For structural and aerodynamic reasons, curved profiles are widely used in the transport industry for manufacturing lightweight structures. In the present work, a curved AA1050 bar with fine grains and high strength was manufactured by a novel forming technique, differential velocity sideways extrusion (DVSE). The evolution of grain structure and micro-texture during DVSE and the mechanical properties of the formed bar were studied, and the grain refinement mechanism was revealed. Due to the severe strains arising in the process, (greater than that for conventional one pass equal channel angular extrusion), significant grain refinement in the curved bar (grain size ∼3 μm) was achieved from the original billet (grain size ∼357 μm) in one extrusion operation. Coarse band-like structures containing subgrains with low angle boundaries in the shearing zone gradually transformed into fine shear band-like structures containing equiaxed (sub)grains with a mixture of low and high angle boundaries. The fine shear band-like structures inclined approximately along the shear intersection planes. Severe plastic deformation induced a high dislocation density that initiated subgrain walls with low angle boundaries, which gradually transformed into grain boundaries with high misorientation, indicating that refinement of AA1050 grains in the DVSE process is due mainly to continuous dynamic recrystallization (cDRX). Due to the appearance of greater effective strain on the inner bend of the extruded bar, the grain refinement degree and high angle boundary fraction of the material on the inner bend are slightly greater than those of the material on the outside. DVSE resulted in a weak C-type shear-texture component which can be determined by a proper rotation of the negative simple shear texture. Compared with the billet, significant increase of the hardness, yield strength and ultimate tensile strength by 134.8%, 354.0% and 116.8% respectively was achieved in the formed curved bar, although the elongation to fracture was decreased by 60.0%.

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Manufacturing a curved profile with fine grains and high strength by differential velocity sideways extrusion

Stretch bending is commonly used in the fabrication of profile-based shapes. However, one of the challenges in many bending-type forming processes is controlling springback upon unloading. Springback appears sensitive to upstream and in-process parameters, making prediction and control difficult. The springback problem is particularly important in forming processes where stringent control strategies are needed to assure product dimension, process flexibility and overall equipment effectiveness. In this research, we present an analytical framework for springback assessment in a stretching-controlled bending process. A new, flexible rotary stretch bending machine is designed and built, which allows the manufacture of complex-shape profiles with varying curvatures, including straight portions, with low tool investments. Furthermore, a Full Moment (FM) analytical model is developed for springback assessment. The modeling strategy employs a full moment distribution along the entire profile upon bending, introducing a stretching-controlled moment transition between different curvature portions such that the springback contribution from the plastic moment outside the theoretically bent portions is considered. Additionally, the influential factors related to material, geometry and process, including the applied stretching strains, are comprehensively considered. In this way, the modeling strategy enables accurate evaluation of springback in the flexible stretch bending process. Moreover, the proposed forming process is validated by a series of experiments conducted for a wide range of stretching levels using rectangular hollow aluminum alloy profiles. This forming strategy demonstrates high capability for controlling springback and dimensional accuracy. By comparing experiments, numerical simulations and analytical calculations, this proves that the developed FM model provides accurate and efficient assessment of springback. The average relative error provided by the FM model is 2.2%, as compared to 10.5% for a classical method used for comparison. Using the analytical model combined with numerical simulation, the crucial deformation characteristics, including stretching-dependent evolution of strain transition, are revealed. Overall, the FM analytical strategy has proven its capability as an effective tool, which can make springback knowledge more explicit, generic and reuseable for stretching-controlled bending processes.

Analytical springback assessment in flexible stretch bending of complex shapes

To produce highly functional tubular parts with locally different properties, tailor welded tubes (TWTs), which are made by tube-to-tube welding of tubes having different thicknesses or mechanical properties, are hydroformed. Notwithstanding the numerous applications of TWT hydroforming processes, TWT presents some disadvantages and considerable risk owing to the presence of a weld seam. This study proposes and characterizes the tailor layered tube (TLT) hydroforming process, which can fabricate hollow tubular parts with dissimilar thicknesses by using TLTs without welding. The proposed TLT hydroforming process was implemented and characterized both numerically and experimentally to validate its feasibility. The optimal loading path to prevent defects including insufficient bulging and wrinkle were analytically determined and verified by experiments. The deformation behaviors of TLT were also characterized at various processing parameters. The results demonstrate that the proposed TLT hydroforming process can be a feasible alternative for achieving tailored properties even with dissimilar thicknesses, with high reliability.

Tailor layered tube hydroforming for fabricating tubular parts with dissimilar thickness

Extruded profiles/sections are increasingly used in the transport industry for lightweight structures. In this paper, a wide thin-ribbed aluminium profile with asymmetric Z-shape, was manufactured by a novel sideways extrusion process proposed by the authors. A comparative study was conducted by utilising the direct/forward extrusion process at the same extrusion temperature and speed, in which the different process mechanics, resulting microstructures and mechanical properties of profiles have been investigated by experiments and finite element modelling. It was revealed that, compared with sideways extrusion, although the design of a die pocket in forward extrusion induces preform and avoids the use of the large-diameter billet and extrusion container/press needed for extruding wide profiles, it requires a greater extrusion force due to work-piece upsetting necessary to fill the die pocket and leads to a lower effective strain in the profile rib. EBSD characterisation of the regions with an equal effective strain indicated that an increased shear strain is more efficient for obtaining fine grains with a higher average misorientation angle. In the same region of the profile rib made from the two different processes, sideways extrusion results in greater grain refinement due to greater effective strains, and a slightly greater texture intensity was found due to the intensive shear deformation. Tensile tests on formed profiles revealed that sideways extrusion leads to a higher yield strength (YS) and ultimate tensile strength (UTS) but a relatively lower elongation to failure, due to the combined effects of grain refinement, GND and texture intensity enhancement. Compared with the billet, the profile formed by forward and sideways extrusion has a YS increased by about 60% and 79% respectively, and an UTS increased by about 74% and 80% respectively in the extrusion direction, demonstrating an advantage of the sideways extrusion process in improving material strength under the same extrusion condition.

