Cylindrical deep drawing deformation process and characteristics
The cylindrical deep drawing deformation process is the plastic deformation process in which the material gradually transforms from a flat blank into a cylindrical part. It can be divided into four stages: flange deformation zone, die radius zone, cylinder wall zone and punch radius zone. There are significant differences in the material deformation behavior and stress state in each stage. Understanding the deep drawing deformation process and characteristics is of great significance for optimizing process parameters, designing mold structure and controlling part quality. The main characteristics of deep drawing deformation include material transfer and distribution, deformation unevenness, work hardening and springback phenomenon, which directly affect the dimensional accuracy and mechanical properties of the drawn parts.
The flange deformation zone is the area of most severe material deformation during the drawing process. Located above the die surface, on the outer ring of the blank, the flange zone experiences the most intense deformation. Under the action of the drawing force, the material in this zone is subjected to a combination of radial tensile stress and tangential compressive stress, causing plastic deformation. This stress gradually shrinks radially and is compressed tangentially, ultimately transforming into the cylindrical wall. Deformation in the flange zone is characterized by radial elongation, tangential shortening, and a slight increase in thickness (approximately 1%-3%), with the flange edge experiencing the most significant increase in thickness. For example, when a low-carbon steel blank (100mm diameter) is drawn into a 50mm diameter cylindrical part, the flange zone shrinks from a radius of 50mm to 25mm, experiencing a radial elongation of approximately 50%, a tangential contraction of approximately 50%, and an increase in flange thickness from 2mm to 2.05mm. Uniform deformation in the flange zone is crucial to drawing quality; uneven deformation can lead to wrinkling or cracking of the flange.
The die radius is the transition zone where the material enters the barrel from the flange. The material is subjected to bending and friction at the die radius, resulting in bending deformation and plastic flow. A smaller die radius increases the bending deformation and frictional resistance, which can easily lead to surface scratches or cracks. A larger radius promotes smoother material flow, but increases the area of the flange, increasing the risk of wrinkling. Deformation in the die radius is characterized by a bending-straightening process, resulting in a slight thickness reduction (approximately 0.5%-1%). Surface roughness is easily affected by the die. For example, with a die radius of 10mm, the material thickness decreases from 2mm to 1.98mm after passing through the radius. If the fillet surface roughness Ra = 1.6μm, slight scratches will occur on the material surface. The die roughness needs to be reduced to Ra = 0.8μm to avoid defects.
The wall region forms the sidewall of a cylindrical component after deep drawing. Material is transferred from the flange region and is the primary area subjected to the drawing force. Its deformation is characterized by unidirectional tensile stress (radial), resulting in axial elongation. The thickness decreases slightly (approximately 1%-2%) due to stretching, and the thickness gradually increases from top to bottom (thinner at the top and thicker at the bottom). For example, the upper portion of a drawn cylindrical component is 1.96mm thick, while the lower portion is 1.98mm thick. This thickness difference is due to the varying forces applied during material flow. The wall region’s strength is enhanced by work hardening, increasing its hardness by 20%-30% compared to the original material. For example, the original hardness of low-carbon steel is 120HB, but after deep drawing, the wall hardness increases to 145-150HB, thereby improving the component’s load-bearing capacity.
The punch radius forms the transition zone between the barrel wall and the bottom. The material is subjected to bending and stretching here, resulting in minimal deformation. However, it is a weak link in the drawn part and prone to cracking. The smaller the punch radius, the greater the bending stress and the greater the likelihood of cracking; the larger the radius, the less flat the bottom. Deformation in the punch radius is characterized by bending deformation, a slight reduction in thickness (approximately 0.5%), and significant stress concentration. For example, with a 5mm punch radius, the material thickness in the radius decreases from 1.98mm to 1.97mm, resulting in a stress concentration factor of approximately 1.2. If the material lacks plasticity, microcracks are likely to form here.
The overall characteristics of cylindrical drawing deformation are characterized by non-uniform deformation. There are significant differences in the degree of deformation, stress state and strain magnitude in different regions, which leads to different mechanical properties and dimensional accuracy of different parts of the drawn part. For example, the elongation of the cylinder wall area can reach 20%-30%, while the bottom has almost no deformation; the tangential compressive stress in the flange area is the largest, and it is prone to wrinkling, and the radial tensile stress in the cylinder wall area is the largest, and it is prone to cracking. In addition, springback will occur during the drawing process, causing the cylinder wall to expand slightly and the bottom to be slightly concave, affecting the dimensional accuracy of the part. By rationally designing mold parameters (such as fillet radius and gap) and optimizing process parameters (such as blank holding force and drawing speed), deformation unevenness can be reduced and the quality of drawn parts can be improved.
Common defects that can occur during the deep drawing process include wrinkling, cracking, uneven thickness, and shape distortion. Wrinkling primarily occurs in the flange area due to excessive tangential compressive stress, leading to material instability. Cracking primarily occurs in the punch radius or on the upper portion of the barrel wall due to radial tensile stress exceeding the material strength. Uneven thickness manifests as thinner at the top and thicker at the bottom, thicker at the edges and thinner in the middle. Shape distortion manifests as a skewed barrel wall and an uneven bottom. For example, insufficient blank holder force can cause wrinkling in the flange area; too low a drawing coefficient (excessive deformation) can cause cracking in the barrel wall; and uneven die clearance can increase the thickness difference in the barrel wall. These defects require appropriate process measures, such as increasing blank holder force to prevent wrinkling, reducing deformation to prevent cracking, and adjusting the die clearance to ensure uniform thickness.
