Annealed Copper Research Articles

Improvement of Mechanical Properties of Multiple Surface Rolled Pure Copper

High strength can be achieved by severe plastic deformation but at the cost of ductility. A novel strategy, which named multiple surface rolling was applied on a homogeneous annealed pure copper to break the strength and ductility trade-off. A combination of high strength and acceptable ductility was achieved in copper strips after submitted to multiple surface rolling. The detail microstructure evolution rolled samples were characterized by EBSD observation and compared with the initially annealed ones. The average grain size does not show significant deviation in both initially annealed and multiple surfaces rolled copper. Detailed observations show a heterogeneous distribution of low angle grain boundaries through thickness direction. The low angle grain boundaries and misorientations revealed the potential strengthening mechanisms in the material. Both microstructural characterization and numerical simulations indicate that multiple surface rolling contributes to strain hardening at the sample surface, while the interior layer was undergoing elastic deformation or partial plastic deformation. This heterogeneous deformation renders copper sheet with a combination of high strength and ductility.

Highly conductive and strong CuSn0.3 alloy processed via laser powder bed fusion starting from a tin-coated copper powder

Despite the high demand, the successful fabrication of fully dense, highly conductive, and strong copper components via laser powder bed fusion (LPBF) is not readily evident. This is mainly due to the low optical absorption of copper, which inhibits the complete melting of copper powders when using commercially available fiber-laser-based LPBF machines. Accordingly, this article proposes a novel approach of using optically absorptive metal-coated copper powders for the fabrication of fully dense, highly conductive, and strong copper components via LPBF. To validate this approach, the surface of the copper powder is modified by applying a very thin (62 ± 14 nm) layer of metallic tin via an immersion plating technique. The application of only a 0.28 wt% of metallic tin coating significantly improved the room temperature powder optical absorption by ∼170 % at the fiber laser wavelength. Consequently, crack-free and fully dense copper parts combining high thermal conductivity of 334 ± 4 (W/(m·K)) and electrical conductivity of 80 ± 1 % international annealed copper standard (%IACS) with a good tensile strength of 256 ± 14 MPa, yield strength of 203 ± 4 MPa, and ductility of 21 ± 2 % have been fabricated using a fiber laser with an output laser power of 500 W. Furthermore, the article also describes the negative influence of the presence of a high amount of (0.091 wt%) sulfur, which originates from the organic additives during sub-optimal coating conditions, on the LPBF processing behavior of CuSn alloys. As such, high sulfur-containing tin-coated copper powders do not allow the fabrication of fully dense parts due to the occurrence of solidification cracks and the formation of pores. Subsequently, the optimum tin coating methodology, which limits the amount of sulfur below 0.0025 wt%, is described. The use of sulfur-free powder is recommended for the successful LPBF processing of fully dense parts made of copper and copper alloys.

In situ synthesis of core-shell W-Cu nanopowders for fabricating full-densified and fine-grained alloys with dramatically improved performance

In this work, a one-step thermal plasma synthesis route was developed to construct core-shell W-Cu nanoparticles with size about 30 nm. As synthesized nanocomposite powders address the issues of homogeneous elemental distribution and short diffusion distance for Cu to form network around W grains, facilitating fast densification with uniform microstructure and suppressed W grain growth. As a result, compact with high relative density of 99.31% and grain size of 260 nm was obtained at low temperature of 1100 °C, further led to the recorded hardness of 562 Vickers-hardness (Hv) and electrical conductivity of 54.32% International Annealed Copper Standard (IACS), respectively.

Copper/functionalized-carbon nanotubes composite films with ultrahigh electrical conductivity prepared by pulse reverse electrodeposition

Carbon nanotubes (CNTs) have been proved a significant role as the reinforcement material in improving the mechanical and electrical properties of metal matrix composites due to their high mechanical properties, excellent electrical and thermal conductivity as well as unique atomic structure. In addition, the dispersion of CNTs has been a key factor in fabricating of metal-based complex especially for copper (Cu) with performance improvement. In the present paper, the well dispersion of functionalized CNTs (F-CNTs) is obtained at the first time, accompanied by using pulse reverse electrodeposition (PRED) technology, leading to formation of the ultrahigh electrical conductivity composite films of Cu/F-CNTs. These composite films exhibit an ultrahigh electrical conductivity of up to 6.1 × 107 S/m (increased by 105.4% of that international annealed copper standard, IACS), but maintain a high hardness of 82.3 HV and tensile strength of 297.1 MPa. It is believed that this work opens new perspectives to develop ultrahigh electrical conductivity composite materials and would role as electric wire for reducing energy loss.

