Rigid-viscoplastic Finite Element Research Articles

Numerical Simulation on Thixo-Forging of Magnesium Matrix Composite

The thixo-forging of magnesium matrix composite was analyzed with computer numerical simulation based on rigid viscoplastic finite element method. The constitutive model of SiCp/AZ61 composite was established in our prior literature. Behavior of metal flow and temperature field were obtained. The differences between traditional forging and thixo-forging processes were analyzed. Results indicated that thixo-forging was better in filling cavity than forging. Simulation results were good agreement with experimental ones.

Numerical Simulation of Microstructure Evolution of Hot Forging

The paper focuses on the research of using the coupled thermo mechanical analysis method of rigid viscoplastic finite element and the evolution of dynamic recrystallization to establish the calculation model coupling factors of deformation, heat conduction, dynamic recrystallization in the DEFORM, and on the hot-forging simulation of AISI4140. Through the simulation, it makes a valid prediction of dynamic recrystallization volume fraction and the grain size of recrystallized during the metal hot-forging process.

High Strength Bolt Manufacturing of Ultra-Fine Grained Aluminium Alloy 6061

Bulk nanostructure of commercially available aluminum alloy of AA6061 was made by an equal channel angular extrusion (ECAE). The effect of the ECAE routes of A, Bc and C on change of microstructure was investigated up to four passes by using the split die set-up. Formability of the ultra-fine grained specimens processed by the ECAE was measured by a compression test. In this test, the specimen made of the conventional material did not buckle when the aspect ratio of the height to the diameter (4 mm) of the specimen was 2.1 but the ultra-fine grained specimens with the same aspect ratio buckled. Considering the limiting value of the aspect ratio obtained from the compression test, a bolt forming sequence was developed into three stages. Load requirement and possible defect formation during the forming process were predicted by employing a rigid-viscoplastic finite element analysis. After manufacturing the high strength bolts using the ultra-fine grained specimens, strength increase was confirmed by the tension test and transmission electron microscopy. Homogeneity of strength increase was also examined by measuring local hardness distribution. In addition, the manufactured bolt with the ultra-fine grained AA6061 was compared to the one manufactured with commercially available AA2024 in terms of strength increase and its homogeneity. According to the present investigation, an innovative approach to utilize the ECAE to manufacture the high strength bolt using the conventional material without applying additional alloying elements or heat treatment is demonstrated. [doi:10.2320/matertrans.M2010309]

An inverse thermal–mechanical analysis of the hot torsion test for calibrating the constitutive parameters

One of the key tasks for mathematical representation of a constitutive model is calibration of its parameters. An inverse computational–experimental solution with the data modelling approach was employed here to estimate the parameters of a hyperbolic sine type constitutive equation. The solution utilizes a multi-layer constitutive model. The computational component of the inverse solution includes a rigid viscoplastic finite element code based on the thermo-mechanical coupling. The hot torsion test data comprises the experimental component of the inverse solution. Determination of the primary constitutive parameters (PCPs) pertinent to a 303 Austenitic stainless steel is presented here as an example. In order to facilitate the calibration of the sub-models, a procedure to provide an initial guess vector for the secondary constitutive parameters (SCPs) is given.

Study on forming defects in the rolling process of large aluminum alloy ring via adaptive controlled simulation

According to the distribution characteristics of equivalent plastic strain (PEEQ) in radial–axial ring rolling, the plastic deformation zones in cross section were established. A 3D rigid-viscoplastic finite element model (FEM) which was controlled adaptively was applied to investigate defects that occurred during ring rolling under ABAQUS software. PEEQ, stress, and temperature distributions in different deformation zones have been analyzed in this study. Strain peaks were found in the cross-section corners. Moreover, it was investigated that non-uniform strain, stress, and temperature distributions in the ring tend to cause non-uniform microstructure and properties. Therefore, forming defects and microstructure damage would appear in cross-section corners due to the high-strain deformation. Based on the new developed FEM of the radial–axial ring-rolling process and comprehensive numerical simulations, the size effects of feed rate and lubrication conditions on strain and temperature distributions and their uniformity were investigated by 3D coupled thermomechanical FE simulation. The results have good agreement with experiment. The achievements of this study can provide basis for quality control and technical guidance.

