Friction stir based welding, processing, extrusion and additive manufacturing

Friction stir welding and processing enabled the creation of stronger joints, novel ultrafinegrained metals, new metal matrix composites, and multifunctional surfaces at user-defined locations. The newly developed friction stir based additive manufacturing methods emerged as transformative technologies since these technologies allow three-dimensional printing of strong dense metal at reduced cost and unprecedented large scales. These technologies have been increasingly adopted in the field of aerospace, shipbuilding, rail transit, automotive, energy, and defense. Since considerable similarities exist in the friction stir technologies, a comprehensive review of the shared fundamentals in these technologies is critical to establish a common background for the entire friction stir community. This paper addressed such needs through (i) a critical assessment of the up-to-date technology innovations about friction stir technologies; (ii) a comprehensive summary of the fundamentals of the friction stir technologies on the aspects of materials flow, heat generation mechanism, microstructural evolution, mechanical properties, process simulation, and specific material issues; and (iii) a systematical analysis of the opportunities and challenges in advancing the friction stir technologies.