Time filter

Source Type

Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing | Li L.-T.,Central South University | Li L.-T.,State Key Laboratory of High Performance Complex Manufacturing | And 4 more authors.
Journal of Alloys and Compounds | Year: 2013

The high-temperature flow behavior of 7075 aluminum alloy was studied by hot compressive tests. Based on the experimental data, the efficiencies of power dissipation and instability parameter were evaluated. Processing maps were constructed by superimposing the instability map over the power dissipation map. Microstructural evolution of 7075 aluminum alloy during the hot compression was analyzed to correlate with the processing maps. It can be found that the flow stresses increase with the increase of strain rate or the decrease of deformation temperature. The high-angle boundaries and coarse precipitations distributing in the grain interior/boundaries, which may result in the deep inter-granular corrosion and large areas of denudation layer, should be avoided in the final products. The optimum hot working domain is the temperature range of 623-723 K and strain rate range of 0.001-0.05 s-1. © 2012 Elsevier B.V. All rights reserved.


Chen M.-S.,Central South University | Chen M.-S.,State Key Laboratory of High Performance Complex Manufacturing | Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing
International Journal of Plasticity | Year: 2013

Large forgings are the essential parts of some nuclear, electrical power generation, rolling mill equipments. Generally, they are directly obtained by forging the large ingots containing some void defects. In this study, the evolution mechanisms for the spherical or spheroidal voids during hot working are investigated by the numerical simulations and experiments. The effects of the initial void size, aspect ratio and positions on the void evolution were discussed. The results show that the closure process of voids can be divided into two stages. i.e.; when the deformation degree is relatively small, the void retains spheroidal. However, the void will not be spheroidal when the deformation degree is relatively large. The changes of void aspect ratio are slightly affected by the void size, but greatly by the initial aspect ratio and position of voids. It also suggests that the strain and stress fields around voids are the key factors influencing the evolution of void aspect ratio. The increase of effective strain contributes to the changes of void aspect ratio. Considering the effects of stress and strain fields on the void evolution, a void aspect ratio evaluation index, which is defined as a function of the stress deviator, effective strain and effective stress, is proposed to describe the changes of void aspect ratio. Based on the results from finite element simulation, a theoretical model is established to predict the changes of void aspect ratio in large forgings during hot working. A good agreement between experimental and simulated results indicates that the proposed void aspect ratio evaluation index and theoretical model can give an accurate description of the void evolution. © 2013 Elsevier Ltd. All rights reserved.


Chen X.-M.,Central South University | Chen X.-M.,State Key Laboratory of High Performance Complex Manufacturing | Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing | And 6 more authors.
Materials and Design | Year: 2014

The dynamic recrystallization (DRX) behavior of a typical nickel-based superalloy is investigated by the hot compression tests. Based on the conventional DRX kinetics model, the volume fractions of DRX are firstly estimated. Results show that there is an obvious deviation between the experimental and predicted volume fractions of DRX when the forming temperature is below 980. °C, which is induced by the slow dynamic recrystallization rate under low forming temperatures. Therefore, the segmented models are proposed to describe the kinetics of DRX for the studied superalloy. Comparisons between the experimental and predicted results indicate that the proposed segmented models can give an accurate and precise estimation of the volume fractions of DRX for the studied superalloy. In addition, the optical observation of the deformed microstructure confirms that the dynamically recrystallized grain size can be well characterized by a power function of Zener-Hollumon parameter. © 2014 Elsevier Ltd.


Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing | Chen X.-M.,Central South University | Chen X.-M.,State Key Laboratory of High Performance Complex Manufacturing | And 4 more authors.
Computational Materials Science | Year: 2014

Due to their excellent properties, nickel-based superalloys are extensively used in critical parts of modern aero engine and gas turbine. The hot deformation behaviors of a typical nickel-based superalloy are investigated by hot compression tests with strain rate of (0.001-1) s-1 and forming temperature of (920-1040) C. Results show that the flow stress is sensitive to the forming temperature and strain rate. With the increase of forming temperature or the decrease of strain rate, the flow stress decreases significantly. Under the high forming temperature and low strain rate, the flow stress-strain curves show the obvious dynamic recrystallization. Based on the stress-dislocation relation and kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of the studied nickel-based superalloy. Comparisons between the predicted and measured flow stress indicate that the established physically-based constitutive model can accurately characterize the hot deformation behaviors for the studied nickel-based superalloy. © 2013 Elsevier B.V.


Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing | Ding Y.,Central South University | Ding Y.,State Key Laboratory of High Performance Complex Manufacturing | And 4 more authors.
Materials and Design | Year: 2013

The hot tensile deformation behaviors of a typical Al-Cu-Mg alloy are investigated by uniaxial tensile tests with the strain rate range of (0.05-0.001)s-1 and temperature range of (673-748)K. The experimental results show that the true stress-strain curves exhibit a peak stress at a very small strain, after which the flow stresses decrease slowly until fracture, showing an obvious dynamic softening behavior. This hot tensile deformation is a thermally activated process, which indicates the competitions of work hardening, dynamic recovery, dynamic recrystallization, and the initiation and growth of voids or cracks. Considering the coupled effects of forming temperature, strain rate, and strain on the material hardening and softening behavior, a new phenomenological constitutive model is proposed to describe the hot tensile deformation behaviors of the studied Al-Cu-Mg alloy under relatively low strain rates. In the proposed constitutive model, the material constants are expressed as functions of forming temperature and strain rate. A good agreement between the predicted and measured results shows that the proposed model can give an accurate estimate of flow stress for the studied Al-Cu-Mg alloy. © 2013 Elsevier Ltd.


Chen M.-S.,Central South University | Chen M.-S.,State Key Laboratory of High Performance Complex Manufacturing | Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing | Chen K.-H.,Central South University
International Journal of Plasticity | Year: 2014

The evolution of voids inside power-law viscous solids is investigated. A representative volume element (RVE) model of an infinite matrix containing an isolated void is applied. In the RVE model, the void is assumed to be elliptic-cylindrical or circular-cylindrical, and the matrix is considered as the isotropic and incompressible power-law viscous material. To obtain the velocity field of RVE, a Ritz procedure is developed using the method proposed by Lee and Mear (1992). Moreover, the results obtained from the Ritz procedure are verified by the finite element simulations. Based on the data obtained from RVE models, the effects of material Norton exponent, remote stress field and void aspect ratio on the changing rate of void aspect ratio are discussed. Especially, when the void's principal axes are parallel to the principal axes of the remote stress, the mathematical models are proposed to relate the changing rate of void aspect ratio to the void aspect ratio and material Norton exponent. The results show that the material Norton exponent, remote stress field and void aspect ratio have a great influence on the changing rate of void aspect ratio. For the remote shear stress and uniaxial compression stress fields, the changing rate of void aspect ratio increases with the increase of void aspect ratio and material Norton exponent. Furthermore, the relationships between the changing rate of void aspect ratio and the void aspect ratio can be represented as the parabolic function and linear function for the remote shear stress field and uniaxial compression stress field, respectively. While the relationships between the changing rate of void aspect ratio and material Norton exponent can be expressed as the first order exponential function for these two remote stress fields. Besides, the changing rate of void aspect ratio can also be expressed as a unified function of void aspect ratio and material Norton exponent. For the remote biaxial compression stress field, the relationships between the changing rate of void aspect ratio and the void aspect ratio can be represented as the parabolic function, in which the coefficients can be expressed as functions of material Norton exponent and remote stress field. The findings of this study can be mainly used to evaluate the aspect ratio of voids inside large ingots during hot working, as well as to model the final densification stage of powder metal compacts. © 2013 Elsevier Ltd. All rights reserved.


Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing | Wen D.-X.,Central South University | Wen D.-X.,State Key Laboratory of High Performance Complex Manufacturing | And 5 more authors.
Materials and Design | Year: 2014

The high-temperature deformation behaviors of a typical Ni-based superalloy are investigated by hot compression tests under the strain rate of 0.001-1s-1and temperature of 920-1040°C. The experimental results show that the deformation behaviors of the studied superalloy are significantly affected by the deformation temperature, strain rate and strain. The flow stress increases with the increase of strain rate or the decrease of deformation temperature. The flow stress firstly increases with the strain to a peak value, showing the obvious work hardening behaviors. Then, the stress decreases with the further straining, indicating the dynamic flow softening behaviors. Considering the coupled effects of deformation temperature, strain rate and strain on the hot deformation behaviors of the studied Ni-based superalloy, the phenomenological constitutive models are established to describe the work hardening-dynamic recovery and dynamic softening behaviors. In the established models, the material constants are expressed as functions of the Zener-Hollomon parameter. The established constitutive models can give good correlations with the experimental results, which confirm an accurate and precise estimation of the flow stress for the studied Ni-based superalloy. © 2014 Elsevier Ltd.