A study on chevron crack formation and evolution in a cold extrusion

A numerical algorithm based on the element deletion method and rigid-viscoplastic finite element approach depending on Cockcroft-Latham and specific plastic work fracture criteria was applied to predict formation and evolution of possible cracking in a cold extrusion of aluminum and steel alloys. The Cockcroft-Latham fracture criterion induced an internal crack while an external crack occurred owing to the specific plastic work criterion in simulations. As a result, the Cockcroft-Latham criterion was found to be valid for predicting chevron cracking in comparison with the experimental observation available in the literature. Using the Cockcroft-Latham criterion, cracking was carefully investigated in terms of the size of the crack and gap distance between cracks depending on the number of elements and boundary condition at the punch interface. The critical damage values for the Cockcroft-Latham fracture criterion were also calculated based on the tensile instability and fracture conditions to investigate their effect on possible cracking. Finally, a processing map based on the Cockcroft-Latham fracture criterion for preventing chevron cracking in the cold extrusion of commercially available steel alloy was developed by considering processing parameters such as reduction in area and semicone angle. According to this investigation, the developed element deletion method with the Cockcroft-Latham fracture criterion was reasonably accurate for carrying out chevron cracking analyses in the cold extrusion with proper selection of a critical damage value.

Three-dimensional finite element analysis of multi-stage hot forming of railway wheels

Three-dimensional finite element analyses has been carried out using DEFORM 3D software on multi-stage hot forming of railway wheels involving the processes of upsetting, forging, and punching of wheels. Thermal analysis related to heating the blank in furnace and all intermediate heat transfer stages between deforming operations have been conducted. Rigid viscoplastic finite element method has been utilized for coupled thermo-mechanical analysis of the processes. Modeling of punching the wheel bore has been carried out using Cockcroft and Latham fracture criterion. Evolution of thermo-mechanical parameters at selected points within the workpiece has been studied in detail. The method of simulating the effects of various process parameters has been explained using relevant mathematical relations. This study shows that design, optimization, and analysis of process perturbations for multi-stage railway wheel manufacturing process can be done efficiently in three-dimensional finite element simulations instead of conventional time and cost intensive trials. It might be necessary to use the results of finite element analysis in shop-floor to enhance productivity and reduce wheel rejection.

Process design of high-strength bolt of fully pearlitic high-carbon steel

In this study, fully pearlitic high-carbon steel, known as a higher delayed fracture strength material, with an ultimate tensile strength of 1410 MPa was used for manufacturing a high-strength M8 bolt. Softening behavior was observed by a compression test of the material. The tensile strength of the material was measured after application of several drawings. On the basis of these test results, two multi-stage manufacturing processes were designed by a knowledge-based expert system in order to develop a high-strength bolt using either the upsetting mode or the extrusion mode for the second preform. The load requirement and possible defect formation during the process were predicted by employing a rigid-viscoplastic finite element analysis. When the second preform was made in a laboratory experiment according to the process design on the basis of the upsetting mode, the specimens cracked at the flange in the final stage of forming. However, the bolt was successfully manufactured using the second preform obtained using the extrusion mode and showed extended tool life in the laboratory test. Finally, the manufactured bolt was confirmed via experiments to have tensile strength of 1600 MPa and delayed fracture strength.

Numerical Simulation for Effects of Friction on Deformation Behaviors in a 3-Dimensional Hot Upsetting Process

The friction between the die and workpiece plays an important role in determining the quality of the finished products during the hot deformation. Based on the experimental results, a rigid-viscoplastic finite element model was made to study the effects of friction conditions on the strain distribution and microstructural evolution in high strength low alloy steel during hot upsetting process. Also, the effects of friction conditions on the forging loadings and shape geometry of the deformed specimen were also investigated. The results show that: (1) the deformation of specimen is inhomogeneous, and the degree of deformation inhomogeneity decreases with the increase of friction coefficient; (2) the dynamic recrystallization volume fraction decreases with the increase of friction coefficient; (3) the distribution of the average grain size is inhomogeneous in the deformed specimen, and the average grain size increase as the friction coefficient is increased; (5) the effects of friction conditions on the peak forging loading and shape geometry of the deformed specimen are significant.

