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AbstractFlexible rolling (FR) is a novel process for manufacturing doubly curved shallow shells with various shapes based on a pair of small-diameter bendable rolls. In this paper, the fundamental reason for bi-directional bending of the sheet metal in FR is explained. For the purpose of predicting the forming radii in the transverse and longitudinal directions more effectively, the exit velocity is divided into linear velocity and interference velocity, with the latter so small as to be considered to cause only elastic deformation of the sheet. The theoretical calculation model of the pre-curvature radius resulted from linear velocity is established. Using the mechanical theory of the doubly curved shallow shell, the transverse and longitudinal curvature changes caused by the interference velocity are derived and solved by the analytical approach, which are added to the transverse and longitudinal pre-curvatures respectively so that the actual curvature radii are obtained. In order to verify the reliability of the proposed shape prediction method of FR, numerical simulations and forming experiments for convex and saddle-shaped parts are carried out. Through measurement and comparison, it is found that the simulated and experimental results are extremely consistent with the theoretical data. Besides, the effect of important parameters on elastic deformation is investigated by means of the deduced formulas. [ABSTRACT FROM AUTHOR]

Disentangled ultrahigh molecular weight polyethylene (dis‐UHMWPE) has excellent processability but can be achieved under extreme conditions. Herein, we report ethylene polymerization with the binuclear half‐sandwich scandium complexes C1‐Sc2 and C2‐Sc2 to afford UHMWPE. C1‐Sc2 bearing a short linker shows higher activity and gives higher molecular weight PEs than C2‐Sc2 containing a flexible spacer and the mononuclear Sc1. Strikingly, all UHMWPEs isolated from C1‐Sc2 under broad temperature range (25–120 °C) and wide ethylene pressures (2–13 bar) feature very low degree of entanglement as proved by rheological test, DSC annealing study and SEM. These dis‐UHMWPEs are facilely mediated solid‐state‐process at 130 °C and their tensile strength and modulus reach up to 149.2 MPa and 1.5 GPa, respectively. DFT simulations reveal that the formation of dis‐UHMWPE is attributed to the binuclear synergic effect and the agostic interaction between the active center and the growing chain. [ABSTRACT FROM AUTHOR]

The surface quality of an aluminum foam sandwich panel (AFSP) is very important for its appearance and application. This paper mainly studies the surface quality of AFSP after plastic forming. Combined with three-dimensional (3D) scanning technology, the normal deviation between the experimental AFSP and the target surface was obtained, and the surface quality parameters S q , S z and d max were calculated to evaluate surface quality. The AFSP models with cubic-spherical (CS) and tetrakaidecahedral (TKD) as foam structures were established respectively. A series of numerical simulations of multi-point forming (MPF) were then carried out. Equivalent strain and deformation characteristics of spherical and saddle-shaped AFSP were discussed. The main surface defects produced by AFSP in plastic forming, such as surface wrinkle, the local straight face effect and surface dimpling were analyzed. Finally, MPF experiments were carried out, and it was found that the numerical simulation results were significantly corresponded to the experimental AFSP in terms of the degree and distribution of normal deviation and surface quality evaluation parameters. The TKD model was more consistent with the experimental results than the CS model. Moreover, the results show that the surface quality can be improved by thicker face sheets and smaller core cell sizes. [ABSTRACT FROM AUTHOR]

The springback is one of the main defects in the flexible 3D stretch-bending process. In this paper, according to the orthogonal design of experiments, the numerical simulation analysis of the springback for the 3D stretch-bending aluminum profile is carried out by the ABAQUS finite element software. And to investigate the effect of material properties on the springback, the range analysis of the orthogonal experiment is performed. The results show that these material properties of the aluminum profile (elastic modulus E, yield strength σy, and tangent modulus E1) might have the biggest influence on the springback of the aluminum profile, and the optimized forming parameters are founded as follows: the horizontal bending degree is 14°, the vertical bending degree is 14°, the number of multipoint stretch-bending dies is 10, the friction coefficient is 0.15, and aluminum alloy grade is 6063. Moreover, the model of the BP neural network for the prediction of the springback is established and trained based on the orthogonal experiment, and the results with the BP neural network model are in good agreement with experimental results. So it is obvious that the BP neural network could predict effectively the springback of 3D multipoint stretch-bending parts. [ABSTRACT FROM AUTHOR]

Aiming at the flexible 3D stretch bending with multipoint dies for the aluminum profile, the numerical simulation analysis of the bending process was carried out by ABAQUS finite element software. In the multipoint stretch bending (MPSB) process, the influence of the number of die units on the springback for the complex section profile was studied. The shape error between the forming parts and the target parts was reduced through the method of die surface modification. The results showed that the springback of the aluminum profile could reach a minimum when the number of die units was 25 under the precondition of saving cost and ensuring the quality of forming parts. In the numerical simulation, the maximum shape error of the forming parts reduced from 18.824 mm to 2.456 mm; in the test, the maximum shape error reduced from 27.26 mm to 6.03 mm through the method of die surface modification. [ABSTRACT FROM AUTHOR]

