Noritoshi Iwata

Improvements in finite-element simulation for stamping and application to the forming of laser-welded blanks

Abstract During stamping-die design, the formability in sheet-metal forming process has been evaluated by the geometrical functions in ‘Die-Face CAD’, which has been developed and improved by Toyota Motor Corporation. When evaluation by these functions is difficult, formability has been estimated by performing experiments using test dies in which the forming defects are similar to those in the actual process. A numerical method has been developed in order to substitute numerical analysis for experiments using test dies for the accurate prediction of defects in sheet-metal forming. The elastic-plastic FEM with the commercial code ‘JNIKE3D’ has been improved in the areas of: (1) the material constitutive equation; (2) the consideration of the pressure distribution on the blank-holder; and (3) the evaluation of breakage initiation. Using the improved method, the square-cup drawing process and the hemming process have been analyzed. Numerical results for strain, breakage initiation, and hemming deflection were in good agreement with experimental results. The formability of laser-welded blanks and the most efficient process to form them were evaluated also using the improved method.

Formability Improvement Technique for Heated Sheet Metal Forming by Partial Cooling

A forming process for heated sheet metal, such as hot-stamping, has limited use in deformable shapes. Higher temperature areas which have not yet come into contact with dies are more easily deformed; therefore, local deformation occurs at these areas which leads to breakage. To improve the formability of heated sheet metal, a deformation control technique utilizing the temperature dependence of flow stress is proposed. This technique can suppress local deformation by partial cooling around potential cracking areas to harden them before forming. In order to apply this technique to a variety of product shapes, a procedure to determine a suitable initial temperature distribution for deep drawing and biaxial stretching was developed with a coupled thermal structural simulation. In this procedure, finite elements exceeding forming limit strain are highlighted, and an initial temperature distribution is defined with areas of decreased temperature around the elements to increase their resistance to deformation. Subsequently, the partial cooling technique was applied to a deep drawing test with a heated steel sheet. The results of the experiment showed that the proposed technique improved 71% in the forming limit depth compared with results obtained using a uniform initial temperature distribution.

Crystal Plasticity Finite Element Analysis Based on Crystal Orientation Mapping with Three-Dimensional X-Ray Diffraction Microscopy

In other study we examined the plastic behavior for polycrystalline iron by three-dimensional x-ray diffraction (3DXRD) experiment. In this study we analyze the behavior by crystal plasticity finite element (CPFE) analysis, to confirm the validity of application to the deformation analysis of engineering steels of a couple of constitutive models. In the CPFE analysis, the observed microstructure and its crystal orientation are modeled with finite elements to take the inter-granular and intra-granular interactions into consideration. The plastic deformation state of the finite element model was computed by means of CPFE analysis based on the {110}<111> slip system in body centered cubic (BCC) crystal. The experiment showed that the most of the grains rotated toward the preferred orientation <110> along the tensile axis and that intra-granular orientation spread and multi-directionally rotated as the tensile strain increased. These results are reproduced by the CPFE analysis, in which the influence of interaction between neighboring grains is taken into consideration.

MRT letter: In situ observation method for microstructural changes of steel during hot deformation.

We report on the result of an in situ method for observing microstructural changes during hot deformation. The observation of microstructural changes of steel at 1,473 K under tensile strain is demonstrated using the reported method. The development of deformed structures and the formation of a new grain boundary, which subsequently moved with increased strain, were clearly observed. The effectiveness of this method was confirmed by the results of several examples.

Initiation and growth of buckling in the biaxial diagonal tensile test on steel sheet

Abstract Initiation and growth of buckling, a main cause of surface deflection in large steel press-formed components such as automobile outer panels, is simulated by the biaxial diagonal tensile test on square steel sheet specimens. The effects of the properties of the steel sheets and the biaxial loading conditions on the initiation and growth of buckling are examined by the tests and by FEM analysis. It is found that both the initiation and the growth of buckling are strongly affected by the yield strength of the material, σ s , and by the tensile stress in the breadth direction, p Y , and that buckling is restrained when p Y / σ s is high: this is because the compressive stress at the center of the specimen, the compressive stress area, and the development of the buckling area are largely affected by these factors.

Calculation for grain growth rate of carbon steels by solute drag model considering segregation effect of each substitutional element

Abstract Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modelled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.

