Forming characteristic of sheet hydroforming under the influence of through-thickness normal stress

Sheet hydroforming is different from conventional sheet forming due to the existence of liquid pressure. Through thickness normal stress induced by liquid pressure is derived by solving the normal force equilibrium equation. The theoretical prediction is in a good agreement with that determined by experimental data at cup wall. As the third principal stress, through thickness normal stress cannot be reflected in two dimensional yield locus. The variables of η and ω as new coordinates were introduced. Accompanied with stress triaxiality β_av and effective strain ?_eq, they were used to describe the change of typical stress states in sheet hydroforming. As liquid pressure increases to infinity, sheet hydroforming covers the stress states from equi-biaxial tension to uniaxial compression. The numerical simulation in Marc shows that, the stress states of non-free bulging zones are away from plane stress in the (β_av, ?_eq) and (η, ω) based spaces.     

织构刀具高速干式切削Al7075-T6的性能研究

表面微织构能够改善刀具的切削性能。为了研究织构刀具高速干切削Al7075-T6时织构参数对切削性能的影响,利用仿真软件建立正交二维切削仿真模型。对比了所选织构参数范围内的最优织构刀具与无织构刀具的主切削力、刀具温度、刀具应力,分析了织构参数对主切削力的影响程度。结果表明:合理的织构参数,可以减小刀具的主切削力,使刀具表现出更好的温度及应力分布梯度;所选织构参数范围内,织构宽度为60μm、织构间距为90μm、织构深度为30μm、织构刃边距为100μm的织构刀具切削性能最好;织构参数对主切削力影响程度从大到小依次为织构刃边距、织构宽度、织构间距、织构深度。     

In situ determination of internal stresses in mixed ceramic cutting tools during friction testing using synchrotron radiation

In order to get a better understanding of wear mechanisms in cutting tool materials, the internal stress states caused by complex external thermo-mechanical loads during cutting processes should be investigated. This can be done using in situ X-ray diffraction with high energy synchrotron radiation. For this reason, in first model tests, the strain state in ceramic cutting tool materials (Al₂O₃–Ti(O,C)) for hard turning applications is determined during friction testing. An experimental set-up is presented which allows the measurement of lattice strain tensor components in the major phases of the mixed ceramic material under a two-dimensional loading condition during friction. Results show that during friction almost no lattice strains in lateral direction can be measured due to strain compensation. In contrast, in normal direction, lattice strains increase due to the addition of strains. Stress tensor components are calculated from lattice strains and show higher internal stress states for Ti(O,C) than for Al₂O₃ due to a higher Young’s modulus of Ti(O,C). The set-up represents a major step forward towards future in situ characterization of internal stress states in cutting tool materials exposed to complex thermo-mechanical loading conditions during cutting processes.     

Determination of workpiece flow stress and friction at the chip–tool contact for high-speed cutting

This paper presents a methodology to determine simultaneously (a) the flow stress at high deformation rates and temperatures that are encountered in the cutting zone, and (b) the friction at the chip–tool interface. This information is necessary to simulate high-speed machining using FEM based programs. A flow stress model based on process dependent parameters such as strain, strain-rate and temperature was used together with a friction model based on shear flow stress of the workpiece at the chip–tool interface. High-speed cutting experiments and process simulations were utilized to determine the unknown parameters in flow stress and friction models. This technique was applied to obtain flow stress for P20 mold steel at hardness of 30 HRC and friction data when using uncoated carbide tooling at high-speed cutting conditions. The average strain, strain-rates and temperatures were computed both in primary (shear plane) and secondary (chip–tool contact) deformation zones. The friction conditions in sticking and sliding regions at the chip–tool interface are estimated using Zorev's stress distribution model. The shear flow stress (k_chip) was also determined using computed average strain, strain-rate, and temperatures in secondary deformation zone, while the friction coefficient (μ) was estimated by minimizing the difference between predicted and measured thrust forces. By matching the measured values of the cutting forces with the predicted results from FEM simulations, an expression for workpiece flow stress and the unknown friction parameters at the chip–tool contact were determined.     

Meso-damage evolution and mechanical characteristics of low-porosity sedimentary rocks under uniaxial compression

Low pore sedimentary rocks (from Guangxi, China) were subjected to uniaxial compression loading experiment under different initial stresses. The rock samples were investigated by nuclear magnetic resonance before and after the loading. The relationships between the mesoscopic rock damage and macroscopic mechanical parameters were established, and the initial damage stress of the low-porosity sedimentary rock was determined. The results showed that this type of rock has the initial stress of damage. When the initial loading stress is lower than the initial stress of damage, the T₂ spectrum area of the rock sample gradually decreases, and the primary pores of the rock are further closed under the stress. The range of the initial stress of damage for this type of rock is 8-16 MPa. When the loading stress exceeds the initial stress of damage, the T₂ spectrum area gradually increases, indicating that the porosity of the rock increases and microscopic damage of the rock appears. The rock damage degree is defined, and the nonlinear function between the rock damage degree and the initial loading stress is established.     

