Pure Copper Specimens Research Articles

Experimental Investigation on Strengthening Effect of Cu2O Film in Micro Sheet Forming of Copper

Materials processed using micro-manufacturing technologies exhibit significantly different properties compared to those produced using conventional macro-manufacturing techniques. In this paper, the uniaxial tensile tests were performed on the thin sheet specimens of pure copper to investigate how the sheet thickness impacts the flow stress. The experimental results show a continuous decrease of flow stress as the sheet thickness reduces from 200 to 100 μm, but an increase of flow stress with further reduction in thickness. Firstly, by introducing the ratio of surface grains, the decrease trend of flow stress was explained on the basis of surface layer model. Secondly, the strengthening effect of Cu2O film was clearly demonstrated by the x-ray diffraction and electrodeposition process. Finally, considering the effects of Cu2O film and free surface layer, the mechanical properties of Cu2O film was studied, which is helpful to explain the material behavior in micro sheet forming.

PS0015-246 デジタル画像相関法による多結晶純銅の不均一変形の評価

To investigate the development of non-uniform deformation behavior of material, uniaxial tensile test of polycrystalline pure copper specimen was carried out. The specimen with different grain sizes were prepared by controlling annealing condition. To cause the non-uniform deformation to the specimen curved gage section was given. Strain field was evaluated by digital image correlation method. Grain size influenced the development of non-uniform deformation.

Mechanism of grain refinement and its effect on Adiabatic Shear Bands in 4340 steel and pure copper during impact

Pre-deformation and post-deformation microstructure characterization was conducted on tempered 4340 steel and commercial pure copper specimens under impact to determine the microstructural changes and the mechanism of grain refinement that occur during the evolution of ASBs. It was observed that the movement and multiplication of dislocations, elongation of grains, breaking of elongated grains, rotation, carbide fragmentation and boundary refinement of broken grains occur simultaneously during the evolution of ASBs in the impacted 4340 steel specimens. The extent of these mechanisms depends on the imposed local strain and strain rate. Extensive grain refinement coupled with high density of dislocations results in the shear band structures being more susceptible to crack nucleation and propagation. In copper, it was observed that sequential occurrence of emergence of dislocations, dislocation cell formations with varying cell boundaries and cell interiors, dynamic recovery and extensive micro-twinning results in the formation of the shear bands. The structure within the evolved shear bands becomes less brittle after the onset of dynamic recovery and micro-twinning. The differences in the mechanism of grain refinement and evolution of the shear bands in both materials is attributed to the differences in the mobility of dislocations, the rate of strain hardening and strain hardening exponents in both materials studied.

Incremental high pressure torsion as a novel severe plastic deformation process: Processing features and application to copper

High pressure torsion is known as one of the most popular severe plastic deformation processes. However, it has certain size limitations, especially regarding the thickness of the processed samples. In this contribution incremental high pressure torsion is introduced as a novel severe plastic deformation process. This further development of conventional high pressure torsion is capable of delivering specimens having an extraordinarily high aspect-ratio of thickness to diameter. The features of this process combined with a case-study on a pure copper specimen with a deformed diameter of 50mm and a thickness of 40mm is presented.

Effect of friction stir processing on the microstructure and mechanical properties of Cu–TiC composite

In this research, a Copper-based composite was fabricated by TiC reinforcement via friction stir processing. Holes with the diameter and depth of 2mm were made on the surface of a pure copper specimen and filled by TiC powders. Friction srir processing was carried out with transverse and rotational speeds of 50mm/min and 1000rpm, respectively. Optical and scanning electron microscope observations revealed that FSP produced a fine grain microstructure with a homogeneous distribution of particles on the surface. Reinforcing particles showed a good bonding with the base metal. Mechanical test results also proved that FSP increased the microhardness and the wear resistance of specimen as a result of grain refinement and the existence of TiC particles in the FSP zone. Maximum hardness in the stir zone was 117 Vickers, while the hardness of pure copper was 65 Vickers. X-Ray analysis proved no intermetallic compounds in the stir zone.

