Oxygen-free Electronic Copper Research Articles

Investigations on the Effects of Bonding and Forming Conditions on the Deformation Behavior of Copper-Steel Bimetallic Rods during the Cold Drawing Processes.

Metal composite parts are widely used in different industries owing to their significant improvement in material properties, such as mechanical strength, electrical conductivity, and corrosion resistivity, compared to traditional single metals. Such composite parts can be manufactured and processed in different ways to achieve the desired geometry and quality. Among various metal forming techniques, drawing is the most commonly used process to produce long composite wires or rods from raw single materials. During the drawing process of composite wires or rods, not only does the core radius ratio change, but the core or sleeve layer may also undergo necking or fracture due to excessive tensile stresses in the softer layer. In this paper, bimetallic rods with AISI-1006 low-carbon steel cores and C10100 oxygen-free electronic copper sleeves are modeled using the finite element software DEFORM. The simulation models are verified by drawing experiments. The effects of initial bonding conditions, the initial core ratio, reduction ratio, semi-die angle, drawing speed, and friction on the plastic deformation behavior of the bimetallic rods are investigated. The results indicate that the initial bonding conditions have a great impact on the deformation behavior of the billets in terms of strain distribution, material flow, residual stress, and the final core ratio. The permissible forming parameters for obtaining a sound product are investigated as well. With the aid of these analyses, the drawing process and the quality of the products can be controlled steadily.

Forming limit diagram of annealed copper OFE thick sheets for optimized hydroforming of superconducting RF cavities

Coated superconducting radio-frequency (SRF) cavities are planned to be implemented in the Future Circular Collider (FCC) at the European Organization for Nuclear Research (CERN). The interest in industrialized and high-performance processes for fabrication of the corresponding copper substrates has thus risen. Tubular hydroforming is a well-known industrial process, but has not been thoroughly applied for ultra-pure oxygen-free electronic copper (Cu-OFE) thick products. To provide a feasibility analysis for the optimized fabrication process of copper substrate seamless cavities, a Forming Limit Diagram (FLD) of the material of interest was studied. In this paper, the methodology, test results and initial benchmark of the material model is presented, for 4 mm and 2 mm thick sheets.

Fabrication of NBI Ion Source Back Plate and its High Heat Flux Experiment

The back plate is an important component of the ion source because of its multiple roles including heat load removal during beam operation. The main components of the back plate are (1) a Type 304L stainless steel (SS304L) magnet positioning plate that holds samarium cobalt permanent magnets required for the confinement of ion source plasma, (2) an oxygen-free electronic copper cooling plate with 35 inner and 8 outer cooling channel grooves (each of which is 4 × 1.8 mm2) that is vacuum brazed with a SS304L magnet positioning plate, and (3) a SS304L magnet cover plate. In this paper, the back plate is successfully fabricated, and a high heat flux experiment is done at the High Heat Flux Test Facility Center with an electron beam power of 200 kW for 458 s. The uniform incident heat flux is 2.5 MW/m2. Demineralized water at 34°C is supplied at the rate of 1 kg/s to the cooling plate at inlet pressure of 8.2 bars to remove the high heat load. The surface temperature of the copper plate is measured by an infrared camera, and three temperature regions are observed. The measured average surface temperature of the cooling plate is ~152°C. The bulk water temperature rise ΔTw is ~39.42°C. The estimated absorbed heat flux is ~2.04 MW/m2, and the heat absorption coefficient is 81.6%. The measured leak rate after the heat flux test is 1.6 × 10−8 mbars∙L/s. These High Heat Flux Test experimental results will be useful to study the thermomechanical performance of the back plate and to understand the effect of increasing the beam pulse length.

