Polycrystalline OFHC Copper Research Articles

Finite Element Simulations for Micromilling of Oxygen-free Copper

Micromilling mechanism studies are the fundamentals for high-quality micro components fabrications. Based on the finite element method (FEM), the simulation model for micromilling of the oxygen-free copper (OFC) was established in this paper. The influences of the key process parameters (feed engagement fz, axial depth of cut ap, radial depth of cut ae, and spindle speed n) on the milling forces were investigated. According to the simulation results, to achieve the minimum undeformed chip thickness, the critical feed engagement was identified to be 2.5 μm/z. when the axial depth of cut increased, the milling force was increasing linearly, but at the same time the milling force took an oscillator decreases due to the increasing of spindle speed. The overall trend of milling force under different radial depths of cut is upward. Therefore, the feed engagement should be greater than 2.5μm/z for micro milling of oxygen-free copper. Small radial depth of cut, small axial depth of cut, and appropriate spindle speed should be selected to obtain a high machining quality. Introduction As an excellent metal with extremely low oxygen content, high conductivity and high corrosion resistance, oxygen-free copper C10100 is widely used in the manufacturing of audio-visual equipment and communication components. With good ductility and no hydrogen embrittlement, it can also be processed into various precision micro parts. Compared with traditional cutting, high-speed micromilling is more suitable for machining micro-small parts. In order to obtain higher dimensional accuracy and better surface quality, it is necessary to study the milling parameters of OFC in the processing of precision parts. In the actual machining process, the changes of milling force caused by the changes of milling parameters can reflect the quality of the final machined parts more intuitively. Therefore, the adjustment of milling parameters can be guided by measuring the milling force in the milling process. In order to reveal the difference in mechanism between traditional cutting and micromilling, scholars have conducted a lot of studies. In the study of cutting poly-crystalling OFC with single crystalline diamond (SOD) micro-tools, X. Ding has proposed that the cutting strategy of reducing cross-feed can improve the cutting performance with using micro-tools. Constant cutting forces, burr size and an improved roughness of the machined surface have been obtained by employing this cutting strategy [1]. Niu has presented an innovative cutting force modeling concept by taking account of micro-cutting dynamics. Experimental cutting trial is conducted to validate the modeling approach and the cutting force model [2]. Qu has studied the impact of MQCL method on the machining results and mechanical properties of the component [3]. Yuan has proposed an innovative uncut chip thickness algorithm and developed the model for determining the instantaneous uncut chip thickness in micro end-milling processes [4]. D.DAS has studied the effect of stress parameters on ratcheting deformation stages of polycrystalline OFHC copper [5]. For the micromilling of pure copper, M. Rahman has studied the failure mechanism and factors which affect the micro end mill [6]. Liu has studied the relationship between size effect and burr in micromilling OFC and constructed a geometric model of exit burrs [7]. Liu has established SPH simulation model and carried out systematic research on formation mechanism of chip [8].For oxygen-free copper materials, Tian has established a constitutive model and carried out simulation analysis based on Abaqus [9]. Cheng has proposed 211

Multiscale modeling of irradiated polycrystalline FCC metals

We propose a set of models for the post-irradiation deformation response of polycrystalline FCC metals. First, a defect- and dislocation-density based evolution model is developed to capture the features of irradiation-induced hardening as well as intra-granular softening. The proposed hardening model is incorporated within a rate-independent single crystal plasticity model. The result is a non-homogeneous deformation model that accounts for defect absorption on the active slip planes during plastic loading. The macroscopic non-linear constitutive response of the polycrystalline aggregate of the single crystal grains is then obtained using a micro–macro transition scheme, which is realized within a Jacobian-free multiscale method (JFMM). The Jacobian-free approach circumvents explicit computation of the tangent matrix at the macroscale by using a Newton–Krylov process. This has a major advantage in terms of storage requirements and computational cost over existing approaches based on homogenized material coefficients in which explicit Jacobian computation is required at every Newton step. The mechanical response of neutron-irradiated single and polycrystalline OFHC copper is studied and it is shown to capture experimentally observed grain-level phenomena.

Ductile failure in polycrystalline OFHC copper

Ductile failure in polycrystalline oxygen-free, high-conductivity copper is explored through uniaxial tension experiments. Specimens obtained through tests interrupted at various stages of deformation and failure evolution are examined through quantitative microscopy to discern the mechanisms of failure and the local strain evolution. The formation of a single rectangular prismatic channel-like cavity, and its subsequent growth as a rectangular cavity, are demonstrated. Fractographic observations are used to suggest that self-similar expansion of the cavity is through an alternating slip mechanism. Local strain levels are estimated from measurements of the change in grain size with deformation and used to indicate that the local values of failure strains are likely to be much larger than that estimated from strains averaged over characteristic specimen dimensions such as the gage length or the specimen diameter.

