At present, the competition in the automobile market is becoming more and more fierce. Consumers are not only confined to a suitable price when choosing products, but also pay more attention to the quality and quality of automobiles. Stamping is the first of the four major automotive processes. The quality problems of stamping parts not only affect the body beauty, but also reduce the corrosion resistance and product life. Therefore, the quality defects control of stamping parts is very important.
Common quality problems in stamping process include wrinkling, cracking, springback, surface quality (collapse, slip, impact) and so on. The above problems account for more than 85% of the quality improvement of stamping parts. Repeated modification and maintenance of dies result in reduced service life, increased shutdown time, and increased production costs due to rework or even scrap of products. Therefore, it is of great practical value to discuss the generation and solution of key defects of stamping parts to improve the forming quality and production efficiency of products.
I. Wrinkles and laminates
Defect Description: Because of the large difference between the thickness direction and the plane direction, the thickness direction is unstable. When the stress in the plane direction reaches a certain degree, the thickness direction becomes unstable, which results in wrinkling phenomenon.
Causes of wrinkling
(1) The material accumulates and wrinkles, and there are too many materials entering the cavity of the die, resulting in wrinkles.
(2) Unstable wrinkling: 1) Weak restraint in the thickness direction of sheet metal leads to wrinkling in compression flange instability; 2) Uneven stress tends to occur in uneven deep drawing position, leading to wrinkling in instability.
(3) The R angle of the workpiece is too large, which results in the punch unable to hold down the material and the material flowing too fast in the drawing process, and then wrinkles.
（4）The setting of pressing bars is unreasonable, or the pressing bars are too small, which can not effectively prevent the material from flowing too fast.
(5) Where the product needs to increase the wrinkle absorption rib, there is no wrinkle absorption rib, or the wrinkle absorption rib is too small, which leads to wrinkle in the process of drawing.
（6）The gap between upper and lower dies is too large, which causes the die to be unable to hold down the material and wrinkle in the process of drawing.
Solutions to wrinkles
(1) Check the rationality of the original product model design; 2) Avoid the saddle shape of the product; 3) Increase the suction bar in the wrinkle-prone part of the product.
（2）Stamping technology: 1) reasonable arrangement of working procedures; 2) check the rationality of pressing surface and drawing supplementary surface; 3) check the rationality of drawing blank, pressing force and local material flow; 4) wrinkle relief by internal rib; 5) improve pressing force, adjust drawing rib, stamping direction, increase forming process, sheet thickness, change product and process modeling to absorb surplus materials, etc. 。
(3) Material: In the case of meeting the product performance, for some wrinkle-prone parts, the material with good formability is used.
The general method to rectify wrinkling is to ensure that the die can hold down the material and ensure a reasonable and normal material flow rate in the process of drawing: if the material flows too fast in the process of drawing, it may cause wrinkling of the parts; if the material flows too slowly, it may cause cracking of the parts.
2, cracking
Defect Description: Cracking is mainly due to the instability of the material due to strain exceeding its limit during deep drawing.
Cracking of stamping parts is the most common quality defect in the process of drawing and flanging. It will directly affect the strength of the parts and reduce the service life of the products. Therefore, when cracking of the parts occurs, scrap treatment will be done, which virtually reduces the production efficiency and increases the production cost. Therefore, effective prevention in the early stage and quick solution after the occurrence of defects are all instant. It is necessary to be patient.
Causes of cracking
(1) The failure of products due to insufficient tensile strength, such as near the corner of convex and concave dies, is caused by excessive local force.
(2) Failure caused by insufficient deformation of material. In bulging deformation, rupture near the top of punch or rupture caused by excessive deformation of flange elongation material flowing into the punch.
(3) Aging crack, that is, material hardening occurs in the complex forming area during the forming process, accompanied by brittleness during the hardening deformation, which results in the cracking of the part under the residual stress of the forming process.
(4) When material is formed, it is bended by deep drawing and then bended back, resulting in rupture, which mostly occurs at the convex rib or the die opening.
（5）Stripe cracks are generally parallel to the rolling direction of sheet metal due to the cracks caused by impurities in the material.
Solutions to Cracking
(1) Material aspect: The material with good drawing property is adopted.
(2) Reducing strain: 1) Choosing reasonable blank size and shape; 2) Adjusting drawing rib parameters; 3) Increasing auxiliary technology, such as cutting; 4) Improving lubrication conditions; 5) Modifying process supplement surface; 6) Adjusting pressing force.
3, rebound
Springback: After stamping, the part is deformed partly or wholly due to the unloading of forming force.
Solution to springback: Because of many factors affecting springback, such as material, pressure, die status, etc., it is difficult to solve in actual production.
In the elastic stage, the higher the yield limit is, the greater the elastic modulus is, the more serious the work hardening phenomenon is, and the greater the springback of bending deformation is. On the contrary, the stronger the resistance to elastic deformation is, the smaller the springback value is. At present, the following methods are commonly used to solve rebound.
(1) Compensation method, its principle is: according to the springback of sheet metal after bending forming, a slope equal to the springback of workpiece is made in advance on the die to compensate the springback of workpiece after forming (according to experience and CAE simulation).
(2) The tension bending method is to apply tension to change the stress state and distribution in the cracked defect sheet of Fig. 2 while bending, so as to make the stress distribution more uniform and reduce the rebound.
（3）The material with good formability is used.
Surface Quality Defects
The main types of surface quality defects of stamping parts are impact line, slip line, collapse, pit, surface distortion and so on. For exterior panel parts, defects on the exterior surface are not allowed.
Causes
The impact line and slip line are mainly caused by the surface scratches caused by friction at the stress concentration point after the contact between the sheet metal and the die during stamping. Collapse, pit and surface distortion are mainly due to inadequate deformation of parts, small strain of local materials and defects after the release of external forces.
Resolvent
(1) To eliminate the impact line, the forming roundness and die roundness can be changed, the drawing depth can be increased, and the blank surface can be changed.
(2) The slip line can be eliminated by changing the product shape (left and right symmetry) and increasing the resistance.
(3) In order to eliminate the collapse and surface distortion, the stress gradient of the part in the deformation area should be known to ensure the uniformity of the plastic deformation of the product as far as possible, and the local forming strain should be increased by increasing the resistance.
Concluding remarks
When a part meets quality defects, the real cause of the problem should be deeply analyzed. Specific problems are analyzed concretely and feasible rectification plans are worked out. In addition, in the implementation of the plan, process records should be made well, typical key issues should be feedback to the early stage, and prevention of recurrence should be done well; in the SE stage, according to the past experience of stamping parts development, problems should be avoided through product shape optimization and adjustment, delivery cycle should be shortened, normal stamping production should be guaranteed, and production efficiency should be improved. This paper elaborates the common quality defect forms, production mechanism, prevention and Countermeasures of stamping parts. It provides ideas and directions for solving such problems in the later stage. It can improve the appearance quality of stamping parts to a certain extent and enhance the market competitiveness of products.