In-situ synthesis via laser metal deposition of a lean Cu–3.4Cr–0.6Nb (at%) conductive alloy hardened by Cr nano-scale precipitates and by Laves phase micro-particles

Conductive and yet strong copper alloys are essential materials in highly mechanically loaded electrical devices. We demonstrate a novel in-situ synthesis approach via laser metal deposition (LMD) in a lean copper alloy, Cu–3.4Cr–0.6Nb (at%). Strengthening in the lean alloy comes from chromium nano-scale precipitates formed in-situ (4 nm diameter; number density 8 × 1023 m−3) and from Laves phase particles (< 1 µm diameter; 2.2 vol%), dispersed across the microstructure. This dual dispersion, in a nearly pure copper matrix, is achieved through a suited combination of chromium alloying and cooling rate during LMD synthesis. The as-synthesized alloy has a conductivity of 68% IACS (International Annealed Copper Standard) and a Vickers hardness of 146, at room temperature. The latter is 11% above the value reported for the strongest lean reference ternary alloy Cu–8Cr–4Nb (at%). The in-situ synthesis approach averts any heat treatment step, which has been an essential step previously in conventional manufacturing, for realizing the property combination in lean Cu–Cr based system.

Effects of Ag on the oxidation kinetics and properties in copper-alumina composites during in-situ fabrication

External oxidation was adopted to fabricate Cu–Al2O3 composites with Ag addition. The influences of Ag on oxidation kinetics, microstructure evolution and properties of Cu–Al2O3 composites were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and optical microscopy (OM). The external oxidation was found to be the combination of surface oxidation and internal oxidation. The parabolic rate constant (PRc), which can reflect the oxidation rate during the internal oxidation process, was analyzed by experimental measurement and theoretical calculation. The measured PRc results during external oxidation have increased by 81% to 2.14×10−8cm2/s at 1073 K and 71% to 2.63×10−7 cm2/s at 1223 K after Ag was added. It indicates that the Ag can remarkably accelerate the internal oxidation process in external oxidation. Moreover, compared with traditional internal oxidation method, the external oxidation was found to be a more efficient way to prepare Cu–Al2O3. The PRc in traditional internal oxidation are 3.58×10−9 cm2/s at 1073 K and 6.72×10−8cm2/s at 1223 K, while the PRc in external oxidation are much higher which are 1.18×10−8cm2/s at 1073 K and 1.54×10−7 at 1223 K. SEM results show that the oxide bands composed of coarsening Al2O3, have disappeared, and the size of Al2O3 precipitates dropped from ∼65.78 nm to ∼45.42 nm after Ag was added. Due to the more homogeneous microstructure caused by Ag, the electrical conductivity had a 9.48% increment to ∼85.27% International Annealed Copper Standard (IACS), and the microhardness climbed from ∼131.8 HV to ∼152.76 HV.

Facile synthesis of high-performance carbon nanosheet/Cu composites from copper formate

Recently, much research has been reported on the fabrication of carbon-nanofiller (e.g., carbon nanotube (CNT), carbon nanosheet (CNS), and graphene)-reinforced copper matrix composites due to their wide potential applications. However, the preparation processes are generally complicated and difficult for commercial applications. Here, we propose a facile method to fabricate CNS-reinforced copper matrix composites by directly pyrolyzing copper formate powder at high temperature and then consolidating by spark plasma sintering. Interestingly, a complex interface consisting of CNSs, Cu nanoparticles, and CuO nanoparticles is found in the composite. The as-sintered CNS/Cu composite containing 0.27 vol% CNSs has a tensile yield strength of 498 ± 2 MPa and an electrical conductivity of 80.8 ± 0.1% International Annealed Copper Standard. The analysis of strengthening mechanism indicates that 51% of the tensile yield strength is contributed by the complex interface. The introduction of H2 during the pyrolyzing copper formate process results in an increase on the elongation and conductivity of CNS/Cu composite. After rolling, the elongation of CNS/Cu composite is as high as 8.5% and the friction coefficient is 0.25. This work provides a new guideline to the facile fabrication and interfacial design of advanced carbon-nanofiller-reinforced metal matrix composites with high performance.

