Tailored process gases for laser powder bed fusion
Doctoral thesis, 2021
The investigated materials displayed different sensitivities to the atmosphere composition. 316L stainless steel had limited differences in terms of composition and strength when processed with high purity argon or nitrogen. Only processing with a built-in nitrogen generator, with which the process starts as soon as 10000 ppm residual O2 is reached, led to the increased oxidation of spatter particles and the appearance of large lack-of-fusion defects. A reduction in residual oxygen down to few ppm allowed to significantly hinder the development of thick Cr- and Al-rich particulate oxides on the surface of Alloy 718 spatter particles exposed to the L-PBF environment. In addition, Ti-6Al-4V had the highest sensitivity to the presence of impurities with significant oxygen and nitrogen pick-ups leading to embrittlement. This could be partially mitigated by limiting heat accumulation with longer interlayer time at the expense of productivity or by decreasing the oxygen level in the build chamber to below 100 ppm. Finally, helium was introduced as a new process gas that allowed to reduce the generation of spatter particles, favouring a stable melt pool, without significantly disrupting the residual stress state of the built part, which is critical for the productivity of L-PBF.
Laser powder bed fusion
AM process productivity
316L stainless steel
Camille Nicole Géraldine Pauzon
Chalmers, Industrial and Materials Science, Materials and manufacture
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.
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4899
Chalmers University of Technology
Virtual Development Laboratory (VDL), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg
Opponent: Prof. Dr. Eric Jägle, Universität der Bundeswehr München, Germany