Tamotsu Nakamura

FEM Simulation of Friction Testing Method Based on Combined Forward Rod-Backward Can Extrusion

A new friction testing method by combined forward rod-backward can extrusion is proposed in order to evaluate frictional characteristics of lubricants in forging processes. By this method the friction coefficient μ and the friction factor m can be estimated along the container wall and the conical die surface in the forward rod extrusion using theoretical calibration diagrams representing the relationship between the punch travel, the forward rod extrusion length and μ or m without requirements of measuring the forming forces and the flow stress of the workpiece. The theoretical calibration curves are obtained by rigid-plastic FEM simulations in a combined forward rod-backward can extrusion process for a reduction in area R b = 25, 50 and 70 percent in the backward can extrusion. It is confirmed that the friction factor m P on the punch nose in the backward can extrusion has almost no influence on the calibration curves. The optimum initial height/diameter ratio of the workpiece is 1.0-1.1. The influence of the workhardening index on the calibration curves is insignificant in the range 0.1 ≤ n ≤ 0.5. Experimental friction tests are carried out in a mechanical press with aluminium alloy A6061 as the workpiece material and different kinds of lubricants. They confirm the analysis resulting in reasonable values for the friction coefficient and the friction factor.

FEM Simulation of a Friction Testing Metliod Based on Combined Forward Conical Can-Backward Straight Can Extrusion

A new friction testing method based on combined forward conical can-backward straight can extrusion is proposed in order to evaluate friction characteristics in severe metal forming operations. By this method the friction coefficient along the conical punch surface is determined knowing the friction coefficient along the die wall. The latter is determined by a combined forward and backward can extrusion of straight cans. Calibration curves determining the relationship between punch travel, can heights, and friction coefficient for the two tests are calculated based on a rigid-plastic FEM analysis. Experimental friction tests are carried out in a mechanical press with aluminium alloy A6061 as the workpiece material and different kinds of lubricants. They confirm that the theoretical analysis results in reasonable values for the friction coefficient.

A Study of the Lubrication Behavior of Solid Lubricants in the Upsetting Process

Four kinds of solid lubricants were tested in order to examine the frictional characteristics and the yield shear stress by the friction testing apparatus developed by the authors. The frictional shear stresses T f increased approximate linearly with the punch pressure p in every solid lubricant, and the friction coefficients μs were approximately constant. The yield shear stress k was also increased with the punch pressure p. In order in investigate lubrication behaviors of solid lubricants in upsetting processes, FEM simulations for upsetting of circular plates have been carried out. Some experimental upsetting tests of circular plate were tried using some kinds of solid lubricants. It has been confirmed that the solid lubricants can lubricate successfully without metal-to-metal contact when μ 1 , at the interface between tool and solid lubricant is relatively low and μ M at the interface between work piece and solid lubricant is relatively high. On the other hand, metal-to-metal contact occurs easily at the peripheral regions of the work piece, when μ D is relatively high and μ M is relatively low.

FEM analysis of contact mechanism in press-forming of lubricant pre-coated steel sheet

Abstract This paper describes the contact situation between the die and the lubricant pre-coated steel sheet in the press-forming by using FEM simulation. The FEM simulation is carried out by supposing that the lubricant pre-coated steel sheet consists of the lubrication layer and thick galvanized layer. Both the lubrication layer and galvanized layer are assumed as rigid–plastic material. The variations in the contact situation between the die and the lubricant pre-coated steel sheet are investigated by changing the friction coefficient between the die and lubrication layer, the thickness and hardness of lubrication layer, the velocity ratio of the relative sliding velocity to pressing velocity, etc. The simulated results show that the contact area ratio is influenced largely by the average contact pressure, the velocity ratio, the thickness of lubrication layer, and friction coefficient.

An analysis of relationship between contact resistance and fracture of oxide film for connector contacts using finite element method

In automotive connectors, electrical current through the tin-plating flows as a result of fracture of the oxide film due to the expansion on the terminal surface area by contact load, and following metal to metal contact. It has been believed that the fracture can easily occur under mechanical contact of two mating terminals, since the oxide film on the tin-plated surface is only several nm thick. However, the whole process, from the fracture of oxide film, metal to metal contact, to the following decrease of contact resistance, has not been visualized by electrical and mechanical simulation. In the present paper, a method to analyze the oxide film fracture and following decrease of contact resistance was developed by using finite element method, to clarify the process of the fracture. It was found by using a single fine projection contact model, that the fracture of the oxide film took place where deformation of tin-plating was concentrated, since the oxide film was subject to tension by the plastic deformation of tin-plating. The change in the contact resistance after the oxide film fracture was also observed by current-flow analysis. Finally, the sudden decrease of the contact resistance observed at the certain contact load in experiment was successfully explained by the combination of single fine projection and multipoint contact models.

