Machinability of ultrafine-grained copper using tungsten carbide and polycrystalline diamond tools

Equal channel angular extrusion (ECAE) is an effective process to produce bulk ultrafine-grained (UFG) materials from regular coarse-grained materials. Such ECAE-processed materials typically excel in strength, wear resistance, ductility, and high strain-rate superplasticity, with promising applications in lightweight transportation and medical industries. Precision machining work is generally indispensable for further applications after bulk materials are produced by ECAE. To effectively and efficiently machine such ECAE-processed materials for further broad applications, machining issues such as machinability and tool material selection should be considered. This study was undertaken to investigate the machinability of ECAE-processed pure copper using both tungsten carbide (WC) and polycrystalline diamond (PCD) cutting tools in order to facilitate broad applications of ECAE-processed UFG coppers. It is found that despite its higher cost, PCD is favored to machine UFG copper based on this study since it has better wear resistance, gives lower cutting forces, yields a better workpiece surface finish, and results in no smearing on the workpiece. In machining UFG copper, depth of cut notching was observed as the wear pattern and abrasion as the wear mechanism for the WC tool, while flank wear was observed as the wear pattern and diffusion as the wear mechanism for the PCD tool.     

Improved fatigue properties of ultrafine-grained copper under cyclic torsion loading

In this study, fatigue behaviors of pure copper with different grain sizes are investigated under cyclic tension–compression and torsion loadings. The fatigue responses of ultrafine-grained (UFG) Cu subjected to equal-channel angular pressing (ECAP) are compared and contrasted with those of coarse-grained (CG) and cold-rolled (CR) Cu. It is found from the S–N curves under the two different loading modes that, in the high-cycle fatigue (HCF) range, the fatigue strength of Cu does not exhibit strong dependence on the grain size under cyclic tension–compression loading, whereas the fatigue strength of UFG Cu is greatly improved over those of CG and CR Cu under cyclic torsion loading. Under cyclic tension–compression loading, the fatigue strength exponent decreases with the refinement of grain size; however, under cyclic torsion loading, with decreasing grain size, its fatigue strength exponent shows the opposite trend and goes up. To explain the phenomena above, the relations between the fatigue strength exponent and fatigue strength coefficient are discussed. Based on the two main stages of fatigue failure (crack initiation and propagation stages), the influences of grain size on fatigue strength exponent and fatigue strength in the HCF range under the two fatigue modes are comprehensively analyzed.     

Microstructure and mechanical properties of ultrafine-grained hardmetals

The interest in ultrafine-grained hardmetals as woodcutting tool materials derives from their excellent mechanical properties compared with those of conventional hardmetals. The aim of this work was to determine the mechanical properties of ultrafine-grained hardmetals and to correlate the measured effects with microstructural parameters. The ultrafine-grained hardmetals (WC grain size 0.3 μm) investigated consisted of different WC powders and different binder systems: Co and complex binder systems. The mechanical properties of ultrafine-grained hardmetals were tested under two different loading conditions: monotonically increasing and cyclic alternating bending loads. It could be shown that the binder systems of different compositions show different behaviours under cyclic loads. Ultrafine-grained hardmetals with Co binder exhibit high bending strength values, but high fatigue sensitivity. Ultrafine-grained hardmetals with complex binders show lower bending strength values but their sensitivity to fatigue is lower. This implies that different damaging mechanisms exist for ultrafine-grained hardmetals with Co and complex binders.     

Electrochemically controlled pitting corrosion in Ni film: A study of AFM and neutron reflectometry

Electrochemical behavior of pitting corrosion of a Ni film, grown on Si substrate by sputtering, prepassivated in a chloride-free sulfuric acid solution and subsequently exposed to chloride above the pitting potential is reported. Specular and off-specular unpolarized neutron reflectometry and Atomic Force Microscopy (AFM) techniques have been used to determine the depth profile of scattering length density and morphology of as-deposited as well as corroded sample. Specular neutron reflectometry measurement of the film after corrosion shows density degradation along the thickness of film. The density profile as a function of depth, maps the growth of pitting and void networks due to corrosion. The AFM and off-specular neutron reflectivity measurements has suggested that the morphology of the film remains same after exposure of the film in chloride solution.     

