Session: 03-04 Metals: Discontinuities and Defects

Paper Number: 93879

93879 - On the Origin of Solidification Cracks and Its Interplay With In-Process Developed Residual Stress in Cm247 Lc Superalloy via Pbf-Lb Technique 

Ni-based CM247LC superalloy is a promising composition used in several hot section components in a gas turbine. However, steep temperature gradients and processing induced residual stresses during additive manufacturing via powder bed fusion (PBF) makes this alloy highly susceptible to solidification cracking. The present study facilitates more extensive insights on the origin of solidification cracking as a function of process parameters through a combination of experimental characterisation and finite element based numerical tools. The scan speed is varied from 0.75 to 1.2 m/s, corresponding to a decrease in the heat input from 440 to 275 J/mm.

Crack density is seen to decrease with increasing scan speed (and decreasing heat input). Correspondingly, the material with higher heat input shows a weak texture, while the one with lower heat input shows a strong texture. The fraction of high angle grain boundaries is higher in the sample with high heat input. The cracking is observed to occur perpendicular to the scan direction. This is correlated to the accumulated in-process residual stress in the mushy zone using a thermo-mechanical finite element model. Significant tensile stress is found to develop for higher heat input samples in the scan direction, leading to cracking in the perpendicular direction. TEM results showed higher dislocation density on the high heat input sample relative to lower heat input. On both samples superlattice diffraction was observed indicating formation of γ’. Based on observed results, it is conclusively stated that the tendency for solidification cracking in additive manufacturing of CM247LC superalloy is a result of two interplaying parameters-microstructural (high fraction of high angle grain boundaries) and thermo-mechanical stresses (in process tensile residual stress developing during fabrication).   

Keywords: Additive manufacturing (PBF-LB), CM247LC, solidification cracking, microstructural evolution, finite element modelling.

Presenting Author: Bikash Kumar Indian Institute of Technology, Bombay

Presenting Author Biography: Dr Bikash Kumar is currently working as Post Doctoral Researcher at Indian Institute of Technology, Bombay. He completed his doctorate from Indian Institute of Technology, Guwahati with expertise in numerical prediction of residual stresses through thermo-mechanical simulations in laser welding. His primary research interest includes thermo-mechanical simulation of layered processes such as welding, additive manufacturing and thermal spray coatings. He also looks into structure-property correlation in additively manufactured metallic materials.

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