Papers by Author: Sergei Alexandrov

Abstract: Compared to conventional metal forming methods, processing by severe plastic deformation is mostly used to improve the mechanical properties and not for the shaping of a product. Processed material usually has an average crystal grain size of less than a micron and as a result, the material exhibits improvements in most of the mechanical properties, such as yield and ultimate tensile strength, microhardness, sufficiently high workability, good corrosion resistance, and implant biocompatibility and others. In this paper, a brief review of the processing by severe plastic deformation was presented, including the benefits, major methods, and the application. Additionally, a brief review of two methods made by authors was made.

Abstract: Fiber metal laminates (FMLs) are widely used in aerospace industry due to their unique high specific strength, fatigue resistance, corrosion resistance and other excellent characteristics. Thermosetting FMLs is generally used for forming large size parts and rarely used as raw material for producing small size and complex shape parts. This study attempts a methodology that stamping thermosetting FMLs to form cylinder shape parts before the curing process. The forming limit height of FMLs were analyzed by choosing different core materials, layup direction and skin layer thickness. And through the optimization of these variables, a better-quality part has been formed.

Abstract: Rheology of polymer-metal powder mixture containing a significant amount of metal powder loading determines its flowability through an extrusion die. To make sheets from such mixtures, geometries based on Coat-hanger, Fishtail and ‘T’ type designs are often used. Flow properties such as flow rate, viscosity, residence time, instantaneous velocity and pressure distribution in different type of dies have been used in simulations in Ansys (CFX or fluent). The present paper describes the special design of the fishtail type die geometry with adjustable part at lip land to make sheets from a polymer-metal powder mixture by extrusion. This also includes the design of primary manifold, tapered pre-land, secondary manifold and lip land geometries to obtain uniform velocity at the exit of the die and as uniform a pressure distribution as possible. It includes designing of suitable die profiles in each of these four zones based on the experimentally established constitutive equations using a Melt Flow Index tester.

Abstract: Rheological study has been performed experimentally by using melt flow index (MFI) tester on a mixture of CI (Carbonyl Iron) powder and HDPE (High-Density Polyethylene) polymer. The rheological properties such as volume flow rate (cm³/s), shear strain rate (s^-1) and viscosity (Pa.s) are investigated for varying conditions of temperature and weight (pressure). This also includes experimental determination of viscosity dependence over parameters like temperature, shear strain rate and CI powder loading by weight added in HDPE. For this experimental conditions selected are temperatures 448⁰K-523⁰K in steps of 25⁰K, weights in MFI tester (ultimately converted to shear strain rate) 0.325, 1.20, 2.16, 3.80, 5.00Kg and carbonyl iron powder loading in binder HDPE (by weight) 80%-92% in steps of 6%. A constitutive equation for viscosity is formulated which considers all factors affecting viscosity with the maximum percentage error of about 4% between experimental value and value predicted by the formulated equation is obtained.

Abstract: In present research work, metal injection molded cylindrical samples containing the mixture of carbonyl iron powder and high-density polyethylene (HDPE) were compressed at various strain rates. Three mixtures of carbonyl iron powder and HDPE were prepared to contain 80 %( Material A), 86 %( Material B) and 92 %( Material C) metal powder by weight. Compression tests were performed on the cylindrical samples at different crosshead velocities of Universal Testing Machine varying from 0.6-25 mm/min. True stress-true strain behavior of these samples under uniaxial compression test was studied. Based on these curves, polynomial equations were formulated to describe the plastic deformation behavior of the various mixtures at various cross head velocities. The Material C samples showed higher strength as compared to samples from the other two materials. However, Material A showed superior deformation behavior.

