Powder Bed Fusion – Laser Beam of a non-weldable Ni-base superalloy: Role of process parameters and scan strategies

Licentiate thesis, 2024

Additive Manufacturing (AM), in particular, Powder Bed Fusion – Laser Beam (PBF-LB) has garnered attention due to its design freedom, near net shape capability, and reduced lead time. Ni-base superalloys are a class of materials used for high temperature applications and widely
utilized in the energy and aerospace sectors. However, only a limited number of Ni-base superalloys can be manufactured defect-free through the PBF-LB process. This is especially true for non-weldable Ni-base superalloys such as CM247LC which are susceptible to hot cracking and solid-state cracking. This is an issue that needs to be addressed for increased utilization of these alloys to manufacture complex components by PBF-LB.

This thesis explores strategies that can enable PBF-LB processing of CM247LC with minimal or no hot cracking (solidification cracking) and low residual stresses. The first part of the thesis explores the impact of main process parameters such as laser power, speed, and hatch spacing on solidification cracking, microstructure, and residual stresses. The results from the first part indicate that low solidification cracks are achieved for low line energy density (ratio of laser power and speed) and low hatch spacing. This is due to the shallower melt pools achieved and its effect on solidification structure and in turn grain morphology/texture. The residual stresses are found to be proportional to the volumetric energy density. The second part of the thesis explores the impact of scan strategies on solidification cracking, microstructure, and residual stresses. This was done as the results from the first part of the thesis indicated that the solidification cracking and residual stresses could not be reduced solely by optimizing laser power, speed, and hatch. Therefore, the study varied the stripe width with optimized laser parameters. The results seemed to be promising for a short stripe width of 0.2 mm that gave lower solidification cracking and residual stresses. The decrease in solidification cracking has been attributed to the modification in melt pool size/shape and the mushy zone length. The lower residual stresses were possibly caused by the increased re-melting which led to residual stress relief.

The results from the thesis provide an improved understanding of solidification cracking and residual stress mechanisms in non-weldable Ni-base superalloys manufactured by PBF-LB. The presented results can enable PBF-LB processing of alloys susceptible to hot and solid-state cracking.