Abstract:

Additive friction stir deposition is an emerging solid-state additive process that enables large-scale additive manufacturing, coating, cladding, and repair for a wide variety of metals and metal matrix composites. By integrating the friction stir principle with a robust material feeding mechanism, this process enables site-specific deposition without concerns of hot cracking, high residual stresses, or the need of post-process annealing. The printed material undergoes severe plastic deformation at elevated temperatures and high shear strain rates. This leads to dynamic recovery and dynamic recrystallization, resulting in refined, equiaxed grains and isotropic mechanical properties. Indeed, additive friction stir deposition is one of the very few metal additive processes that give rise to wrought, rather than cast, microstructures.

Additive friction stir deposition is carried out under extreme thermomechanical conditions, in which the processing domains defined by deformation profile, temperature history, and energy input level are unexplored by conventional manufacturing routes. As a result, novel dynamic phase and microstructure evolution phenomena may arise. In this talk, I will show a few examples of microstructures resulting from dynamic phase formation and continuous/discontinuous dynamic recrystallization. For dynamic phase formation, the focus is on forming high volume fraction (~50 vol%), dense, submicron intermetallic particles during the printing of thermodynamically immiscible metal-metal composites. For dynamic recrystallization, the focus is on the effect of stacking fault energy and the processing conditions on grain size, grain boundary character, and grain size distribution (monomodal or bimodal distribution).  Given the unexplored processing domains and far-from-equilibrium nature of additive friction stir deposition, knowledge based on conventional thermomechanical processing like hot rolling and forging is not readily applicable. To fully understand the processing science underlying these phenomena, therefore, new theories or models based on systematic experimental investigations into microstructure evolution during printing are essential—which may comprise advanced, in situ and quasi-in situ characterization approaches. This provides great collaboration opportunities for unraveling new processing science under extreme conditions between Argonne National Laboratory and Virginia Tech.