The microstructure and tensile properties of as-extruded AZ91 alloy with 1.0 wt% Ca, 0.5 wt% Si, 0.1 wt% La and 0.1 wt% Ce additions (was named AZXSE91100 alloy) were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The addition of multiple elements results in various compounds forming in as-cast alloys. On one hand, the massive large particles are good for grain refinement of as-extruded alloy, which can promote dynamic recrystallization (DRX) during extrusion process via the mechanism of particle stimulated nucleation (PSN). On the other hand, through hot extrusion, the coarse second phases are fragmented into finer dispersed particles including lots of heat-resistant compounds, such as CaMgSi, Mg 2 Si, Al 2 RE, which play an important role in hindering dislocation motions and grain boundary sliding at elevated temperature. Accordingly, the best tensile properties at room temperature are obtained from the peak-aged as-extruded AZXSE91100 alloy, and yield stress (YS), ultimate tensile stress (UTS) and elongation (EL) are 310 MPa, 370 MPa and 8.9%, respectively. The best tensile properties at elevated temperature are obtained from as-extruded AZXSE91100 alloy, and YS, UTS and EL at 423 K reach 222 MPa, 230 MPa and 25.9%, respectively. [ABSTRACT FROM AUTHOR]

Three-dimensional rolling is a novel forming process for three-dimensional surface parts, which combines the rolling process with multi-point forming technology. This process employs a pair of forming rolls as a forming tool; the residual stress of sheet metal makes the sheet metal generate three-dimensional deformation by controlling the nonuniform distribution of roll gap of the forming rolls. In this article, two types of forming processes are investigated for three-dimensional surface parts with the same target shape in different roll adjusting radius. The roll adjusting radius of a forming process is much larger than the target transverse curvature radius of the forming part, and the roll adjusting radius of another forming process is equal to the target transverse curvature radius of the forming part. Finite element analysis models are established; spherical and saddle surfaces are simulated. The corresponding experimental results are obtained. The dimensional accuracy of the forming parts using the two types of forming processes is compared, and the difference between the two types of forming processes for forming parts is analyzed through simulated results. The bendable roll rotates around its bent axis easily if its bending deformation is small; therefore, the forming process that the roll adjusting radius is much larger than the target transverse curvature radius of the surface part has relatively extensive application prospect. The comparison and analysis of the forming results may provide useful guidance on optimizing the three-dimensional rolling process for three-dimensional surface parts. [ABSTRACT FROM PUBLISHER]

• The dislocation motion is the dominant deformation mechanism of 300 M steel treated by LSP. • The compressive residual stress field reduces the crack propagation rate. • Multi-pass LSP improves the impact and fatigue properties of 300 M steel. • A strengthening model is established to predict the contribution of combined strengthening on microhardness. 300 M steel was treated by multi-pass laser shock peening (LSP). The microstructures were characterized by transmission electron microscopy (TEM), electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). In addition, the surface roughness, microhardness, residual stress, impact toughness and mechanical properties after LSP were measured. The results showed that high-density dislocations were generated after multi-pass LSP and the dislocation motion dominated the deformation process. The intensity of texture decreased while low-angle grain boundaries increased, and the grain refinement was not achieved after LSP. The residual stress transformed from tensile stress (+20 MPa) to compressive stress (−556 MPa) and the microhardness was 24.8% higher than the unpeened sample after LSP. Based on the linear superposition rule, a strengthening model was established to predict the contribution of dislocation and grain boundary strengthening on microhardness. Additionally, the fatigue life of peened samples increased by 165% compared with the unpeened ones. The improvement of high-cycle fatigue (HCF) properties was attributed to the rewarding compressive residual stress and microstructural evolution. [ABSTRACT FROM AUTHOR]

The multi-point forming (MPF) process of spherical surface parts of titanium alloy retiary sheet and titanium alloy sheet metal with different thickness and curvature radius was simulated by an explicit finite element software. Contradistinctive analysis between retiary sheet and sheet metal forming parts with different modes were done. The simulation results show that under the same forming conditions, titanium alloy retiary sheet is not easy to wrinkle and springback, whereas it is easy to form. The reason for differences in the formability of above-mentioned sheet metal is also analyzed. A non-wrinkling limited graph and a fracture critical graph for spherical surface parts of retiary metal sheet and metal sheet were obtained. Finally a forming test of titanium alloy cranial prosthesis was done in MPF press. Testing results indicate the customized 3D curved surface of prosthesis can be adequately shaped and the forming quality was guaranteed. [ABSTRACT FROM AUTHOR]