Microstructure Evolution and Properties Tailoring of Rheo-Extruded Al-Sc-Zr-Fe Conductor via Thermo-Mechanical Treatment.

Low-cost heat-resistant Al-Sc-Zr-Fe conductor wires were successfully manufactured by continuous rheo-extrusion process, and the mechanical and conductive properties of the materials were analyzed and compared after three different thermo-mechanical treatment methods. The coarse plate-shape Al3Fe phase transformed to small sized rod-like phase after solid solution treatment at 630 °C for 21 h. Direct aging treatment at 300 °C for 24 h led to the refinement and spheroidization of Al3Fe phase with a diameter of 200 nm. After the subsequent aging treatment at 300 °C for 24 h, the tensile strength and conductivity of the alloy wire significantly increased due to the homogeneous precipitation of the coherent spherical Al3(Sc, Zr) phase with an average size of 15 nm. The tensile strength, elongation, and conductivity of the alloy conductor wire after optimized thermo-mechanical treatment reached 165.7 MPa, 7.3%, and 60.26% International Annealed Copper Standard (IACS), respectively. The thermal resistance of the present alloy wire was superior to that of standard AT1 type alloy conductor according to IEC international standard.

Precipitate phase transformation behavior, microstructure, and properties of Cu–Cr–Co–Si alloy

Abstract

Effect of ECAP Process and Subsequent Annealing on Microstructure and Properties of Cu-0.25Se-0.25Te Alloy

Cu-0.25Se-0.25Te alloy has been melted in a vacuum induction furnace and then heat treated. Afterwards, equal-channel angular pressing (ECAP) and subsequent annealing were carried out on the obtained samples. The effects of the ECAP process and subsequent annealing on the microstructure and properties of the alloy have also been investigated by scanning electron microscopy, Vickers microhardness testing, and eddy-current conductivity measurements. The results suggest a morphological evolution of the grains during the ECAP process. The equiaxed grains of the heat-treated alloy initially developed into shapes elongated along the shearing plane, then a series of smaller streamlined strip-like grains appeared and were eventually crushed into fine and equiaxed grains with a size of about 5 μm after different numbers of ECAP passes (one, two, four, or eight), along with a deformed strain e. Furthermore, the microhardness (HV) rose sharply from 68 N/mm2 to 140 N/mm2 and then climbed steadily to 169 N/mm2 under the comprehensive effect of e and the refinement of deformed grains. By contrast, the electrical conductivity decreased slowly from 96.2% international annealing copper standard (IACS) to 93.5% IACS. The microhardness and the electrical conductivity of the samples were found to vary contrarily with the change of annealing temperature and stabilized at 500°C. Notably, the electrical conductivity increased gradually, whereas the hardness of the alloys that were subject to a higher number of ECAP passes decreased remarkably with increasing annealing time at 300°C.

The microstructures and properties of graphite flake/copper composites with high volume fractions of graphite flake

Graphite flake (average lateral size of 292 μm and average thickness of 13 μm)/copper composites with high volume fractions (72.08%-93.34 %) of graphite flake were produced by a vacuum hot pressing method. Results show that the composites are anisotropic due to the alignment of the surface plane of graphite flake perpendicular to the pressing direction. With increasing the volume fraction of graphite flake, the density of the composites decreases from 4.07 to 2.63 g cm−3, the relative density decreases apparently when the volume fraction of graphite flake are more than 82.6%, the in-plane electrical conductivity decreases from 14.71% to 2.45% of the international annealed copper standard, the in-plane coefficient of thermal expansion decreases from 6.6 to 2.2×10−6 /K, the in-plane bending strength decreases from 42.48 to 14.63 MPa, the in-plane compressive strength decreases 45.75 to 20.46 MPa while the in-plane thermal conductivity exhibits a maximum of 663.73 W m−1 K−1 at the volume fraction of GF 82.6%. The maximum in-plane thermal conductivity is caused by inter-flake pores that are not fully infiltrated by Cu. The in-plane and out-of-plane thermal conductivity agree well with the modified layers-in-parallel model and modified layers-in-series, respectively.