Elastic-plastic Behavior of WC-Co Cemented Carbide Used for Forging Tool Considering Anisotropic Damage and Stress Unilaterality:

Elastic—plastic constitutive equation of hard metal for cold forging tools such as WC-Co cemented carbide with anisotropic damage is proposed to predict a precise service life of cold forging tools. A second rank symmetric damage tensor is introduced in order to express the anisotropic material damage and damage-induced stress unilaterality, namely a salient difference in uniaxial behavior between tension and compression. The conventional framework of irreversible thermodynamics is used to derive the constitutive equation. The Gibbs potential is formulated as a function of stress tensor, damage tensor, isotropic hardening variable, and kinematic hardening variable tensor. The elastic-damage constitutive equation, conjugate forces of damage, isotropic hardening, and kinematic hardening variable is derived from the potential. For the kinematic hardening variable, the superposition of three kinematic hardening laws is employed in order to improve the cyclic behavior of the material. For the evolution equation of the damage tensor, the damage is assumed to progress by fracture of the Co matrix—WC particle interface and by the mechanism of fatigue, i.e., the accumulation of microscopic plastic strain in matrix and particles. By using the constitutive equations, calculation of uniaxial tensile, and compressive test is performed and the results are compared with the experimental ones in the literature. Furthermore, finite element analysis on the behavior of cemented carbide as a die-insert of cold forward extrusion die set was carried out. The proposed constitutive equation was implemented to commercial FE software MSC.Marc2005 using User Subroutines. It is found that the anisotropic damage component can describe the different failure modes of the die-insert, namely fatigue crack and forced rupture.

Synthesis of single-phase polycrystalline Ca2Si powder and sintered compacts

Abstract Orthorhombic polycrystalline Ca2Si sintered compact bulk crystals were synthesized via a two-step treatment, and the experimental evaluation of the electrical and thermoelectric properties of Ca2Si is reported for the first time. First, single-phase Ca2Si powders were synthesized by the heat treatment of Mg2Si powders under a Ca vapor. Then Ca2Si sintered compacts were synthesized by the spark plasma sintering technique using the Ca2Si powders. It was found that single-phase Ca2Si sintered compacts were obtained when the sintering was carried out at temperatures lower than 673 K with high pressure of 300 MPa. The structural, electrical and thermoelectric properties of the Ca-silicide sintered compacts and their dependence on process conditions are discussed.

Effect of Workpiece Surface Topography on Friction in Cold Forging Using Environmentally-Friendly Lubricant

An optimal surface topography for the use of an environmentally friendly lubricant was elucidated, by making a wide combination of surface roughnesses Ra and RSm representing height and frequency of surface asperity, using the wet blast treatment. A forward conical can - backward straight can extruded friction test method proposed by the present authors was used. Coulomb friction coefficient was evaluated using a calibration curve representing the relationship between the surface friction coefficient of the conical punch and the front/back extrusion height of the workpiece. It was found that the surface topography of workpiece largely affects the lubricating property when the friction coefficient is large. Especially, the surface treatment with larger Ra and smaller RSm is effective for improving the friction. Furthermore, the correlation between the friction coefficient and the proposed parameter for the surface topography Ra/RSm was shown to be better than the Ra.

Elastic‐Plastic Constitutive Equation of WC‐Co Cemented Carbides with Anisotropic Damage

Elastic‐plastic constitutive equation of WC‐Co cemented carbides with anisotropic damage is proposed to predict a precise service life of cold forging tools. A 2nd rank symmetric tensor damage tensor is introduced in order to express the stress unilaterality; a salient difference in uniaxial behavior between tension and compression. The conventional framework of irreversible thermodynamics is used to derive the constitutive equation. The Gibbs potential is formulated as a function of stress, damage tensor, isotropic hardening variable and kinematic hardening variable. The elastic‐damage constitutive equation, conjugate forces of damage, isotropic hardening and kinematic hardening variable is derived from the potential. For the kinematic hardening variable, the superposition of three kinematic hardening laws is employed in order to improve the cyclic behavior of the material. For the evolution equation of the damage tensor, the damage is assumed to progress by fracture of the Co matrix — WC particle interf...

Finite Element Analysis of V-Bending of Polypropylene Using Hydrostatic-Pressure-Dependent Plastic Constitutive Equation *

Finite element analysis of V-bending process of polypropylene was performed using hydrostatic-dependent elastic-plastic constitutive equations proposed by the present authors. Kinematic and isotropic hardening rule was employed for the plastic constitutive equations. The kinematic hardening rule was more suitable for the expression of the stress reversal in uniaxial stress - strain relation than the isotropic hardening. For the result of the finite element analysis of V-bending, the kinematic hardening rule was able to predict the experimental behavior of springback more properly than the isotropic hardening. Moreover, the effects of hydrostatic pressure-dependence were revealed by examining the calculated distribution of bending plastic strain, bending stress and the width of the bent specimen.