Key Engineering Materials Vols. 410-411

Abstract: To study material behaviour under conditions encountered in ISF operations tensile tests have been carried out on material taken from the walls of pyramidal products. The shape of the stress-strain curves depend on orientation. Tests in the direction of punch movement show an overshoot indicating a change in strain path, tests across that direction do not. From this it is concluded that the major direction of deformation in the walls is perpendicular to the direction of punch movement. There is no indication of a severe deformation in the direction of punch movement, either stretch or shear. The level of hardening in the material is less than expected from the macroscopic changes in dimensions. Apparently the forming operation in ISF causes additional softening of the material

Abstract: Asymmetric incremental sheet forming (AISF) is a new sheet metal forming process in which sheet metal parts are produced by CNC-controlled movements of a simple ball-headed forming tool. Despite its flexibility and successful application in many cases, AISF has not yet been established in an industrial context due to some still existing process limits such as severe thinning, which strongly depends on the inclination of the part surface, as well as a limited geometric accuracy due to springback. Furthermore, there is little knowledge available about the properties of parts produced by AISF, especially in comparison to deep-drawn parts. The aim of the present paper is to compare cylindrical cups manufactured by deep-drawing and AISF regarding the resulting strain and thickness distribution. For AISF, different forming strategies were applied. Comparisons of the wall thickness and surface strain distributions show similar results for the cup produced by deep-drawing and the best cup produced by AISF, but the surface strains and the sheet thinning in the parts formed by AISF were larger than in the deep-drawn part.

Abstract: The main influence on the dimensional accuracy in incremental sheet metal forming results from the compliance of the involved machine structures and the springback effects of the workpiece. This holds especially for robot based sheet metal forming, as the stiffness of the robot’s kinematics compared to a conventional machine tool is low, resulting in a significant deviation of the planned tool path and therefore in a shape of insufficient quality. To predict these deviations, a coupled process structure model has been implemented. It consists of a finite element (FE) approach to simulate the sheet forming and a multi body system (MBS) modeling the compliant robot structure. The forces in the tool tip are computed by the FEA, while the path deviations due to these forces can be obtained using the MBS model. Coupling both models gives the true path driven by the robots. Built on this path prediction, mechanisms to compensate the robot’s kinematics can be implemented. The current paper describes an exemplary model based path prediction and its validation.

Abstract: The numerical simulation of the Single Point Incremental Forming process (SPIF) is time consuming due to the necessity to take into account various non-linearity such as the material behaviour, large strain deformation and the evolution of the tool-flange contact. Classical contact algorithms give good agreement with experimental results, but are time consuming. In this paper, we investigate the development of a procedure to simplify the management of the contact interface between the tool and the sheet. Nodes with imposed displacements are determined by a geometrical approximation of the deformed sheet. In order to have a better approximation of the local stresses in the flange, a pressure is applied on the tool side of the elements in the contact zone. The pressure value is obtained by an analytical model. A classical contact algorithm and the present simplified approach are compared in terms of an incremental forming benchmark. It has been shown that, for the benchmark problem studied here, a CPU time reduction of approximately 65% can be achieved while at the same time have good simulation results.

Abstract: Nowadays, several researchers all over the world on Incremental Sheet Forming are focused on formability analysis and, in particular, on the development of a model able to predict material response also in the design phase. Focalising the attention on this aspect and taking into account the literature review, it is possible to establish that both numerical and analytical approaches were utilised to justify the experimental evidences. Anyway, many efforts are spent in order to define new models to predict material breakage during incremental forming of simply shaped components. On the contrary, the availability of a predictive model able to detect failure insurgence, based on the products geometry, is a relevant issue to improve industrial application of incremental forming process. At the same time, it still represents an open point of the current scientific research. According to this aim, a preliminary analysis was executed to detect the factors that deeply impact on the process performance. In a subsequent analysis, a robust Neural Network was built in order to define a more reliable tool for calculating the allowable component height when a multi-slope trajectory is imposed. The reliability of the proposed approach was tested executing some experimental results. All the details are accurately discussed in the paper.

