To address multi-dimensional geometric deviations in extruded profiled thin-walled hollow members, which are challenging to correct using traditional processes, this study proposes an axial hydro-extrusion method based on a novel stress-transformation mechanism for synchronous control of multi-dimensional deviations. Unlike traditional tensile-based forming, this process induces global plastic extrusion deformation through axial feeding under internal pressure, transforming non-uniform axial, circumferential, and shear stresses into a nearly uniform and predominantly compressive stress state. This shift eliminates stress gradients, thereby controlling deviations in different dimensions and enabling synchronous outer contour correction of the hollow member. First, the effect of stiffeners on wrinkling and shape accuracy of the outer contour of the hollow member in axial hydro-extrusion is analysed, establishing a critical instability internal pressure model. Next, stress states generated by different geometric deviations during fitting die deformation and their changes during axial extrusion are investigated through theoretical and simulation methods, clarifying the reasons for deviation changes in different dimensions. Additionally, a critical extrusion displacement model incorporating work hardening is developed to quantify the stress transformation process and analyse the effects of initial deviations on critical extrusion displacements. Finally, a specialised axial hydro-extrusion forming platform is designed for single-cavity and multi-cavity hollow members with various deviations, achieving over 95% reduction in dimensional deviations. This study provides a new strategy for solving the problem of multi-dimensional geometric deviation synchronisation control in complex thin-walled hollow members.

The tube hydroforging process (THFG) is an advanced technology for manufacturing tubular components with complex cross-sections. The positive-curvature arc is one of the most fundamental and difficult-to-form features of complex cross-sections. However, its wrinkling mechanism in the THFG process cannot be explained by the existing theory. This restricts the application of the technology. First, because of the bending deformation caused by the excessive circumferential force, compression instability occurs at the positive-curvature arc part. This results in wrinkling similar to that in the conventional linear part. In addition, owing to the existence of the positive-curvature arc, the circumferential force produces a component force along the vertical direction that causes rigid displacement of the materials. This yields another new instability model: motion instability. The corresponding critical pressures for the two instability models were determined by adopting static method and energy methods respectively. Theoretically, motion instability is dominant in the early stages of compression, whereas compression instability is dominant in the subsequent stages. However, considering actual production, the correlations between the critical pressures of the different parts were compared. The wrinkling of the linear part inhibits the occurrence of compression instability in the positive-curvature arc. Thus, wrinkling of the arc can be caused only by motion instability. Therefore, the critical pressure for motion instability is defined as the critical pressure required for the positive-curvature arc. In addition, a forming window that considers the critical pressure of each part was established successfully.

The hydro-piercing process is an emerging approach to the direct punching of holes on complex hollow components. During tube hydro-piercing, the deformation in the region adjacent to the pierced hole may range from having a substantially flat form to having a countersunk form. To improve the understanding of deformation behavior in square hole hydro-piercing, an experimental setup was designed and the effects of internal pressure and punch corner radius on the deformation sequence, as well as the collapse behavior, were investigated in this study. At the same time, a numerical simulation was conducted using the Abaqus/Explicit software 6.13. The results showed that the degrees of collapse at three characteristic points were different when the internal pressure was low and that the differences in the degrees of collapse could be reduced by increasing the internal pressure. It was demonstrated that the collapse was related to the internal pressure but had little dependence on the punch corner radius.

