Interaction of scanning strategy, microstructure, and principal stresses in laser powder bed fusion of the nickel-based superalloy Inconel 718.
Source: BAM
Additive manufacturing offers unique capabilities in the realization of complex shaped parts. Among these processes, laser powder bed fusion is one of the most widely used in the context of metal alloys. In essence, a laser selectively melts loose powder particles contained in a powder bed which is repeated layer-by-layer to build the 3D structure. One can imagine that the resulting heat input from the laser is enormous and must dissipate through the part into a cooler baseplate upon which the part is built. The inevitable outcome during processing is the formation of high internal stress driven by these thermal gradients, which upon cooling results in locked in residual stresses. These residual stresses can be detrimental during the manufacturing or subsequent application; therefore, it is crucial to understand their formation. In this study, we varied the laser scan strategy (i.e., the direction how the laser melts the powder bed) during manufacturing of a simple prism geometry (Figure 1). In this way, we controlled the direction of heat input into the material. As expected, different scanning strategies resulted in the variations of microstructures and transition zones of 0.1 - 0.2 mm between them. Determining the residual stress by neutron diffraction, using the crystal lattice as a strain gauge, we found that the direction of the highest (principal) stress depends on the underlying scan strategy. When the scan vectors are aligned with the x- and y sample coordinate axes, the principal stress directions were found to co-align. In the presented geometry, a nearly transverse isotropic stress states (equal in all directions within a plane) develops when the scan vectors are either aligned at 45° to x and y or continuously rotated by 67° between each layer. The maximum magnitude of the residual stresses was found to be around -700MPa, close to the yield strength of the material (Inconel 718) and always aligned along the build direction (z).
Impact of scan strategy on principal stresses in laser powder bed fusion
Jakob Schröder, Tobias Fritsch, Vladimir Luzin, Bruno Ferrari, Juan Simón-Muzás, Alexander Evans, Giovanni Bruno
Materials & Design, Volume 244, 2024