Study on the Internal Stress and Thermal Anisotropy in Magnesium Alloys Using a Thermal Elastic Viscoplastic Self-Consistent Model.

A thermal elastic viscoplastic self-consistent model is utilized to examine the thermal stress induced by the thermal anisotropy of single crystals during heat treatments. This model considers temperature-dependent elastic constants and critical resolved shear stress associated with thermal dilation. Simulation results demonstrate that under cooling, the elastic lattice strain increases significantly when constrained compared to unconstrained cooling. The deformation mechanism observed under cooling with constraint resembles tension along the constrained direction at room temperature. Polycrystals offer more deformation mechanisms to accommodate thermal anisotropy compared to single crystals, resulting in lower applied stress at the constrained boundary. Among the various observed textures, the maximum amplitude of residual lattice strain follows the following order: rolled > extruded > random. Lower thermal anisotropy in the entire polycrystal structure leads to reduced internal stress. For a single crystal within aggregates, the {00.2} plane experiences tensile lattice strain, while the {10.0} and {11.0} planes undergo compressive lattice strain due to the greater contraction of single crystals along the direction compared to the direction during cooling.

Quenched Residual Stress Reduction in Pentagon-Curved Aluminum Alloy Forgings Using the Bulging Process.

Quenched residual stress in pentagon-curved forgings (PCGs) often leads to severe deformation during subsequent machining operations. This study aims to mitigate the quenched residual stress in PCGs through the implementation of the bulging method. The edge distance ratio (e/D), a geometric characteristic of PCGs, is defined and considered in the established thermo-mechanical model, which incorporates the effects of quenched residual stress. Increasing e/D resulted in amplified maximum internal stresses and surface stresses. To address this issue, a bulging finite element (FE) model was developed to effectively alleviate the quenched residual stress. The stress reduction in surface stress and internal stress was qualified using average stress reduction (Ra) and peak stress reduction (Rp), respectively. Notably, stress reduction exhibited an inverse relationship with e/D, indicating that decreasing e/D yields greater stress reduction. Furthermore, an overall stress reduction assessment was conducted for different bulging ratios, revealing that the stress reduction increased as the bulging ratio increased. A comprehensive comparison of different bulging ratios highlighted 2% as the most optimal bulging ratio for stress reduction in PCGs. X-ray diffraction measurement and the contour method were employed to determine surface stress and internal stress, respectively. The experimental results were in agreement with the simulation outcomes, validating the high accuracy of the FE model.

Dynamic Constitutive Relationship of Mg-Gd-Y-Zr-Ag Alloy during High Temperature Deformation Process.

The thermal deformation behavior of the Mg-Gd-Y-Zr-Ag alloy was studied by isothermal hot compression tests at high temperatures. The flow stress increased with increased strain rates and decreased temperatures, first increasing and finally remaining stable with increased strain. A hot processing map was built. Using the processing map and microstructural analysis, the temperature should remain at 673-773 K for this alloy to ensure the deformation quality. The primary softening mechanism is discontinuous dynamic recrystallization (DDRX). Rising temperatures and declining strain rates facilitated the emergence and growth of Dynamic recrystallization (DRX) grains. An original JC (O-JC) model and a modified JC (M-JC) model were established. The M-JC model indicated a better prediction than the O-JC model. Still, it was deficient in predicting flow stresses with insufficient coupling effects. Hence, based on the M-JC model, a newly modified JC (NM-JC) model, which further enhances the interaction between strain and strain rate as well as strain and temperature, is proposed. Its projected values can better align with the tested values.

Study on Residual Stress and Optimization of Welding Parameters in Linear Friction Welding of TC17 Titanium Alloy.