Development and application of a miniaturized tensile testing device for in situ synchrotron microtomography experiments

The nondestructive nature of X-ray microtomography (µCT) associated with its suitable temporal resolution, obtained with the use of high energy polychromatic radiation from electron acceleration rings, allows in situ investigations of the damage evolution process during tensile loading in composite materials. In this context, the development of adequate scientific instrumentation as well as the understanding of the corresponding technology by research groups is paramount to guarantee reliability of the obtained results. In the present work, a miniaturized universal testing machine (tensile, compression and fatigue) was developed to be applied in in situ µCT experiments using synchrotron radiation with innovative technology. The equipment has a uniaxial load capacity of up to 300 N with 1 N resolution and can impose 0.01 mm resolution displacements following controlled deformation rates. In order to validate the developed equipment, tensile test results performed on sub-sized and standard annealed copper specimens were compared and in situ µCT tensile tests were performed in glass-fiber-reinforced polypropylene matrix composites at the Laboratorio Nacional de Luz Sincrotron (LNLS) in Campinas (SP, Brazil). Data analysis was performed by applying image processing techniques to successive tomograms in order to monitor crack propagation in the initial stages of damage evolution. The results obtained allowed the clarification of crack propagation mechanisms in the investigated composite materials and demonstrated the viability of the equipment’s intended application.

Liquid Crystal-Based Enclosed Coplanar Waveguide Phase Shifter for 54–66 GHz Applications

A 0–10 V bias voltage-driven liquid crystal (LC) based 0°–180° continuously variable phase shifter was designed, fabricated, and measured with insertion loss less than −4 dB across the spectrum from 54 GHz to 66 GHz. The phase shifter was structured in an enclosed coplanar waveguide (ECPW) with LC as tunable dielectrics encapsulated by a unified ground plate in the design, which significantly reduced the instability due to floating effects and losses due to stray modes. By competing for spatial volume distribution of the millimeter-wave signal occupying lossy tunable dielectrics versus low-loss but non-tunable dielectrics, the ECPW’s geometry and materials are optimized to minimize the total of dielectric volumetric loss and metallic surface loss for a fixed phase-tuning range. The optimized LC-based ECPW was impedance matched with 1.85 mm connectors by the time domain reflectometry (TDR) method. Device fabrication featured the use of rolled annealed copper foil of lowest surface roughness with nickel-free gold-plating of optimal thickness. Measured from 54 GHz to 66 GHz, the phase shifter prototype presented a tangible improvement in phase shift effectiveness and signal-to-noise ratio, while exhibiting lower insertion and return losses, more ease of control, and high linearity as well as lower-cost fabrication as compared with up-to-date documentations targeting 60 GHz applications.

Modification of Electrical and Mechanical Properties of Selective Laser‐Melted CuCr0.3 Alloy Using Carbon Nanoparticles

Herein, the influence of carbon nanoparticle addition on the selective laser melting (SLM) behavior and the resultant properties of gas‐atomized CuCr0.3 powder are investigated. The carbon addition neither affects the powder flowability, nor its packing density, but it significantly enhances the optical absorption for infrared radiation. Despite the improved optical absorption of the powder bed after carbon addition, a comparable SLM behavior is observed for both virgin and carbon‐mixed CuCr0.3 powders. Furthermore, the addition of carbon nanoparticles facilitates in situ deoxidation of copper and chromium oxides present in the CuCr0.3 powder. Accordingly, a larger fraction of chromium is obtained in its metallic form in the as‐built carbon‐mixed CuCr0.3 part. The presence of metallic chromium allows tailoring of the mechanical and electrical properties by postheat treatments (HTs). The best combination of mechanical (tensile strength of 298 ± 19 MPa, yield strength of 208 ± 18 MPa, ductility of 21 ± 4%) and electrical (87 ± 1% international annealed copper standards [% IACS]) properties is realized for the carbon‐mixed CuCr0.3 alloy after a direct age hardening HT. The improvement in mechanical properties is attributed to the precipitation of nanometer‐sized, coherent, and metallic chromium precipitates in the copper matrix.

