Control of principal processing parameters for “defect suppression model” for fixed pin friction-stir-welding (FSW)

Abstract

Friction-Stir-Welding (FSW) involves many critical process variables that affect the quality of the weld; consequently, it is difficult to accurately predict the most favorable process parameters to suppress the defects and acquire high quality defect-free welds. Weld defects in friction stir (FS) welded 8.13mm thick aerospace grade Aluminum Alloy AA2219-T87 were correlated with weld process parameters from an assortment of different weld schedules. The welds were classified into three categories (hot, nominal, and cold welds) based on three critical welding parameters (plunge force, spindle rotational speed, and welding speed), non-destructively evaluated weld defects, and utilizing tensile test results. Underfill and small voids were found in hot welds. Internal cavities, i.e., wormholes (WH), surface cavities, i.e., trenching (TR) defects, and incomplete penetration (IP) were found in cold welds. Weld schedules comprising spindle rotational speed, welding speed, and plunge force can be linked to characteristic defects or to lack of defects (sound weld criterion). In the current investigation, a FSW process parameter window and an empirical correlation was established for effective joining of AA2219-T87. The developed empirical correlation (i.e., Empirical Force Index) relates three critical welding parameters and was found to successfully distinguish hot, nominal, and cold weld schedules. Strengths and defect structures of 85 friction-stir-butt welds with dimensions mentioned above were determined as three parameters, spindle rotational speed, feedrate, and plunge force, were varied. The data were analyzed in terms of a simplified physical model yielding temperature/plunge force, upper and lower bounds separating a nominal, defect-free parameter window from hot/high pressure defect and cold/low pressure defect regions. Defect suppression requires control of all three variables to maintain “defect-free” window conditions.