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Large-dimension complex integral thin-shell components are widely used in advanced transportation equipment. However, with the dimensional limitations of raw blanks and the manufacturing process, there are inhomogeneous geometric and mechanical properties at welded joints after welding, which have a significant effect on the subsequent forming process. Therefore, in this paper, the microstructure of welded joints with a sharp property change was accurately characterized by the proposed isothermal treatment method using the BR1500HS welded tube as an example. In addition, an accurate constitutive model of welded tubes was established to predict the deformation behavior. Firstly, the heat-treated specimens were subjected to uniaxial tensile tests and the stress–strain curves under different heat treatment conditions were obtained. Then, the continuous change in flow stress in the direction of the base metal zone, the heat-affected zone and the weld zone was described by the relationship between the microhardness, flow stress and center angle of the welded tube. Using such a method, a continuous constitutive model of welded tubes has been established. Finally, the constitutive model was compiled into finite-element software as a user material subroutine (VUHARD). The reliability of the established constitutive model was verified by simulating the free hydro-bulging process of welded tubes. The results indicated that the continuous constitutive model can well describe the deformation response during the free hydro-bulging process, and accurately predicted the equivalent strain distribution and thickness thinning rate. This study provides guidance in accurately predicting the plastic deformation behavior of welded tubes and its application in practice in hydroforming industries. [ABSTRACT FROM AUTHOR]

We propose an experimental method to identify anisotropic coefficients in non-principal axis directions of thin-walled tubes. The method involves extracting specimens from the parent tubes and machining a hole in the axial center. The specimens are then inserted into a tube without a hole. The inner diameter of the specimen is theoretically equal to the outer diameter of the inner tube. The double-layer tube undergoes free bulging under internal pressure in our self-developed experimental equipment, with the hole on the specimen expanding simultaneously. The stress states around the hole are uniaxial, and the hole deformation can reflect the anisotropic plastic flow characteristics of the tube. Furthermore, based on the information obtained from the proposed experimental method, a hybrid numerical–experimental method was used to identify the anisotropic coefficients of tubes. Through FE simulations, the relationships between the thickness, stress, and strain states around the hole, the hole shape, and anisotropic coefficients of non-principal axis directions are revealed, and the factors that affect the hole deformation are analyzed. Finally, the hole bulging experiments and FE simulations of AA6061-O extruded tube were conducted, and modeled with Hill48 and calibrated by uniaxial tensile and hoop tensile tests. Its in-plane anisotropy coefficients in any direction are given for the first time which first increase and then decrease from 0° to 90°, reaching a maximum of 1.13 in 60° and a minimum of 0.69 in 0°. This work can provide the key experimental data for establishing an accurate anisotropic plastic constitutive model of thin-walled tubes. [ABSTRACT FROM AUTHOR]

To address the challenge of manufacturing NiAl intermetallic thin-walled components, an integrated manufacturing process using the preform prepared by laying/winding of metal foil strips was proposed. This method entails the initial fabrication of a stacked foil preform via the laying/winding of metal foil strips, followed by pre-sintering to prepare a laminated preform. Subsequently, precise shaping is achieved through hot bulging of the laminated preform, while the NiAl intermetallic phase is obtained through reaction synthesis of Ni and Al foils under hot pressing. The microstructure of Ni/Al/Ni laminated preforms under varied sintering parameters and different laminating methods were analyzed. Additionally, the deformation behaviors of these preforms at both room and high temperatures were investigated via tensile tests and bulge tests. Results show: (1) The micro-cracks and Ni-Ni interface delamination occurred in the brittle Ni 2 Al 3 layer of the laminated preform. The deformation capability of laminated plates decreased as the thickness of the brittle Ni 2 Al 3 layer increased with sintering time. (2) Tiny gaps between foils due to laying path and foil size deviations, causing reduced deformability and fractures when the tensile direction was perpendicular to these gaps. (3) These gaps and micro-cracks can be progressively eliminated through hot pressing sintering processes. The findings verify the preliminary feasibility of the proposed novel manufacturing process and provide a novel solution for manufacturing NiAl intermetallic thin-walled components. [Display omitted] • Propose a novel process for intermetallic thin-walled components using the preform prepared by metal foil strips. • Determine the optimal sintering parameters and deformation parameters of the Ni/Al/Ni laminates. • Clarify the relationship between the deformation behavior of Ni/Al/Ni laminates and their microstructures. • The viability of the integrated manufacturing process for intermetallic thin-walled components was validated. [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]

The precise characterisation of hot flow behavior of titanium alloys is of vital importance for practical hot forming processes. To precisely determine the hot flow behavior of titanium alloys under the forming conditions, Gleeble hot tensile tests are usually performed to simulate the forming processes by accurately controlling the deformation temperatures and strain rates under designed conditions. However, there exists a non-uniform temperature distribution during the Gleeble tests, which leads to inaccuracies in the determined hot flow behavior. To overcome such an issue, this paper proposed a new strain-based correction method for Gleeble hot tensile tests, enabling the mitigation of the non-uniform temperature-induced stress-strain curve inaccuracies. The non-uniform temperature zones have been successfully excluded in the calculation of the true strain levels. A series of hot uniaxial tensile tests of TA32 at temperatures, ranging from 750 °C to 900 °C, and strain rates, 0.01/s~1/s, were carried out. The obtained stress-strain correlations for a large gauge zone were characterized using the new correction method, which was further used to evaluate the hardening behavior of titanium alloys. The results have shown that the ductility, strain hardening component (i.e., n), strain rate hardening component (i.e., m) and uniform strain value (i.e., ε u ) are over-estimated, compared to conventional method. Higher strain rates and lower temperature leads to enhanced hardening behavior. This research provides an alternative correction method and may achieve more accurate stress-strain curves for better guidance of the hot forming process for titanium alloys. [ABSTRACT FROM AUTHOR]

• A novel controlled biaxial tensile testing system was developed using digital image correlation (DIC) as a measurement tool. • The constitutive model of tubular materials considering the evolution of anisotropic characteristics was calibrated accurately. • The accuracy of the calibrated constitutive model was verified in engineering practices by compiling the model into VUMAT for simulating typical free tube hydro-bulging. • The developed DIC-based controlled biaxial tensile test is an effective method to determine an appropriate anisotropic constitutive model for tubular materials. The reliability of numerical simulation results vitally depends on the accurate determination of the constitutive model, which requires accurate and sufficient experimental data. To address the current limitations of the range and accuracy of the experimental data of tubular materials, a novel controlled biaxial tensile testing device was developed using digital image correlation (DIC) system as the deformation measuring tool. Using such a device, arbitrary biaxial tensile stress path can be accurately applied to tubular specimens, and the strain can be accurately and continuously measured over the whole deformation process. Firstly, the accurate experimental data of AA6061-O tubes were obtained through the controlled biaxial tensile tests. Then, using the experimental data, the constitutive model considering the evolution of anisotropic characteristics was calibrated based on the Yld2000-2d yield criterion. Finally, the accuracy of the calibrated constitutive model was verified in engineering practices by compiling the constitutive model into finite element software as a user material subroutine (VUMAT) for simulating tube hydro-bulging. Excellent agreements were achieved between experimentally-determined and predicted bulge height, bulge profile, and pole thickness of bulged tubes over the whole hydro-bulging process. It is demonstrated that the DIC-based controlled biaxial tensile test is effective to accurately calibrate the constitutive model of tubular materials, which can be used for both the research of plastic deformation behavior and practical application of hydroforming industry. Image, graphical abstract [ABSTRACT FROM AUTHOR]