Abstract: This paper deals with Incremental Sheet Forming (ISF), a sheet metal forming process, that knew a wide development in the last years. It consists of a simple hemispherical tool that, moving along a defined path by means of either a CNC machine or a robot or a self designed device, locally deforms a metal sheet. A lot of experimental and simulative researches have been conducted in this field with different aims: to study the sheet formability and part feasibility as a function of the process parameters; to define models able to forecast the final sheet thickness as a function of the drawing angle and tool path strategy; to understand how the sheet deforms and how formability limits can be defined. Nowadays, a lot of these topics are still open. In this paper, the results obtained from an experimental campaign performed to study sheet formability and final part feasibility are reported. The ISF tests were conducted deforming FeP04 deep drawing steel sheet 0.8 mm thick and analyzing the influence of the tool path strategy and of the adopted ISF technique (Single Point Incremental Forming Vs. Two Points Incremental Forming). The part feasibility and formability were evaluated considering final sheet thickness, geometrical errors of the final part, maximum wall angle and depth at which the sheet breaks. Moreover, process forces measurements were carried out by means of a specific device developed by the Authors, allowing to obtain important information about the load acting on the deforming device and necessary for deforming sheet.

Abstract: Single point incremental forming (SPIF) is a promising new production technique in which a metal sheet is formed stepwise by a spherical tool. However, the technique still shows some particularities. It is observed that the final geometry of a SPIF part can deviate significantly from the programmed tool path. As illustrated in this paper, elastic springback is only to a minor extent responsible for this phenomenon. The goal of the presented paper is to illustrate the gradual emergence of unintended deviations as measured by means of a Digital Image Correlation (DIC) technique. Two CCD cameras were used to take the necessary in-process images. The mechanism of deformation in function of the forming depth is documented and discussed.

Abstract: Friction Stir Welding (FSW) is an energy efficient and environmentally "friendly" welding process. The parts are welded together in a solid-state joining process at a temperature below the melting point of the workpiece material under a combination of extruding and forging. This technology has been successfully used to join materials that are difficult-to-weld or ‘unweldable’ by fusion welding methods. In the paper a neural network was set up and trained in order to predict the final grain size in the transverse section of a FSW butt joint of aluminum alloys. What is more, due to the relationship between the extension of the “material zones” and the joint resistance, the AI tool was able to furnish indications for the design of the welding process. Experimental tests and subsequent microstructure observations were developed in order to verify the numerical predictions.

Abstract: The severe double curvatures often encountered in aerospace fuselage and nacelle skins often necessitate multistage stretch forming operations. Each stage adds value to the component and rejection, particularly at later stages, must be avoided. Therefore process optimisation using accurate modelling tools to predict the strain levels is essential. This paper presents a novel method for modelling the aforementioned stages of the process using a tailored material model based on strain history and heat treatment. The modelling process is implemented within the PAMSTAMP Finite Element code, by modifying the material properties in the input file using an Excel based algorithm. The proposed method is validated using profiled stretch formed specimens, thereby imposing a variable strain gradient in a single specimen without having to resort to the complexity of double curvature. The model results are shown to represent the observed behaviour well.

Abstract: An improved two-dimensional constitutive model for shape memory alloys (SMAs), which can describe both the shape memory effect (SME) and super elasticity effect (SE) of the SMAs, is developed in the paper based on the previous work of Boyd and Lagoudas, who used the thermodynamics theories of free energy and dissipation energy to derive the constitutive law of the SMAs. The improved model, which will combine the ideas of Brinsion’s one-dimensional constitutive law and the concepts of Boyd and Lagoudas’ two-dimensional one, has a simple but accurate expression. Two examples are used to numerically validate the efficiency of the improved model and the results of the simulations show that the developed constitutive model can qualitatively describe the thermo-mechanical behaviors of two-dimensional SMAs.