Evaluation of strain and stress states in the single point incremental forming process

Single point incremental forming (SPIF) is a promising manufacturing process suitable for small batch production. Furthermore, the material formability is enhanced in comparison with the conventional sheet metal forming processes, resulting from the small plastic zone and the incremental nature. Nevertheless, the further development of the SPIF process requires the full understanding of the material deformation mechanism, which is of great importance for the effective process optimization. In this study, a comprehensive finite element model has been developed to analyse the state of strain and stress in the vicinity of the contact area, where the plastic deformation increases by means of the forming tool action. The numerical model is firstly validated with experimental results from a simple truncated cone of AA7075-O aluminium alloy, namely, the forming force evolution, the final thickness and the plastic strain distributions. In order to evaluate accurately the through-thickness gradients, the blank is modelled with solid finite elements. The small contact area between the forming tool and the sheet produces a negative mean stress under the tool, postponing the ductile fracture occurrence. On the other hand, the residual stresses in both circumferential and meridional directions are positive in the inner skin of the cone and negative in the outer skin. They arise predominantly along the circumferential direction due to the geometrical restrictions in this direction.The authors would like to gratefully acknowledge the financial support from the Portuguese Foundation for Science and Technology (FCT) under project PTDC/EMS-TEC/1805/2012. The first author is also grateful to the FCT for the postdoctoral grant SFRH/BPD/101334/2014.info:eu-repo/semantics/publishedVersio

Force reduction in bending of thick steel plates by localized preheating

A trend towards higher tonnage press brakes, suitable for processing thick plates, can be observed. In this paper a dynamic method for preheating of thick steel plates, limited to the bending area, is presented, facilitating bending with a significantly reduced total force. The dynamic heating method thus allows to substantially extend the process window of press brakes. The method was experimentally tested on low carbon steel plates, characterised by a relatively high required temperature rise in order to achieve a favourable yield behaviour. A prototype setup developed for this purpose is described. Reported test results include achievable temperature distributions, force measurements as a function of temperature and spring back measurements. A derived simulation model allows to extrapolate the results to higher thickness plates.status: publishe

Force Analysis for single point incremental forming

Single point incremental forming is a new forming process for small batch production and rapid prototyping of sheet metal parts. The forces induced during single point incremental forming are unknown process parameters the magnitude of which is of importance for the development of process models. The purpose of the current study is to develop a method for predicting forces for complex part geometries based on the measured forces for parts with uniform wall angles. Firstly, a set of experiments, planned applying techniques for design of experiments, was performed with the aim of establishing a relation between the forces and the main process parameters for uniform angle parts. A regression equation of the forces was then obtained in terms of four parameters: the vertical step size between consecutive contours, the diameter of the tool, the wall angle of the part and the thickness of the sheet metal. In a second step, a technique of assembling the force curves obtained from the regression equation was employed to predict the forces for other shapes with varying wall angles.status: publishe

Some considerations on force trends in incremental forming of different materials

Today, incremental Forming challenges are mainly related to formability limits and precision. High achievable strain levels, together with the possibility to form complex shapes without need for dedicated dies, probably represent the main process advantages. However the attention on material formability is always very relevant. Taking into account both the formability and the process accuracy, the knowledge of the forces generated between the punch and the clamped sheet supplies strategic information to the analyst. In fact, the force level is not only relevant for the equipment deflection but also influences the precision. In fact, in previous publications the authors demonstrated that there is a strict correlation between the force trend and the material failure approaching. In this paper, a broader analysis on AA1050-O, AA3003-O and DC04 drawing steel is carried out, highlighting the force trends depending on the process parameters and the relationship with formability limits. © 2007 American Institute of Physics.status: publishe

Comparison of the tests chosen for material parameter identification to predict single point incremental forming forces

peer reviewedSingle Point Incremental Forming is a sheet forming process that uses a smooth-ended tool following a specific tool path and thus eliminates the need for dedicated die sets. Using this method, the material can reach a very high deformation level. A wide variety of shapes can be obtained without specific and costly equipment. To be able to optimize the process, a model and its material parameters are required. The inverse method has been used to provide material data by modeling experiments directly performed on a SPIF set-up and comparing them to the experimental measurements. The tests chosen for this study can generate heterogeneous stress and strain fields. They are performed with the production machine itself and are appropriate for the inverse method since their simulation times are not too high

A method for force reduction in heavy duty bending

A trend towards thicker and larger sheet metal parts can be observed. This leads to the construction of huge press brakes. A method to reduce the bending force is to preheat locally and quickly the bending area of the thick steel plate. For this purpose a dynamic contact method has been chosen: the steel is heated by being put in contact with preheated 'contact elements'. This method was experimentally tested on low carbon steel plates with a prototype setup. Reported results of the tests include achievable temperature distributions, and associated bending force. An achievable press force reduction of over 50% was demonstrated. The proposed preheating process has been simulated using the Finite Element method. Referring to the measured temperature evolutions, an appropriate value for the heat transfer coefficient could be determined. This allowed simulating the process for other combinations of plate thickness and contact element size. Copyright © 2008 Inderscience Enterprises Ltd.status: publishe