Stand offʹs indirect estimation in GMAW

This work was aimed at developing a methodology for the estimation of the contact tip-to-workpiece distance (stand off) in the gas metal arc welding (GMAW) process. The proposed methodology was based on the measurement of the electrical resistance between the contact tip and the workpiece during the short circuits in the dip transfer mode of metal transfer (dip resistance). The time periods in which the short-circuits were occurring, were identified based on the positive cycles of the time derivative of the welding current signal, together with the analysis of the welding voltage. The experimental part of this work was carried out in three basic parts: (a) determination of the working envelop for the welding parameters; (b) acquisition of welding data from controlled welding trials and the development of multiple regression models; and (c) validation of the chosen model. The weld beads were laid using an AWS A5.18/79 welding wire of 0.8 mm diameter, C25 shielding gas (Ar + 25% CO2) on 1/4 in. (6.25 mm) mild steel plate workpieces (ABNT 1020). A multiple linear regression model was developed having as independent variables the wire feed speed, the average welding current (measured on-line), the average dip-resistance (measured on-line) and some second order interactions between these variables, and the predicted stand-off as the dependant variable. The standoff obtained from the validation weld runs were in good agreement with the predicted values. Errors around 4% were observed.