Finite element simulation of stretch forging using a mesh condensation method

In order to reduce the computation time of finite element simulations of stretch forging process, a mesh condensation method is presented and applied to a three-dimensional rigid-viscoplastic finite element program. In this method, a conventional mesh for the whole zone of a workpiece is condensed to a computational mesh for the active deformation zone. Two vital problems are solved, which are automatic construction of the computational mesh and treatment of interfaces between the deformation zone and the rigid zone. The mesh condensation method is compared with conventional finite element method by simulations of a six-bite stretch forging process. Some simulation results including forging load, temperature distribution and effective strain distribution are illustrated. The efficiency and accuracy of this method are verified.

Numerical Simulation of Semi-Solid AZ91D Magnesium Alloy in Thixoforming Process

In the process of metal plastic forming severe plastic deformations occur to the workpiece. As a result the constitutive relations between material and geometry are nonlinear. One of the most common tools to solve such problems is the finite element method. In thixoforming processes of semi-solid metal many factors affect thixoforming fluidity. Therefore it is important to simplify the problem and predict the metal flow properties. For that reason the thixoforming process of semi-solid AZ91D magnesium alloy was modeled with the theory of rigid viscoplastic finite element method using the commercial finite element software DEFORM-3DTM. The fluid and effective stress-strain fields in the thixoforming process were obtained and the relationships among stress, strain rate and temperature were analyzed.

Development of severe plastic deformation by various asymmetric rolling processes

Rolling is the most appropriate process to produce a severe plastic deformation in the sheet or plate type of materials. Compared to symmetric rolling, asymmetric rolling is more efficient in producing plastic deformation since it develops additional shear strain for a given reduction in thickness. In the present investigation, various asymmetric rolling processes were analyzed by the rigid-viscoplastic finite element method. The results of the analyses were compared to each other in terms of plastic deformation: effects of asymmetries due to the differences in size, rotational speed and frictional condition between upper and lower rolls. Rolling pressure distribution, rolling force and rolling torque were also compared.

Coupling Thermomechanical and Microstructural FE Analysis in Plate Rolling Process

Using three-dimensional rigid-viscoplastic finite element method (FEM), a coupling multivariable numerical simulation model for steel plate rolling has been established based on the physical metallurgy microstructural evolution rule and experiential equations. The effects of reduction, deformation temperature, and rolling speed on the deformation parameters and microstructure in plate rolling were investigated using the model. After a typical rolling process of steel plate 16Mn is simulated, the strain, temperature, and microstructure distributions are presented, as well as the ferrite grain transformation during the period of cooling. By comparing the calculated ferrite grain sizes with measured ones, the model is validated.

Two-dimensional finite element approximation of the hexagonal bolt forming process

In cold forging industry, it is well known that deformation mechanism of the billet and forming tools should be known during forging to properly design the dies and estimate the tool life. Since three-dimensional simulations of forging the geometrically complex forged parts involve with large computation time and effort, two-dimensional (2D) axi-symmetric approximation might be beneficial for an initial die design and selection of the machine capacity and die material. In the present study such an approximation was made for the two-stage hexagonal bolt forming process of two different carbon steels for comparative study. For simulations, in-house rigid-viscoplastic finite element (FE) programs were used for the deformation analysis with 2D and 3D approaches with hexahedral and tetrahedral elements to compare computation time, load requirement, distributions of the effective strain and stress, and interface boundary die force among three cases. The elastic analysis of tools was also carried out for comparison. It was found out that the load values and strain distributions were similar between 2D and 3D simulation results. However, elastic stress distributions in lower dies for 2D and 3D cases showed a relatively reasonable difference incurring the variation of tool life. For 3D simulations hexahedral element was more effective under the present investigation condition.

