Metal Deformation Processes Research Articles

Pore-scale numerical modelling of large deformation behaviour of sintered porous metals under compression using computed microtomography

This work concerns the numerical modelling of the deformation process of porous sintered metals, considering the shape of pores on the mesoscopic scale based on microtomography. Sintered 316L steel with various porosity values was used. Limited microtomography device accuracy for porous metals studies results in a lack of fissure mapping and pores with smaller dimensions than the pixel size of the tomographic image. For this reason, two methods were used in this work to compensate for the influence of not mapping the geometric details of porous mesostructures onto the results of numerical calculations. Based on the microtomographic cross-sections obtained, three-dimensional geometric models mapping the shape of the porous structure of the studied materials were created. The models were used for numerical calculations of sintered steel compression behaviour using the finite element method. The modelling also considers self-contact of the porous structure and the effect of closing the pores during compression. As a result, macroscopic stress-strain curves for the studied porous structures and distributions of stress and strain in the deformed material were obtained. Based on the analysis of the numerical calculation results, the deformation process for porous sinters in the range of large plastic strain was described and the influence of porous structure deformation on the mesoscopic scale on the material behaviour in the macroscale was determined. The work also compares the results of tests obtained by three calculation models and points out the advantages and disadvantages of their application to numerical modelling of the deformation process of porous metals.

Acquisition of Dynamic Material Properties in the Electrohydraulic Forming Process Using Artificial Neural Network.

Electrohydraulic forming is a high-velocity forming process that deforms sheet metals with velocities above 100 m/s and strain rates more than 100 s−1. This experiment was conducted in a closed space because of safety concerns related to the high-velocity conditions; therefore, we were not able to examine the deformation process of the sheet metal. To observe the electrohydraulic forming process in detail, we performed virtual numerical simulations using accurate material properties. Therefore, in this paper, we obtained the material property of a sheet metal from a numerical estimation by using a surrogate model based on the reduced order model and the artificial neural network. The Cowper–Symonds constitutive equation was selected for the Al 6061-T6 sheet metal, and two strain rate parameters were adopted as the unknown parameters. From the two sampling techniques, the training and test samples were extracted from the specific ranges of two unknown parameters, and a numerical simulation was performed for these samples by using the LS-DYNA program. The z-axis displacements of the deformed sheet metal were obtained from the results of the numerical simulation, and two basis vectors were extracted by using principal component analysis. In addition, to predict the weighting coefficients of the two basis vectors at the defined range of parameters, we used the artificial neural network technique as a surrogate model. By comparing the surrogate model and the experimental results and calculating the root mean square error value, we estimated the optimal parameter for Al 6061-T6. Finally, the reliability of the obtained material parameters was proved by comparing the experimental results, the surrogate model, and LS-DYNA.

Dynamic high pressure torsion: A novel technique for dynamic severe plastic deformation

Abstract Severe plastic deformation (SPD) processing of polycrystalline metals is often used to obtain ultra-fine grained and nano-crystalline microstructures. Increasing the strain rate during the SPD process is known to have a favorable effect on both grain fragmentation and mechanical properties, mainly through its effect on dislocation densities and twinning. However, dynamic SPD is barely explored, mainly because of the lack of suitable experimental techniques. In the present paper, a novel dynamic high pressure torsion facility is presented which imposes a dynamic, torsional deformation to a sample subjected to high hydrostatic pressure. The setup combines basic features of both split Hopkinson torsion bar (SHTB) and static high pressure torsion (HPT) devices. Torsional speeds up to 30,000 rpm can be reached, giving rise to strain rates in the sample up to 10,000/s. The first series of tests have been performed on commercially pure aluminum. The effect of DHPT processing on the mechanical properties and microstructure is evaluated by hardness and electron back-scatter diffraction measurements, respectively. The properties and microstructural features are compared with those obtained in conventional, statically deformed HPT samples.

Numerical analysis in a beverage can utilizing tube hydroforming process

Numerical analysis for Tube Hydroforming (THF) was developed in this work to predict the behavior of extruded aluminum tube in a forming die for beverage can applications. THF is a metal forming process dependent of three parameters: friction between the tube and the die, internal pressure, and material properties of the tube. Strain hardening is a governing phenomenon that occurs in the plastic deformation process of metals. Hollomon’s equation offers a mathematical description of the metal behavior in the plastic zone. For a proper simulation, experimental determination of the mechanical properties of aluminum 6061-T5 were conducted and test specimens where obtained directly from the aluminum tube. Experimental data were necessary because no sufficient data of the mechanical properties of the tube were available in the literature. Numerical simulations of THF were performed, and the results were compared with analytical results for validation purposes with less than 10% of error.

