Ultrafine-grained Copper Research Articles

The influence of graphene oxide on the microstructure and properties of ultrafine-grained copper processed by high-pressure torsion

New metal matrix nanocomposites with enhanced thermal stability were produced in a three step process consisting of mechanical milling, spark plasma sintering and High-Pressure Torsion (HPT). The nanocomposites consisted of a copper matrix and the addition of 1 wt% Graphene Oxide (GO) as a reinforcement. A nanocrystalline microstructure, enhanced hardness and improved thermal stability were achieved. The grain size of the nanocomposites was ∼55 nm which is almost four time smaller than for Cu HPT at 210 nm. Hardnes and ultimate tensile strength of the nanocomposites reach 250 Hv and 700 MPa, respectively, which was more than three times higher than for the initial material. The most important result is that the nanocomposites remained ultrafine-grained up to 500 ⁰C whereas the Cu HPT fully recrystalized after annealing at 300 ⁰C The report also includes an investigation of the electrical conductivity of the copper-based composite which was slightly better than for copper after HPT together with the wear behaviour of this material. This is one of the first reports on copper reinforced with graphene oxide composites produced by HPT and it gives information on its thermal stability, electrical conductivity and wear behaviour together with the microstructural characteristics and mechanical properties.

Revealing the high strength and high thermal stability of a nano-lamellar Cu-0.1 at.% Zr alloy

Nano-lamellar structured Cu-0.1 at.% Zr alloys are prepared by the combination of equal channel angular pressing and subsequent cryogenic rolling. The advanced processing route enables a systematic reduction of the lamellae boundary spacing down to 45 nm and a concurrent increase of yield strength up to 626 MPa. Subsequent annealing at temperatures up to 673 K induced recovery processes and uniform coarsening through triple junction motion, avoiding a catastrophic degradation of the microstructure and performance and preserving the yield strength at a level comparable to ultrafine-grained copper. Simultaneously, the tensile uniform elongation experienced a substantial recovery up to 11.9 %. The high thermal stability is mainly attributed to the low mobility of grain boundary and the relaxation of non-equilibrium grain boundaries, both of which are facilitated by the microalloying of Cu with Zr. Based on the theory of dislocation pile-up at boundaries, the Hall-Petch equation is adopted and extended to the lamellar structured Cu-0.1 at.% Zr alloy, revealing a lower-critical bound of 90 nm for lamellae boundary spacing, corresponding to an effective boundary spacing of about 150 nm.

Friction welding of UFG copper using the W2Mi prototype machine

When welding ultra-fine-grained metals using conventional methods, the microstructure in the joint area is degraded (mainly due to recrystallization) in the heat-affected zone and a significant deterioration of mechanical properties, including joint strength. The aim of the research presented in this article was to identify the possibility of obtaining joints with strength close to initial material using friction welding of metal materials with ultra-fine grain. For this purpose, UFG (ultra-fine-grained) material was produced from technically pure M1Ez4 copper using a hybrid SPD (severe plastic deformation) process. The welding process was carried out on a machine with a prototype design that allows minimizing the welding time, while generating high force. The process parameters used on the prototype machine resulted in an increase in the hardness of the material by 4% in the joint area. The strength of the joint compared to the base material decreased slightly by 2%. The tests carried out proved that, using appropriate process parameters, it is possible to obtain a UFG metal joint without a decrease in its mechanical strength.

Bulk ultrafine grained microstructures with high thermal stability via intragranular precipitation of coherent particles

Producing ultrafine-grained (UFG) microstructures with enhanced thermal stability is an important yet challenging route to further improve mechanical properties of structural materials. Here, a high-performance bulk UFG copper that can stabilize even at temperatures up to 750 °C (∼ 0.75Tm, Tm is the melting point) was fabricated by manipulating its recrystallization behavior via low alloying of Co. Addition of 1 wt.%–1.5 wt.% of Co can trigger quick and copious intragranular clustering of Co atoms, which offers high Zener pinning pressure and pins the grain boundaries (GBs) of freshly recrystallized ultrafine grains. Due to the fact that the subsequent growth of the coherent Co-enriched nanoclusters was slow, sufficient particles adjacent to GBs remained to inhibit the migration of GBs, giving rise to the UFG microstructure with prominently high thermal stability. This work manifests that the strategy for producing UFGs with coherent precipitates can be applied in many alloy systems such as Fe- and Cu-based, which paves the pathway for designing advanced strain-hardenable UFGs with plain compositions.

