Experimental and finite element investigation of ultrasonic metal forming applications

The research reported in this thesis demonstrates the effects of applying longitudinal and radial ultrasonic vibration to forming tools in metal forming processes using aluminium, In the study of ultrasonic vibration assisted tension tests, and ring and cylindrical specimen compression tests, finite element (FE) models under quasi-static loading were first developed to gain insight into material and interface mechanisms. FE simulations of the metal material included the effects of elasticity, plasticity, large deformation and a Coulombic interface friction boundary condition in the presence and absence of lubricants. Agreement was achieved between the FE results and those obtained from tension and compression experiments with respect to the stress-strain relationships and deformed configurations for a range of friction coefficients. The coefficients of friction for static and ultrasonic ring compressions were estimated for dry and lubricated surfaces by matching the experimental data with the FE results for quasi-static deformation in the form of friction calibration curves. Analysis of the surfaces of deformed ring specimens was conducted using two techniques; profilometry and scanning electron microscopy, and the experimental results were compared with the estimated friction condition. By measuring the oscillatory force response of the longitudinal mode ultrasonic loading as well as the static force, it was shown that the experimentally derived stress-strain data from tension and compression tests do not satisfy the description of a simple oscillatory stress superposition model. Subsequently FE models of tension and compression tests were developed, and a description of the contact friction condition was included for the compression test model. For ultrasonic tension tests, it was found that by superimposing ultrasonic excitation for an interval during plastic deformation, the measured stress- strain relationship could be closely matched by finite element model data where the model was adjusted to a softer material during the interval. For ultrasonic compression tests, by combining a softer material model description with a change in the coefficient of friction at the contact surface, only during the interval of ultrasonic excitation, the FE model predicted stress-strain data matched the experimentally derived stress-strain data. By superimposing a radial mode ultrasonic vibration on static compression tests, the experimental and FE results suggest that the stress-strain relationship is dominated by a reduction in interface friction, with little effect due to material softening. Finite element models of two metal forming processes were subsequently developed; extrusion of an aluminium solid cylinder and die necking of an aluminium hollow thin cylinder. The models were validated against experimental data reported in the literature. By developing the FE models it was possible to show how a reduction in the extrusion force can be achieved if ultrasonic excitation of the die results in a reduction in the coefficient of friction at the die-material interface. For the die necking operation, the FE model showed how a diameter reduction could be achieved without buckling of the hollow cylindrical body if a reduction in the coefficient of friction was modelled under ultrasonic excitation of the die. However, by comparing with the published experimental data, the calculated extrusion force reduction achieved by a change in coefficient of friction estimated from the FE model is not sufficient alone to explain the effects of ultrasonic excitation in extrusion. The experimental and FE models developed in this study have provided some insights into the contribution of reduced friction to measured reductions in forming force in ultrasonic forming processes. The evidence suggests that, as well as changes in the interface friction condition, material properties are also affected by ultrasonic excitation applied to forming aluminium