Porous Blanks Research Articles

Исследование кинетики формообразования деталей сферического подшипника скольжения из коррозионно-стойких сталей, полученных объемной штамповкой пористых заготовок

Introduction. Spherical powder sliding bearings are widely used in various branches of mechanical engineering. Therefore, the development of a promising method of production of spherical sliding bearing parts from powders of corrosion-resistant steels with specified properties is an urgent task. Purpose of work: is to study the kinetics of forming during cold die forging of spherical sliding bearing parts from stainless steel powder blanks, and to assess the effect of the chemical composition of lubricants and the design of the pressing tool on the structure and properties of the bearing outer ring. Materials from sprayed powders of stainless chromium-nickel steels obtained by cold die forging of sintered blanks coated with lubricants are studied in the work. The following research methods were used: mechanical tensile testing, metallographic studies and cold die forging process simulation. Results and its discussion. It is revealed that the resistance and work of deformation, as well as the kinetics of forming of the outer ring of the spherical sliding bearing are influenced by chemical composition of powders and lubricants, microstructure and mechanical properties of the blank material, configurations of the end surfaces of punches. The top and bottom edges of the outer bearing are most intensively sealed when the punch faces are made with a chamfer angle of 30–40 degrees. With an increase in the relative strain degree by height up to 0.30–0.35 its residual porosity amounted to 0.5–2.0 %. The features of definition of strain state and calculation of strain energy in the implementation of the offered method and the choice of technological parameters of the cold die forging process of sliding bearings parts are shown. A simple method for calculating and experimentally determining the coefficient of contact friction in the process of cold die forging of porous stainless steel blanks is developed, which allows to establish the effect of lubricant composition on the strain resistance at different values of the degree of radial deformation and to develop optimal methods of cold die forging of porous blanks in the production of parts of different complexity.

Kinematic Characteristics of Deformed Porous Structures

Experimental and computational methods of studying the stress state in the plastic region are characterized by various methods and accuracy of measurements, methods of mathematical processing of experimental information, and interpretation of results. The experimentally determined kinematics as a starting point is the most widely used method to study the stress-strain state in the plastic region. When studying the process of plastic deformation of porous blanks, the model of a rigid-plastic isotropic-strengthening porous body with a loading surface that has the shape of an ellipsoid with semi-axes. It depends on the amount of porosity and the ratio of the associated flow law as a mechanical model of the material. In the axisymmetric extrusion of porous blanks, the viscoplasticity method was used to determine the field of flow velocities based on the results of experimental studies. R-functions were applied to approximate experimentally obtained values. The problem of finding approximations was formulated in a variational statement. Cubic splines of one argument were used to interpolate functions. As a result, an approach was proposed, which consists of a particular sequence of calculating the derivatives of the coordinates of the nodes in time in combination with the R-functions approach. All the calculations were performed in Euler variables, eliminating the need to switch from Lagrangian variables and simplifying the solution. Additionally, this method allowed for working with an irregular and non-rectangular grid in areas with any shape of boundaries. This approach is more effective from the point of view of the approximation’s accuracy and the speed of calculations. Finally, the equation for calculating the porosity in the volume of the deformable workpiece based on the information about the distortion of the dividing grid elements was obtained. For stationary axisymmetric processes, a technique was developed that allowed for replacing the calculation of the accumulated deformation of the base material along the deformation trajectory by integration over the region. A technique was developed for determining the stress-strain state at unstable and stable stages of axisymmetric plastic deformation of porous blanks. The calculation results were compared based on the proposed experimental and calculation techniques and the finite element method.

