Residual stress effects on near-threshold fatigue crack growth in friction stir welds in aerospace alloys

Abstract

Friction stir welding (FSW) is being explored as a potential tool for manufacturing aluminum aerospace structures. Several joint configurations, butt, lap and fillet joints have been made in the production of such exploratory structures. Research work on S–N fatigue and fatigue crack growth in the weld zone is required to provide an understanding and tools to assess the damage tolerance issues in friction stir welded joints and structures. In this work, results of a study conducted on near-threshold fatigue crack growth in friction stir welded aluminum alloy 7050-T7451 and a titanium alloy Ti–6Al–4V are presented. Tests were conducted on weld coupons as a function of specimen geometry (compact tension, eccentrically loaded single edge and center-crack tension) and stress ratio to understand the effects of residual stresses in the heat affected zone (HAZ) of the alloy. Residual stresses were measured on samples machined from the friction stir welded plates prior to testing. The crack growth results show that residual stresses play a key role in the crack growth parallel to the weld-path in the HAZ. Although friction stir welding process induced low residual stresses in the welds, they are found to produce large effects on the near-threshold fatigue crack growth. In general, the magnitude of the shift in the fatigue threshold in the friction stir welded coupons is a function of microstructure, residual stresses and specimen geometry. However, for a constant microstructure, fatigue thresholds at low stress ratios, were specimen geometry dependent. The thresholds were either higher or lower than those of the parent material. At high stress ratio, the differences due to the specimen geometry vanish. Stress ratio studies show that the center-crack tension geometry is less sensitive to the residual stress effects compared to the compact tension geometry. Crack growth analysis using equivalent residual stresses was used to determine the residual stress intensity factor and predict stress ratio effects. The role of microstructure on the fatigue threshold in the welds is also discussed.