A novel variable rake angle tool design for fabricating ultrafine-grained pure copper strips with improved formability and strain controllability

Extrusion machining (EM) is an efficient and convenient method to prepare ultrafine-grained (UFG) materials. As a vital parameter in the EM process, tool rake angle (α) is critical to control the material's shear strain and grain size. The grains tend to refine with decreasing α, whereas a small α leads to difficult cutting, chip breakage, and poor surface quality. To achieve the controllable strain distribution, grain size adjustment, and improved formability of UFG pure copper strips, this paper proposed a novel design of the variable rake angle (VRA) tool. Combining experiments and finite element simulation, extrusion machining with variable rake angle tools (VRA-EM) was investigated. The results implied that a deviating angle (β) for the material flow existed during VRA-EM. The β increased as the cutting thickness (tc) increased, resulting in decreased plastic deformation, strain, and microhardness. The microstructure was also characterized by electron backscatter diffraction system (EBSD) and transmission electron microscopy (TEM) techniques. A dead metal zone was formed using the VRA tool, in which the material flow behavior was quite weak. The obtained UFG strip was further divided into the low-strain zone (LSZ) and high-strain zone (HSZ) based on strain distribution. And a new theoretical model of shear strain was established and conformed to experimental values. Accordingly, VRA-EM process was able to improve the mechanical properties and strain controllability, showing promising prospects for efficiently fabricating UFG materials.

Achieving high fatigue strength of large-scale ultrafine-grained copper fabricated by friction stir additive manufacturing

The preparation of large-scale bulk materials and the limitation of fatigue strength improvement are two crucial obstacles restricting the industrial applications of ultrafine-grained (UFG) materials. In this study, we successfully fabricated the large-scale UFG pure copper by using the water-cooling assisted friction stir additive manufacturing (FSAM) method and investigated its high cycle fatigue (HCF) properties. The microstructural characteristics before and after fatigue were almost the same, proving the high microstructure stability of FSAM Cu during the HCF deformation. Therefore, the fatigue strength of FSAM Cu was as high as 130 MPa, and the fatigue ratio (0.30) reached the same level as coarse-grained Cu. This study can provide an efficient method to fabricate large-scale bulk materials with high fatigue resistance, bringing possibility to the engineering application of UFG materials.

Strain localization and ductile fracture mechanism of micro/mesoscale deformation in ultrafine-grained pure copper

To realize the application of ultrafine-grained (UFG) materials in micro-forming, the micro/meso-deformation and fracture behavior of UFG pure copper processed using equal-channel angular pressing were analyzed by making in/ex-situ observations during tensile testing. The evolution of surface cracks and inner voids was apparent in the in-situ scanning electron microscopy (SEM) images and synchrotron radiation X-ray tomograms, respectively. The results indicate that the surface-crack evolution of UFG pure copper can be correlated with strain localization when the flow stress decreases after peaking. The effects of grain size and shear texture on damage evolution during the fracture process are also discussed. This work comprehensively explores the plasticity and fracture/failure mechanisms in UFG Cu samples during the micro-forming process.

