The friction stir additive manufacturing (FSAM) experiment was conducted with 2mm thick sheets of 2195-T8 Al-Li alloy by using five tools with various shapes. The influence of stir tool shape and additive process on the formation, the interface defects and the hardness distribution of friction stir additive manufactured build was mainly investigated by metallographic observation and hardness testing. The results show that there is no obvious mixing of the materials across the interface of the builds manufactured by the cylindrical pin and the conical pin with three flats. However, the TrivexTM pin and the pin with three concave arc grooves are good for mixing of the materials across the interface and reducing hooking defects. Dense and defect free metallurgical connection occurs at the interface on the advancing side of the nugget zone. However, the materials across the interface on the retreating side are not sufficiently mixed, hence the hooking defect on the retreating side is easy to extend into the nugget zone, and the weak-bonding defect is originated from the hooking defect on the retreating side. For the four-layer build, except for the top layer, the welding method that the welding directions of two adjacent layers are opposite can make the hooking defects on both sides of the other layers bend to the outside of the nugget zone and improve the weak-bonding defects. The hooking defect on the retreating side of the top layer extends into the nugget zone. There is an obvious softening phenomenon in the nugget zone of the builds. The distributions of the hardness of the nugget zones of the builds manufactured by different welding processes are uniform, which shows that the build with uniform properties can be obtained by friction stir additive manufacturing. Compared with the single pass welding method, the back and forth double passes welding method further softens the material of the nugget zone of the one-layer build. For the four-layer build, the closer the layer is to the top, the higher the average hardness of the nugget zone is.
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