In a variety of industries, Additive Manufacturing has revolutionized the whole design–fabrication cycle. Traditional 3D printing is typically employed to produce static components, which are not able to fulfill dynamic structural requirements and are inappropriate for applications such as soft grippers, self-assembly systems, and smart actuators. To address this limitation, an innovative technology has emerged, known as “4D printing”. It processes smart materials by using 3D printing for fabricating smart structures that can be reconfigured by applying different inputs, such as heat, humidity, magnetism, electricity, light, etc. At present, 4D printing is still a growing technology, and it presents numerous challenges regarding materials, design, simulation, fabrication processes, applied strategies, and reversibility. In this work a critical review of 4D printing technologies, materials, and applications is provided.

Selective laser melting (SLM) enables the production of metal complex shapes that are difficult or impossible to obtain with conventional production processes. However, the attainable surface quality is insufficient for most applications; thus, a secondary finishing is frequently required. Barrel finishing is an interesting candidate but is often applied without consistent criteria aimed at finding processing parameters. This work presents a methodology based on Bagnold number evaluation and bed behavior diagram, developed on experimental apparatus with different charges and process parameters. The experimentation on an industrial machine and the profilometric analysis allowed the identification of appropriate process parameters and charge media for finishing the investigated materials (Ti6Al4V and Inconel718). Two case studies, characterized by complex shapes, were considered, and consistent surface measures allowed understanding the capability of the technology.

Lightweight structures with an internal lattice infill and a closed shell have received a lot of attention in the last 20 years for satellites, due to their improved stiffness, buckling strength, multifunctional design, and energy absorption. The geometrical freedom typical of Additive Manufacturing allows lighter, stiffer, and more effective structures to be designed for aerospace applications. The Laser Powder Bed Fusion technology, in particular, enables the fabrication of metal parts with complex geometries, altering the way the mechanical components are designed and manufactured. This study proposed a method to re-design the original satellite structures consisting of walls and ribs with an enclosed lattice design. The proposed new structures must comply with restricted requirements in terms of mechanical properties, dimensional accuracy, and weight. The most challenging is the first frequency request which the original satellite design, based on traditional fabrication, does not satisfy. To overcome this problem a particular framework was developed for locally thickening the critical zones of the lattice. The use of the new design permitted complying with the dynamic behavior and to obtain a weight saving maintaining the mechanical properties. The Additive Manufacturing fabrication of this primary structure demonstrated the feasibility of this new technology to satisfy challenging requests in the aerospace field.