Experimental investigations on Ultrasonic Single Point Incremental Forming (USPIF)

Abstract

The Incremental sheet forming (ISF) process has gained the attention of researchers in the field of sheet metal forming due to the high formability achieved in the process and the capability to produce prototypes of new products at low cost and minimum lead time. The single point incremental forming (SPIF) and two point incremental forming (TPIF) are the main variants of the process. The research effort is going on to enhance the capabilities of the process so that it can be accepted as a commercial production process. In this thesis, first, the progress and recent development in the field of incremental sheet forming have been reviewed and the current status of the process has been recognized. Based on the literature review, the research gap has been identified and the research objectives have been framed. Ultrasonic vibration has found widespread engineering applications. The use of ultrasonic vibrations in the machining and welding processes has reached the commercially viable stage. However, the application of ultrasonic vibrations in sheet metal forming applications is still in the research phase. This thesis addresses the application of ultrasonic vibrations in the single point incremental forming process. The set-up has been developed for the ultrasonic vibration-assisted single point incremental forming (USPIF) process using the vertical machining centre. The fixture required to clamp the ultrasonic stack in the spindle of the VMC machine is developed in-house. The fixtures required to form various geometrical shaped components have also been developed in-house. Moreover, to achieve the longitudinal mode of vibration in the tool cone, its design is carried out by performing the modal analysis in the ANSYS software. In a single point incremental forming process, components are produced by moving the hemispherical ended tool along the predefined tool path. The single-stage tool path strategies are used to form the components but it leads to problems related to formability, geometric accuracy, and non-uniform thinning especially in the case of components having a high wall angle. A multi-stage tool path strategy can be used to circumvent the same, however, it leads to the formation of the stepped feature due to rigid body translation. In this thesis, the multi-stage tool path strategy has been proposed to circumvent the issue related to stepped feature formation for the hemispherical shaped component which is difficult to produce using a single-stage forming strategy due to 90° tool entry angle at the base. Moreover, the proposed strategy doesn’t require the calculation of rigid body translation and subsequent tool path modification. The conventional single-stage tool path strategy has been used to form other geometric shapes like straight groove, cone, and circular groove. The Excel VBA program has been developed to generate the NC part program for forming the v component on the VMC machine. In addition, the program incorporates the tool path strategy used for a particular shape and generates the Time vs. Position (X-, Y-, and Z-) file required for the FE simulation in LS-DYNA software. The proposed multi-stage forming strategy has been used in forming the hemispherical shaped component with the SPIF and the USPIF process. The enhancement in formability using the proposed multi-stage tool path strategy has been investigated in terms of depth of forming, angle of tool entry, uniform strain distribution, geometric accuracy, and thinning. The experiments have been carried out to study the stage wise evolution of the thickness and strain. The ultrasonic hardening effect is also confirmed by forming the hemispherical shaped component. The USPIF process being the latest development, is not completely established from the point of view of the underlying deformation mechanism and the effect of the process variables on the formability of the component. In this research work, the effect of amplitude of vibration, feed, and step-depth has been investigated by performing a straight groove test using a design of experiments method. The fracture forming line (FFL) for the SPIF process is also proposed based on the experimental method by forming the different shaped components. The empirical relationship for fracture forming line for the USPIF process is also determined and validated by performing the confirmatory experiment. Moreover, critical values of the amplitude of vibration and the ultrasonic intensity have been identified which demarks the ultrasonic softening zone and the ultrasonic hardening zone. Keywords: Incremental sheet forming, ISF, Single Point Incremental Forming, SPIF, Two Point Incremental Forming, TPIF, Formability, forming limit curve, FLC, Fracture Forming Line, FFL, Ultrasonic vibration, Tool cone, Longitudinal vibration, multi-stage forming, hammering frequency, Thinning band, Strain distribution, rigid body translation, protrusion defect, ultrasonic hardening, ultrasonic softening.