Breaks in the Hall–Petch Relationship after Severe Plastic Deformation of Magnesium, Aluminum, Copper, and Iron

Strengthening by grain refinement via the Hall–Petch mechanism and softening by nanograin formation via the inverse Hall–Petch mechanism have been the subject of argument for decades, particularly for ultrafine-grained materials. In this study, the Hall–Petch relationship is examined for ultrafine-grained magnesium, aluminum, copper, and iron produced by severe plastic deformation in the literature. Magnesium, aluminum, copper, and their alloys follow the Hall–Petch relationship with a low slope, but an up-break appears when the grain sizes are reduced below 500–1000 nm. This extra strengthening, which is mainly due to the enhanced contribution of dislocations, is followed by a down-break for grain sizes smaller than 70–150 nm due to the diminution of the dislocation contribution and an enhancement of thermally-activated phenomena. For pure iron with a lower dislocation mobility, the Hall–Petch breaks are not evident, but the strength at the nanometer grain size range is lower than the expected Hall–Petch trend in the submicrometer range. The strength of nanograined iron can be increased to the expected trend by stabilizing grain boundaries via impurity atoms. Detailed analyses of the data confirm that grain refinement to the nanometer level is not necessarily a solution to achieve extra strengthening, but other strategies such as microstructural stabilization by segregation or precipitation are required.

Fretting wear behaviour of ultrafine-grained copper produced by cryogenic temperature extrusion machining

Cryogenic temperature extrusion machining (CT-EM), as a new process, was first applied for preparing ultrafine-grained (UFG) copper strips. The microstructure evolution and mechanical properties (hardness and fretting wear) were characterised by electron backscatter diffraction and transmission electron microscopy methods. Refined grains (450 nm) and high dislocation density could be obtained by CT-EM because of effectively suppressing heat generation and the annihilation of dislocations. As a result, the CT-EM sample exhibited enhanced fretting wear properties, e.g. a much lower friction coefficient (reducing 0.1–0.2) and a lower wear loss (decreasing 32–45%) were achieved due to its enhanced hardness and refined microstructure. In-depth, the delamination and oxidation wear proved to be the primary wear mechanism of the CT-EM sample under dry wear conditions.

Friction stir additive manufacturing enabling scale-up of ultrafine-grained pure copper with superior mechanical properties

Bulk manufacturing has always been a big barrier to realize industrial application of ultrafine-grained (UFG) material for a long time. In this study, a new friction stir additive manufacturing (FSAM) method was developed, and three-dimensional large-scale bulk UFG pure Cu with uniform microstructure and high mechanical property was successfully manufactured.

Characterization of the hydroextruded oxygen-free copper via microindentation in the temperature range 77–300 K

The micromechanical properties of annealed coarse-grained (CG, average grain size of 1–10 μm) and ultrafine-grained (UFG) oxygen-free copper have been studied in the temperature range of 77–300 K. UFG samples were obtained in two ways: by direct hydroextrusion with a change in the diameter of the billet from 50 mm to 13 mm (DE, average grain size of 0.6 μm) and as a result of equal channel angular hydroextrusion of the billet with a diameter of 13 mm (ECAE, average grain size of 0.5 μm). The microhardness at the periphery of the extruded billets was 25% less than in the center. The maximum hardening as a result of extrusion expressed as the ratio of the microhardness of the UFG samples to the microhardness of the annealed CG sample was approximately 2 and 2.3 for the DE and ECAE cases, respectively. The temperature dependence of the microhardness of CG and UFG copper indicates that, upon indentation, plastic flow at temperatures of 77–300 K occurs without a change in the mechanism which is evidently the intersection of mobile dislocations with statistically stored (as forest) dislocations, and small values of the activation volume coincide with their high density. It is shown that the UFG structure of copper is stable; the micromechanical characteristics remain unchanged during long-term storage of extruded samples at room temperature. The results of indentation tests are compared with the data on the tensile deformation of similar samples. The correlation between microhardness and yield strength of DE and ECAE copper in the temperature range 77–300 K is considered.