Experimental analysis of subsurface integrity during fine turning of OFE copper for radiofrequency cavity manufacturing

Machining oxygen-free electronic (OFE) copper could be challenging but is not widely studied because few industrial or critical components requires to master the machined sub-surface characteristics. CERN radio frequency cavities are one of the applications, especially because the turned surface is not the functional one of the final products. The niobium coating post process, which gives superconductive properties to the cavity, largely depends on the machined surface characteristics. The present study relies on an experimental approach of the cutting process, through thermal and mechanical probing of high precision, pollution free, turning. Cutting forces and thermal load on the tool are detailed for finish turning. The critical uncut chip thickness, defined at macroscale as the limit between cutting and ploughing behavior, is also a frontier at microscale. Consequently, surface integrity is evaluated by advance microstructural analysis (EBSD and FIB), imposed by the thinness of the affected layer. Grain recrystallization appears in the first 0.6 micrometers below the surface and deformed grains are observed up to 4 micrometers for cutting regime, while the thickness of the layers is three time larger in case of ploughing regime. Hence surface integrity of OFE copper finish turning is characterized and optimal cutting conditions are defined. The research shows that simple cutting tests can quickly narrow down to optimal cutting condition, which are then confirmed through metallurgical analysis, even in the edge case of pure OFE copper, hence relevant to other material.

Plasma sprayed alumina-yttria composite ceramic coating for electrical insulation applications

A composite ceramic coating consisting of 50:50 weight fraction of alumina (Al2O3)-yttria (Y2O3) blend was deposited as an electrically insulating topcoat on Oxygen-Free Electronic Copper (OFEC) substrates with a cermet interlayer of 30 wt% NiAl-70 wt% Al2O3 blend, both deposited by Atmospheric Plasma Spray (APS) process, and with a NiCrAlY bond coat deposited by High-Velocity Oxy-Fuel (HVOF) process for application in Direct Current (DC) conduction pump of the sodium-cooled fast reactor. The cermet interlayer in between topcoat and bond coat is beneficial for a reduction in the thermal expansion mismatch stresses and further contribute to the improvement in the thermal fatigue life of the multilayer coating system. The coated samples were subjected to a thermal fatigue cycle between 600 °C and 200 °C, simulating sodium-cooled fast reactor temperature patterns in regular operation and transient conditions. The multilayer composite ceramic coating with cermet interlayer successfully endured up to 1000 thermal cycles with no cracking, spallation, or delamination of the topcoat. Surface cross-sectional micrographs before and after the thermal cycle revealed neither significant changes in the splat and lamellae morphology nor distribution of phases, except for the densification effects. X-ray Diffraction (XRD) analysis revealed neither phase change nor the formation of any new phase upon thermal cycling. The adhesion test as per ASTM-C633 revealed that the bonding strength of the coatings lower by ~20% after thermal cycling. The insulation resistance of thermal cycled samples showed a two order reduction in the magnitude, compared to that of the as-coated samples.

Mechanical properties of degraded OFE copper subjected to electron irradiation

• SPT specimens of OFE copper are subjected to electron irradiation in accelerator. • Experimental results are used to get mechanical properties as a function of dose. • Fracture toughness indicates a sharp drop in ductility beyond 62.5 MGy. • Such observation is validated by variation of YS, biaxial-fracture-strain and specimen energy. • A New correlation between biaxial-fracture-strain and specimen energy is derived. The mechanical and fracture properties of aged materials degrade due to metallurgical and micro- structural changes during service life. Miniaturized specimen testing methodology has been evolved over the years to quantify such degradation mechanisms. Oxygen Free Electronic (OFE) Copper has wide applications in nuclear industry, particularly in fusion reactors and accelerators. Such material when subjected to nuclear irradiation during service life experiences degradation of ductility and fracture toughness. In the present study, linear accelerator is used to irradiate OFE Copper specimens up to varying dose of irradiation. Specimens are subjected to electron irradiation dose which varies from 11 MGy to 91.5 MGy. The specimens are then tested, and the load-displacement curves thus generated are used to quantify changes in mechanical and fracture properties, viz. yield stress, ultimate stress, specimen energy at fracture, bi-axial fracture strain and fracture initiation toughness of irradiated specimens. It is observed that there is a loss of ductility of OFE copper beyond 62.5 MGy irradiation dose as indicated by decrease in biaxial fracture strain, specimen energy and fracture initiation toughness. Such changes should be taken into consideration for calculating service life during the design of components made up of OFE Copper subjected to irradiation.

