Kaveh A. Tagavi

Investigation of the morphology of internal defects in cross wedge rolling

Abstract Since internal defects in the cross wedge rolling (CWR) process can weaken the integrity of the final product and may ultimately lead to catastrophic failure, it is necessary to investigate the mechanisms of their generation and growth. Using a specially designed CWR experimental apparatus, experiments were performed at more than 50 different operating conditions. The cross-sectional profiles of the workpiece specimens were examined and compared at each condition. Based on the experiments, the influence of three primary parameters in CWR process—the forming angle α , the stretching angle β , and the area reduction Δ A were determined. From the experimental results, the morphology of void generation and growth in CWR is ascertained and discussed. Through the definition of a non-dimensional deformation coefficient e , a method for predicting the likelihood of void formation is also established and discussed with respect to optimizing CWR tooling design.

Analysis of stress in cross wedge rolling with application to failure

In the present investigation, a previously developed three-dimensional finite-element model for the cross-wedge rolling (CWR) process has been used to characterize the workpiece material stress and deformation behavior. Particular attention has been paid to the center and mid-radius points of the billet where internal defects (i.e. internal cracks and porous voids) often occur. Several failure criteria in the solid mechanics theory are summarized. The effect of three important CWR parameters, namely the forming angle, the area reduction, and the friction coefficient, on the field variables has been investigated, including the first principal stresses, maximum shear stresses, etc. A total of 14 rolling conditions are analyzed for the billet material aluminum alloy 1100. After initially verifying the numerical results, several tendencies for the CWR process, as related to failure, are ascertained and discussed.

Analysis of interfacial slip in cross-wedge rolling: a numerical and phenomenological investigation

Abstract Friction and slip between the workpiece and the tool in metal forming processes are important issues in tool design and product development. This paper characterizes the tool–workpiece interfacial slip in a flat-wedge cross-wedge rolling process (CWR) using an explicit dynamic finite element method. An experimentally validated finite-element model of CWR is used to investigate the effects of the friction coefficient, the forming angle, and the area reduction on the tool–workpiece interfacial slip. A total of 14 rolling conditions are analyzed. An analytical CWR model for the workpiece rotational condition, which predicts the onset of rotation, is derived. Using this model, the critical area reduction and friction coefficient are compared with the failure conditions that occurred in the finite-element modeling and the prototype experiments. The variation of interfacial slip with CWR design parameters is found to provide insight regarding the CWR process design.

Analysis of interfacial slip in cross-wedge rolling: an experimentally verified finite-element model

Abstract Friction and interfacial slip in metal forming processes are very important parameters when considering tool design, tool wear, and finished product integrity. This paper investigates the interfacial slip between the forming tool and workpiece in a relatively new metal forming process, cross-wedge rolling (CWR). After a brief description of CWR is given, a three-dimensional finite-element model (FEM) is introduced which realistically characterizes the interfacial slip that occurs during a flat-wedge CWR process. Finite-element results, which are generated for various workpiece area reductions, are verified using experimental data obtained from a CWR prototype machine that was specially designed and constructed for understanding the deformations encountered in CWR. From the close agreement between the experiment and numerical results, it is shown that all of the important physical phenomena in the nonlinear deformation process of CWR are included in the FEM. The relevance of developing such a model, as applied to automating CWR tool design, is subsequently discussed.

Influence of material properties and forming velocity on the interfacial slip characteristics of cross wedge rolling

By means of the explicit dynamic finite element method, the relationship between the tool-workpiece interfacial slip and several cross wedge rolling (CWR) variables is investigated for a flat-wedge CWR process. After defining the components of interfacial slip and area reduction, an experimentally validated finite element model of CWR is introduced. This model is used to analyze a total of 189 distinct operating conditions by varying workpiece material (aluminum 1100, steel 1018 and brass C21000), forming velocity (0.4∼4.0 m/s), area reduction (25 percent, 40 percent and 55 percent) and forming angle (20 deg, 30 deg and 40 deg). The numerical results indicated that forming velocity was an important variable in determining the interfacial slip characteristics of the CWR process analyzed. Additionally, the area reduction and forming angle were found to have a significant influence on the interfacial slip under the conditions considered.

