Synchrotron diffraction measurement of residual stresses in friction stir welded 5383-H321 aluminium butt joints and their modification by fatigue cycling

This paper presents two-dimensional information on the residual stresses in 8 mm 5383-H321 aluminium plates joined by double pass (DP) friction stir welding (FSW). It considers the inherent variability in residual stress magnitudes along 0.5 m lengths of weld pass, and their modification under a sequence of applied fatigue loads. This represents one of a planned series of experiments aimed at illuminating the effects of fatigue cycling on residual stress fields. In this particular case, the magnitudes of the bending fatigue loads (R = 0.1) were chosen to correlate with the measured proof strengths of the weld metal (approximately 160 MPa) and the parent plate (approximately 260-270 MPa). In four-point bend S-N tests at R = 0.1 on 40 mm wide FS welded specimens of this alloy and plate thickness, these peak stress levels correspond to lives of around 10 5 cycles and 10 7 cycles, respectively. Results from the work indicate that significant variability exists among welded plates in peak compressive stress magnitudes (a range of perhaps -50 MPa to -140 MPa), although peak tensile stresses were relatively low and more consistent (from around 0 to 30 MPa). Fatigue loading accentuates the peak-to-valley stress change and causes an overall translation of the stresses to become more positive. Peak tensile stresses increase several-fold during fatigue cycling.