POD Preprocessing of IR Thermal Data to Assess Heat Source Distributions

Infrared thermography is a useful imaging technique for analyzing the thermomechanical behaviour of materials. It allows, under certain conditions, surface temperature monitoring and, via a diffusion model, estimation of heat sources induced by dissipative and/or thermally coupled deformation mechanisms. However, the noisy and discrete character of thermal data, the regularizing effect of heat diffusion and heat exchanges with the surroundings complicate the passage from temperature to heat source. The aim of this paper is to show that the prior use of reduced-basis projection of thermal data improves the signal-to-noise ratio before estimating the heat source distributions. The reduced basis is generated by proper orthogonal decomposition (POD) of physically-admissible thermal fields. These fields are solutions of ideal diffusion problems related to a set of putative heat sources. preprocessing is applied to different direct methods (finite differences, spectral solution, local least-squares fitting) already used in the past. The gain of this preprocessing is determined using a numerical penalizing benchmark test. The methods are finally compared using data extracted from a dynamic cyclic test on a pure copper specimen.

Dissipation Assessments During Dynamic Very High Cycle Fatigue Tests

This paper presents an experimental device developed to detect and estimate dissipated energy during very high cycle fatigue tests (VHCF) at high loading frequency (20 kHz) and low stress (i.e. far below the yield stress). Intrinsic dissipation is computed using local expressions of the heat diffusion equation and thermal data fields provided by an infrared focal plane array camera. The results obtained from tests performed on pure copper specimens show that dissipated energy exists whatever the attainable stress range and show that the dissipated energy rate is not constant throughout the test. Both findings are respectively incompatible with the concepts of fatigue limit based on elastic shakedown or on stabilized cyclic state associated with the mechanical hysteresis loop (viscoplastic shakedown).

Very high cycle fatigue of copper: Evolution, morphology and locations of surface slip markings

The surfaces of commercially pure polycrystalline copper specimens subjected to interrupted 20kHz fatigue tests in the very high cycle fatigue regime were investigated. The stress amplitude needed to form the early slip markings was found twice lower than the stress amplitude required to fracture which confirmed the results obtained by Stanzl-Tschegg et al. (2007). Three types of slip markings were classified according to their morphology and their location in the polycrystalline material. They are compared to slip markings observed during fatigue tests at frequencies lower than 100Hz and numbers of cycles lower than 107. For 20kHz fatigue tests, stress amplitudes ranging from 45MPa to 65MPa produce straight and long early persistent slip markings located along twin boundaries. Stress amplitudes lower than 45MPa produce clusters of fine early persistent slip markings mainly located at triple junctions.

An Experimental Investigation on the Fabrication of Micro/Meso Surface Features by Metallic Roll-to-Plate Imprinting Process

Metallic components with large-area functional surface micro/mesostructures have been increasingly utilized in various industrial fields, such as friction/wear reduction, viscous drag reduction, and energy efficiency enhancement. Roll-to-plate (R2P) imprinting process is an efficient and economical method in fabricating micro/mesofeatures on the large-area surface of the metal parts. However, process design methods based on scale law cannot be directly used due to size effects. Its formability is greatly influenced by tool feature size and material grain size. In this study, a lab-scale R2P imprinting system was developed to fabricate the microsructures on the surface of metallic materials. The specimens of pure aluminum and pure copper with various size grains were prepared. Rigid die with geometric dimensions was fabricated and series of experiments were conducted. The microfeature height of the imprinted workpiece was measured to evaluate the effects of tool feature dimensions (width, spacing, and fillet) and metal grain sizes. It is found that the groove width and fillet had more significant effect on the microfeature formation among the die cavity geometric parameters. Wider groove could enhance the microforming ability and large fillet could improve the flowing ability. From the viewpoint of polycrystalline material, grain structures significantly affected the microfeature formation. When the grain size was smaller than the groove width, the material flowed more easily into the die cavity with increasing of the grain size because of the decrease of grain boundary strengthening effect.