The evolution of the compaction process and the deformed state of porous blanks during their hot forging in the open die

The article presents the results of the study of the evolution of the deformed state of the workpieces, the energetic parameters of the process and the distribution of porosity over the volume of the forging at all stages of hot forging of porous powder forgings in an open die. Modeling of the process was carried out using the finite element method using the DEFORM 2D/3D software complex. As a result of the simulation, it was found that at the initial stage of the process, the blank is mainly compacted with minimal radial flow of the material. Noticeable flow of metal into the groove groove area begins only when the forging material reaches an average relative density exceeding 90%. A significantly different nature of the distribution of axial er and radial ez deformations over the volume of the forging was noted. A zone with increased levels of values of axial deformations is formed in the central layer of the forging, equidistant from the upper and lower surfaces of the die cavity, and the values of radial deformations decrease with distance from the zone bordering the free side surface of the workpiece in the radial (to-center) direction. The minimum values of ez and er appear in the upper and lower angular stagnant zones of the forging. It is shown that the graph of the dependence of the deformation force on the displacement of the punch is marked by the presence of at least three characteristic sections due to the relationship between the processes of compaction and forging shape change at each stage of the process.

Моделювання процесів виготовлення тонкостінних втулок з поруватих заготовок з використанням прямого видавлювання та радіального ущільнення

The deformation process of powder materials thin-walled bushings manufacture was investigated by computer modeling. Two shape formation bushings schemeswere considered (direct extrusion and radial compaction).A continuum approach was used to create a modeling method. The method is based on rheological models of porous body plastic deformation and the finite element method. The accepted material rheological model allows describing the deformation of both powder and porous blanks. It takes into account the different resistance of these materials in tension and compression. Modeling of the deformation process was carried out in stages, using the method of successive loads. The elastic stresses were determined, the plastic potential was calculated and, if it was necessary, the stresses and material parameters of the model were corrected at each load step. The porosity value is reach maximum in blank and area, that is free from the loads, and the accumulated deformation is reachminimum in direct extrusion. The effect of back pressure leads to a more uniform distribution of these parameters, a decrease in porosity and an increase in the accumulated deformation of the solid phase. During radial compaction of thin-walled bushings, deformation of the material occurs locally. Porosity in the product section increases with increasing radius. Increasing the number of technological transitions with a gradual increase in the forming tool diameter reduces the uneven distribution of residual porosity and its value. However, the unevenness of the porosity distribution over the radius remains. In the process of radial compaction, a burr is formed on the ends of the product. The burr can be reduced by changing the initial shape of the blank. The process of direct extrusion allows obtaining more uniform distribution of residual porosity and accumulated plastic deformation of product material. However, this technological process requires the higher loads application, which leads to less stability of the tool. The radial compaction method (which characterized by local deformation) requires not high loads and allows not powerful equipment using. However, the distribution of residual porosity over the radius of the bushing is uneven. Keywords: plasticity theory, powder materials, computer modeling, finite element method, stress-strain state, porosity distribution.

Аналитическая модель равноканального углового прессования титановой губки

Introduction. The use of equal-channel angular pressing (ECAP) of metal powder makes it possible to obtain practically non-porous blanks with high hardness, with a high level of accumulated deformation and with the formation of an ultra-fine-grained structure. A relevant issue for the study of the semi-continuous ECAP process remains a reliable assessment of the energy-power parameters of the process and the prediction of the porosity of compressed materials. This, in turn, is due to the need to develop sufficiently accurate, reliable and simple mathematical models for practical application. The purpose of the work is to develop an analytical model of the process of equal-channel angular pressing of porous material. Powdered screening of spongy titanium of the TG-100 brand was selected as a model of the material for the study. The object of the study is the process of semi-continuous equal-channel angular pressing of axisymmetric porous briquette of titanium sponge in the channel of the mold. It is assumed that the ECAP uses a punch to create back pressure. For the solution, a process scheme, a statically permissible load scheme on a layer of intense deformation and a kinematically permissible flow scheme of a plastically compressible medium in a layer are determined. A system of equations is constructed in accordance with the accepted schemes. The equation power balance is applied. The analytical equation is solved by the method of successive approximations. Finite element simulation of the porous titanium ECAP process was carried out at the angles of intersection of the mold channels at 45°, 50°, 55° and 60°. Results and Discussion. The porosity of the blank is determined at different stages of the ECAP process. A diagram of the change in pressure on the punch using the analytical solution and finite element simulation is obtained. It is revealed that the results of the analytical solution are consistent with the data of the finite element simulation. The highest stress level occurs in the process of equal-channel angular pressing at α = 45°, however, the distribution of relative density over the cross section is most uniform. The maximum value of the pressure on the working punch decreases with an increase in the angle α. Rational technological parameters of pressing porous blanks should provide the maximum permissible pressure on the deforming tool. From this condition, in each specific ECAP process, it is possible to determine the optimal angle value from the analytical solution.

