Plastic Flow Of Metals Research Articles

High-resolution measurement of contact stresses and friction in indentation and machining by full-field photoelasticity

An experimental study has been made of the sliding plastic contact in wedge indentation and machining of ductile metals. The use of full-field photoelasticity has allowed direct determination of interface pressure and shear stress distribution along the tool (indenter) and workpiece sliding contact at micron-scale resolution. Sapphire, a transparent photoelastic material that possesses sufficiently high hardness to indent and machine metals, is used as the tool material. It is shown that the Coulomb friction model applies only at the edges of the contact where the interface pressure falls below a certain value associated with the material's flow stress. Within the plastic contact zone, the mode of friction as well as the relationship between interface pressure and shear stress are very complex. In both cases, it is shown that shear stress neither remains constant nor varies in proportion to the pressure. It is proposed that the complex nature of friction and stress field at the sliding contact can be understood through simultaneous observations of the metal plastic flow at and near the interface. Using in situ measurements of the velocity field near the interface and local stress distribution at the contact from photoelasticity, friction contribution to the overall work is calculated and shown to be substantial (greater than one-third) in the case of machining.

Junction formation rates, residence times, and the rate of plastic flow in FCC metals

During plastic flow in metals, dislocations from slip systems with different glide planes collide to form junctions. After being in-residence within the dislocation network for some period of time, these junctions then break, thereby liberating the attached dislocation lines. In this work we use random forest discrete dislocation dynamics simulations to quantify the junction formation rate and junction residence time as a function of stress for all junction types in face-centered cubic metals. We then relate these quantities to the dislocation link-length distribution, which is found to exhibit an exponential form. This enables us to quantify the mean junction strength and also the slip system interaction coefficients. Finally, using the link-length model we obtain a flow rule for our systems which is physics-based with all parameters determined from DDD simulations. The insights here provide a path forward for a dislocation network theory of plastic flow based on the link-length distribution.

Simultaneous enhancement in mechanical and physical properties of graphene-oxide-reinforced Al matrix composites by trace-Mg-alloying strategy

This paper proposes a trace-Mg-alloying strategy to fabricate high-performance graphene oxide (GO)-reinforced Al metal matrix composites (MMC). A minute quantity (0.4 wt%) of elemental Mg was introduced in Al–Si alloy powders. Individual GO nanosheets were uniformly decorated onto the surface of the AlSiMg particle through electrostatic attractions via hetero-agglomeration. A GO network was formed at the primary boundaries of AlSiMg particles through spark plasma sintering of GO/AlSiMg mixed powders. In this process, elemental Mg reacted with the oxygen atoms within GO nanosheets, resulting in the in situ production of high-quality reduced GO. Subsequent hot extrusion disrupted the GO network owing to the metal plastic flow, causing the GO nanosheets to realign in a single direction. Consequently, the tribological behavior, tensile response, and thermal and electrical conductivities of the GO/AlSiMg composites were simultaneously enhanced. Hot extrusion led to stretching of the overlapping GO nanosheets and increased surface contact area of AlSiMg powders. Thus, the extruded samples exhibited peak electrical conductivity at 0.2 wt% GO, whereas the sintered samples demonstrated peak thermal conductivity at 0.1 wt% GO. Overall, the findings of this study highlight the potential of merging trace powder alloying and powder metallurgy techniques to create multifunctional nanocarbon/MMCs without a trade-off between mechanical and physical performance.

Development of a method for measuring pulse currents of large magnitude

The paper presents the results of a study on the search for correct methods for measuring a high-value current pulse, which will be used to conduct research on the electroplastic effect. The electroplastic effect is the effect of electric current pulses on the plastic flow of metals. Electroplastic metal forming technology is a relatively new metal forming process that is energy efficient, environmentally friendly and versatile. In particular, it can be used to process metals or alloys that are difficult to process using conventional manufacturing processes. For the experimental study of the electroplastic effect, it became necessary to measure pulse currents of large magnitude, not only in amplitude, but also in the shape of the pulse. The pulsed current causes the formation of an alternating electromagnetic field near the conductors, so it can be measured with a Rogovsky current transformer. The results of the work present a schematic electrical diagram and a photograph with the appearance of an experimental installation for the study of the electroplastic effect. The results of measurements of the current value, the voltage drop on the sample and the dependence of the peak voltage values on the sample on the peak current value are shown. After making calculations and renormalising the data for the voltage drop on the sample according to the peak value of the current obtained on the transformer, the authors obtained the desired current values. The error of this method is estimated by calculating the total capacitance of capacitors, which does not exceed 2%.