Micro/Meso Roll-to-Plate (R2P) Imprinting Process to Fabricate Micro Channel/Riblet Features

Functional metallic surface micro/meso structures such as micro channels/grooves or micro riblets have been increasingly applied in many fields, such as optics, tribology, fluid-dynamics and heat exchange or mass tranfer. Micro/meso forming has the potential of efficiently and economically fabricating functional microstructures over a large surface area. A mciro/meso roll-to-plate imprinting technique was proposed and the process system was self-developed. The pure copper specimens with various grains were imprinted to investigate the effects of mould cavity dimensions (width, spacing, and fillet) and grain size on the formation of features. It was concluded that the width and grain size had significant effects on the feature formation. Wider groove or larger size grain was helpful to enhance the flowability of materials. Moreover, the technique was validated for high throughput, low cost, low emission, and flexible fabrication technique.

An analysis on necking effect and stress distribution in round cross-section specimens of pure copper with different diameters

The size dependence of the tensile properties and necking effect of pure copper exposed to various heat treatments are investigated in this paper using round bar cross-section specimens through experimental and 3D finite element methods (FEM). The results show that the total and post-necking elongation increased dramatically as the specimen diameter increased. The size effects on necking and stress distribution of different diameter specimens are analysed. These results show that the post-necking deformation length is only dependent on the diameter of the specimens and is independent of the specimen’s gauge length, which results in the size dependence of the post-necking elongation. Moreover, FEM simulations show that the size dependence of the tensile properties can be eliminated when the ratio of the gauge length to the diameter is larger than 10.

Correlation between wear resistance and subsurface recrystallization structure in copper

Wear of conventional metallic materials involves various complex solid state processes of which the dominant process is elusive. From a thorough experimental investigation on the worn subsurface structure evolution in pure copper specimens with various microstructures, we conclude that the transformation from the subsurface dynamic recrystallization (DRX) structure into the top nanostructured mixing layer (NML) is the most important process which could determine the wear rate. A pronounced correlation is identified that wear rate increases significantly with an increasing grain size or a decreasing hardness of the DRX structure adjacent to the NML. This result points out an effective approach to make materials better against wear by stabilizing the deformed structure against DRX.

Statistical characterization of nanostructured materials from severe plastic deformation in machining

Endowing conventional microcrystalline materials with nanometer-scale grains at the surfaces can offer enhanced mechanical properties, including improved wear, fatigue, and friction properties, while simultaneously enabling useful functionalizations with regard to biocompatibility, osseointegration, electrochemical performance, etc. To inherit such multifunctional properties from the surface nanograined state, existing approaches often use coatings that are created through an array of secondary processing techniques (e.g., physical or chemical vapor deposition, surface mechanical attrition treatment, etc.). Obviating the need for such surface processing, recent empirical evidence has demonstrated the introduction of integral surface nanograin structures on bulk materials as a result of severe plastic deformation during machining-based processes. Building on these observations, if empirically driven, process–structure mappings can be developed, it may be possible to engineer enhanced nanoscale surface microstructures directly using machining processes while simultaneously incorporating them within existing computer-numeric-controlled manufacturing systems. Toward this end, this article provides a statistical characterization of nanograined metals created by severe plastic deformation in machining-based processes that maps machining conditions to the resulting microstructure, namely, the mean grain size. A specialized designed experiments approach is used to hypothesize and test a linear mixed-effects model of two important machining parameters. Unlike standard analysis approaches, the statistical dependence between subsets of experimental grain size observations is accounted for and it is shown that ignoring this inherent dependence can yield misleading results for the mean response function. The statistical model is applied to pure copper specimens to identify the factors that most significantly contribute to variability in the mean grain size and is shown to accurately predict the mean grain size under a few scenarios.