Fabricating micro-dent on the surface of ultrafine-grained pure copper by laser-induced shock wave and its deformation behavior

Fabricating micro-dent on the surface of ultrafine-grained pure copper by laser-induced shock wave and its deformation behavior

Formability enhancement of ultrafine-grained pure copper sheets produced by accumulative roll bonding aided by electromagnetic forming

Formability enhancement of ultrafine-grained pure copper sheets produced by accumulative roll bonding aided by electromagnetic forming

Machinability of pure copper before and after processing by severe plastic deformation method

For using ultrafine-grained metals and alloys in industry, further machining sequences and studying their possible effects are necessary to attain these products in their ultimate dimensions. In this work, the machinability of ultrafine-grained pure copper and the corresponding coarse-grained counterpart investigated systematically. The results showed that the processed copper could be machined as efficiently as its initial one. Also, the machining of the Cu sample with the polycrystalline diamond tools requires much less force than tungsten carbide tools. The produced cutting forces were smaller for the processed copper as compared to the initial copper. Also, similar tool wear mechanisms were detectable for the copper specimen before and after the process. Chip morphology altered less through the machining, and the surface quality enhanced during machining the ultrafine-grained copper. Eventually, the polycrystalline diamond tool increased the surface roughness of initial and processed specimens since it caused less than 50% roughness compared to the tungsten carbide tool.

Anomalous high strain rate compressive behavior of additively manufactured copper micropillars

Microscale dynamic testing is vital to the understanding of material behavior at application relevant strain rates. However, despite two decades of intense micromechanics research, the testing of microscale metals has been largely limited to quasi-static strain rates. Here we report the dynamic compression testing of pristine 3D printed copper micropillars at strain rates from ∼0.001 s−1 to ∼500 s−1. It was identified that microcrystalline copper micropillars deform in a single-shear like manner exhibiting a weak strain rate dependence at all strain rates. Ultrafine grained (UFG) copper micropillars, however, deform homogenously via barreling and show strong rate-dependence and small activation volumes at strain rates up to ∼0.1 s−1, suggesting dislocation nucleation as the deformation mechanism. At higher strain rates, yield stress saturates remarkably, resulting in a decrease of strain rate sensitivity by two orders of magnitude and a four-fold increase in activation volume, implying a transition in deformation mechanism to collective dislocation nucleation.

Effect of strain energy on corrosion behavior of ultrafine grained copper prepared by severe plastic deformation

Effect of strain energy on corrosion behavior of ultrafine-grained (UFG) copper prepared by severe plastic deformation (SPD) was investigated in terms of microstructural evolution. The SPD processed material showed an ultrafine-grained (UFG) structure after grain refinement for several time processes, which will affect mechanical and corrosion behavior Homogeneity can be obtained efficiently through the pressing process commonly known as simple shear extrusion (SSE), which is one of the SPD techniques. Pure copper was processed by SSE for two, four, eight, and twelve passes. The structure of SSE treated sample was observed by laser microscope and transmission electron microscope as well as X-ray diffraction. The corrosion behavior by potentiodynamic polarization curve was observed in modified Livingstone solution, 1 M NaCl, and sulphuric solution. The structure of SSE processed sample showed that the first pass of the SSE processed sample displayed large deformation by developing the elongated grain and sub-grain structure. By increasing the SSE pass number, the grain shape became equiaxed due to excessive strain. The X-ray broadening related to ultrafine-grained (UFG) structure processed SSE on the copper sample, leading to smaller crystallite size, higher microstrain, and higher dislocation density. More homogeneous passive film was developed on the material with UFG structure appearance. However, the current density in 1 M NaCl was decreased by an increment of pass number due to the dissolution of copper metal. The UFG structure has more boundaries than coarse grain structure, and these phenomena show why Cu dissolve ability influences the current density. The grain boundary behaves as the cathodic site.