Probing joining mechanism of Ti6Al4V - SS316L steel rods in pressure-controlled joule-heat forge welding

Ti6Al4V-SS316L steel joints are vital in automotive and nuclear industries due to lightweight, creep, and corrosive resistivity; however, their fusion-based weldments readily form brittle phases under uncontrolled melt-solidification dynamics. Plasticity-induced solid-state welds are recourses, but uneven softening and metal upset remain intricate and cause failure. Pressure-controlled joule-heat forge welding offers plastic deformation with the maneuvered pressure, current and forging strokes. Experimental studies reveal local homogeneous plastic flow of dissimilar metal interfaces for a high-strength metallurgical bond at low temperatures using this process. However, the underlying weld mechanics remains to be uncovered. Thus, to address this shortcoming, finite-element-based calculations of concurrent electric resistive heating and pressure-displacement-induced plastic flow of dissimilar interfaces are modeled by a coupled electric, thermal and dynamic stress analysis. The estimated transient thermal histories and different weld geometries provide insight into the joining procedure and are later validated to the measured results. The mechanics of locally controlled plastic deformation are illustrated with evolved stress-strain transients that reasonably indicate differential responses of dissimilar cross-sections to the applied forging, except at the interface that forms metallic joints. Further, the computed results show a homogeneous temperature increment rate at the center and periphery of the deformed cross-sections. The microscopically observed weld structures indicate dynamic recrystallization and smooth hardness transit across bimodal α-β phases with elongated titanium α-grains and refined annealed twin austenite in steel.

Additional effect on the deformation zone during plastic metal flow

In the article, on the basis of previously conducted theoretical and experimental studies, a phys-ical model of the process of additional impact on the basis of non-uniform plastic deformation is developed and investigated. On the basis of the physical model, a mathematical model of the additional impact is developed, the generalizing factor of which in the case of asymmetric de-formation is the external moment from the side lines.

Application of recurrence quantification analysis of acoustic emission time series to analysis of a plastic flow of metals.

As a result of the application of recurrence quantification analysis to acoustic emission time series obtained in uniaxial tensile testing of copper and silver, we detected the existence of a characteristic interval in which the Shannon information entropy (as a parameter of the quantitative analysis of recurrence plots) increased rapidly. Using statistical analysis of the behavior of the dislocation ensemble, we established a relation between the physical parameters of the given interval and the global stability loss parameters of the plastic flow of metals, indicating the predictability of the distinctive point determined long before the critical state was attained.

Numerical and experimental investigation of pressure-controlled joule-heat forge welding of steel tubes

Joining metallic tubes without melting is an enroute to impermeable and high-strength joints for lightweight and transport applications. Friction-based joining processes avoid melting and solidification defects, although the generated metallurgical bonding remains critical due to the uncontrolled differential interface heating and plastic deformation. Pressure-controlled joule-heat forge welding obtains a joint in a solid state with uniform plastic deformation of faying interfaces. A short-duration high-current pulse is discharged across the metallic tube assembly under the electrically conducting copper-steel plates. A high-pressure contact and thermal softening allow a radially outward metal plastic flow with the expulsion of oxide films. The joint evolution is further controlled with the pushing rate to a set distance. This study marks the novel effort to examine the thin-walled steel tubes joining with an integrated experimental and numerical approach. The computational analysis based on the finite element approach reveals the time instants of the interfacial contact pressure, thermal softening, and plastic flow behavior for the joint formation. The welded and upset-out region indicates uniform recrystallization of radially aligned grains with a consistent microhardness distribution. The actual welded geometry dimensions matched fairly well with the calculated dimensions.