Temperature and strain rate dependent friction model for hot forming of Ti-6Al-4V titanium alloy

Abstract Friction plays a significant role in hot deformation processes of metals. The effect of friction is especially important during hot deformation of titanium alloys. In this study friction factors in Levanov and shear laws during deformation of the Ti-6Al-4V titanium alloy at the temperature of 925 °C without lubrication, with lamellar graphite, tungsten disulfide and hexagonal boron nitride lubrication were studied. Effect of temperature in range of 825 °C–925 °C and strain rate in range of 0.01 s−1–1 s−1 on the friction factor with hexagonal boron nitride lubrication also was studied.

Process mechanism in laser peen texturing artificial joint material

Friction and wear of artificial joints seriously restrict the service life of them. Surface texturing has been recognized as an effective means to reduce friction coefficient and improve wear resistance of artificial joints. In this study, a new and promising technology named laser peen texturing (LPT) is proposed to fabricate dimple textures on artificial joints. Process applicability of LPT on artificial joint material 316 L stainless steel is studied by experiments. Experimental results show that LPT is capable of fabricating dimple arrays on 316 L stainless steel and has good controllability and repeatability. Fundamental process mechanism of LPT is explored by numerical simulation. A 2D axisymmetric and semi-infinite finite element analysis (FEA) model has been developed and validated to predict the stress distribution and deformation process of the biomedical metal. The simulated dimple profiles are well consistent with the measured ones. Stress analysis shows obvious residual compressive stress after LPT. The maximum residual compressive stress and the thickness of the residual compressive stress zone are sensitive to LPT processing parameters. The local dynamic elasto-plastic deformation process of the material is discussed based on the deformation analysis at different time.

Viscous Shear Banding in Cutting of Metals

Shear banding is a type of plastic flow instability with often adverse implications for cutting and deformation processing of metals. Here, we study the mechanics of plastic flow evolution within single shear bands in Ti- and Ni-based alloy systems. The local shear band displacement profiles are quantitatively mapped at high resolution using a special micromarker technique. The results show that shear bands, once nucleated, evolve by a universal viscous sliding mechanism that is independent of microstructural details. The evolution of local deformation around the band is accurately captured by a momentum diffusion equation based on a Bingham-type flow rule. The predicted band viscosity is very small, compared to those of liquid metals. A plausible explanation for this small viscosity and fluid-like behavior at the band, based on phonon drag, is presented.

Microstructure and Strain Rate-Dependent Tensile Deformation Behavior of Fiber Laser-Welded Butt Joints of Dual-Phase Steels

The microstructure and tensile deformation behavior of the fiber laser-welded similar and dissimilar dual-phase (DP) steel joints over a wide range of strain rates from 10−3 to 103 s−1 were investigated for the further applications on the lightweight design of vehicles. The high strain rate dynamic tensile deformation process and full-field strain distribution of the base metals and welded joints were examined using the digital image correlation method and high-speed photography. The strain rate effects on the stress–strain responses, tensile properties, deformation, and fracture behavior of the investigated materials were analyzed. The yield stress (YS) and ultimate tensile strength (UTS) of the dissimilar DP780/DP980 welded joints were lying in-between those of the DP780 and DP980 base metals, and all materials exhibited positive strain rate dependence on the YS and UTS. Owing to the microstructure heterogeneity, the welded joints showed relatively lower ductility in terms of total elongation (TE) than those of the corresponding base metals. The strain localization started before the maximum load was reached, and the strain localization occurred earlier during the whole deformation process with increasing strain rate. As for the dissimilar welded joint, the strain localization tended to occur in the vicinity of the lowest hardness value across the welded joint, which was in the subcritical HAZ at the DP780 side. As the strain rate increased, the typical ductile failure characteristic of the investigated materials did not change.

Revealing the atomistic deformation mechanisms of face-centered cubic nanocrystalline metals with atomic-scale mechanical microscopy: A review

Nanocrystalline metals have many functional and structural applications due to their excellent mechanical properties compared to their coarse-grained counterparts. The atomic-scale understanding of the deformation mechanisms of nanocrystalline metals is important for designing new materials, novel structures and applications. The review presents recent developments in the methods and techniques for in situ deformation mechanism investigations on face-centered-cubic nanocrystalline metals. In the first part, we will briefly introduce some important techniques that have been used for investigating the deformation behaviors of nanomaterials. Then, the size effects and the plasticity behaviors in nanocrystalline metals are discussed as a basis for comparison with the plasticity in bulk materials. In the last part, we show the atomic-scale and time-resolved dynamic deformation processes of nanocrystalline metals using our in-lab developed deformation device.