Linear friction welding (LFW) is a kind of advanced manufacturing technology and used mainly in the manufacturing of aircraft engine bladed disks (blisks) currently. However, the residual stress evolution of TC17 titanium alloy during LFW is complex and its distribution is difficult to characterize. In this study, the residual stress of welding was studied using numerical simulation and experimental methods. The results showed that the maximum temperature on the welded surface was up to 1000 °C and the cooling rates in the lengthwise, widthwise, and normal direction with the same distance from the center of the weld were 456 °C/s, 448 °C/s, and 232 °C/s, respectively. The lengthwise stress on the welding surface was the largest, followed by the widthwise stress and normal stress. Among the three factors affecting welding stress, the upsetting force played a leading role, followed by the vibration amplitude and frequency of the welded parts. By optimizing the process parameters: upsetting force 18.2 kN, vibration amplitude 2.5 mm, vibration frequency 40 Hz, a 30% decrease of the maximum residual stress could be achieved compared to that without optimization. The residual stress before and after welding parameter optimization was measured by the contour method, and the measured results were in good agreement with the simulation results, which verified the effectiveness of parameter optimization on residual stress controlling.

Vibration and Trajectory Tracking Control of Engineering Mechanical Arm Based on Neural Network.

We offer a neural network-based control method to control the vibration of the engineering mechanical arm and the trajectory in order to solve the problem of large errors in tracking the path when the engineering mechanical arm is unstable and under the influence of the outside world. A mechanical arm network is used to perform tasks related to learning the unknown dynamic properties of a engineering mechanical arms keyboard without the need for prior learning. Given the dynamic equations of the engineering mechanical arm, the dynamic properties of the mechanical arm were studied using a positive feedback network. The adaptive neural network management system was developed, and the stability and integrity of the closed-loop system were proved by Lyapunov's function. Engineering mechanical arm motion trajectory control errors were modeled and validated in the Matlab/Simulink environment. The simulation results show that the management of the adaptive neural network is able to better control the desired path of the engineering mechanical arm in the presence of external interference, and the fluctuation range of input torque is small. The PID control has a large error in the expected trajectory tracking of the engineering mechanical arm, the fluctuation range of the input torque is as high as 20, and the jitter phenomenon is more serious. The use of detailed comparisons and adaptive neural network monitoring can perform well in manipulating the trajectory of the engineering mechanical arm. The engineering mechanical arm uses an adaptive neural network control method, in which the control precision of engineering mechanical arm motion trajectory can be improved and the out-of-control phenomenon of mechanical arm motion can be reduced.

Effects of Quenching Cooling Rate on Residual Stress and Mechanical Properties of a Rare-Earth Wrought Magnesium Alloy.

To investigate the effect of quenching rate on microstructure, residual stress (RS) and mechanical properties of a rare-earth wrought magnesium alloy Mg-Gd-Y-Zr-Ag-Er, RS in 20 °C water quenching (WQ (20 °C)), 100 °C water quenching (WQ (100 °C)) or air cooling (AC) conditions were measured and compared with the simulation results, corresponding mechanical properties and microstructure in quenching and aging state were studied. The decrease of quenching rate has little effect on the grain size but makes the twinning disappear, precipitates increase and the texture weakened, leading to easier brittle fracture after aging. WQ (100 °C) is the best quenching condition in this study, with a significant decline in RS and only 4.9% and 3.7% decrease in yield stress (YS) and hardness compared with WQ (20 °C). The results make it feasible to invent an appropriate quenching method of greatly reducing RS while maintaining mechanical properties. The research conclusions would be beneficial to the application of the alloy.

Comparison of Constitutive Models and Microstructure Evolution of GW103K Magnesium Alloy during Hot Deformation.

The characteristics of constitutive behavior and microstructure evolution of GW103K magnesium alloy were investigated via hot compression tests at a strain rate of 0.001-1 s-1 and a temperature of 623-773 K. The rheological stress of GW103K alloy decreased with increasing temperature or decreasing strain rate during hot deformation. Three models including the Johnson Cook (JC) model, the strain-compensated Arrhenius (SCA) model and back-propagation neural networks (BPNN) were applied to describe the constitutive relationships. Subsequently, the predictability and precision of the models were compared by evaluating the correlation coefficient (R), root mean square errors (RMSE), and relative errors (RE). Compared with the JC and SCA models, the BPNN model was more efficient and had higher prediction accuracy in describing flow stress behavior. Furthermore, EBSD maps confirmed that magnesium alloy easily causes dynamic recrystallization (DRX) during hot deformation. The volume fraction and size of DRX grains increased with decreasing strain rate and/or increasing temperature.

