查看更多>>摘要：In micro electrical discharge machining (EDM) drilling with a liquid dielectric, the bubble flushing effect is significant to the dielectric exchange and debris evacuation. Clarifying its mechanism is critical to guild the machining and avoid the deterioration of machining environment, which is a common problem in deep hole drilling with micro EDM. However, the investigation of bubble flushing is always disturbed by debris, which affects bubble production through disturbing the discharge process, because the debris is always conductive as a byproduct of metal materials. In this paper, the bubble flushing effect is systematically investigated from the in situ observation to statistical analysis to clarify the driven force for bubble movement or escape in the narrow gap and assess the evolution of bubble flushing effect with increasing hole depth. Moreover, the influence of debris on the bubble flushing effect is quantitatively analyzed through the contrast experiments in single crystal SiC and stainless steel SUS304 (SS304). It is confirmed in this paper that the debris of SiC has nearly no effect on the discharge due to its special material property. The visible and quantitative investigations reveal the nature bubble flushing effect in micro EDM drilling. The contrast analysis of bubble flushing effect under two cases (with/without influence of debris) clarifies how bubble and debris affect one another and their influences on the drilling process. This study provides new insights into the mechanism of micro EDM drilling and benefits to optimize the actual machining processes for the industry and manufacturer.
查看更多>>摘要：A deflectable pulsed fibre nanosecond laser was employed in systematically studying the profiling small-angle bevel-edge contour diamond grinding wheels. There is taper error in laser profiling grinding wheel technology as with other laser removal technologies and little research on the formation factors and suppression measures. In this paper, the mathematical model of the laser energy variation on bevel contour was first proposed, which theoretically explained the reasons for the angle error of laser profiling the bevel-edge contour. The ways to boost the contour accuracy and enhance surface quality were investigated. The results demonstrated that the smaller the bevel profile angle, the more scattered the focused laser irradiated on the grinding wheel surface, and the higher the reflectivity of the laser on the profiled surface. The above factors caused an extreme decrease in the laser power density irradiated on the grinding wheel, which led to taper error. It was found that the surface quality deteriorated, the obvious ablation marks appeared on the profiled surface as the overlap rate decreased, and adjusting the laser deflection angle could effectively eliminate the angle error of the bevel contour and improve the profiling accuracy. After laser profiling, the purpose of precision profiling the bevel contour of 11 could be achieved by setting the laser deflection angle of about 1, and the PV value of the bevel contour reached 5.812 mu m. The mathematical model of laser profiling bevel contour established in this work can also be applied to other laser processing courses.
Stubblefield, G. G.Fraser, K. A.Van Iderstine, D.Mujahid, S....
15页
查看更多>>摘要：In this study, the temperature and strain rate dependent physics of Additive Friction Stir Deposition (AFS-D) are elucidated for the first time. Candidate constitutive models are selected to account for the severe plastic deformation and wide ranges of temperature and strain rate inherent to AFS-D. Torsion mechanical tests are performed to capture AA6061 stress dependency on temperature and strain rate. Four constitutive models, including a low fidelity-calibrated one, are applied to single-layer AFS-D simulations. The robust meshfree method, smoothed particle hydrodynamics (SPH), provided the framework for constitutive model comparison. Utilizing the SPH simulations, peak values, temperature contours, effective stress contours, effective plastic strain contours, build profiles, and simulation run time are all compared to reveal hidden mechanisms of AFS-D processing. This work highlights the importance of constitutive model selection, which can lead to improved simulation results by accurately capturing plastic stress dependence on strain, strain rate, and temperature.
