Zhong Ning Guo

Microstructure and Corrosion Properties of AlCoCrFeNi High Entropy Alloy Coatings Deposited on AISI 1045 Steel by the Electrospark Process

Electrospark deposition (ESD) was employed to clad the AlCoCrFeNi high-entropy alloy (HEA) on AISI 1045 carbon steel. The relationship between the microstructure and corrosion properties of the HEA-coated specimens was studied and compared with that of the copper-molded cast HEA material. Two major microstructural differences were found between the cast HEA material and the HEA coatings. First, the cast material comprises both columnar and equiaxed crystals with a columnar-to-equiaxed transition (CET), whereas the HEA coatings consist of an entirely columnar crystal structure. The CET phenomenon was analyzed based on Hunt’s criterion. Second, unlike the cast HEA material, there was no obvious Cr-rich interdendritic segregation and nano-sized precipitate distributed within the dendrites of the HEA coating. With regard to corrosion properties, the corrosion current of the HEA-coated specimen was significantly lower than for the 1045 steel and the cast HEA material. This was attributed to the ESD specimen having a relatively high Cr oxide and Al oxide content at the surface. Moreover, for the ESD specimen, the absence of Cr-rich interdendritic phase and second-phase precipitation resulted in a relatively uniform corrosion attack, which is different from the severe galvanic corrosion attack that occurred in the cast specimen.

Experimental investigation into shaping particle-reinforced material by WEDM-HS

Abstract Particle-reinforced material is a kind of difficult-to-machine material. In general, it is inefficient if conventional methods are used to shape it, with worse surface quality and serious wear of the cutting tool. In this paper, the machineability when the material is machined by wire EDM is studied. A method of orthogonal design has been adopted to determine the main factors that affect the machining process. The result shows that the electric discharge energy is closely related to machining stability. A good machining effect can be attained when the electrical parameters are properly selected, otherwise, the machining may be unstable resulting in wire breakage.

Molecular Dynamics Simulation for Ultrafine Machining

This article surveys the advances of molecular dynamics (MD) simulation in the research of ultrafine machining and related technologies. Modeling methods, including interatomic potentials and boundary conditions, are addressed. Algorithm strategies for MD simulations are discussed. By applying simulations with Tersoff potential, a case study of the material removal mechanism of the polishing based on coupling vibrations of liquid (PCVL) is presented.

Development of Ultra-Short Pulse Power Supply Applicable to Micro-ECM

A nanosecond pulse power supply for micro-ECM (Electrochemical Machining) has been developed, and the minimum pulse on-time is 50 ns. After the influence of ultra-short pulse on micro-ECM was analyzed, complementary chopper circuit was presented to avoid waveform distortion, which can achieve higher pulse frequency. A fast protect circuit was designed, and its response time is less than 5μs, which can avoid damaging workpiece and electrical components by switching off machining voltage and step motor as soon as a short circuit was detected. A series of ECM experiments using the power supply have been carried out, and results of test have proved that high frequency, short pulse width power supply helps to achieve better quality surface, higher machining accuracy.

Smoothing CVD Diamond Films by Wire EDM with High Travelling Speed

A technique of processing CVD diamond films by wire EDM with high traveling speed (WEDM-HS) has been developed, with which the smoothing of CVD diamond films can be efficiently achieved. A CVD diamond film is coated with a thin layer of electrically conductive material in advance, and then the wire electrode moves across the coated surface with high traveling speed. As a result, peaks on the surface of the diamond film are removed rapidly. In machining, graphitization of diamond enables the electro-discharge process to continue. The finite element method is employed to analyze the coating material. An experiment for the single pulse discharge was made to test the theoretical results from the FEM. It is shown that the material of the coated layer can evidently affect the removal behavior of the CVD diamond films. Compared with the machining of ordinary metal materials, the process of ED machining of CVD diamond films possesses a quite different characteristic. The removal mechanism of the CVD diamond films is discussed. A polishing experiment was performed using different machining parameters.

Computer simulation and characteristic analysis of electrode fluctuation in wire electric discharge machining

Abstract In wire electric discharge machining, the vibration of the wire electrode has a significant influence on the performance and stability of the machining process. A method of computer simulation has been adopted in this paper to study the vibration of the wire electrode under the action of successive discharges, by which the effect of wire fluctuation on the distribution of the discharge points is also analysed. The result shows that the discharge points can be distributed much more evenly along the span of the wire when an optimum condition is reached between discharge energy, discharge frequency, wire tension and wire span. Under such a condition, it is possible that the hazard of wire breaking can be avoided.

Investigation into polishing process of CVD diamond films

A new technique used for polishing chemical vapor deposition (CVD) diamond films has been investigated, by which rough polishing of the CVD diamond films can be achieved efficiently. A CVD diamond film is coated with a thin layer of electrically conductive material in advance, and then electro-discharge machining (EDM) is used to machine the coated surface. As a result, peaks on the surface of the diamond film are removed rapidly. During machining, graphitization of diamond enables the EDM process to continue. The single pulse discharge shows that the material of the coated layer evidently affects removal behavior of the CVD diamond films. Compared with the machining of ordinary metal materials, the process of EDM CVD diamond films possesses a quite different characteristic. The removal mechanism of the CVD diamond films is discussed.

Microscopic machining mechanism of polishing based on vibrations of liquid

A molecular dynamics method has been applied to study the mechanism of polishing based on vibrations of liquid. Movements of polishing particles and formations of impact dents are simulated and discussed. The abrasive effect between particle and machined substrate is evaluated empirically. Polishing qualities, including roughness and fractal character under multiple impacts, are obtained by numerical methods. Results show that the particle will vibrate and roll viscously on the substrate. Press, tear and self-organization effects will be responsible for the formation of impact dents. Simulation results are compared with experimental data to verify the conclusions.

Formation of porous hydrophobic stainless steel surfaces by maskless electrochemical machining

Surface topography is an important feature, which directly affects the friction, corrosion and flow characteristics of a workpiece. In this study, a new approach was explored to produce a microporous structure by maskless electrochemical machining (ECM). A rough surface with a microporous structure was created on a stainless steel substrate, and the principle of the micropore formation was investigated. It was found that the microporosity could form, grow and interconnect when the potential of the anodic surface was higher than the pitting potential of the material. The electric field is an important factor, and the surface topography can be altered by changing the electrical parameters. The porous microstructure was observed by laser scanning confocal microscopy. The rough and porous surface, modified with 1-dodecanethiol, exhibits hydrophobic characteristics, with a static contact angle of 137° for water droplets. This is a significant improvement in the wetting properties for stainless steels.

Non-periodic boundary model with soft transition in molecular dynamics simulation

The imposition of non-periodic boundary conditions is a challenging issue in molecular dynamics (MD) simulation because of the difficulty in reconstructing the detailed particle dynamics from the few state variables. In this paper, a general scheme for the imposition of this kind of boundary conditions is presented. The complete model includes the imposition of Dirichlet boundary conditions and applying the boundary force to ensure the continuity of flow properties. Both the flux and the boundary force are introduced in a consistent way with the microscopic dynamics of the molecular system. Moreover, a soft constraint mechanism is included to correct the instantaneous deviation of the local state. A uniform flow and a Poiseuille flow of Lennard-Jones fluid is simulated using the boundary model. The results demonstrate that the fluctuation of flow properties near the boundary is kept at a low level, and the model is stable even subject to internal disturbance.