The Effect of Magnet-to-Coil Distance on the Performance Characteristics of EMATs.

The poor conversion efficiency and obvious lift-off effect of the electromagnetic acoustic transducer (EMAT) are commonly known to be problems for its practical application. For the purpose of enhancing the performance of EMATs, numerical simulations were performed in order to analyze the effect of various parameters. The results indicate that only the magnet-to-coil distance can effectively enhance the conversion efficiency and weaken the lift-off effect at the same time. When the magnet-to-coil distance is 2 mm, the lift-off effect will continue to be weakened as the magnet-to-coil distance increases, whereas the decrease of the lift-off effect is inconspicuous and the conversion efficiency starts to decline at this time. Therefore, to get the best performance of this specific EMAT, the suitable magnet-to-coil distance is 2 mm. The experiment effectively verifies the improvement of EMATs with a magnet-to-coil distance of 2 mm.

Static Softening Behavior and Modified Kinetics of Al 2219 Alloy Based on a Double-Pass Hot Compression Test.

In this paper, the static softening mechanism of a 2219 aluminum alloy was studied based on a double-pass isothermal compression test. For the experiment, different temperatures (623 K, 723 K, and 773 K), strain rates (0.1/s, 1/s, and 10/s), deformation ratios (20%, 30%, and 40%), and insulation periods (5 s, 30 s, and 60 s) were used. Based on the double-pass flow stress curves obtained from the experiment, the step rate expressed by the equivalent dynamic recrystallization fraction is dependent on the deformation parameters, which increases with the increase in strain rate and insulation time, while it decreases with the increase in temperature and strain. Based on the microstructure observed using electron backscattered diffraction (EBSD), the static softening mechanism of the Al 2219 alloy is mainly static recovery and incomplete static recrystallization. A new expression for the static recrystallization fraction is proposed using the reduction rate of the sub-grain boundary. The dependent rule on the deformation parameters is consistent with the step rate, but it is of physical significance. In addition, the modified static recrystallization kinetics established by the new SRX fraction method was proven to have a good modeling and prediction performance under given deformation conditions.

Effect of Shot Peening on Redistribution of Residual Stress Field in Friction Stir Welding of 2219 Aluminum Alloy.

Welding is one of the essential stages in the manufacturing process of mechanical structures. Friction stir welding structure of aluminum alloy has been used as a primary supporting member in aerospace equipment. However, friction stir welding inevitably generates residual stress that promotes the initiation and propagation of cracks, threatening the performance of the welded structure. Shot peening can effectively change the distribution of residual stress and improve the fatigue properties of materials. In this paper, friction stir welding and shot peening are performed on 2219 aluminum alloy plates. The residual stress fields induced by friction stir welding and shot peening are measured by using the X-ray diffraction method and incremental center hole drilling method, and the distribution characteristics of residual stress fields are analyzed. The effect of the pellet diameters and pellet materials used in shot peening on the redistribution of welding residual stress field are investigated. The pellet diameter used in the experiment is in the range of 0.6-1.2 mm, and the pellet material includes glass, steel, and corundum. This study provides guidance for the application of shot peening in friction stir welding structure of 2219 aluminum alloy.

Residual Stress Relief in 2219 Aluminium Alloy Ring Using Roll-Bending.

Relieving the residual stress in components is essential to improve their service performance. In this study, a roll-bending process was proposed to reduce the quenching residual stress in a large-size 2219 Al alloy ring. The roll-bending effect on quenching residual stress was evaluated via the finite element method (FEM) combined with experiment. The effect of radial feed quantity, friction coefficient, and roller rotational speed during the roll-bending process on quenching residual stress was analyzed. A set of optimized roll-bending parameters with radial feed quantity, friction coefficient, and roller rotational speed was obtained. The results reveal that the best reduction rates of circumferential and axial residual stress reached 61.72% and 86.24%, respectively. Furthermore, the difference of the residual stress reduction effect between the roll-bended ring and the three-roller bended beam was analyzed.

Finite Element Prediction of Residual Stress and Deformation Induced by Double-Pass TIG Welding of Al 2219 Plate.