• The 2D residual stress on specimen surface prior to and post to fatigue crack growth tests was measured. • A significant crack retardation was observed only at high residual stresses levels. • The fatigue lives of LSP specimens with square and circular spots were increased by 1.68 and 2.38 times, respectively. • The superposition principle combined with weight function method was used to predict the fatigue crack growth rate. This study investigates fatigue crack growth behavior in residual stress fields induced by laser shock peening (LSP) and its effect on the fatigue life of Ti6Al4V titanium alloy. The two-dimensional residual stress distribution on the specimen surface prior to and post to fatigue crack growth tests was measured by X-ray diffraction. Laser peening with square spot and circular spot was performed on the fatigue specimens. The fatigue lives of LSPed specimens with square spot and circular spot were 1.68 and 2.38 times those of the unpeened specimens, respectively. The result showed that significant crack retardation was observed in specimens only at high compressive residual stresses levels, which was attributed to the reduction of local stress. The superposition principle combined with the weight function method was used to predict the fatigue crack growth rate in residual stress fields. [ABSTRACT FROM AUTHOR]

This paper studies the plastic forming of sandwich panels and proposes a universal elastoplastic equivalent method suitable for sandwich panels. To verify the generality of the equivalent method, according to the different core structures, the cores of bi-directional trapezoidal sandwich (BTS) panels and aluminum foam sandwich (AFS) panels are equated to orthotropic and isotropic (special orthotropic) single-layer panels respectively. Through the finite element (FE) numerical simulation of the mesoscopic model of the sandwich panel, the elastoplastic constitutive relationship of the equivalent core model is established, and then the macroscopic equivalent model of the sandwich panel is established. The FE numerical simulation of plastic forming was carried out for the mesoscopic model and equivalent model of BTS panel and AFS panel, and plastic forming experiments were conducted for the sandwich panel through a multi-point forming (MPF) test machine. The results show that the relative errors of the section average stress at the same position of the equivalent model and the mesoscopic model of sandwich panels are all within 4%; compared with the experimental results, the equivalent model of the sandwich panel has high forming accuracy and small shape error, which verifies the high accuracy and generality of the equivalent method. Moreover, using the sandwich panel equivalent model effectively reduces the calculation time of the numerical simulation. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation on the plastic forming of doubly curved surfaces of aluminum foam sandwich panel (AFSP). A mesoscopic 3D Voronoi model that can describe the structure of closed-cell aluminum foam relatively realistically was established, and a series of numerical simulations using the model of the sandwich panel with a Voronoi foam core were conducted on the plastic forming of two typical doubly curved surfaces including spherical and saddle-shaped surfaces of AFSPs to analyze the deformation behaviors and the forming defects in detail. Multi-point forming experiments of spherical and saddle-shaped AFSPs with different target radii were implemented and the doubly curved panels with good forming quality were obtained. The simulated results of the surface illumination maps, the face sheet profiles, and the maximum strain differences in selected areas of the face sheet and the experimental results indicated that the Voronoi AFSP model can reflect the actual defects occurred in the plastic forming of doubly curved sandwich panels, and the high forming accuracy of the sandwich panel model was also demonstrated in terms of the shape error and the thickness variation. [ABSTRACT FROM AUTHOR]

Continuous roll forming (CRF) is a new method for the rapid forming of three-dimensional (3D) surfaces developed in recent years, and the significant advantage of CRF compared with traditional die forming is that the longitudinal dimension of the sheet metal is not limited. By controlling the curvature radius and gap shape of upper and lower bending rolls, three-dimensional parts with different shapes and sizes can be precisely formed. When the elastic deformation is ignored during the forming process, the transversal curvature radius of the three-dimensional surface is consistent with the radius of the roll gap centerline. Therefore, the calculation of longitudinal curvature radius is the key to improve the accuracy of the 3D surface in CRF. In this paper, the basic principle of CRF is described. The modified formulas for calculating the longitudinal curvature radius of convex and saddle surfaces based on the quadratic relationship between the strain and coordinates are deduced in detail, and the corresponding design method of the roll gap is derived. Furthermore, the mathematical equations of convex and saddle surfaces are given. Through numerical simulation and theoretical analysis, it is found that the relative errors of the longitudinal centerline radius are reduced from 13.67% before modification to 4.35% after modification for a convex surface and 6.81% to 0.41% for a saddle surface when the transversal curvature radius is 800 mm and the compression ratio is 5%. The experimental and measured results indicate that the shapes of formed parts are more consistent with the target parts after modification, which further proves the applicability of the modified formulas. [ABSTRACT FROM AUTHOR]

