Key Engineering Materials Vols. 622-623

Abstract: Forming is a manufacturing process by which the geometrical shape and size of sheet and plate metals are changed by means of either an external force using mechanical presses and dies or induced thermal stresses by external heat. This study reports on the finite element analysis of mechanically formed steel components using the ANSYS commercial package version 14.5. The samples of the steel sheets were mechanically formed to about 120 mm curvatures using a 20 ton capacity eccentric mechanical press at room temperature. The results showed that the steel samples were successfully formed to the curvature of about 120 mm and the finite element modelled results confirmed the experimental measured curvature. Key words: Forming, Mechanical presses, Finite Element Simulation and Sheets.

Abstract: The paper reports the results of experimental and theoretical work leading to the construction of a multiscale mathematical model describing the phenomena accompanying the steel deformation in semi-solid state as well as at extra-high temperatures. Conducted experiments and simulations confirms the need to seek new methods to obtain precise characteristics in the context of detailed computer simulations. The investigations presented in the current paper has shown, that temperature distribution inside the controlled semi-solid volume is strongly heterogeneous and non-uniform. Axial-symmetrical model (core of the old methodology) does not take into account all the physical phenomena accompanying the deformation. Finally, the error of the predicted strain-stress curves can still be improved. The proposed solution of the presented problem is application of both fully three-dimensional solution and more adequate solidification model taking into consideration evolution of forming steel microstructure. Contrary to the current model the new approach should allow to better capture the physical principles of semi-solid steel deformation in micro-scale. Additionally, the new complex methodology should allow to transfer the characteristics of the material behaviour between the micro-and macro-scale. As a consequence the final results should be more precise and accurate.

Abstract: Magnesium-calcium alloys with increased bio-compatibility are applied in medicine for sake of high compatibility and solubility in human body. Production of surgical threads to integration of tissue may be one of the applications of those types of alloys. A new manufacturing process of thin wires made of biocompatible Mg alloys, including drawing in heated dies, was developed in Authors previous works. Conducting drawing process in conditions, in which recrystallization occurs, is the basis of the process. This allows for multi-pass drawing without intermediate annealing. Control of recrystallization after every pass using experimental method is complex so numerical simulation seems to be a rational method to design the process parameters. The purpose of the paper is developing a mathematical model of recrystallization for MgCa08 alloy, its implementation into the finite element (FE) code that simulates wire drawing and experimental verification of the numerical calculations. The first part of work was focused on the development of mathematical model of wire drawing process of Mg alloys in heated die. Proposed model takes into account thermal phenomena in the wire and in the die, plastic flow of the material, stress-strain state and recrystallization. The fracture criterion was implemented into FE code to eliminate the possibility of damage. The second part of the work was focused on experiments including upsetting and tensile tests for calibration of recrystallization and fracture models. Recrystallization model was calibrated on the basis of flow curves only what is a limitation. Therefore, experimental wire drawing on drawing bench developed by the Authors was the final stage of the work performed to validate the model. Recrystallization during wire drawing was studied. The developed computer program enables prediction of the recrystallization kinetics during wire drawing in heated die for MgCa08 alloy. The model of static and dynamic recrystallization of this alloy and complex model of the drawing process were proposed in this work, as well.

Abstract: In this work, the formation mechanisms of surface defects in multistage cold forging of axisymmetrical parts have been studied through FEM simulations. As case history, the industrial production of an heating pipe fitting by cold forging has been analyzed. Based on simulated flow behaviour of material, several types of surface defects are identified and attributed to plastic instability of the work-material, inappropriate axial/radial flow ratio, excessive forming-pressure and uncorrect tooling design. The results of the FE model are finally compared with those obtained from real forging process and good agreement is observed.

Abstract: Prediction of life of compound die is an important activity usually carried out by highly experienced die designers in sheet metal industries. In this paper, research work involved in the prediction of life of compound die using artificial neural network (ANN) is presented. The parameters affecting life of compound die are investigated through FEM analysis and the critical simulation values are determined. Thereafter, an ANN model is developed using MATLAB. This ANN model is trained from FEM simulation results. The proposed ANN model is tested successfully on different compound dies designed for manufacturing sheet metal parts. A sample run of the proposed ANN model is also demonstrated in this paper.

Abstract: The friction caused power losses and bulging deformation are always inevitable. The rotating compression forming can reduce the compression force and bulging deformation in metal forming. Furthermore, the bounded double-layer metal material is able to have advantage in proper material cost saving and meeting the required strength for customer. Accordingly, this present study proposes the analysis based on Slab Method (SM) and Finite Element Method (FEM) under constant shear friction to predict the compression force and the rotating moment in the rotating compression forming. Moreover the bulging deformation profiles, the effective stress, the effective strain, the velocity field, the compression force, the rotating moment, and the twist angle can be obtained from FEM simulations. It is also shown that the predicted results have the same trend to verify the acceptance of analysis models.