Exploring the size effect in scaling up ECAP using the theory of similarity

The size effect of equal-channel angular pressing (ECAP) is investigated systematically, by examining the similarity of geometrical dimensions, mechanical parameters and thermal behaviors. The strain and strain rate distribution, microstructure and mechanical property evolution upon scaling up were characterized experimentally, followed by finite element simulations and theoretical analyses of thermal behavior. Steady and consistent microstructure recovery and mechanical properties softening observed in the eight-pass ECAP processed ultrafine-grained (UFG) copper upon scaling-up, demonstrate a significant size effect. The billet temperature profile rises upon scaling-up in simulation that accounts for the observed size effect. Theoretical analyses show that neither the heat conduction within the billet, nor the heat exchange between the billet and its surroundings could maintain similarity upon scaling. In other words, the size effect in ECAP is inherently rooted in the laws of thermal physics. Plunger velocity scaling down was proposed as an effective strategy to control the temperature rise and size effect upon scaling up. The fundamental discoveries uncovered by present work will benefit the scaling of ECAP for commercialization. And the frame of similarity applied to the examination of the ECAP process is believed to be also applicable to the scaling of other thermal mechanical processing.

Characterization of ultrafine-grained copper joints acquired by rotary friction welding

Copper rods with ultrafine-grained microstructure, obtained by multi-turn ECAP processing, were subjected to Direct Drive Rotary Friction Welding using various processing parameters, such as rotational speed and pressure, which resulted in different energy and heat input. Even though friction welding is a high energy process, by a proper selection of processing parameters it was possible to maintain grain size at around 0.7 µm in the weld zone and preserve the UFG microstructure. These microstructural features translated into mechanical properties: the YS for those specimens was around 330 MPa. Processing parameters that resulted in a larger heat input caused an increase in grain size to around 2 µm; this, however, increased ductility and led to a uniform elongation exceeding 5%. Corrosion resistance in the stir zone increased, as was evident in the higher open circuit potential and higher corrosion potential in comparison with base material; the observed differences were about 50 mV. These changes can be explained by the higher fraction of HAGBs in the SZ.

Pre-strain assisted low heat-input friction stir processing to achieve ultrafine-grained copper

In this study, the effects of pre-rolling and friction stir processing (FSP) on the microstructure and mechanical behavior of pure copper were investigated. With increasing the tool rotational speed from 400 to 800 rpm, the average grain size increased for both 0% (annealed) and 60% (pre-rolled) samples. Also, pre-rolling of copper led to a reduction in the average grain size of the FSPed samples. It was found that homogeneously distributed fine grains structure in the 400–100 (0%) (400 rpm–100 mm/min – with 0% pre-rolling), 400–100 (60%), and 800–100 (60%) samples were observed due to the large heat transfer caused by the copper backing plate. The application of rolling and FSP caused an improvement in hardness, yield strength, and ultimate tensile strength values due to the grain refinement mechanism. Among the FSPed samples, the 600–100 (60%) sample showed a better strength-ductility balance than the other samples. In the initial and FSPed samples, many dimples were observed, revealing the ductile fracture. With increasing the rotational speed, the dimples morphology became more homogeneous. Also, the pre-rolled samples had dimples smaller than the annealed samples.

Solid-state welding of ultrafine grained copper rods

The article focuses on the Direct Drive Rotary Friction Welding of ultrafine-grained copper rods, which feature increased mechanical properties and good electrical properties, yet are limited in size. The use of UFG metals is often limited by the too small dimensions of semi-finished elements produced by SPD methods. Therefore, the production of finished machine parts from UFG metals is currently economically unjustified. Dismissal of dimensional limitations can be done by introducing joining to technological processes. The proposed joining method does not lead to a melting of the material in the joining zone or excessive degradation of the UFG microstructure. To obtain the best results, the research used the method of low-energy welding of two kinds of specimens: with a flat or a conical contact surface. In the article, the authors present, by means of metallographic microsections and microhardness measurements, the influence of rotational speed, welding pressure and conical shape contact surface on the quality of the obtained joints. The conducted research made it possible to obtain good quality joints whose microhardness is reduced only by about 10% in comparison with the base material and the tensile strength dropped from only 397–358 MPa.