Cold Spray Additive Manufacturing: Microstructure Evolution and Bonding Features

ConspectusMost metal additive manufacturing (AM) methods involve the melting or sintering of feedstock powder or wire using an energy source (laser, electron beam, or electric arc). Solid-state AM, sometimes also known as non-beam-based AM, is a process in which the deposited material does not melt and is built up layer-by-layer, typically through severe plastic deformation. Initially considered to be a coating technique, by virtue of its high deposition rate, cold spray additive manufacturing (CSAM) is attractive as a solid-state AM technology. In the CSAM process, metal powder particles are impacted onto a substrate at a supersonic velocity and relatively low temperature. The CSAM process reduces or eliminates many problems associated with melting or beam-based AM methods, making the CSAM technically attractive for a wide range of applications, such as in the aerospace, automobile, marine, biomedical, machinery, and energy sectors.In this Account, the author briefly reviews the setup of a cold spray system and discusses the strengths and drawbacks of CSAM compared with thermal sprays and beam-based AM processes, as well as applications in relevant industries. The author summarizes the bonding mechanisms proposed for the cold spray process. The focus of this Account is to review the microstructure evolution of several typical model metals (copper, nickel, aluminum, and titanium) during the cold spray process. The author shows a large variety of microstructure characteristics (recrystallized grains, annealing twins, shear bands, submicron-sized grains, deformation twins, and nanometer-sized grains) in cold-sprayed copper, dynamic recrystallization of cold-sprayed nickel, and the formation of refined grains even below 10 nm in size in cold-sprayed aluminum. The magnitude of the stacking fault energy of as-sprayed materials considerably influences the microstructure after cold spray and postprocessing. Due to the relatively low thermal conductivity and ductility of titanium, cold spraying of titanium shows both a high deposition efficiency and relatively high porosity of as-sprayed parts, as well as a heterogeneous microstructure. Moreover, the Account introduces the postprocessing heat treatment and nanoindentation characterization of cold-sprayed materials. A fundamental understanding of microstructure evolution during and after the cold spray process is essential for optimal mechanical properties. Lastly, the author provides a perspective on applying old lessons and taking advantage of new techniques and materials, including powder characterization, hybrid additive manufacturing, machine learning for searching process windows, nanomechanical testing, and emerging alloys (high-entropy alloys, nanocrystalline alloys, and quasicrystals), to advance the research and applications of CSAM.