Performance improvement studies for cutting tools with perforated surface in turning of titanium alloy

In turning process, the cutting tool is essential for shaping materials. The cutting tools with various perforated surfaces help to increase the cutting tool life. Also, advances in CNC machining technologies have enhanced the productivity of machining process. One of the best or futuristic approaches in modern manufacturing engineering is the use of FEM Simulation for the metal cutting process. FEM simulation helps in understanding the metal deformation process and also helps in the reduction of experiments. The simulation helps the researchers to predict the major influencing cutting variable values without carrying out any experiment which is time-consuming and expensive. This research presents the simulation study of the performance of micro-hole patterned Polycrystalline Diamond cutting insert in machining Titanium alloy (Ti-6Al-4V). Micro-holes are drilled using Electrical Discharge Wire Drilling machine on the rake face of Polycrystalline Diamond (PCD) cutting inserts. FEM analysis is carried out to evaluate the effect of perforations on the mechanical integrity of insert. The micro-hole patterned insert is modeled in PRO-E modeler and simulated using DEFORM-3D software. The effective stress, strain, and temperature distribution are analyzed and the results are compared with the normal insert.

Performance improvement studies for cutting tools with perforated surface in turning of titanium alloy

In turning process, the cutting tool is essential for shaping materials. The cutting tools with various perforated surfaces help to increase the cutting tool life. Also, advances in CNC machining technologies have enhanced the productivity of machining process. One of the best or futuristic approaches in modern manufacturing engineering is the use of FEM Simulation for the metal cutting process. FEM simulation helps in understanding the metal deformation process and also helps in the reduction of experiments. The simulation helps the researchers to predict the major influencing cutting variable values without carrying out any experiment which is time-consuming and expensive. This research presents the simulation study of the performance of micro-hole patterned Polycrystalline Diamond cutting insert in machining Titanium alloy (Ti-6Al-4V). Micro-holes are drilled using Electrical Discharge Wire Drilling machine on the rake face of Polycrystalline Diamond (PCD) cutting inserts. FEM analysis is carried out to evaluate the effect of perforations on the mechanical integrity of insert. The micro-hole patterned insert is modeled in PRO-E modeler and simulated using DEFORM-3D software. The effective stress, strain, and temperature distribution are analyzed and the results are compared with the normal insert.

Ultrasonic testing of grain distortion direction in cold formed aluminium profile

Cold deformation process of metals causes distortion of crystalline structure. When a material is formed, the grains are usually distorted and elongated in one or more directions which make the material anisotropic. Anisotropy can be defined as a difference in a material's physical or mechanical properties in different directions of taking of testing samples. In exploitation, it is very important to recognize the direction of grain elongation so that the best orientation of working part can be chosen. For that purpose ultrasonic testing is used. The ultrasonic velocity of the material is determined by using a digital oscilloscope under condition that thickness of the material is known. Testing shows the difference in wave velocity for different orientations of grain structure. In order to determine whether there is a significant difference in the ultrasonic velocity, the measurement results were statistically analyzed and graphically presented. The results are verified by performing the same measurement procedure on annealed homogenous testing samples. The result of this work leads to a non-destructive, simplified way of anisotropy recognition, without more expensive, destructing, testing by cutting out a large number of testing samples.

Advances in Constitutive and Failure Models for Sheet Forming Simulation

Non-Associated Flow Rule (Non-AFR) can be used as a convenient way to account for anisotropic material response in metal deformation processes, making it possible for example, to eliminate the problem of the anomalous yielding in equibiaxial tension that is mistakenly attributed to limitations of the quadratic yield function, but may instead be attributed to the Associated Flow Rule (AFR). Seeing as in Non-AFR based models two separate functions can be adopted for yield and plastic potential, there is no constraint to which models are used to describe each of them. In this work, the flexible combination of two different yield criteria as yield function and plastic potential under Non-AFR is proposed and evaluated. FE simulations were carried so as to verify the accuracy of the material directionalities predicted using these constitutive material models. The stability conditions for non-associated flow connected with the prediction of yield point elongation are also reviewed. Anisotropic distortion hardening is further incorporated under non-associated flow. It has been found that anisotropic hardening makes the noticeable improvements for both earing and spring-back predictions. This presentation is followed by a discussion of the topic of the forming limit & necking, the evidence in favor of stress analysis, and the motivation for the development of a new type of forming limit diagram based on the polar effective plastic strain (PEPS) diagram. In order to connect necking to fracture in metals, the stress-based necking limit is combined with a stress- based fracture criterion in the principal stress, which provides an efficient method for the analysis of necking and fracture limits. The concept for the PEPS diagram is further developed to cover the path-independent PEPS fracture which is compatible with the stress-based fracture approach. Thus this fracture criterion can be utilized to describe the post-necking behavior and to cover nonlinear strain-path. Fracture modeling in the stress space including mean stress contribution is also conducted and compared with M-C criterion. Strain-path effect on fracture criteria and the role of a damage parameter have been also discussed.

In situ observation of deformation processes in nanocrystalline face-centered cubic metals.