A new anisotropic-asymmetric yield criterion covering wider stress states in sheet metal forming

The stress state-dependent plastic anisotropy of sheet metals has drawn significant attention. Nevertheless, existing phenomenological plasticity models limitedly capture the distinctive yield strengths and plastic flow (r-values) under a wide range of stress states covering uniaxial tension (UT), equi-biaxial tension (EBT), uniaxial compression (UC), plane strain tension (PS), and pure shear (SH). This study aims to propose a new anisotropic-asymmetric plasticity model with enhanced flexibility based on the non-associated flow rule. The yield function is formulated based on the additive coupling of two stress potentials: coupling between a new anisotropic pressure-sensitive fourth-order polynomial function, and an isotropic stress invariant-based function. Then, all the anisotropic/asymmetric parameters of the yield function can be analytically identified from 7 yield stresses measured from UT tests along 0°, 45°, and 90° to the rolling direction (RD), UC tests along 0° and 90° to the RD, EBT test, and SH test along 45° to the RD. Within the scheme of no-associated flow rule, a new asymmetric fourth-order plastic potential function is proposed to analytically describe the plastic flow of sheet metals under UT, UC, EBT and SH. The additively coupled stress invariant-based term is introduced to adjust the yield stresses and plastic flow under biaxial tension including PS, which leads to an independent description of the yield stresses and plastic strain directions under SH and PS states. Various automotive sheet metals such as advanced high-strength steels, aluminum alloy, and magnesium alloy with strong asymmetry and anisotropy are investigated to verify the proposed yield criterion. The results demonstrate remarkable flexibility and accuracy of the proposed yield criterion against other existing models. Evolving yield surface and plastic potential of a quenching and partitioning steel are accurately captured by the proposed anisotropic-asymmetric yield criterion. Besides, the advantages of new model are further discussed in terms of the proof of yield surface convexity, identified parameters, and the regulation between anisotropy and isotropy in comparison with existing multiplicative coupling method.

Methodology and technological equipment development for treatment processes metal plastic flow research. Part 2. Special equipment and methodology development for experiment research by digital image correlation method

The metal plastic flow regularities experimental estimation, as well as the residual stresses (RS) values in the treatment processes is an important task to correctness justification of obtained finite element modeling solutions. However, the currently known and used plastic flow regularities and RS experimental determination methods in most cases have a significant disadvantages: labor intensity, low accuracy, sample preparation necessity and difficult to apply on real parts. This work purpose is to develop special equipment and methods for researches by the digital image correlation method for conducting experimental metal plastic flow and RS researches during free orthogonal cutting and surface plastic deformation (SPD) under the technological treatment factors variation. The paper substantiates orthogonal loading schemes in the metal plastic flow research in the cutting and SPD processes, research accessories and equipment is developed. The machine movement errors were determined, the sample sizes and loading routines were determined and justified. Using the digital image correlation method, the metal plastic flow experimental researches in the deformation zone during free orthogonal cutting and surface plastic deformation processes were performed. For experimental samples RS determinination after free orthogonal cutting and surface plastic deformation, the digital image correlation and probing hole drilling methods combination was used for the first time, which idea is to determine the material particles movements on the sample top from two images: the sample surface having residual stresses and the same surface after high-speed probing hole.

Monitoring of vacuum diffusion welding process of titanium alloy using mathematical modelling

The results of evaluation and analysis of technological modes of diffusion welding in vacuum of titanium alloy VT6 using mathematical modeling are presented. Within the framework of the general concept of mathematical modeling existing in the description of powder metallurgy processes, a mathematical model was used to solve planar problems of the plastic flow of metal, which allows us to study the stress-strain state of real materials in the process of their formation. To solve the flat problems of plastic metal flow, a developed technique was used, which allows calculating and having experimental rheological dependencies, as well as mechanical test data, to choose the deformation modes optimal from the point of view of the quality of joints when they are obtained by diffusion welding in vacuum. The results obtained made it possible to optimize the parameters of diffusion welding of titanium powder blanks.