A new method, by directly utilizing original measured data (OMD) of the stress–strain relation in the Marciniak–Kuczynski (M–K) model, was proposed to predict the forming limit curve (FLC) of an aluminum alloy sheet. In the groove zone of the M–K model, by establishing the relations of the equivalent strain increment, the ratio of shear stress to the first principle stress and the ratio of the second principle stress to the first principle stress, the iterative formula was established and solved. The equations of theoretical forming limits were derived in detail by using the OMD of the stress–strain relation. The stretching specimens of aluminum alloy 6016-T4 were tested and the true stress–strain curve of the material was obtained. Based on the numerical simulations of punch-stretch tests, the optimized specimens' shape and test scheme were determined, and the tests for FLC were carried out. The FLC predicted by the proposed method was more consistent with the experimental results of FLC by comparing the theoretical FLCs based on OMD of the stress–strain relation and of that based on traditional power function. In addition, the influences of anisotropic parameter and groove angle on FLCs were analyzed. Finally, the FLC calculated by the proposed method was applied to analyze sheet formability in the stretch-forming process, and the predicted results of FLC were verified by numerical simulations and experiments. The fracture tendency of the formed parts can be visualized in the forming limit diagram (FLD), which has certain guiding significance for fracture judgment in the sheet-forming process. [ABSTRACT FROM AUTHOR]

Uniform-contact stretch forming based on loading at multi-position (UC-SF) was designed to substitute for conventional stretch forming (C-SF) in the manufacturing of qualified three-dimensional surface parts of a specified shape. Since the integral rigid clamps are replaced by discrete clamps, the sheet metal can be bent in a transverse direction (perpendicular to the stretching direction), and the sheet metal can be automatically warped to the die surface during the loading process. In this paper, finite element numerical simulations were performed to research the contact state evolution and its effect on forming precision by two kinds of loading modes (UC-SF and C-SF). The evolutions of contact state for spherical and saddle-shaped parts were analyzed in different steps, and the results reflect that, in UC-SF, the contact region of curved surface parts is gradually extended in a long strip, and the effective formed regions of the final parts can be in contact with the die surface. However, in C-SF, it is difficult for the final parts to be completely in contact with the die surface, especially spherical parts of a large curvature. Moreover, it is found that the noncontact region of the saddle-shaped part is susceptible to wrinkling in C-SF. Conversely, in UC-SF, the sheet metal can be constrained by contact with a die surface, such that the noncontact region and wrinkle defect disappear and high-precision parts are formed. Finally, stretch forming experiments were carried out and the perfect curved surface part was formed; thus, the experimental results verify the feasibility and effectiveness of UC-SF. [ABSTRACT FROM AUTHOR]

The effect of gadolinium (Gd, 1 wt%) and zinc (Zn, 1 wt%) additions on microstructures and mechanical properties of Mg-4Sm-3Nd-Zr alloy have been investigated by optical microscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The additions of Gd and Zn changed the kinds of precipitates in the T6-state alloy, leading to improvement of tensile properties. The prismatic precipitated phases β ′ and β ″ existed in the peak-aged Mg-4Sm-3Nd-Zr alloy, while β ′ phase and γ phase were dominant precipitates in peak-aged Mg-4Sm-3Nd-Gd-Zn-Zr alloy, and the strip-shaped ZrZn phase could also be observed after solution treatment at 798 K for 8 h. The peak-aged Mg-4Sm-3Nd-Gd-Zn-Zr alloy exhibited good heat resistance with ultimate tensile properties 213 MPa and elongation 8.6% at 573 K, which was attributed to β ′ phase, γ phase and ZrZn phase hindering dislocation and grain boundary motions. [ABSTRACT FROM AUTHOR]

Federated learning is an emerging learning paradigm where multiple clients collaboratively train a machine learning model in a privacy-preserving manner. Personalized federated learning extends this paradigm to overcome heterogeneity across clients by learning personalized models. Recently, there have been some initial attempts to apply transformers to federated learning. However, the impacts of federated learning algorithms on self-attention have not yet been studied. In this article, we investigate this relationship and reveal that federated averaging (FedAvg) algorithms actually have a negative impact on self-attention in cases of data heterogeneity, which limits the capabilities of the transformer model in federated learning settings. To address this issue, we propose FedTP, a novel transformer-based federated learning framework that learns personalized self-attention for each client while aggregating the other parameters among the clients. Instead of using a vanilla personalization mechanism that maintains personalized self-attention layers of each client locally, we develop a learn-to-personalize mechanism to further encourage the cooperation among clients and to increase the scalability and generalization of FedTP. Specifically, we achieve this by learning a hypernetwork on the server that outputs the personalized projection matrices of self-attention layers to generate clientwise queries, keys, and values. Furthermore, we present the generalization bound for FedTP with the learn-to-personalize mechanism. Extensive experiments verify that FedTP with the learn-to-personalize mechanism yields state-of-the-art performance in the non-IID scenarios. Our code is available online https://github.com/zhyczy/FedTP.