Use of equations of the theory of plasticity of a porous solid for determining stresses in steady-state processes of plastic working of P/M materials

Conclusions A model of a plastic porous solid is proposed based on Beltrami's hypothesis concerning the transition to the plastic state, and equations are derived relating stresses to deformation rates. A method is described, based on the use of coordinate grids for finding deformation rate fields and of equations obtained in this work for the plastic flow of a porous solid, enabling stresses to be determined in steady-state processes of plastic working of P/M materials. The results are presented of a calculation, by this method, of stresses generated in the plastic broaching of holes in porous bushings from PZh2M2 iron powder.Translated from Poroshkovaya Metallurgiya, No. 6(210), pp. 15–21, June, 1980.     

Features of drawing axially symmetric composite wares with a filament core made of nonferrous metals and alloys

The mechanism of plastic deformation of axisymmetric composite wares with dissimilar components during drawing is more complex than with a similar treatment of a monometal. By virtue of the specifics of the deformation mode, the constant nonuniformity of deformation is inherent to axisymmetric drawing; it depends on the reduction in the passage, the die cone angle, the relative length of the calibrating die rib, the friction ratio between the wire and die in the deformation region, the coefficient of connection strength of the core and sleeve interfaces, the relative core size, and the ratio of stresses of the plastic flow of the sleeve and the core. The features of drawing the composite wares with a monofilament and a multifilament core and implementation conditions of the uniform proportional flow of both components without the formation of fractures of the core continuity, the surface sleeve fracture, and the fracture of the ware itself are established by experimental investigations.     

A simplified numerical analysis of the sheet tensile test

A simplified numerical analysis of the tensile test for sheet metal specimens is presented with particular emphasis on the relationship between plastic flow parameters and tensile ductility. This analysis is based on a one-dimensional, “long wavelength” approximation in which the stress state is assumed to be uniaxial throughout the deformation. To account for the influence of necking and triaxiality on the flow behavior, however, an extended Bridgman correction for sheet specimens deformed under conditions of plane stress is employed. The governing equilibrium equation and boundary conditions are discretized to obtain computer solutions. Engineering stress-strain curves and strain distributions in deformed tensile specimens are compared to results from a more complex formulation previously published in the technical literature and show good agreement with it.     

Investigation of the heat resistance and thermal stability of micaceous ceramic materials with additions of boron nitride

Conclusions On the basis of complex investigations it has been established that Brand UMB-5KT materials, having a high heat resistance and satisfactory thermal stability, can be recommended for use in the flowthrough parts of high-head compressors, with a temperature of the gas flow equal to 600–700°C, and in certain types of turbines with a working temperature up to 950–1000°C.It has been established that the requirements for densified materials are most fully satisfied by Brand UMB-5KT material with 4–8% boron nitride.Special graphite K70/30-3 and aluminum-asbestos-graphite layers have sufficient heat resistance only up to 500°C, and cannot be recommended for long-term operation at higher temperatures.Deceased.Translated from Poroshkovaya Metallurgiya, No. 9 (81), pp. 80–86, September, 1969.     

Effect of plastic working on the structural stability of composite (nickel-chromium alloy)-tungsten materials

Conclusions The character of diffusional interaction of the components of tungsten-reinforced heat-resisting sheet composite materials produced by hot forging with subsequent plastic working depends on the latter's degree of plastic deformation. The loss of chromium from the surface of such a CM varies depending on the degree of plastic deformation and on the composition of the matrix alloy. Tungsten fibers stabilize the composition of a CM by acting as obstacles in the paths of chromium diffusion fluxes.Translated from Poroshkovaya Metallurgiya, No. 3(255), pp. 40–44, March, 1984.     

Structure and properties of sintered nickel-molybdenum strip

Conclusions The powder rolling method enables NMoZh-30-5 type alloys to be produced whose mechanical properties are comparable to those of cast and rolled alloys. Uniformity of microstructure and properties in finished strips is achieved by sintering rolled bars for a long time (not less than 10–18 h). By employing cold plastic working with reductions of 20–70% and varying the grain size in the range 0.01–0.03 mm, it it possible to regulate the mechanical properties of such an alloy.Translated from Poroshkovaya Metallurgiya, No. 1(217), pp. 35–41, January, 1981.     

Effect of plastic deformation on the physicomechanical properties of tungsten carbide-cobalt hard alloys

Summary A study was made of the effect of prior plastic deformation on the hardness, strength, coercive force, and electrical resistivity of tungsten carbide-cobalt hard alloys containing 10–25% cobalt. Plastic deformation decreases the hardness of the alloys. Up to a deformation of about 5–6%, all the alloys investigated showed a marked drop in hardness. Further deformation did not decrease the hardness of alloy VK25; for the alloys with lower cobalt contents, the hardness decrease was less pronounced.     

