In recent years, many attempts have been made to explore the protective capability of graphene as an anti-wear coating for nanotribology. By the molecular dynamics simulations of the nanoindentation ...of a graphene/Fe (110) system, we provide a new insight into the atomistic mechanism of how graphene protects a metal substrate during a rough contacting process. We demonstrate that, in rough contacts, graphene can increase the loading area and homogenize the contact stress. It can weaken the effect of the surface roughness, reduce the magnitude of the contact stress and change its distribution. Finally, graphene can inhibit the nucleation of the surface dislocation and improve the load bearing capacity. These findings demonstrate that graphene holds great promise as effective protective coatings for contact damage.
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Micro-milling force and specific cutting energy play an important role in revealing the micro-milling mechanism. However, it is quite difficult to compare the micro-milling force values from ...different experiments due to the lack of a representative cutting force parameter to comprehensively evaluate the micro-milling force. Especially, there is no unified formula to accurately calculate the specific cutting energy in micro-milling due to variable chip cross-sectional area and periodically varying micro-milling force. In this work, the micro-milling force was systematically analyzed with fast Fourier transform spectrum analysis, curve shape, and representative parameter evaluation. The quasi-dynamic cutting force, which is represented by the P–V value of cutting force, was adopted to comprehensively evaluate the micro-milling force. The specific cutting energy was calculated with the ratio of quasi-dynamic cutting force and the average undeformed chip thickness. Moreover, the variable regularity of quasi-dynamic cutting force and specific cutting energy on cutting parameters were obtained with the micro-milling experiment. The results show that the quasi-dynamic cutting force first decreases and then increases with the increase of feed per tooth due to the chip accumulation effect. With the increase of spindle speed and depth of cut, the quasi-dynamic cutting force decreases and increases, respectively. The minimum undeformed chip thickness is between 0.3 and 0.5 μm, which is around 0.19 to 0.32 of tool edge radius in micro-milling Ti6Al4V. With the increase of spindle speed and depth of cut, the specific cutting energy shows a decreasing trend and changes a little, respectively. With the decrease of the feed per tooth, the specific cutting force shows a nonlinear increase. Our findings are of great significance for further scientific understanding the micro-milling mechanism from the perspective of cutting force and specific cutting energy.
Burr formation is still the major challenge in slot micro-milling of titanium alloy Ti6Al4V due to its difficult-to-machine material properties and hard deburring processing. In the present work, ...chip and burr formation were investigated by surface morphology observation of slot micro-milling Ti6Al4V, with an emphasis on the top burr and side burr formation mechanism, in which the side burr is defined as the burrs at the bottom edge of the sidewall. Especially, the effect of cutting parameters on burr formation was also analyzed. The results show that the chip shows the segmental in slot micro-milling Ti6Al4V. The up-milling top burr is mainly correlated with the extrusion effect of cutting-edge radius. The extrusion effect of cutting-edge radius, chip turnover induced by micro-milling cutter’s helix angle, and chip separation’s tearing effect all contribute to the down-milling side top burr formation in micro-milling Ti6Al4V. The up-milling side burrs are general adhesive chips and residual tear burrs due to the falling off of adhesive chips from the tool rake face and plowing-shear-plowing chip removal mechanism of the single cutting pass, respectively. The down-milling side burrs are general residual tear burrs and mental debris due to the transformation of the chip removal mechanism for the single cutting pass and tool deflection from up-milling to down-milling in the micro-milling. The down-milling top width and the side burr width have all shown a decreasing trend with the increase of feed per tooth. It is recommended to adopt the large feed per tooth for minimizing top burr width and side burr width. Our findings can provide a guideline for deburring and suppressing the burr formation in slot micro-milling Ti6Al4V.
Material microstructure in micro-cutting, such as the phase boundary for multiphase material, plays a dominant role in governing the surface integrity that strongly influences the fatigue life of ...micro-components with complex structures. In the present study, surface formation, subsurface deformation, and subsurface microstructure evolution for the machined surface were investigated by slot micro-milling multiphase titanium alloy Ti6Al4V in terms of chip morphology, burrs formation, surface roughness, surface defects, and subsurface deformation. In particular, the effect of β-Ti was considered in the study. The results show that the machined surface shows the obvious feed marks and adhesive materials accompanied by squeezed β-Ti, split β-Ti and plastic side flow β-Ti. The subsurface of the machined surface can be divided into the amorphous layer (around 24 nm), nanocrystalline layer (around 124 nm), elongated grain layer (around 197 nm), and bulk material. The deformation layer thickness of the β-Ti zone is larger than the α-Ti zone. In the deformation layer of the machined subsurface, the synergetic deformation between α-Ti and β-Ti can lead to the formation of a large number of nanocrystalline grains and some enlarged grains in the phase boundary of the α-Ti side, and the β-Ti side is full of elongated grains and sporadic dynamic recrystallization nanocrystalline. Our study enhances the understanding of the surface formation and subsurface formation mechanism in micro-milling Ti6Al4V, which can provide a theoretical basis and practical reference for achieving high surface integrity for polycrystalline or multiphase material by micro-milling.
