Oxygen Free High Conductivity Research Articles

Laser Powder Bed Fusion Additive Manufacturing of Mo and TZM Exoskeleton with Cu Infiltration for New Heat Sinks Configuration

This study reports the fabrication and characterization of molybdenum (Mo) metal and titanium‐zirconium‐molybdenum (TZM) alloy exoskeletons with honeycomb cavity structures (HCS) that are infiltrated with oxygen‐free high conductivity (OFHC) Cu under an inert atmosphere as a potential replacement for Cu‐Mo‐Cu laminate in heat sink applications for power electronics semiconductors like GaAs. The thermal expansion behavior and the thermal loading of the starting Mo and TZM structures, and of the Cu‐infiltrated parts are evaluated. The fabricated Mo and TZM structures with density >99% and Mo‐based heat sinks with improved CTE (6.6 × 10−6 K−1) when compared to conventional Cu‐Mo‐Cu laminated heat sinks (CTE = 7.6 × 10−6 K−1). The new Mo and TZM structures promise superior performance due to their closer CTE to that of GaAs and similar semiconductors (CTE = 5.7 × 10−6 K−1). Exposure to temperatures up to 1073 K did not affect the Mo microstructure due to the inherent resistance to recrystallization, while exposure to 1373 K did reduce hardness. In contrast, TZM exoskeletons showed resistance to recrystallization even at 1373 K. The fabricated composite heat sinks showed thermal diffusivity (≈61 × 106 m2 s−1) that is within the upper limits of those reported for commercial laminated heat sinks (45 to 65 × 106 m2 s−1).

Secondary electron yield (SEY) evolution of laser processed oxygen-free high conductivity copper induced by different ultrasonic cleaning durations

Laser treated material surface has highly regular curves and greatly reduces the secondary electron yield of surface. As one of the most typically used materials in the vacuum system of particle accelerators, the surface of oxygen-free high conductivity copper (OFHC) is processed by laser. Moreover, ultrasonic cleaning can effectively remove powders and impurities produced by laser ablation. The secondary electron yield (SEY), surface morphology and surface composition of laser treated oxygen-free copper samples before and after ultrasonic cleaning were compared. Experiments were conducted by using the secondary electron yield test device, X-ray diffractometer, X-ray photoelectron spectroscopy, and scanning electron microscope to characterize the SEY, crystal structures, surface composition and surface morphology of laser-treated oxygen-free copper samples, respectively. The SEY evolution of the samples was investigated and analyzed at electron doses of 7.6 × 10−6 C mm−2: after 15-min ultrasonic cleaning, the δmax of sample #1, #2 and #3 increased from 1.09, 1.23, and 1.38 to 1.19, 1.2, and 1.63, respectively, with the maximum increase of 18.12 %. Ultrasonic cleaning could greatly reduce the accumulation of spherical particles and the fibrous or flocculent structures produced by laser processing, thereby reducing the capture of secondary electrons.

Thermal Design of the Conduction-Cooled High Temperature Superconducting Magnet for Pulsating Magnetic Field

A conduction-cooled high temperature super-conducting (HTS) solenoid is fabricated to produce pulsat-ing 4 T magnetic field for magnetic refrigeration. HTS con-ductor of 245 m is wound as a solenoid with 112 layers each of which consists of 12 turns of the conductor. For a stable ramping operation of the magnet, AC loss inside the wind-ing pack should be effectively rejected to the heat sink. The thermal drains are inserted between the layers and con-nected to the magnet flange which is cooled by a commercial two-stage Gifford-McMahon cooler. They are made of Ox-ygen-Free High Conductivity (OFHC) copper, and has a width of 6 mm and a thickness of 50 μm. Thermal analysis is conducted to confirm the effect of the thermal drain on the AC loss heat rejection. AC loss of the magnet is esti-mated by using T-A formulation (current vector potential T and magnetic vector potential A). The ramp rate of the mag-net and the heat sink temperature are set to 0.1 T/s and 4.5 K, respectively. In the thermal diffusion calculation, each winding layer is considered as the lumped capacitance. In addition, thermal conduction between the adjacent lay-ers and that through the thermal drains are considered. The thermal analysis confirms that the conductor temperature of the winding is maintained lower than the operating limit of 20 K. Based on the results of the thermal analysis, the stable pulsating operation of the conduction-cooled super-conducting magnet is predicted to be feasible and is also confirmed by the experiment.