Hydromechanical compression of a porous blank

Conclusions Using a model of plastic flow of porous solids proposed in [1], expressions have been derived with the aid of which it is possible to determine the porosity and geometric dimensions of a cylindrical specimen subjected to hydromechanical compression with a constant lateral pressure and in a closed volume of liquid. To attain the maximum specimen density, deformation should be performed at the lowest possible lateral pressure which does not yet lead to the rupture of the specimen.Translated from Poroshkovaya Metallurgiya, No. 2(278), pp. 11–13, February, 1986.The authors wish to thank V. Z. Spuskanyuk and Yu. A. Palant for helpful discussion.     

Contact phenomena in a sphere-plane single - Crystal system

Conclusions Theoretical considerations, experimentally established facts, and elementary estimates lead to the conclusion that the initial stage of the sintering process may be accompanied by plastic deformation, while a later stage is controlled by diffusion-dislocation flow, in which mass transport responsible for growth of the contact neck takes place under conditions of self-consistency of the dislocation loop generation and dissolution processes. This article is concerned only with monocrystalline objects, but the conclusions arrived at in it can be extended also to real polycrystalline systems. This will necessitate regarding the structural elements of a polycrystal as obstacles to dislocation motion characterized by different degrees of penetrability. An opportunity will thus arise in particular to obtain fuller information on the initial stage of sintering of powder compacts, which are characterized by high sintering activity.Published to stimulate discussion.Translated from Poroshkovaya Metallurgiya, No. 10(214), pp. 14–24, October, 1980.     

Production of sintered deformable (Wrought) nickel-molybdenum alloys I. Investigation of the sintering and structure of alloys and determination of their hot-working conditions

Conclusions A method has been developed for producing nickel-molybdenum alloys with 28–50% Mo by a powder metallurgy process comprising hydrostatic pressing of mechanical Ni + Mo mixtures, sintering of resultant blanks, and their subsequent hot plastic working (forging or rolling) into rods and sheets. A study was made of the workability of nickel-molybdenum alloy blanks in forging and hot rolling. It was found that such alloys with 28–40% Mo are characterized by satisfactory, and NM45 alloy by poorer, workability, while NM50 alloy is not amenable to plastic working. Thus, the nonworkable (when produced by the conventional metallurgical process) 36–40% Mo alloys acquire satisfactory workability through the use of the powder metallurgy technique, which enables them to be employed in industry instead of molybdenum for operation in highly corrosive environments.Translated from Poroshkovaya Metallurgiya, No. 11(239), pp. 30–38, November, 1982.     

Variation of the yield stress of a porous metal during cold deformation

Conclusions An examination is made of individual factors involved in the variation of the yield stress of a porous material during deformation, and a method is proposed for determining them using empirical relationships between the mechanical properties and density of porous materials and information concerning the true variation of the yield stress of the relevant nonporous metal. The method proposed, which allows for the variation of density during deformation and gives values of yield stress which are in good agreement with experimental results, may be recommended for the determination of deformation forces in various production processes for the plastic working of porous materials.Translated from Poroshkovaya Metallurgiya, No. 1(193), pp. 7–13, January, 1979.     

Effect of Purity Levels on the High-Temperature Deformation Characteristics of Severely Deformed Titanium

In the present investigation, high-temperature compression tests were conducted at strain rates of 0.001 to 0.1 s^?1 and at temperatures of 873 K to 1173 K (600 °C to 900 °C) in order to study the hot deformation characteristics and dynamic softening mechanisms of two different grades of commercial purity titanium after severe plastic deformation. It was observed that the effects of deformation rate and temperature are significant on obtained flow stress curves of both grades. Higher compressive strength exhibited by grade 2 titanium at relatively lower deformation temperatures was attributed to the grain boundary characteristics in relation with its lower processing temperature. However, severely deformed grade 4 titanium demonstrated higher compressive strength at relatively higher deformation temperatures (above 800 °C) due to suppressed grain growth via oxygen segregation limiting grain boundary motion. Constitutive equations were established to model the flow behavior, and the validity of the predictions was demonstrated with decent agreement accompanied by average error levels less than 5 pct for all the deformation conditions.     

Model of Plastic Deformation of Powder Materials Taking Account of the Proportion of Contact Volume

A structural model is suggested that takes account of the contact nature of plastic deformation of a powder body. It is shown that in averaging local stresses and strain rates the volume of averaging can be presented as a combination of straight cylinders constructed at all contact areas of a particle and having a common nucleus formed by the intersection of cylindrical bodies. On the whole a plastically deformed powder body is an orientated contact-rod system composed of cylinders that are in contact by their bases and that experience uniform tension-compression deformation. The volume fraction of powder body contact volume determined using Bal'shin's and Zhdanovich's equations can be a quantitative measure of the volume fraction of plastically deformed material. The contact-rod model satisfies boundary conditions for the poured state of powder and it is good agreement with experimental data for isostatically compacted metal powders.     

Creep of titanium monocarbide in its homogeneity range

Conclusions The mechanism operative in the plastic deformation of titanium carbide depends largely on testing conditions. For nonthreshold creep, the rate of deformation is limited by directional diffusion of the metal atoms. In the homogeneity range of this compound, the rate of creep of titanium carbide varies nonmonotonically.Translated from Poroshkovaya Metallurgiya, No. 7 (139), pp. 69–74, July, 1974.     

