Mechanics of Ultrasonic Tube Hydroforming
BUNGET, CRISTINA JANETA. Mechanics of Ultrasonic Tube Hydroforming. (Under the direction of Gracious Ngaile.)
Tube hydroforming is a manufacturing process which applies controlled internal pressure and axial feed to expand the tube to desired shapes. The main advantages are: part consolidation, weight reduction, fewer secondary operations, and tighter tolerances. However, it has disadvantages due to many variables, such as loading paths, material formability, and tribological conditions, which limit its applicability and influence parts failure (excessive thinning, wrinkling, buckling or bursting).
This research presents ultrasonic technology as a method of improving formability and tribological conditions. The superimposing of ultrasonic oscillations was already proved to have benefits for other metal forming processes, such as reduction in the forming load and frictional stresses. The objectives of this research work are to develop an analytical model to predict the state of stress and strain for the tube expansion under internal pressure and friction conditions, for ultrasonic and non-ultrasonic processes, observe the effects of the vibration on the deformation pattern, design a set of tooling and conduct experiments.
An analytical model was derived for both conventional and ultrasonic tube hydroforming processes, using equilibrium of forces, geometric relationships, material flow law and yield criterion. The square dies were chosen for this study, due to the simplicity of plane strain conditions. In conventional process the tube is expanded under internal pressure in the presence of friction. In the ultrasonic process, vibrations are imposed on the die, resulting in alternating gaps at the die/tube interface. The gaps open and close after each oscillation. Two different states of stress alternate during one oscillation in an element of the tube wall.
The analytical model was used to predict the internal pressure required, the corner radius, the thickness distribution, and the state of stress and strain. For the conventional process, the influence of some parameters on the deformation pattern and the forming load, such as strain hardening and friction conditions, was studied. Lower friction coefficient is required for more uniform tube wall thickness and lower pressure.
In the ultrasonic process, more uniform thickness distribution and state of stress and strain were predicted, as compared to the classical process. When the internal pressure is maintained, the corner radius obtained is smaller. The reduction in the corner radius was between 2.4 and 9%. More uniform thickness and less thinning, as well as smaller corner radius, indicate improvement of the formability of the material. If ultrasonic oscillations are used and the pressure exerted on the tube due to vibration is less than a critical value (equal to the internal pressure), there is a decrease in the maximum internal pressure needed for the same expansion. The most effective ultrasonic pressure is 0.1 MPa.
Finite element method was used to approximate the ultrasonic pressure and to design a set of tooling for ultrasonic process at 20 kHz. Four models were proposed and analyzed for three square die sizes. Modal analysis was used to observe the die vibration and the possible useful effects on the forming process. Harmonic response analyses were conducted to evaluate the amplitude of vibration in the deformation zone and the stress in the tooling. Based on the displacement distribution, a method of approximating the ultrasonic pressure was proposed. Most of the average pressure values were found to vary from 5 MPa to 25 MPa.
In order to observe the effects of ultrasonic oscillations on tube hydroforming, experiments were conducted with and without vibration. The ultrasonic tests resulted in smaller corner radii as compared to the conventional test, with a reduction of 5.2-7.7%, implying increase in forming capability due to vibration.
Advisor:Gracious Ngaile; Stefan Seelecke; Kara Peters; Michael Shearer
School:North Carolina State University
School Location:USA - North Carolina
Source Type:Master's Thesis
Date of Publication:12/05/2008