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Shi D.,Beihang University | Dong C.,Beihang University | Yang X.,Beihang University | Zhang L.,Beijing Aviation Manufacturing Engineering Institute | And 2 more authors.
Materials Science and Engineering A | Year: 2012

Experiments were carried out in order to investigate the creep rupture strength of vacuum brazed joints in a directionally solidified (DS) superalloy. Vacuum brazed joint specimens were tested with and without post-brazing heat treatment. Specimens were crept at 980 °C in laboratory air at applied stress levels between 124. MPa and 318. MPa. After creep testing, specimens were analyzed by means of SEM, OM and EDXS in order to understand failure mechanisms. The results showed that: (1) under the same test conditions, all brazed joints exhibited a lower creep life compared with a typical DS superalloy. In addition, the life of heat treated brazed joints was longer than that of non heat treated braze joints; (2) all the brazed joints fractured in the brazing seam; (3) the Larson-Miller equation can be used to characterize the creep rupture strength of the DS superalloys and the brazed joints. (4) the post-brazing heat treatment had an important impact on the creep behavior and microstructure of brazed joints; and (5) creep loading contributed to interdiffusion of the elements and decrease of the brazing pores in the brazed joints. © 2012 Elsevier B.V.

Yang X.,Beihang University | Lang L.,Beihang University | Liu K.,Beihang University | Liu B.,Beijing Aviation Manufacturing Engineering Institute
Chinese Journal of Aeronautics | Year: 2015

To obtain the influence of fluid pressure and temperature on warm hydroforming of 5A06-O aluminum alloy sheet, the unified mechanics equilibrium equations, which take through-thickness normal stress and friction into account, were established in spherical coordinate system. The distribution of through-thickness normal stress in the thickness direction was determined. The relation between through-thickness normal stress and fluid pressure was also analyzed in different regions of cylindrical cup. Based on the method of subtracting one increasing function from another, the constitutive equation of 5A06-O applied to warm hydroforming was established and in a good agreement with uniaxial tensile data. Based on whether the thickness variation was taken into account, two mechanic models were established to do the comparative study. The results for the studied case show that the calculated stress values are pretty close according to the two models and consistent with results of finite element analysis; the thickness distribution in flange computed by the second model conforms to the experimental data. Finally, the influences of fluid pressure on the flange thickness and radial stress were analyzed. © 2015 The Authors.

Lang L.,Beihang University | Du P.,Beihang University | Liu B.,Beijing Aviation Manufacturing Engineering Institute | Cai G.,Beihang University | Liu K.,Beihang University
Journal of Alloys and Compounds | Year: 2013

A viscoplastic model considering the influence of microscopic evolution and macroscopic deformation has been developed to represent the deformation behavior of material in warm sheet hydroforming process. Based on a hydroforming environment, a set of rate dependent constitutive equations, which is constructed by using the pressure rate, the evolution of dislocation density and kinematic isotropic hardening, has been proposed to predict stress-strain response of material. The non-linear Chaboche's isotropic hardening criterion is also modified to characterize the instantaneous state of hardening considering microstructure evolution. Different from traditional sheet plastic forming process, the rate of fluid pressure variation is a significant factor that can determine the forming speed. Therefore, the pressure rate is applied to be one factor that can influence the deformation of material in warm hydroforming. The hydraulic bulge experiments on aluminum alloy at warm temperature indicated that the deformation behavior of material is sensitive to pressure rate. The genetic algorithm optimization technique was used to determine the optimum values of a set of free material constants associated with the proposed constitutive model. The computed data has been in a good agreement with the test data on the basis of the optimized material constants. © 2013 Elsevier B.V. All rights reserved.

