Investigation of formability of material in incremental sheet metal forming process

ABSTRACT: Currently, Incremental Sheet Forming (ISF) is known as a novel technology, which promises a higher flexibility in process, lower production price and lead time, in addition to improved formability of material compared to conventional forming processes. The beneficial aspects of ISF are mainly ascribed to the essence of this technique that is progression of forming process by propagation of localized plastic deformation. However, this special characteristic concurrently induces the major imperfection of the process that is long processing time. Furthermore, this attribute of ISF along with nonlinear behavior of material, lead to complexity in simulation of the process. Accordingly, to achieve better understanding of the process and to extend its application over different industrial fields, a great deal of efforts has been focused on experimental investigation of ISF during the last decade. However, a few number of research work are reported to aim at optimization of the process by consideration of process parameters. Moreover, the interaction effects of participating parameters are mostly neglected by these studies. This thesis, taking an experimental-statistical approach, attempted to investigate the individual and interactional effect of predictor parameters, namely tool and step size, feed rate, spindle speed and blank thickness upon formability of material in ISF. To do so, a set of screening trials of 16 runs conducted prior to the main experimental campaign of 72 runs in order to realize the reasonable levels of aforesaid participating factors. The preliminary and the main tests were designed and later analyzed according to Two-Factorial and Optimal Design of Experiment (DOE) by application of scholarly accredited commercial software, Design Expert. For the sake of preciseness, a novel sensor system was developed and employed to detect the crack, once it appeared on the specimens. Based on the literature review and the knowledge of process acquired through this research, a new indicator proposed to effectively measure the formability of material in ISF. In addition, Individual and interactive effect of associated factors up on the new criterion were assessed by development of a quadratic and also modified cubic response surface models. More specifically, the positive effect of elevated spindle speed (2400 to 3000 rpm) on formability was highlighted in this study. However, feed rate (up to 5000 mm/min) was found to produce no significant effect, which means the process time can be remarkably reduced. Blank thickness was considered as a continuous numerical factor along with the rest of factors to provide an empirical model; hence it portrayed the greatest individual effect upon formability. Regarding the interaction effects, it was explored that the factors of spindle speed and blank thickness interdependently create the strongest effect on formability. At the next stage, interaction of the tool and step size was depicted to play a key role. Finally, the process was optimized in terms of maximum achievable formability, minimum processing time, and minimum sheet thickness. By optimized process parameters, it was demonstrated that approximately 96% of the maximum formability can be attained, using a moderate sheet thickness (1.26 mm), while the average forming time can be reduced to about 5.5 min. Maintaining the same criteria, but setting the spindle speed at zero, the most optimized situation revealed a reduction of about 29% in formability and an increase of about 62% in processing time. Keywords: Incremental sheet forming, formability, design of experiments, process parameters, individual effects, interaction effects, optimization. …………………………………………………………………………………………………………………………