Laser powder-bed fusion of metallic alloys with enhanced microstructures and properties

Laser powder-bed fusion (LPBF) is an additive manufacturing (AM) process that uses a laser to selectively melt a powder bed. This process can build dense samples with complicated geometries from a wide variety of metallic alloys. In this thesis, LPBF of alloys with enhanced properties, such as a Ni-based superalloy (CM247LC), a Zr- and a Pd-based bulk metallic glass (BMG), and a noble high entropy alloy (HEA), have been studied.In CM247LC, the strengthening mechanism is the precipitation of the ordered secondary phase (γ′). The cracking mechanisms are all relayed to the presence of tensile residual stresses (TRS) induced by LPBF. Laser shock peening (LSP) is used to convert TRS into compressive residual stresses (CRS) in the near-surface region of the parts. Since LSP can be applied any time during the part fabrication, the new hybrid process is called 3D LSP. The results showed that LSP was very effective in reducing crack density, through a crack healing mechanism promoted by a combination of CRS and LPBF induced heating.BMGs refer to metallic alloys that have the ability to solidify in an amorphous state. BMGs produced by conventional methods are limited in size due to the high cooling rates required to avoid crystallization and the associated detrimental mechanical properties. LPBF is a potential solution to this problem as the interaction between the laser and the powder is short and confined to a small volume. However, producing amorphous parts with this technique with mechanical properties comparable to as-cast samples remains a challenge for most BMGs, and a complete understanding of the crystallization mechanisms is missing. Two BMGs based on Zr and Pd were studied in this thesis. Highly amorphous and crack-free parts were achieved, showing good properties of hardness, wear resistance, compression, tension, and bending strengths, but a low fatigue limit. The crystallization mechanism of the Zr-based BMG was investigated by considering thermal finite element (FEM) simulations and Fast DSC measurements.A noble HEA (PdPtRhIrCuNi) was fabricated via LPBF, and was shown to be a two-phase face-centered cubic solid solution with high cracking sensitivity. Although microcracks were present in the microstructure, the fabricated specimens could deform up to a true strain of 1.27 and reach a high ultimate compression strength of 2284 MPa. Two distinct cracking mechanisms were identified, and LSP treatments with a similar strategy as for CM247LC were successful in reducing crack density.