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Key factors that should be paid attention to in mold design to prevent forgings from folding

Posted by: steel world 2023-10-27 Comments Off on Key factors that should be paid attention to in mold design to prevent forgings from folding

Abstract: The folding defect of aluminum alloy die forgings is the most important defect among many defects and the main waste among die forging waste products. Conduct analysis and research on this problem to find out the causes of folding and propose solutions. In actual production, the yield of die forgings has been greatly improved and obvious economic benefits have been achieved.
Keywords: aluminum alloy; die forging; folding; die design; wool die chamber; pre-forging die chamber; final forging die chamber
The folding of aluminum alloy die forgings destroys the continuity of the metal and reduces the load-bearing capacity of the forgings, which is the main waste product in the production of forgings. Based on years of production practice and experimental research, the causes of folding and elimination methods are analyzed, and measures that should be taken in mold design are proposed to reduce folding and improve forging yield.
1 Folding defects of die forgings and their causes
During the die deformation process, metal always follows the law of least resistance and flows in the direction of least obstruction, causing the surface metal to flow into the inside of the forging in a local area of the die forging. This flow from the surface to the inside of the forging removes impurities such as the oxide layer of the skin and lubricant. Fold together into the interior of the forging to cause folding [1-2]. The reasons for its occurrence: unreasonable design of die forgings, concave radius (that is, the convex radius of the mold) is too small, and the changes in each section are too large; improper coordination of the wool die chamber, pre-forging die chamber and final forging die chamber, and metal distribution Unreasonable, too much or too little metal locally, resulting in uneven deformation during final forging; for forgings with complex shapes, directly use round
The blank is formed in the final forging die, and no pre-forging die or rough die is used; the blank selection is unreasonable, the shape is improper, and the reduction amount is too large; there is too much oil or uneven oiling; the edges of the forging blank are too sharp, or the last time Incomplete repairs after molding, etc.
When the molded parts with folding defects are cut open for low-magnification structural inspection, it can be found that there is an obvious black line from the surface of the die forging to the fold inside the forging, which is called a crease, as shown in Figure 1. The distance from the surface of the die forging to the end of the fold, that is, the length of the fold, is called the fold depth.

Figure 1 Typical folding of die forgings (low magnification structure)
Folding has a serious impact on the quality of die forgings. First, folding destroys the integrity of the surface of the die forging, greatly reducing the load-bearing area of the part. The folding itself is a gap in the part, which causes stress concentration during use and becomes a source of fatigue, which may cause fatigue fracture of the component. Secondly, there are lubricants or other impurities in the folds, and acid and alkali residues remain in the folds during the subsequent etching and cleaning process, which will cause excessive corrosion of the parts at the folds. Judging from the internal structure of the die forging, the flow patterns of the metal at the folds on the surface will produce eddy currents or cross-flows. The more severe the folding, the less smooth the metal flow lines will be.
The generation of folding defects consists of three processes: surface folding, folding generation, and folding development. In the initial stage of folding, the tail end of the fold becomes a small rounded corner (see Figure 2). If after a large amount of deformation, the tail ends of the folds can become sharp cracks or fork-shaped cracks (see Figure 3). Extremely severe folding will break the entire rib shape of the die forging (see Figure 4). Generally, a fold at a certain location is mainly composed of one main fold. Folds composed of a single crease are deeper, while folds composed of multiple creases are shallower.

Figure 2: Primary folding, the tail end of the fold is in the shape of a small round corner

Figure 3 The sharp crack at the folded tail end
Figure 4 Severe folding and broken ribs
A large number of light alloy die forgings produced on site are ordinary open die forgings. There is a certain margin between the forgings and their parts, as well as the die forging slope and large rounded corners. Therefore, although some die forgings have local folding defects, as long as the folding depth does not exceed the machining allowance, that is, the final folding part remains on the main body of the part, it is still allowed, and it meets the technical requirements of the product. Therefore, in on-site production, the method of cutting open wounds is used to determine whether the forgings are scrapped. Years of practice have proven that this method of cutting wounds is an effective and convenient inspection method to identify whether folding defects constitute scrapping of forgings.
Folding can be divided into four types according to its position: rib root folding, rib top folding, corner folding, and edge folding. According to its production mechanism, it can be divided into five types: rib-penetrating folding, covering folding, return folding, converging folding and stacking folding. There are many causes and influencing factors of folding, which can be summarized into three aspects: factors considered in the design process from part drawings to die forging drawings; factors in production process preparation; factors in specific production operations. The factors that should be considered during the mold design process to eliminate forging folding will be discussed below.
2 Necessary shape simplification in die forging design
The shape of the die forging should be as consistent as possible with the shape of the part, but in order to facilitate the smooth completion of the die forging and eliminate die forging defects such as folding, necessary shape simplification must be carried out.
Production practice has proved that the main reason for folding is often that after the metal has filled the die cavity, because there is still too much metal, the underpressure amount of the forging (referred to as underpressure amount) is too large. In order to reduce the underpressure amount, , even if too much metal is discharged in the form of burrs, pressure must continue to deform. In this way, when too much metal flows out, it is the stage where folding is most likely to occur, and these folds are concentrated at the fillet where the rib root of the module connects to the web.
Die forgings with frame-shaped closed ribs are prone to folding at the inner rib roots. It can be seen from Figure 5a that the eight positions indicated by arrows are prone to folding. In order to reduce folding, the shape must be simplified in the design of die forgings. Although a little processing allowance is added, folding can be greatly reduced. As shown in Figure 5b, there are only two easy folding positions. If the shape is further simplified as shown in Figure 5c, the folding in Figure 5a can be completely avoided.