Quantification of softening kinetics in cold-rolled pure aluminum and copper using High-Temperature Scanning Indentation

The High-Temperature Scanning Indentation (HTSI) method enables the continuous monitoring of various mechanical properties, including Young's modulus, hardness, and creep properties, during specific thermal cycles. In this study, the HTSI method is applied to cold-rolled samples of commercially pure aluminum and oxygen-free high conductivity (OFHC) copper. The observed variations in hardness during heating are attributed to the underlying restoring mechanisms, namely static recovery and static recrystallization. Inverse analyses are performed using established metallurgical models of restoration. The results highlight the influence of the initial deformation state and heating rate on the kinetics parameters. The findings are further supported by Electron-Back Scattering Diffraction measurements. The study concludes by demonstrating the HTSI method's capability to quantify restoration parameters as a function of temperature through a limited number of well-designed HTSI experiments.

A multi-length-scale investigation of the applicability of ductility laws for annealed and work-hardened copper

Small-scale mechanical testing provides unique advantages over conventional testing in investigating the mechanical response of nuclear materials. While small-scale testing enables accelerated materials research, it raises the question of comparability to large-scale properties for engineering design considerations. Among different mechanical properties for structural design considerations such as yield and ultimate tensile stress, ductility is of critical importance with respect to structural component health. The effectiveness of using small-scale mechanical testing for probing these mechanical properties is dictated by understanding how the mechanical responses translate from the microscale to the engineering scale. Therefore, a better understanding of size scaling effects via gathering and analysis of experimental data is required to bridge length-scales. This work builds upon previous reports to provide statistical data on how mechanical response changes as the size of the specimens are reduced from conventional sizes. One hundred twenty mesoscale tensile coupons of oxygen-free high thermal conductivity (OFHC) copper in work-hardened and annealed state have been manufactured using micro-wire electrical discharge machining (EDM) and tested under uniaxial tension. The mesoscale specimens have dimensions dictated by pre-selected geometric ratios (i.e., gauge section length over the square root of the cross-section area) with gauge length dimensions of 3 mm (or less) and sub-millimeter gauge widths and thicknesses. The mesoscale responses were compared to bulkscale properties to reveal the effect of specimen size on the mechanical response as a function of grain size and material state for a single phase face-centered cubic metal.

Investigation of thermal contact conductance of bare copper-copper and aluminum 1070-aluminum 1070 interfaces at low temperatures between 10 and 70 K

The prediction model of thermal contact conductance (TCC) for bare OFHC-OFHC and Al 1070-Al 1070 flat interfaces is presented. OFHC (Oxygen free high conductivity copper) and Aluminum are widely used in cryogenic systems. Since the thermal conductivity of pure copper or aluminum materials has a highly non-linear characteristic with its peak value at around 20 K, it is difficult to predict TCC of those materials at this temperature range. The TCC model in this paper proposes the dimensionless correlation through 16 experiments considering various conditions such as surface roughness, materials, pressure, and temperature. The model has been established by the modification on the previous CMY (Cooper, Mikic, Yovanovich) model with additional parameters. The first important feature of the model is to adopt the contact thermal conductivity (kc). It can be calculated by using the size effect theory at the micro-scale contact single spot. The second feature is the dimensionless temperature (T/TD). An experimental setup is constructed inside of the conduction-cooled cryostat to measure the thermal conductivity of bulk material and the TCC simultaneously for the temperature range between 10 K and 70 K. OFHC and Al 1070 specimens with various surface roughness are prepared and tested with contact pressure of 6 to 13.5 MPa in vacuum environment. The predicted TCC by the proposed model shows a reasonable agreement with the experimental results in low temperature.