Effect of Intermetallic Compound Layer on Peel Strength and Crack Propagation Behavior in Cu/Al/Cu Clad Composites

The effects of interfacial modification in tri-layered Cu/Al/Cu composites by heat treatment on interface stability and crack propagation were investigated. In order to investigate the crack path during the peel test, the intermetallic compound layer with the propagating crack was examined using electron backscatter diffraction (EBSD) analyses. The increase of peel strength from 7.8 to 9.1 N/mm in the tri-layered Cu/Al/Cu composite in the presence of thin discontinuous intermetallic compounds with heat treatment at 200–300 °C was accompanied by the increase of electrical conductivity from 65.3% IACS (International Annealed Copper Standard) to 66.8% IACS. Continuous intermetallic layers consisting of Al2Cu, AlCu, and Al4Cu9 were found in Cu/Al/Cu heat-treated at temperatures above 350 °C and its thickness increased rapidly and reached up to 35.2 μm at 500 °C. The peel strength drastically decreased to 5.75 N/mm after heat treatment at 400 °C, and it gradually increased as the heat treatment temperature was increased to 450 °C (5.91 N/mm) and 500 °C (6.16 N/mm). The increased peel strengths after heat treatment at 450 and 500 °C were accompanied by pronounced serrations of the peel strength–displacement curves. The amplitude of serration increased substantially with increasing annealing temperature from 400 to 500 °C. The major crack along the interface propagated, mostly along the Al2Cu/AlCu boundary with some inclined cracks, propagated through the AlCu and Al4Cu9 intermetallic compound layers. The repetition of crack propagation along the interface and crack deflection through the intermetallic layer as an inclined crack induced the serrated surface on the peeled-off Cu plate, enhancing the interface toughening.

High strength and electrical conductivity of copper matrix composites reinforced by carbon nanotube-graphene oxide hybrids with hierarchical structure and nanoscale twins

Cu matrix composites reinforced with carbon nanotube (CNT)-reduced graphene oxide (RGO) hybrids were prepared by molecular-level mixing (MLM). Hierarchical structure consists of carbon-rich region and carbon-poor region are formed through the self-assemble process by controlling the pH value during MLM process, and nanoscale twins in the Cu matrix are also formed during reduction and sintering process. It was demonstrated that both the hierarchical structure and nanoscale twins contribute to the mechanical properties and electrical conductivity of the composites. The ultimate tensile strength of 2.5 vol% CNT-RGO/Cu composite is 601 MPa, much higher than that of 2.5 vol% RGO/Cu composite (450 MPa) and pure Cu (294 MPa). The obvious grain refinement effect and higher load-transfer efficiency of CNT-RGO hybrids were believed to contribute to the high strength of CNT-RGO/Cu composite. At the same time, satisfied electrical conductivity of 83% and 85% International Annealed Copper Standard (IACS) are retained in 2.5 vol% CNT-RGO/Cu and RGO/Cu composites, respectively. This work provides a better understanding of the architecture design of Cu composites with hierarchical structure and contributes to optimize their mechanical and electrical properties.

A property-oriented design strategy for high performance copper alloys via machine learning

Traditional strategies for designing new materials with targeted property including methods such as trial and error, and experiences of domain experts, are time and cost consuming. In the present study, we propose a machine learning design system involving three features of machine learning modeling, compositional design and property prediction, which can accelerate the discovery of new materials. We demonstrate better efficiency of on a rapid compositional design of high-performance copper alloys with a targeted ultimate tensile strength of 600–950 MPa and an electrical conductivity of 50.0% international annealed copper standard. There exists a good consistency between the predicted and measured values for three alloys from literatures and two newly made alloys with designed compositions. Our results provide a new recipe to realize the property-oriented compositional design for high-performance complex alloys via machine learning.

