Materials Science Forum Vols. 773-774

Abstract: Computer simulation/optimization is widely applied to hot forming processes nowadays. A crucial point of any kind of computer simulation is the input data. There are implemented in all FEM codes some basic material database, but usually for limited materials and material states. As the input data decide about the output results quality, it is always necessary to pay attention, what are the simulation inputs. The best solution, but not always possible, is direct measurement of materials parameters for considered material and state. As this is in some cases not possible, materials data are searched in alternative ways. One possibility is survey of published data in literature. A reliability of published data is difficult to assess, as there are usually not available sufficient information about the material investigated in publications. Other possibility how to obtain material data nowadays is utilization of programs predicting material behavior on the basis of chemical composition. One of currently available software for material properties prediction is JMatPro. The program is able to calculate a broad range of properties and behavior with the use of established thermodynamics models on the basis of chemical composition and the initial state of the considered material state. There can be for example determined thermophysical and physical properties (from room temperature to the liquid state), time-temperature-transformation/ continuous-cooling transformation diagrams, stress/strain diagrams, proof and tensile stress, hardness, coarsening of γ and γ , and creep. The current paper is dealing with the simulation of hot forming of steam blade made of Ti-based alloy. As an input material data used for FEM simulation are conventionally measured data and data obtained from JMatPro. With the use of both material data sets, a sample component forming is simulated. The results obtained from both simulations are subsequently compared with the results obtained from physical forming of the sample component. Performance of both simulations is discussed here, taking into account results of physical forming results.

Abstract: The process parameters of aluminium alloy hot stamping produce an importantly effect on production forming quality. In the case of a door impact beam inside the car doors, the fi-nite-element model of aluminium alloy hot stamping is set up. Based on the model, the forming quality is investigated under usual process condition. Using the Latin hypercube method, we sampled the data points from design space of process parameters. Data points are imported into finite-element model to calculate the forming quality indices. According to their responding values, the quadratic response surfaces between process parameter inputs and forming quality indices are initialized. By optimized the response of the process parameters exercising multi-objective genetic algorithm—NSGA-II (non-dominated sorting genetic algorithm), the Pareto combinations of blank hold force and stamping velocity are obtained. Finally, by comparison with the results of stamping trial and numerical simula-tion, it is concluded that the finite-element model can be used to predict forming defects and is consistent with actual condition. Thus the optimization method is feasible.

Abstract: In this paper, to design the piercing punch for thick and uneven steel in piercing process, FE simulation technique has been proposed to predict the deformation behavior of CVJ cage through the simulation of the piercing process. In this research, an optimizing technique based on artificial neural network has been applied for the thick and uneven steel in piercing processes to construct database of the dimension precision depending on the shape of punch. It is indicated that shape of punch plays a significant role to keep the dimension precision and to increase the tool life in the uneven piercing process.

Abstract: An in-situ laboratory based X-ray diffraction technique has been developed to directly measure lattice strain and stress evolution associated with {10.2} < 10.1 > twin nucleation and growth in rolled and extruded Mg alloys during tensile loading. A transmission diffraction geometry was utilised to measure peak position and intensity for the (10.0), (00.2) and (10.1) lattice planes while the sample was loaded in uni-axial tension. Lattice re-orientation arising from deformation twinning is utilizedto estimate the twin volume fraction by measuring the increase in the (10.0) peak intensity along with a simultaneous decrease in the (00.2) peak intensity as a function of applied load. From observation of the lattice strain plotted against applied stress for different orientations it was found that the (10.1) orientation displayed the anticipated linear behaviour within the whole stress range. Yielding in the (10.2) and (10.3) orientations was identified at around  75 and 90 MPa respectively, indicating theonset of basal slip. Twin nucleation was observed at at a stress of approximately 110 MPa.

Abstract: Experimental and numerical investigations using Forming Limit Curve (FLC) and Forming Limit Stress Curve (FLSC) were carried out for two Advanced High Strength Steel (AHSS) grades DP780 and TRIP780. The forming limit curves were experimentally determined by means of Nakazima stretching test. Then, both FLC and FLSC were analytically calculated on the basis of the Marciniack-Kuczinsky (M-K) model. The yield criteria Barlat2000 (Yld2000-2d) were employed in combination with the Swift and modified Voce strain hardening laws to describe plastic flow behavior of the AHS steels. Hereby, influence of the constitutive models on the numerically determined FLCs and FLSCs were examined. Obviously, the forming limit curves predicted by the M-K model applying the Yld2000-2d yield criterion and Swift hardening law could fairly represent the experimental limit curves. The FLSCs resulted from the experimental data and theoretical model were also compared.

