| Elbow tubes with small relative bending radius(R/D≤0.5) are widely used innarrow space to connect hydraulics and fuel piping, in aero turbine engines andunmanned air crafts.The light materials like aluminum and titanium alloys areutilized to meet the demand of weight reduction. However, the tubes with smallbending radius cannot be successfully manufactured due to the limit ofcompressive-extrusion deformation mechanism by traditional bending methods. Inorder to solve the critical problem, a shear hydro-bending process was proposed. Ittakes the advantage of shear deformation to forming the ultra-small radius bendingtubes. The shear hydro-bending of aluminum and titanium alloy tubes will besystematically investigated with experiments, numerical simulation andmicrostructure observation. It is expected to provide theory foundation andtechnology support for the application of this new process.The mechanical properties of 5A02 aluminum alloy tubes and TA18 titaniumalloy tubes were measured by uniaxial tensile tests, which served as the fundamentalproperty parameters for experimental and FEM simulation. The target specimen wasdesigned, associating with the experimental device. The strain analysis of eachspecimen was made with the ASAME automatic strain measuring system. Thecorresponding microstructure was observed to reveal the deformation mechanism.The shear hydro-bending of 5A02 aluminum alloy tube and strain measurementwere carried out to investigate the influences of internal pressure and feeding ratio onthe deformation defects, thickness distribution and corner radius. The processwindow was obtained. The shear hydro-bending tubes were achieved in differentinternal pressures and feeding ratios, the distribution of effective strain was broughtto light. The bursting pressure was measured by free bulging test.The microstructure in typical deformation area was observed by electronbackscatter diffraction(EBSD) and transmission electron microscopy(TEM). Thecorresponding microstructure morphology and grain distribution were analyzed. Theobvious microstructural refinement could be obtained due to the shear deformation.The mechanism of grain refinement was clarified by plasticity theory and dislocationtheory. Meanwhile, the micro-hardness was measured to reveal the relationshipbetween shear deformation with grain refinement.The shear hydro-bending process was numerically simulated based on dynamicexplicit finite element method in Abaqus 6.10. The distributions of shear stress andshear strain were analyzed. It was proved that the shear stress was only found on theside. The analysis formula of shear strain in shearing area was obtained.Muti-step shear hydro-bending was proposed for the shear hydro-being oftitanium alloy. One-step and two-steps shear hydro-bending of TA18 titanium alloytube were conducted. Two typical defects, shear fracture and tension fracture wereobtained in the one-step shear hydro-bending process. The fracture morphology wasobserved by SEM. The effects of internal pressure and feeding ratio on limittransverse feeding was investigated. The optimal process parameter in one-step shearhydro-bending process was obtained. The microstructure and grain sizes wereinvestigated by EBSD and TEM. The microscopic mechanism in shear deformationarea was revealed, accompanying with low-angle grain boundaries increasing, twinboundaries decreasing and grain refinement degree improving. The performanceproperties of titanium alloy tube components were measured by uniaxial tensile test,micro-hardness test and free bulging test. It was obvious that the higher strength andlower elongation was achieved in the shear deformation area relative to theundeformed area. The mechanisms of work hardening and grain refinementstrengthening appeared in the shear deformation area were revealed. The TA18 titanium alloy tube with out radius of 11 mm, relative bending radius Rb=0.37, wasformed successfully by two steps shear hydro-bending with inter annealing. |
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