Precipitation in AA6063 produced from swarf using additive friction stir deposition

Abstract

• AA6063 via additive friction stir deposition (AFSD) refines the harmful Fe containing particles present in the swarf and exhibits a relatively refined and equiaxed grain structure. • A precipitation gradient is formed during AFSD but post heat treatment readily produces a uniform structure. • Heat-treated AFSD AA6063 shows a wrought-like property. Additive friction stir deposition (AFSD) is an emerging additive manufacturing technology with the unique capability to create large-scale, free-form depositions without melting. It has the added benefit of accepting input material that is readily formed from swarf. The present work addresses questions that have arisen over the impact of the process on the precipitation in 6063 aluminium alloys (AA6063). The mechanical properties and microstructure throughout the deposit was assessed and characterized in T4, T5 and T6 conditions using a combination of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). In the as-deposited condition, the degree of precipitation was highly heterogeneous across the different printed layers, resulting in a significant hardness gradient from the bottom to the top of the deposit. Such gradients in hardness and precipitation could be eliminated through the use of a T6 heat treatment. Additionally, the EBSD result shows that the deposited AA6063 exhibited a relatively refined and equiaxed grain structure, which evolved via continuous dynamic recrystallization during deposition. These results emulate those observed for 6xxx-series alloy deposits formed from primary material. This work demonstrates the capacity of AFSD as a viable method of solid state Al recycling, creating depositions with wrought-like properties when an adequate post-deposition heat treatment is conducted.