International Journal of Material Forming最新文献

Establishing a unified constitutive model to simulate the hot deformation behaviors, microstructure evolution and fracture behaviors under different stress states during the hot forming of titanium alloy is indispensable. The high temperature tensile tests were first carried out on different stress states of forged Ti-6Al-4 V alloy specimens to analyze the flow behaviors, microstructure evolution and fracture mechanism. The results show that the effect of temperature on fracture elongation is more significant than strain rate. High temperature and low strain rate will increase the dynamic recrystallization (DRX) volume fraction and softening effect, which inhibits the nucleation and growth of voids, thereby enhancing the plastic deformation ability of the alloy. The DRX volume fraction, grain size and stress triaxiality were introduced into the unified Gurson-Tvergaard-Needleman (GTN) damage model using the internal state variables. The parameters of GTN model were modified by the Response Surface Method (RSM) and compared with the high temperature tension. Finally, the established GTN damage model was successfully applied to finite element (FE) simulation under different stress states. The correlation coefficient R of predicted stress is 0.989, and the maximum errors of DRX volume fraction and grain size are 9.86% and 6.54%. The research results can provide a basis for the performance control in hot working of titanium alloy.

Nowadays, the requirements on metallic materials have become more comprehensive, which gradually exceed the capability of monolithic metals. One of the solutions is the composite metal, where different properties of the constituents are integrated as one. In industrial practice, hot roll bonding has been frequently employed to produce laminated composite metals thanks to its high adaptivity. However, the bonding mechanism and the bond strength models have not been thoroughly investigated and parametrized. In a recent publication, a semi-empirical bond strength model has been developed, which quantitatively considers the influence of various influencing factors on the bond strength.

In this paper, this new model is applied in FE simulations of lab-scale hot roll bonding of multiple passes to achieve a better understanding of the process and the bonding behaviours. Firstly, this new model is adapted for macroscopic process simulations, implemented in FE environment via Abaqus subroutines, and evaluated by the simulations of the truncated-cone experiments. Secondly, the FE setup is applied in the process simulation of hot roll bonding. Eight roll bonding passes are simulatively reproduced and good accordance with experiment is achieved. The strain distribution in thickness, evolution of temperature and bond strength, bonding status and cause of local temporary de-bonding are analysed by this simulation. Finally, the influences of the thickness ratio of metallic plates, height reduction, rolling velocity, and material combination with different bonding properties are tested in simulative studies. The process simulations provide a promising way to facilitate the design and optimization of hot roll bonding by FE simulations.

This study proposes a machine learning-based methodology for evaluating the formability of sheet metals. An XGBoost (eXtreme Gradient Boosting) machine learning classifier is developed to classify the formability of the TV back panel based on the forming limit curve (FLC). The input to the XGBoost model is the blank thickness and cross-sectional dimensions of the screw holes, AC (Alternating Current), and AV (Audio Visual) terminals on the TV back panel. The training dataset is generated using finite element simulations and verified through experimental strain measurements. The trained classification model maps the panel geometry to one of three formability classes: safe, marginal, and cracked. Strain values below the FLC are classified as safe, those within 5% margin of the FLC are classified as marginal, and those above are classified as cracked. The statistical accuracy and performance of the classifier are quantified using the confusion matrix and multiclass Receiver Operating Characteristic (ROC) curve, respectively. Furthermore, in order to demonstrate the practical viability of the proposed methodology, the punch radius of the screw holes is optimized using Brent's method in a Java environment. Remarkably, the optimization process is completed swiftly, taking only 3.11 s. Hence, the results demonstrate that formability for a new design can be improved based on the predictions of the machine learning model.

The flow stress model, the dynamic recrystallization (DRX) model, the grain growth (GG) model and the Normalized Cockcroft-Latham (NC-L) ductile fracture criterion are integrated into the finite element (FE) model to simulate the physical field and DRX evolution of the AZ80 magnesium (Mg) alloy wheel forming process by the rotating back extrusion (RBE) process. The deformation behavior of the AZ80 Mg alloy wheel during the forming process is calculated quantitatively when the angular velocity ((omega)) is 0 to 80°/s. Findings revealed that the RBE process increases the deformation heat and effective strain in the forming process of the wheel, and refines the grain size of the whole wheel. However, excessive angular velocity ((omega) > 40°/s) is not conducive to the DRX of the wheel bottom, which makes the grain at the wheel core grow abnormally and reduces the uniformity of the microstructure distribution at the wheel bottom. The damage factor value at the upper rim increases with the increase in (omega), i.e., the tendency of the upper rim to crack increases. Therefore, the (omega) of the Mg alloy wheel produced by the RBE process within the scope of this study should be set at 40°/s. The RBE process of the Mg alloy wheel can provide a new idea for the plastic forming of Mg alloy wheels.

The production of high-performance composite parts with non-crimp fabrics (NCFs) requires a profound understanding of the material’s behavior during draping to prevent forming defects such as wrinkling and gapping. Simulation methods can be used to model the complex material behavior of NCFs and predict their deformation during the draping process. However, NCFs do not intrinsically deform under pure shear like most woven fabrics, but often under superimposed shear, transverse tension and in-plane roving compaction. Therefore, non-standard characterization methods have to be applied besides typical picture frame tests or bias-extension tests. Off-axis-tension tests (OATs) utilize a simple setup to characterize a fabric’s membrane behavior under different ratios of superimposed shear, transverse tension and in-plane compaction. OATs at three different bias angles (30(^circ ), 45(^circ ) and 60(^circ )) are conducted to investigate a unidirectional and a bidirectional NCF. A method is presented to measure the fiber curvatures in addition to the occurring strains. The investigations reveal a relatively symmetrical, shear-dominated behavior with limited roving slippage for the Biax-NCF. The behavior of the UD-NCF strongly depends on the stitching load during tests and is characterized by an asymmetric shear behavior as well as significant roving slippage. The off-axis-tension test results can be used as the basis for the development and validation of new simulation methods to model the complex membrane behavior of NCFs.

