Incremental sheet forming (ISF) is a promising manufacturing process in which flat metal sheets are gradually formed into 3D shapes using a generic forming tool. Since the process features benefits of reduced forming forces, enhanced formability and greater process flexibility, it has a great potential to achieve economic payoff for rapid prototyping applications and for small quantity production in various applications. Although substantial research has been performed in the past decades on ISF, there is still a lack of an efficient prediction of forming force to facilitate the production design and optimization of the process. Also, unsatisfactory part quality obtained still hampers its wide use in industrial applications. Therefore, the work presented in this thesis is mainly focused on two aspects of ISF: efficient forming force prediction and process investigation and its improvement.
The first research aspect of this thesis has been focused on the development of an efficient force prediction model that includes the development of both finite element (FE) and analytical models. (i) FE modelling. Two types of FE models have been established using an explicit code LS-DYNA to investigate the deformation mechanism in ISF which is the basis for developing the force prediction model as well as further improving the forming process. First, FE models with shell elements for the groove forming process were set up and the strain behaviour and thickness distribution with different tools were evaluated and compared with experimental results. The strain behaviour of elements at different positions has been studied and the maximum strain has been found near the end of the groove corresponding to the failure location. The thickness distributions of the groove formed with different sized tools were predicted and it was found that the groove formed by the 20 mm tool is thinner than that formed by the 30 mm tool. Second, to further investigate different deformation modes and their evolution history in ISF, a FE model with fine solid elements for the cone-forming process has been established. The FE model was verified by experimental work with forming forces to allow a quantitative study of deformation behaviours of stretching, bending and shearing during the process. The evolution history of all the strain components along with the effective strain was presented. Moreover, the characteristic of each strain component and its contribution to the total effective plastic strain during the cone-forming process were investigated in detail. It was confirmed from FE simulations that the deformation mechanism in the ISF process is a combination of shearing, bending and stretching though the quantitative contributions in two directions are varied. (ii) Force prediction modelling and its validation. Based on the understanding of the deformation mechanism during the forming process, an efficient analytical model for tangential force prediction has been developed. Initially, deformation modes including shear, bending and stretching were analytically considered separately in the proposed sub-models.
A preliminary combined model has been constructed that can provide a prediction of tangential force with an average error less than 11 % with varying wall angle from 30˚ to 70˚. Subsequently, the force prediction model was further improved to balance the contribution of shear and bending on the total plastic deformation. The enhanced model was successfully validated through a comprehensive experimental campaign with two geometric shapes (truncated cone and pyramid) and various process parameters (step down, wall angle, tool radius and thickness). Finally, the analytical model was further extended to capture the changing of local curvature and wall angle during forming to deal with more complex shapes. The truncated ellipsoidal cup was selected as the target shape which has continuous local curvature and wall angle variations in each contour. It was verified that the extended model is able to provide a reasonably accurate variation of tangential force in each contour compared to experiments.
The second research aspect in this thesis is the process investigation and improvement of forming forces, geometric accuracy, forming efficiency and surface roughness. (i) Forming forces. Forming forces have been recorded for forming various shapes including straight groove, truncated cone, truncated pyramid and truncated ellipsoidal cup. The influences of different process parameters (i.e. wall angles, sheet thicknesses, step-down sizes, tool radius, tool-path types and sheet orientation) on forming forces were extensively studied. Particular attention has been paid to the relationship between converted tangential forces and forming parameters for the validation of the analytical model. The steady tangential forces at the second stage of the process demonstrate an increasing trend with the increase of step-down size and wall angle. However, tangential forces vary in a concave manner with the variation of tool diameter from 10 to 30 mm with a minimum occurring between 20-25mm. (ii) Geometric accuracy. The effects of various process parameters on geometric accuracy have been investigated by performing a Box-Behnken design of 27 tests that considers four factors at three levels. An empirical model has been developed with the most influential forming parameters and it was concluded that the geometric quality is largely determined by the quadratic effect of wall angle, the linear effect of sheet thickness and the interaction effect of thickness and step down. (iii) Forming efficiency. The effects of process parameters (step over (spiral tool path), feed rate, sheet thickness and tool diameter) on forming time have been studied through a Design of experiments (DOE) together with Taguchi method. Energy consumptions during the forming process were calculated based on measured forming forces. It was found that reducing sheet thickness, increasing step-down size with a limited range as well as decreasing the wall angle are effective approaches for energy saving. (iv) Surface roughness. The surface topography formed by both sliding and rolling tools were examined through SEM images which showed that a rolling contact condition causes less local damage and scratching of the surface.
Furthermore, an empirical model has been developed to predict the overall roughness of parts formed by ISF using the design of experiments (DOE) and it was found that sheet thickness has the most influential effect on overall surface roughness followed by the step down.
The work in this thesis explores various aspects of ISF research, although the most important contribution is efficient forming force prediction and its validation.