Numerical and experimental study of influence function in magnetorheological finishing (MRF) of freeform surface

In the present work, a dynamic viscosity model is proposed based on the varying magnetic flux density of the neodymium permanent magnet along its width. The proposed viscosity model is helpful in analysing the influence function in a better way as compared to standard non-Newtonian fluid models. During the finishing process, the research of influence function aids in establishing the right range of process parameters. MR fluid flow on the freeform surface is examined numerically and experimentally to better understand the influence function. The flow behaviour of the MR fluid is investigated using the three-dimensional (3D) computational fluid dynamics (CFD) simulation, wall shear stress and dynamic pressure are estimated on the Oxygen-free high conductivity (OFHC) copper for various process parameters (working gap and rotational speed). On the convex and concave sections of a freeform surface, indentation depth is estimated for various process parameters using the results of dynamic pressure and empirical relation. Finishing tests are performed to investigate forces (tangential and normal) and indentation depth on the workpiece, and the results are evaluated.

Case study of advanced processed OFHC copper by dry sliding wear test

The wear behaviour of copper material processed by ECAP (Equal Channel Angular Pressing) and orbital forging (OF) is presented in this study. Dry sliding wear tests were carried out for the wear behaviour of the investigated system. Oxygen-free high thermal conductivity (OFHC) copper was used for testing. The new combination of metal forming processes was used because of ease of fabrication. Additionally, wear rate, friction coefficient and wears mechanisms were observed. The friction resistance is caused by the destruction of the adhesion between surface asperities in metal friction. Moreover, increased asperity interactions connected with wear particle entrapment gradually increase the friction coefficient. These results show the metal forming process's positive influence in reducing interfacial adhesion and asperity deformation. Finally, the combinations of newly used advanced processing demonstrated excellent wear characteristics of copper.

Finishing performance of polyurethane-based flexible tools fabricated using solvent casting and particulate leaching method

Oxygen-free high thermal conductivity (OFHC) copper is a common material for mirrors in different optical applications. As it is a soft material and easy to scratch, the nanofinishing using traditional finishing processes is difficult. A flexible abrasive tool that can adapt to the shape of the component and apply uniform and gentle pressure is required to achieve a high-quality surface finish on the soft materials. Flexible abrasive tools are fabricated using the solution casting and dissolution method with open porous microstructure and uniformly dispersed silicon carbide particles in a polyurethane polymer matrix. Mechanical and finishing characteristics of the fabricated tools are obtained under dry and wet conditions. The developed tools are self-lubricating and self-replenishing, reducing tool wear and eliminating burn marks on finished surfaces. Scanning electron micrographs and energy dispersive spectroscopy are used to investigate the tool microstructure and abrasive particle distribution before and after finishing. The mechanical behaviour of tools is studied using a dynamic mechanical analyser. Porosity is measured using the interconnected void volume, and fluid absorption rate is calculated through weight measurement before and after fluid absorption. The surface finish of finished surfaces is analysed using an optical surface profilometer. Results show that the flexible abrasive tool performs the best under wet conditions. In a single pass, a wet salt-leached tool achieved a reduction of 42.2 % on a surface having a lower initial roughness, whereas a wet pure polymer tool achieved a reduction of 45.7 % on a surface having a higher initial roughness. Finally, areal surface roughness of 34 nm is achieved in 16 mins of finishing experiment using pure polymer tool followed by salt-leached tool.