Nanoscale deformation of crystalline metals: Experiments and simulations

In the present work, we performed experiments and molecular dynamic simulations to study the atomic physical process and deformation mechanism involved in nanoscale deformation of crystalline metals. By contact deforming bulk metals using hard molds with different cavity sizes at various temperatures and different stress levels, the flow of metals (e.g., Au and Ag) into nanocavities is quantified by the aspect ratio of molded micro-/nanorods, based on which the dependence of deformability on cavity size, temperature and stress is revealed. It is found that the critical forming pressure is determined by the entering barrier. Once the entering barrier can be overcome, quantified by the aspect ratio of metal nanorods, the molding efficiency increases with the cavity size decreasing. Moreover, three deformation modes are uncovered in the nanoscale plastic flow of metals and directly related with the atomic physical processes, i.e. atomic diffusion, dislocation nucleation, multiplication and movement. In the dislocation dominated temperature regime, a transition of deformation mode from burst growth to continuous growth is observed as the cavity size increases. When the mold cavity size is very small (100-101 nm), the metal atoms flowing in a mold cavity will arrange amorphously even at low temperature, which usually only occurs at high temperature for large cavity sizes. Therefore, reducing the size shows the same effect as increasing the temperature, making nanomolding easier.

Incipience of Plastic Flow in Aluminum with Nanopores: Molecular Dynamics and Machine-Learning-Based Description

Incipience of plastic flow in nanoporous metals under tension is an important point for the development of mechanical models of dynamic (spall) fracture. Here we study axisymmetric deformation with tension of nanoporous aluminum with different shapes and sizes of nanopores by means of molecular dynamics (MD) simulations. Random deformation paths explore a sector of tensile loading in the deformation space. The obtained MD data are used to train an artificial neural network (ANN), which approximates both an elastic stress–strain relationship in the form of tensor equation of state and a nucleation strain distance function. This ANN allows us to describe the elastic stage of deformation and the transition to the plastic flow, while the following plastic deformation and growth of pores are described by means of a kinetic model of plasticity and fracture. The parameters of this plasticity and fracture model are identified by the statistical Bayesian approach, using MD curves as the training data set. The present research uses a machine-learning-based approximation of MD data to propose a possible framework for construction of mechanical models of spall fracture in metals.

Rate-Controlling Microplastic Processes during Plastic Flow in FCC Metals: Origin of the Variation of Strain Rate Sensitivity in Aluminum from 78 to 300 K

The thermodynamic response of dislocation intersections with forest dislocations and other deformation products is recorded using the Eyring rate relation wherein the application of shear stress increases the probability of activation at a given strain rate and temperature. The inverse activation volume, 1/ν, can be directly determined by instantaneous strain-rate change and its dependence on flow stress, τ, defines the strain-rate sensitivity, S, through the Haasen plot slope. A linear slope over a large strain interval is observed even for a heterogeneous distribution of obstacles that could be of more than one type of obstacles encountered by the gliding dislocation. It was deduced that ν and τ at each activation site are coordinated by the internal stress resulting in constant activation work (k/S). The stress changes from down-rate changes become larger than that from up-rate changes due to the formation of weaker obstacles, resulting in a composite S, whereas only forest dislocations are detected by the up-change. The additivity of 1/ν was used to separate obstacle species in specially prepared AA1100 and super-pure aluminum from 78 to 300 K. The deduction that repulsive intersection is the rate-controlling process and creates vacancies at each intersection site depending on temperature was validated by observing the pinning and depinning of dislocations via pipe diffusion above 125 K. A new method to separate S for dislocation-dislocation intersections from the intersections with other obstacles and their temperature dependence is presented and validated.

Effect of Surface Dissolution on Dislocation Activation in Stressed FeSi6.5 Steel.