Study on Formability of Micro-Feature in the Coining Process

In micro-manufacture field, micro-forming is paid much attention due to its efficiency for mass production. However, owing to the particularity of deformation, mass researches for specialized micro-forming processes to direct the industrial production are of great urgency. As a similar process with the micro-forming, the forming property of micro-feature forming in coining process was investigated by the experimental research and numerical analysis. Firstly, utilizing the workpieces with different thicknesses and micro-feature sizes, the coining processes were numerically simulated to study the forming property of micro-feature. The height of micro-feature was selected as the evaluation criteria for the deformation behavior. Then, the pure copper specimens with different thicknesses were coined experimentally, using a die with a micro-hole by the universal testing machine to verify the simulation results. Finally, a modified scheme was successful to be proposed which could improve the manufacturability of the processes. The research results could provide technical references for manufacturing micro-parts and those with micro-features.

Experimental and numerical investigations of the plane strain compression of an oligocrystalline pure copper specimen

To evaluate if the deformation in the bulk of an oligocrystalline sample during a plane strain compression test can be accurately simulated numerically with the aid of a crystal plasticity model a detailed comparison between the experimentally found and numerically predicted deformation is made. In the experiment an incremental plane strain compression (PSC) test is conducted on an oligocrystalline specimen. The incremental PSC test is complemented by electron backscatter diffraction (EBSD) measurements. The experimental results enable qualitative observation of the specimen's microstructure and orientation distribution at intermediate stages of the plastic deformation. This incremental PSC test is numerically simulated with the aid of a crystal plasticity finite element model. The numerical model accounts for the specimen's microstructure and the orientations of the crystals. Mapping the microstructure and the grain orientations on the workpiece in the finite element model provides realistic boundary conditions. Both, the experiment and numerical simulation show that the deformation in the oligocrystalline specimen with only a few grains in the deformation zone considerably differs from that of well known continuum like materials. Qualitative comparisons of the incremental PSC test and its numerical simulation showed that most of the experimentally observed behaviour of the grains and their mutual interaction can be seen in the results of the CPFEM simulation as well. From quantitative comparisons it is concluded that the two-dimensional CPFEM simulation can predict the rotations of grains in the bulk of the three-dimensional microstructure of the oligocrystalline specimen only in a qualitative manner. The lack of quantitative agreement is most probably caused by the fact that this two-dimensional, plane strain CPFEM model does not account for the interaction of the grains in the microstructure beyond the analysed cross sectional surface.

Biaxial Fatigue Life Predicted by Crack Growth Analysis in Various Material Microstructures Modeled by Voronoi-Polygons

Fatigue life is affected by the crack growth behavior that depends on the material microstructure as well as the stress biaxiality. By considering such effects on crack growth, a numerical procedure for predicting failure life in biaxial fatigue of materials with different microstructures was proposed in this study. Such a procedure will be helpful in the material design for higher performance of fatigue resistance in a material. The microstructure of a material was first modeled using Voronoi-polygons, and the crack initiation was analyzed as the result of slip-band formation in individual grains in the modeled microstructure. In the analysis, stress states in individual grains were randomized so that the average stress state should be equivalent to the bulk stress state. An algorithm for the crack growth analysis was established as a competition between the crack-coalescence growth and the propagation as a single crack. The failure life was statistically predicted based on the crack growth behavior simulated for 40 distinct microstructural configurations, which were generated by randomizing shapes of Voronoi-polygons for the same material. By applying the proposed procedure, simulations were conducted for experimental conditions of fatigue tests, which had been conducted under axial, torsional, and combined loading modes using circumferentially notched specimens of pure copper, medium carbon steel, and (α + β) and β titanium alloys. In this case, 40 different failure-lives were obtained for each combination of material and loading mode. It was revealed that the failure lives observed in experiments were almost covered by the life-ranges between the minimum and the maximum lives given in simulation. Statistical characteristics in simulated life-distributions were investigated using Weibull distribution function and its related statistical parameters.

