Studying the effect of laser beam profile on as-built residual stress of Inconel 718 manufactured by laser-based powder bed fusion process

Narges Mirzabeigi1,2*, Peter Holfelder-Schwalmer 2, Fabian Sonnen2, Bojan Podgornik3, Irena Pulin3, Crtomir Donik3, Matjaz Godec3, Katrin Wudy1

1 Professorship of Laser-based Additive Manufacturing-Technical University of Munich, Germany, 2 EOS GmbH Electro Optical Systems, Munich, Germany, 3 Institute of Metals and Technology, Ljubljana, Slovenia

*narges.mirzabeigi@tum.de, narges.mirzabeigi@eos.info

Beam shaping technology shows great potential in enhancing productivity [1;2], reducing spatter formation [3], and controlling melt pool morphology by redistributing laser intensity from Gaussian laser beam profile [4;5]. This redistribution influences key microstructural characteristics such as grain size, morphology, and orientation, ultimately affecting tensile properties and anisotropy [1;6].

Simulation studies indicate that modifying the laser intensity distribution alters the thermal gradient within the melt pool, which in turn influences grain growth behaviour [7]. Since both the thermal gradient in the melt pool [8] and the resulting microstructure [9] are closely linked to as-built residual stress, beam shaping could play a significant role in as-built residual stress management. However, only a few studies have briefly explored the relationship between beam shaping and residual stress [8;10] leaving an important area for further investigation.

Figure 1. IPF maps, measured areal porosity level, area weighted average grain diameter (GS) and texture index of samples built with different beam laser beam profiles. These parameters are selected for qualitative and quantitative residual stress analysis

In this study, we examine two axially symmetric beam profiles—Gaussian and Ring—along with one axially asymmetric profile, which is a simulation-optimized design aimed at achieving a homogeneous melt pool temperature [11] referred to as the chair-shaped beam. All beam profiles have a diameter of 250 µm, measured using the second-moment method, with corresponding process parameters leading to a build rate within the range of 11.2 to 14.4 mm3/s and porosity levels below 3%. Figure 1 presents the resulting microstructures obtained under different laser intensities, visualized through inverse pole figures (IPF) measured via electron backscatter diffraction (EBSD) showing effect of beam shapes on texture index, grain size and morphology. To qualitatively assess residual stress and observe resulting deformations, double-sided cantilever specimens were fabricated. Additionally, for a quantitative evaluation of residual stress induced by different beam shapes, the hole-drilling method [12] was applied to selected process parameter sets. Both qualitative and quantitative analyses show a significant change in the degree of deformation and measured residual stress.

References

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