High-Strength Cu–Ta–W Composite

High-strength copper conductor composites have been used in pulse magnet applications and as internal reinforcement in superconductor wires. The required property is high strength while maintaining high conductivity. Heavily worked pure copper has a strength of 450-500 MPa. Increasing the strength of copper by alloying additions without affecting the conductivity drastically is a significant challenge. Mixing pure copper with a higher modulus, higher strength tantalum alloys is an option to meet the challenge. This work reports on the use of a tantalum-tungsten alloy to strengthen high-purity copper through a multi-filament approach. The tantalum tungsten has a high modulus and very low solubility in copper. A composite wire containing 133 filaments of Ta-W (25% by volume) has a conductivity of 80% International Annealed Copper Standard and ultimate tensile strength of over 650 MPa with a modulus over 140 GPa. The wire has good ductility and toughness. These results are comparable or better than current options, and the potential to improve this product is explored.

Theoretical and experimental analysis of the deformation zone and minimum thickness in single-roll-driven asymmetric ultrathin strip rolling

Although asymmetric rolling is employed extensively to produce ultrathin strips in micromanufacturing, microelectronics, and other high precision engineering fields, very limited attention has been given to the deformation zone, which is a direct result of the material thickness reduction behavior during asymmetric rolling. In this study, theoretical analysis of the minimum thickness of single-roll-driven asymmetric rolling is performed by combining FE (finite element) and theoretical analytical models to complete a comprehensive investigation. Based on the FE model calculation results, the deformation zone contour varies with reduction ratio, strip thickness, and yield strength; the Fleck model can be used to describe single-roll-driven asymmetric ultrathin strip rolling when the thickness is small enough. Different from symmetric rolling, a cross-shear zone is inevitably formed during asymmetric rolling, but the cross-shear force resulting from asymmetric rolls decreases and approaches 0 with smaller strip thickness, smaller reduction ratio, and higher yield strength, which means the rolling status of single-roll-driven asymmetric ultrathin strip rolling is very similar to symmetric rolling when the strip reaches its limiting minimum thickness. Therefore, a minimum thickness model is developed for single-roll-driven asymmetric ultrathin strip rolling with different roll diameters, mean tensions and strip yield strengths, and the elastic modulus and Poisson ratio of the two rolls are considered. Eventually, the minimum thickness for single-roll-driven asymmetric rolling of ultrathin strips is estimated using the new minimum thickness model, with further experimental verification by the single-roll-driven asymmetric rolling of various materials, including aluminum, 304 stainless steel, and both non-annealed and annealed copper. Compared with experimental results, the model is shown to be a possible alternative for more accurate minimum thickness prediction in single-roll-driven asymmetric rolling.

Comparative studies on microstructures and properties of Cu–Ni–M alloys controlled by strong interaction between elements

The heat resistance of copper alloys is desirable to improve while it services at high temperature as electrically and thermally conductive components. In present paper, the strengthening pattern of nickel-based superalloys is expected to be applied in copper alloys. Here the Cu50Ni37.5M12.5 (M = Al, Cr, Mo) alloys were prepared by vacuum arc melting and heat-treated in the corresponding conditions to obtain three different kinds of typical microstructure. The room temperature (RT) properties (hardness and resistivity) and high temperature properties (melting point, softening temperature and variable temperature resistivity) of the three alloys are contrasted and analyzed in detail. The Cu50Ni37.5Al12.5 alloy strengthened through the γ′ phase coherent precipitation shows the best conductivity and highest hardness (5.47 %IACS (International Annealed Copper Standard) and 310.1 HV). The splitting of the γ′ phase is contributing to hardening at high aging temperature. The resistivity-temperature behavior of the three alloys shows that the resistivity with rising temperature consists of the ideal lattice resistivity and the resistivity increment due to the defects. The Cu50Ni37.5Al12.5 alloy has the smallest room temperature resistivity and its resistivity maintains the lowest during the whole heating process (RT-1077K). Therefore, the coherent precipitated γ′ phase is expected to improve the heat resistance of the copper alloys while maintaining its electrical conductivity.