Hirohiko Takuda

Modelling on flow stress of Mg–Al–Zn alloys at elevated temperatures

Abstract For the evaluation of flow stress of magnesium-based alloys AZ31 and AZ91 in hot working processes, a formula is derived by analysing the stress data measured at various temperatures and strain rates. The formula is expressed in a simple form by means of the temperature-compensated strain rate, i.e. the Zener–Hollomon parameter, Z. It is demonstrated that a linear equation of the logarithmic Z fits the flow stress of the alloys at elevated temperatures.

The application of some criteria for ductile fracture to the prediction of the forming limit of sheet metals

Abstract The possibility of the application of ductile fracture criteria to the formability prediction of sheet metals is examined. Some criteria in simple forms are employed and combined with the finite element simulation of sheet metal forming. From the calculated histories of stress and strain in each element, the fracture initiation site and the forming limit are predicted by means of the criteria for ductile fracture. The material constants in the criteria are determined from uniaxial and plane-strain tension tests. The calculations are carried out for the axisymmetric deep drawing of various aluminium alloy and steel sheets. A comparison between the calculated and experimental results shows that the present approach using the ductile fracture criteria gives fairly good predictions.

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.

Finite element simulation of warm deep drawing of aluminium alloy sheet when accounting for heat conduction

Abstract The deformation behaviour and the temperature change in cylindrical deep drawing of an aluminium alloy sheet at elevated temperatures are simulated by the combination of the rigid-plastic and the heat conduction finite element methods. The comparison with the experimental results shows that the forming limits and the necking sites are successfully predicted by the simulation. It is clarified that the appropriate distribution of flow stress depending on temperature must exist in the sheet for the higher limiting drawing ratio. The numerical as well as the experimental results show that the limiting drawing ratio in the warm deep drawing increases with the die profile radius.

Finite-element analysis of the formability of a magnesium-based alloy AZ31 sheet

Abstract To examine the formability of a magnesium-based alloy AZ31 sheet, the deep drawing and the Erichsen tests are carried out and they are simulated by the finite-element method. A recently proposed approach is applied to the prediction of forming limit of the AZ31 sheet, which latter breaks suddenly without any obvious necking phenomenon. From the histories of stress and strain calculated by the finite-element simulation, the fracture initiation is predicted by means of a criterion for ductile fracture. Comparison with experimental observations shows that the fracture initiation site and the critical punch stroke in forming processes of the AZ31 sheet are successfully predicted by the combination of the simulation with the ductile fracture criterion.

Prediction of forming limit in bore-expanding of sheet metals using ductile fracture criterion

Abstract To predict the forming limit in sheet metal forming, a criterion for ductile fracture is introduced into the finite element simulation. From the histories of stress and strain in each element calculated by the finite element simulation, the initiation of fracture is predicted by means of the fracture criterion. Calculations are carried out for axisymmetric bore-expanding processes of mild steel and high strength steel sheets using flat-, hemispherical- and conical-headed punches. Comparison with experimental results shows that the fracture initiation site and the critical stroke are predicted successfully by the present approach.

Prediction of forming limit in deep drawing of Fe/Al laminated composite sheets using ductile fracture criterion

Abstract Axisymmetric deep drawing processes of laminates composed of mild steel and various aluminium alloy sheets are simulated by FEM. From the calculated stress and strain histories of elements in each layer, the fracture initiation site and the forming limit are predicted by using the ductile fracture criterion. The predictions so obtained are compared with experimental observations. The results exhibit that various types of fracture initiations in deep drawing of the laminated composite sheets are successfully predicted. Furthermore it is found that the drawability is improved by setting the mild steel sheet on the punch side for the case of aluminium alloy sheet with comparatively high ductility, and by sandwiching the aluminium alloy sheet with the mild steel sheets for the case of low ductility.

Deformation Process of a Water Droplet Impinging on a Solid Surface

We are concerned with numerical simulations of the deformation behavior of a liquid droplet impinging on a flat solid surface, as well as the flow field inside the droplet. In the present situation, the case where a droplet impinges on the surface at room temperature with a speed in the order of a few [m/s], is treated. These simulations were performed using the MAC-type solution method to solve a finite-differencing approximation of the Navier-Stokes equations governing an axisymmetric and incompressible fluid flow. For the first case where the liquid is water, the liquid film formed by the droplet impinging on the solid surface flows radially along it and expands in a fairly thin discoid-like shape. Thereafter, the liquid flow shows a tendency to stagnate at the periphery of the circular film, with the result that water is concentrated there is a doughnut-like shape. Subsequently, the water begins to flow backwards toward the center where it accumulates in the central region. For the second case where a n-heptane droplet impinges the surface, the film continues to spread monotonically up to a maximum diameter and there is no recoiling process to cause a backwards flow towards the central region.

Experimental Study of Deformation Mechanism of a Water Droplet Impinging on Hot Metallic Surfaces Above the Leidenfrost Temperature

This paper is concerned with the collision dynamics of a water droplet impinging on three kinds of smooth surfaces (Inconel alloy 625, stainless-steel, and silicon) heated to above the Leidenfrost temperature (500°C). It has been found that the time histories of the droplet diameter, the height and the distance between the bottom of droplet and the hot surface after rebounding are almost unchangeable regardless of the kind of surface material, when the Weber number is kept so low that the droplet does not break up into some parts. However, the critical Weber number, whether or not the droplet is disintegrated into some pieces during deformation, has been confirmed to be changeable depending upon the kind of surface material. For relatively low Weber number cases, but above the critical one, the droplet breaks up into some parts after the droplet reaches a maximum diameter on the surface. As the Weber number is increased further, the droplet disintegration occurs during the spreading process. Also, the droplet disintegration mechanism has been discussed from an experimental point of view.

Tensile properties of a few Mg-Li-Zn alloy thin sheets

The tensile properties of experimentally produced Mg-6Li-1Zn, Mg-9.5Li-1Zn and Mg-12Li-1Zn alloy thin sheets at room temperature are investigated in this study. Uniaxial tension tests are carried out for various strain rates between 1.4 × 10−5 and 8.3 × 10−2 s−1, and the microstructural and textural changes during the tests are examined. The Mg-6Li-1Zn sheet is composed mainly of the α (hcp) phase and inferior to the other sheets in ductility. The β (bcc) phase is dominant in the Mg-9.5Li-1Zn and Mg-12Li-1Zn sheets, and they have a considerable sensitivity to strain rate. It is observed that the grains are elongated with textural change mainly in the β phase at low strain rates, and the Mg-9.5Li-1Zn and Mg-12Li-1Zn sheets have sufficiently high ductility at low strain rates. The Mg-9.5Li-1Zn sheet composed of (α + β) two phase is superior to the Mg-12Li-1Zn sheet of β single phase in the tensile strength.