The atomistic mechanisms active during plastic deformation of nanocrystalline metals are still a subject of controversy. The recently developed approach of combining automated crystal orientation mapping (ACOM) and in situ straining inside a transmission electron microscope was applied to study the deformation of nanocrystalline PdxAu1−x thin films. This combination enables direct imaging of simultaneously occurring plastic deformation processes in one experiment, such as grain boundary motion, twin activity and grain rotation. Large-angle grain rotations with ≈39° and ≈60° occur and can be related to twin formation, twin migration and twin–twin interaction as a result of partial dislocation activity. Furthermore, plastic deformation in nanocrystalline thin films was found to be partially reversible upon rupture of the film. In conclusion, conventional deformation mechanisms are still active in nanocrystalline metals but with different weighting as compared with conventional materials with coarser grains.

Использование накопленной энергии деформирования при численном моделировании разрушения конструкций из стали

В данной работе предложена модель баланса энергии в неупруго деформируемом металле. Ее отличительной чертой является учет структурных изменений в материале при помощи введения тензорного параметра плотности дефектов, совпадающего по смыслу с деформацией, обусловленной дефектами. Введение такого параметра позволило авторам работы предложить способ расчета величины накопленной энергии и сформулировать на ее основе критерий разрушения. Рассмотрены два способа вывода определяющих соотношений, позволяющих рассчитывать эволюцию пластической и структурной деформации. Первый способ основан на применении принципов линейной неравновесной термодинамики, второй способ представляет собой аналог теории пластического течения. Предложенная модель позволяет не только описывать напряженно-деформированное состояние, но и открывает возможности для использования энергетических критериев разрушения. На основе анализа экспериментальных исследований процесса накопления скрытой энергии деформирования в металлах предполагается, что разрушение в материале происходит при достижении накопленной энергией некоторого критического значения. Применение этого подхода к описанию разрушения проиллюстрировано на двух примерах. Первый пример посвящен моделированию распространения трещины при циклическом кручении трех цилиндрических образцов с трещинами, ориентированными к оси цилиндра под разными углами, второй пример описывает процессы зарождения и распространение трещины в опоре подшипника. При решении этих задач использовался расширенный метод конечных элементов. Полученные результаты согласуются с ранее опубликованными результатами.

Rayleigh-Taylor instability in the high-speed collision of metal plates

On the basis of numerical simulation the authors investigate dynamic processes occurring in the high-speed impact of the two metal plates. The calculations showed the presence of the Rayleigh-Taylor instability, growing on the boundary of metal impact. The article presents the comparative characteristic of the metal deformation processes in plane and spatial cases, calculations with different equations of state.

On the theory of plastic structure formation of metals

Plastic structure formation in AD1 aluminum has been studied experimentally and theoretically using a physical phenomenological model of cyclic and nearly cyclic plastic deformation with large strain intensities accumulated in one and several cycles. Based on the results obtained, the proposed model of the process was verified, the concept of severe plastic deformation was refined, its role in structure formation was investigated, and the existence of a limit of grain refinement during deformation processing of metals was shown.

Deformation Processes of Porous Metals and Metallic Foams (Review)

Although fabrication techniques of porous metals or metallic foams were widely reported, studies on plastic working and metal forming of the materials have been limited. As pores may be closed so that advantages, e.g., lightweight or large specific surface area, may decrease in deformation processes. However, deformation process of porous metals has several positive advantages. In this paper, past studies reported by the authors and other researchers are reviewed. It is shown that deformability of porous metals is generally high and sufficient for bulk metal forming. Applications only feasible with porous metals are described. It is concluded that deformation process of porous metals and metallic foams is advantageous for industrial applications.

Simulation of Cold Extrusion Process of Hollow Pieces

The impact of friction factor, thickness of the cold extrusion, semi-cone angle and other factors on cold extrusion was analyzed. Various process parameters were set and optimized. Cold extrusion process of hollow pieces was simulated by DEFORM-3D software. Through the simulation of the virtual velocity field, strain field, stress field, load - stroke curve and metal deformation process, the flow velocity of the metal and the deformation of the workpiece were studied. The metal flow routes and the parts that are easy to wear in die were gained. The above research results provide valuable references for the processing of hollow pieces.

Causality in the Bauschinger effect generation and in other deformation processes in metals

Abstract The phenomenon of Bauschinger effect and of other generally known deformation processess of metals is explained and modeled on the mesoscale. The basic feature of the model consists in the use of tensorial internal variables that have explicit physical meaning – internal mesomechanical stresses. The metallic materials under consideration are modeled as two-phase media with two substructures, one compliant and the other resistant. It is shown that Bauschinger effect and other deformation phenomena observed in most metallic materials can be explained and described as results of interplays of these substructures.