Deuterium absorption in Mg70Al30 thin films with bilayer catalysts: A comparative neutron reflectometry study

We present a neutron reflectometry study of deuterium absorption in thin films of Al-containing Mg alloys capped with a Ta/Pd, Ni/Pd and Ti/Pd-catalyst bilayer. The measurements were performed at room temperature over the 0-1 bar pressure range under quasi-equilibrium conditions. The modeling of the measurements provided a nanoscale representation of the deuterium profile in the layers at different stages of the absorption process. The absorption mechanism observed was found to involve spillover of atomic deuterium from the catalyst layer to the Mg alloy phase, followed by the deuteration of the Mg alloy. Complete deuteration of the Mg alloy occurs in a pressure range between 100 and 500 mbar, dependent on the type of bilayer catalyst. The use of a Ti/Pd bilayer catalyst yielded the best results in terms of both storage density and kinetic properties.     

Fatigue properties of friction stir welded joint of ultrafine-grained 2024 aluminium alloy

Ultrafine-grained (UFG) 2024 aluminium alloy prepared by the equal channel angular pressing was friction stir welded (FSW). The high cycle fatigue and crack growth behaviour of the FSW joint were investigated in air and NaCl solution, respectively. This study demonstrated that FSW was a viable technique for joining UFG materials. The UFG microstructure was retained in the nugget zone (NZ). Compared with the UFG base metal (BM), FSW joint exhibited lower ultimate tensile strength and hardness, and the minimum hardness value was located in the heat affected zone (HAZ). NaCl solution significantly reduced the fatigue strength of FSW joint. Fatigue crack propagation rates in the NZ and HAZ were slower than that in the BM in the whole fatigue life.     

Prediction of intergranular strains in cubic metals using a multisite elastic-plastic model

A novel approach is adopted for determining the elastic and plastic strains of individual grains within a deformed polycrystalline aggregate. In this approach, termed “multisite modeling”, the deformation of a grain does not merely depend on the grain lattice orientation. It is also significantly influenced by the interaction with one or several of the surrounding grains. The elastic-plastic constitutive law is integrated by identifying iteratively which dislocation slip systems are activated within the grains, and the local stress tensor is shown to be the solution of a linear equation set. Several micro–macro averaging schemes are considered for the distribution of the macroscopic load over the polycrystalline aggregate. These averaging schemes are tested by simulating the development of intergranular strains during uniaxial tension of MONEL-400 as well as commercial purity aluminium. Neutron diffraction measurements of the elastic lattice strains are used as a reference in order to discriminate between the various predictions. The results demonstrate the relevance of “multisite” grain interactions in f.c.c. polycrystals.     

Non-destructive investigation of aluminum alloy hemmed joints using neutron radiography and X-ray computed tomography

Methods for the non-destructive analysis of aluminum alloy hemmed joints for automotive applications were investigated to visualize the void size, volume fraction and shape within the adhesive fill. These defects can adversely affect the performance of hemmed joints by decreasing fatigue strength and corrosion resistance. Thermal neutron radiography and X-ray computed tomography were applied to visualize the voids. The void fraction within the adhesive fill was determined from the neutron images with the use of an image analysis thresholding method.     

不同温度下超细晶铜的准静态压缩力学行为

利用电子万能实验机对超细晶铜(UFG-Cu)进行温度范围为77～573 K的准静态压缩实验(应变率为1×10-3s-1),研究温度对材料流动应力和应变硬化行为的影响.结果表明:与退火粗晶铜相比,超细晶铜在压缩过程中的流动应力显著增大,但是由于材料的位错密度已经饱和,其应变硬化能力却几乎丧失,应变硬化率对应变和温度的依赖...     

退火处理对超细晶铜-锗合金力学性能的影响

超细晶Cu-0.1at％ Ge合金在液氮温度下轧制后在150℃退火1h,分别研究了超细晶铜锗合金在退火前后的显微硬度及力学性能.结果表明,通过低温退火,超细晶铜锗合金的显微硬度和强度得到提高,而均匀伸长率下降.     

Determination of frictional behavior in sheet metals using orthogonal arrays

Frictional behavior of two sheet materials, aluminum-killed drawing-quality steel (AKDQ) and electro-galvanized (zinc) drawing-quality steel (AKDQ-EG), is examined under conditions of varying die material, die radius, crosshead speed, and lubricant. Tests are conducted using a special apparatus designed to measure front and back tension on uniform tensile strip specimens pulling over a circular die, simulating both frictionless and frictional conditions under certain sheet-metal-forming conditions. Use of a specially designed test apparatus with four contact angles for the same test condition minimizes the error associated with the use of single measurements for the determination of friction coefficient. Lubricant and die material play important roles among different factors examined in determining the coefficient of friction. Die radius has the most pronounced effect on the coefficient of friction. Implication of these results on actual sheet forming processes are discussed.     