Some experimental investigations in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates

In this paper, a concept of delamination factor F_d (i.e. the ratio of the maximum diameter D_max in the damage zone to the hole diameter D) is proposed to analyze and compare easily the delamination degree in the drilling of carbon fiber-reinforced plastic (CFRP) composite laminates. Experiments were performed to investigate the variations of cutting forces with or without onset of delamination during the drilling operations. The effects of tool geometry and drilling parameters on cutting force variations in CFRP composite materials drilling were also experimentally examined. The experimental results show that the delamination-free drilling processes may be obtained by the proper selections of tool geometry and drilling parameters. The effects of drilling parameters and tool wear on delamination factor are also presented and discussed.Cutting temperature has long been recognized as an important factor influencing the tool wear rate and tool life. An experimental investigation of flank surface temperatures is also presented in this paper. Experimental results indicated that the flank surface temperatures increase with increasing cutting speed but decreasing feed rate. Optimal cutting conditions are proposed to avoid damage from burning during the drilling processes.     

Optimization of process parameter of residual stresses for hard turned surfaces

Hard turning has been recognized as a substitute for abrasive-based processes due to its operational flexibility, economic benefit, and low environmental impact. Besides these advantages, hard turning can induce compressive residual stresses, which increase the fatigue life of the workpiece. In this paper, we investigate the process parameters of cutting speed, depth of cut, and feed rate on inducing subsurface compressive residual stress. Using a designed experiment based on a Taguchi L₉ (3⁴) array, we varied process parameters over a feasible space. The resulting residual stresses were examined and evaluated by X-ray diffraction. Using the smaller-is-better objective function for residual stress, we then identified the optimal set of process parameters.     

The corrosion behavior of plasma sprayed Fe2Al5 coating in molten Zn

Aluminum coating was plasma sprayed on Fe-0.14-0.22 wt.% C steel substrate, and heat diffusion treatment at 923 °C for 4 h was preformed to the aluminum coating to form Fe₂Al₅ inter-metallic compound coating. The corrosion mechanism of the Fe₂Al₅ coating in molten zinc was investigated. SEM and EDS analysis results show that the corrosion process of the Fe₂Al₅ layer in molten zinc is as follows: Fe₂Al₅ → Fe₂Al₅Zn_x (η) → η + L(liquid phase) → L + η + δ(FeZn₇) → L + δ → L. The η phase and the eutectic structure (η + δ) prevent the diffusion of zinc atoms efficiently. Therefore the Fe₂Al₅ coating delays the reaction between the substrate and molten zinc, promoting the corrosion resistance of the substrate.     

涂层刀具微喷砂表面处理技术的进展

介绍了微喷砂表面处理技术进展及工作原理,分析了微喷砂处理对涂层刀具表面完整性、切削性能的影响。研究发现,微喷砂能够改善涂层刀具表面的粗糙度并提高涂层表面的残余压应力,进而提升刀具的切削性能并延长使用寿命。总结了微喷砂表面处理技术对涂层刀具表面的影响,并且对微喷砂表面处理技术进行了展望。研究结果为涂层刀具的表面处理和绿色智能制造提供了参考。     

Prediction of fracture limits of Ni–Cr based alloy under warm forming condition using ductile damage models and numerical method

The stretch forming and the deep-drawing processes were carried out at 300 and 673 K to determine the safe forming and fracture limits of IN625 alloy. The experimentally obtained strain-based fracture forming limit diagram (FFLD) was transformed into a stress-based (σ-FFLD) and effective plastic strain (EPS) vs triaxiality (η) plot to remove the excess dependency of fracture limits over the strains. For the prediction of fracture limits, seven different damage models were calibrated. The Oh model displayed the best ability to predict the fracture locus with the least absolute error. Though the experimentally obtained fracture limits have only been used for the numerical analysis, none of the considered damage models predicted the fracture strains over the entire considered range of stress triaxiality (0.33<η<0.66). The deep drawing process window helped to determine wrinkling, safe and fracture zones while drawing the cylindrical cups under different temperature and lubricating conditions. Further, the highest drawing ratio of 2 was achieved at 673 K under the lubricating condition. All the numerically predicted results of both stretch forming and deep drawing processes using the Hill 1948 anisotropic yielding function were found to be good within the acceptable range of error.     