A numerical procedure for solving 2D phase-field model problems

We present a general 2D phase-field model, but without anisotropy, applied to freezing into a supercooled melt of pure nickel. The complete numerical procedure and details of assigning the numerical parameters are provided; convergence of the numerical method is demonstrated by conducting grid function convergence tests. The physics of solidification problems such as conditions for nucleation and crystal growth rate are discussed theoretically and shown to display at least qualitative agreement numerically. In particular, comparison of the computed critical radius with the theoretical one and the consistency of the computational dendrite structure for different Stefan numbers, the relationship between the growth rate and the Stefan number, etc., with the theoretical and experimental evidence indicate that phase-field models are able to capture the physics of supercooled solidification.

Two-Dimensional Phase-Field Model Applied to Freezing into Supercooled Melt

Cryopreservation of living cells is a necessary part of many medical procedures such as organ transplants and preservation of sperm and oocytes of endangered species. However, there is at least one apparent contradiction between the concept of cryopreservation and experimental findings that cells and tissues can be damaged by the cryopreservation process itself. Successful cryopreservation was made possible by the addition of glycerol as a cryoprotective agent (CPA). A major portion of the damage is due to and occurs during the supercooling of tissues and cells and their environment. Therefore, a detailed understanding of how supercooling impacts biological environments is important to preventing damage to cells and tissues during cryopreservation. Studies of supercooling are complicated due to the inherent instability associated with supercooling and the influence of surface tension as a stabilizing factor and other parameters associated with the liquid-solid phase-change front. The only method that effe...

Homogeneous nucleation of vapor by depressurization at constant volume

An analysis has been carried out to determine the thermodynamic requirements for homogeneous nucleation of a vapor bubble when the pressure drops inside a constant-volume container of liquid. This situation can occur in phase change processes when a rigid vessel filled with liquid is cooled. The nucleation equations at constant volume have a somewhat different character from their more familiar constant-pressure counterparts that reflects the change in the boundary conditions for bubble formation. Both the physical and mathematical implications of the new solution are explored in detail, and it is shown to reduce to the well known constant-pressure result in the limiting case of a very large container volume. To illustrate an application of the new equations, some numerical examples have been worked out for homogeneous nucleation of water with various container sizes and initial liquid temperatures. In addition to increasing fundamental understanding of homogeneous nucleation, these results should prove valuable for calculation purposes when vapor nucleation takes place under isochoric rather than isobaric conditions.

Local Discrete-Operator Interpolation Solution of the Phase-Field Model

This paper reports continuing work on application of discrete-operator interpolation (DOI) in solving the one-dimensional phase-field model applied to melt-front tracking. DOI is a numerical technique for computing function values not computed at the original grid points of a finite-difference (or finite element) scheme so as to satisfy the discrete governing equations at the new points. The previous study showed that the DOI technique works quite well for the phase-field model problem. The shortcoming of earlier work was global (in space) application of DOI. Due to the fact that at any instant in time, the melt-front of the phase-field model exists within only a small region of space, it is more efficient to employ a local DOI technique. Local DOI interpolates the numerical solutions only in the melt-front region while a standard numerical method is applied in other regions. In this paper, we describe the phase-field model together with the details of the local DOI method and their numerical implementations. The results of the phase-field model are obtained using a Crank-Nicolson finite-difference scheme. The local DOI results are compared with direct numerical simulation results obtained on a very fine grid to demonstrate the advantages of this method.Copyright

An introduction to the derivation of surface balance equations without the excruciating pain

Abstract Analyzing complex fluid flow problems that involve multiple coupled domains, each with their respective set of governing equations, is not a trivial undertaking. Even more complicated is the elaborate and tedious task of specifying the interface and boundary conditions between various domains. This paper provides an elegant, straightforward and universal method that considers the nature of those shared boundaries and derives the appropriate conditions at the interface, irrespective of the governing equations being solved. As a first example, a well-known interface condition is derived using this method. For a second example, the set of boundary conditions necessary to solve a baseline aerothermodynamics coupled plain/porous flow problem is derived. Finally, the method is applied to two more flow configurations, one consisting of an impermeable adiabatic wall and the other an ablating surface.