Abstract: Ductile damage inadvertently exists in the steel during hot tension. The ductile damage during hot forming directly influences the mechanical properties of 25CrMo4 steel for high-speed railway axle. To investigate the grain growth/refinement rule and damage features of 25CrMo4 in hot forming, grain growth test and grain refinement test were conducted using the thermal mechanical simulator Gleeble-1500. In the grain growth test, the specimens were compressed to ensure that the initial austenitic grain size was small enough, then held at the deformation temperatures (1223K, 1273K, 1323K and 1373K) for 0min, 10min, 20min and 30min, respectively, to study the grain growth rule. In the grain refinement test, the specimens were stretched to different strain level at three temperatures (1313K, 1373K and 1433K) with two strain rates of 1.0/s and 10.0/s to study the grain refinement rule. The micro-voids and micro-cavities were found in tensile specimens during grain refinement test. Based on damage evolution mechanisms, damage constitutive equations are formulated to model the evolution of micro-voids and micro-cavities for 25CrMo4 under hot forming conditions. Partial experiment data were used to determine the material constants in damage constitutive equations by using the Genetic Algorithm (GA) method. To validate the model, the experimental data and computed curves of effective stress and grain size were compared. Close agreements were found between the experimental and prediction results. The developed viscoplastic damage equations are able to characterize the deformation behaviour of 25CrMo4 in hot tension process.

Abstract: During phase transformation of steels, when stress is applied, significant large strain can be observed even though the applied stress is smaller than their yield stresses. This phenomenon is called Transformation Plasticity or TRansformation Induced Plasticity (TRIP). Transformation plasticity is known to play an important role during steel producing processes. Although its importance, the phenomenon is not fully understood because of complicated coupled effect of metallurgical, thermal and mechanical behaviour during phase transformation. There are several explanations which account for the phenomenon. Among those, Greenwood-Johnson effect appears to be appropriate explanation especially for diffusive phase transformation. According to Greenwood-Johnson effect, volume change during phase transformation causes locally heterogeneous stress variation and it results in the macroscopic strain together with small applied stress. Along with the notion, Leblond et. al. developed an analytical model which describes well the phenomenon of transformation plasticity. On the other hand, the authors have developed a micromechanical model of polycrystalline materials using discrete FFT (Fast Fourier Transform) method with diffusive phase transformation. In this study, volume expansion along with phase transformation (Greenwood-Johnson effect) is taken into account in the model in order to evaluate the transformation plasticity and micromechanical behaviour during phase transformation. The results by FFT confirm linear relation between applied stress and transformation plastic strain, only if the applied stress does not exceed a half the value of yield stress of the parent phase. In contrast, if applied stress is relatively large (more than half of yield stress of weaker phase), the linear relation is never satisfied. The numerical results are compared with those of experimental and of Leblond model. Furthermore, pre-deformation (deformation just before phase transformation) effect on transformation plasticity is investigated. As a first step, uniaxial tensile followed by phase transformation simulation is carried out. Back stress develops in the course of tensile process and thus the material will be macroscopically anisotropic. It is found that the pre-deformation causes anisotropic dilatation during phase transformation. The mechanism of this anisotropic dilatation will be discussed in the micromechanical point of view.

Abstract: In this work the Warm Hydroforming (WHF) process for the production of a 6xxx series Al alloy component has been investigated using a numerical/experimental approach: both experimental and numerical hydroforming tests were carried out using the alloy AC170PX, a pre aged (T4 condition) Al alloy often adopted for automotive applications. In order to evaluate both the mechanical and strain behaviour of the material, tensile tests were carried out at different temperature and strain rate levels using the Gleeble system 3180, keeping also into account the ageing effect; in addition, formability (Nakazima) tests in warm conditions were performed by means of a specific equipment and the Forming Limit Curves at different temperature levels were evaluated according to the ISO standard 12004-2. Hydroforming experiments were carried out using a prototypal press machine specifically designed for WHF and SuperPlastic Forming tests. Such tests, scheduled by a DoE approach, were aimed at investigating the suitability of using the investigated Al alloy in the WHF process: attention was thus focused on those parameters mainly affecting the aging phenomenon (temperature, heating time and cycle time). In order to overcome the actual physical limitation of the hydroforming facilities, a Finite Element (FE) model of the WHF process was also created implementing experimental data (flow stress curves and FLCs) and tuned using data from preliminary WHF tests. In particular, after setting the Coefficient Of Friction (COF) according to temperature and verifying the robustness of numerical simulations, the FE model was used for investigating: (i) the influence of the Blank Holder Force (neglected in the experimental campaign); (ii) the adoption of quite smaller values of the parameter cycle time (being the aim to determine higher strain rates in the material). Through the definition of proper response variables (Flatness, Bursting Pressure and Thickness Ratio) both experimental and numerical results were analyzed by means of polynomial Response Surfaces in order to evaluate the optimal process conditions.

Abstract: Thin-walled tubes have always considered as energy absorption systems by researchers. This paper presents a new technique for energy absorption system which is simpler than other designs in production. This novel model is a thin-walled tube with perforation. During manufacturing process, equal numbers of holes are created in rows and columns in order to increase the energy absorption ability. In this article two different workpieces with the same geometry, one with holes and the other one with grooves, are compared to validate the model in accordance with other presented ones. For this purpose, specimens were modeled in finite element software ABAQUS with the same conditions and the amount of energy absorption, the initial decay, and the weight ratio of energy absorption (SEA) were evaluated. Then results which obtained from simulation are compared with experimental ones. Results confirmed that specimens with perforation have better decay symmetry rather than ones with grooves. In addition, force absorption in workpieces with hole is as twice as ones with grooves. The amounts of absorbed energy and SEA in workpieces with perforation are 56% and 46% more than workpieces with grooves, respectively.