Min Hong Seo

On the die corner gap formation in equal channel angular pressing

A die internal corner gap is usually found during equal channel angular pressing (ECAP) of materials. Finite element analysis using the DEFORM2D code is carried out in order to investigate the corner gap formation between the die and workpiece during the plane strain ECAP process. The comparison of the deformation and the corner gap formation behaviour between the strain hardening material and the quasi-perfect plastic material was made. The mechanism of the corner gap formation is described in conjunction with the strain hardening behaviour and local flow velocity of the workpiece in the deforming zone. The adjustment of the corner angle from the die corner angle to the arc curvature of the workpiece is necessary for a better prediction of the strain during ECAP.

Plastic deformation analysis of metals during equal channel angular pressing

Abstract Severe plastic deformation (SPD) methods have been the subject of intensive investigation in recent years not only due to the superior physical and mechanical properties inherent to various ultrafine-grained materials, but also to several advantages of severely plastic-deformed materials compared to nano-structured materials manufactured by other methods through powder forms. Among various severe plastic deformation methods, equal channel angular pressing (ECAP), which involves a large simple shear plastic deformation by moving a workpiece through two intersecting channels has been the subject of intensive study due to its capability of producing fully dense samples containing ultrafine grain size. In this process, knowledge of the plastic deformation behaviour of the workpiece during the process is very important for the determination of the optimum process conditions such as die design, speed, temperature, friction and preform design. In this study, the rigid–plastic two-dimensional finite element method using the DEFORM2D code is presented for the better understanding of the plastic deformation behaviour of the workpiece. The material flow stress is considered to be the strain rate and strain dependant. The deformation within the workpiece is inhomogeneous unlike the ideal pure shear deformation. The pressed workpiece is divided into three zones (front, steady and end zones). It was also found that a lesser shear deformation zone occurs in the outer part of the workpiece, which is attributed to the faster flow of the outer part compared to the inner part within the main deformation zone. In particular, inhomogeneous deformation of the workpiece with respect to the effect of the friction factor is investigated.

Finite element analysis of equal channel angular pressing of strain rate sensitive metals

Abstract In recent years, equal channel angular pressing (ECAP) has been the subject of intensive study in recent years due to its capability of producing fully dense samples having a ultrafine grain size. In this process, knowledge of the internal stress, strain and strain rate distribution is fundamental to the determination of the optimum process conditions for a given material. The properties of the ECAPed materials are strongly dependent on the shear plastic deformation behavior during ECAP, which is controlled mainly by die geometry, material properties, and process conditions. In this study, we applied the finite element method to obtain a better understanding the plastic deformation behavior of the workpiece during ECAP in terms of material flow properties and die geometries. The material flow stress, i.e. hardening behavior, was considered to be both strain rate dependent and strain dependent. The deformation within the workpiece is inhomogeneous unlike the ideal pure shear deformation. The mesh size dependence on the local deformation behavior in the deforming zone was addressed. The corner gap formation between the die and workpiece during the plane strain ECAP process was investigated. The mechanism of the corner gap formation is described in conjunction with the hardening behavior and local flow velocity of the workpiece in the deforming zone.

Preform Effect on the Plastic Deformation Behavior of Workpieces in Equal Channel Angular Pressing

Preform design is an effective means of achieving the homogeneous deformation of workpiece materials and decreased load in metal forming. However, this approach has not been applied to equal channel angula. pressing (ECAP). In this paper, plastic deformation behavior of workpieces having four different preform shapes during ECAP was investigated using finite element analyses. The results indicated that a preform design of the workpiece head has a beneficial effect on homogeneous deformation, reducing the maximum pressing load at the initial stage and eliminating folding defects at strain concentration points.

Creep Analysis of Cr-Mo Steels Using a Dislocation Based Constitutive Modelling

In order to analyze the creep behaviour of Cr-Mo steels, an elasto-viscoplastic constitutive model based on dislocation density considerations is described. A combination of a kinetic equation, which describes the mechanical response of a material at a given microstructure in terms of dislocation glide, and evolution equations for internal variables characterising the microstructure provide the constitutive equations of the model. Microstructural features of the material are implemented in the constitutive equation. The internal variables are associated with the total dislocation density. The model has a modular structure and can be adjusted to describe a particular type of materials behaviour and metal forming processes. In this paper, the predicted creep behaviour of Cr-Mo steels is compared with the experimental results.