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for predicting possibility of bad arc ignition and possibility of undercut were applied
directly, without modification.
In adapting the models, new welding trials were carried out using the
BDH550. The range of welding parameters used in the trials was selected to cover the
whole range of conditions normally used for gas metal arc welding thin sheet steel.
The voltage levels were chosen to produce a stable process in all the trials. Models
were then developed to map the behaviour of the BDH550, based on the set-up
welding parameters and on the statistical features (see equations 2.25 to 2.32)
extracted from each welding trial. The welding data was analysed using the multiple
regression analysis tools provided by Statgraphicss and the model structures proposed
by Ogunbiyi [ref. 51] were adopted. In order to reduce the number of experimental
trials, the welding speed was fixed at 500 mm/min.
Simple linear regression models were used to calibrate the leg length and weld
penetration prediction models developed by Ogunbiyi [ref. 51] so that they could fit
the corresponding measurements obtained in this work. This model calibration was
necessary' to compensate or account for differences in the heat sink provided by the
jigging system, the welding position and the welding power source. It should be noted
that Ogunbiyi's models were developed for horizontal-vertical fillet joints whereas in
this work the flat position was used. Due to the different welding positions and in
order to reduce the masking effect caused by bead misalignment, the calibration
functions were obtained considering the average values of the geometrical features of
interest (i. e. average leg length and average penetration).
The BDH550 models and the bead geometry calibration models (see section
6.1) were successfully used in the welding parameter generator. Although calibrated,
the penetration model was still imprecise, a fact which was also observed by Ogunbiyi
[ref. 51]. However, considering that the quality criteria for penetration are quite
flexible [refs. 107,190] and that penetration is normally assessed as either adequate or
inadequate, the output of the model was found to be acceptable.
The models developed were used to predict bead geometry and welding
parameters in both dip and spray metal transfer modes. Although the bead and the
penetration profile in both modes are essentially different, a single model was used to
map welds from both modes of metal transfer. This was due to the fact that it is
difficult to establish a limit above which a determined mode of metal transfer should
predominate and, in addition, it is sometimes beneficial to use mixed mode transfer.
8.3.2 Stand-off estimation models
The stand-off estimation model proposed by Ogunbiyi [ref. 51] (see equations
2.21 to 2.24) was initially implemented. Taking into consideration the differences in
power sources, a new welding current model was built for the BDH550 using the
model structure utilised by Ogunbiyi [ref. 51]. This model was a function of the power
source set-up voltage, the stand-off and the wire feed speed (see equation 6.4).
However, during the stand-off model (see equation 2.22) validation trials (see
5 PC software for statistical analysis
6A similar approach has been previously used by Doherty and Plummer (rcf. 1981 to calibrate a
welding rig.
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