Surface energies and relaxation of NiCoCr and NiFeX (X=Cu, Co or Cr) equiatomic multiprincipal element alloys from first principles calculations

First principles calculations of the energies and relaxation of unreconstructed low-index surfaces, i.e. (001), (011) and (111) surfaces, in NiCoCr and NiFeX (X=Cu, Co or Cr) equiatomic multi-principal element alloys are presented. The calculations were conducted for twelve-layer slabs represented by special quasi-random supercells using the projector augmented wave method within the generalized gradient approximation. While experimental predictions are unavailable for comparison, the calculated surface energies agree fairly well with those from thermodynamic modeling and a bond-cutting model. In addition, the calculations unveil an important surface structure, namely, that the topmost surface layer is in contraction except for the (001) surface of NiFeCr alloy, the next layer below is in extension, and the bulk spacing is gradually recovered from the subsequent layers down. Additionally, the surface contraction is the most pronounced on the (011) plane, being about 4-10% relative to the bulk spacings. The results presented here can provide an understanding of surface-controlled phenomena such as corrosion, catalytic activities and fracture properties in these equiatomic multi-principal element alloys.

Effect of Graphene nanoplatelets (GNP) reinforcement on grindability of zirconium diboride ceramics

The grindability of graphene nanoplatelets (GNP) reinforced ZrB2 was studied using resin bonded diamond grinding wheel under dry and wet conditions. A comparative study of grinding forces was performed at selected wheel surface speeds and depth of cuts for surface grinding. ZrB2-GNP showed lower normal grinding forces due to the reduced hardness. The presence of GNP reinforcement in ZrB2 resulted in lower tangential forces and reduced specific grinding energy due to the role of GNP as solid lubricant. The measured forces showed good correlation with the micro cutting model for ZrB2 and ZrB2-GNP under dry condition. The tangential forces showed same trend as normal forces at different depth of cuts and wheel surface speeds for both ZrB2 and ZrB2-GNP with average force ratios of 0.3 and 0.35 respectively. The presence of porosity in ZrB2 increased the normal grinding forces during wet grinding. Scanning Electron Microscope (SEM) images of the grinding chips indicated a mixture of both the ductile mode and the brittle mode of material removal with predominantly brittle fractured chips. Energy Dispersive Spectroscopy (EDS) confirmed the presence of GNPs in ZrB2-GNP grinding chips. The topography of the grinding wheel showed higher wheel loading after the dry grinding than that of wet grinding. The wet grinding resulted in relatively lower surface roughness (Ra values) compared to that of dry grinding.

Model of Ploughing Cortical Bone with Single-Point Diamond Tool

Generating topological microstructures on the surface of cortical bone to establish a suitable microenvironment can guide bone cells to achieve bone repair. Single-point diamond tools (SPDTs) have advantages in efficiency and flexibility to fabricate surface microstructures. However, the cutting force during ploughing cannot be predicted and controlled due to the special properties of cortical bone. In this paper, a novel cutting model for ploughing cortical bone using an SPDT was established, and we comprehensively considered the shear stress anisotropy of the bone material and the proportional relationship between the normal force and the tangential force. Then, the orthogonal cutting experiment was used to verify the model. The results show that the error of calculated value and the experimental data is less than 5%. The proposed model can be used to assist the fabrication of microstructures on cortical bone surface using an SPDT.

Temperature Distribution Simulation, Prediction and Sensitivity Analysis of Orthogonal Cutting of Cortical Bone

Bone cutting plays an important role in spine surgical operations. The power devices with high speed employing in bone cutting usually leads to high cutting temperature of the bone tissue. This high temperature control is important in improving cutting surface quality and optimizing the cutting parameters. In this paper, the bone-cutting model was appropriately simplified for finite element (FE) based modeling of 2D orthogonal cutting to discuss the change law of cutting temperature of cortical bones for cervical vertebra, and to study the orthogonal cutting mechanism of the anisotropic cortical bone, a 3D FE simulation model had been also established in which longitudinal, vertical, and transversal cutting types were accomplished to investigate the effect of osteons orientation. Secondly, this response surface method was used to regress the simulation results, and establishes the prediction model of maximum temperature on cutting depth, cutting speed, and feed speed. Then, the Sobol method was used to analyze the sensitivity of the milling temperature prediction mathematical model parameters, in order to clarify and quantitatively analyze the influence of input milling parameters on the output milling temperature. Finally, the cutting temperatures obtained with the simulations were compared with the corresponding experimental results obtained from the bone milling tests. This study verifies the influence of key variables and the cutting parameters on thermo mechanical behavior of the bone cutting. The obtained cutting temperature distribution for the bone surfaces could be employed to establish a theoretical foundation for research on thermal damage control of bone tissues.

