In-situ induce reinforcement in WAAM of Al-matrix composite using active N₂ as shielding gas followed by friction stir processing

Economic and active nitrogen (N₂) is used as a shielding gas for the wire-arc additive manufacturing (WAAM) of Al-matrix composite (AMC). Although no crack was found, large flaky nitrides up to 500 μm are generated in the matrix of the Al alloy, leading to a significant deterioration in both plasticity (reduced by 81.4%) and tensile strength (reduced by 17.5%) compared with the deposit protected by argon (Ar), which is to be modified. Then, FSP was performed on the matrix of the Al alloy. The influence of friction stir processing (FSP) on the microstructure and mechanical properties in the stirred zone (SZ) of WAAM 5356 Al alloy deposit, prepared under a N₂ environment, was studied. Flaky nitrides were observed in the Al matrix, and were broken due to FSP. The large flaky nitrides with a length of 500 μm were broken into granular nitrides with sizes lower than 2 μm. The grain size in the SZ decreased by 83% after FSP, and the microhardness in the SZ became more uniform, with an average value of 91.9 HV. Owing to the refinement of AlN by FSP, the tensile strength and elongation of the AMC were 349 MPa and 19.9%, much more than that before FSP of 231 MPa and 5.5%, showing an increase of 51.1% and 261.8%, respectively. The plasticity has obvious improvement, which is higher than the common used stir casting AMC.