An interrupted tensile testing at high strain rates for pure copper bars

A high-speed tensile facility (HSTF) invented by us was applied to interrupting the tests for pure copper specimen bars controlled locally at different levels of elongation. It was realized to isolate and identify the different stages of the dynamic fracture process of the pure copper specimen bar under impact tension. The results of scanning electron microscopical (SEM) investigation of the recovered pure copper specimens show that the void evolution near the surface of the minimum cross-section of the necking area is more severe than that at the middle of the necking area, which may be connected with the findings discussed by Alves and Jones [J. Mech. Phys. Solids 47, 643 (1999)]. The constitutive models in a certain range of strain determined from the tensile split Hopkinson bar optimized by us were employed and adjusted in numerically simulating the large deformation of the pure copper specimen in the interrupted tensile tests on HSTF. The dependence of the instability strain of thermoviscoplastic materials in simple tension on material parameters delineated by Batra and Wei [Int. J. Impact Eng. 34, 448 (2007)] was inspected in predicting the diffuse necking of the specimen bar. The axisymmetric necking rod model with a central void under static tension presented by Ragab [Eng. Fract. Mech. 71, 1515 (2004)] was extended to predicting the local necking and fracture of the specimen bar under impact tension.

Creep damage characterization using a low amplitude nonlinear ultrasonic technique

This paper describes a new nonlinear ultrasonic (NLU) technique for creep damage characterization. Traditionally, nonlinear material response is evaluated at high amplitude levels. The present paper evaluates the nonlinear response of samples at much lower amplitude levels than used in any previous study of nonlinear material behavior. The newly observed nonlinear behavior occurs when dislocations are constrained between two quiescent lattice planes as defined by Cantrell. Three parameters are used to characterize the low amplitude nonlinear response during the progression of creep damage in 99.98% pure copper specimens studied here; these are parameters that characterize (a) the static displacement, (b) the second harmonic and (c) the third harmonic components of the ultrasonic through-transmitted signal. Various features of the received NLU amplitude vs. input amplitude to the ultrasonic transducer were observed to correlate well with micro-void concentrations caused by creep damage.

Life prediction based on biaxial fatigue crack growth simulated in different microstructures modeled by using Voronoi-polygons

Fatigue life is significantly affected by the crack growth behavior that depends on the material microstructure as well as the stress biaxiality. By considering such effects of microstructure and stress state on crack growth, a methodology to predict failure life in biaxial fatigue of materials with different microstructures was proposed in the present work. For more appropriate modeling, an aggregate of Voronoi-polygons was adopted to express microstructural features of polycrystalline materials. In the microstructure modeled by an aggregate of Voronoi-polygons, the crack initiation was quantitatively analyzed by considering the slip deformation on slip plane which is randomly set in each Voronoi-polygon. The algorithm for the crack growth was established as a competition between the growth by crack coalescences and the propagation of a dominant crack as a single crack. The coalescence growth under the assumed criteria was also taken into account among initiated and/or propagating cracks during the whole fatigue process. The failure life was statistically predicted based on the crack growth behavior simulated for forty distinct microstructural configurations, which were formed by generating different combinations of randomized shapes of Voronoipolygons for the same material. By applying the proposed analytical procedure, simulations were conducted according to the same conditions as experimental ones in fatigue tests, which had been carried out under axial, torsional and combined loading modes by using circumferentially-notched specimens of pure copper, medium carbon steel, and (α+β) and β titanium alloys. In this case, forty different failure-lives were obtained for each combination of material and loading mode. It was revealed that the failure lives observed in experiments were almost covered by the life-ranges between the minimum and the maximum lives given in simulation. It was also clarified that a dispersion of simulated lives was found to be larger at higher stress level.

The formation of adiabatic shear bands in copper during torsion at high strain rates

The formation of adiabatic shear bands, which are narrow deformation bands with intense strains, in copper at high strain rates has long been questioned due to ductility of copper and its ability to exhibit uniform deformation at relatively large strains. However, in some instances when copper is tested in torsion at shear strain in excess of 103 s−1, it has been reported to develop adiabatic shear bands. This study examines the occurrence of adiabatic shear bands in torsion specimens of commercial pure copper (99.94% Cu) as a function of the initial torque and angle of twist. Adiabatic shear bands (ASBs) were observed using scanning electron microscopy. It is concluded that a mesomechanical property such as adiabatic shear bands can be induced in ductile materials such as polycrystalline copper. The source of these bands is correlated to the extensive shear deformation before fracture.