Numerical analysis of shear deformation localization using a double-variable damage model

A double-variable damage model was introduced into the constitutive equations to demonstrate the effect of the material damage for the isotropic elastic, hardening, and damage states, and for the isothermal process. The shear damage variable D _s and the bulk damage variable D _b may be, respectively, used to describe the effect of shear damage and bulk damage for material properties without the superfluous constraint, D _b=D _s, that is found in the single-variable damage model. The double-variable damage model was implemented to form the finite element code for analyzing the effect of shear damage and bulk damage. In this article, two numerical simulation examples were completed to model the whole process of initiation and propagation of shear bands in an aluminum alloy. The numerical computational results are coincident with the experimental results.     

Mg/Zn界面的AF超动力学中的阿累尼乌斯方程及两步法模拟机制(英文)

加速因子(AF)法是解决固体界面原子深层扩散简单且适用的方法。在AF超动力学模拟的框架内,以Mg/Zn界面为例,首先研究原子沿界面z轴方向上的一维扩散系数Dz与温度的关系,进而探讨加速因子A对扩散常数、扩散激活能的影响;然后,在AF法的基础上,通过两步法对Mg/Zn界面的原子扩散及相形成过程进行模拟。结果表明,不同加速因子下的扩散系数Dz均较好地遵循Arrhenius方程;在没有发生相变的情况下,当扩散机制一定时,扩散常数D0的值与A无关;采用AF超动力学模拟时原子的扩散激活能Q*与原激活能Q的关系符合Q*=Q/A。据此,构建了AF超动力学模拟下的一维阿累尼乌斯方程:Dz=D0exp[Q/(ART)]。同时,AF法可以对界面相形成过程及其变化趋势作预测,且只有两步法才能确切得到不同传质阶段的体系平衡结构及平衡能量。     

Modelling the initiation of cleavage fracture of ferritic steels

The initiation of the cleavage fracture of ferritic steels at cracked grain boundary carbides situated in a plastic zone is modelled, using a 2D discrete dislocation simulation of the plastic zones associated with a microcrack under triaxial loading. Fracture is assumed to occur when the local stress intensity factor for the microcrack equals K_IC. For microcracks in the size range 0.1–10 μm, the applied stress at fracture, σ_F, is found to be independent of the yield stress (in the range 200–1600 MPa) and therefore of temperature. The dependence of σ_F on the crack size deviates from a Griffith type relation. These predictions are consistent with experimental data on steels in the literature. The values of σ_F predicted are in good agreement with the experimental values. The results suggest that only a small fraction of carbides are in configurations leading to fracture.     

Effects of local damage on asymptotic stress field of a growing creep crack

A parametric study on the effects of local damage field on the crack-tip stress field of a growing Mode I creep crack is performed in the framework of Continuum Damage Mechanics (CDM). According to the results of creep crack growth analysis based on CDM and Finite Element Method, the damage distribution1-(D/D _cr)=h(θ)r^m represented by a power law function of the radiusr from the crack tip is postulated for the damage variableD. The damage effects are incorporated into the Norton creep law by means of the hypothesis of strain equivalence of CDM. The resulting two-point boundary value problems of differential equations for the growing creep cracks in the states of plane strain and plane stress are solved by means of a shooting method. For a given creep exponentn of the Norton law, the exponentp of the asymptotic stress field σ _ij ∞r ^p is found to be governed by the exponentm of the power law damage distributionr ^m.     

Obtaining a submerged arc welding flux of the MnO–SiO2–CaO–Al2O3 – CaF2 system by fusion

Plate temperature and heat input in an oxyfuel gas cutting process with H₂/LP gas and LPG flame are calculated by three-dimensional FE heat conduction analyses. FE analyses are performed by using moving coordinates, and cutting groove temperature is determined by iterative calculation. The two-dimensional groove temperature distribution determined by Matsuyama's theory is chosen as the initial values in this iterative calculation. The heat transfer properties of the preheating flame are determined by using the genetic algorithm-based heat transfer estimation technique proposed in the previous report. The validity of the proposed numerical procedure and the accuracy of the determined groove temperature are examined by comparing the calculated and measured plate temperature and heat-affected zone sizes. Heat input due to preheating, q_G, and that due to self-burning of steel, q_B, are estimated in these analyses, and they are compared with the heat inputs estimated by Wells' and modified Wells' equations. The relation between the heat transfer characteristics of the preheating gas flame and plate temperature distribution is examined, and the cutting performance improvement mechanisms of hydrogen preheating are discussed. As a result, the followings are found: (1) the three-dimensional groove temperature distribution can be calculated by performing the iterative analyses procedure proposed in this study; (2) the critical cutting speed can be estimated once the gas heat transfer parameters are known; (3) it is not appropriate to evaluate the magnitude of cutting thermal deformation only from the preheating gas's total calorific value; (4) under the conditions chosen, the heat generated by self-burning is inadequate to maintain the cutting process, and it is essential to supplement heat by preheating; (5) the faster cutting speed and smaller total heat input of H₂/LP gas are results of the larger local heat transfer coefficient below the gas ejection hole. It is supposed that the improvement in oxyfuel gas cutting performance can be achieved by modifying the heating apparatus so that the local heat transfer coefficient becomes larger.     

