Study on Stamping-Bulging Process of Thin-Walled Superalloy Diaphragm for S-Shaped Bellows.

A combined stamping-bulging forming process was proposed to achieve high-precision forming of large-diameter, ultra-thin-walled, superalloy welded S-type corrugated diaphragms. The underlying principle is to enhance the diaphragm's forming accuracy by increasing the plastic deformation region and reducing springback. Using the ABAQUS version 6.14 finite element analysis software, finite element models were constructed for the stamping, hydraulic bulging, and combined stamping-bulging forming processes of the welded S-type metal corrugated diaphragms. A comparative analysis was conducted on the forming processes of the welded S-type metal corrugated diaphragms under the three forming methods, focusing on equivalent stress, distribution of wall thickness, and forming accuracy. This analysis determined the optimal forming process and the corresponding process parameters for superalloy welded S-type metal corrugated diaphragms. The results show that under a constant drawing force, as the bulging pressure increases, the plastic deformation of the straight sections of the diaphragm becomes more pronounced, resulting in improved shape accuracy. The combined stamping-bulging forming process guarantees the highest degree of shape accuracy for the diaphragm. The optimal process parameters were identified as a 30 t force and a 5 MPa pressure, with a maximum shape error of 0.02 mm. Concerning a plate thickness of 0.3 mm, the maximum deviation rate was found to be 6.7%, which represents a 30% improvement over traditional stamping processes. The maximum wall thinning rate was found to be 3.3%, a 1% reduction compared to traditional stamping processes, confirming the process's feasibility.

The Development of a Continuous Constitutive Model for Thin-Shell Components with A Sharp Change in the Property at Welded Joints.

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.

A New Regression Model for the Prediction of the Stress-Strain Relations of Different Materials.

Experimental flow stress-strain data under different stress states are often used to calibrate the plastic constitutive model of anisotropic metal materials or identify the appropriate model that is able to reproduce their plastic deformation behavior. Since the experimental stress-strain data are discrete, they need to be mathematically returned to a continuous function to be used to describe an equivalent hardening increment. However, the regression results obtained using existing regression models are not always accurate, especially for stress-strain curves under biaxial stress loading conditions. Therefore, a new regression model is proposed in this paper. The highest-order term in the recommended form of the new model is quadratic, so the functional relationships between stress-strain components can be organized into explicit expressions. All the experimental data of the uniform deformation stage can be substituted into the new model to reasonably reproduce the biaxial experimental stress-strain data. The regression results of experimental data show that the regression accuracy of the new model is greatly improved, and the residual square sum SSE of the regression curves of the new model reduced to less than 50% of the existing three models. The regression results of stress-strain curves show significant differences in describing the yield and plastic flow characteristics of anisotropic metal materials, indicating that accurate regression results are crucial for accurately describing the anisotropic yielding and plastic flow behaviors of anisotropic metal materials.

Effect of Initial Orientation on the Anisotropy in Microstructure and Mechanical Properties of 2195 Al-Li Alloy Sheet during Hot Tensile Deformation.

The 2195 Al-Li alloy, as one of the representative third-generation Al-Li alloys, has extensive applications in lightweight aerospace structures. In this paper, the anisotropy in mechanical properties and microstructure evolution of 2195 Al-Li alloy sheets were investigated under a strain rate of 0.01, 0.1, 1 s-1 and a temperature of 440 and 500 °C. Experimental results showed that the hot tensile properties of the 2195 Al-Li alloy sheet exhibited a strong dependence on loading directions. The peak stress (PS) and elongation (EL) along the rolling direction (RD) were larger than the transverse direction (TD). For the tests carried out at 440 °C-1 s-1, the PS values of the sheets stretched along the RD and TD are 142.9 MPa and 110.2 MPa, respectively. And, most of the PS anisotropy values are larger than 15%. The anisotropy in EL is less significant than in PS. All the differences are about 10%. Moreover, dimples in the samples stretched along RD were more and deeper than those along TD at 440 °C. The fracture morphology along RD and TD were similar, and both were cleavage fractures at 500 °C. Particularly, the fractions of high angle grain boundaries (HAGBs) along TD were all about 5% larger than those of RD. And, there were more small-sized continuous dynamic recrystallization (CDRX) grains inside the initial grains and discontinuous dynamic recrystallization (DDRX) grains featured with the local bulge of grain boundaries along TD. This was due to the smaller average Schmid factor and the vertical EL trend of the initial grains when the samples were stretched along TD. A model of grain evolution during the dynamic recrystallization (DRX) along RD and TD was proposed based on EBSD results. The Schmid factor and banded structure had a more prominent effect on the hot ductility of the 2195 Al-Li alloy compared with the degree of DRX, thus presenting a higher EL and better hot ductility along RD.

Identification of Anisotropic Coefficients in the Non-Principal Axis Directions of Tubular Materials Using Hole Bulging Test.

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.

Experiment for Measuring Mechanical Properties of High-Strength Steel Sheets under Cyclic Loading by V-Shaped Double-Shear-Zone Specimens.

