Abstract:
The feasibility of fully automatic GMAW processes may rely on the
development of sophisticated equipment to emulate the manual welding
torch oscillation pattern or on the development of high level methods of
control to prevent the appearance of defects, especially the lack of sidewall
fusion. An intermediate solution is to optimise the weaving
parameters of a conventional pattern oscillator in such a way as to
minimise the level of rejection.
A prototype of a computerised system to work with Pulsed-GMAW
equipment, in the vertical-up position, was proposed to produce a minimal
level of rejection for welds in plates up to 25 mm thick. The system
basically consists of optimised mode control algorithms, based on
theoretical and experimental models of weld pool behaviour. Three tasks
are performed by the system; the selection of parameters for an optimum
working point, an off-line simulation of the operation and real-time error
monitoring of the process.
Statistical experimental modelling was applied in order to build
most of the optimised models, because of the large number of variables
to be treated and their complex inter-correlation. The welding variables
were correlated with single responses. Partial and Correlation Analysis
techniques were used to discover the relationship between the variables
and the responses. Regression Analysis was then applied as a means of
obtaining the 'weight' of the most significant variables. Finally, since
some variables were found to be collinear, a corrective technique for
biased variables was employed.
Acceptance criteria for bead shapes were proposed and assessed.
The effect of the oscillation parameters and other welding variables on
the bead formation was analyzed and an operational 'envelope' for the
parameters determined. A theoretical approach to predict the occurrence
of poorly shaped beads, due to the lack of metal bridge between the joint
walls, was successfully developed and applied in parallel with the
statistical experimental methods. Equations for optimising the bead shape
and for determining the operational envelope contours were subsequently
generated and evaluated.
An extension of the system to an actual adaptive control scheme
was discussed and sensors and signals to be used were evaluated. Finally,
a process instability phenomenon in long test plates was identified and
investigated. This instability may prevent the use of GMA W in some
conditions in the vertical-up position.