Numerical Analysis of the Die Forging of Porous Blanks in a Die with the Implementation of Active Friction Forces

This paper provides the results of simulating the hot die forging of porous powder preforms with active friction forces applied along the lateral surface of the blank under deformation due to internal cohesion in the die–material system. The study covers the evolution of the distribution of the relative density over the blank cross section at different stages of deformation, the stress-strain state, and the total strain force while varying the boundary loading conditions by changing the initial compression force applied to elastic elements that prevent the die from displacement. It is shown that the active friction forces that act on the periphery of the forged piece adjacent to the die inner side result in areas with a significantly higher deformation intensity compared to deformations in the center of the blank volume. In this case, the volume of the high deformation intensity area and the maximum values of deformation increase with a decrease in the initial compression force of the springs and, accordingly, with an increase in the die displacement value during the deformation. The automatic displacement of the die due to the internal cohesion in the die–deformed material system leads to a decrease in the total deformation force, and the deformation force increases with a decrease in the die displacement value during deformation.

The effect of sodium microalloying on the rolling contact fatigue and mechanical properties of hot-deformed powder steels

Rolling bearing rings offer great opportunities for expanding the powder metallurgy production. At present, these opportunities are not fully realized. Hot forging of porous preforms makes it possible to obtain high-density materials for the manufacture of heavy-duty products, in particular rolling bearing rings. The problem of hot-forged bearing ring manufacturing is associated with a large amount of impurities in initial powders, as well as residual one-side open pores and microcracks in the surface layer of parts caused by cooling down of heated porous blanks in such process operations as hot repressing. The paper considers a potential improvement of mechanical properties and rolling contact endurance of hot-deformed steels with eutectoid composition obtained on the basis of chrome-molybdenum iron powder, as well as unalloyed iron powders with various impurity contents due to microalloying by sodium. Sodium was doped as bicarbonate. The method proposed previously for producing high-density iron-based chromium-bearing powder material was used in order to reduce the probability of heated porous preform oxidation during hot forging (HF). The method involves obtaining cold-pressed blanks with a porosity of 10–12 % with their sintering in a vacuum furnace and subsequent HF. 10×10×55 mm prismatic samples were obtained for mechanical test and structural analysis. Rolling contact endurance was studied using ∅ 26 × 6 mm cylindrical samples. The tests were carried out by running the flat surfaces of cylindrical samples with balls. Doping Na microadditives can significantly increase the rolling contact endurance of powder steels compared to unalloyed samples, as well as with respect to check test pieces made of ShKh15 heat-treated steel due to a decrease in austenite grain size, an increase in the quality of interparticle jointing and a decrease in surface porosity. Carbonaceous powder steels containing the optimum amount of sodium microadditive (0.2 wt.%) can be used to manufacture structural products operating under contact loads.

Oxynitride dendritic strengthening in pre-melted aluminium flakes through controlled in situ nitridation

In situ nitridation of pre-compacted aluminium flakes is shown to be a key to lightweight composites for many engineering applications such as more efficient and cost-effective construction vehicles. This new composite has the mechanical properties of high-strength streels while being ∼3.4 times lighter. This composite was produced by: (i) compaction of micrometric Al flakes into porous blanks, (ii) partial melting through rapid heating to 870 K in N2 and (iii) annealing of the pre-melted blanks at 750 K in N2, where different Al flake deoxidisers were used to control a sufficiently low level of H2O/O2 activating in situ nitridation. The nitrided composites contained stellar dendrites composed of Al, AlN and γ-Al2O3, in which the ceramic fraction increases over the nitridation stage. The traces of H2O in the N2 atmosphere were found to be beneficial for strengthening, due to oxidation and suppression of the composite dendrites from overgrowing and segregation.

