Determining the minimum grain size in severe plastic deformation process via first-principles calculations

Although the stacking fault energy (SFE) is a fundamental variable determining the minimum grain size (d_min) obtainable in severe plastic deformation (SPD) processes, its accurate measurement is difficult. Here we establish the SFEs of binary Pd–Ag, Pd–Cu, Pt–Cu and Ni–Cu solid solutions using the axial interaction model and the supercell model in combination with first-principles theory. The two models yield consistent formation energies. For Pd–Ag, Pd–Cu and Ni–Cu, the theoretical SFEs agree well with those from the experimental measurements. For Pt–Cu no experimental results are available, and thus our calculated SFEs represent the first reasonable predictions. We discuss the correlation of the SFE and d_min in SPD experiments and show that the d_min values can be evaluated from first-principles calculations.     

A predicted milling force model for high-speed end milling operation

In this study, a predicted milling force model for the end milling operation is proposed. The speed of spindle rotation, feed per tooth, and axial and radial depth of cut are considered as the affecting factors. An orthogonal rotatable central composite design and the response surface methodology are used to construct this model. The milling force per spindle revolution period obtained from each treatment is equally divided into suitable sections. The extreme value of the milling force in each section is selected to build the predicted model so as to predict the extreme force in each section for any cutting conditions within the specified range of the design database, including the speed of spindle rotation, feed per tooth, and axial and radial depth of cut. Moreover, the predicted extreme force in each section is applied to reconstruct the milling force waveform by means of the expansion of the Fourier series. The predicted model presented in this paper is adequate for a 95% confidence interval, and shows good correlation between experimental and predicted results.     

A statistical model for predicting the mechanical properties of nanostructured metals with bimodal grain size distribution

A statistical analysis is employed to investigate the mechanical performance of nanostructured metals with bimodal grain size distribution. The contributions of microcracks in the plastic deformation are accounted for in the mechanism-based plastic model used to describe the strength and ductility of the bimodal metals. The strain-based Weibull probability distribution function and percolation analysis of microcracked solids are applied to predict the failure behavior of the bimodal metals. The numerical results show that the proposed model can describe the mechanical properties of the bimodal metals, including yield strength, strain hardening and uniform elongation. These predictions agree well with the experimental results. The stochastic approaches adopted in the proposed model successfully capture the failure behavior of bimodal coppers that are sensitive to grain size and the volume fraction of coarse grains in addition to the corresponding threshold for percolation. These results will benefit the optimization of both strength and ductility by controlling constituent fractions and the size of the microstructures in materials.     

A model for thread milling cutting forces

This paper presents a mechanistic model for prediction of the thread milling forces. The mechanics of cutting for thread milling is analyzed similar to the end milling process but with modified cutting edge geometry. The chip thickness and cutting force models are developed considering the unique geometry of the tool. The model has been calibrated for 6061 Aluminum and validated. The effects of tool and thread geometry have been studied using the model.     

Predictive cutting force model in complex-shaped end milling based on minimum cutting energy

A force model is presented to predict the cutting forces and the chip flow directions in cuttings with complex-shaped end mills such as ball end mills and roughing end mills. Three-dimensional chip flow in milling is interpreted as a piling up of the orthogonal cuttings in the planes containing the cutting velocities and the chip flow velocities. Because the cutting thickness changes with the rotation angle of the edge in the milling process, the surface profile machined by the previous edge inclines with respect to the cutting direction. The chip flow model is made using the orthogonal cutting data with taking into account the inclination of the pre-machined surface. The chip flow direction is determined so as to minimize the cutting energy, which is the sum of the shear energy on the shear plane and the friction energy on the rake face. Then, the cutting force is predicted for the chip flow model at the minimum cutting energy. The predicted chip flow direction changes not only with the local edge inclination but also with the cutting energy consumed in the shear plane cutting model. The cutting processes with a ball end mill and a roughing end mill are simulated to verify the predicted cutting forces in comparison with the measured cutting forces.     

Simulation of cutting process in peripheral milling by predictive cutting force model based on minimum cutting energy

The cutting force and the chip flow direction in peripheral milling are predicted by a predictive force model based on the minimum cutting energy. The chip flow model in milling is made by piling up the orthogonal cuttings in the planes containing the cutting velocities and the chip flow velocities. The cutting edges are divided into discrete segments and the shear plane cutting models are made on the segments in the chip flow model. In the peripheral milling, the shear plane in the cutting model cannot be completely made when the cutting point is near the workpiece surface. When the shear plane is restricted by the workpiece surface, the cutting energy is estimated taking into account the restricted length of the shear plane. The chip flow angle is determined so as to minimize the cutting energy. Then, the cutting force is predicted in the determined chip flow model corresponding to the workpiece shape. The cutting processes in the traverse and the contour millings are simulated as practical operations and the predicted cutting forces verified in comparison with the measured ones. Because the presented model determines the chip flow angle based on the cutting energy, the change in the chip flow angle can be predicted with the cutting model.     

A cutting force model for face milling operations

A procedure for the simulation of the static and dynamic cutting forces in face milling is described. For the static force model, the initial position errors of the inserts and the eccentricity of the spindle are taken into consideration as the major factors affecting the variation of the chip cross-section. The structural dynamics model for the multi-tooth oblique cutting operation is assumed as a multi-degrees of freedom spatial system. From the relative displacement of this system, based on the double modulation principle, the dynamic cutting forces were derived and simulated. The simulated forces were subsequently compared to measured forces in the time and frequency domains.     