To overcome the difficulties of forming S-shaped bellows by conventional hydroforming, such as excessive thinning and high internal pressure, and to obtain S-shaped aluminium alloy bellows with large diameters and large expansion rates with minimal reduction in wall thickness, the axial hydroforging method was proposed. In this method, the deformation area produced bending deformation and gradually fitted the die by the end axial feeding under the support of the internal pressure. First, a finite element model was created to investigate the effect of initial internal pressure, final forming internal pressure and axial feeding on the forming quality of 5A03 aluminium alloy S-shaped bellows after springback from the wall thinning ratio and profile accuracy. The results demonstrated that the overall wall thickness of bellows decreased significantly during the bulging and forming stages, whereas the local thickening of the convolution crown occurred during the forging stage, which caused a shift in the position of the maximum wall thinning ratio and a decrease in the wall thinning ratio. The increase in the initial internal pressure increased the maximum wall thinning ratio, convolution height and convolution pitch of the bellows and decreased the convolution thickness. The increase in the final forming internal pressure increased the maximum wall thinning ratio and convolution height of the bellows and decreased the convolution thickness and convolution pitch. The increase in axial feeding increased the convolution height of the bellows and decreased the maximum wall thinning ratio, convolution thickness and convolution pitch. Finally, the experimental setup was designed, and the S-shaped bellows with a small wall thinning ratio and high profile accuracy were successfully manufactured based on the best simulation parameters, which verified the accuracy of the finite element model and the feasibility of the axial hydroforging process. It is essential to develop the bellow hydroforming technology and improve the quality of S-shaped bellows.

Axial hydro-forging process is a promising alternative for the forming of variable-diameter tubes, in which the tube is subjected to bending and compression deformation. Consequently, wrinkling becomes the primary defects. Based on the corner wrinkling phenomena in corner filling stage of axial hydro-forging process, an analytical model was established to investigate the mechanism of the corner wrinkling and calculate the critical pressure. Subsequently, experiments and numerical analysis for axial hydro-forging were carried out. The effects of initial corner radius, thickness, and friction coefficient on critical pressure were investigated. It is found that the critical pressure decreases with increasing of the initial corner radius, but increases with increasing of the thickness. Through the analytical model, the critical loading path was obtained and compared with the experiments. The results validate that corner wrinkle can be suppressed when the internal pressure is above the critical pressure; otherwise, wrinkle occurs. It also indicates that the proposed analytical model is of practical significance for the hydro-forging process.

A novel modification of Weng’s secant approach to model the elastoplastic stress-strain response of tailor-tempered 22MnB5 steel with heterogeneous material properties when subjected to plastic deformation is described. In the tailored tempering process (TTP) of high-strength boron-manganese steel, different distributions of mechanical properties in the same component can be realized. The elastoplastic constitutive relation is an urgent issue when developing components with customized material properties. The capabilities of classical mixture rule methods and improved homogenization methods to describe the mechanical behavior of inhomogeneous products containing various volume fractions of constituent phases are extensively studied. Weng’s secant model is modified by using the exponential function instead of the power law relation to calculate the flow stress of tailor-tempered 22MnB5 steel. To correctly predict the higher hardness of the bainite around martensite than that inside the bainite structure, which is due to plastic deformation of the surrounding local bainite induced by the volume expansion accompanying martensite transformation, the phase boundary considering the hardening of surrounding local bainite is introduced into the secant model. Furthermore, Vickers hardness parameters instead of phase volume control parameters are used in the modified secant method. The predictability of the overall response obtained using the novel micromechanical-based secant method shows good agreement with experimentally obtained elastoplastic deformation behaviors.

Five sets of boron steel sheet blanks are heat treated and their microstructures are close to mixture of ferritic and pearlitic, upper bainitic, lower bainitic and fully martensitic, respectively. It shows that the micro-hardness of the blank that quenched in the water-cooled die is of 527 HV0.1, while intermediate hardness of 401 HV0.1, 322 HV0.1 and 280 HV0.1 maintained in the heated die as well as 181 HV0.1 cooled in the open furnace. Various constitutive models such as Ghosh, extend Swift, modified Voce, et al., are calibrated and compared using the stress-strain data of the five material grades. The constitutive parameters of each grade are calibrated using the standard uni-axial tensile tests at quasi-static strain rates and at room temperature. The inverse analysis method coupled with optimization algorithm is used to calibrate the parameters of the hardening laws. It is concluded that the Voce + Voce model can accurately predict the stress-strain relation of the five kinds of materials at the same time. The Gurson-Tvergaard-Needleman (GTN) model and the Johnson-Cook model are used to describe the damage behavior of the ductile boron steel B1500HS grades. The simulated force-displacement curves are compared with those of the experiments, showing considerable good coincidence.

