Progress in Solid-State Additive Manufacturing based on Additive Friction Stir Deposition

Despite the popular usage of beam-based metal additive manufacturing today, such as powder bed fusion and directed energy deposition, fundamental challenges exist in these high-energy and high-cost processes. Enormous thermal gradient, devastatingly high residual stresses, and high risk of hot cracking—just to name a few. During the last few years, a series of solid-state metal additive manufacturing technologies have emerged, which exploit the deformation bonding rather than melting and rapid solidification to enable material addition and adhesion. Among them, additive friction stir deposition (brand name MELD) stands out as a purely additive technology that enables site-specific build-up of metals with good quality, fine equiaxed microstructures, and excellent mechanical properties. Still at an early stage, a good understanding of the physical processes underlying additive friction stir deposition has been elusive, especially temperature evolution and material flow. This critically prevents us from control of the microstructure and properties of the as-printed metal. In this talk, I will discuss the most recent understanding of process fundamentals of additive friction stir deposition, based on complementary in situ monitoring of (i) temperature evolution, (ii) material deformation flow, and (iii) feed force and torque evolution. The focus on how the intrinsic properties of materials, such as stacking fault energy and dynamic recovery rate, dictate the material flow behavior and heat generation mechanisms. These findings pave the road toward material-orientated processing control in solid-state additive manufacturing. I will also introduce several niche applications of additive friction stir deposition, such as large-scale repair, dissimilar material cladding, material reuse and recycling, and selective area reinforcement on sheet metals.