Finite element (FE) analysis of welding residual stress and deformation is one of the essential stages in the manufacturing process of mechanical structures and parts. It aids in reducing the production cost, minimizing errors, and optimizing the manufactured component. This paper presents a numerical prediction of residual stress and deformation induced by two-pass TIG welding of Al 2219 plates. The FE model was developed using ABAQUS and FORTRAN packages, Goldak's heat source model was implemented by coding the nonuniform distributed flux (DFLUX) in user subroutine to represent the ellipsoidal moving weld torch, having front and rear power density distribution. Radiation and convection heat losses were taken into account. The mechanical boundary condition was applied to prevent the model from rotation and displacement in all directions while allowing material deformation. The FE model was experimentally validated and the compared results show good agreement with average variations of 18.8% and 17.4% in residual stresses and deformation, respectively.

Effects of Deformation Parameters on Microstructural Evolution of 2219 Aluminum Alloy during Intermediate Thermo-Mechanical Treatment Process.

To explore the effective way of grain refinement for 2219 aluminum alloy, the approach of 'thermal compression tests + solid solution treatment experiments' was applied to simulate the process of intermediate thermo-mechanical treatment. The effects of deformation parameters (i.e., temperature, strain, and strain rate) on microstructural evolution were also studied. The results show that the main softening mechanism of 2219 aluminum alloy during warm deformation process is dynamic recovery, during which the distribution of CuAl2 phase changes and the substructure content increases. Moreover, the storage energy is found to be decreased with the increase in temperature and/or the decrease in strain rate. In addition, complete static recrystallization occurs and substructures almost disappear during the solid solution treatment process. The average grain size obtained decreases with the decrease in deforming temperature, the increase in strain rate, and/or the increase in strain. The grain refinement mechanism is related to the amount of storage energy and the distribution of precipitated particles in the whole process of intermediate thermal-mechanical treatment. The previously existing dispersed fine precipitates are all redissolved into the matrix, however, the remaining precipitates exist mainly by the form of polymerization.

A Physically Based Constitutive Model and Continuous Dynamic Recrystallization Behavior Analysis of 2219 Aluminum Alloy during Hot Deformation Process.

The isothermal compression tests of the 2219 Al alloy were conducted at the temperature and the strain rate ranges of 623⁻773 K and 0.01⁻10 s-1, respectively, and the deformed microstructures were observed. The flow curves of the 2219 Al alloy obtained show that flow stress decreases with the increase in temperature and/or the decrease in strain rate. The physically based constitutive model is applied to describe the flow behavior during hot deformation. In this model, Young's modulus and lattice diffusion coefficient are temperature-dependent, and the creep exponent is regarded as a variable. The predicted values calculated by the constitutive model are in good agreement with the experimental results. In addition, it is confirmed that the main softening mechanism of the 2219 Al alloy during hot deformation is dynamic recovery and incomplete continuous dynamic recrystallization (CDRX) by the analysis of electron backscattered diffraction (EBSD) micrographs. Moreover, CDRX can readily occur under the condition of high temperatures, low strain rates, and large strains. Meanwhile, the recrystallization grain size will also be larger.

Comparisons of Different Models on Dynamic Recrystallization of Plate during Asymmetrical Shear Rolling.

Asymmetrical shear rolling with velocity asymmetry and geometry asymmetry is beneficial to enlarge deformation and refine grain size at the center of the thick plate compared to conventional symmetrical rolling. Dynamic recrystallization (DRX) plays a vital role in grain refinement during hot deformation. Finite element models (FEM) coupled with microstructure evolution models and cellular automata models (CA) are established to study the microstructure evolution of plate during asymmetrical shear rolling. The results show that a larger DRX fraction and a smaller average grain size can be obtained at the lower layer of the plate. The DRX fraction at the lower part increases with the ascending speed ratio, while that at upper part decreases. With the increase of the offset distance, the DRX fraction slightly decreases for the whole thickness of the plate. The differences in the DRX fraction and average grain size between the upper and lower surfaces increase with the ascending speed ratio; however, it varies little with the change of the speed ratio. Experiments are conducted and the CA models have a higher accuracy than FEM models as the grain morphology, DRX nuclei, and grain growth are taken into consideration in CA models, which are more similar to the actual DRX process during hot deformation.