Performance of micro-beam X-ray absorption spectroscopy at beamline 7.2W of Synchrotron Light Research Institute

The existing BL7.2W beamline at SLRI, initially developed for Macromolecular Crystallography, was upgraded with a Si-drift X-ray detector, sample micro-stage, and polycapillary X-ray lens to provide possibilities for experiments using X-ray fluorescence (XRF) imaging and micro-beam X-ray absorption spectroscopy (μXAS) techniques. The micro X-ray beam was characterized. The beam diameter of approximately 25 μm was determined from the knife-edge scan method. The measured photon flux was 3.8 × 109, 1.8 × 109, and 6.4 × 108 photons/s for 8.0, 10.0, and 12.0 keV X-rays, respectively. Imaging XRF and μXAS measurements were successfully performed on a welding joint of oxygen-free high thermal conductivity copper and AISI304 stainless steel. This opens opportunities for SLRI users to utilize this new measurement setup for elemental and chemical characterizations of small areas on various samples.

Dynamic indentation testing and characterization of metals based on the split Hopkinson pressure bar (SHPB) device

The classical quasi–static indentation testing theory and technique have been developed to a relatively mature degree, while the related studies on dynamic indentation testing and characterization are relatively few and incomplete but of significant importance in the actual conditions subjected to local dynamic loading. In this study, a dynamic indentation test device is established based on the split Hopkinson pressure bar test system. The “three–wave method” is proposed and validated through numerical simulations to effectively acquire the dynamic indentation response data (indentation depth and indentation load) of metallic materials, and dynamic indentation tests are carried out on the representative metal of oxygen free high conductivity copper (OFHC) with different striker velocities. Combining the dynamic indentation contact stiffness analysis with related indentation testing theory, the real–time dynamic indentation hardness of OFHC during the loading stage under different striker velocities are determined and certain rate sensitivity is shown. Related research lays a foundation for further studies on dynamic indentation testing and characterization of metals.

Grain size dependence of modified material constitutive model for OFHC copper

Grain refinement occurs during manufacturing process, and grain size has a significant impact on mechanical properties. It is essential to establish the relationship between grain evolution and plastic behavior. The quasi-static and dynamic compression experiments for three kinds of oxygen-free high-conductivity (OFHC) copper with different grain sizes have been carried out. The true stress–strain curves were corrected to the isothermal curves, in which the temperature dependence of the specific heat capacity was used. It was found that the grain size has a significant influence on the yield stress, strain rate sensitivity, and thermal softening. A new phenomenon-based constitutive model considering the grain size and the coupling between strain hardening and strain rate was developed, and the results of the quasi-static and dynamic compression experiments were used to fit the parameters. The experimental data and the predicted results of the newly developed constitutive model agree well, which proves the constitutive model has a high prediction accuracy. The fitted constitutive model depicted that the strain rate sensitivity increases and thermal softening degree declines as the rise of the grain size of OFHC copper. The developed constitutive model could be used to the numerical simulation for the manufacturing process of OFHC copper considering the grain refinement.

Finishing of OFHC copper using fluid filled open-cell porous flexible abrasive impregnated tools

Super finishing of oxygen-free high thermal conductivity (OFHC) copper disks is of great interest as copper is widely used material for high power rated mirrors in laser and other optical systems. It is difficult to super finish copper using traditional finishing processes as it is soft and can be scratched easily. Flexible tools that conform to component shape and maintain low as well as uniform pressure are needed for finishing of soft materials. In this work, open-cell porous flexible abrasive tools are developed using solvent casting method with polyurethane as the base polymer and SiC particles as abrasives. The open-cell porous tools are saturated with deionized water before use. The tools are self-replenishing as the abrasives are uniformly dispersed in the tool and the fluid trapped in pores acts as coolant as well as lubricant. The viscoelastic behavior of the fabricated tools with different abrasive concentrations is characterized using a dynamic mechanical analyzer (DMA). Spot finishing experiments are carried out to optimize the tool RPM, tool compression, and abrasive concentration for better surface finish. The average areal surface roughness (Sa) of copper disk reduced to 54 nm from 946 nm.

