Direct observation of crack formation mechanisms with operando Laser Powder Bed Fusion X-ray radiography

Laser powder bed fusion (L-PBF) is a versatile additive manufacturing (AM) process that can print geometrically complex metal parts for a variety of applications. However, poor control of defect formation during processing hampers its widespread industrial adoption. Many materials suffer from a high crack susceptibility during L-PBF, which results in degraded mechanical properties, and is an obstacle to the certification of critical parts.

The CM247LC nickel-based superalloy is a γ´-strengthened Ni superalloy, which is of particular interest due to its excellent mechanical, creep, wear, and oxidation properties at both ambient and elevated temperatures. However, this alloy contains high amounts of Al and Ti and is considered vulnerable to cracking due to L1₂ Ni₃ (Al, Ti) precipitation. 

To unveil the actual cracking mechanisms, we have employed for the first time high-speed synchrotron X-ray radiography in combination with a miniaturized L-PBF setup that reproduces real processing conditions. These  operando experiments were performed at the TOmographic Microscopy and Coherent rAdiology experimenTs (TOMCAT) beamline of the Swiss Light Source. Two distinct crack-related features could be observed: 1) the formation, growth, and remelting of cracks after interaction with the melt pool, and 2) the growth of existing cracks after interaction with the heat-affected zone (HAZ).

The operando radiography experiments are complemented by electron microscopy observations, operando X-ray diffraction at MicroXAS and finite element simulations. This unique combination  allows to unequivocally identify and separate solidification and liquation cracking mechanisms. It is anticipated these results will stimulate the development of more accurate numerical simulations of the L-PBF process, cracking criteria, and new approaches to achieve crack-free L-PBF nickel-based parts, thereby unraveling the full industrial potential of AM technologies for these alloys.