Development and beam commissioning of a continuous-wave window-type radio-frequency quadrupole

The radio-frequency quadrupole (RFQ) group at Peking University has built a window-type RFQ, operating at 162.5 MHz in continuous-wave (cw) mode. It is designed to accelerate a 50 mA deuteron beam from 50 keV to 1 MeV with a vane length of 1.809 m. The cavity was fabricated in two segments using 100% oxygen-free electronic (OFE) copper. Using an iterative assembly and measurement procedure for the precise alignment of the two segments, we reduced the assembly errors to within $\ifmmode\pm\else\textpm\fi{}0.05\text{ }\text{ }\mathrm{mm}$. The radio frequency (rf) measurements of the whole cavity show excellent rf properties, with the measured intrinsic Q-value of 8962, which equates to 96% of the simulated value for OFE copper. We also investigated field fluctuations caused by misalignment between the two segments, and studied their impact on the beam transmission using beam dynamics simulations. During field tuning, we compiled a set of unique tuning rules for the window-type RFQ. After tuning, the maximal field unflatness of the single quadrant is within $\ifmmode\pm\else\textpm\fi{}2%$, and the asymmetry of the four quadrants is within $\ifmmode\pm\else\textpm\fi{}1%$. During rf conditioning, the cw power of the cavity reached 55 kW within 32 hours, and we have recorded nearly seven hours of stable running at a cw power of 50 kW. The measured bremsstrahlung spectrum shows that the accelerator needs 49.9 kW to generate the intervane voltage of 60 kV, with a specific shunt impedance of $130.5\text{ }\text{ }\mathrm{k}\mathrm{\ensuremath{\Omega}}\text{ }\mathrm{m}$. An ${\mathrm{H}}_{2}^{+}$ ion beam extracted from an electron cyclotron resonance ion source was used for the beam commissioning, because deuteron beam acceleration will bring a serious radiation field. We achieved stable and robust acceleration of about 1.5 mA cw ${\mathrm{H}}_{2}^{+}$ beam for one hour.

Influence of Exposure to Multiple Brazing Cycles on the Integrity of OFE Copper Brazed Joints

A 10 MeV, 5 kW S-band travelling wave electron linear accelerator has been developed at Raja Ramanna Centre for Advanced Technology for industrial applications. The electron accelerating structure is made of oxygen-free electronic (OFE) copper and comprises of fifty-six brazed joints and is fabricated in multiple stages, as per the process and testing requirements. The accelerating structure is often required to be repaired through an additional brazing cycle with the same filler. The present study evaluates possible microstructural and mechanical property degradation of OFE copper brazed joints, as a result of its exposure to multiple brazing cycles. The study was performed on an OFE copper cavity stack brazed assembly, made through a four-stage brazing using BVAg-8 as braze filler. The resultant brazed assembly did not exhibit any leak at a sensitivity of 1 × 10−10 mbar.L/s. The results of the study demonstrate that OFE copper brazements can be safely subjected to an additional brazing cycle without causing significant degradation in its microstructure and strength. However, brazed joint’s exposure to any additional brazing cycles results in the formation of voids and cracks due to intergranular penetration of silver, with associated drop in tensile strength from about 210 MPa to about 150-180 MPa. Based on the results of study, it is recommended to qualify the accelerating structure, fabricated through multiple brazing cycles, by a suitable non-destructive technique. The results of the study are important for design, fabrication and maintenance of electron linear accelerator components.

Process Development for Vacuum Brazed Niobium–316L Stainless Steel Transition Joints for Superconducting Cavities

The paper describes process development for producing sound, strong, and ductile Nb pipe–316L stainless steel (SS) flange brazed joint suitable for application in superconducting radiofrequency (SRF) cavities. The developed transition joints, made with BVAg-8 braze filler metal (BFM), were free of brittle intermetallic compounds, in contrast to the existing global brazing practice of using oxygen-free electronic copper as BFM which results in the formation of a continuous layer of Fe–Nb brittle intermetallic compound at Nb–braze interface. In view of the large difference in the mean thermal expansion coefficients between niobium and 316L stainless steel, a new design for manufacturing and assembly (DFMA) has been developed to ensure achievement of desired joint thickness with uniformity in circumferential and longitudinal directions. An environment-friendly prebraze cleaning procedure has been qualified and implemented. DFMA has resulted in (i) significant reduction of the out-of-roundness errors (≤10 μm) while machining of the niobium pipe, (ii) simplified clearance fit prebraze assembly at room temperature (RT), and (iii) uniformity of joint thickness. A process flow chart has been developed to ensure repeatability of joint characteristics. The brazed joint, of niobium pipe and 100CF knife edge 316L SS flange made by standardized practice, displayed helium leak tightness better than 5 × 10−10 mbar l/s at RT and at liquid helium temperature (LHT). The braze-joint sustained 873 K/10 h postbraze hydrogen degassing treatment and thermal cycling between RT and LHT without any loss in hermeticity.

