and shape of forgings. In production, folding mostly occurs in the web and ribs, ribs and edges of forgings.
1.2 The causes of folding defects are: (1) unreasonable design of wool, resulting in differences in metal distribution. The forging process chooses a long bar with a diameter of 180 mm *420 mm. The forging is made according to two or more stages until 120 mm *180 mm *480 mm. Then the middle part of the bar is drawn out and finally formed on the press. From its shape, it can be seen that the upper and lower ribs are symmetrical, although the shape of the blank is similar to that of the forging, but the upper and bottom of the blank are not uniform, the bottom metal distributes more, the height is not enough, and the forging is symmetrical. During the die pressing, the upper rib is filled with the cavity, and the lower metal is not full. As the deformation occurs, a hole is formed in the upper corner of the cavity, and finally gold is located here. It is joined with the lower part to fill and fold. (2) From the analysis of the fillet radius of web and rib joint, because of the small fillet radius, in die forging, the upper and lower dies keep closing after filling the two ribs, the surface metal passes through the groove along the direction of less resistance, and drives the surface metal outflow, so that the ribs and webs overlap together, resulting in folding. (3) When considering the die pressing from the metal flow direction, the metal filling cavity does not flow in and out of the corner, so that the metal first contacts with the relative side wall, then contacts with the bottom, and metal backward flow occurs to the corner, which makes the positive flow coincide with the backward metal surface, and then forms folding. (4) When the last die pressing was finished, the repair was not thorough, and the folding was not quickly repaired so that it could enter the next die pressing, so that both inside and outside of the forging could be folded.
2. Folding Defects of Shaft Forgings
Most of the forgings are axle forgings. The forming die is wrestling die, which consists of upper and lower parts. In forging, the forgings rotate around the axis without flying edges, such as clamping die and straightening wrestling die. The characteristics of wrestling die forming are that the lateral flow of metal is restricted by the side pressure of the die, which makes the metal extend along the axis. Compared with free forging, the drawing rate increases by 20%-40%. At the same time, the internal longitudinal crack can be prevented by the long internal pull-out stress. In production, it is found that folding defects often occur in step parts when step shaft forgings are forged by common wrestling dies.
2.1 Following the folding defect is the half-axle forging process, which illustrates the defect in the process of forming step axle and the solution after improving the die. Half-axis forming steps are as follows: ① shoulder diameter is 130 mm; ② drawing two ends to diameter 130 mm, free forging forming; ③ shoulder diameter 118 mm at one end; ④ drawing one end to diameter 115 mm, free forging forming; ⑤shoulder diameter 108 mm; ⑥drawing the other end to diameter 105 mm, free forging forming; ⑦adjusting and straightening forming, straightening and throwing die. When using the wrestling die, folding defects are easy to occur in steps①, ③, ⑤and⑦, especially in straightening the wrestling die.
2.2 Problem analysis now takes straightening and wrestling die as an example to analyze the causes of defects and process improvement. Steps are generated in different parts of the model cavity, and their height is determined by the difference of diameter, which makes the steps not be metal that has severe deformation and flow impediment during forging. When the upper and lower modules are closed, a step gap is created at the contact step by setting the fillet on the cavity to avoid stress concentration and smooth the flow, which further affects the metal flow. In this way, in the step part, besides most of the metal flowing according to the tooling, the remaining metal is not flowing smoothly because of the change of diameter. Some of the metal flows into the gap between the upper and lower modules, forming the "flying edge" when forging. When the forging is rotated, the remaining metal of this part is bent and affixed to the surface of the forging because of its thin thickness. When further forging, it will be stuck in the forging and folded. Therefore, there are two reasons for folding defect in common wrestling die: one is the step with different diameter on the cavity; the other is the fillet of the cavity. According to these two factors, it is analyzed that: (1) the fillet of the cavity can not be cancelled; and (2) folding begins to appear at the junction of the module, which indicates that the main reason for folding is the steps near the contact part.
