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The adequate filling and quenching of small corner features are major challenges in manufacturing complex-shaped boron steel tubular parts during the hot metal gas forming process. Considering that the tube forming process involves closed and invisible features, a single-sided die quenching experiment of boron steel sheets was proposed to simulate the in-die quenching process of tubes. The results confirmed that for a given sheet thickness, the critical size of the non-contact zone of achieving complete martensite transformation could be determined. A simple demonstrator geometry was designed to analyze the effects of bulging temperature and pressurizing rate on the corner filling, microstructure, and mechanical properties in the hot metal gas forming process, which are then correlated with the cooling rate in single-sided die quenching experiment. The corner filling was significantly improved with the increase in the bulging temperature and the pressurizing rate. At the bulging temperature of 900 °C, when the pressurizing rate increased from 1 to 3 MPa/s, the obtainable minimum corner radius decreased only from 24 to 16 mm. Under the above increased pressurizing rate, the width of the non-contact zone was 12.23 mm, which corresponded to a cooling rate that could reach 47 °C/s in the corner zone. The limited corner filling resulted from a significant temperature drop during hot metal gas forming. Decreasing the cooling rate of the tube or increasing the pressurizing rate can extend the range of reasonable process parameters in the boron steel tubes' hot metal gas forming. [ABSTRACT FROM AUTHOR]

To avoid wrinkling and cross-sectional distortion in thin-walled tube bending, a novel bending process called tube press bending under gas internal pressure (abbreviated as TGPB) is proposed. In the TGPB process, internal pressure serves a supportive function and generates an additional tensile stress, which can reduce the axial compressive stress on the inner side of the bend, avoiding the occurrence of wrinkles and cross-sectional distortions. Compared with the previous tube bending technologies, the support pressure inside the tube will not change suddenly with the volume change of the tube cavity due to the compressibility of gas. It is suitable for thin-walled tube bending with large, variable axial curvatures and high strength as the shape of the target part is determined by the die cavity. A theoretical prediction model of critical support pressure for TGPB without wrinkling defects is established. The critical support pressure, wall thickness, and section distortion of tubes with different curvatures bent by TGPB are analyzed through FE simulations and experiments, and tubes with different material properties are also done through FE simulations. The results indicate that the tube can be stably bent to the desired shape under the predicted critical support pressure, which is significantly affected by the bending radius and material characteristics, increases with strength coefficient, and decreases with bending radius and strain hardening exponent. A high-strength steel prebending part with complex variable axial curvatures has been successfully formed by TGPB technology under a support pressure of 5 MPa; the minimum and maximum bending radii of which are 846 mm and 2373 mm, respectively. [ABSTRACT FROM AUTHOR]

Hot gas forming (HGF) is an advanced technique for fabricating complex-shaped hollow tubular parts. Practically, multi-step pre-forming involving pre-deformation is often necessary prior to HGF. This paper performs an experimental investigation to simulate the pre-forming operation evaluating the effect of pre-strain on the subsequent HGF. Firstly, the dislocation density was accumulated with uniaxial pre-stretching with different strains (5%, 10%, and 15%) at room temperature, simulating the pre-forming operations. Secondly, the sub-sized specimen from the pre-stretched sample was characterized at different temperatures (350, 400, and 450 °C) to evaluate the effect of pre-strain on successive hot deformation for simulating practical HGF. The experimental results proves the coupled influence of pre-strain and temperature on the flow stress and stress-strain variations. To thoroughly understand the micro-mechanisms, EBSD analysis of grains and grain boundary angles was carried out under different pre-deformation levels and temperatures, which shows the recrystallization phenomenon at 15% pre-strain and 450 °C temperature. Finally, a physical mechanism constitutive model is established based on the determined macro and micro results where the pre-strain effect shows the accurate modeling of stress flow behavior of the material. [ABSTRACT FROM AUTHOR]

