Common Defects in the Forming Process of Automobile Stamping Parts and Analysis

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Influencing factors of defects arising from the stamping and forming process

Material parameters

In the sheet metal stamping and forming process, the forming quality is generally judged by the forming limit curve (FLD).

（The most influential factors in the process of curve generation are the thickness anisotropy coefficient r and the hardening index n. The change of these parameters has a double effect: the continuous increase of the hardening index n can effectively improve the extension uniformity of the material, but at the same time, the continuous increase of the hardness will make the sheet metal more prone to pulling and cracking, and of course, the continuous decrease of the n parameter will have the opposite effect. The opposite effect is caused by decreasing the n parameter. If the thickness anisotropy coefficient r increases, the pulling point will also change to a certain extent, and when it increases to a certain degree, generally when it is greater than 1, the deformation in the thickness direction of the sheet will be reduced to a certain extent, and the component will not be so easily pulled and cracked, and it will be less likely to produce wrinkles in the stretching process, but its ductility will be affected to a certain extent.

Die surface parameters

The die surface (die, rounded corners, shape, etc.) has an important influence on the formation of the part. The function of the die fillet is illustrated by the example of a cylindrical deep-drawn part. In the deep drawing process of cylindrical parts, the key factor for success is the proper size of the die fillet, whose main function is to control the material flow and speed. The value of the die corner has a certain relationship with the thickness of the material. When the thickness is less than 1mm, the radius of the corner is not less than 5 times the thickness of the material; when the thickness is between 1 and 2.5mm, the radius of the corner is not less than 4 times the thickness of the material; when the thickness is greater than 2.5mm, it must be determined on a case-by-case basis. If the radius of the corner is too small, it will break easily during the drawing process. When designing the radius of the corner of the mold, the radius of the rounded corner can be increased or decreased according to the flow of the material, firstly based on the calculation results and experience, so as to control the molding process.

Process parameters

Process parameters refer to parameters other than the mold surface parameters and the material's own parameters, including edge pressure, friction, stretching speed and lubrication conditions. These parameters also play a decisive role in the formation of the part. When the pressure side is too large, the material flows poorly during the forming process and can make the sheet thinner or even cracked. Conversely, if the edge force is too low, the material will be uncontrolled in the molding and the sheet will accumulate quickly, which will affect the part formation and even destroy the mold. Crimp rings or crimp rings are usually used to control the crimping force. Friction is mainly the direct contact force between the sheet metal and the punch and die. It is similar to the crimping force and causes wrinkling of the part and leads to cracking of the part. In addition, excessive friction can cause scratches on the part surface, damaging the die face and reducing the life of the die. During the drawing process, the drawing speed has a major impact on the formation of the part, too fast or unevenly can lead to part cracking. Lubrication conditions are also important for forming parts that primarily affect material flow. Poor lubrication can lead to part breakage due to poor flow; excessive lubrication not only causes sheet buildup but also tends to create closed vacuums and affects part formation.

Methods to solve the defects

(1) Optimization of the die: arrange the tension bars and give a reasonable radius mainly according to the shape of the part and the location of the defect; calculate the reasonable size of the blank pressing force; design the size of the convex and concave die circle according to the nature of the plate; ensure the reasonable clearance of the convex and concave die.

(2) Optimization of flat plate: Reasonable size of blank; Give reasonable positioning of blank; Poor performance material should be treated simply before forming.

(3) Other aspects: improve lubrication conditions and carry out stamping and forming process under ideal environment (temperature, humidity) as far as possible; improve the quality and business level of workers.

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