Study on requirement of nickel electroplating in OFE copper-316L stainless steel brazed joints

The paper describes an experimental study to evaluate the need of nickel electroplating on stainless steel part in vacuum brazed oxygen-free electronic (OFE) copper-type 316L stainless steel transition joints with silver copper eutectic braze filler metal (BVAg-8 of AWS A5.8). The results of the study demonstrated that nickel electroplating is not required to obtain hermetically sealed sound and strong OFE copper-316L stainless steel transition vacuum brazed joints. The only pre-treatment requirement for stainless steel part is simple ultrasonic cleaning, although pickling treatment has been found to have a favorable effect in improving its wettability for the braze filler metal. Brazed joints made with unplated stainless steel (SS) not only displayed (i) required level of hermeticity (helium leak rate ≤2 × 10−10 mbar·l/s) and bakeablility at 523 K for ultra-high vacuum application and (ii) reasonably good tensile strength comparable to those made with Ni-plated SS, but also exhibited significantly suppressed extent of intergranular penetration of braze filler metal into copper. Important factors responsible for the development of sound OFE copper-316L stainless steel brazed joints with ultrasonically cleaned stainless steel part are excellent wettability of OFE copper which compensated for poor wettability of stainless steel surface and presence of hydrocarbon-free high vacuum environment during brazing.

Quantifying height of ultraprecisely machined steps on oxygen-free electronic copper disc using Fourier-domain short coherence interferometry

The internal shape and alignment of accelerator discs is crucial for efficient collider operation at the future compact linear collider (CLIC). We applied a calibrated custom-made Fourier-domain short coherence interferometer to measure the height of 40 and 60 μm ultraprecisely turned steps (surface roughness Ra≤25 nm, flatness ≤2 μm) on an oxygen-free electronic copper disc. The step heights were quantified to be (39.6±2.6) μm and (59.0±2.3) μm. The uncertainties are quoted at 95% confidence level and include contributions from calibration, refractive index of air, cosine error, surface roughness, and thermal expansion in comparison to standard temperature of 20°C. The results were verified by measuring the same steps using a commercial white light interferometer Veeco—NT3300. Our instrument can ensure that the accelerator discs of the CLIC are aligned within the tolerance required for efficient collider operation.

Hybrid Bishop-Hill model combined finite element analysis for elastic-yield limited design

Purpose – Microstructure-sensitive design (MSD), for optimal performance of engineering components that are sensitive to material anisotropy, has largely been confined to the realm of theory. The purpose of this paper is to insert the MSD framework into a finite element environment in order to arrive at a practical tool for improved selection and design of materials for critical engineering situations. Design/methodology/approach – This study applies the recently developed Hybrid Bishop-Hill (HBH) model to map the yield surface of anisotropic oxygen free electronic copper. Combining this information with the detailed local stresses determined via finite element analysis (FEA), a “configurational yield stress” is determined for the entire component. By varying the material choice/processing conditions and selecting the directionality of anisotropy, an optimal configuration is found. Findings – The paper provides a new FEA-based framework for MSD for yield-limited situations. The approach identified optimal directionality and processing configurations for three engineering situations that are particularly sensitive to material anisotropy. Research limitations/implications – The microstructure design space for this study is limited to a selection of eight copper materials produced by a range of processing methods, but is generalizable to many materials that exhibit anisotropic behavior. Originality/value – The introduction of MSD methodology into a finite element environment is a first step toward a comprehensive designer toolkit for exploiting the anisotropy of general materials (such as metals) in a way that is routinely undertaken in the world of fiber-based composite materials. While the gains are not as sizeable (due to the less-extreme anisotropy), in many applications they may be extremely important.

