Key Engineering Materials Vol. 958

Abstract: AM technologies have been developed for different applications in sectors such as aeronautics, automotive, or healthcare. Additionally, the range of materials that can be 3D printed has increased considerably since these technologies began to be used in the 80s of the XX^th century. Nowadays, it is possible to use both polymers and composite materials (ceramic-filled, metal-filled, etc.) in extrusion technologies, both with fused filament fabrication (FFF) and with direct ink writing (DIW). Unfortunately, even though 3D printing technologies offer much more freedom than conventional manufacturing technologies (molding, machining, etc.), before printing it is necessary to optimize the process, regarding the printing parameters that are recommended with each material. In addition, post-processing techniques are often required. This study aims to show not only the different parameters that are to be considered to optimize material extrusion 3D printing of ceramic materials, mainly regarding dimensional accuracy and surface finish, but also the types of structures that can be manufactured, as well as the challenges that are nowadays faced regarding 3D printing of ceramics.

Abstract: Great advances have emerged in recent years around additive manufacturing techniques, with an increasing number of different materials (polymers, ceramics, metals). However, metal part manufacturing has always been one of the most demanded in engineering. That is due to its ability to create final functional parts with good mechanical properties. One of the most widely used technique is Selective Laser Melting (SLM). The SLM process uses a laser power source to selectively melt metal powder layer by layer. Typically, this manufacturing technique requires mechanical post-processing operations, not only to split the parts from the build-plate, but also to improve the mechanical properties and surface finish of parts or the dimensional accuracy of specific regions to ensure assembly and interchangeability. In particular, sandblasting is a method of mechanical abrasion cleaning commonly used and very useful for improving the surface topology of SLM printed parts. Besides, the laser scanning strategy used in this additive manufacturing process influences the surface quality of parts. Therefore, in this work, the sandblasting post-process has been optimized for surface roughness improving in parts printed using the most common laser scanning strategies (normal, hexagonal, concentric). The role that sandblasting pressure and time plays in the surface quality of parts, indispensable to optimize this SLM post-process, has been evaluated. Thus, surface roughness of different specimens subjected to different sandblasting parameters has been measured to optimize both values related to the laser scanning strategy used in SLM manufacturing. The material used is 17-4PH stainless steel, an alloy that presents an excellent combination of high strength and good corrosion resistance, high hardness, good thermal properties, as well as excellent mechanical properties at high temperatures. This precipitation-hardened steel has important applications in the aerospace sector, chemical and petrochemical industry, energy sector, surgical instruments, high wear components, and general metallurgy, among others.

Abstract: Today, the design of polymer parts by additive manufacturing subjected to large stress has undergone a clear advance with the new concept of continuous fiber-reinforced thermoplastic composites (CFRTPC). The optimization of fiber alignment is the main asset to improve strength and has been widely discussed by the research community. In this work, the effect of the intercalation of the polymeric matrix (ONYX) between layers of continuous fiberglass reinforcement has been analyzed in shear specimens. Onyx is nylon mixed with short carbon fibers. The results show how inserting polymeric layers among the continuous fiber ones improves the shear strength up to 25% and the stiffness modulus up to 18%.

Abstract: Multicriteria decision techniques, among which the AHP represents one of the most widely used, allow the influence of a set of criteria to be considered simultaneously when making a certain decision. And they do so through the design of a hierarchical structure, in which the considered criteria are distributed, and the establishment of judgments that define the relative importance of each of them in the final decision. The results of any manufacturing process are conditioned by a set of parameters. At the same time, the different pieces of equipment that can carry out a certain process present differences in their performance, both in relation to said parameters and to the results that they allow to obtain. From this point of view, multicriteria decision techniques offer opportunities to channel the simultaneous consideration of all these aspects in order to make decisions in productive contexts. This work faces the initial phase of the design of a methodology for the selection of the most suitable FFF-SLA additive manufacturing equipment for different manufacturing scenarios. For this, the authors design an artefact capable of assessing the performance of the equipment considered as alternatives in relation to different relevant parameters for the decision. Then, main results and conclusions are discussed.

Abstract: In several sports, such as martial arts, point scoring depends on applied impact. Embedded sensors in wearable protection devices help to quantify the impact intensity. In parallel, there is a growing interest in developing products with a reduced ecological footprint, using natural materials such as cork. Considered a light, resilient, flexible, and hypo-allergenic material, cork has been proving to be a material with interesting impact absorption properties to be integrated into personal protection, as is the case with helmets and vests. Silicon rubber has also been proposed as an alternative elastomer material with adequate sealing and electrical isolation properties, while resistant to ageing and impact. Since injuries occur more frequently in lower extremities, presented work is part of a project that aims to explore the interest of replacing foams and synthetic leathers usually applied in foot protections with smaller CO₂ footprint materials while increasing their impact absorption properties. At this stage, a part of a protection prototype consisting of a plate made from natural cork, coated with silicone rubber protection containing embedded force sensors, was studied. The objective is to explore the potential of such materials to replace those conventionally applied in commercial foot protections. Firstly, the adhesion between imprinted silicon and cork was confirmed through peeling tests. Then, considering the opportunity of inserting sensors within the additive manufacturing of the parts, different silicone coating configurations and sensor placements were tested regarding their impact performance. Comparison between impact absorption performance was carried out using an impact test device, assessing sandwich composite performance and force sensing reading accuracy. Silicone printability during the robocasting extrusion technique demonstrated to facilitate the sensor placement without requiring deposition path modification. It showed to be an easy way to manipulate the sensor position within the composite layers and, by that, to modify its impact performance. This is an important contribution to the main goal of the undergoing project, which is to completely replace commercial materials with this new sandwich composite.

