Design and Mechanical Characterization of Voronoi Structures Manufactured by Indirect Additive Manufacturing

Abstract

Additive manufacturing (AM) is a production process for the fabrication of three-dimensional items characterized by complex geometries. Several technologies employ a localized melting of metal dust through the application of focused energy sources, such as lasers or electron beams, on a powder bed. Despite the high potential of AM, numerous burdens afflict this production technology; for example, the few materials available, thermal stress due to the focused thermal source, low surface finishing, anisotropic properties, and the high cost of raw materials and the manufacturing process. In this paper, the combination by AM of meltable resins with metal casting for an indirect additive manufacturing (I-AM) is proposed. The process is applied to the production of open cells metal foams, similar in shape to the products available in commerce. However, their cellular structure features were designed and optimized by graphical editor Grasshopper®. The metal foams produced by AM were cast with a lost wax process and compared with commercial metal foams by means of compression tests.Materials 2020, 13, 1085 2 of 11 AM processes for metallic materials represent an interesting technology in manufacturing applications. The most applied AM processes for metals include laser beam melting (LBM), electron beam melting (EBM), and laser metal deposition (LMD). Metallic parts produced by AM are more suitable for industrial applications compared to polymeric parts. However, expensive machineries, as well as low surface finishing and demanding process settings, limit the application of these methodologies in industrial environments [7]. Metal foams are composed of biphasic and cellular materials, which combine good mechanical resistance with excellent thermal and acoustic properties [8].In particular, high specific strength [9][10][11][12][13] and strain [14][15][16], excellent energy absorption [17,18], acoustic insulation [19], and heat dissipation media [20][21][22][23][24] make this class of materials increasingly useful for several multifunctional applications. The main problem afflicting metal foams regards the manufacturing process, and specifically the porosity distribution [25,26], as well as the connection with other components. The latter is a critical factor in structural and heat-exchange devices, because welding and brazing processes are time-consuming, costly, and not suitable for the most common materials exploited in metal foams production. This burdens their feasibility in industrial applications [27,28]. The cellular configuration design is fundamental, as its purpose is the definition of a commercial foam-like structure that matches specific application requirements. As a consequence of their random structure, the foams offer very valuable materials for filters [29][30][31] or heat exchange processes [32][33][34][35].

Numerical Investigation of Effective Thermal Conductivity of Strut-Based Cellular Structures Designed by Spatial Voronoi Tessellation

“…However, for anisotropic structures such as RV10 and RV30, it is meaningless to discuss the influence of the number of seed points on the effective thermal conductivity, because the orientation of the structure is also one of the main factors affecting its effective thermal conductivity. While existing studies have shown that the PPI is inseparable from the mechanical properties of the random Voronoi structure, especially the energy absorption capacity [ 25 ]. As has been mentioned above, the effective thermal conductivity of the structure is inversely proportional to the porosity.…”

“…Nie et al [ 24 ] investigated the pressure drop of open-cell foams created by LVT with various pore densities and porosities. Almonti et al [ 25 ] proposed an indirect additive manufacturing method that combined the AM with metal casting to fabricate the open-cell foams designed by Voronoi tessellation and compared the resulting mechanical properties with commercial metal foams. These studies revealed that the porosity and pore size are significant structural parameters that are closely related to the property of the open-cell foam.…”

“…Previous researchers have studied the heat transfer characteristics of additive manufacturing (selective laser melting and selective laser sintering) strut-based cellular structures using experimental research and theoretical analysis methods, and mainly pay attention to their heat transfer characteristics [ 39 , 40 , 41 ]. The influence of the porosity, pore size and pore size distribution of 3D Voronoi structure on heat transfer has been extensively studied [ 24 , 25 , 35 ]. However, the research on the influence of the number of seed points on the heat transfer in the strut-based structure may be ignored.…”

Evaluation of the effects of the metal foams geometrical features on thermal and fluid-dynamical behavior in forced convection

“…The samples produced were characterized by different PPI values and branches thickness (r) as showed in Table 1; also, two different kinds of edges (smooth/regular) were considered. The structure of the samples was designed with the ad hoc method described in references [41,42]. A Spatial Voronoi Tessellation with the Rhinoceros plug-in GrassHopper was used to design the structure.…”

“…Almonti et al produced a foam structure made of a foundry resin using a 3D stereolithography laser printer. Following, this model was reproduced with a casting process to achieve a metal foam with the designed structure [41,42].…”