Pool oscillations and cast variations - penetration control for orbital tig welding of austenitic stainless steel tubing.

摘要

Pool oscillations in tungsten inert gas welding pools have been used in a closed-loop control system for orbital welding of ultra high purity tubing, determining atarget level of penetration by altering the welding current in real-time. Thetechnique is ideally suited to this application since it is does not contravene thecleanliness requirements for the inner bore and can be implemented outside thesmall orbital heads that are commonly used. The results presented in this thesisshow how clear pool oscillation signals in extremely small molten pools can bemonitored by optimising the welding conditions and signal processing of the arcvoltage signal. As an indicator of the likely variation in cast behaviour presentparticularly in austenitic stainless steels, a 'time-to-penetrate' characterisation wasmade of the materials, using the time of the transition from the Mode 1 to theMode 3 oscillation behaviour as the measured variable. By applying the testacross a range of welding currents, significant insight was obtained into the castand associated penetration behaviour. Late transitions indicated casts thatexhibited significantly different responses to the more usually applied weldingprocedures, especially at the lower levels of welding current (highlighting theirpotentially more problematic penetration behaviour). It was shown that theestablished theoretical models were difficult to apply with certainty to movingweld pools, and consequently a fuzzy logic model was used in the controlstrategy. The closed-loop system comprised a user-interface PC, a control rackand commercial welding power source - control signals were applied every 2 to3 Hz. Mode 3 pool oscillations were found to offer a more than satisfactorysensitivity to the inner bead width created for the various casts of 1.65 mm wallthickness materials studied.