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Iranian Journal of Materials Forming - Volume:2 Issue: 1, Spring and Summer 2015

Iranian Journal of Materials Forming
Volume:2 Issue: 1, Spring and Summer 2015

  • تاریخ انتشار: 1394/10/20
  • تعداد عناوین: 6
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  • M. A. Moazam, A. Ghasemi, M. Moradi, H. Monajatizadeh Page 1
    Final step of rail manufacturing is cold rolling straightening process and it has significant effects on mechanical properties, straightness, flatness and development of residual stresses. Measurement of residual stresses after straightening process is obligated by rail manufacturing standards. In the present investigation, an attempt has been made to evaluate residual stresses after straightening process by finite element method and strain gage sectioning technique. The straightening process has been simulated here using the FE package ABAQUS. All the input data were extracted from experimental tests according to rail manufacturing standard and homogenous and isotropic behavior of material were considered. Moreover, initial camber of the rail was measured after hot rolling and cooling process. Obtained results of numerical calculation has been validated by strain gage sectioning technique and compressive residual stresses in head and web and tension residual stress in foot of the rail has been observed. The roller arrangement used in this investigation could reduce the amount of residual stresses comparing with the previous results. Furthermore, straightness of the rail after straightening was satisfactory.
    Keywords: Rail, Straightening process, Camber, Numerical calculation, Strain gage
  • A. Nayebi, A.H. Khosravi Page 11
    Formability of automotive friction stir welded TWB (tailor welded blank) sheets is numerically investigated in biaxial stretching based on hemispherical dome stretch (HDS) test in four automotive sheets of Aluminum alloy 6111-T4, 5083-H18, 5083-O and DP590 steel, having different thicknesses. The effects of the weld zone modeling and the thickness ratio on formability are evaluated. In order to carry out the numerical simulations, mechanical properties are considered according to Chung et al. [11] experimental results. von-Mises and Hill’48 quadratic yield functions are used to compare the isotropic and anisotropic behaviors of the used sheets. In order to simplify the problem, the anisotropy of the weld zone is ignored. The FEM results are compared with experimental results of [11]. Anisotropic assumption for base materials and varying thickness for the weld zone give more accurate prediction. Numerical results are in good
    agreement with the experimental results. Failure onset locations and patterns are accurate. Since the formability is dependent on the stress concentration, asymmetric distribution of strength and complexity of weld zone properties, the thickness ratio in TWB affect formability.
    Keywords: TWB, HDS, Anisotropic, Forming, FSW
  • M. Seyed Salehi, N. Anjabin, R. Mansoori Page 18
    In the present study, the microstructural changes of a Nickel based superalloy Nimonic
    80A during a non-isothermal deformation were studied. Therefore, microstructure evolution during hot side pressing test was predicted with combined methods of finite element analysis and processing map of the material. The predicted results were validated through experimental microstructural studies. The results show that the distribution of deformation parameters (i.e. strain, strain rate, and temperature) is non-uniform in the deformed samples. The severity of this non-uniformity depends on the amount of sample reduction. High reduction value at one step forging can cause flow localization and non-uniform dynamic recrystallization, which results the formation of adiabatic shear bands, while using the lower reduction value at each forging step, leads to more uniformly distribution of the deformation parameters and thus uniform the dynamic recrystallization with the stable flow. Hence the workability and microstructure of the Nimonic 80A alloy are mainly depends on the deformation path.
    Keywords: Hot deformation, Nimonic 80A, Processing map, Microstructure, finite element analysis
  • A.A. Khamei, K. Dehghani, S. Bakhshi, K. Kalayeh Page 30
    The full annealing AA6061 aluminum alloy was subjected to severe plastic deformation
    via the combination of equal channel angular pressing (ECAP) and cold rolling (CR) in order to refine its microstructure and to improve its mechanical properties. According to the results of hot and cold tensile tests, the combination of ECAP and CR significantly affected the final strength and ductility of studied AA6061. Four passes of ECAP followed by 90% reduction in rolling led to about 5.4 and 3.15 times increase in the yield and ultimate tensile strengths, respectively. In addition, the hot ductility and strain rate sensitivity were increased by applying ECAP plus CR. The changes in mechanical properties were attributed to the enhanced dislocation density and to the reduced grain size. The results show that a decrease in grains/subgrains size (0.37 μm) and an increase in the fraction of high angle grain boundaries, exhibited significant effect on the hot ductility of higher severe plastic deformed sample.
    Keywords: Severe plastic deformation, Ultrafine, Grained (UFG), 6061 Al alloy, Rolling, Hot ductility
  • G. R. Ebrahimi, A. Momeni, H. Eskandari Page 43
    Strain induced precipitation in HSLA-100 steel was investigated by conducting hot compression and relaxation tests at temperature range of 850°C to 1100°C and strain rate of 0.001 s-1 to 1s-1. The absence of dynamic recrystallization at temperatures below 1000°C was attributed to the influence of dynamic precipitation. The stress relaxation tests showed that strain induced precipitation is possible over a wide range of temperatures from 850°C to 1050°C. The starting and finishing times of precipitation were sensitive to temperature than strain rate. At temperature range of 950-1000°C and strain rate of 0.1s-1 the lowest times for precipitation were observed. The combination of precipitation-time-temperature and recrystallization-time temperature diagrams showed that at high temperatures and low strain rates, precipitation precedes dynamic recrystallization, whereas at the opposite condition, dynamic recrystallization goes in advance. The starting and finishing times for dynamic precipitation were approximated about 40 percent lower that those for strain induced precipitation.
    Keywords: Hot deformation, Cu, bearing HSLA steel, Dynamic recrystallization, Hot compression, Stress relaxation
  • A. Khosravifard, M. M. Moshksar, R. Ebrahimi Page 51
    The dynamic behavior of three high manganese steels with very different stacking fault
    energy (SFE) values (4-30 mJ/m2) were studied using high strain rate torsional tests. The hotrolled microstructure of the steel with the lowest SFE of 4 mJ/m2 consisted of a duplex mixture of austenite and ε-martensite, but those of the other two steels were fully austenitic. The deformed microstructures were studied by optical and electron microscopy. The quasi-static deformation of the low-SFE steel was accompanied with profuse martensitic transformation. However, when this steel was deformed at high strain rates (> 500 /s), martensite formation was reduced due to the adiabatic temperature rise and the increased SFE of the steel. The deformation of the steel with
    moderate SFE of 18 mJ/m2 at all the tested strain rates was mainly controlled by the formation of mechanical twins that was leading to an excellent ductility of about 55% even at the highest strain rate of ~1700 /s. In contrast, dynamic deformation of the steel with the highest SFE of 30 mJ/m2 led to the appearance of some shear bands. This was ascribed to the decreased twinning and work hardening rate in this steel. Finally, the topographic studies showed that the fracture surface of the low-SFE steel contained relatively larger cleavage areas and smaller dimples suggesting a relatively more brittle fracture. This was related to the presence of brittle ε and α` martensite phases in this steel.
    Keywords: Stacking fault energy, High Manganese, TRIP, TWIP, High strain rate