Deformation analysis of deep drawing
Deep drawing is a forming process that uses a die to stamp a flat blank into an open, hollow part. The deformation process is complex, involving the plastic flow of the material, stress-strain distribution, and die-material interaction. In-depth analysis of the deformation characteristics of deep drawing is crucial for understanding forming mechanisms, predicting defects, and optimizing process parameters. The primary characteristic of deep drawing is that, under the action of radial tensile stress and tangential compressive stress, the material undergoes plastic deformation, resulting in radial elongation and tangential contraction, gradually transforming the blank into a hollow part with a certain height.
During the drawing process, the material’s deformation zone is primarily concentrated in the annular region at the die opening (i.e., the flange portion of the blank). Under the action of the punch, the material in this region is continuously pulled into the die to form the cylinder wall. The material in this deformation zone undergoes a transition from a flat surface to a curved surface, with radial fibers elongated and tangential fibers compressed. This deformation causes a change in material thickness: radial elongation thins the material, while tangential contraction thickens it. Ultimately, the thickness of the cylinder wall exhibits an uneven distribution, with the thinning typically occurring most severely at the transition radius between the wall and the bottom (a reduction rate of up to 10%-20%). For example, when drawing a low-carbon steel cylindrical part, the wall thickness remains essentially unchanged in the middle, thickens slightly near the top opening, and thins most significantly near the bottom radius. This area is a high-risk area for deep-drawn rupture.
The stress state during deep drawing is complex. The material in the deformation zone is primarily subjected to radial tensile stress (provided by the punch) and tangential compressive stress (generated by the mutual compression of the materials). In addition, it is subject to normal compressive stress and friction from the die and blank holder. Radial tensile stress is the primary driving force for material flow into the die, increasing with drawing depth. Tangential compressive stress causes tangential contraction of the material, which can easily lead to wrinkling in the flange. In the barrel wall, the material is primarily subjected to axial tensile stress, transmitted by the punch, to overcome resistance and friction in the deformation zone. When axial tensile stress exceeds the material’s tensile strength, the barrel wall will rupture. For example, excessive blank holder force during drawing increases resistance in the deformation zone, increasing axial tensile stress in the barrel wall and potentially leading to rupture. If the blank holder force is too low, tangential compressive stress can cause wrinkling in the flange, increasing resistance to material flow and similarly causing overload rupture of the barrel wall.
The unevenness of drawing deformation is the main cause of part quality defects. This unevenness is reflected in the degree of deformation, stress distribution and thickness changes in different parts. For rotating parts, the unevenness of deformation is mainly manifested as differences in the radial and tangential directions; for non-rotating parts (such as rectangular parts), it is manifested as differences between corners and straight edges. Uneven deformation can cause defects such as uneven wall thickness, springback, and distortion in parts. For example, when a rectangular part is drawn, the degree of deformation at the corners is greater than that of the straight edges, resulting in more severe thinning of the wall thickness at the corners, which is prone to rupture first. In addition, the anisotropy of the material will also aggravate the uneven deformation. The longitudinal and transverse mechanical properties of rolled steel plates are different. During deep drawing, the flow rate and degree of deformation of the material in different directions are different, resulting in irregular wavy shapes at the mouth of the part.
The limit of deep drawing deformation is determined by the material’s plasticity and process conditions. It’s often expressed as the drawing coefficient (drawing coefficient m = finished product diameter / blank diameter). The smaller the drawing coefficient, the greater the deformation. The greater the material’s plasticity (e.g., mild steel), the smaller the drawing coefficient can be; the higher the material’s strength (e.g., high-strength steel), the larger the drawing coefficient required. Process conditions (such as die clearance, blank holder force, and lubrication) also affect the drawing limit. Proper die clearance and blank holder force can reduce deformation resistance and improve drawing capacity. For example, the initial drawing coefficient for mild steel can be as low as 0.5-0.55, while the initial drawing coefficient for high-strength steel must be ≥0.65. If the deformation exceeds the material’s drawing limit, the part will crack. Therefore, multiple drawing cycles are required to gradually achieve larger deformations. Each drawing coefficient must exceed the corresponding limit and be increased gradually.
Analysis methods for deep drawing deformation include theoretical calculations, experimental studies, and numerical simulations. Theoretical calculations establish mechanical models, analyze stress and strain distributions, and predict deformation trends and limits. Experimental studies verify theoretical analysis by observing the deformation process through test punches and measuring key parameters (such as thickness change and stress values). Numerical simulations utilize finite element software (such as ABAQUS and AutoForm) to create three-dimensional models, simulating material flow, stress and strain, and defect generation. This intuitively displays the dynamic changes in deformation and provides a basis for process optimization. For example, numerical simulations can predict the location and timing of potential fractures during the drawing process, allowing process parameters to be adjusted in advance (such as increasing the fillet radius and reducing the blank holder force) to avoid defects. By combining various analytical methods, a deep understanding of the laws governing deep drawing deformation can be achieved, providing scientific guidance for process design and mold optimization.
