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The integration of metals and polymers in the same component to couple the best properties and advantages of both these classes of materials can be useful for engineering applications. In this scenario, the aim of this paper is to integrate additive manufacturing of composites with cold spray deposition of metal particles: in particular, 3D-printed carbon fibre-reinforced plastics (CFRP) panels were coated with aluminium particles to couple the lightweight and high mechanical properties of CFRP with high wear resistance and hardness of metals. CFRP panels have been produced by using the Fused Filament Fabrication (FFF) technology, then the panels have been coated with aluminium particles by using a low pressure cold spray facility. Panels with various stratification sequences have been produced to investigate the feasibility of the process and to highlight the influence of the lay-up strategy on the whole process; the parameters of the cold spray process have been also optimized to ensure an effective deposition. Adhesion tests, distortion measurements, cross-section observations and surface coverage measurements have been chosen as response outputs to evaluate the effectiveness of the process and to assess the influence of the process parameters. The integration between FFF of CFRPs and cold spray deposition of aluminium particles has been proved, values of surface coverage close to 100% and good values of adhesion strength (close to 4 MPa) have been achieved; moreover, the presence of the fibres, giving a higher stiffness to the substrate, avoids the occurrence of distortion phenomena during the deposition. It was also proved that the printing parameters influence the deposition: an increase in the infill density from 30% to 50% leads to an increase in the adhesion strength up to 40%. In summary, the results obtained proved the feasibility of the process and allowed to enucleate directions of future research.

Polymer matrix composites (PMCs) are the most used in aerospace fields due to their special mechanical properties coupled with reduced weight. Metallic structures can be manufactured on the surface of PMCs producing hybrid constructions with intriguing characteristics. The cold spray technology (CS) seems to be the best solution for the manufacturing of metallic coatings on polymer-based substrates. However, the last researches on PMCs metallization through CS were carried out without taking into account a proper manufacturing of the substrates. Therefore, the customized manufacturing of PMCs used as substrates for CS deposition represents a crucial aspect to be considered, the target surface should be designed and manufactured to fit better the CS requirements. In this scenario, the Fused Filament Fabrication method (FFF) is the best solution for tailored composites manufacturing. Therefore, this work aims to develop a novel manufacturing route to produce innovative composite structures by using the FFF technology for the 3D-printed polymer-based substrate and the cold spray technology for the 3D-printed metal deposition. The novelty is that, for the first time, the polymer-based substrates will be designed and manufactured “ad hoc” for the cold spray metallization by using the rapid prototyping 3D-printing process successfully integrated with CS technology.

Cold spray deposition, which can be framed in the wider family of additive manufacturing, is a manufacturing technique that is able to produce coatings on diverse types of substrates through the deposition of feedstock powder. As a low-temperature process, cold spray represents a potential solution for the metallization of temperature-sensitive materials, i.e. polymers and polymer matrix composites. The study of the cold spray technology for the metallization of polymers is still in its early stage, and the deposition mechanisms of metals on polymer-based materials are not thoroughly understood yet. On these premises, a review on this topic is needed to systematically depict the actual state of the art and to provide a reliable and well-organized overview discussing all the theories arisen in these years. In summary, this review aims: i) to collect all the available literature and enucleate the most discussed and interesting points (the most prevailing theories regarding the bonding mechanisms, the influence of the different process parameters and the main characteristics of cold-sprayed coating), providing a reliable and well-organized state of the art; ii) to define the open questions and to delineate the directions of future work.

Cold spray (CS) is a low-temperature process that can be used for the metallization of temperature-sensitive materials, such as polymers or polymer matrix composites, so coupling the lightweight of polymers with the wear resistance, physical properties and hardness of metals. The study of the cold spray of metal particles applied to polymers is still in its early stage and the deposition mechanisms underlying the process are not thoroughly understood yet. Moreover, numerical studies of cold spray of metal-to-polymer are almost completely absent in literature. Therefore, aiming to fulfill this gap of knowledge, the scope of this work is to develop a numerical FE model capable of predicting the impact and the adhesion of a micron size metallic particle onto a polymeric substrate. The results from the model were compared with the experimental outcomes found in literature to establish the effectiveness of the model that was used as a powerful tool to better understand the bonding mechanisms and all the related phenomena ruling the CS process of metal-to-polymer.

In this research activity, the manufacturing of metallic coatings on polymer-based panels through the low pressure cold spray technique was studied. Aluminium particles were sprayed for the metallization and carbon fibre composite materials with thermoplastic matrix (PP, PA66) were used as substrates. Different deposition parameter combinations have been set to investigate the feasibility of the process; SEM analyses were carried out to study the coating surface morphology and the interface interaction between particle and substrate. The results showed that the CS process is an effective technique to create aluminium coatings on polymeric matrix composite materials: compact coatings can be obtained if the process parameters are properly set.

