Year

2000

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunications Engineering

Abstract

The performance of the short-circuit gas metal arc welding (GMAW) process under the control of two current-controlled techniques is investigated in detail using a custom built, high performance power source. The objective of these techniques is to provide significant improvements in process control compared to that obtained by using conventional constant voltage power sources.

The first control technique, described as "open loop", has a fixed, preprogrammed response to key events within the process that define various states of the weld cycle. The response parameters are adjusted to suit the wire feed speed, welding travel speed and shielding gas mixture. Adaption to the requirements of the process occurs automatically, since the timing of the responses is determined solely by events at the weld. This technique produces stable, low-spatter welds with excellent bead appearance across a very wide range of wire feed speeds and contact tip to workpiece distance (CTWD values. Operation does not depend on synchronising the weld cycle frequency with the natural frequency of the weld pool. The technique also significantly decouples key welding parameters such as peak arc length, fusion area, and short-circuit metal transfer characteristics.

Following investigation of this technique, a new approach is proposed. This "closed loop" control aims to regulate the size of the droplet formed at the end of the electrode on a cycle-by-cycle basis. A model of the instantaneous melting rate is developed to predict droplet size. A novel method is used to estimate the CTWD using "through the arc" sensing techniques. The objectives of this technique are to increase process stability and to further reduce spatter. These aims were achieved under a limited set of operating conditions. The performance of this second technique is influenced by weld pool oscillation but the key benefit of parameter decoupling is retained.

The theoretical development of both techniques and experimental validation of their performance is detailed in this thesis.

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