Thermal expansion of two-phase texturized composite materials

Conclusions Theoretical and experimental studies were made of the effects of temperature and composition on the thermal expansion of deformed (texturized) Mo-Cu two-phase composite materials, whose component phases differ markedly in their properties. The anisotropy of thermal expansion of such composite materials was investigated, and it was established that their volume expansion shows very little variation with deformation. It is demonstrated that two-phase composite materials can be formulated exhibiting thermal expansion characteristics controlled by their composition and the type and degree of plastic deformation.Translated from Poroshkovaya Metallurgiya, No. 3 (111), pp. 82–89, March, 1972.     

An experimental and numerical study of cyclic deformation in metal-matrix composites

The cyclic stress-strain characteristics of discontinuously reinforced metal-matrix composites are studied both experimentally and numerically. The model systems used for investigation are aluminum alloys reinforced with SiC particulates and whiskers. Finite element analyses of the fatigue deformation of the composite are performed within the context of axisymmetric unit cell formulations. Two constitutive relations are used to characterize the matrix of the composite: the fully dense Mises model of an isotropically hardening elastic-viscoplastic solid and the Gurson model of a progressively cavitating elastic-viscoplastic solid (to simulate ductile matrix failure by the nucleation and growth of voids). The brittle reinforcement phase is modeled as elastic, and the interface between the ductile matrix and the reinforcement is taken to be perfectly bonded. The analyses provide insights into the effects of reinforcement shape and concentration on (1) constrained matrix deformation under cyclic loading conditions, (2) cyclic hardening and saturation, (3) the onset and progression of plastic flow and cavitation within the matrix, and (4) cyclic ductility. The numerical predictions of flow strength, strain hardening, evolution of matrix field quantities, and ductility under cyclic loading conditions are compared with those predicted for monotonic tensile deformation and with experimental observations. formerly Visiting Scientist, Division of Engineering, Brown University     

Film Boiling and Water Film Ejection in the Secondary Cooling Zone of the Direct-Chill Casting Process

Accurate thermal modeling of the direct-chill casting process relies nowadays on increasingly complex boundary conditions for the secondary cooling zone. A two-dimensional axisymmetric finite-element model of the direct-chill casting process was developed to quantify the importance of secondary cooling at the surface compared with internal heat conduction within the billet. Boiling water heat transfer at the surface was found to dominate and be the governing factor only when stable film boiling or water film ejection take place; all other cases were dominated by internal heat conduction. The influence of various parameters (casting speed, cooling water flow rate, and thermophysical properties of the cast material) on the occurrence of water film ejection was analyzed. An exponential relationship was found between the cooling water flow rate and the minimum casting speed at which water film ejection takes place.     

Scanning electron microscopical investigation of the fracture of reinforced magnesiumand aluminum-base composites

Conclusions As a result of a fractographic investigation of fracture surfaces of Mg/steel, Mg/B, Al/steel, and Al/B composites it was established that during the extension of a composite material consisting of a ductile matrix and ductile fibers (e.g., Mg/steel or Al/steel) in the temperature range 20–400°C the matrix fails in a tough manner and no significant changes are observed in the morphology of structure of its fracture. The fracture of the fibers occurs with waist formation. Raising the testing temperature intensifies the processes of plastic flow in the fibers, and the latter can therefore be regarded as playing a dominant role in the fracture of the material as a whole. In a composite material consisting of a ductile matrix and brittle fibers (e.g., Mg/B or Al/B) the matrix ruptures in a tough manner while the fibers fail in a brittle manner, by cleavage. At higher testing temperatures the matrix deformation processes become intensified, in consequence of which the fibers experience multiple fractures and the strength of the material falls.Translated from Poroshkovaya Metallurgiya, No. 2(206), pp. 87–90, February, 1980.     

Some features of the fiber-matrix reaction occurring during the manufacture of an aluminum-boron composite material

Conclusions The reaction of the components in the Al-B system may commence at the boundaries of matrix alloy grains emerging to the foil surfaces. In the formation of the matrix-fiber reaction zone during the pressing of the FCM a key role is played by plastic deformation of the foil, which leads to rupture of the surface oxide films along the grain boundaries of the matrix alloy. A physicochemical reaction resulting in the formation of intermetallic compound phases can be induced by plastic deformation of the aluminum foil at low temperatures. The relative size of the reaction zone in the A1-B system can be controlled by varying the grain size of its matrix.Translated from Poroshkovaya Metallurgiya, No. 1(265), pp. 34–36, January, 1985.     

Kinematics and stress-strain state of metal powder issuing from a hopper device. Part I

Conclusions On the basis of CM hypotheses a mathematical problem has been formulated on determining the flow parameters and stress-strain state of a powder issuing from a hopper under the action of gravity and frictional forces. In the solution of the problem incompressibility conditions, equations of motion, and kinematic boundary conditions are satisfied absolutely, while relationships between stresses and deformation speeds having a physical meaning (model of the medium, friction laws) are satisfied approximately by the mismatch minimization method.Translated from Poroshkovaya Metallurgiya, No. 9 (285), pp. 19–22, September, 1986.