Outgassing rate testbed for in-operation analysis of powered and heated assemblies

We introduce a new apparatus for measuring the outgassing rate of assemblies and functional units. Outgassing rates given in the literature, if available, are mostly insufficient to plan and set up a vacuum system, more so if commercial parts, preassembled items, or complex electronics with unknown components are deployed. Outgassing rates have a severe impact on the required pumping speed in ultrahigh vacuum systems and they change if the samples are powered or heated. The newly built apparatus uses the throughput method to measure outgassing rates. It can house probes with a size of 0.10×0.10×0.13m3 and supplies electrical currents of up to 5 A. In addition, a heater can establish temperatures of up to 200 °C directly. A procedure is given to identify the main gas inputs and to retrieve data with a lower limit of 1.5×10−8Pam3s−1 after 100 h in vacuum. In this paper, we present the experimental setup and first results for samples of polytetrafluorethylene, polyamide (PA6), Viton (1A), and oxygen-free high thermal conductivity copper at room temperature.

Influence of Material Microstructure on Machining Characteristics of OFHC Copper C102 in Orthogonal Micro-Turning

Micro-cutting is different from conventional cutting in its mechanics. The workpiece material is not considered to be homogeneous in the micro-cutting process. As a result, it is critical to comprehend how microstructure affects surface integrity, cutting forces, and chip formation. In this paper, we experimented with micro-turning on oxygen-free high-conductivity (OFHC) copper with different microstructures after annealing. Feed rate parameters were smaller than, larger than, and equal to the grain size, respectively. Experimental results show that when the feed rates are equivalent to the grain size, the surface roughness of the machined surface is low and the width of the flake structure on the free surface of chips is minimal, and the explanations for these occurrences are connected to dislocation slip.

Shock initiation of low density polymer bonded explosive LX-14: A study of two morphologies

A series of six shock initiation experiments have been carried out on low density LX-14 powder in order to simulate a shock insult on the heavily damage polymer bonded explosive, LX-14. Two distinct morphologies were studied (tap density molding powder and machined swarf), both at the same density of 0.942 g/cm3, or 50.1% theoretical maximum density. The purpose of these experiments was to provide data to help make an assessment of the effects that damage has on the material sensitivity to a planar shock. This was achieved primarily by providing shock sensitivity data in the form of a Pop plot, and also reactants equation of state data, which aids in the determination of input conditions for both the experiments performed in this work and also future experiments of this material type. The experiments were of a cut-back format, consisting of four sample heights on each shot and diagnosed with optical velocimetry. The experiments were carried out at the Technical Area 40 Chamber 9 gas gun facility at the Los Alamos National Laboratory, where the LX-14 targets were subjected to Al 6061 and Oxygen Free High Conductivity copper impactors launched to velocities up to 2.14 km/s. Time corrected reactive growth wave profiles are presented in this paper along with the derivation of the following Hugoniot parameters for this explosive, where the molding powder and machine swarf are represented by the following linear equations, respectively, US=1.64(±0.64)up+2.25(±0.49) and US=0.87(±0.60)up+4.08(±0.49). The results show that the steady increase in shock sensitivity with increasing void fraction reaches an inflection point beyond which the shock sensitivity begins to decease. This inflection point lies between 65% and 50% of the theoretical maximum density of the LX-14. In both cases, the damaged LX-14 was not as sensitive as expected, with a relative shock sensitivity of the molding powder being less than the pressed LX-14, and the machined swarf having a shock sensitivity that is comparable to pressed LX-14.

Taylor Impact Tests with Copper Cylinders: Experiments, Microstructural Analysis and 3D SPH Modeling with Dislocation Plasticity and MD-Informed Artificial Neural Network as Equation of State

