Cellular solids have gained extensive popularity in different areas of engineering due to their unique physical and mechanical properties. Recent advancements in manufacturing technologies have led to the development of cellular solids with highly controllable microstructures and properties modulated for multiple functionalities at low structural weights. The concept of density gradation in cellular solids has recently gained attention due to its potentials in opening new doors to the development of lightweight structures that offer optimal physical and mechanical properties without compromising their favorable characteristics. Herein, a comprehensive insight into the fundamental concepts, fabrication, and current and potential applications of density‐graded cellular solids in various areas of science and engineering is provided. Cellular solids are broadly classified into two main categories: foams and lattice structures. An overview of the fundamental concepts in each category is presented, followed by details on the characterization approaches and some of the most novel processing techniques utilized in fabricating the structures. The uses of density‐graded structures in load‐bearing, acoustic, and biomedical applications are highlighted. The state of the art in each category and the current trends in application‐specific optimization of density‐graded structures are discussed. The review concludes with an outlook of the future directions in this exciting field.

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Based on the cold spray technique, the solvent-free and solid-state deposition of glassy polymers is envisioned. Adiabatic inelastic deformation mechanisms in the cold spray technique are studied through high-velocity collisions (<1000 m/s) of polystyrene microparticles against stationary target substrates of polystyrene and silicon. During extreme collisions, a brittle-to-ductile transition occurs, leading to either fracture- or shear-dominant inelastic deformation of the colliding microparticles. Due to the nonlinear interplay between the adiabatic shearing and the thermal softening of polystyrene, the plastic shear flow becomes the dominant deformation channel over brittle fragmentation when increasing the rigidity of the target substrate. High molecular weights (>20 kDa) are essential to hinder the evolution of brittle fracture and promote shear-induced heating beyond the glass transition temperature of polystyrene. However, an excessively high molecular weight (∼100 kDa) reduces the adhesion of the microparticles to the substrate due to insufficient wetting of the softened polystyrene. Due to the two competing viscoelastic effects, proper selection of molecular weight becomes critical for the cold spray technique of glassy polymers.

Cold spray deposition is a process in which bonding between solid microscale particles and a substrate occurs due to high energy impact. In this process, adhesion forms between the impacted particle and the substrate within a certain multi-parameter (material, temperature, particle size, etc.)-dependent impact velocity range. The present work analyzes the effects of particle diameter (for a range of 10-60 lm) on the adhesion of thermoplastic PEEK particles on PEEK substrates using a combined numericalanalytical approach. Our findings indicate that a maximum critical velocity (i.e., impact velocity required for a successful bond formation) of *360 m/s is required for cold spraying of 10 lm PEEK particles at room temperature. The critical velocity decreases at higher temperatures and for particle diameters in the range of 10 to 40 lm. The predicted critical velocity is shown to increase again for particles with 60 lm diameter. For PEEK particles with diameters from 10 to 60 lm, the critical velocity window is determined to be in the range of 277-360 m/s. The velocity window decreases to 208-270 m/s if the spray temperature is increased to 150°C, i.e., slightly above the nominal glass transition temperature of PEEK.

Polymer cold spray has gained considerable attention as a novel manufacturing process. A promising aspect of this technology involves the ability to deposit uniform polymer coatings without the requirements of solvent and/or high-temperature conditions. The present study investigates the interplay between shear instability, often considered to be the primary mechanism for bond formation, and fracture, as a secondary energy dissipation mechanism, collectively governing the deposition of glassy thermoplastics on similar and dissimilar substrates. A hybrid experimental-computational approach is utilized to explore the simultaneous effects of several interconnected phenomena, namely the particle–substrate relative deformability, molecular weights, and the resultant yielding versus fracture of polystyrene particles, examined herein as a model material system. The computational investigations are based on constitutive plasticity and damage equations determined and calibrated based on a statistical data mining approach applied to a wide collection of previously reported stress–strain and failure data. Results obtained herein demonstrate that the underlying adhesion mechanisms depend strongly on the molecular weight of the sprayed particles. It is also shown that although the plastic deformation and shear instability are still the primary bond formation mechanisms, the molecular-weight-dependent fracture of the sprayed glassy polymers is also a considerable phenomenon capable of significantly affecting the deposition process, especially in cases involving the cold spray of soft thermoplastics on hard substrates. The strong interplay between molecular-weight-dependent plastic yielding and fracture in the examined system emphasizes the importance of molecular weight as a critical variable in the cold spray of glassy polymers, also highlighting the possibility of process optimization by proper feedstock selection.