Effect of stress parameters on ratcheting deformation stages of polycrystalline OFHC copper

ABSTRACTEngineering stress‐controlled ratcheting tests under different sets of stress amplitudes and mean stresses show that ratcheting deformation in polycrystalline OFHC copper occurs in three different stages. A plateau region with almost no accumulation of inelastic strain follows general ratcheting deformation during initial loading cycles. With breakdown of the plateau region inelastic ratcheting deformation occurs at an increasingly rapid rate. The effect of the stress amplitude on the ratcheting process is found to be more than mean stress effect. Reconstruction of the ratcheting curves clearly separates the conditions for stress‐controlled low cycle fatigue with zero mean stress and ratcheting with tensile mean stress.

Effect of material anisotropy on shear angle prediction in metal cutting—a mesoplasticity approach

Material anisotropy plays an important role in the formation of shear angle in metal cutting. Crystallographic textures contribute to an important source of material anisotropy. A simplified mesoplasticity model is proposed in this paper to predict the effect of crystallographic orientations on the shear angle formation in machining a polycrystalline work material. The most likely shear angle is the one at which the Taylor factor is minimum. A good agreement is found between the predicted shear angle in machining a polycrystalline OFHC copper and the experimental data reported in the published literature. The assumptions made in the model approximate well the cutting conditions commonly encountered in single point diamond turning process.

Measurement of deformation fields in polycrystalline OFHC copper

Validation of continuum crystal plasticity models for polycrystalline materials requires experiments with sub-grain scale resolution, performed on materials with realistically small grain sizes. To date only limited experiments have been performed to investigate displacement fields with resolution well below the grain scale over a wide field for polycrystals with realistic grain sizes. This paper explores high-resolution in-plane material stretch and rotation fields measured at the sub-grain scale for ensembles of 10–20 grains in 70 μm grain-size OFHC Cu. Using a novel photolithographic technique, a series of micron-scale grids were deposited onto the surface specimens that were subsequently subjected to compression or torsion to effective strains up to unity. The resulting grid patterns for compression exhibit a tendency toward localization of stretch and rotation fields, with larger grains less highly deformed. For the case of shear, there is a reduced tendency towards microstructural scale localization of the in-plane stretch and rotation fields.

A constitutive model for fcc crystals with application to polycrystalline OFHC copper

Based on the results of a series of experiments on commercially pure OFHC copper (an fcc polycrystal), a physically based, rate- and temperature-dependent constitutive model is proposed for fcc single crystals. Using this constitutive model and the Taylor averaging method, numerical calculations are performed to simulate the experimental results for polycrystalline OFHC copper. The model calculation is based on a new efficient algorithm which has been successfully used to simulate the flow stress of polycrystalline tantalum over broad ranges of temperature, strain rate, and strain (Nemat-Nasser, S., Okinaka, T., Ni, L., 1998. J. Mech. Phys. Solids 46, 1009). The model effectively simulates a large body of experimental data, over a broad range of strain rates (0.001–8000 s −1), and temperatures (77–1096 K), with strains close to 100%. Few adjustable constitutive parameters of the model are fixed at the outset for a given material. All other involved constitutive parameters are estimated based on the crystal structure and the physics of the plastic flow.

H processing of copper surfaces

Changes in the crystallographic structure of polycrystalline copper surface due to the atomic hydrogen treatment are described. Samples of polycrystalline OFHC copper plates were exposed to a flux of thermal hydrogen atoms with the density between 1×1021 atoms/m2s and 1×1023 atoms/m2s. Hydrogen atoms partially accommodate and recombine in the potential well on the surface. During these processes copper atoms on the surface displace, which leads to a recrystallization of the samples. The effects were investigated by a scanning electron microscope (SEM).

Fatigue crack initiation and stage-I propagation in polycrystalline materials. I: Micromechanisms

The microscopic mechanisms involved in fatigue crack initiation and stage-I fatigue crack propagation in polycrystalline materials are investigated and models of the observations are proposed in this first paper of a two-paper series. A qualitative description of changes in crack profile in the transition region between the initiation and propagation is also proposed. The results of an experimental study that involved a combination of replication techniques and post-mortem fractographic surface examination by scanning electron microscopy carried out on smooth cylindrical specimens of polycrystalline OFHC copper at room temperature provide support for the models proposed.