Corrosion inhibition using superhydrophobic films

Neutron reflectivity (NR) was used to study the effectiveness of superhydrophobic (SH) films as corrosion inhibitors. A low-temperature, low-pressure technique was used to prepare a rough, highly porous organosilica aerogel-like film. UV/ozone treatments were used to control the surface coverage of hydrophobic organic ligands on the silica framework, allowing the contact angle with water to be continuously varied over the range of 160° (SH) to <10° (hydrophilic). Thin (∼5000 Å) nano-porous films were layered onto aluminium surfaces and submerged in 5 wt% NaCl in D₂O. NR measurements were taken over time to observe interfacial changes in thickness, density, and roughness, and therefore monitor the corrosion of the metal. NR shows that the SH nature of the surface prevents infiltration of water into the porous SH film and thus limits the exposure of corrosive elements to the metal surface.     

Tensile behavior of TRIP-aided multi-phase steels studied by in situ neutron diffraction

TRIP-aided multi-phase steels were made by thermo-mechanically controlled process, where the ferrite grain size and the amount of the retained austenite were changed by controlling process conditions. The tensile behavior of four steels was studied by in situ neutron diffraction. It is found that the retained austenite bearing about 1.0 wt% C is plastically harder than the ferrite matrix. The steel with a ferrite grain size of ≈2.0 μm showed tensile strength of 1.1 GPa and a uniform elongation of 18.4%, in which stress-induced martensitic transformation occurs during plastic deformation but a considerable amount of austenite remains even after the onset of necking. It is concluded that the enhancement of uniform elongation is caused mainly by the work-hardening due to the hard austenite and martensite, where the contribution of the transformation strain is negligible.     

Analysis of antiphase domain growth in ternary FeCo alloys after different cooling rates and annealing treatments using neutron diffraction and positron annihilation

FeCo alloys are industrially important engineering materials which play an outstanding role in applications requiring soft magnetic materials. The challenge is to include ternary elements to improve the mechanical properties. Here noble elements as Pt or Pd were used for these experiments. With neutron diffraction and positron annihilation technique Fe₆₇Co₃₀Pt₃ and Fe₆₇Co₃₀Pd₃ (at. pct.) samples were measured to study the influence of different cooling rates on ordering and disordering. The ordering and disordering process is responsible for the mechanical properties in dependence of temperature. The correlation of ordering and defect density is described.     

Brazing diamond grits onto a steel substrate using copper alloys as the filler metals

Surface-set diamond tools were fabricated by an active metal brazing process, using bronze (Cu-8.9Sn) powder and 316L stainless steel powder mixed to various ratios as the braze filler metals. The diamond grits were brazed onto a steel substrate at 1050 °C for 30 min in a dry hydrogen atmosphere. After brazing practice, an intermediate layer rich in chromium formed between the braze filler metal and diamond. A braze filler metal composed of 70 wt % bronze powder and 30 wt % stainless steel powder was found to be optimum in that the diamond grits were strongly impregnated in the filler metal by both mechanical and chemical types of holding. The diamond tools thus fabricated performed better than conventional nickel-plated diamond tools. In service, the braze filler metal wore at almost the same rate as the diamond grits, and no pullout of diamond grits or peeling of the filler metal layer took place.     

Investigation of residual strains by neutron diffraction in an AZ31 direct chill cast billet

Neutron diffraction data was collected, showing the strain distributions along radial, axial and hoop directions in a direct chill cast billet of AZ31 magnesium alloy. Strain measurement by neutron diffraction is a non-destructive technique that uses the diffraction of a beam of thermal neurons to determine the atomic spacing within a small gauge volume inside polycrystalline materials. The benefits of using neutrons versus X-rays lie in the increased penetration depth achieved with neutrons, which can allow measurement of the internal strains several centimeters away from the material surface. A data-processing technique was proposed to assess and remove the irregular points, and the point-to-point fluctuations were evaluated. Residual strain measurements on the as-cast billet contribute direct observation of the stress/strain state in the billet. And the results also provide the data necessary to validate a thermo-mechanical model that predicts the evolution of stress/strain during the DC casting and subsequently to investigate the cracking defects in the billets.