Residual Stress Distributions in AA6061 Material Produced by Additive Friction Stir Deposition

This paper provides the first description of residual stress distributions in a 66-mm-thick AA6061 deposit produced by additive friction stir deposition (AFSD). The AFSD process is being quickly developed as a solid-state additive manufacturing technique. Residual stresses in additively manufactured components can affect the susceptibility to fracture and fatigue, so there is a need to understand the residual stress distributions in the AFSD deposits. In this research, a large (194 × 49 × 66 mm) AA6061 deposit was produced using AFSD to study the residual stress distributions through its thickness. Three components of residual stresses were independently measured using neutron diffraction on the VULCAN instrument at Oak Ridge National Laboratory. At the starting end and the center region in the AA6061 deposit, the longitudinal residual stresses were generally tensile and ranged between − 5 and 91 MPa, and were consistently larger than the transverse residual stresses, ranging between − 5 and 76 MPa. The balancing compressive residual stresses were present at the finishing end of the deposit between − 127 and − 46 MPa. Residual stresses were homogeneously distributed throughout the deposit except at the top few layers (top 10 mm), which were exposed to the least number of thermal cycles from the AFSD process. The measured residual stresses in the AFSD AA6061 deposit were still significant in light of the low yield strength of AFSD AA6061 in its as-deposited state.