On the development of a novel benchmark design for crack quantification in additive manufacturing

Solidification cracking (SC) is a defect that has been extensively studied in welding and casting and, consequently, standardized testing methods to quantify cracking have been developed for these processes. However, additive manufacturing (AM) processes currently lack any such test. The objective of the current study is to outline the development of a first-of-its-kind solidification cracking test for AM in the form of a benchmark specimen which aims to standardize quantification of solidification cracking in AM. This test serves as a novel method of crack quantification that specifically addresses the unique process characteristics of AM, such as scan strategy, geometric limitations, and layer reheating, as opposed to adopting tests designed for traditional manufacturing processes. The benchmark design utilized self-restraint to induce cracking at pre-defined locations, and was printed from Inconel 625 and 718 using laser directed energy deposition. Crack severity was quantified by optical microscopy, and it was found that the ratio of the linear crack length to the total crack length was consistently between 0.8 and 0.9. Further characterization revealed that the cracks propagated transgranularly along the melt pool boundaries. The commercially available finite-element software package Simufact Welding was used to simulate printing of the benchmark specimen using the directed energy deposition module and confirmed high levels of stress at the crack initiation locations. Based on the characterization and simulation results, it was determined that the cause of the observed cracks was likely due to ductile fracture rather than solidification cracks. Nevertheless, the benchmark was able to show a difference in the level of cracking between alloys and the ability to initiate cracks at pre-defined locations using geometrically induced restraint. Thus, it was concluded that while the existing benchmark design demonstrated progress towards a standardized test, further refinement to the design in order to improve reliability of SC formation is required.