Investigation of Weld Bead Cooling Rate during Startup of Gas Metal Arc Welding

摘要

Most rejectable gas metal arc weld defects occur during weld initiation. Suchdefects are probably caused by nonequilibrium thermal conditions during weld startup which result in higher than normal weld bead cooling rates. This report describes experimental studies to investigate the feasibility of decreasing the initial weld bead cooling rate. A control model that allows independent setting of weld heat and mass input was used in combination with a proportional-integral controller. Startup and steady-state weld bead cooling rates have been established for a range of heat inputs. The controller was then used to delay welding torch movement for 4 s, and to apply a higher than normal heat input during the first 7 s of the weld. Heat input was then reduced to a predetermined lower value while maintaining constant mass input. The combination of delayed torch movement and 12.5% higher initial heat input reduced the weld bead cooling rate by at least 40% during weld startup. Present results confirm the feasibility of modifying the initial weld bead cooling rate by appropriate combinations of equipment operating parameters, but an extra heat source is needed to achieve an initial cooling rate equal to the subsequent steady-state rate. (jes)