Four-Parameter Hybrid–Bishop–Hill Model Applied to OFE Copper for the Evaluation of Elastic/Yield Limit

This study employs a novel stress-based Hybrid–Bishop–Hill yield model approach to evaluate the yield surface of oxygen-free electronic copper samples. The local yield surface is determined from three parameters of crystal orientation and one parameter of geometrically necessary dislocation (GND). All four local state variables can be rapidly determined by analysis of measured electron backscatter diffraction patterns. Estimates for the polycrystalline yield surface are obtained by standard averaging procedures. The shape of the yield surface is most influenced by the texture of the material, while the volume of the envelope scales with the average GND density. However, correlations between crystal orientation and GND content modify the yield surface shape and size. While correlations between GND density and crystal orientation are not strong for most copper samples, there are sufficient dependencies to demonstrate the benefits of the detailed four-parameter model. The four-parameter approach has potential for improving estimates of elastic-yield limit in all polycrystalline materials.

Development and beam test of a continuous wave radio frequency quadrupole accelerator

The front end of any modern ion accelerator includes a radio frequency quadrupole (RFQ). While many pulsed ion linacs successfully operate RFQs, several ion accelerators worldwide have significant difficulties operating continuous wave (CW) RFQs to design specifications. In this paper we describe the development and results of the beam commissioning of a CW RFQ designed and built for the National User Facility: Argonne Tandem Linac Accelerator System (ATLAS). Several innovative ideas were implemented in this CW RFQ. By selecting a multisegment split-coaxial structure, we reached moderate transverse dimensions for a 60.625-MHz resonator and provided a highly stabilized electromagnetic field distribution. The accelerating section of the RFQ occupies approximately 50% of the total length and is based on a trapezoidal vane tip modulation that increased the resonator shunt impedance by 60% in this section as compared to conventional sinusoidal modulation. To form an axially symmetric beam exiting the RFQ, a very short output radial matcher with a length of $0.75\ensuremath{\beta}\ensuremath{\lambda}$ was developed. The RFQ is designed as a 100% oxygen-free electronic (OFE) copper structure and fabricated with a two-step furnace brazing process. The radio frequency (rf) measurements show excellent rf properties for the resonator, with a measured intrinsic $Q$ equal to 94% of the simulated value for OFE copper. An ${\mathrm{O}}^{5+}$ ion beam extracted from an electron cyclotron resonance ion source was used for the RFQ commissioning. In off-line beam testing, we found excellent coincidence of the measured beam parameters with the results of beam dynamics simulations performed using the beam dynamics code TRACK, which was developed at Argonne. These results demonstrate the great success of the RFQ design and fabrication technology developed here, which can be applied to future CW RFQs.

Room Temperature Recovery in Rolled Polycrystalline Copper after many Years

Eight years ago recrystallization of OFE (oxygen-free electronic) copper was examined in detail using various techniques. In 2008 exactly the same material was measured using EBSD microscopy. The deformed state and fully recrystallized state have been analyzed and compared with data obtained eight years ago. The stored energy (SE) estimated by Image Quality (IQ) analysis was compared in these two cases. A significant amount of recovery took place in the sample, but only in some texture components. Some others present more or less the same SE as eight years ago. The textures of recrystallized samples were compared. We observed that the difference in SE distribution between the two deformed state has an influence on the final textures after recrystallization. Our study confirms the hypothesis that if a grain (orientation) has distinctly lower SE than other orientations - it has the highest growth preference (threshold hypothesis). Such grains (orientations) are dominant in the recrystallization texture.

Hybrid Bishop-Hill Model for Elastic-Yield Limited Design With Non-orthorhombic Polycrystalline Metals

A method is presented for adapting the classical Bishop-Hill model to the requirements of elastic/yield-limited design in metals of arbitrary crystallographic texture. The proposed Hybrid Bishop-Hill (HBH) model, which will be applied to ductile FCC metals, retains the “stress corners” of the polyhedral Bishop-Hill yield surface. However, it replaces the ‘maximum work criterion’ with a criterion that maximizes the projection of the applicable local corner stress state onto the macroscopic stress state. This compromise leads to a model that is much more accessible to yield-limited design problems. Demonstration of performance for the HBH model is presented for an extensive database for oxygen free electronic copper. The design problem considered is a hole-in-a-plate configuration of thin sheets loaded in uniaxial tension in arbitrary directions relative to the principal directions of material orthorhombicity. Results obtained demonstrate that HBH-based elastic/yield limited design is capable of predicting complex and highly nonintuitive behaviors, even within standard problems.