Predicting polygonal-shaped defects during hot ring rolling using a rigid-viscoplastic finite element method

We applied a rigid-viscoplastic finite element method to investigate polygonal-shaped defects that occur during ring rolling and developed an improved analysis model with relatively fine finite elements near the roll gap to reduce the computational time, along with a scheme to minimize the volume change. We also simulated the formation of a polygonal-shaped defect during hot ring rolling of a ball bearing outer race. The results were in good qualitative agreement with actual experimental phenomena.

계층적 접촉 탐색방법을 이용한 3-D 초소성 성형/확산접합의 공정설계(I)

Superplastic forming/diffusion bonding (SPF/DB) processes were analyzed using a 3-D rigid visco-plastic finite element method. A constant-triangular element based on membrane approximation and an incremental theory of plasticity are employed for the formulation. The coulomb friction law is used for interface friction between tool and material. Pressure-time relationship for a given optimal strain rate is calculated by stress and pressure values at the previous iteration step. In order to improve the contact searching, hierarchical search algorithm has been applied and implemented into the code. Various geometries including sandwich panel and 3 sheet shape for 3-D SPF/DB model are analyzed using the developed program. The validity fer the analysis is verified by comparison between analysis and results in the literature.

Finite Element Simulation of Hot Strip Continuous Rolling Process Coupling Microstructural Evolution

Using the nonlinear rigid-viscoplastic finite element method (FEM), a finite element simulation of the hot strip continuous rolling process was done, which completely integrates different phenomena such as the metallurgical behavior of the strip and the thermo-mechanics in the strip based on the physical metallurgical microstructural evolution law. By combining with the process parameters of certain 2 050 mm hot strip rolling, an actual rolling process of low carbon steel SS400 was simulated using the FEM model. Based on the simulation results, the distributions of the strain field, the temperature field, and the microstructure were presented. Meanwhile, the simulated rolling force, temperature, and microstructure are in good agreement with the measured results.

Research on deformation and microstructure evolution during forging of large-scale parts

Based on the rigid-viscoplastic finite element method (FEM), an investigation on large-scale forging parts deformation was carried out with the mode of twist-compression deformation adopted in this study. The processing model coupled thermal, mechanical and microstructural phenomena, in which some empirical equations for recrystallization evolutions were obtained using the Gleeble-3500 thermomechanical simulator. Through FEM simulation, the distributions of different field-variables, such as force, strain and grain size and so on, were obtained. After different forging parameters were employed, one can know that twist-compression deformation style can decrease the deforming pressure effectively and make deformation more homogeneous through analyzing these results. And it is worth mentioning that compared with upsetting this changed forming style makes grain sizes in the metal inner decrease and their distributions more uniform. Thus, the comprehensive properties of large-scale forging parts will be improved.

6시그마 기법과 컴퓨터 시뮬레이션 기술을 이용한 금속유동선도를 고려한 열간 단조공정의 최적화

In this paper, the six sigma scheme together with the rigid-viscoplastic finite element method is employed to obtain the optimal metal flow lines of a hot forging according to the six sigma processes, i.e., five steps such as define, measure, analyze, improve and control. Each step is investigated in detail to meet customer's requirements through improvement of product quality. A forging simulator is used for analysis of the metal flow lines of the hot forging, manufactured by a hot press forging machine, under various conditions of major factors determined at each step. The analyzed results are examined in order to reveal the effects of major factors on the metal flow lines and the formed shapes. The effects are then used to find an optimal process and the optimal process with die is devised and tested. The comparison between the required metal flow lines and the experiments shows that the approach is effective for optimal process design in hot forging considering metal flow lines.

A coupling thermal mechanical and microstructural FE model for hot strip continuous rolling process and verification

Based on physical metallurgy rules and experiential equations, a module for calculating microstructural evolution in the metal hot-forming process has been developed in this article. Meanwhile, with practical technologies of some plant integrated, a coupling multi-variable simulation model for hot strip continuous rolling (HSCR) has been built using non-linear rigid-viscoplastic finite element method (FEM). After an actual 2050 mm HSCR process on JIS-SS400 was simulated using the above model, evolution laws of strain, temperature and microstructure were analyzed. Simulated results of rolling force, temperature and microstructure have a good agreement with the measured values.