Microstructure-Based Numerical Simulation of the Tensile Behavior of SiCp/Al Composites

Modeling and prediction of the damage evolution in particle reinforced composites is a complex problem. Microstructure characters such as the particle morphologies, sizes, and distribution significantly affect the damage evolution in composites. A numerical simulation has been performed to investigate the damage evolution of SiC_p/AA2009 composites. Tensile deformation in SiC_p/AA2009 composites was simulated using the microstructure-based model constructed from the metallograph. Matrix damage, particle cracking, and interface debonding were simulated combining the ductile damage model, the normal stress criterion, and the maximum stress ratio criterion. The simulation results show that under tensile loading, damage initiates at the interface, and then propagates along the weakest direction. The simulation microstructures agree well with experimental results in which interface debonding, particle cracking, and matrix damage co-exist. In addition, the effects of component properties on the damage evolution are examined for various situations.     

Microstructural evolution during partial remelting of AM60B magnesium alloy refined by MgCO3

AM60B magnesium alloy was refined by MgCO₃ and its microstructural evolution was investigated during partial remelting. The results indicate that MgCO₃ is an effective grain refiner for AM60B alloy and can decrease the grain size from 329 μm of the unrefined alloy to 69 μm. A semisolid microstructure with small and spheroidal primary particles can be obtained after being partially remelted. The microstructure evolution can be divided into four steps: the initial rapid coarsening, structure separation, spheroidization and final coarsening. Correspondingly, these four steps result from the phase transformations of β→α, α+β→L and α→L, α→L and two reverse reactions of α→L and L→α, respectively. One spheroidal primary particle in the semisolid microstructure usually originates one dendrite in the as-cast microstructure. The variation of primary particle size with holding time does not obey the LSW law, D_t³?D₀³=Kt, after the semisolid system is in its solid-liquid equilibrium state. Longer heating duration makes the primary particles more globular, but it makes their size larger at the same time.     

盲孔法测量精度的研究─—边界及孔间距的影响

本文根据弹性这柯西解及三维弹性有限元分析，对通孔及盲孔测量时，边界与孔中心距离和孔与孔间距问题进行了研究讨论，得到边界及孔间距是在一定范围内有影响的，孔与孔之间的影响与载荷方向有关，并且通过实验对其进行了验证。由此确定了误差小于２％－３％的横向最小间距ｄｉｍｉｎ（垂直国荷），纵向最小距ｄｘｍｉｎ（平行于载荷方向）和孔与边界的最小距离ｄｍｉｎ。为盲孔测量获得比较可靠而又完整的应力数值提供了依据。     

Anisotropic mechanical properties of amorphous Zr-based foams with aligned,elongated pores

Equal-channel angular extrusion is used to consolidate a blend of amorphous Zr_56.3Nb_5.1Cu_15.6Ni_12.9Al_10.0 and crystalline W powders into dense composites. Chemical dissolution of the crystalline phase results in amorphous foams with elongated pores, aligned at a 22–28° angle with respect to the extrusion direction, whose compressive properties are studied for various orientations. As the angle between the pore long direction and the applied stress direction increases from 0° to 68°, there is a significant decrease in loading stiffness and peak stress, as expected from predictive analytical models; however, the observed increase in stiffness and peak stress observed when the pores are oriented 90° to the direction of loading is not predicted by all of the models. Foams with pores aligned 24–68° to the direction of loading show increased plastic bending in individual walls and accumulation in microscopic damage without failure, leading to increased compressive ductility and absorbed energy over other orientations.     

不同加载速率下氧化铝纳米线弯曲力学行为的分子动力学研究（英文）

采用LAMMPS软件建立弯曲加载下α-Al₂O₃纳米线的分子动力学(MD)模型,计算并分析不同加载速率下α-Al₂O₃纳米线的原子应力和应变,揭示加载速率对其弯曲力学行为的影响规律。研究结果表明：α-Al₂O₃纳米线在不同加载速率下的最大表面应力-转角曲线均可分为弹性变形、塑性变形和破坏3个阶段,弹性极限点可通过加、卸载循环下的曲线对称性来确定;加载速率对α-Al₂O₃纳米线的塑性变形影响很大,但对弹性模量影响较小;当加载速率增加时,塑性变形阶段缩短,材料更易发生脆性断裂,且弹性极限和强度极限(由直接和间接MD法确定)更加接近。MD模拟结果与改进的弯曲环路试验结果吻合良好,从而验证分子动力学建模和计算方法的有效性。直接MD法是确定在任意加载速率下(无论发生脆性还是韧性断裂)纳米晶须弹性极限和强度极限的有效方法。