Cutting Performance of Randomly Distributed Active Abrasive Grains in Gear Honing Process

In power gear honing, the random distribution of abrasive grains on the tooth surface of the honing wheel is the main factor that influences the machining performance of high-quality hardened gears. In order to investigate the micro-edge cutting performance of the active abrasive grains on the workpiece gear, the real honing process is simplified into a micro-edge cutting model with random distribution of active abrasive grains in the cells of the meshing area by obtaining the random distribution states such as the position, orientation and quantity of the honing wheel teeth. The results show that although the active abrasive grains are distributed at different locations, they all experience three types of material removal—slip rubbing, plowing and cutting—allowing the gear honing process to have the combined machining characteristics of grinding, lapping and polishing. The active abrasive grains in first contact produce high honing force, high material removal efficiency and poor surface roughness on the machined workpiece, while the latter ones have the opposite effects. The dislocation angle affects the chip shape and chip discharging direction, and the highest honing force and material removal efficiency is achieved with a dislocation angle of 135°. The higher the number of active abrasive grains in a given contact area, the higher the material removal efficiency.

Comparison Analysis Between Simulation and Experiment of Cutting Force and Cutting Stress as Well as Chip Morphology During the Macro and Nano Cutting of Single Crystal Copper

In this paper, the simulation and experiment comparison of cutting force, cutting stress and chip morphology during the macro and nano cutting of single crystal copper are carried out. Firstly, a finite element method based on Johnson-Cook metal strength and failure model were used to establish a macro cutting model, and the cutting force, cutting stress, cutting displacement and chip morphology were obtained. Then a molecular dynamics simulation was used to establish a nano cutting model, and the cutting force, von mises stress and chip morphology were obtained. Afterwards, a comparative analysis of the two was carried out. Finally, the external turning experiment was used to verify the simulation results of the macro cutting model. The results show that: 1. The change trend of the cutting force in x and y directions are different,but the corresponding ratios of cutting forces in x and y directions in macro and nano cutting process are very close, and the corresponding ratios of the average macro and nano cutting forces in x and y directions are also very close. 2. The cutting stress in nano cutting process was about 100 times of macro cutting stress. 3. The chip length in macro cutting process is larger than the chip length in nano cutting process, and the shape is more regular. 4. The experimental cutting force change trend is very similar to the simulation cutting force change trend, but there exits difference in the value between the experiment cutting forces and simulated cutting forces.

A novel multiscale material plasticity simulation model for high-performance cutting AISI 4140 steel

The achievable machined surface quality relies significantly on the material behavior during the high-performance cutting process. In this paper, a multiscale material plasticity simulation framework is developed to predict the deformation behaviors of AISI 4140 steel under various high-performance cutting conditions. The framework was built by coupling a three-dimensional discrete dislocation dynamic (3D-DDD) model with a finite element method (FEM) through the optimization of a dislocation density-based (DDB) constitutive equation (compiled as a user-defined subroutine in ABAQUS). The movement of edge and screw dislocations such as generation, propagation, siding, and their interactions, was performed by 3D-DDD, and the statistical features of dislocations were used to optimize the critical constants of the DDB constitutive equation. For validation, a classic FEM cutting model (Johnson-Cook constitutive equation) was employed as a reference. The simulation results indicated that the proposed multiscale model not only can precisely predict the stress, strain, cutting force, and temperature as those predicted by the classic FEM simulations, but also capture the microstructure characteristics such as grain size and dislocation density distributions under the tested cutting conditions. Severe dynamic recrystallization phenomena were found at the core shear zones. The recrystallization process reached a dynamic equilibrium at the machined surfaces when the cutting speed is larger than 280 m/min or the external-assisted temperature is between 200 and 350°, indicating an optimal range of machining parameters for improved surface integrity.

Factor Analysis of Machining Parameters On Surface Integrity of CFRP Composites Based On A Microscopic Finite Element Cutting Model

In the paper, a three-dimensional (3D) micromechanical finite element (FE) cutting model with three phases was developed to study the surface integrity of CFRP composites. The surface roughness and the depth of subsurface damage were predicted by using the FE cutting model, which were used to characterize the surface integrity. The machined surface observations and surface roughness measurements of CFRP composites at different fiber orientations were also performed for model validation. It is indicated that the 3D micromechanical model is capable of precisely predicting the surface integrity of CFRP composites. To investigate the complex coupling influences of multiple machining parameters on the surface integrity, the factor analysis of multiple machining parameters was performed, and then the effects of these machining parameters on the surface roughness and subsurface damage depth were obtained quantitatively. It was found that the fiber orientation angle and cutting speed are the most significant factors affecting the surface roughness, and the fiber orientation and edge radius are the main factors affecting the subsurface damage depth. The results also reveal that coupling effects of depth of cut and edge radius should be considered for improving the surface integrity of CFRP composites.