Search

Laser powder bed fusion (LPBF) includes a few printing techniques widely used, in recent years, concerning the additive manufacturing of Ti6Al4V alloys. These produced parts, typically utilized in sectors such as aerospace and biomedical, are characterized by very high added value. It is therefore fundamental to identify the influence of process parameters typical of LPBF technology on the occurrence of warping leading to process failure. This study deals with the characterization of single-track and “micro-scale” level warping phenomena which may lead to protrusion of material over the powder bed and process failure before normal termination. This phenomenon was investigated as a function of process parameters, referring also to the strength and ductility characteristics of the manufactured samples. With this purpose, several samples were printed using variable process parameters both in terms of line energy density (LED) values and in terms of laser power and speed combinations such as to guarantee constant LED values. For the samples that did not show significant micro-warping phenomena, in addition to the transversal and longitudinal geometric characterization of the single track, tensile tests were performed to determine both the resistance of the material and the ductility characteristics. The single tracks, for given process parameters, were printed on a homogeneous material substrate. For every single track, a microstructural and morphological transverse and longitudinal characterization has been carried out and the measured geometrical features were correlated to the process parameters. The obtained results allowed the identification of a new threshold parameter, indicating the limit operating conditions beyond which significant warping phenomena and process failure occur.

Titanium alloy Ti6Al4V is one of the most utilized alloys in the field of additive manufacturing due to the excellent combination of mechanical properties, density and good corrosion behavior. These characteristics make the use of this material particularly attractive for additively manufacturing components with complex geometry in sectors such as aeronautics and biomedical. Selective Laser Melting (SLM), by which a component is fabricated by selectively melting of stacked layers of powder using a laser beam, is the one of most promising additive manufacturing technologies for Ti6Al4V alloy. Although this technique offers numerous advantages, it has some critical issues related to the high thermal gradients, associated with the process, promoting the formation of a metastable martensitic microstructure resulting in high tensile strength but poor ductility of the produced parts. The formation of microstructural defects such as balling and porosity can occur together with the presence of residual stresses that may significantly affect the mechanical characteristics of the component. Specific process parameters and geometries can determine heat accumulation phenomena that result in a progressive decrease in thermal gradients between layers. These heat accumulation phenomena are influenced by the number of layers deposited, but also by the building orientation that, for a given geometry, determines a variation of the deposited surface for each layer.

Laser powder bed fusion is one of the additive manufacturing technologies which has developed more rapidly in recent years as it enables the production of very complex geometries. Titanium alloys are among the most popular materials in the aerospace industry thanks to excellent mechanical and corrosion resistance. The corrosion behavior and mechanical properties of samples made of Ti6Al4V and characterized by the geometrical features typical of brackets were investigated taking into account the effects of process parameters on porosity and microstructure. A comparison between the corrosion resistance of samples with complex geometry (3D) and specimens characterized by simple geometry (FLAT) was carried out to understand the role of the part geometry in the passive film formation mechanism, also highlighting the effect of the lack of passivation film formed naturally by exposure to air. Finally, the samples with complex geometry were anodized in a fluoride-free aqueous solution and the corrosion resistance was evaluated.

THALASSA is a large research project, developed in the field of the naval structures. It aimed at studying innovative solutions of significant impact in increasing environmental sustainability through a weight reduction, a careful production planning and a circular approach to the entire life cycle of the parts, from design to the disposal/recycle/reuse. The project is led by NAVTEC Technological District and it is composed of a large team counting more than 300 researchers of several centres among which the CNR institutes, the Universities of Messina, Palermo, Catania, Roma “La Sapienza”, and Udine. Innovative solutions for industrial processes of greatest interest for some major national shipbuilding operators such as Azimut Benetti and Fincantieri or for shipowner such as Caronte&Tourits have been investigated, and players such as ATRIA have been assisted in the formulation of innovative coatings. The presence of NAVTEC District has allowed to concentrate the field of action on themes of strong industrial interest and at the same time to widen the research action on a number of different topics with a view on the entire product life cycle. The district allowed to network skills, laboratories and resources that individually would not have led to carry out activities on such a high number of variables, in terms of materials and joint types. Several issues have been addressed, by identifying solutions thanks to skills of the industrial players, such as: joining technologies (i.e. etching / texturing laser, clinching, self piercing riveting, co-curing, bonding, friction stir welding); eco and bio-sustainable composite reinforced by natural (i.e. vegetable or mineral) and hybrid fibres; degree of recoverability of traditional or bio-resins; corrosion of conventional metal structures and processes for increasing the useful life; and innovative coatings that are combined with the functional protection needs of the structures.