Experimental investigation and parametric optimization of friction stir powder additive manufacturing process for aerospace-grade Al alloy

Details of experimental investigation on friction-stir powder additive manufacturing (FSPAM) process for aerospace-grade aluminum alloy Al 6061 powder and identification of its optimum parameters using grey relational analysis (GRA) are presented in this paper. External feeding of deposition powder and external pre-heating of substrate generate good consolidation of Al alloy 6061 generating 383 degrees C as max. temperature which is much below its melting point. Full factorial 27 experiments were conducted on an indigenously developed experimental FSPAM apparatus by varying tool rotational speed and substrate traverse speed at 3 levels each and using 3 geometrical features on tool bottom (i.e., flat bottom, radially grooved, and circumferentially and radially grooved). Depositions have onion rings for all tool geometries, and they move towards advancing side than retreating side giving rough surface with smoother edge to former and smoother surface with rough edge to latter. GRA identified tool rotational speed of 1200 rpm, substrate traverse speed of 25 mm/min, and circumferentially and radially grooved tool as optimum FSPAM process parameters and optimum deposition was obtained using these parameters. Microstructure study of the optimum deposition showed substantial grain refinement with an average sub-grain size of 1.6 mu m which is attributed to dynamic recrystallization. It consists of splat boundaries due to severe plastic deformation. It is defect-free with absence of voids or porosities and having maximum height and arithmetical average roughness as 8.4 mu m and 2.5 mu m respectively. EDS revealed uniform distribution of all the major alloying elements of Al 6061 powder throughout the optimum deposition. XRD revealed retention of all the phases of the Al 6061 powder in the optimum deposition. Geometrical features on tool bottom led to better surface finish but deteriorated microhardness of deposition. ANOVA of grey relational grades revealed that tool rotational speed is the most dominant parameter affecting surface quality and surface integrity of the deposition. This study proves FSPAM to be a viable alternative to fusion-based additive manufacturing processes particularly for aluminum alloys.