Figure 5 The impact of different design solutions on folding
In Figure 6a, the distance a between the two ribs is smaller than the rib height b, and severe folding is most likely to occur at the locations indicated by the arrows. Simplifying it to Figure 6b would eliminate these folds.

Figure 6 The impact of necessary shape simplification in mold design on folding
From the above examples, it can be seen that in the mold design process, taking necessary shape simplification measures for different forgings or different parts is of great significance to ensure that folding defects are avoided during the forging and forming process of die forgings.
3. The influence of the selection of parting surfaces on folding
In ordinary open die forging, the correct selection of the parting surface will not only directly affect the smooth completion of molding, the length of die life, and the level of production efficiency, but also have an important impact on the quality of forgings. The factors that should be considered when selecting the parting surface are explained below only from the perspective of their impact on folding.
Figure 7 illustrates the same die forging. If three different parting surfaces are selected, different folding results will occur: Figure 7a The parting surface is selected in the middle of the web. The distance between the rib root and the web surface is only h2. In this way, the circle The gold dollars at the corners are very easy to participate in the outflow, and as a result, they are most likely to cause folding. The parting surface in Figure 7b is set on the end surface of the web, so that the distance between the rib root and the web surface from the parting surface is H. Compared with the metal in Figure 7a, the fillet R is farther away from the parting surface. Although the metal at the mold surface and rib fillets is not as easy to flow out and cause folding as shown in Figure 7a, it is still difficult to avoid folding. As shown in Figure 7c, the parting surface is changed to the top of the rib, so that the molding conditions are changed from the press-in molding of Figures 7a and 7b to reverse extrusion. When the excess metal is removed, it flows in sequence, so Figures 7a and 7b can be completely avoided. Possible folding in 7b.

Figure 7 Different effects of different parting surfaces on folding (P are the same, ↓ represents the parting surface)
Figure 8 illustrates the folding effects of selecting different parting surfaces for parts with I-shaped cross-sections. The parting surface in Figure 8a is selected in the middle, which will easily cause folding at the four rounded corners where the ribs and webs are connected, and the metal streamlines on the cross section will also flow through. In Figure 8b, the parting surface is selected at the top of the rib, so that the fillet connecting the rib and the web will not fold.

Figure 8 The impact of folding when selecting different parting surfaces for the I-shaped section
From the above examples, it can be seen that in order to eliminate folding, the impact of folding on the parting surface should be fully considered while taking into account other factors when designing the mold.
4. Selection of fillet radius for die forgings
In order to facilitate metal filling and minimize die wear, all edges and corners of die forgings are designed to have a certain R rounded corner for smooth transition. Each surface of all die forgings is transitionally connected by internal fillet, external fillet, horizontal internal fillet and horizontal external fillet.
Figure 9 shows various fillets of a certain forging. Among them, the inner fillet is generally not prone to folding, and only some forgings may have rib top covering folding. It is not easy to cause tendon folding (reinforcement folding) at the outer rounded corners. There is generally no folding on the horizontal fillet, only at the connection between the horizontal fillet and the tendon root and web at the lower part a′-a′ in the figure, because it is located at the turning point where the geometry of the forging changes. , at the ends of the ribs on both sides, the obstruction of the lateral flow of the web metal by the ribs on both sides suddenly disappears, and a large amount of lateral flow of metal occurs, resulting in the unformed forging. As the deformation continues, the unformed parts are formed by the adjacent parts.
The nearby webs and ribs are filled with metal, and finally folded. The horizontal outer corners d, e, and f are all angles equal to 90° and are generally not easy to fold. However, as for d, since it is at the corner of the entire forging, the metal on the longitudinal and transverse ribs have longitudinal and transverse angles respectively. flow (see Figure 10). These two flow components in different directions combine into a 45° outward resultant force at this corner. This resultant force will be the main factor leading to folding. If the radius of this horizontal outer corner is increased from the design, the hazard of folding can be reduced.

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