Numerical Prediction of Springback Shape of Severely Bent Sheet Metal

In the sheet metal forming simulation, the shell element widely used is assumed as a plane stress state based on the Mindlin‐Reissner theory. Numerical prediction with the conventional shell element is not accurate when the bending radius is small compared to the sheet thickness. The main reason is because the strain and stress formulation of the conventional shell element does not fit the actual phenomenon. In order to predict precisely the springback of a bent sheet with a severe bend, a measurement method for through‐thickness strain has been proposed. The strain was formulated based on measurement results and calculation results from solid element. Through‐thickness stress distribution was formulated based on the equilibrium. The proposed shell element based on the formulations was newly introduced into the FEM code. The accuracy of this method’s prediction of the springback shape of two bent processes has been confirmed. As a result, it was found that the springback shape even in severe bending can b...

Investigation of Hardness Change for Spot Welded Tailored Blank in Hot Stamping Using CCT and Deformation-CCT Diagrams

When an outer panel of a B-pillar is manufactured with the hot stamping process, reinforcements are spot welded on its inner side. Before reinforcements are added, the microstructure of the outer panel is martensite. However, reheating during spot welding changes the martensite to ferrite, which has a lower hardness in the heat-affected zone than in other areas. If spot welding is conducted before hot stamping for making a spot welded tailored blank, the microstructure in the spot welded tailored blank after hot stamping is martensite. This sequence of processes avoids hardness reduction due to spot welding. In this study, the hardness and microstructure around spot welded parts of the tailored blank were investigated. The results clearly showed that areas close to the spot welded parts are severely stretched during hot stamping. In addition, stretching suppresses the martensitic phase transformation and reduces the hardness. To characterize this phenomenon, a simulation was conducted that considered the effects of pre-strain on the phase transformation. A continuous cooling transformation (CCT) diagram and a deformation continuous cooling transformation (DCCT) diagram were made in order to quantify the effect of the cooling rate and pre-strain on the phase transformation and hardness. The hardness was then calculated using the experimentally measured CCT and DCCT diagrams and the finite element analysis results. The calculated hardness was compared with the experimental hardness. Good agreement was found between the calculated and experimental results.

Maximization of Strengthening Effect of Microscopic Morphology in Duplex Steels

An inverse analysis method based on nonlinear finite element analysis is developed to find an optimized morphology of periodic microstructure for improving the macroscopic mechanical properties in duplex elastoplastic solids. Here a gradient-based computational optimization method and two types of homogenization methods are employed. In this study, the optimization problem is defined as the maximization of the sum of macroscopic external works for several macroscopic deformation modes, enabling us to obtain a high strength material. The morphologic strengthening effect is discussed through a comparison with experiments and classical theories.

Surface evaluation method and stamping simulation for surface deflection of automotive outer panels

In designing dies of automotive outer panels, the most difficult process is to modify surface deflection. To fabricate high-quality outer panels without modifying dies, it is important to develop an evaluation method and a numerical analysis method for surface deflection of outer panels. In this study, we have developed a new evaluation method that uses the maximum value of curvature calculated using reflecting curves in the surface. This new evaluation method made the examiners evaluation to conform with the digital evaluation. The evaluation results with the new method shows better agreement with the sensory value than those with the conventional methods. We have proposed the new analysis method to predict surface deflection correctly. By the proposed simulation method, plastic deformation is calculated in consideration of stress in thickness direction, and restriking conditions have been examined. We have applied our methods to the fabrication of automotive outer panels, and verified that these are useful and practical.In designing dies of automotive outer panels, the most difficult process is to modify surface deflection. To fabricate high-quality outer panels without modifying dies, it is important to develop an evaluation method and a numerical analysis method for surface deflection of outer panels. In this study, we have developed a new evaluation method that uses the maximum value of curvature calculated using reflecting curves in the surface. This new evaluation method made the examiners evaluation to conform with the digital evaluation. The evaluation results with the new method shows better agreement with the sensory value than those with the conventional methods. We have proposed the new analysis method to predict surface deflection correctly. By the proposed simulation method, plastic deformation is calculated in consideration of stress in thickness direction, and restriking conditions have been examined. We have applied our methods to the fabrication of automotive outer panels, and verified that these are us...