The simple shear test shows significant advantages when measuring the hardening and shear properties of thin sheet metal at large strains. However, previous shear tests had an end effect caused by local stress concentration and a boundary effect caused by deformation overflow, resulting in non-uniform strain distribution in the shear zone. Therefore, a unique V-shaped double-shear-zone specimen is proposed to measure the Bauschinger effect under cyclic shear loading conditions in this paper. Simple shear experiments and three different types of cycle shear experiments are conducted to analyze the uniformity of deformation in the shear zone and the effect of pre-strain and the number of cyclic loads on the Bauschinger effect of Q890 high-strength steel sheets. The results indicate that the proposed V-shaped double-shear-zone specimen can still maintain uniform shear deformation in forward/reverse cyclic loading experiments, even at large strains. Q890 high-strength steel exhibits a significant Bauschinger effect, which is more pronounced with the increase in shear pre-strain and loading cycles. The results of this paper provide a new approach for studying the hardening characteristics under large strain and the mechanical properties under cyclic shear loading for metal sheets.

Forming Limit Analysis of Thin-Walled Extruded Aluminum Alloy Tubes under Nonlinear Loading Paths Using an Improved M-K Model.

To meet the requirement of lighter weight and better performance in tube hydroforming, one of the most important tasks is to accurately predict the forming limit of thin-walled tubes under nonlinear loading paths. This work established the M-K+DF2012 model, a combination of the M-K model and the DF2012 ductile fracture criterion, for the forming limit prediction of thin-walled tubes under nonlinear loading paths. In this model, the failure of the groove is determined by the DF2012 criterion, and the corresponding strains in the uniform region are the limit strains. The limit strains of an AA6061 aluminum alloy tube under a set of linear loading paths and two typical nonlinear loading paths were tested. Parameter values of the M-K+DF2012 model for the tube were determined based on the experimental limit strains under linear loading paths, and the limit strains under the two nonlinear loading paths were predicted. Then the strain-based forming limit diagram (ε-FLD) and the polar effective plastic strain FLD (PEPS-FLD) of the tube under different pre-strains were predicted and discussed. The results show that the limit strains of the tube are obviously path-dependent, and the M-K+DF2012 model can reasonably capture the limit strains of the tube under both linear and nonlinear loading paths. The predicted ε-FLD shows a strong dependence on the pre-strain, while the predicted PEPS-FLD is weakly strain path-dependent and almost path-independent on the right-hand side for the AA6061 tube.

A Homogeneous Anisotropic Hardening Model in Plane Stress State for Sheet Metal under Nonlinear Loading Paths.

In sheet metal forming, the material is usually subjected to a complex nonlinear loading process, and the anisotropic hardening behavior of the material must be considered in order to accurately predict the deformation of the sheet. In recent years, the homogeneous anisotropic hardening (HAH) model has been applied in the simulation of sheet metal forming. However, the existing HAH model is established in the second-order stress deviator space, which makes the calculation complicated and costly, especially for a plane stress problem such as sheet metal forming. In an attempt to reduce the computational cost, an HAH model in plane stress state is proposed, and called the HAH-2d model in this paper. In the HAH-2d model, both the stress vector and microstructure vector contain only three in-plane components, so the calculation is significantly simplified. The characteristics of the model under typical nonlinear loading paths are analyzed. Additionally, the feasibility of the model is verified by the stress-strain responses of DP780 and EDDQ steel sheets under different two-step uniaxial tension tests. The results show that the HAH-2d model can reasonably reflect the Bauschinger effect and the permanent softening effect in reverse loading, and the latent hardening effect in cross loading, while the predictive accuracy for cross-loading softening remains to be improved. In the future, the HAH-2d model can be further modified to describe more anisotropic hardening behaviors and applied to numerical simulations.

Friction Stir Welding of 5754 Aluminum Alloy with Cover Sheet.

Friction stir welding can realize high-strength aluminum alloy joints. In this study, friction stir welding with cover sheet (CFSW) is proposed to solve the thinning caused by the tool shoulder and reduce the heat-affected zone. The microstructures and mechanical properties of CFWS were also studied. After the cover sheet was added, a reinforcement was formed on the weld surface, which compensated the thinning caused by the friction of the tool shoulder. As the cover absorbed heat from the shoulder, the width of the heat-affected zone of the welded sheet became smaller than the diameter of the shoulder. Without milling the cover sheet, the tensile strength of the 5754 aluminum alloy joint reached 94% of that of the base metal. The fracture position was the heat-affected zone of the forward-side weld joint. After the cover sheet was added, the stress concentration shifted from the thinning area of traditional friction stir welding to the outside of the welding seam.

Wireless Optofluidic Systems for Programmable In Vivo Pharmacology and Optogenetics.

In vivo pharmacology and optogenetics hold tremendous promise for dissection of neural circuits, cellular signaling, and manipulating neurophysiological systems in awake, behaving animals. Existing neural interface technologies, such as metal cannulas connected to external drug supplies for pharmacological infusions and tethered fiber optics for optogenetics, are not ideal for minimally invasive, untethered studies on freely behaving animals. Here, we introduce wireless optofluidic neural probes that combine ultrathin, soft microfluidic drug delivery with cellular-scale inorganic light-emitting diode (µ-ILED) arrays. These probes are orders of magnitude smaller than cannulas and allow wireless, programmed spatiotemporal control of fluid delivery and photostimulation. We demonstrate these devices in freely moving animals to modify gene expression, deliver peptide ligands, and provide concurrent photostimulation with antagonist drug delivery to manipulate mesoaccumbens reward-related behavior. The minimally invasive operation of these probes forecasts utility in other organ systems and species, with potential for broad application in biomedical science, engineering, and medicine.