A method and apparatus for determination of the ultrasonic-assisted forming limit diagram
Nima Najafizadeh, Majid Rajabi, Ramin Hashemi, Saeid Amini
Abstract
It was recently demonstrated that ultrasonic vibrations could result in lower forming forces and a better surface finish in a wide range of metal forming processes. The Nakazima stretch-forming test is a long-established experimental procedure for the formability evaluation of sheet metals. Today, the development of a standard test is critical for assessing sheet metals’ formability enhancement due to ultrasonic vibrations, which can potentially unlock the material selection limit in various manufacturing applications. This study aims to present a method and apparatus for effectuating the forming limit diagram (FLD). At the same time, high-frequency ultrasonic vibrations are combined with the movement of the forming tool. Taking St14 steel as an example, the mechanical and microstructural properties such as formability, Micro-Vickers hardness, and grain sizes were systematically investigated. Furthermore, the conventional FLD, as well as the novel “Ultrasonic-Assisted Forming Limit Diagram” (UA-FLD), were attained and compared with a nonlinear regression-based approach. The results have indicated that superimposing ultrasonic vibrations with the amplitude of 15µm at the frequency of 20 kHz to the tool would cause a notable enhancement in forming limit diagram, a maximum of 28% increase in hardness, and a 23% reduction in average grain size of the specimens.