Li Miaoquan

An adaptive prediction model of grain size for the forging of Ti-6Al-4V alloy based on fuzzy neural networks

Abstract In this paper, an adaptive model of grain size, has been established with the help of the fuzzy neural networks (FNNs), based on experimental results for Ti–6Al–4V titanium alloy with homogeneous deformation under various process technological parameters. The data of Teacher’s samples has been obtained from the experimental results. By the comparison of the calculated results with the experimental data of the Teacher’s samples and the testing samples, it has been verified that the model proposed in this paper can be applied to compute the grain size evolution during the deformation of Ti–6Al–4V titanium alloy.

Microstructural evolution and mechanical properties of the semisolid Al–4Cu–Mg alloy

Abstract Microstructural evolution and the mechanical properties of the semisolid Al–4Cu–Mg alloy were investigated, and a grain coarsening equation of this semisolid alloy in the semisolid state was provided based on Ostwald ripening theory. The experimental results showed that the process parameters, including height reduction, isothermal temperature and holding time, affect grain size, fractal dimension and distribution of grain size to some extent. The grain size decreases with an increase of the height reduction, a decrease of the isothermal temperature and/or the holding time. The spheroidization of grains is promoted with an increase of height reduction, isothermal temperature or holding time. The grain size has a Gaussian distribution under different experimental conditions. There will be a more uniform distribution of grain size at high height reduction, low isothermal temperature and short holding time. The pretreatment of samples with cold or hot deformation before isothermal heat treatment is necessary. Good ultimate strength and yield strength could be obtained by the conditions of suitable height reduction, low isothermal temperature and short holding time. Finally, a kinetic equation of grain coarsening was put forward, which could be applied to the semisolid Al–4Cu–Mg alloy system over a high volume fraction of separated grains range from 0.75 to 0.90.

Microstructure Evolution during High Temperature Deformation of Ti-6Al-4V Alloy

Abstract The effect of processing parameters, including deformation temperature, strain rate and deformation degree on the microstructure evolution and the microstructure variables (grain size and volume fraction of primary α phase) was investigated based on the microstructure observation and the quantitative metallography of the isothermally compressed Ti-6Al-4V alloy. The results show that the curves of grain size of primary α phase are oscillatory in the α+β two-phase region with increasing of the deformation temperature, and meanwhile the volume fraction of primary α phase decreases. The effect of the strain rate on the morphology of primary α phase is significant, while its effect on the microstructure variables of isothermally compressed Ti-6Al-4V alloy is dependent on deformation temperature. The grain size of primary α phase decreases with increasing of strain rate above 1203 K, but below 1203 K it fluctuates. The curves of volume fraction of primary α phase are oscillatory with increasing of strain rate above 1223 K, but below 1223 K the volume fraction decreases. The grain size of primary α phase increases slightly after a drop with increasing of the deformation degree, and meanwhile the secondary α phase decreases gradually. The effect of deformation degree on the volume fraction of primary α phase is not significant.

Effect of Hydrogen on Microstructure of Ti-6Al-4V Alloys

Abstract Hydrogenation of Ti-6Al-4V titanium alloys was carried out in a special furnace through the accumulated flux method, and OM, XRD and TEM techniques were used to investigate the microstructure evolution of the hydrogenated Ti-6Al-4V titanium alloy. The hydrogenation of the Ti-6Al-4V titanium alloy with the hydrogen content of 0.30 wt% makes the α phase fraction decrease compared with that of the as-received Ti-6Al-4V titanium alloy, but it makes the β phase fraction increase. The δ hydride (TiH 2 phase) occurs in the hydrogenated Ti-6Al-4V titanium alloy when the hydrogen content is more than 0.3 wt%. The shear deformation is main pattern in the transformation processes from β -Ti(H) phase to α -Ti phase and δ hydride (TiH 2 phase). The phase transformation temperature of the hydrogenated Ti-6Al-4V titanium alloy decreases by 180 °C compared with that of the as-received Ti-6Al-4V titanium alloy, and it is related to the phase fraction and phase transformation.

Finite element simulation of deformation behavior in friction welding of Al-Cu-Mg alloy

Friction welding is one of the most effective and widely used solid-state joining methods in modern industries. Plastic deformation of interface material is the essence of friction welding, and welding process parameters affect the welding quality greatly. To understand the friction welding process better, it is important to calculate the temperature, stress, and strain fields of welding interface material in the welding process. In this paper, continuously driven friction welding of Al-Cu-Mg alloy round bars that are commonly used in aerospace structures are calculated with the finite element method (FEM). FEM calculations and results are explained and discussed in much detail. For example, depending on experiments as reference, FEM results show that a temperature of 490 °C, which is below the low value of Al-Cu-Mg alloy melting point, is obtained at the end of 0.6 s of friction welding. During the whole process of friction welding, the calculated equivalent strains increase monotonously, and the equivalent strain at the center of circular section of interface material is the largest.

