| This study is an investigation of the effects of weld parameters and post weld heat treatment on 6xxx series friction stir welds. This study was originally desired by the sponsors in order to quantify the effects of post-weld natural aging time on attainable weld properties. Welds were produced in AA6019-T651, AA6056-T651, and AA6056-T451 at low and high advance per revolution resulting in low and high power welds with low and high peak temperatures, respectively. For AA6019, tool rotation speeds of 720 and 1680 rpm were used with a tool traverse speed of 10.16 mm s-1. For AA6056, tool rotation speeds of 960 and 1680 rpm were used with a tool traverse speed of 10.13 mm s -1. These welds were naturally aged for periods ranging from 24, to 1200 hours prior to artificial aging to peak strength. A wide range of tests and analyses were performed in order to characterize the welding processes and the weld properties attained. These included: weld response variable analysis, nugget grain size measurements, weld zone hardness mapping, transverse and longitudinal tensile tests using digital image correlation to determine full-field strain, fracture toughness measurement, fractography, and FEA modeling of weld mechanical response.;Results showed that, in general, natural aging time was not a significant factor in determining the mechanical properties of the weld. The effect of advance per revolution on weld nugget asymmetry was pronounced for the AA6019 welds and was evidenced by grain size mapping, hardness distributions, and the spatial distribution of response to post-weld aging. Taken together, the results confirm that there is a definite temperature gradient, decreasing from the advancing to retreating sides of the nugget, most likely due to the predicted strain and strain rate gradients. Based on review of the literature, evidence for such gradients is common in precipitation hardening aluminum alloys; however, the nugget asymmetry is only obvious if the solution heat treatment temperature is attained over only part of the nugget. |
