Investigation of microstructure, hardness and residual stresses of wire and arc additive manufactured 6061 aluminium alloy

Wire and arc additive manufacturing cold metal transfer® enable the defect-free soldering of poorly weldable alloys, such as AA6061. Thin-walls have been successfully built with two deposition strategies that is either a back and forth movement of the torch (alternate deposition vectors) or a repeating movement along one direction (unidirectional deposition vectors). The obtained macro- and micro-structures are highly dependent on the WAAM process parameters and the deposition strategy. Both deposition strategies produce stacked structures consisting of successive alternating layers of columnar and equiaxial grains. The AA6061 alloy contains iron-rich intermetallic compounds and β-phase. The precipitates of the latter have been melted during the WAAM deposition process leading to an even higher hardness as compared to the commercial T6 material. At the nanoscale, Transmission Electron Microscopy analyses reveal β’ phase, β’’ phase and small dispersoids at T6 state, as well as a multitude of phases (L, Q’) in the studied material. It shows the coexistence of β’’- and β’- phases. The conventional T6 treatment applied to the WAAM material resulted in a slight over-ageing of this material. Neutron diffraction has been used for residual stress measurements showing tensile stresses (up to 130 MPa) in the built parts and compressive stresses (up to −80 MPa) in the substrate. Despite a difference between the generated microstructure (grain shape orientation and subsequent crystallographic texture) by the two building strategies, no significant variation has been observed on their mechanical properties.