Popular Science Summary
Significant growth of additive manufacturing, also called 3D printing, during the last decade, caught the attention of various industrial segments and disrupted traditional manufacturing approaches. Complex-shape high-performance components can be produced with geometries not feasible using conventional manufacturing technologies, with high material utilization and short time-to-market. Several technological solutions under the additive manufacturing umbrella allow for the processing of many different materials, from metals to polymers and composites.
In metal additive manufacturing, laser powder bed fusion (L-PBF) represented the largest share of the market, approaching 10000 systems installed worldwide. The most popular materials are Ti-6Al-4V, stainless steels, and nickel-based alloys, followed closely by aluminium alloys. This process uses the energy from a laser to selectively melt a bed of powder particles of tens of microns in size, slightly smaller than a human hair. This step is repeated in a layer-wise manner to build a 3D component. Great effort is devoted to developing robust L-PBF process and the material portfolio to address a wide range of applications. Integration of additive manufacturing within industrial production schemes, making it an economically interesting manufacturing alternative, is another important challenge nowadays. This demand is also associated with the need for productivity increases and material properties’ control. In this context, gaining a better understanding of the physical phenomena involved during L-PBF and optimizing the process is necessary.
This thesis focuses on the effect of the process atmosphere on the interaction between laser and powder bed and the resulting microstructure, process stability and productivity, as well as, spatter formation and their characteristics. Typically argon or nitrogen are used as processing gases, filling the process chamber where the laser scans the powder bed. This variable, the process gas, has been largely neglected in favour of first order parameters, such as the laser power or speed. This work demonstrates a strong influence of both the type of gas and the purity achieved in the process chamber on the microstructure and properties of the produced material, as well as, the powder exposed to the processing conditions. In addition, the results highlight that guidelines associated with the process atmosphere have to be formulated considering the sensitivity of the alloy produced. Furthermore, helium and argon-helium mixtures were investigated as an approach to stabilize the process and showed potential toward increasing process stability, allowing to increase build rates and thus productivity.
Examiner: Lars Nyborg, IMS
Opponent: Professor Eric Jägle, Institute of Materials Science, University of Bundeswher, Munich , Germany