Hot gas forming with an integrated heat treatment was utilized to form complex-shaped components in one operation that obtain full strength. The effects of the pressurizing rate on the deformation and strengthening behaviors were critical. Free-bulging tests were conducted at different pressurizing rates (0.0088–2.37 MPa/s) to reveal the corresponding deformation and strengthening behaviors of an Al–Cu–Li alloy sheet and for choosing appropriate pressurizing conditions. The profile and thickness were analyzed to describe the deformation behaviors. The Vickers hardness under different pressurizing and deformation conditions was measured to evaluate the strengthening behavior. The microstructure evolution was observed to reveal the deformation and strengthening mechanisms. An excellent bulging ability was obtained under the full solution condition, with a maximum strain of 0.88 at a pressurizing rate of 0.034 MPa/s. The thickness distribution became more uniform under rapid pressurizing conditions because of the increasing strain and strain-rate hardening abilities. The hardness increased with increasing deformation and was not affected by the pressurizing rate. Dynamic recovery was the main deformation mechanism, and dynamic recrystallization occurred at a low pressurizing rate. Fine precipitates comprising the T1 phase (Al2CuLi, hexagonal structure) were the main strengthening phase. The pressurizing condition was only considered to obtain the maximum improvement in formability and microstructure, regardless of its effect on strengthening during hot gas forming with an integrated heat treatment. [ABSTRACT FROM AUTHOR]

The formation of local features in complex-shaped components is very difficult during rapid hot gas forming. To characterize the local formability of an Al-Mg-Si alloy sheet, free-bulging test and corner-filling test at different temperatures and pressurizing conditions were conducted in this study. The effects of temperature (400 ~ 500 °C) and strain rate (0.1 ~ 10 s-1) on the local formability were analyzed in detail. The local formability was reflected by the ultimate bulging ability and corner filling ability. The reason for local filling behavior was revealed from the theoretical point of view. The sheet had good bulging formability at elevated temperatures and high strain rates. The ultimate bulging ability was almost similar to the decreasing diameter of bugling die. The ultimate strain could reach 0.78 when the temperature was 500 °C, the strain rate was 1.5 s-1, and the diameter of the bugling die was 10 mm. The filling ability increased as the temperature and pressure increased. A corner radius of 2.50 mm was obtained within 1 s when the temperature was 500 °C and the pressurizing rate was 10 MPa/s. In practice, complex-shaped components with local features could be formed within several seconds by hot gas forming. [ABSTRACT FROM AUTHOR]

In the hot metal gas forming process, the deformation conditions, such as temperature, strain rate and deformation degree, are often prominently changed. The understanding of the flow behavior of α-Ti seamless tubes over a relatively wide range of temperatures and strain rates is important. In this study, the stress-strain curves in the temperature range of 973-1123 K and the initial strain rate range of 0.0004-0.4 s−1 were measured by isothermal tensile tests to conduct a constitutive analysis and a deformation behavior analysis. The results show that the flow stress decreases with the decrease in the strain rate and the increase of the deformation temperature. The Fields-Backofen model and Fields-Backofen-Zhang model were used to describe the stress-strain curves. The Fields-Backofen-Zhang model shows better predictability on the flow stress than the Fields-Backofen model, but there exists a large deviation in the deformation condition of 0.4 s−1. A modified Fields-Backofen-Zhang model is proposed, in which a strain rate term is introduced. This modified Fields-Backofen-Zhang model gives a more accurate description of the flow stress variation under hot forming conditions with a higher strain rate up to 0.4 s−1. Accordingly, it is reasonable to adopt the modified Fields-Backofen-Zhang model for the hot forming process which is likely to reach a higher strain rate, such as 0.4 s−1. [ABSTRACT FROM AUTHOR]

Internal high pressure forming of tube is widely used to form complex tubular part with different cross sections and curved axis. The ability of tube to undergo expansion deformation is crucial for the forming process. To evaluate the formability of AZ31B extruded tube, bulging test was carried out at different temperatures from RT up to 480 °C. Bursting pressure and maximum expansion ratio (MER) of the tube were obtained. The fracture surface after bursting was analyzed and compared with that obtained by tensile test along axial direction. Hardness changes in different positions along axial direction were also measured. Results show that the MER value remain almost unchanged from RT to 100 °C. In the temperature interval between 100 and 480 °C, an oblique N model can be used to describe the variation of MER value. The first peak and the bottom MER value occurred at 160 and 330 °C, respectively, and about 30.3% expansion ratio was reached at 480 °C. The bursting pressure decreased almost linearly as testing temperature increased. The fracture mode also changed from intercrystalline fracture to gliding fracture. However, burnt structure happened when the forming temperature was about 480 °C. The hardness value of the tube decreased significantly after the bulging test. [ABSTRACT FROM AUTHOR]