To differentiate the ductile fractures caused by deformation incongruities between phases, 22MnB5 boron steel sheets consisting of diverse bainite-martensite fractions were studied. The microstructural characteristics of each phase in tailor-tempered boron steel were examined in tandem with the ductile fracture morphologies after undergoing varied stress states. A micromechanical model was proposed, including microstructural reconstruction using a 2D representative volume element (RVE) approach and a constitutive formulation of a dislocation density-based strain hardening model. The microstructure-based RVE model can be utilized to study the effects of stress states and the influence of microstructural characteristics on the failure mode of tailor-tempered 22MnB5.

With the increasing demand of the reduction of energy consumption and enhancement of automobile safety, the steels with ultra-high strength are widely utilized in automobile industry. Because of the high achievable strength, hot stamping process can be used to supply promising automobile components. The increasing use of high-strength steels leads to great challenges to model the fracture evolution because of their high strength and low ductility. The paper is concerned with plasticity and ductile fracture modelling of hot stamped boron steel 22MnB5, where martensite phase with high yield strength, work hardening and ductility are present during the quenching process. The Lou's ductile fracture criteria which involve void nucleation, growth and coalescence in metallic materials are theoretically reviewed and extended and implanted using users' subroutine VUMAT in ABAQUS/Explicit. Parameters of the selected fracture criterion are determined using calibration tests with various kinds of specimens. Dogbone-like uniaxial tensile specimens are tested to determine the deformation hardening parameters, and specimens with central hole, in-plane shear specimens and notched specimens, balanced biaxial tensile specimens are tested to identify the fracture behavior in various loading stresses. Digital Image Correlation (DIC) method is applied to monitor the surface pattern and record the strain evolution of the specimen surface during the experiment. Because of fully austenitized microstructure, the Mises isotropic yield function is adopted. The hybrid method is used to obtain the fracture strain in various stress directions and implemented to determine the material parameters of fracture model. The constructed ductile fracture surfaces calibrated with fracture strain using DIC calculation and hybrid method are compared in Lode parameter and triaxiality coordinate as well as the principal stress space, respectively. The comparison reveals that the fracture strain obtained by hybrid method is more reasonable. The determined ductile fracture criterion is applied to forecast the fracture onset for seven kinds of specimens and compared with the experiment data. The comparison indicates the simply extended Lou's criterion of ductile fracture can precisely describe the evolution of ductile fracture from uniaxial tensile to biaxial tensile stress state for hot stamped boron steel 22MnB5.

An anisotropic modified Mohr–Coulomb (MMC) ductile fracture criterion was developed to characterize the ductile fracture behavior of the Ti-6Al-4V titanium alloy at room temperature. The stress triaxiality is inherently coupled within the ductile fracture criterion and shows an exponential effect on the material ductility decay. The effect of the Lode angle is considered as well, considering a parabolic trend. The fracture criterion was developed based on the direction-independent plastic strain-rate increment called isotropic equivalent plastic strain-rate increment. To incorporate the influence of directionality on the ductile fracture behavior, the fourth order linear transformation tensor was successfully introduced. The Ti-6Al-4V fracture behavior under uniaxial tension, in-plane shear, plane strain along various loading orientations, and equi-biaxial tension states of stress was successfully predicted using the newly developed fracture criterion. Furthermore, the theoretical Fracture Forming Limit Curve (FFLC) predicted by the anisotropic ductile fracture criterion was compared with experimental forming limit curves from literature. The results show that the developed phenomenological anisotropic MMC ductile fracture criterion has a great advantage to predict the ductile failure of sheet metals characterized by strong anisotropy.