1. wall rupture
This kind of defect usually appears on the side wall near the corner of the square cylinder, and usually appears near the radius of the die fillet (rcd). In the stage of mold design, it is generally difficult to predict. That is, inverted W-shaped, with a 45-degree cross grid above it. Cross-grid is like crossing with a scriber. When looking for the cause of wall rupture, if you don't pay attention to it, you will not see it. It is a relatively clear and rare defect.
In square tube deep drawing, the deformation of straight edges and corners is uneven. With the deep drawing, the plate thickness increases only at the corner. Therefore, in the ground blank holder ring, the blank holder force is concentrated at the corner, and at the same time, the work hardening is promoted.
Therefore, the force needed in bending and straightening increases, and the drawing load is concentrated in the corner. The peak load of the stroke load curve of this drawing appears twice.
The first peak corresponds to deep drawing fracture, and the second peak corresponds to wall fracture. In terms of average load, the first peak is the highest. As far as the corner is concerned, since the drawing load is obviously concentrated towards the corner in the later stage of processing, the wall crack often occurs at the second peak.
Compared with carbon steel plate (mild steel plate), 18-8 series stainless steel is prone to wall cracking due to severe work hardening. Even if a uniform product like a cylinder is drawn, it will often break.
Picture
The reasons and elimination methods are as follows
(1) product shape
(1) Deep drawing is too deep. Because this defect is produced during deep drawing, it can be solved by reducing the depth of drawing. However, when drawing must be carried out according to the drawing size requirements, there are many examples of other solutions.
②rd and rc are too small. Because this defect occurs when the corner radius (rc) of the square tube is too small, rc should be increased. When deep drawing is carried out due to the small radius of die fillet (rd), there is also the danger of wall cracking. If cracks occur, grind (rd) well to enlarge them.
(2) Stamping conditions
① The blank holder force is too large, so long as it does not wrinkle, it can reduce the blank holder force. If wrinkling is the cause of cracking, it is necessary to reduce the blank holder force carefully. If a thin wrinkle occurs on the whole flange, and it shines at the cracked place, it may be due to some local reasons, such as the height of the buffer pin is not processed well, the precision of the die is poor, the precision of the press is low, the parallelism of the binder ring is not good, and the impact occurs.
Corresponding measures must be taken. Whether the above factors exist or not can be judged by the impact marks. If the impact marks are normal, the shape will be neat; if it is not neat, it means that there must be something wrong somewhere.
② The selection of poorly lubricated processing oil is very important. The method to distinguish whether lubricating oil is suitable is to reconsider the selection of lubricating oil and lubricating method if the temperature of the product is too high to be touched by hand when the product is taken out of the mold.
In the process of drawing, one of the most important factors is that the oil film of lubricating oil cannot be broken. The failure is caused by the softening of the material caused by the rising temperature of the side wall of the punch. Therefore, when deep drawing is carried out, the friction caused by deep drawing should be reduced as much as possible, and the active cooling scheme should be considered at the same time.
(3) Improper blank shape According to experience, when the wall cracks in the trial drawing stage, the defects can be eliminated only by changing the blank shape. There are many such examples. When drawing square tube, square blank is used for drawing first. If the rd part breaks, the four corners of the blank are chamfered.
At this stage, if the inverted W-shaped fracture and grid defects occur, it means that the tangential angle of the four corners is too large. The shape of cutting corners, such as notches at the four corners of flanges during deep drawing, can be eliminated if the amount of cutting corners is appropriately reduced, and cracking can also be prevented.
(4) Even if the cutting angle of poor positioning is appropriate, if the blank positioning is incorrect, it will still break like the cutting angle is too large. In addition, when the three-point positioning device is used in mass production, the positioning depends entirely on the operator's hand feeling, and at this time, wall breakage often occurs.
⑤ Poor contact of buffer pin If the length of buffer pin is properly adjusted, the defect can be eliminated.
(3)Mold problem
① Rough die surface and poor contact can improve the surface finish by grinding the concave die surface, and at the same time, achieve the matching state without forming concentrated load.
② Error of parallelism and verticality of dies When deep drawing is carried out, the verticality and parallelism of punch or die are poor due to the increase of die height, and when approaching the bottom dead center, the change of fit and clearance becomes the cause of cracking. Therefore, the parallelism and perpendicularity of the mold must be checked after it is manufactured.
③ Poor position and shape of drawbead weakens the function of drawbead at corner during square tube drawing.
(4) Materials
(1) insufficient tensile strength.
② If the grain size is too large, wall cracks will easily occur, so the grain size of materials should be reduced.
(3) The deformation limit is insufficient, so the material with high R value should be replaced.
(4) increase the thickness of the plate and carry out trial drawing.
2. Longitudinal crack of side wall
If it is subjected to compression deformation at the initial stage of processing and tensile deformation at the later stage of processing, longitudinal cracks may occur.
(1) product shape
(1) drawing too deep and bulging beyond the limit and causing longitudinal cracks; In addition, when finishing, if the longitudinal or transverse bulging exceeds the limit, it will also cause cracking. In a word, the direct cause of rupture is consistent with the overrun of bulging. Therefore, if the deformation limit is exceeded, the fracture will be drawn too deep. If the drawing depth is reduced, the forming conditions will become better.
② The radius (rd) of concave die is too small. Because it is bulging deformation, if it exceeds the deformation limit of the material, it will break. Therefore, a reasonable rd can not only prevent cracks in the flange, but also supplement materials. As one of the methods to improve the inflow conditions of materials, it is to increase the radius (rd) of the die fillet. Although increasing rd prevents cracking, rd is larger than the drawing size at this time. In order to make rd meet the drawing requirements, a finishing process should be added.
(2) Stamping conditions
① The most basic way to adjust the drawing force is to adjust the blank holder force. If cracks occur and the flange part shines, it is because the blank holder force is too large. Therefore, when there is a risk of breakage, the blank holder force can be slightly reduced to observe the change of the product.
② Insufficient lubrication on die surface With the increase of blank holder force, the oil film strength of lubricating oil should be improved accordingly to minimize friction.
(3) Poor blank shape If the blank is larger, the forming conditions will become worse and worse. Therefore, it is necessary to minimize the blank. That is to say, when approaching the bottom dead center, the blank should pass over the drawbead, and then try to draw.
(3) Mold problem
(1) The shape and position of drawbead are incorrect. Although the use of drawbead can prevent the flange from wrinkling, its side effect is that it hinders the inflow of materials. Therefore, if the reason for the rupture is that the inflow resistance of materials is too high, it is necessary to comprehensively analyze the position and shape of drawbead together with the blank shape in order to facilitate the inflow of materials.
(2) Poor machining If the die surface is poorly machined, the blank holder force can not be improved. Therefore, it needs to be polished with a grinding wheel.
(4) If the material exceeds the deformation limit, it needs to be replaced with a more advanced material. In addition, it is necessary to increase the plate thickness.