The effects of surface dissolution on dislocation activation in FeSi6.5 steel are quantitatively studied by analyzing the stress relaxation data using the thermal activation theory of dislocation. The stressed FeSi6.5 steel sample in acid solutions (H2SO4 or HCl) exhibits a much higher rate of stress reduction with time compared with that in air or deionized water. As the stress relaxation time is prolonged to 20 min, the relaxation rates are 0.055 MPa·min-1 in water and 0.074, 0.1, 0.11 MPa·min-1 in H2SO4 solutions with pH 4, 3, and 2, respectively. In a NaCl solution, a slight increase in the relaxation rate compared with air was found. Higher acidity (lower pH) of the solution inducing higher stress relaxation rate implies the softening is associated with the anodic dissolution of the surface layer and the accelerated (additional) flow of dislocations. The analyses using the thermal activation theory of dislocation during relaxation reveal the mechanism for the accelerated plastic flow induced by the corrosive medium. The variations of these parameters are related to the relaxation of the stress field of dislocations and the weakening of interaction between slip dislocations and short-range obstacles. The chemomechanical effect, including a reduction in apparent activation energy and a decrease in waiting time for dislocation to obtain sufficient thermal activation energy to cross obstacles, causes an increase in the stress relaxation rate (plastic strain rate). The study confirms that surface dissolution accelerates the plastic flow of metals and supports the view that surface dissolution facilitates dislocation slip. It is helpful to improve the formability of brittle metals.

Quasiplastic deformation in shocked nanocrystalline boron carbide: Grain boundary sliding and local amorphization

Materials respond to shock in many ways such as plastic flow in metals and amorphization in ceramics. It is very challenging to characterize the deformation mechanisms of ceramics under shock compression due to extreme loading conditions. Here we report the shock response of nanocrystalline boron carbide (n-B4C) using large-scale molecular dynamics simulations with a machine-learning force field. We identify three quasiplastic deformation mechanisms in shocked n-B4C: grain boundary (GB) sliding, intergranular amorphization, and intragranular amorphization. As the deformation mechanism changes from GB sliding to intergranular amorphization at ∼40 GPa, a bilinear Hugoniot behavior occurs, consistent with experimental observations. At higher pressure (>∼100 GPa), intragranular amorphization becomes dominant, causing a complete loss of shear strength. These quasiplastic mechanisms may play an important role in the shock behaviors of ceramics.

Деформации в нестационарной стадии прессования прутка из алюминиевого сплава с малым коэффициентом вытяжки

Introduction. It is noted that extrusion is the main procurement process in the aluminum alloys forming operations. At the same time, the process has such a disadvantage as the nonstationarity of the metal plastic flow. The work aim is to establish the inhomogeneity deformation level of the pressed rod front part by numerical simulation using the finite element method. The study objectives are to formulate the extrusion process boundary conditions, to obtain a solution and to evaluate the inhomogeneity degree. Research methods: the finite element method was used to evaluate the deformed state. The actions sequence included the creation of primary deformation zone shape and the tool configuration. The mutual movement of the tool and the deformable material is set using the appropriate boundary conditions. The deformable medium is a ductile material with power-law hardening, the physical and mechanical properties correspond to the aluminum alloy of the 6000 series. Results and discussion: It is revealed that the strain degree in the pressed rod front part is extremely nonuniform distributed; differences above 300% are recorded. The strain degree distribution dependences in the rod cross sections are constructed depending on the distance from the front end at different relative radial coordinates. It is revealed that the rod central layers acquire a constant level of the strain degree earlier than the peripheral layers. The stationary process is achieved with less metal motion. The work result application scope is the technological study of rational metal cutting of aluminum alloys at the extrusion final stage in order to use recyclable waste more rationally. Conclusions. In the extrusion process with a low elongation ratio, the strain degree is distributed nonuniform both along the press rod cross and along its length. The rod front part remains weakly deformed both at the periphery and in the center in the nonstationary initial extrusion stage. It often forces to send for remelting due to the insufficiently developed metal structure. At the same time, if the limits on the minimum possible degree of deformation are set, then using the results of the calculation by the finite element method, the minimum length of the metal to be removed can be set, thereby reducing the mass of waste sent for remelting.