Abstract: Tooling feature size to minimum thickness becomes small in micro scale products and its ratio affects the deformation behavior in micro sheet forming significantly. In this study, the effect of this relative tooling feature size on drawing characteristics and effects to improve the drawability, such as friction holding effect, hydrodynamic lubrication effect and compression effect by blank edge radial pressure, in micro hydromechanical deep drawing (MHDD) are investigated using plasticity theory and numerical simulation. The results show that the micro drawing characteristics in MHDD can be improved by applying counter pressure. However, the required fluid pressures for friction holding and hydrodynamic lubrication effects increase as the relative punch diameter and/or die shoulder radius to thickness decrease, although the compression effect by radial pressure on the blank edge is independent of the relative tooling feature size.

Abstract: Gradient theories of plasticity play an important role in the description of inelastic behavior of materials. Usually, these theories involve space derivatives of stress or strain. On the other hand, conventional theories of plasticity can be divided into two groups, flow and deformation theories. Each of these groups has its own area of applications. The main conceptual difference between the theories belonging to the different groups is that the primary kinematics variables in deformation theories are displacements (or strains) whereas in flow theories velocities (or strain rates). Therefore, it is of interest to propose a gradient theory of plasticity involving space derivatives of a measure of strain rate (strain-rate gradient theory of plasticity) and to compare qualitative behavior of solutions for the strain-rate gradient theory of plasticity and an existing strain gradient theory of plasticity. One possible strain-rate gradient theory of plasticity is proposed in the present paper. The equivalent strain rate (second invariant of the strain rate tensor) is used as a measure of strain rate. The Laplacian operator is adopted to introduce the gradient term. An analytic solution for expansion of a hollow sphere is given for two strain-rate gradient theories of plasticity and one strain gradient theory. Comparison of the solutions shows that some qualitative features of the solutions for the strain-rate gradient theories are in better agreement with general physical expectations than those for the strain gradient theory.

Abstract: The paper reviews several theoretical and experimental methods for the assessment of ductile fracture criteria and for their application to the fracture prediction in metal forming processes. In particular, distinguished features of two widely used ductile fracture criteria are demonstrated in the case of free surface fracture. Conventional empirical ductile fracture criteria are not compatible with behaviour of plastic solutions in the vicinity of maximum friction surfaces. An approach to overcome this difficulty is discussed. Finally, a theoretical/experimental method to reveal a possible effect of geometric singularities on the applicability of ductile fracture criteria is reviewed.

Abstract: Theoretical solutions for several rigid plastic models used to describe plastic flow in metal forming processes are singular in the vicinity of maximum friction surfaces. In particular, velocity gradients and the equivalent strain rate approach infinity near such surfaces. Such singular behavior can be excluded from consideration by choosing another friction law or material model. However, a different approach is proposed in the present paper. The starting point of this approach is that many experiments show that velocity gradients are very high in the vicinity of surfaces of high friction and that a narrow material layer is formed near such surfaces whose properties are very different from the properties in the bulk. Taking into account that the equivalent strain rate has a significant effect on the evolution of material properties, this experimental fact suggests that a theory based on the singular plastic solutions can be developed to describe the formation of the aforementioned material layer. In the present paper such a theory is proposed to describe the evolution of grain size. It is assumed that, in addition to the equivalent strain rate, the material spin has an effect of the evolution of grain size. It is then shown that the solutions for the material spin are singular as well. The interrelation between the present theory and strain gradient theories of plasticity is discussed. It is shown that it is necessary to account for the strain rate gradient to propose a more adequate theory to deal with the material flow near surfaces of high friction. Some experimental results on the formation of the narrow layer of ultra-fine grains in the vicinity of the fraction surface in extrusion are presented. An illustrative example to relate these experimental results and the new theory is given.

Abstract: The objective of the present paper is to show that predictions of the conventional strain gradient theories do not coincide with some general physical expectations when large strains and geometry changes should be considered. As an alternative, it is proposed to use strain rate gradient theories of plasticity. One possible theory of this type is formulated as a formal modification of a strain gradient theory of plasticity. The problem of hollow sphere expansion at large strains is solved for both the strain gradient and strain rate gradient theories of plasticity. Comparison of these solutions reveals advantages of the strain rate gradient theory of plasticity for a class of problems.