Additive Manufacturing (AM) is referred to technologies that produce directly 3D parts in a layer-by-layer fashion. Laser Powder Bed Fusion (L-PBF) is one of the most important AM processes, enabling production of very complex metallic objects from a computer-aided design (CAD) model. In this research, the L-PBF process has been studied at different scales, going from laser/matter interaction to the identification of thermomechanical conditions undergone by the material, to optimization of process parameters, and identification of cracking mechanisms. The studied alloys were bronze, red gold, 316L stainless steel and the CM247LC Ni-based superalloy.
The first part of the thesis deals with the optimization of L-PBF parameters of one alloy, red gold, taking advantage of known optimized parameters for bronze. Printing gold samples by laser melting processes is usually difficult because of their high reflectivity and thermal conductivity. It is, therefore, proposed to use an alternative material, CuSn8 (bronze), as a test material before printing gold alloys, which is cheaper and has physical properties such as melting point and absorptivity close to the gold alloy. In addition, an experimental L-PBF station capable of working with small amounts of precious powder has been developed. The processing of 18-Carat gold alloy was optimized in two steps. The first step consisted in finding the best parameters for printing bronze samples with the highest density. Relative densities as high as 99.8% were obtained. In a second step, gold alloy samples were printed by scaling the bronze best parameters. The concept of normalized enthalpy was used to take into account the differences in thermal and optical properties among the different materials. A translation rule was derived for the prediction of optimal processing conditions, based on the ones found for the test material. One important input for this translation rule is the powder absorptivity, which was measured at the appropriate laser wavelength and at room temperature. This approach eventually leads to the highest reported density for an additive manufactured 18-carat gold alloy (99.81% relative density).
The second part of the thesis looks at laser/matter interaction and process parameters effects on the melt pool formation. Numerical finite element simulations identified precise thermal histories, which were used to understand phenomena such as phase transformation in red gold, or cracking in CM247LC.
CM247LC is susceptible to cracking during manufacturing by L-PBF process. In order to unveil the mechanisms of crack formation, both Gleeble testing of LPBF samples, and high-speed synchrotron X-ray imaging in combination with a miniaturized L-PBF set-up that reproduces real processing conditions have been employed. Hot cracking could be differentiated from liquation. Reduction in crack density could be obtained by 3D laser shock peening (LSP), process optimization, and chemical composition modification.
The impact of the disordered/ordered phase transformation on the microstructure of additively manufactured red-gold alloy has been studied. 3D printed red-gold samples with different post processing treatments were subsequently heat treated at 250 ºC, and characterized by Electron backscatter diffraction (EBSD). Textures after heat-treatment were shown to strongly depend on the stress state in the early stages of the phase transformation, during the manufacturing process.