Metal Additive Manufacturing Conference 2019

The selective laser melting process involves very high cooling rates and consequently results in non-equilibrium microstructures of the printed metal. Examples of this are the cellular sub-grain structure observed in austenitic stainless steels and the very fine hierachical martensite formed in Ti6Al4V titanium. Both stainless steels and titanium alloys are commonly heat treated in order to tailor the microstructure, so the desired final properties can be obtained. However, the successful application of these heat treatments usually relies on the microstructure after conventional processing, which often includes deformation and which is radically different from the non-equilibrium microstructure obtained with additive manufacturing. Since one of the main benefits of additive manufacturing is to realize the final design without material waste or specialized post-machining, deformation is often not a desired post-process. Therefore, the conventional heat treatments need to be tailored to the initial microstructure of selective laser melted materials. This research focuses on the effect of heat treatments on the characteristic features of the microstructures (e.g. cells or martensite formation) and tailoring the heat treatments in order to achieve a specific microstructure by either preserving parts of or completely altering the initial state. The investigated heat treatments are inspired by conventional heat treatments for these materials, however adapted to fit the unique qualities of selective laser melting. Both high and low temperature treatments are considered to provide different microstructures. The effect of the treatments is characterized using microscopy, x-ray diffraction and Vickers microindentation.