Finite element analysis and optimization of deep drawing processes using progressive die

Deep drawing is a kind of sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by a punch. The process is considered "deep" drawing when the depth of the drawn part exceeds its diameter. This can be achieved by a single step or multiple steps redrawing the part through a series of dies. The flange region experiences a radial drawing stress and a tangential compressive stress due to the material retention property. These compressive stresses (hoop stresses) result in flange wrinkles. Wrinkles can be prevented by using a blank holder, the function of which is to facilitate controlled material flow into the die radius. Higher blank holder force causes cracking in workpiece and lower blank holder force causes wrinkling so that a proper and optimal blank holder force should be used to achieve a high quality drawing process. Progressive dies are known as multi-stage dies. Punches and dies are designed so that successive stages in the forming of the part are carried out in the same die on each stroke of the press. Throughout this study, both theoretical and experimental studies are applied over deep drawing process. A sheet metal goes into a progressive die machine with a uniform shape and leaves the line as a finished part. The part is objected to eleven operations in the machine; cutting, deep drawing, drilling in sucession. In this operation, the general shape of the part is created by applying three deep drawing processes. Deep drawing process in three steps are analized by finite element method. ANSYS "Static Structural", an implicit solver is used in the analysis. During the experiments DC04 sheet metal with 0,6 mm wall thickness is used. DC04 is widely used material in aerospace, automative, white goods and many other different areas in industry. It is a suitable sheet metal for extra deep drawing processes. In order to determine its mechanical properties tensile experiments are applied. The tensile tests are done considering anisotropy of the material so that the tests are applied in three directions which are 0, 45 and 90 degress to rolling direction. It is seen that, material shows the same elastic and plastic behavior in all directions. Because, their stress-strain curves are so similar to each other so that their behaviors are assumed as if they harden isotropically. This is a very important issue in finite element analysis because this manufacturing is modelled considering tensile test data. The basic mechanical parameters such as Elastic modulus, Poisson ratio and Density of material is evaluated considering steel properties as 210 GPa, 0,298 and 7,85 E-6 kg/mm3 respectively. Another properties like yield strength, tensile strength, strain hardenin exponent, strength coefficient, elongation at rupture are obtained from tensile experiment. These paremeters are very close to each other for each direction to rolling. Their average values are evaluated as yield strength 171 MPa, tensile strength 348 MPa, strain hardening exponent 0,248, strength coefficient 613 MPa, elongation at rupture 0,5. In large deformation, use of true stress-true strain data gives more real results. Through engineering stress-strain curve, the material behavior can be modelled up to necking point. So that, engineering stress-engineering strain data is converted into true stress-true strain data using Holloman strain hardening equation to model material's behavior after necking point. Whether a sheet metal can be deep drawn successfully into a round cup-shaped part has been found to be a function of the normal anisotropy (plastic anisotropy) for the sheet metal. In order to determine the magnitude of normal anisotropy, a tensile test specimen is prepared and subjected to an elongation of 15% to 20%. Because cold-rolled sheets generally have anisotropy in their planar direction, the normal anisotropy of a specimen cut from a rolled sheet depends on its orientation with respect to the rolling direction of the sheet. For this condition an average anisotropy values is calculated as 1,772. This shows that the material shows high ressistance in thickness direction. The other phenomenon faced during deep drawing process is earing. Planar anisotrpy determines the amount of earing to be occurred during drawing process and is found as 0,375. Deep drawing with progressive die has some unknowns like friction coefficient between surfaces and blankholder force. In order to provide a proper analysis these parameters must be obtained. To do this, the length, width, thickness and depth of the 20 parts are measured after first deep drawing process. Then the analysis of first deep drawing process is launched until the length, width, thickness and depth of the part match with the part obtained from the experiment. After obtaining friction coefficients and blank holder force, the analysis of second and third deep drawing analysis are applied. Deep drawing process are repeated by applying first and second, second and third operations in order to observe if number of deep drawing processes can be decreased or not. The same procedure, analysis on decreasing of deep dawing process, is repeated for lower sheet thickness without any failure. Forming limit diagrams are widely used to evaluate the formability of thin sheet metals. For each analysis, forming limit diagram is constructed with principal strains obtained from analysis. For each analysis, from first to third, from second to third, fist-second-third operations, forming limit diagrams are created. In the same way, diagrams are reconstructed for different sheet thickness, 0,5 and 0,4 mm. In FLDs, as the thickness of the sheet increases limit curve shifts upward and as the thickness of the sheet decreases forming limit curve shifts downward. Shifting upward shows the material has high formability property and shifting downward shows the material has low formabilility property. Strain hardening exponent also affects the forming curve same as sheet thickness. The tensile experiments show that DC04 has high strain hardening exponent and this means it has the property of high formability. For 0,6 mm sheet thickness, forming limit diagrams show that workpiece does not subject to any failure in first-second-third, first-to-third, second-to-third operations. Because none of the principal strains passes above the forming limit curve. But there are some data close to forming limit curve whose thickness decreases 20% or more so that, slimming of the sheet metal above 20% notifies about the failure risk. Even so, it is seen that the number of press can be decreased for sheet metal with 0,6 mm wall thickness because all the principal strains are located below the forming limit curve for first-to-third and second-to-third operations. The analysis done for 0,6 mm sheet thickness is repeated for lower sheet thickness; 0,5 and 0,4 mm. It is seent that, the first deep drawing process can be successfully achieved without any damage for both 0,5 and 0,4 sheet thicknesses. When the deep drawing is done by three operations, both of the metals fails. When the operation is done by decreasing the number of steps from three to two for fist-to-third press, both 0,5 and 0,4 mm thick metals fail. When two-to-third press is apllied, as 0,4 mm sheet suffers damage but 0,5 mm does not suffer damage. Generally, sheet with 0,4 mm thickness suffers more damage than the sheet with 0,5 mm thickness. This is an expected result because thinner sheets are subjected more stress than thicker sheets. So that, both decreasing number of press and the sheet thickness deep drawing operation can be provided successfully. Examination of sheet metal slimming is very important in the case of sheet metal failures. It is seen that, the punch radius plays very important role in sheet metal slimming. As the punch radius increases the sheet metal gets less thinner. As the punch radius decreases the steet metal gets more thinner. The radius of first punch is 3,5 mm and the second one is 2 mm and the third one is variable from 2 mm to 1 mm. So, when the first deep drawing is applied using the first punch and die, the amount of decrease in 0,6 mm sheet thickness is about %16. When the second deep drawing is applied using the second punch and die, the amound of decrease in 0,6 mm sheet thickness is about %21. It shows that, for one step deep drawing operation, use of sharper punch causes more slimming. However, for multi-stage deep drawing process, things can change. When first-to-third two step deep drawing analysis is applied, the amount of decrease in 0,6 mm sheet thickness is about %42. But when second-to-third two step deep drawing analysis is applied, the amount of decrease in 0,6 mm sheet thickness is about %25. Here, things are different. So, it can be inferred that, in multi-stage deep drawing processes, use of similar punch form to final punch form in the first press leads sheet metal to becoming less thinner. In progressive die machines, the rate of production is so high. Any improvement in his production line brings about very considerable benefits. Performed studies show that, one press operation can be removed using 0,6 mm sheet, and sheet thickness can be decreased to 0,5 mm by removing the first press operation from the manufacturing line. Removing one die saves one die cost, and decreasing sheet thickness provides very high amount of profit.