Taylor impact tests involving the collision of a cylindrical sample with an anvil are widely used to study the dynamic properties of materials and to test numerical methods. We apply a combined experimental-numerical approach to study the dynamic plasticity of cold-rolled oxygen-free high thermal conductivity OFHC copper. In the experimental part, impact velocities up to 113.6 m/s provide a strain up to 0.3 and strain rates up to 1.7 × 104 s−1 at the edge of the sample. Microstructural analysis allows us to find out pore-like structures with a size of about 15–30 µm and significant refinement of the grain structure in the deformed parts of the sample. In terms of modeling, the dislocation plasticity model, which was previously tested for the problem of a shock wave upon impact of a plate, is implemented in the 3D case using the numerical scheme of smoothed particle hydrodynamics (SPH). The model includes an equation of state implemented in the form of an artificial neural network (ANN) and trained according to molecular dynamics (MD) simulations of uniform isothermal stretching/compression of representative volumes of copper. The dislocation friction coefficient is taken from previous MD simulations. These two efforts are aimed at building a fully MD-based material model. Comparison of the final shape of the projectile, the reduction of the sample length and increase in the diameter of the impacted edge of the sample confirm the applicability of the developed model and allow us to optimize the model parameters for the case of cold-rolled OFHC copper.

An Internal State Variable Elastoviscoplasticity-Damage Model for Irradiated Metals

Abstract This study presents an irradiation-dependent internal state variable (ISV) elastoviscoplasticity-damage constitutive model that accounts for nuclear irradiation hardening and embrittlement of the irradiated polycrystalline materials. The irradiation effects were added to the coupled plasticity-damage kinetics with consideration of the structure–property relationships. The present irradiation-dependent elastoviscoplasticity-damage model was compared with the lab deformation experimental data of irradiated oxygen-free high conductivity (OFHC) copper, modified 9Cr-1Mo steel, and Ti-5Al-2.5Sn. The results show excellent agreement over the entire stress–strain curves at various irradiation doses. Because the ISV model, before the irradiation plasticity-damage addition, had been used on over 80 different metal alloys, it is anticipated that this nuclear irradiation supplement will also allow for application to many more irradiated metal alloys.

Angular particle impact on ductile materials using the Lagrangian gradient smoothing method

The impacts of single angular particles on ductile materials can result in large deformations and material chip separation, which cannot be effectively simulated using conventional mesh-based numerical methods. In this study, a novel particle-based mesh-free method called the Lagrangian gradient smoothing method (L-GSM) is first applied to simulate the dynamic process of single diamond-shaped particles impact on metallic surfaces. Based on the theory of L-GSM, a numerical model is established by incorporating the Johnson–Cook plasticity and failure models. Oxygen-free high-conductivity (OFHC) copper which is represented by a series of discrete particles, is selected as the material of the target block. Simulation results show that the L-GSM model can reproduce the typical erosion mechanisms such as cutting, micro-machining and ploughing. The large deformation of plastic craters and material removal are successfully simulated without using any additional techniques to handle the distorted domain. In addition, the computational accuracy and efficiency of the L-GSM are discussed by comparison with different mesh-free models. This study provides an effective numerical model with better accuracy and efficiency, helping to reveal the fundamental erosion mechanisms resulting from the impacts of angular particles.

The Effect of Strain Rate and Initial Grain Size on Deformation Behavior of OFHC Copper at Elevated Temperatures

Understanding of plastic deformation mechanisms and/or microstructural changes of metals and alloys at elevated temperatures makes possible to control their hot working behavior and final mechanical properties. The aim of the present work is to optimize the conditions to achieve maximum ductility in terms of initial grain size, process temperature and deformation rate. In this study, the OFHC (oxygen-free high conductivity) copper samples of different initial grain sizes (25, 50, 100 and 150 μm) were subjected to tensile tests at temperatures 300, 405, 500 and 700 °C (0.42 - 0.75 Tm) and cross-head speeds of 1, 2, 5, 10, 20 and 50 mm/min (strain rates of 5.6x10-4 - 2.8x10-2 s-1). Experimental results indicated that particular conditions (initial grain size of 50 µm; 700 °C of working temperature and 5.6x10-3 s-1 of strain rate) should be provided in terms of process temperature and deformation rate depending upon initial grain size for dynamic recrystallization and also maximum ductility.