Due to the size effect phenomenon, the conventional friction models commonly used in metal forming are not accurate for use in micro-metal forming. In this study, the frictional size effect, as one of the most important phenomena in micro-metal forming, has been investigated. Different frictional models developed based on open and closed lubricant pockets theory have been investigated in both dry and lubricated frictional conditions. Those models use a scale parameter to quantify friction on the micro-scale and a real contact area to calculate the friction force at contacting surface. The models were implemented into ABAQUS finite element package via the VFRIC_COEF subroutine interface. The ring compression test with specimens of different sizes was used to determine the parameters of the models. By reducing the dimensions of the specimens in the ring compression test, no size effect was observed in dry friction conditions. However, in the lubricated frictional conditions, it was observed that the coefficient of friction increased significantly with reducing the specimen size. As the dimensions of the specimen decrease and the scale parameter approaches 1, the gap between the coefficient of friction curves increases significantly, and the coefficient of friction converges to those obtained in dry friction conditions.

Multi-point forming with individually controlled force–displacement (MPF-ICFD) is a novel multi-point forming process with characteristics of good deformation uniformity and high forming accuracy. The process has two different deformation sequences: positive forming (PF) and negative forming (NF). The shape accuracy of a part is significantly different when the deformation order is changed. To reveal the influence mechanism of the deformation sequence on shape accuracy, experiments and numerical simulations are used to assess shape accuracy during multi-point forming. The deformation behaviours of a cylindrical surface, sail surface and saddle surface in PF and NF processes are investigated to obtain the shape accuracy characteristics of a sheet under different deformation sequences. In addition, the strain distribution characteristics of the cylindrical surface are given quantitatively. The influence mechanism of the deformation sequence on the shape accuracy is revealed. The results show that the amount of plastic deformation on the part is significantly increased and the shape accuracy is significantly improved during the PF process. When the loading conditions are identical, the maximum strain of the cylindrical parts is increased by 73.4%, and the amount of springback is decreased by 90.0%. The above research indicates that the PF process has good application prospects in sheet metal forming.

3D printing has offered cost-effective, lightweight, and complex parts. To extend their applications, 3D printed parts need to be welded in order to form the larger functional assemblies. For this purpose, Laser Transmission Welding (LTW) is a promising joining technology. This paper aims to investigate the light scattering effect on the intensity profile of the laser heat source during the transmission through the 3D printed laser-transparent part. Indeed, the inherent design of the 3D printing technology results in a complex heterogeneous microstructure with a significant amount of porosity inside the printed parts. Such structure induces the optical diffusion (i.e. light scattering) of the laser beam within the 3D printed parts. This phenomenon leads to the reduction of the transmitted energy arriving at the weld interface, which directly influences the quality of the joint and its mechanical performance. The approach adopted in this paper is to propose a ray-tracing model to simulate the optical paths of the laser beam through the 3D printed laser-transparent part, which is able to evaluate changes in the laser heat source at the weld interface directly linked with the light scattering effect within the microstructure of the parts. Experimental measurements are performed to assess the transmitted intensity flux distribution using an image processing technique, instrumented with a digital camera and macro lens. The numerical results show good accordance with the experimental one, which proves the confidence of the proposed ray-tracing model. Finally, 3D transient thermal model of the LTW process is performed using the FEM software COMSOL Multiphysic® to confirm the influence of the scattering effect on the temperature field and thus on the quality of the weld.

Recently a novel multi-container extrusion method has been proposed and proved feasible to simultaneously extrude multiple billets for producing thin-walled wide profiles with reduced force by experiments. Due to its different die structure compared to conventional extrusion methods, the effects of tooling geometries on the multi-container extrusion performance should be comprehensively analysed as the base of die design optimisation. In this study, the original three-container extrusion design and six modified designs were established to investigate the influence of three key geometrical variables, including container diameter, upper die height and welding chamber height, on the extrusion performance through finite element simulation. The considered extrusion performances include metal flow uniformity, extrusion force, welding pressure, die lifetime and material yield. The drawbacks of the original die design were revealed after the primary analysis of the key influencing variables and a three-step way of design improvement was proposed accordingly. Container block and upper die were optimised for the first step; welding chamber height and die bearing length were modified during the second and third steps respectively. Compared with the original design, the final optimised design can decrease the unevenness of the extrudate front shape by 75% and double the material yield, while the extrusion force, die lifetime and welding quality were basically unchanged.

Sandwich sheets comprising continuous carbon fiber reinforced plastics (CFRP) are applied mainly in the aerospace industry due to their light weight and high rigidity. However, sandwich sheets require separate formation and bonding of the face sheets and core, resulting in high labor costs and early fracture due to delamination of the adhesive layer. The purpose of this study is to overcome these problems by integrating sandwich sheet using additive manufacturing. The mechanical properties of the integrally formed sandwich sheets were compared with those of adhesively formed sandwich sheets using a three-point bending test. The strain distribution was captured by digital image correlation (DIC) during the test. Additionally, the geometric design parameters of a core with superior mechanical properties were investigated. The test results showed that the integrally formed specimens exhibited superior properties compared to those of the adhesively formed specimens. It was also observed that the larger the width angle of the corrugated core, the better the mechanical properties.