Abstract: This paper presents results of theoretical and experimental research works on metal forming process of a hub. A typical technology of forging on hammer of this part with flash was discussed. Two new processes of a hub forging were proposed, characterized by large material savings in comparison with typical technology. The first process is based on forming without flash of a forging with axial cavity. The second one is connected with forming of forging from pipe billet. The realization of these processes is possible at the application of a press with three movable working tools. Theoretical research works were done on the basis of simulations by means of finite element method. Simulations were made mainly in order to determine kinematics of material flow in forging processes and precision of shape and dimensions of obtained products. The first of the proposed processes was experimentally verified and a product of good quality was obtained. Material consumption of the analyzed processes and other factors acting on their effectiveness were also compared.

Abstract: Stainless steels are increasingly today applied in industrial use. The metastable structure of austenitic stainless steels enables strain induced martensite formation during plastic deformation. Thus, in order to effectively apply these steels in incremental sheet forming (ISF), it is essential to know their α-martensite transformation tendency in the process. For the four different austenitic stainless steels in the present study, the transformation was found to be very sensitive to the applied process parameters. The martensite formation was more profound with the unstable grades, however, with external heating the martensite formation could be diminished. By optimizing the ISF process, the amount of transformed martensite can be controlled and products with exceptional property combinations can be produced. The novelty of the present paper is to, first, provide information on the influence of strain induced martensite on the incremental forming process and product properties. In addition, based on the observations, propose means to control the transformation. Furthermore, the paper establishes that ISF favours a moderate rate of martensite transformation for extreme formability.

Abstract: Twin-roll strip casting is a concerned technology for economically producing magnesium alloys sheets. In this paper, numerical simulation of the twin-roll strip casting of an AZ61 magnesium alloy was carried out and the optimal process parameters were obtained. Then, under the conditions obtained through simulation, AZ61 strips of good surface quality were successfully manufactured. The microstructure of the alloy by twin-rolled strip casting is obvious refined compared with that by conventional casting.

Abstract: ncremental sheet forming (ISF) is a new promising technology due to its flexibility and low-cost tooling properties compared with conventional forming processes. However, it is only suitable for small-batch production because of its incremental feature inducing relative long forming time. Presently, widespread usage of the process is restricted by a lack of predictive understanding of the process due to its complexity. In this paper, the aspect of forming time is studied by investigating the effects of four distinctive process parameters (step over, feed rate, sheet thickness and tool diameter). An effective analysis tool, Taguchi method together with design of experiment (DOE) and analysis of variance (ANVOA) is utilized to study the effects of the four process parameters on forming time and further to optimize parameter combinations in order to minimize forming time. Using these techniques, experimental results show that the step over of spiral tool path is the most important process parameter affecting forming time followed by feed rate. Sheet thickness and tool diameter have little effect on forming time. The comparison between the prediction of optimized parameter combination and the confirmation test result has further demonstrated the effectiveness of the proposed method. It is worth noting that the results of this study will indicate a further direction on how to optimize process parameters to find a balance between forming efficiency (forming time) and forming quality (forming accuracy and surface roughness).

Abstract: Creep age forming (CAF) is a combined creep forming and age hardening treatment process. How to control the springback after forming is one of the key problems for the process. In this paper, Creep tests were conducted for different stress levels at 160°C for 25h, which is the suitable parameters for CAF process of aluminum alloy 7050T451. Based on experimental results, a set of mechanism-based creep constitutive equations was formulated. The six material constants of the constitutive equations were determined by non-linear least squares fitting methods. The creep age forming process for aluminum alloy 7050T451 plate was simulated by using FE software ABAQUS through the subroutine CREEP. The effects of the forming parameters on the springback were analyzed. Finally, experimental research was performed. It is found that the developed numerical simulation can be used to simulate the whole process of creep age forming process. The maximum relative error is 6.9%.