IDENTIFICATION OF PARAMETERS FOR CHARACTERIZATION AT HIGH-TEMPERATURE OF 38MnVS6 STEEL USED IN HOT FORGING PROCESSES

Finite element reliability is highly sensitive to the input data quality and constitutive equations used. In this article, two constitutive equations for modeling the plastic flow of metals are used. Two equations commonly employed in metal forming finite element analysis were selected, e.g., the Hensel-Spittel and Johnson-Cook. Compression tests were conducted at various temperatures and strain rates to obtain the plastic flow stress curves. These experimental results were used as a target to fit the constitutive equation parameters. The parameter identification procedure was conducted by solving the so-called inverse problem. An objective function gives a value (error) that indicates how well a parameter set makes the equation fits the experimental results. This error value is minimized by applying optimization techniques; in this case, by using a Genetic Algorithm followed by a gradient-based strategy. Two objective functions are used; the first is based on the well-established least square approach, and the second is proposed. The proposed objective function shows a faster performance than the least square. In addition, the proposed objective function also gives the best fit for the Hensel-Spittel equation. The Johnson-Cook equation is fitted using only the proposed function. The Hensel-Spittel equation shows several local minimums, whereas the Johnson-Cook shows a global one. Keywords: Hot forging, Parameter identification, constitutive equations, finite element analysis.

DETERMINATION OF KINEMATIC CHARACTERISTICS OF METAL PLASTIC FLOW UNDER CONDITIONS OF AXIAL SYMMETRY OF THE DEFORMATION PROCESS

With axisymmetric deformation, the deformable body and the load have a common axis of symmetry. Such deformation occurs in numerous technological operations. About 70% of parts obtained by cold extrusion are deformed under conditions of axial symmetry. The plane problem of the theory of plasticity is reduced to a solution in the framework of a two-dimensional statement, when the motion of points in the section of a workpiece is analyzed. Each point can move only in the section plane, and its velocity can be decomposed into two mutually perpendicular directions along the coordinate axes. For plane deformation, it is assumed that the velocity in the direction of the third coordinate axis is equal to zero. The presence of axial symmetry allows us to confine ourselves to studying the behavior of points located on the plane of the meridional section of the workpiece. In this case, each point can move only in the section plane and its velocity can be decomposed into two orthogonal directions: along the axis and along the radius. The component of the velocity vector in the circumferential direction is equal to zero, so only four of the six independent components of the strain rate tensor remain. In this regard, axisymmetric problems of the theory of plasticity are of considerable interest from the point of view of solving applied problems. The article discusses the possibility of using mixed Euler and Lagrange coordinates to determine the components of the strain rate tensor in plastic deformation processes characterized by the axisymmetric nature of metal plastic flow. The vector field of displacements at each point in space reflects the transformation of the initial (non-deformed) configuration into the current one, and therefore determines the configuration of the deformed body in space in a certain frame of reference. The deformation process is first considered in a single Cartesian coordinate system, and then in a cylindrical coordinate system, the use of which is more appropriate for axisymmetric deformation. It is assumed that the functions of the Euler coordinates from the Lagrange coordinates are obtained by approximating the experimental data.

The Influence of Tool Shape on Plastic Metal Flow, Microstructure and Properties of Friction Stir Welded 2024 Aluminum Alloy Joints

In this paper, the effect of different shapes of tool pin on the plastic flow of 2024-T6 aluminum alloy during friction stir welding was studied. In order to observe the plastic flow of materials more clearly, we chose the method of friction stir welding of dissimilar materials, considering the different corrosive characteristics of aluminum alloys made of different materials when exposed to the same corrosive liquid. By studying and comparing the temperature field, macro and microstructure, microhardness and tensile properties of welded joints, the results indicated that the metal in the weld nugget zone (WNZ) mainly came from the base metal of the advancing side, the thread being the driving force of the downward movement of the FSW plastic metal. The deep groove thread tool pin had the strongest ability to drive the metal downward. The conical cam thread tool pin had the strongest stirring effect on materials and the best metal fluidity. The macroscopic morphology, microstructure, mechanical properties and fracture morphology of different joints were analyzed, and the results showed that all joints could form an excellent union, with an onion ring pattern appearing in cross-section. The minimum grain size of the WNZ formed by the conical cam thread stirring head was 7~12 μm; the hardness was least at the junction of the heat affected zone (HAZ) and the thermo-mechanically affected zone (TMAZ). However, the hardness of the weld formed by the conical cam thread at this point was higher than that of other stirring heads; the tensile strength of all joints was more than 80% of the BM, and the maximum tensile strength of the joint welded by the conical cam thread tool pin was 364.27 MPa, accounting for 86.73% of the base metal (BM). The elongation after break was 14.95%. Tensile fracture morphology analysis showed that all joints were fractured by plastic fracture.