Norio Takakura

FINITE ELEMENT ANALYSIS OF LIMIT STRAINS IN BIAXIAL STRETCHING OF SHEET METALS ALLOWING FOR DUCTILE FRACTURE

To predict limit strains in biaxial stretching of sheet metals, a criterion for ductile fracture is combined with the finite element simulation. The limit strains are determined by substituting the values of stress and strain obtained from the finite element simulation into the ductile fracture criterion. Material constants in the criterion are obtained from the fracture strains measured in the biaxial stretching tests. Calculations are carried out for various strain paths from balanced biaxial stretching to uniaxial tension of aluminium alloy sheets, and compared with the experimental results. The predicted limit strains are in good agreement with the measured ones not only just at the fracture site but also at outside of the fracture site. It is demonstrated that the forming limit diagrams are successfully predicted by the present approach.

Compaction and sintering characteristics of composite metal powders

Abstract The compaction and sintering characteristics of composite metal powders are examined using aluminium-copper, aluminium-iron and copper-iron powders. The density of a composite green compact is predicted based on the compacting pressure-density curve of each powder. The effects of the volume fraction of the mixed powders and the sintering temperature on the density and hardness of a sintered part are also examined. Experimental results show that the density of the composite green compact can be predicted by the law of mixtures. The density change in the sintering of the composite green compact depends on the combination and the volume fraction of the composite powders as well as on the sintering temperature. For copper-iron mixed composite powders, the density of the sintered part increases with increasing sintering temperature and volume fraction of copper powders, whilst, for the combination of aluminium-copper powders, anomalous behaviour is found; when the green compact (50 vol.% aluminium-50 vol.% copper) is sintered at a temperature of about 800K, the density of the sintered part becomes smaller than that of the green compact. However, the hardness of the sintered part increases remarkably; the micro-Vickers hardness of the sintered part is about 600, which is approximately 15 to 20 times that of the sintered pure copper and aluminium, respectively. This should be emphasized as one of the important characteristics of sintered aluminium-copper composite material.

Redrawing analysis of aluminum–stainless-steel laminated sheet using FEM simulations and experiments

Abstract The behavior of two-layer aluminum–stainless-steel (AL-SUS) laminated sheets during deep drawing, direct and reverse redrawing processes (first and second drawing stages), have been examined by simulations and laboratory experiments. For the simulation a rigid-plastic finite element program has been used. The results of simulations are presented as the variation of drawing ratios with respect to various thickness ratios and setting conditions. They show that to achieve the highest drawing ratios in direct and reverse redrawing, the thickness ratio should be about 1 3 (one-layer aluminum and three-layer stainless-steel) and the setting conditions are opposite to each other. Considering the FEM results, laminated sheets with a thickness ratio of 71.3% aluminum and 28.7% stainless-steel were used to prepare deep drawing and redrawing experiments. The results of experiments are presented as the variation of thickness strain distribution in the drawn cup and punch load–stroke curves with respect to the setting condition. Results show that while in direct redrawing, contact of stainless-steel with the punch leads to the maximum drawing ratio, in reverse redrawing, aluminum should contact the punch in order to achieve the highest drawing ratio. An explanation for this finding is offered based on the thickness strain distribution, and punch load–stroke curves.

Consideration of the re-drawing of sheet metals based on finite-element simulation

Abstract The effects of the work-hardening characteristics of blank material, the first-stage drawing ratio, the tool shape and interstage annealing on the limiting re-drawing ratio have been considered, based on the results of the rigid-plastic finite-element simulation of the direct and reverse re-drawing processes. In an attempt to increase the drawing limit, re-drawing experiments have also been carried out using a partially thickened blank, the thickness of which is somewhat greater at the punch-head portion than at the flange portion. The simulation results show that the success or failure of the re-drawing process depends not only on the re-drawing ratio but also on the material and process parameters described above. It is predicted also that the difference in the limiting re-drawing ratio achieved by the direct and reverse re-drawing processes results from the severity of bending and unbending at the profile radius portions of the die and the cup holder. The use of a partially-thickened blank contributes appreciably to an increase in the drawing limit. For steel sheet, for instance, the total drawing ratio achieved by the first and second drawing processes can be increased from 2.94 for a normal blank with uniform thickness to 3.45 for a partially-thickened blank.