Liu K.,Beihang University | Lang L.,Beihang University | Cai G.,Beihang University | Yang X.,Beihang University | And 2 more authors.
International Journal of Mechanical Sciences | Year: 2015

Abstract Precise estimation of various mechanical properties of sheet metal with strong non-linearities has always been a major problem in process simulation by Finite Element Method (FEM). The hydraulic bulge test as prescribed in ISO standard 16808 is well-known for its capability of obtaining flow stress at higher strain levels compared to uniaxial tensile test, but a more applicable method is also needed to calculate the flow stress properties of bulge test especially at elevated temperature. In this study, a set of explicit integral format formulas based on the classical plastic flow rules were deduced to calculate thickness distribution from the aspect of the mechanics analysis. Afterwards an iterative method combined with integration formulas was established to evaluate hardening curves for specimen. Finally, A group of hydraulic bulge tests were performed using sheet metal of AA7075 alloy at room and elevated temperatures with a constant pressure increasing rate, supplementary tensile tests were also carried out at corresponding conditions to compare with the stress-strain curves from the bulge tests. Result shows that the proposed method can evaluate the flow stress curves of the test specimen with reasonable accuracy and with higher strain range over the temperatures ranging from cold to elevated values. © 2015 Elsevier Ltd.

Yang X.,Beihang University | Dong C.,Beihang University | Shi D.,Beihang University | Zhang L.,Beijing Aviation Manufacturing Engineering Institute
Materials Science and Engineering A | Year: 2011

Due to the different low cycle fatigue (LCF) properties and fatigue fracture behavior between DZ125 base metal and the brazed joint, the LCF tests are carried out systematically using tension cycling under stress amplitude control conditions (stress ratio R= 0) at elevated temperature in laboratory air. The present paper sets out to investigate the cyclic deformation response of DZ125 base metal and the brazed joint in two aspects, i.e. fatigue life and fatigue fracture behavior, with the comparative method. Furthermore, the comparative method on the typical fatigue fracture surface features (including fatigue source zone, crack propagation zone and fatigue fracture zone) of DZ125 base metal and the brazed joint cycled to failure is conducted in detail. Based on both the macro mechanical behavior and macro and micro fracture observations, experimental results show that: (1) for the brazed joint, the softening is not obvious at lower stress ranges. But from 640 to 720. MPa, it is very significant; (2) under the same test condition, the brazed joint shows lower fatigue life compared with DZ125 base metal and all brazed joints are fractured in the brazing seam observed by the Scanning Electron Microscope (SEM); and (3) there are many distinctive differences of the fracture phenomena between DZ125 base metal and the brazed joint as follows: (1) the crack initiation mode; (2) the crack propagation behavior; and (3) the morphology of dimple pattern at the fatigue fracture zone. © 2011 Elsevier B.V.

Liu B.,Beihang University | Lang L.,Beihang University | Du P.,Beihang University | Zeng Y.,Beijing aviation manufacturing engineering institute | Liu K.,Beihang University
International Journal of Mechanical Sciences | Year: 2012

The formability can be improved in warm/hot sheet hydroforming due to two important factors of temperature and through thickness normal stress. An extension of M-K model is presented in this paper with consideration of the liquid pressure induced through thickness normal stress. The through thickness normal stress is introduced by using the general form of Hill48 yield criterion. The effects of temperature and through thickness normal stress on the formability improvement can be predicted by using the modified M-K model with the combination of a proposed temperature dependent constitutive equation. The Newton-Raphson method is used and the numerical procedure is approved stable and correct. The yield loci show a significant dependence on temperature and through thickness normal stress. Size shrinking caused by the elevated temperature and location shifting due to the increasing thickness normal stress are observed. Comparison of the experimental FLDs of AISI1012 at plane stress and room temperature, STKM-11A for tube hydroforming at room temperature and 5A90 at plane stress and elevated temperature with the theoretical results show good agreements. The key parameters, such as inhomogeneity factor f 0, n value, m value and initial thickness T 0, grain size d, initial surface roughness R 0, show a strong dependence on FLDs and increase of FLD 0. The increase of FLD 0 is formulized in a full quadratic form, which is a function of temperature and through thickness normal stress. The cylindrical cup warm hydromechanical deep drawing was carried out and the effects of temperature and through thickness normal stress on formability can be observed. © 2012.

Liu B.,Beihang University | Lang L.,Beihang University | Zeng Y.,Beijing Aviation Manufacturing Engineering Institute | Lin J.,Imperial College London
Journal of Materials Processing Technology | Year: 2012

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 ε 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, ε eq) and (η, ω) based spaces. © 2012 Elsevier B.V. All rights reserved.

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