The evolution of damage in tritium exposed copper

Severe microstructural damage has been observed in polycrystalline OFHC copper specimens thermally exposed to high pressure tritium gas at temperatures ⋚200 °C, but not at 300 °C. No such damage occurs in single crystal specimens exposed under identical conditions, regardless of temperature. In the polycrystals, the damage takes the form of very flat, crack-like intergranular cavities. It is found that the cavitation evolves slowly with time. For short exposure times, cavities as small as 0.1 μm are observed. In specimens subjected to the longest aging times, the cavities grow and link until entire grain boundary facets fail. The driving force for the growth of these cavities is attributed to the internal gas pressure of helium-3 generated by the decay of tritium. The growth kinetics of the cavity microstructure are described by a coupled grain boundary, surface self-diffusion process. The tritium exposure profoundly affects the mechanical properties of the polycrystalline material, inducing a severe loss in ductility. In concert with the observed ductility loss is a change in fracture morphology from transgranular ductile rupture to intergranular fracture. Examination of the resulting grain boundary facets reveals a dimple structure. The spacing of these dimples can be correlated with the spacing of the exposure-induced grain boundary cavities.

Dislocation structure and work hardening in polycrystalline ofhc copper rods deformed by torsion and tension

The dislocation structure during work hardening in copper deformed by torsion and tension is investigated by X-ray line broadening and TEM. In the case of torsion the equivalent tensile flow stress and the rate of work hardening is lower than that obtained in tensile experiments. At the same time the dislocation density at the same equivalent strain is considerably larger in the torsionally deformed material and the TEM microstructure indicates a double-slip deformation mechanism. In the tensile deformed samples asymmetric X-ray line broadening indicates long-range internal stresses with relatively lower dislocation densities. In this mode of deformation multiple slip takes place. The lower equivalent flow stress and the smaller rate of work hardening in the torsionally deformed material is correlated with the restricted number of slip systems and the absence of strong long-range internal stresses, which leads to the relative ease of generating high dislocation densities.

Logarithmic Creep of Copper and Aluminium

Transient creep of polycrystalline OFHC copper and aluminium of 99·95 % purity was studied by the method of incremental loading, in tension at room temperature. All specimens were initially annealed, but some were also prestrained 10% either in tension or in compression before creep. The creep strain ε was expressed by ε = S log (1 + t/t o), and the dependence of the parameters s and to on the level of the applied tensile stress and on the sign of the prestrain was examined. The structure sensitivity of both parameters, indicated by the results, was discussed in terms of a simple stochastic model; the latter is based on the concepts of a spectrum of energy barriers to dislocation motion and the tendencyof dislocation assemblies to assume minimum-energy configurations.The observations point to the potential usefulness of such relatively simple investigations in assessing the creep resistance and studying structural detail of metals and alloys.

The dislocation cell size and dislocation density in copper deformed at temperatures between 25 and 700°C

A dislocation cell structure was developed in polycrystalline OFHC copper by deforming tensile specimens 10 per cent, one at each of the following temperatures: 25, 250, 400, 500, 550, 600, 650 and 700°C. The dislocation cell size and the dislocation density were measured by transmission electron microscopy. The mean cell diameter increased with increasing temperature from 1.4 μ at 25°C to 6.0 μ at 700°C. The dislocation density ρ decreased with increasing temperature, from 118.0 × 10 8cm/cm 3 at 25°C to 8.5 × 10 8 cm/cm 3 at 700°C. The thickness of the cell walls decreased with increasing temperature of deformation. It was found that the mean cell diameter d was related to the dislocation density ρ through the relation d = 16ρ −1 2 . The occurrence of such a relation supports a theory in the literature of dislocation cell formation. The flow stress τ was given by τ = 10.5 Gb/ d and τ = 0.57Gb ρ 1 2 where G is the shear modulus and b is the Burgers vector.

Preyield Plastic Deformation in Copper Polycrystals

Preyield dislocation motion and multiplication were studied in large-grained polycrystalline OFHC copper by the etch-pit technique. Specimens were loaded in compression and dislocation arrangements were observed in both the stressed and unstressed conditions. The dislocation loops generated by the first few active sources on both primary and secondary slip systems generally traversed the entire cross section of the grain. The observations were consistent with the idea that jog density is an important factor in determining the mobility of a dislocation segment. The external surface was found to be a preferential site for dislocation multiplication even when grown in segments, where they cut the surface, were immobile, and stress concentrations were carefully avoided. It is suggested that the rapid increase in length per unit volume of moving dislocation line that is associated with macroscopic yield requires motion of dislocation segments of high jog density.