In this work, the manufacturing through cold gas dynamic spray (cold spray or CS) of metallic composite coatings of Al-Al2O3 on organic composite substrates with thermoplastic PLA matrix and hemp fibers was studied. Alumina powders, with a mean diameter of 50 μm, were used blended with aluminum powders in three different weight concentration percentages (0, 15, 20, and 45%) as feedstock material in order to highlight and discuss the variations of the coating surface properties depending on the alumina percentage. The coatings were produced by using a low-pressure cold spray equipment. A detailed experimental campaign, including microstructural observations and confocal microscopy, was carried out to study the structure of the coatings. Moreover, the tribological behavior of the coatings was studied through both scratch test and pin-on-disk test. The experiments showed that a small addition of alumina improves the compactness of the coating and its resistance to scratch and wear behavior.

Polymer matrix composites are finding never-ending widespread uses in the last decades, one recent tendency is to metallize their surface to further widen their field of application. Cold-spray deposition is one of the most promising techniques that can be adopted to this aim. Cold-spray deposition on polymers is in its early stage and more experimental work is required to fully understand the phenomena ruling the deposition. In this paper, the results of nanoindentation measurements on cold-spray coatings on various substrates will be presented and discussed. Polypropylene was used as matrix while carbon and glass fibers have been used as reinforcement, both steel and aluminum have been used as feedstock material for the cold-spray deposition. Nanoindentations tests have been then carried out on all the different samples, the influence of the fibers and of the powders sprayed on the behavior of the coatings is discussed in light of the experimental outcomes.

The deposition of steel powders on glass fiber reinforced polymer (GFRP) with the Cold Spray technique is investigated. Metallization of composite materials is still challenging when high-strength and dense powders, such as steel, are used due to excessive erosion of the polymer or disruption of the fibers during the deposition. To overcome these issues, low-pressure cold spray equipment has been used for the experimental campaign shown in this paper. Different coatings were obtained by using nitrogen as the carrier gas and different sets of pressure and standoff distance. Optimization of the process parameters, including the stratification of the laminate, has been carried out. Results of an initial study of the feasibility of the process and the influence of the different chosen spray conditions are exposed. In particular, the bonding mechanisms of steel powders and their morphology are discussed.

Cold spray deposition of aluminum powders on polyethylene reinforced with glass fibers has been studied. Different processing conditions, including different stratifications of the fiber-reinforced polymer used as a substrate, have been investigated to better understand the deposition mechanism. The experimental campaign has been conceived and tailored to highlight the influence of several process parameters on the deposition of the first layer of particles. The results obtained allowed to highlight and clarify the influence of standoff distance and gas pressure and also gave some insights concerning the effect of the substrate stratification. Guidelines regarding the design of the manufacturing process have been also released.

Recently, the growing attraction to the development of new eco-sustainable composite materials is driving the research interest toward the replacement of synthetic reinforcing fibres with natural ones and exploiting the inherent recyclability of thermoplastic resins even for uses in which thermosetting matrices are well consolidated (e.g. naval and aeronautical fields). Among the natural fibres, a growing interest of the research is addressed to basalt fibres. Focusing the attention on thermoplastic composites, many experimental findings already available in literature highlight the outstanding mechanical properties of composite materials including basalt fibres and their potentiality concerning glass ones. On the other hand, some issues are related to the surface properties of the bio-laminates: in particular, the wear ones, the flame resistance and the aesthetic appearance have to be improved to extend the application fields of these materials. Aiming to these goals, this paper deals with the study of the deposition ofaluminium coating through cold spray process on polypropylene/basalt fabric composite laminates. The specimens were obtained by film stacking, and compression moulding technology and their performances were studied in terms of low-velocity impact behaviour, considering the influence of the surface modification with the aluminium coating. The results obtained from the reference laminates and the coated ones are compared in terms of impact parameters: the aluminium deposition seems to affect the damage mechanism propagation even if the impact response seems to be similar in both conditions.

In this activity, customized panels for the CS deposition have been produced and aluminium has been deposited using air as a carrier gas. The custom-made panels were made with different matrix thicknesses on the surface in order to point out the influence of the fibres in the deposition and to study the feasibility and the growth of the coating. The polymer chosen for the matrix is polypropylene, widely used in the automotive sector, while for the reinforcements glass fibres have been used. Different process parameters have been examined in order to point out the influence of temperature, stand-off distance and pressure chosen, on the compaction and densification of the coating produced. In addition, morphological investigations were performed to further characterize the coatings deposited. It has been noticed that aluminium powders can be effectively deposited on polypropylene panels, leading to significantly better results when the thickness of the matrix is ​​sufficient to prevent the powders bouncing elastically on the fibres, while benefiting from the stiffening effect of the latter.