A comparative study on deformation mechanisms, microstructures and mechanical properties of wide thin-ribbed sections formed by sideways and forward extrusion

Achieving small-radius bending of difficult-to-form tubular materials with large-diameter, thin-walled (LDTW, D/t > 20) geometry — as well as the properties of high-strength poor-hardening (HSPH) — is a continual target and challenging issue in the aerospace, marine, and healthcare industries. Based on the theoretical analysis of neutral layer shifting reconstruction (NLSR) by positively utilizing the temperature-dependent tension-compression (T-C) asymmetry of tubes, an innovative differential heating-based rotary draw bending (DH-RDB) method is designed, i.e., applying differential working temperatures at the intrados (Tin) and extrados (Tout) of tubes during forming to proactively coordinate the non-uniform deformation and thus prevent multiple forming defects to break the bending limit. To verify the above concept, the temperature-dependent evolution of the T-C asymmetry of titanium tubes was characterised — using commercial pure titanium (CP–Ti) tubes as case materials — and numerically implemented into the thermo-mechanical coupled three-dimensional finite element (3D-FE) model of the DH-RDB process. The influence of two heating schemes–i.e., overall heating and differential heating at the extrados and intrados of the tube — on bending formability was quantitatively assessed regarding multiple bending defects, such as wrinkling, over-thinning, and cross-section flattening. It was found that by introducing differential heating, the design and optimisation space for NLSR in tube bending is enlarged. The overall heating increases the work hardening of titanium tubes and thus restrains wrinkling during bending. Furthermore, the coupling effects of the increased compression-tension flow stress ratio (CTFSR) and decreased friction force under differential heating of Tin

Breaking bending limit of difficult-to-form titanium tubes by differential heating-based reconstruction of neutral layer shifting

In tube free-bending, the connection of the bending die to the guide has a great influence on the stability of mechanical movement and the forming limit of the bent tube. The general connection of the bending die to the guide cannot realize the continuous stable movement of the bending die. Spherical connection of the bending die to the guide can guarantee a continuous changed tipping of the bending die, but has a limited minimal bending ratio (ratio of bending radius to tube outer diameter) of R/D0 ≥ 3 depending on the geometrical constrains. In this paper, a new spherical connection of the bending die to the guide was designed through geometric derivation, which can lead to a greater deflection of the bending die, to realize the bending ratio of down to R/D0 = 2.5. Based on the designed spherical connection, the bending ratio of down to R/D0 = 2.43 and R/D0 = 2.78 of brass tube were realized in the finite element simulation and bending test respectively. And with the analytical model, simulation model and experimental bending results, the forming characteristics of bent tube with small bending radii based on the spherical connection structure were investigated. The main results show that the equivalent stress, displacement of the strain neutral layer (NL) increase with the decrease of bending radius, and the wall thickness thinning in outer bend shows little differences under tight and big bending radii due to the increased axial thrust.

Forming characteristics of tube free-bending with small bending radii based on a new spherical connection

The invention discloses a three-axis free bending die and guiding mechanism matching optimization design method and belongs to the technical field of metal plastic forming. The optimization design method is characterized in that the matching form (stand-alone type or straight-line type) is changed into a spherical contact type. Compared with a traditional form, the movement of the bending die in the pipe forming process is more stable under the matching form, the rotating angle theta can be accurately controlled, and the minimum relative bending radius (R/D0) of the three-axis free bending equipment can be reduced to 2.5.

Three-axis free bending die and guiding mechanism matching optimization design method

The invention provides a device and a method for spinning reducing and space free bending synchronously forming. The device comprises a propelling mechanism, a pipe fitting clamping mechanism, a pipefitting guiding device and a bending mold which are sequentially connected along the axial direction of a pipe blank; a spherical bearing is arranged outside the bending mold, and the bending mold isinternally provided with a spinning reducing mechanism; and the left side and the right side of the spinning reducing mechanism are in contact with the bending mold through two conical bearings and are in high-speed rotation in the bending mold. According to the device and the method, the spinning reducing mechanism arranged in the bending mold is used for reducing the pipe diameter, three rotating wheels which rotate at a high speed around the axis are used for replacing an arc part which is in contact with a pipe fitting of an original bending mold, so that the pipe fitting is subjected to three-dimensional free bending deformation while reducing the diameter, and a freely-bent hollow component with the required pipe diameter is obtained.

Ma Ziqi

Papers

Forming characteristics of tube free-bending with small bending radii based on a new spherical connection

Three-axis free bending die and guiding mechanism matching optimization design method

Device and method for spinning reducing and space free bending synchronously forming