Li J.,Central South University | Liu L.,State Key Laboratory of High Performance Complex Manufacturing | Deng L.,State Key Laboratory of High Performance Complex Manufacturing | Ma B.,State Key Laboratory of High Performance Complex Manufacturing | And 2 more authors.
IEEE Electron Device Letters | Year: 2011

The interfacial microstructures of the Cu-wire bonding to an Al pad are investigated first by using an X-ray microdiffractometer and high-resolution transmission electron microscopy. It was found that the intermetallic compounds hardly formed at the Cu/Al interface during the thermosonic Cu-wire bonding process. However, when heating temperature is elevated to 340 °C which increases energy levels of Cu/Al, the intermetallic phases Al2Cu and Al4Cu9 can form and reach to 130 nm thick within 20 ms due to atomic interdiffusion and reaction activated by ultrasonic energy and heat at the Cu/Al interface. The Al side of the interface is aluminum-rich Al 2Cu with lattice parameters a = 6.067 Å and c = 4.864 Å, and the Cu side is copper-rich Al4Cu9 with lattice parameter a = 8.706 Å. Bonding strength and bondability increase significantly after forming the Cu/Al intermetallic phases. © 2011 IEEE.


Deng J.,Central South University | Deng J.,State Key Laboratory of High Performance Complex Manufacturing | Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing | And 5 more authors.
Materials and Design | Year: 2013

The hot tensile deformation and fracture behaviors of the hot-rolled AZ31 magnesium alloy were studied by uniaxial tensile tests with the temperature range of 523-723K and strain rate range of 0.05-0.0005s-1. Effects of deformation parameters on the strain hardening rate, strain rate sensitivity, microstructural evolution and fracture morphology were discussed. The results show that: (1) The flow curves show a considerable strain hardening stage and no obvious diffuse necking stage under the relatively low temperatures (523 and 573K). (2) The elongation to fracture increases with the increase of the deformation temperature. But, the sharp drop of the elongation to fracture under 723K and 0.0005s-1 results from the synthetical effects of the grain growth, inverse eutectic melting reaction (α+β=L) and the incipient melting of α matrix. (3) For the case with the deformation temperature of 623K and relatively low strain rates, the fracture mechanism is the combination of the void coalescence and intergranular fracture. (4) Under the deformation temperature of 723K, the fine recrystallized grains experienced a rapid growth and the deformation mechanism is the dislocation creep with the help of inverse eutectic liquid phase. (5) The presence of proper amount of liquid phase on the grain boundary changes the deformation mechanisms, and makes great contribution to the high ductility. However, it will deteriorate the material ductility if the amount of liquid phase is too much. © 2013 Elsevier Ltd.


Lin Y.C.,Central South University | Lin Y.C.,State Key Laboratory of High Performance Complex Manufacturing | Chen X.-M.,Central South University | Chen X.-M.,State Key Laboratory of High Performance Complex Manufacturing | And 3 more authors.
International Journal of Fatigue | Year: 2013

The uniaxial low-cycle fatigue behavior of hot-rolled AZ91 magnesium alloy was investigated by asymmetric cyclic stress-controlled experiments at room temperature. The effects of the sampling direction, peak stress and stress ratio on the fatigue life were discussed. The fatigue life increases with increasing the stress ratio or decreasing the peak stress. Due to the anisotropic property, the specimen in transverse direction shows superior fatigue resistance. Considering the effects of mean stress on the fatigue strength coefficient and fatigue strength exponent, a modified Basquin model was proposed and validated to evaluate the fatigue life of AZ91 magnesium alloy. © 2012 Elsevier Ltd. All rights reserved.

Loading State Key Laboratory of High Performance Complex Manufacturing collaborators
Loading State Key Laboratory of High Performance Complex Manufacturing collaborators