Identification of optimized constitutive model parameters at a high strain rate using electromagnetic ring expansion test results

In this article, parameters of the material constitutive model have been identified at high strain rate electromagnetic ring expansion test using experimental data taken from literature for oxygen-free electronic copper (OFE Cu). The experimental data were processed using a finite-element optimization procedure in which the measured deformation has been applied to a specimen. An optimal set of material constants for JohnsonCook constitutive model have been computed by minimizing the standard deviation of differences between experimental and calculated stressstrain curves. The applicability of identified parameters has been validated successfully by using simulation of reverse Taylor impact test results. The presented method in this article was found to be an effective method for identifying the material model parameters at high strain rates.

Instrumented Taylor anvil-on-rod impact tests for validating applicability of standard strength models to transient deformation states

Instrumented Taylor anvil-on-rod impact tests have been conducted on oxygen-free electronic copper to validate the accuracy of current strength models for predicting transient states during dynamic deformation events. The experiments coupled the use of high-speed digital photography to record the transient deformation states and laser interferometry to monitor the sample back (free surface) velocity as a measure of the elastic∕plastic wave propagation through the sample length. Numerical continuum dynamics simulations of the impact and plastic wave propagation employing the Johnson-Cook [Proceedings of the Seventh International Symposium on Ballistics, 1983, The Netherlands (Am. Def. Prep. Assoc. (ADPA)), pp. 541–547], Zerilli-Armstrong [J. Appl. Phys. C1, 1816 (1987)], and Steinberg-Guinan [J. Appl. Phys. 51, 1498 (1980)] constitutive equations were used to generate transient deformation profiles and the free surface velocity traces. While these simulations showed good correlation with the measured free surface velocity traces and the final deformed sample shape, varying degrees of deviations were observed between the photographed and calculated specimen profiles at intermediate deformation states. The results illustrate the usefulness of the instrumented Taylor anvil-on-rod impact technique for validating constitutive equations that can describe the path-dependent deformation response and can therefore predict the transient and final deformation states.

Anisotropy in mechanical properties of high-purity copper processed by equal channel angular extrusion

High-purity oxygen-free electronic copper has been processed by equal channel angular extrusion for up to four passes at room temperature, using 90° tooling, by either route B c or C. Small tensile specimens and compression cubes were sectioned by electro-discharge machining and tested in the as-processed condition. The equal channel angular extrusion (ECAE)-processed material was mechanically anisotropic, with the strength in compression in the direction perpendicular to the plane formed by the inlet and outlet channels being greater than that in the other directions, for both routes. Work softening occurred after a strain of about 1% in this direction, whereas slight but positive work hardening occurred in the other directions, for both routes. In the extrusion direction, the strength in compression was greater than the strength in tension, for both routes. Modeling which assumes that active slip planes harden more rapidly than inactive planes can predict the measured behavior reasonably well, apart from the work softening.

Experimental study of rf pulsed heating on oxygen free electronic copper

When the thermal stresses induced by rf pulsed heating are larger than the elastic limit, microcracks and surface roughening will occur due to cyclic fatigue. Therefore, pulsed heating limits the maximum surface magnetic field and through it the maximum achievable accelerating gradient. An experiment using circularly cylindrical cavities operating in the ${\mathrm{TE}}_{011}$ mode at a resonant frequency of 11.424 GHz was designed to study pulsed heating on oxygen free electronic (OFE) copper. An $X$-band klystron delivered up to 10 MW to the cavities in $1.5\text{ }\ensuremath{\mu}\mathrm{s}$ pulses at 60 Hz repetition rate. One run was executed at a temperature rise of 120 K for $56\ifmmode\times\else\texttimes\fi{}{10}^{6}$ pulses. Cracks at grain boundaries, slip bands, and cracks associated with these slip bands were observed. The second run consisted of $86\ifmmode\times\else\texttimes\fi{}{10}^{6}$ pulses with a temperature rise of 82 K, and cracks at grain boundaries and slip bands were seen. Additional information can be derived from the power-coupling iris, and we conclude that a pulsed temperature rise of 250 K for several million pulses leads to destruction of copper. These results can be applied to any mode of any OFE copper cavity.