Behavioral simulations and experimental evaluations of stress induced spatial nonuniformity of dynamic bulk modulus in additive friction stir deposited AA 6061

Additive friction stir deposition (AFSD) is a rapidly developing solid-state additive manufacturing (AM) tech-nology that enables the fabrication of customized products on a relatively large scale. The unique forge-liked process offers high quality products with refined equiaxed microstructure and exceptional mechanical perfor-mance. However, in AFSD, the spatially distributed mechanical properties within a deposited layer have not been established so far, due to the lack of a suitable large-scale mapping method. In the present work, an ultrasonic dynamic-elasticity mapping method was employed to map the distribution of physical properties in a AFSD 6061 Al. Strong spatial asymmetries in elastographies were mainly due to the residual stresses. The origin of the asymmetries was investigated through thermo-mechanical simulations of the AFSD process. It was observed that majority of the asymmetries were associated with the mechanical residual stresses instead of the symmetric thermal residual stresses during linear deposition of the AFSD process. However, the thermal residual stresses can be manipulated to become asymmetric with a 90 degrees turn in the linear deposition path, due to the difference be-tween the levels of cooling along the edges of the inner and outer corners. Hence, the mechanical and thermo-mechanical residual stresses can be additive or subtractive depending on the direction of the turns. Such numerically predicted behaviors were also experimentally observed on the AFSD fabricated samples by ultra-sonic elastography. Furthermore, additional preliminary studies on multi-layer AFSD fabrications show the asymmetric mechanical stresses generated during the deposition process on different layers generate constructive or destructive interference which results to spatially variable dynamic bulk modulus depending on the deposition orientations of layers.

Automated summary

Additive friction stir deposition (AFSD) is a solid-state additive manufacturing technology that allows the production of customized products on a large scale. Through the use of ultrasonic dynamic-elasticity mapping, the spatial distribution of physical properties in AFSD 6061 Al was analyzed. Experimental and numerical studies showed that mechanical and thermo-mechanical residual stresses can generate constructive or destructive interference, resulting in spatially variable dynamic bulk modulus in multi-layer AFSD fabrications.