Sheet metal forming simulation

Sheet metal forming, which is often referred to as stamping, is a process in which a piece of sheet metal, referred to as the blank, is formed by stretching between a punch and a die.

The most painful and most frequent defects are wrinkles, thinning, springback and splits or cracks. Few methods are being used around the industry to cope with the main defects, based on the experience of the technicians. However, the correct process is the most vital, since it involves the correct geometry followed by number of steps to reach at final geometry. Which demands for specific experience or higher number of iterations.[2]

Deformation of the blank is typically limited by buckling, wrinkling, tearing, and other negative characteristics which makes it impossible to meet quality requirements or makes it necessary to run at a slower than desirable rate.

Wrinkling in a draw are series of ridges form radially in the drawn wall due to compressive buckling. Practically these are duo to low blank holder pressure due to which material slips and wrinkles formed. The optimum blank holding pressure is the key, however in certain cases it doesn't work. Then draw beads are the solutions, the location and shape of draw bead is the challenge, which can be analysed with FEA during design stage prior to tool manufacturing.[2]

Crack in the vertical wall due to high tensile stresses, some small radius block the material flow and results in excessive thinning at that point usually more than 40% of the sheet thk. result in cracks. In some cases it may happen due to excessive blank holder pressure, which restrict the metal flow. Somewhere it might be due to wrong process design, like try to make a more deep draws in a single stage, which otherwise feasible only in two stages.[2]

Thinning is a Excessive Stretching in the vertical wall due to high tensile stresses cause thickness reduction specifically on the small radius in the metal parts, however up to 20% thinning is allowed due to process limitations.[2]

Springback is a particularly critical aspect of sheet metal forming. Even relatively small amounts of springback in structures that are formed to a significant depth may cause the blank to distort to the point that tolerances cannot be held. New materials such as high strength steel, aluminum and magnesium are particularly prone to springback.[3]

Sheet metal forming is more of an art than a science. The design of the tooling, stamping process and blank materials and geometry are primarily done by trial and error.

Nowadays the simulation software's comes under CAE (computer aided engineering), used the finite element analysis to predict the common defects in design stage, prior to die manufacturing.[2]

The traditional approach to designing the punch and die to produce parts successfully is to build try-out tools to check the ability of a certain tool design to produce parts of the required quality. Try-out tools are typically made of less expensive materials to reduce try-out costs yet this method is still costly and time-consuming.[4]

The first effort at simulating metalforming was made using the finite difference method in the 1960s to better understand the deep drawing process. Simulation accuracy was later increased by applying nonlinear finite element analysis in the 1980s but computing time was too long at this time to apply simulation to industrial problems.

Rapid improvements over the past few decades in computer hardware have made the finite element analysis method practical for resolving real-world metal forming problems. A new class of FEA codes based on explicit time integration was developed that reduced computational time and memory requirements. The dynamic explicit FEA approach uses a central different explicit scheme to integrate the equations of motion. This approach uses lumped mass matrices and a typical time step on order of millionths of seconds. The method has proved to be robust and efficient for typical industrial problems.

As computer hardware and operating systems have evolved, memory limitations that prevented the practical use of Implicit Finite Element Methods had been overcome.[5] Using the implicit method time steps are computed based on the predicted amount of deformation occurring at a given moment in the simulation, thus preventing unnecessary computational inefficiency caused by computing too small time steps when nothing is happening or too large a time step when high amounts of deformation are occurring.

Chaboche type material models are sometimes used to simulate springback effects in sheet metal forming. These and other advanced plasticity models require the experimental determination of cyclic stress-strain curves. Test rigs have been used to measure material properties that when used in simulations provide excellent correlation between measured and calculated springback.[8]

Many metal forming operation require too much deformation of the blank to be performed in a single step. Multistep or progressive stamping operations are used to incrementally form the blank into the desired shape through a series of stamping operations. Incremental forming simulation software platforms addresses these operations with a series of one-step stamping operations that simulate the forming process one step at a time.[9]

Another common goal in design of metal forming operations is to design the shape of the initial blank so that the final formed part requires few or no cutting operations to match the design geometry. The blank shape can also be optimized with finite element simulations. One approach is based on an iterative procedure that begins with an approximate starting geometry, simulates the forming process and then checks deviation of the resulting formed geometry from the ideal product geometry. The node points are adjusted in accordance with the displacement filed to correct the blank edge geometry. This process is continued until the end blank shape matches the as-designed part geometry.[10]

Metal forming simulation offers particular advantages in the case of high strength steel and advanced high-strength steel which are used in current day automobiles to reduce weight while maintaining crash safety of the vehicle. The materials have higher yield and tensile strength than conventional steel so the die undergoes greater deformation during the forming process which in turn increases the difficulty of designing the die. Sheet metal simulation that considers the deformation of not only the blank but also the die can be used to design tools to successfully form these materials.[11]

Tata Motors engineers used metal forming simulation to develop tooling and process parameters for producing a new oil pump design. The first prototypes that were produced closed matched the simulation prediction.[12]

Nissan Motor Company used metal forming simulation to address a tearing problem in a metal stamping operation. A simple simulation model was created to determine the effect of blank edge radius on the height to which the material could be formed without tearing. Based on this information a new die was designed that solved the problem.[13]

There are lots of sheet metal programs available in the industry as SolidWorks and LITIO.[14]

Nowadays FEA software's such as LS DYNA, AUTOFORM, HYPERFORM, PAMSTAMP are very good for virtual process simulations prior to product manufacturing. The defects such as Wrinkles, thinning and cracks can be seen in the design stage right just before the process design, results in correct process selection and reduction in lead time and save valuable money, which otherwise invested in hectic manufacturing iterations.[15]