A note on grain size dependent pinning

A rigorous approach to grain size dependent pinning is proposed by accounting for special grain boundary locations such as quadruple points. Using this approach abnormal grain growth is rationalized for a high strength low alloy (HSLA) steel with a stable particle size distribution.     

Grain rotation by dislocation climb in a finite-size grain boundary

We investigate the kinetics of grain rotation in a bicrystal with a tilt grain boundary by studying the relaxation of an edge dislocation wall in a discrete-dislocation approach. The boundary is infinitely extended in one direction and of finite size in the orthogonal one. The relaxation process is simulated numerically by solving the equations of motion of the dislocations, assuming climb by diffusive transport in the boundary plane. Surprisingly, we find that boundaries never rotate all the way into coincidence. Instead, the final state is a metastable array with 18 dislocations and, hence, with a finite misorientation that depends on the boundary length and the Burgers vector. All boundaries with fewer than 18 dislocations are also metastable. The relaxation time to reach the metastable configuration is found to be proportional to the logarithm of the number of dislocations and to the cube of the length of the boundary. We give a critical discussion of image force arguments that underlie earlier work on grain rotation, and verify that the present analysis of image forces does satisfy the boundary conditions at the free surfaces. The results have implications for the kinetics of rotation of nanoparticles on a substrate and for the stability of grain and subgrain boundaries in thin metal films.     

基于位错演化模型纳米晶块体材料晶粒的演变

利用试验及数值仿真方法研究了工业纯铝在等径角挤压变形过程中晶粒的尺寸变化。数值仿真过程引入位错演化模型,对纯铝等径角挤压过程中晶粒尺寸的演化进行了模拟。研究结果表明,基于位错演化模型,经4道次剪切变形仿真后纯铝出现纳米级晶粒。数值仿真结果与试验对比显示,两者结果一致,说明利用数值仿真可以预测等径角挤压块体材料晶粒尺寸的演变过程。     

Effect of the initial grain size on the final grain size of materials of bellows

Conclusions The final grain size of the material of measuring bellows is determined by the conditions of mechanical and heat treatment, and it depends little on the initial grain size; this makes it possible to eliminate acceptance control of the grain size of material for bellows in the state as supplied.Belorussian Polytechnic Institute, Pilot Plant of the Research Institute of Heat-Treatment Instruments (NIIteplopribor), Smolensk. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 50–51, January, 1985.     

A cutting power model for tool wear monitoring in milling

This paper describes a cutting power model in face milling operation, where cutting conditions and average tool flank wear are taken into account. The cutting power model is verified with experiments. It is shown with the simulations and experiments that the simulated power signals predict the mean cutting power better than the instantaneous cutting power. Finally, the cutting power model is used in a cutting power threshold updating strategy for tool wear monitoring which has been carried out successfully in milling operations under variable cutting conditions.     

A dislocation pile-up model for the yield stress of a composite

A numerical model to simulate yielding in a composite is developed for the transmission of slip across a dissimilar interface through the formation of co-planar dislocation arrays in both phases. A pile-up of dislocations in the soft phase is assumed to nucleate dislocations in the hard phase in which movement is dictated by lattice friction stress. The polycrystalline composite yield stress is calculated by determining the equilibrium positions of the dislocation arrays as a function of the length scales, elastic constants and Burgers vectors in the two phases, with particular reference to melt oxidized Al–Al₂O₃, in which homophase boundaries are absent, and to the commercially important system Co–WC. The hardness values predicted from this model are in good agreement with experimentally measured values in the above systems. The implications of these results for the design of hard composite microstructures are elucidated.     

核电用304不锈钢静态再结晶模型的建立

利用Gleeble-1 500 D热力模拟试验机,对304不锈钢进行了热压缩模拟试验。研究了变形温度在950℃~1 200℃之间,变形速率为1×10-3 s-1、1×10-2 s-1、1×10-1 s-1,变形量为50%的变形行为及组织特征,建立了304钢的静态再结晶晶粒尺寸模型。结果表明：变形量、变形温度、应变速率因素的影响较大;应变量越大,变形温度越低,应变速率越高,晶粒越细;304钢静态再结晶晶粒尺寸模型的建立为大锻件静态再结晶数值模拟分析提供了可靠的判据。     

A voxel-based kinematic simulation model for force analyses of complex milling operations such as wobble milling

Mechanical machining of fiber reinforced plastics poses special challenges due to the heterogeneous and anisotropic material composition. Process strategies for the generation of drill holes, which aim at directing the resultant process forces toward the center of the workpiece, have been shown to obtain good machining results with less process induced damage. However, these strategies (e.g. wobble milling) might involve complex multiaxial tool movements and are thus very difficult to analyze and optimize. A voxel-based kinematic simulation program has been set up, which allows analyses of process forces for arbitrary milling operations based on the time-resolved determination of the cutting thickness and multivariate process force regression models. A basic analysis of the process of wobble milling is presented as well. It confirms that the resultant process forces are directed toward the center of the workpiece when the outer material layers are machined. The resultant force is directed in a favorable direction throughout the complete cut during the actual process step of wobble milling.