Increase in forming limit of sheet metals by removal of surface roughening with plastic strain (Balanced biaxial stretching of aluminium sheets and foils)

Abstract When forming a sheet metal, the free surface of the sheet becomes rough with increasing plastic strain. This is well known as orange peel phenomena and has a great effect not only on the surface quality of a product but also on the forming limit. In an attempt to increase the forming limit of thin sheet metals, in the present paper, a new forming technique is proposed in which surface asperities generated during the forming process are polished to eliminate the weak portions of the sheet which lead to the onset of a localized necking. Balanced biaxial stretching of aluminium sheets and foils is carried out in this manner, and how the forming limit can be increased is examined. It is shown that the forming limit of sheet metal can be appreciably improved by a simple method of polishing the surface roughness during the stretching process. The polishing of surface roughness at the strain somewhat before the onset of localized necking is recommended to efficiently increase the forming limit. The effect of polishing surface roughness on the forming limit becomes large as the thickness of sheet decreases. For aluminium sheets with 0.1 and 0.2 mm in thickness, for example, the forming limit strains achieved have been about 1.4 times as large as those for the ordinary balanced biaxial stretching without the interstage polishing. When such processes of stretching and subsequent polishing are repeated in turn 2 or 3 times, further increase in the forming limit can be achieved.

Attempts to facilitate low volume production of soft aluminium cups with large drawing ratios by deep drawing based on Maslennikov's technique

Abstract When producing cylindrical cups by conventional deep drawing with metal tools, the maximum first-stage drawing ratio seldom exceeds 2.2. For a larger drawing ratio, therefore, re-drawing in one or more stages is necessary. Since a punch and die are required for each stage, tooling costs increase relatively as the total drawing ratio increases and as the number of products decreases. Two methods, based on Maslennikovs process, are investigated to facilitate low volume production of cylindrical cups with very large drawing ratios. Firstly, a conventional direct re-drawing is carried out as a subsequent operation to increase the total drawing ratio and to correct the shape and dimensions of cups produced by Maslennikovs process. Secondly, a modified form of Maslennikovs process employing a metal punch is examined and the additional effects resulting from the introduction of the punch are discussed. When these methods are applied to soft aluminium sheets of 1 mm thickness, a fully-drawn cup with an extremely large drawing ratio can be produced using only one metal punch and one or two dies throughout the drawing process. The process does not seem to be too slow for the production of cups with drawing ratios of less than about 6. The shape, dimensional accuracy and surface finish of the cups produced are also adequate for practical use.

A novel technique of friction aided deep drawing using a blank-holder divided into four segments

Abstract A new process of friction aided deep drawing has been developed, in which the friction force between the blank and the blank-holder is used to aid the drawing deformation of the blank. This has been achieved by using a divided blank-holder, which consists of four segments and can move radially under axial pressure. The drawing process can be combined with a supplemental punch which gives a constant punch force during the drawing process. The experimental results show that there is no fracture at the flange of deformed blank, which is often observed in Maslennikov’s process. The present new process is a very good trial to secure the advantages of deep drawing with elastic tools, but by using rigid tools. Very deep cups can be produced by repeating the drawing process. Theoretical analysis based on the energy and slab methods has also been conducted to study the possibility and the main features of this new process.

EFFECTS OF STRAIN RATE AND TEMPERATURE ON DEFORMATION RESISTANCE OF STAINLESS STEEL

ABSTRACT Upsetting tests of a 13Cr stainless steel are carried out using a computer controlled high speed hydraulic press which generates a constant strain rate throughout an upsetting process. The effects of strain rate and temperature on the flow stress are mainly examined. For high speed upsetting, the temperature rise during the upsetting process is compensated by employing an assumption of the adiabatic plastic deformation. An empirical expression of the flow stress is given in terms of strain, strain rate and temperature. Stress-strain curves calculated from the empirical expression are described on a three dimensional space of stress-strain-temperature coordinates. This description is useful for explaining the work-softening behaviour of metals which is often observed in high speed upsetting at room temperature.

Numerical Investigation of Flexural Properties of Sheet Metals Subjected to Embossing and Restoration Process

In this study, numerical simulations of flexural tests of sheet metals subjected to embossing and restoration process are carried out using LS-DYNA3D. Several models are created varying the number, position, and pitch of the emboss or restoration points. The emboss height and sheet thickness are also varied. Results show that improvement in rigidity of sheet metals can be optimized by taking into considerations several parameters as discussed in this paper.

Production of long rods by sequential extrusion of wood powders

Abstract Wood-based materials are environment-friendly resources as they eventually decompose into carbon dioxide and water after disposal. However, there are a number of productivity and workability related problems: it takes many years for a tree to grow to a useable size, and wood materials are more difficult to process than metal and plastic materials. To overcome these problems, sequential extrusion of cryptomeria powders mixed with Japanese cypress was performed in an attempt to make long wooden rods. In sequential extrusion, the powder replenishment and short-stroke extrusion steps are alternated until the desired rods are obtained. In this process, a strong extrusion seams were required between the new powder batch and the extruded portion of the rod. A steel container with a device for controlling replenishment temperature was used to increase the bonding strength of extruded rods. The results showed that there exist an optimal replenishment pressure and temperature, and that, under these optimal conditions, long rods with the strength of natural wood can be obtained. Finally, it is found that the presence of back-pressure is key in producing sound, high-strength rods.