Rolling contact fatigue of hot-deformed powder steels with calcium microadditives

Fatigue damage of the surface layers of metal is a characteristic cause of failure of rolling bearings, gears and a number of other machine parts operating under cyclically repeated contact loads. Resistivity to the development of contact damage of steels obtained by hot forging porous blanks is determined by the presence of cohesive bonds between the particles of the base powder, as well as by the presence of non-metallic inclusions and grain size. The possibility of increasing the contact endurance of hot-deformed powder steels due to micro-doping with calcium has been studied. Iron powders with various content of impurities, as well as atomized powder of low-alloyed chromium-molybdenum steel were used as the basis for preparation of the blends. Calcium was doped as calcium carbonate. Mixing was performed in a planetary centrifugal mill. Samples for mechanical testing were obtained by hot forging porous blanks. After hot forging the samples were carburized to compensate for the loss of carbon in the surface layer. It has been established that doping calcium microadditives is favourable for increasing the energy content of damage under the conditions of exposure to contact-fatigue and bending loads. This is due to a decrease in the size of austenite grains at the expense of inhibition of their growth during the adsorption of calcium at the grain boundaries. Microalloying with calcium changes the localization of seats of contact fatigue damage. In samples-witnesses without microadditives of calcium cracks originate near non-metallic inclusions of sharp-angled shape in the near-surface zone. In microalloyed specimens the cracks are located in the subsurface layer in the area of Hertz maximum shear stresses.

Application of ultrasonic testing method for determining defects of hot-deformed powder materials

The article is devoted to the analysis of elastic and plastic characteristics of composite materials during hot stamping. The purpose of this work is to offer optimal conditions for hot plasticity of composite porous material with determination of temperature conditions of hot stamping excluding the appearance of defects in the structure. Production of details of the difficult form by method of hot stamping from preparations of the cylindrical form is followed by development of barrel on a peripheral surface. Sludge sintered porous blanks, and sediment compact material, accompanied by a nonuniform height lateral deformation. In connection with the action of friction forces on the contact surfaces, this leads to the formation of a “barrel”. The heterogeneity of the deformed state is associated with the appearance of tangential tensile stresses on the free surface of the workpiece. If they exceed some critical degree of transverse deformation, cracks appear on the side surface, which leads to gas saturation (oxidation) of the inner layers of the forging, to the ingress of grease into them and its pressing into the volume of the part during hot stamping. In the end, this significantly reduces the properties of hot-stamped parts. Conclusion: the methods of determining the elastic characteristics depending on the geometric parameters of the workpieces, the applied strain energy, body density and temperature dependence of the plasticity characteristics of the hot deformation of the powder material are сonsidered.

Influence of Different pH and Fluoride Addition on the Corrosion Behavior of the Sintered CoCr Alloy Ceramill Sintron Compared to the Cast Alloy Girobond NB

In addition to conventionally casting, dental metallic framework can be manufactured using different CAD/CAM technologies (selective laser melting, milling). The milling of porous CoCr blanks followed by sintering under protective gas is a new 2012 introduced dental technology called Ceramill Sintron. For this new material so far, there exist few studies on the corrosion behavior. The aim of this study was to investigate the influence of different pH values as well as fluoride additions on the corrosion behavior of the sintered CoCr alloy compared to the cast condition by electrochemical corrosion measurements according to ISO 10271.

Polymer composites based on polytetrafluoroethylene modified with motor oil

The production of wear-resistant polymer composites from polytetrafluoroethylene modified with motor oil is considered. Porous polymer blanks are steeped with motor oil and then heated.

Analysing the Twist Extrusion of Porous Blanks Using Modified Theories of Plasticity for Porous Bodies

Features of the twist extrusion of blanks containing pores and plane cracks are considered. The effect of flat cracks on the porosity distribution and rate of local loosening during deformation is analyzed. The analysis is conducted using a modified theory of plasticity for porous bodies and the finite-element method. It is established that the porosity distribution is sensitive to the concentration of flat defects. At a certain concentration of plane defects, the porosity distribution is inverted: the porosity is higher at the periphery than at the center. The effect of flat defects on the accumulated strain and, as a result, on the structure refinement is less significant. It is confirmed that using the counter-pressure allows suppressing the growth of defects, reducing the refinement efficiency.

