Nobuki Yukawa

Prediction of Dimensional Difference of Product from Tool in Cold Backward Extrusion

Abstract The effects of forming stresses and generated heat on the dimensional change of punch, die and work piece during forging are discussed analytically. The change in outer and inner diameter of backward extruded cup is investigated numerically using thermo-elastic-plastic FEM code according to the actual forging sequence, namely extruding, unloading of punch force, and ejection and air cooling of extruded cup. The calculated results of outer and inner diameters of product are in good agreement with the experimental results. The simulation can be used to determine the initial tool dimensions for precision parts in the tool design process of cold forging.

Analytical Approach to Elimination of Surface Micro-Defects in Forging

Initial surface micro-scratches due to cropping, rolling or handling change that cause crack or lap defects during forging and reduce the quality of forged products. Elimination of surface micro-defects in forging is pressing problem in forging industry. A new method of finite element analysis featuring effective meshing and an adaptive remeshing system is applied to clarify the mechanism of the formation or the disappearance of surface micro-defects in forging. V-shaped and rectangular scratches on the surface of billet are simulated under the axisymmetric assumption and the basic deformation of scratch during upsetting and the influence of friction on the deformation of micro-defects are discussed. Shallow V-shaped scratch on the top surface of billet tends to disappear and deep scratch becomes lap defect during upsetting. Defects are more likely to disappear under low friction. It is possible to predict that an initial surface micro-scratch on the billet remain or not as a defect after forging.

Computer Simulation and Control of Microstructure and Mechanical Properties in Hot Forging

An analytical procedure for predicting the microstructure and mechanical properties of forged microalloyed steels is presented. Empirical relationships between recrystallized grain size, temperature and strain as well as the relationship between austenite grain size and ferrite grain size are obtained. These relations are expressed as an incremental formula and incorporated into thermo-coupled FEM code. By applying this analysis to practical multiple stage hot forging of flange companions or automotive parts made of microalloyed steel, the microstructure and hardness of forged parts can be predicted. The results of these predictions are in good agreement with the experimental results. The simulation is used to investigate the thermo-mechanical processes for forging functional parts with partially fine grain and high hardness.

Effect of Extrusion Conditions on Metal Flow and Microstructures of Aluminum Alloys

Abstract Study of metal flow in extrusion billet is fundamental and useful to understand extrusion technology. Investigation of behavior of extrusion billet skin and microstructure of products is indispensable to maintain qualities of extrusions. In order to research the behavior, experiments and FE analysis of clad billet extrusion were performed. The analytical results of billet skin deformation were similar to that in experiments. Friction between back end of billet and ram affects the deformation of skin and its penetration into billet. Secondly, microstructure of an extruded product is predicted and the condition of recrystallization and grain refnement is revealed.

Formability of superplastic aluminum alloy processed by mechanical alloying

Abstract The superplastic deformation of mechanically alloyed (MA) AlCuMg and AlCuMgZn alloy has been observed at high strain rates of 10 0 –10 1 s, which correspond to the speed of a general mechanical press. Very fine and stable grains of sub-micron size are obtained by the mechanical alloying process. The formability, and especially the forgability, of these materials are examined by use of the upsetting test and the spike-forging test. The upsetting limit and the forging limit of these materials on a mechanical press are substantially greater than those on a low-speed hydraulic press, so one of the obstacles with conventional superplastic materials, that superplasticity appears only at very low strain rates, is eliminated.

Determination of Ductile Damage Parameters by Notched Round Bar Tension Test Using Image Analysis

In this study, a method was devised for deciding material properties relating to ductile fracture (damage parameters) through a simple experiment. To predict void nucleation, a void nucleation critical strain model was proposed. Gradual changes in the necking shape were measured by using notched round bar tension test using image analysis. From the results, strain and stress history at the necking section were obtained by Bridgman’s correction. From the result, the decision of the ductile damage parameters were carried out on some ductile fracture criteria. Additionally, the relationship between void volume fraction and strain was investigated using an experimental two‐step tension test.

Optimization of Pass Schedule from the View Point of Shape and Profile of Cold Rolled Strip

Abstract The paper aims at finding analytically an optimum pass schedule for producing strip with uniform thickness across the width and dead flat shape. The profile of strip is calculated by a three-dimensional analysis in which the elastic deformation of rolls and the plastic deformation of strip are considered as one system. The flatness defect is considered as the elastic buckling by the the residual stress which is calculated from the stresses at the exit of roll gap.

Application of Adaptive Remeshing Method to Simulation of Material Forming Processes

The problem in simulation of metal forming processes using Lagrangian type finite element method is an excessive mesh distortion which causes a large discretization error. In this paper, an adaptive remeshing method is applied to a simulation of metal forming processes based on a rigid-plastic model with 6 node triangular elements to control the accuracy of the analysis. The algorithm used is described, and some calculated examples are shown. The effectiveness of adaptive remeshing method on finite element analysis of metal forming processes is discussed.

Experimental Approach for Investigation of Micro Void Behavior on Dual Phase Steel during Plastic Deformation

In this research, micro void behavior of DPS, which has different phase volume fraction, is investigated experimentally. Prepared specimens have different amount of alloy element and heat treatment condition. Although both these specimens have almost the same strength and total elongation, these showed quite different local elongations, hole expanding ratio and bending ductility. In order to observe micro void behavior specimens, that have different strain levels, are prepared. These sectional areas are observed by laser microscope after cutting and polishing. The change ratio of micro void volume fraction of DPS, which showed high local ductility, is lower than that of the other DPS. Concerning low ductility DPS, small size void (less than 3µm diameter) increases rapidly not only at grain boundary but in the ferrite grain, especially at initial stage of deformation. On the other hand, nucleation of micro void of high ductility DPS is inhibited.

Prediction of the Change of Surface Micro‐Defects in Plate Rolling

A new method of finite element analysis featuring an adaptive remeshing system was applied to calculate the change in surface micro‐defects in plate rolling. In the analysis, small meshes of sub‐millimeter size were used around the defects. Rolling operations performed on plates having surface defects of several types; for instance, V‐shaped and rectangular defects, were simulated and the defects remaining after rolling were investigated. Experiments were performed in order to verify the validity of the simulation results. Changes in defect shape in rolling were found to fall under two main categories: opening type and closing type. The influences of rolling conditions (roll diameter, lubrication, pass schedule, etc.) on existing defects after rolling were revealed. The proposed method for predicting surface defects is useful for producing defect‐free products in rolling and forging processes.