查看更多>>摘要：The gas film plays a key role in electrochemical discharge machining (ECDM). However, the impact force of the electrochemical discharge affects the gas film and hence can lower the machining performance. To improve the stability of the gas film, a novel ECDM approach based on a non-Newtonian fluid electrolyte, i.e. the mixture of Polyacrylamide and KOH is proposed in this study. The experimental results show that, compared to the traditional Newtonian fluid KOH electrolyte, the non-Newtonian fluid electrolyte can significantly weaken the effect of the impact force on the gas film, and thus the gas film was more stable. The theoretical model and experimental results both show that stable electrochemical discharge and a lower critical voltage can be achieved with a non-Newtonian fluid electrolyte condition for its' gas film is thinner and more stable than with the KOH electrolyte. With the non-Newtonian fluid electrolyte, the heat-affected zone and entrance overcut of the microchannels were both smaller than with the KOH electrolyte. In this study, a complex microchannel with a depth of 100 mu m and a spacing of 30 mu m was successfully fabricated. This is the smallest spacing reported so far in the literature for a microchannel fabricated in glass using ECDM. Moreover, a smoother machining surface with Ra 0.45 mu m was obtained with the non-Newtonian fluid electrolyte. It can be concluded that using the non-Newtonian fluid electrolyte is a simple and effective way to enhance the stability of gas film and thus, improve the micromachining performance of ECDM.
查看更多>>摘要：TC4 titanium alloy was joined to T2 copper through vacuum diffusion bonding by a two-step slow cooling heat treatment in the vacuum furnace, which could reduce the residual stress and avoid the interface cracking from the large difference in linear expansion coefficients between titanium and copper. The effect of bonding temperature and bonding time on the microstructure and mechanical properties of the joint was investigated. The through crack formed at the interface with traditional cooling process, but disappeared with this two-step slow cooling process. The typical interfacial microstructure of bonded joints was beta-Ti+Ti2Cu+Ti3Cu, Ti2Cu, TiCu, TiCu2, TiCu3 and TiCu4 phases, in diffusion zones I to VI at bonding temperature below 880 degrees C. The joints obtained by the traditional cooling process failed during natural aging, with nearly null shear strength. The maximum shear strength of joints at 840 degrees C for 45 min by the slow cooling heat treatment reached 111 MPa. The growth activation energy obtained from zones I to VI was 190 kJ/mol, 446 kJ/mol, 240 kJ/mol, 147 kJ/mol, 849 kJ/mol and 79 kJ/mol, respectively. Fracture occurred mainly in the interface of TiCu and TiCu2 phases, with many dimples, revealing clearly ductile mode. Cu element near the interface had a larger diffusion capacity and diffused faster than Ti element in the diffusion process. The results in this paper provide an effective strategy for the joining of titanium alloy and copper in high precision friction components of aerospace engines.
查看更多>>摘要：As-built surface morphology and texture of Additively Manufactured (AM) metal parts reduce their mechanical and corrosion properties. One chemically accelerated vibratory finishing (CAVF) technique employs a chemically-based stepwise process to gradually remove surface roughness without the need for significant manual manipulation. While this procedure is effective at producing smooth surfaces, the corrosion response of the resulting surface is unknown. This study evaluates the effect of this surface finishing technique on the corrosion response and mechanisms of 316L stainless steel fabricated using Laser Powder Bed Fusion (L-PBF) AM techniques. The results show that the CAVF process does not obviously change the microstructure but imparts residual compressive stresses on the surface which improve the breakdown potential compared to other specimens evaluated. Further, the process removed surface Cr3C2 precipitates formed during heat treatment. CAVF improves surface finish and mechanical properties with an added benefit of enhancing the corrosion response of processed parts.
Zhang, HanleiWang, YuankangDe Vecchis, Rafael RodriguezXiong, Wei...
12页
查看更多>>摘要：Precipitation hardening regulated with heat treatments is a critical technique for tuning the mechanical properties of Ni-based superalloys fabricated with additive manufacturing, where mechanical deformation is no longer viable for microstructural refinement. As one of the key precipitates, carbides prevail in the additively manufactured alloys, whose precipitation kinetics remain largely vague. This work studies the influence of heat treatments on the evolution of MC- and M23C6-type carbides within a Haynes (R) 282 superalloy prepared by wire arc additive manufacturing (WAAM). SEM-EDS analysis was performed to identify the evolution pathways of the carbides, and TEM and XRD characterizations were carried out to track the crystallographic features. The MCtype carbides are originally present in the as-printed alloy, which distribute uniformly across the whole material and remain largely unchanged in size during the heat treatments. By contrast, the incipiently non-existent M23C6-type carbides precipitate during an aging treatment at 1010 degrees C, which majorly extend along the grain boundaries as kinetically feasible sites. Further experimental and computational analysis confirms the enhancing effect of the M23C6-type carbides on the Vickers hardness. For the first time, this work revealed the evolution pathways of carbides within a Haynes (R) 282 superalloy prepared by WAAM, providing critical information for tunning the microstructure and mechanical properties. The carbide evolution kinetics established in this work can be potentially extended to other alloys prepared with WAAM, providing critical information to prevent mechanical failures associated with carbide coarsening.