Компьютерное моделирование процесса горячего изостатического прессования пористых заготовок

Existing mathematical models for the process of HIP porous blanks are based on the solution of approximately differential equations of the equilibrium of quasi-continuous medium. The continual approach is used for computer modeling of the compressible body HIP process. Partial differential equations of motion for a quasi-continuous packing medium and physical equations for a viscous-plastic isotropic porous material subjected to work hardening are taken as the basis for the mathematical model by means of these simultaneous equations. Besides the equations of motion and the rheological equation, the equation of continuity deformation and the equation of heat flow are used. Numerical calculations for additional packing of a high-speed steel powder billet preliminary pressed in the hydrostat are performed. Numerical calculation of the problem for hardening the cylindrical high-speed steel billet with a mild steel shell is done using Lagrange’s method by means of the difference scheme of continuous calculation of the Wilkins’ type. Computer modeling allows to control the process of hardening and changing the form of a porous body during the HIP process.

The structure and phase composition of boride coatings on sintered powder steels

Results of metallographic and X-ray diffraction analyses of boride coatings produced by saturation of porous blanks from iron–carbon powder alloys are provided. It is established that the structure and phase composition of boride coatings made of iron–carbon powder alloys depend on the composition of borating mixtures: the amount of diffusion activator AlF3 in the borating mixture essentially influences the structure, phase composition, and hardness of the coating and a higher amount of the activator intensifies the borating process, leading to the formation of a very hard coating and borocarbides in the transition zone.

Simulation of the Equal-Channel Angular Extrusion of Porous Blanks using Different Deformation Patterns

The results from simulation of equal-channel angular extrusion of sintered porous blanks with use of different deformation patterns are presented. It is shown that the maximum equidensity and the minimum volume of the poorly compacted area are observed in the pattern with a movable bottom plate of the die horizontal channel and with backpressure on the sample portion being extruded. This deformation pattern is also determined by the maximum values of deformation force.

A New Technology of Hardening Porous Materials of Titan Powders

A technology of hardening porous materials of titan powders has been elaborated. The technology is based on passing alternating current with duration of ~10-1…101 s through porous (35…40%) blanks made by method of Sintering by Electric Discharge (SED) by passing a pulse of current with duration of ~10-5…10-3 s. The influence of technological regimes of porous blanks treatment on their structure and properties is investigated. Geometry and dimension of contact necks between powder particles of obtained samples are evaluated. Variations of porosity and strengths as well as microstructure of porous samples materials before and after treatment are investigated. Optimum range of treatment technological regimes is determined within which porosity of 30…35% with maximum strength values.

Side milling cutters with working elements made of high-speed steel powder. II. Properties of cutting materials

Investigations were carried out into the materials for blanks of side milling cutters produced from high-speed steel powder by hot upsetting ring-shaped porous blanks with the centripetal flow of the material. A method of evaluating the quality of material on the basis of the density and specific work of failure of specimens, cut from the discs in different areas and in different directions, was developed and applied for this purpose. The use in the blank of a central insert produced from rolled structural steel results in higher properties (“the head” effect). The results of testing the durability of cutters with working elements produced from high-speed steel powder show that their durability decreases less rapidly with increasing cutting speed than that of the cutters made of rolled material.

Modeling deformation of a porous blank in open-die forging

We present the results of modeling porous cylindrical blanks for open-die forging. We have found that for each size and type of the porous blanks, there are optimal ratios of the initial porosity to the initial clearance between the dies which ensure both complete densification of the blanks and a minimum amount of the material wasted as flash.

Hot forging of sintered porous blanks: L.A. Barkov et al (Technical University, Chelyabinsk, Russia)

Hot forging of sintered porous blanks: L.A. Barkov et al (Technical University, Chelyabinsk, Russia)