Abstract: In recent years, the potential of application of auxetic structures, with negative Poisson's ratio, is gaining interest due to the increasingly widespread use of additive manufacturing. Additive technologies allow the manufacture of lightweight and complex shapes, and among them, auxetic cellular three-dimensional structures stand out for their unique behavior and with applications of interest in fields such as aerospace, medical or construction engineering. In the present work, 3D re-entrant honeycomb auxetic structures are designed and manufactured with Stereolithography (SLA) and Fused Filament Fabrication (FFF) with different geometrical parameters to analyse their impact in the mechanical behaviour of these complex structures. To this aim, an Ultimaker S5 and a Formlabs Form3 printer, respectively, have been used. Design variations are approached considering the following parameters: the length of the vertical strut (H), the length of the re-entrant strut (L), the re-entrant angle between the re-entrant and the vertical strut (θ) and the diameter of the struts cross sections (d). The designed structures shape behavior is evaluated with mechanical tests including compression tests and digital image correlation technique, and numerical simulations, The results show that lower Poisson ratio’s values are identified with slimmer profiles and with higher effective lengths and, therefore, corresponds to slender struts. On the other hand, lower angles between vertical and oblique struts show lower Poisson ratios, associated to the load distribution and its effect on the structure’s node displacements.

Abstract: The design process of street furniture nowadays focuses on aspects that go beyond functionality. In this type of product, total aesthetic integration with the space is sought in order to minimise aesthetic pollution. People's quality of life is directly related to the environmental, visual and sound impact of street furniture. Hence the search for sustainability through materials and finishes based on biophilic design. In the context of recycled materials, one branch of research focuses on the search for applications such as street furniture. In turn, 3D printing technology makes it possible to manufacture customised products and use recycled materials. This work focuses on the creation of organic textures for the creation of street furniture printed with recycled materials. The aim is to analyse the state of the art in this field and propose a series of textures and reliefs based on organic shapes, their manufacture by additive manufacturing of fused material (FFF) and their metrological study of the manufactured pieces with respect to the models created by computer-aided design.

Abstract: Cellular structures such as lattice or triply periodic minimal surfaces (TPMS) present interesting applications in different industry sectors due to their high specific strength (the ratio strength/weight), elastic modulus and energy absorption. Some applications focus on new geometries for heat exchangers due to a higher cooling performance. In addition, different sectors demand lighter materials and structures maintaining component mechanical properties but reducing its weight, which is the case of microstructures. Microstructures composed by rigid skeletons are able to maintain the global component stiffness. The main challenge is designing with optimized microelements size, shape, and topology. The internal cavities and complex geometries of these parts are an impossible barrier for most of the manufacturing processes. Thus, one of the most suitable processes for these types of components is Additive Manufacturing, yet these microstructures formed by a heterogeneous skeleton with cavities is still a real challenge. In this work, different case studies for microstructures manufactured by Laser Powder Bed Fusion (L-PBF) are presented and both, manufacturing process parameters and components analysis, are performed.

Abstract: Direct ink writing (DIW) is an extrusion additive manufacturing (AM) technique in which inks are extruded through a nozzle and then deposited layer-by-layer. This technology allows 3D printing many different materials such as ceramics, metals, food, etc. In this work, the performance of zirconia pastes is addressed. The pastes are composed of yttria stabilized zirconia (YSZ) powder and a polymeric binder. Ceramic content is a mix of two components: A and B. Both the total content of ceramic and the content of component A in the paste are varied, according to a 3² design of experiments. The paste was characterized regarding Densification (%) and Elastic modulus G’ (Pa). A new parameter w³/G’ is defined to evaluate the viscosity of the inks. In the tests, the ceramic percentage is limited by the pressing force of the plunger that will be used to extrude the pastes. On the other hand, the binder concentration is also limited, because it requires to be in a gel form in order to be properly extruded. The results showed that Densification depends mainly on ceramic content, while the w³/G’ parameter is related to percentage of component A. In this work, the properties of the pastes prior to 3D printing are assessed. However, in the future, the pastes will be used to extrude complex parts with medical applications. AM extrusion processes constitute a possible way to overcome the difficulties to obtain complex geometries with conventional methods such as machining, in which zirconia parts can break due to their brittleness. Thus, the results of this work will help to manufacture complex shapes with porous areas in zirconia, when the DIW technology is employed.

Abstract: The interest in aluminium alloys for Additive Manufacturing (AM) processes has increased significantly during the last years. Thanks to the freedom of design offered by AM technologies, specifically for Powder Bed Fusion – Laser/Metal (PBF-L/M), aluminium alloys have shown a high potential for their implementation in several industrial sectors. The combination of a reduced density together with high specific mechanical performance (e.g. Rp0.2 stress against density) and other materials properties, like a high thermal and electrical conductivity, or improved corrosion resistance, between others, make AM Al-alloys a great choice for the production of several applications, in particular for the aerospace industry. There are currently available several commercial Al-alloys in the market, although their maturity for producing final Al-products with the high quality required for the aerospace industry still need to be investigated. This paper explores the processability of different Al-alloys manufactured by PBF-L/M. This is analyzed by the development of a Design of Experiment (DoE) campaign for achieving the best processing parameters in order to produce full dense materials. There, bulk and surface density, void content and surface roughness are the main variables to be characterized at this initial step. Later, different thermal treatments are applied and evaluated for each alloy, aiming to produce the best mechanical characteristics, but also analysing other relevant aspects such the electrical resistivity and thermal conductivity.