Measurements of the changes in microstructure during superplastic deformation

Abstract In this paper, changes in the volume fraction of cavities, in the fractal dimension of cavities, in the grain size and aspect ratio (the ratio of the major axis to the minor axis) of grains and in the dislocation density during superplastic deformation have been measured quantitatively. The experimental results show that the volume fraction of the cavities, the fractal dimension of the cavities and the dislocation density increase with increase in the degree of deformation. At the early stage of deformation, the increase in the volume fraction of the cavities varies slightly with the increase in strain, but beyond a particular strain, the volume fraction of the cavities increases sharply. The stress state has a considerable influence on the growth rate of the volume fraction of the cavities: at the same strain, the volume fraction of the cavities under the plane equi-tensile stress state is greater than that under uniaxial tension. However, the volume fraction of the cavities at fracture under the plane equi-tensile stress state is smaller than that under the uniaxial tensile-stress state. The grain size after superplastic deformation increases generally, but grain fining after superplastic deformation is also discovered sometimes, due depending upon the type or commercial state of the metal: of course, it is related to the stress state in addition. The aspect ratio of the grains tends to the increase with the increase of strain, but it increases only slowly.

Numerical computation of cavity damage and failure during the superplastic deformation of sheet metals

Abstract Superplastic deformation is considered as a thermo-viscoplastic flow. The deformation and failure of superplastic sheet metals are a result of a combination and interaction process between tensile instability and internal cavity evolution, which are controlled by the rheological parameters (i.e., the strain-rate sensitivity index m, the strain-hardening exponent n, and the visco-plastic anisotropy parameter) and the cavity growth rate of the materials. Based on Gursons constitutive relationship for porous ductile materials, with some modifications, and Hills normal anisotropic (plane isotropy) yield criterion being quadratic in the stress components, a thermo-viscoplastic anisotropic damage-instability model is proposed. It includes strain hardening, strain-rate hardening, the anisotropy parameter and the internal cavity volume fraction. The superplastic sheet metals are modelled using this thermo-viscoplastic damage-instability constitutive relationship that accounts for strength degradationresulting from the growth of cavities. The current stress components and their ratio ( α = σ 2 σ 1 ), the stress triaxiality ratio ( σ m σ ), and the cavity volume fraction (f) during superplastic deformation of sheet metals for any strain path between uniaxial tension and biaxial equitension, are studied numerically. Finally, taking the occurence of localized instability ( d e 2 = 0 ) or the cavity volume fraction reaching the critical value (fc) as a fracture criterion, the limit strain and the maximum uniform strain are predicted. The rheological parameters (m, n, r), the initial cavity volume fraction and other material constants used in the calculations are determined experimently. Comparisons of the calculations with experimental results indicate that the thermo-viscoplastic damage-instability model can provide good estimations of the cavity volume fraction, the strength reduction induced by cavity growth, the deformation and instability behaviour, and the limit strain under various strain histories.

A study of cup-cup axisymmetric combined extrusion by the upper-bound approach I. Upper-bound solutions for the deformation force

Abstract In this paper, several kinematically admissible velocity fields for cup-cup axisymmetric combined extrusion are advanced. On the basis of these velocity fields, the upper-bound approach is applied to analyze the process of combined extrusion and to estimate the average uppper-arm pressures at different stages of deformation. It is found that the average upper-ram pressure varies with the reduction in area of the backward-extrusion part in a manner somewhat similar to a parabolic function, and that it varies monotonously with the reduction in area of the forward-extrusion part. During the extrusion process, with progressive penetration of the upper ram, the deformation flow may change from steady flow in the early stages into non-steady flow in the final stages. Moreover, both in the early stages and in the final stages, there are three possible flow patterns: simple forward extrusion; simple backward extrusion; and forward-backward extrusion. Good correlation is found between the theorical and experimental results. The results presented in the paper can be used for process design.

Effects of isothermal heat treatment on microstructural evolution of semisolid Al-4Cu-Mg alloy

The authors investigated the effects of the isothermal heat-treatment conditions on the microstructural evolution and composition distribution of semisolid Al-4Cu-Mg alloy during isothermal heat treatment. The experimental results show that the microstructural evolution and composition distribution of semisolid Al-4Cu-Mg alloy are controlled by atom diffusion during the isothermal heat treatment process. Grain growth and spheroidization were promoted with the increase of the isothermal temperature and/or the holding time. Moreover, the higher the isothermal temperature, or the longer the holding time, the more segregation constituent elements occurred to the grain boundaries. The low melting structure at grain boundary is greatly affected by Cu. The microstructural evolution in the isothermal heat-treatment process is as follows: recovery, recrystallization, fragmentation, spheroidization, and coarsening. Such fragmentation, spheroidization, and grain growth of coalescence and Ostwald ripening are involved as main mechanisms in the isothermal heat-treatment process.

Adiabatic Shear Band of TC4 Alloy during Warm Compression

Abstract The adiabatic shear band (ASB) forming in the warm compression of TC4 titanium alloy was investigated at a strain rate of 50 s−1 and the deformation temperatures ranging from 560 to 660 °C. The results show that the deformation temperature affects significantly the formation of the ASBs. The shear band width increases from 85 μm to 140 μm as the deformation temperature increases. The corresponding Vickers micro-hardness values of the ASB are higher than that of the matrix due to the strain hardening and fine grains in ASB. Meanwhile the microstructure evolution mechanism within the ASB was discussed. Large strain and high temperature make the occurrence of dynamic recrystallization (DRX) in ASB, leading to the formation of equiaxed DRX grains with a grain size about 200 nm. The ASBs in this study have both the characteristics of deformed band and transformed band.