Laser powder bed fusion of Fe-based shape memory alloys for novel structural metamaterials and "smart" structures

Laser powder bed fusion (LPBF) is a powder-based additive manufacturing (AM) technique, which shows great potential in the production of complex-shaped parts with unprecedented design freedom. In addition, it allows for an active manipulation of the microstructure of the fabricated parts through the control of the material's solidification behavior via laser parameter selection. Thus, LPBF is seen to be highly beneficial for processing iron-based shape memory alloys (Fe-SMAs), whose deformation behavior and shape memory properties strongly depend on several microstructural factors. Nevertheless, there is a lack of data focusing on the possibility of AM for Fe-Mn-Si-based SMAs. Up until now, the majority of Fe-Mn-Si-based components have been manufactured using traditional methods, such as melting and casting in facilities equipped with high vacuum or high purity shielding gas. To achieve the desired final shape, additional processes like hot-forging and cold-rolling are usually necessary, limiting the production to components with simple geometries like strips or bars. AM technologies like LPBF offer a potential solution to overcome the constraints of conventional manufacturing. By fully melting the raw metal powder, intricate parts with tunable functionalities can be created in a single production step.With this work, the feasibility of LPBF fabrication of bulk parts made of Fe-Mn-Si-based SMAs and exhibiting shape memory effect and pseudo-elasticity is demonstrated. Special focus is placed on the understanding of the local microstructural changes introduced via modification of the LPBF processing parameters and their effect on the mechanical and shape memory properties of the fabricated components. In-situ characterization techniques are applied in order to find the correlation between processing parameters, components' microstructure and components' performance. The obtained knowledge is exploited to locally manipulate the microstructure of the LPBF parts with the aim of achieving the desired thermo-mechanical and functional properties. Finally, 3D complex parts made of Fe-SMAs are fabricated to demonstrate that the material-inherent functional behaviors (shape memory capabilities) can be combined with additional functionalities deriving from specifically designed geometries. Components showing shape memory properties together with auxetic behavior, pronounced energy absorption capabilities and extraordinary specific strength are in this way produced.