Tailored process gases for laser powder bed fusion
Doctoral thesis, 2021

Metal laser powder bed fusion (L-PBF) allows for production of complex components using the energy from a laser to locally melt micron-sized powder following a layer-wise approach. Considerable scientific efforts are focused on addressing the influence of the process parameters on the melting stability and the control of material properties, while developing necessary monitoring and characterization tools. The importance of the process atmosphere has largely been undermined in favour of first order parameters connected to the laser scanning. The role of the atmosphere has been limited to the reduction of the operating residual oxygen level down to typically 1000 ppm. This thesis focuses on providing knowledge on the influence of the process atmosphere on the laser – metal powder interaction during L-PBF and the resulting properties of the built material in terms of generated defects, microstructure and mechanical properties. Different purities and compositions of generated atmospheres have been investigated to manufacture the most used materials in the field, namely 316L stainless steel, Alloy 718 and Ti-6Al-4V. The scope of process gases was extended from the traditionally employed argon to also include nitrogen, helium and mixtures of argon and helium. Purities from the typical 1000 ppm O2 threshold down to a few ppm were achieved using external monitoring of the atmosphere on both industrial- and laboratory-scale production machines.

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.

Helium

Laser powder bed fusion

AM process productivity

Spatter particles

Residual oxygen

316L stainless steel

Argon

Nitrogen

Additive manufacturing

Process atmosphere

Process stability

Residual stresses.

Ti-6Al-4V

Alloy 718

Virtual Development Laboratory (VDL), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg
Opponent: Prof. Dr. Eric Jägle, Universität der Bundeswehr München, Germany

Author

Camille Nicole Géraldine Pauzon

Chalmers, Industrial and Materials Science, Materials and manufacture

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.

Subject Categories

Materials Engineering

ISBN

978-91-7905-432-8

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4899

Publisher

Chalmers University of Technology

Virtual Development Laboratory (VDL), Chalmers Tvärgata 4C, Chalmers University of Technology, Gothenburg

Online

Opponent: Prof. Dr. Eric Jägle, Universität der Bundeswehr München, Germany

More information

Latest update

3/4/2021 4