查看更多>>摘要：Light emission is a unique machining feature during high-speed cutting of Zr-based bulk metallic glass (BMG), which significantly deteriorates the machining quality and efficiency. However, its generation mechanism is still unclear. In this study, the light emission of Zr-based BMG during high-speed cutting was quantified photometrically, and its correlation with cutting energy was investigated. The chemical composition change and the formation process of luminescent particles were analyzed. The generation mechanism of light emission was subsequently revealed. It was found that the light emission is due to the spontaneous ignition in air of Zr-based BMG particles with large specific surface area, produced by ductile tearing of the bottom of the chip. The friction energy of the tool-chip interface flowing into the chips is the dominant factor that triggers particle combustion. Therefore, from the perspective of controlling tool wear, machining strategies with controllable light emission were proposed. In order to suppress light emission, the primary method is to decrease the cutting speed and feed. The second is to improve the chip removal ability and the wear resistance of the tool. Cutting oil is only effective for suppressing light emission at low cutting speeds, while liquid nitrogen is not suitable for cutting Zr-based BMG due to increased tool wear. The findings of this work contribute to a deeper understanding of the generation mechanism of cutting features in glassy metals, and provide guidance for high-speed cutting process optimization of Zr-based BMG.
查看更多>>摘要：Electropolishing, as a damage-free and highly efficient surface finishing method, has been widely used for finishing of metal components. In this study, the electropolishing mechanism of tungsten in the electrolyte with H2SO4 and CH3OH was investigated from the perspective of etching isotropy. The anodic dissolution behavior of tungsten demonstrated that anisotropic and isotropic etching occurred under the activation polarization and mass transfer polarization, respectively. The evolution of surface morphology, roughness, and electric current density during electropolishing has been experimentally investigated and analyzed. According to the changes in surface morphology and current density, there was a transformation from anisotropic etching to isotropic etching during the electropolishing due to the time-consuming accumulation of reactive products. The application of a pulse power supply with a duty cycle of 10% resulted in a rough surface on tungsten due to the mass transfer polarization being inhibited. Similar results were also obtained in the electrolyte with 2 wt% NaOH. The presented findings experimentally demonstrate the importance of the isotropic etching mode for electropolishing, which is of great value for further revealing the electropolishing mechanism.
查看更多>>摘要：Micro diamond tools are indispensable for machining microstructured arrays. The cutting edge durability and consistency of micro diamond tools are the determinants of the microstructure quality and accuracy, in addition to the motion accuracy of the machine tool. A strength distribution model of the working area including the cutting edge and rake and flank faces was established considering diamond anisotropy and chip flow direction. Comprehensive wear resistances of micro diamond tools with different crystal orientation combinations were analyzed based on the model, and the wear prone areas of different tools were successfully predicted. The evolution processes of the sharpness and wear topography were monitored for every micro diamond tool in the micromachining experiments. The morphologies, profile errors and topological characteristic of the micro-structures machined with different micro diamond tools with increasing cutting distance were analyzed. Finally, a conclusion was drawn that the wear resistances of the micro diamond tools in ascending order are A(gamma){100} A(alpha){100}, A(gamma){100}A(alpha){110}, A(gamma){110}A(alpha){100}, and A(gamma){110}A(alpha){110}. The three working areas of the A(gamma){100} A(alpha){100} tool are prone to wear; in contrast, those of the A(gamma){110}A(alpha){110}tool are resistant to wear. The tool wear of A(gamma){100}A(alpha){110}is caused by flank face wear, and that of A(gamma){110}A(alpha){100} is caused by rake face wear.
NSTL
Elsevier