Influence of ultrasonics on upsetting of a model paste

Abstract

This paper describes a preliminary study of the influence of ultrasonics on the boundary conditions associated with the equipment walls in a soft solid forming operation using Plasticine as a material model. A detailed finite element analysis is described involving the upsetting of a cylindrical specimen between two parallel rigid dies with kinematics and ultrasonic oscillatory loading conditions. A series of squeeze flow experiments has been conducted to validate the finite element models. The oscillation parameters were measured using a 3D laser Doppler vibrometer to complement the measurement of reaction forces.

Introduction

Many benefits of applying high power ultrasonics in forming processes have been reported, and research demonstrating cost savings and process improvements has been underway for about 40 years [1], [2]. Previous studies [3], [4] have demonstrated the benefits of applying low frequency coaxial vibration of forming tools in an upsetting process using Plasticine as a model elasto-viscoplastic soft solid material. It was established that vibration assisted upsetting can result in a significant reduction in resistance of the forming material to deformation, by a combination of stress superposition and an attenuation in interfacial friction. However, vibration is more effectively applied to tooling when the system is resonant. Practically, this means applying ultrasonic vibration, at a frequency in the low ultrasonic range (20–40 kHz), via a series of tuned components, and it also involves the design of tuned tooling.

The influence of ultrasonic oscillation on friction was examined by many fundamental studies [5], [6]. It was reported that reduced frictional forces can be obtained when the oscillating direction is parallel to the direction of motion of the die. It has been postulated that beneficial changes in interfacial friction conditions are maximised at ultrasonic frequencies with little effect on the bulk characteristics of the workpiece [1], [2]. Many high power ultrasonic processes are characterised by significant localised rises in surface temperature with insignificant bulk heating of the specimens [1], [2]. Also, it has been shown that if the tools are heated in an upsetting process, hot material can act as a lubricant causing the colder bulk material to undergo biaxial extension accompanied by little or no shear, or bollarding behaviour can occur where the maximum velocity profile near the tool wall cause the diameter of specimen faces, which are in contact with the tools, to deform more than the diameter at the centre [7]. However, these effects have not been investigated for ultrasonically activated tools. This paper investigates the relationships between temperature rise during ultrasonic forming and changes in the friction coefficient.

This current study arises from the availability of modelling and experimental facilities that are now sufficient to define a new generation of ultrasonic technologies, which can be applied to the processing of soft solids. The initial results rely on the study of the fundamental benefits of ultrasonic oscillation assistance in the upsetting of a model soft solid and, in particular, the influence of oscillation on the interfacial friction boundary condition and the thermal effects during the process. Numerical results are presented to show the effects of variation in several parameters on the die–specimen interface for Plasticine. The rheological characteristics of Plasticine have been demonstrated to be typical of soft solids [8]. Plasticine is therefore adopted as a generic elasto-viscoplastic material in this paper.