Deep drawing is a complex forming process, in which the sheet metal is drawn into a forming die by a punch, so that a different kind of stresses are applied on the material blank. If the depth of the part is greater than its diameter, the operation is considered to be deep. This can be reached by a redrawing of a part using a series of dies or press line.
This process has found a broad application in the industry, in particular in an automotive and aircraft production, as allows to manufacture complicated and seamless shapes. In addition, it can be set up for mass production. On the other hand, a relatively high price of the equipment, a need for qualified technicians, and high sensitivity for material quality are the main drawbacks of this technology [1-4].
Deep drawing process concept
There are three basic types of a deep drawing processes: with active parts, with active energy, and with tools, which is the most common in the automotive industry [1]. The process utilizes rigid tools such as a punch, a die, and a blank holder that usually are made of tool steel, carbon steel or cemented carbides.
Firstly, a material blank is fixed between the holder and the die, and subsequently deformed by a punch, moving downwards into the die until the required shape is obtained. The process is usually accompanied by other operations, such as piercing, trimming, marking, and many other. A broad range of materials can be drawn, among other aluminium, unalloyed steel, stainless steel, brass, bronze or copper.
The main parameters affecting the process are: blank-holding down pressure (clearance-type or pressure-type) that prevent wrinkling when drawing; drawing ratio, which is the relation between the blank diameter and the throat diameter of the die; radial clearance between a die and a punch determining a maximum drawing force; a punch and a die profile radii [5, 6].
The most common defects in deep drawing operations are wrinkling, galling, orange peeling, earing, and tearing. Their occurrence is related to numerous factors, such as blank hardness, lubrication system, quality of the die surface, grain size and anisotropy of the drawn material, holding down pressure, and clearance between the punch and die.
In order to avoid the apparition of these defects, the material can be subjected to the heat treatment process or another method of grain refinement. In addition, the drawing setup needs to be adjusted, namely, the clearance between the punch and die, considering the blank thickness. The lubrication system is utilized to reduce the friction, and furthermore, simplifies a drawpiece removing from the die [5, 6].
For a detailed knowledge of the forming process, it is necessary to perform a numerical simulation of each deep drawn part, supported by material knowledge including the plasticity, anisotropy and hardening.
As the tool production is quite expensive, the validation of material behaviour including the sheet metal flow using experimental die is recommended. Within the tests using the experimental die, the complexity of a serial part can be reproduced and evaluated on a simple geometry, but resembling and adjusting all boundary conditions, which can be used to upgrade the numerical simulation. The most used experimental die are cup, cross shape, square die and triangular shape (ETH). The various geometries generate different stresses and deformations leading to different forming limits.
To compare the results of numerical simulation and deep drawing using experimental die with the real production part, a photogrammetric system can be used for detailed analysis of material formability. With GOM’s ARGUS system the part is recorded with high resolution images, which subsequently are processed by sophisticated algorithms to get a surface strain distribution based on initial printed pattern (Figure 1) grid on undeformed, initially flat sheet [7].
Fig. 1: The experimental drawpiece covered with the grid pattern (prepared for ARGUS analysis).
The thickness and Forming Limit Diagram can be created from the forming results that can be directly compared with the imported Limit Curves, which are obtained from the measurement based on standard ISO12004. With such an innovative approach, the full design cycle can be performed and the sheet metal forming process can be adjusted and optimised.
In the frame of the FormPlanet project, deep drawing tests and ARGUS analysis were performed in cooperation with the industrial partner ESTAMP. The methodology used consisted of several stages. Firstly, the smooth metal sheets were delivered to COMTES FHT, where the initial grid pattern was created. Then, the sheets were subjected to embossing, and subsequently, scanned using ARGUS. Secondly, the embossed sheets were deep drawn and scanned again. Based on the displacement of the pattern’s dots, the colourful deformation maps were created. The results of ARGUS analysis were presented as the distribution of major/minor strain or thickness reduction along chosen sections. The data points illustrated in the major vs. minor strain diagram were compared with the FLC obtained experimentally, according to the standard ISO 12004 with a 20% safety margin added. The results revealed that all recorded values were well below the limits established by forming limit diagrams.
The experimental deep drawing with the deformation analysis by ARGUS is one of the services to be offered by FormPlanet. The application of this system allows the detection of critical deformation regions on a final product and indication, if the deformation is still within the safety margins, which can be visualised in terms of Forming Limit Curve, Thickness Reduction Curve and others (Figure 2). The companies can validate the results of numerical simulation since the data from ABAQUS, ANSYS and LS-DYNA can be easily imported and compared in the GOM’s software. This system is helpful in material selection and optimisation of forming tools [4].
Figure 2. The analysis of strain distribution using ARGUS by GOM.
Sylwia Rzepa
PhD student in Biomechanics. She is R&D engineer at Mechanical Testing and Thermophysical Measurement Department at COMTES FHT (Czech Republic). Her research is focused on mechanical characterisation of additively manufactured metallic biomaterials.
Miroslav Urbánek
He has completed his Ph.D. at the University of West Bohemia in Pilsen. Since 2006, he is an R&D researcher at Numerical Simulation Department at COMTES FHT. His research is focused on the design and numerical simulation of forming processes. He has published more than 20 research papers, 4 patents, and 30 utility models.
References
[1] https://www.autoform.com/en/glossary/deep-drawing/
[2] https://www.manufacturingguide.com/en/deep-drawing
[3] https://quintustechnologies.com/whatisdeepdraw/
[4] https://www.gom.com/en/topics/deep-drawing
[5] A. I. O. Zaid, Deep drawing mechanism, parameters, defects and recent results: state of the art, IOP Conf. Ser.: Mater. Sci. Eng. 146 012009, 2016
[6] Z. Marciniak, J. L. Duncan, S. J. Hu, Mechanics of sheet metal forming, Butterworth-Heinemann, 2002