Modelling and simulation has become a general tool in product development of mechanical products. Building mathematical models of subsystems and components is one of the most important tasks in the ...analysis, design and optimization of any mechanical systems. Multibody system serves as a basis for many modern mathematical models of dynamic systems and has been applied in many areas of science. In the last decade, many algorithms and numerical manipulation tools have been developed to meet the increasing demands in the modeling and simulation of advanced mechanical systems in the industry. There are different methods used to define the body orientation in the spatial domain. Among these, Reference Point Coordinate Formulation with Euler Angles (RPCF-EA) and Reference Point Coordinate Formulation with Euler Parameters (RPCF-EP) are the most common ones. The main difference between them is that (RPCF-EA) defines the body orientation by using three successive angles, while (RPCF-EP) defines the same orientation using four parameters. In this paper, the formulation change of the equations of motion and the mapping of generalized forces into cartesian perspective are presented. In addition, three numerical examples are used to discuss the differences between using RPCF-EA and RPCF-EP in multibody systems with respect to the type of application. The first example demonstrates the suitability of each coordinates to model those systems subjected to a combination of holonomic and non-holonomic constraints. Second example, illustrates the differences between the two methods when modeling the types of joints that constraints the rotational motion, or make the relative rotation very small. Final example discusses the effectiveness of implementing RPCF-EA and RPCF-EP onto systems with gyroscopic motion, which has some numerical integration problems due to gimbal lock.
Surface microtexturing has been widely used due to its good hydrophobic or drag reduction characteristics, and become an effective method to improve product performance and reduce energy consumption. ...This paper mainly discusses the improvement of microtextures on the dynamic pressure characteristics of hydrostatic bearings, and explores the effects of texture parameters on carrying capacity, macroscopic wall two-plane shear force, cavity area and other factors. In the oil film model calculation of the smooth wall surface of the radial hydrostatic bearing under the action of high speed and large external load, the oil film divergent wedge often has a negative pressure area, which is obviously not in line with the actual situation, so the cavitation effect needs to be considered. The CFD analysis method of the “gas-oil” two-phase flow model was carried out by using the mixture model to seek the optimal texture model scheme and thus to improve the load carrying capacity (LCC) and reduce the wall shear force. The effects of the texture area arrangement and geometric parameters on the lubrication characteristics were compared and analyzed. It is found that the carrying capacity of local texture is better than that of global texture, and different texture arrangements can achieve better drag reduction rates. The work presented in this paper studies the lubrication of the surface texture of a hydrostatic bearing. Taking the oil film carrying capacity and shear force as the target parameters, the factors, such as texture morphology, geometric parameters, texture distribution and cavitation phenomenon, are investigated through simulation and experimental methods. The surface textured hydrostatic bearing is expected to obtain the maximum oil film carrying capacity and the minimum friction resistance. The analysis results show that by arranging the partial streamwise texture at the rear end of the diverging wedge, the maximum shear force of the wall can be reduced by about 15%, and the LCC can be increased by about 18%.
The anisotropy exhibited by single-crystal silicon in nanometric cutting is very significant. In order to profoundly understand the effect of crystal anisotropy on cutting behaviors, a large-scale ...molecular dynamics model was conducted to simulate the nanometric cutting of single-crystal silicon in the (100)0–10, (100)0-1-1, (110)−110, (110)00–1, (111)−101, and (111)−12-1 crystal directions in this study. The simulation results show the variations of different degrees in chip, subsurface damage, cutting force, and friction coefficient with changes in crystal plane and crystal direction. Shear deformation is the formation mechanism of subsurface damage, and the direction and complexity it forms are the primary causes that result in the anisotropy of subsurface damage. Structurally, chips could be classified into completely amorphous ones and incompletely amorphous ones containing a few crystallites. The formation mechanism of the former is high-pressure phase transformation, while the latter is obtained under the combined action of high-pressure phase transformation and cleavage. Based on an analysis of the material removal mode, it can be found that compared with the other crystal direction on the same crystal plane, the (100)0–10, (110)−110, and (111)−101 directions are more suitable for ductile cutting.
This article presents an experimental investigation on high-efficiency machining monocrystalline silicon by polycrystalline diamond (PCD) end mills with a tool diameter of 5 mm. The milling ...experiments were carried out on a self-made five-axis numerical control machine tool. A super-high magnification zoom lens 3D microscope, white light interferometer, and dynamometer were applied to qualitatively and quantitatively characterize the performance of processed surfaces and cutting force. A surface roughness of Ra = 9.9 nm, better than most previously reported value on silicon, was obtained. Also, greater plastic removal efficiency was achieved under a small axial depth of cut and feed rate (less than 60 and 0.3 μm/tooth, respectively) and high spindle speeds (50,000 rpm). Moreover, surface characterization studies were explored which reveals that faster spindle speed, lower feed rate, and smaller cut depth can provide an easy access to ductile machining. At last, note that the shape structure and amplitude of cross-cutting force kept a close relation with the processing mode of single-crystal silicon, and a stable cross force contributed to improve surface quality as well as inhibiting micro-crack initiation.
This article presents an experimental investigation on ductile-mode micro-milling of monocrystalline silicon using polycrystalline diamond (PCD) end mills. Experimental results indicate that the ...irregular fluctuation of cutting force always induces machined surface failure, even in ductile mode. The internal mechanism has not been investigated so far. The multiscale discrete dislocation plasticity framework was used to predict the dislocation structure and strain evolution under the discontinuous cutting process. The results showed that a mass of dislocations can be generated and affected in silicon crystal. The dislocation density, multiplication rate, and microstructure strongly depend on the milling conditions. In particular, transient impulse load can provide a great potential for material strength by forming dislocations entanglement structure. The continuous irregular cutting process can induce persistent slip bands (PSBs) in substrate surface, which would result in stress concentration and inhomogeneous deformation within grains.
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