Relationship between associated acoustic emission and crack position during directed energy deposition of a metal matrix composite

Laser-based directed energy deposition (DED) is a versatile additive manufacturing (AM) technique capable of depositing high-quality coatings, repairing components, and fabricating complex metal matrix composite structures. The DED process, however, is prone to defects, particularly cracking, due to dynamic thermal gradients and residual stresses inherent in the process. Conventional monitoring methods, such as optical and thermal imaging, primarily focus on surface defects and often fail to detect subsurface cracks, that can significantly affect the structural integrity of fabricated structures. This study presents a novel acoustic emission (AE)-based monitoring method capable of detecting and quantifying both surface and subsurface cracks during the DED process. By exploiting the exponential decay of the unique acoustic emissions due to DED, the second-order derivative of the acoustic signal is invariant, thereby filtering extraneous noise sources and hence yielding a robust methodology for relating DED-based cracking initiation times and their associated positions. The results reveal that crack formation timing and location vary significantly with energy density. The novel techniques were used to show that higher energy density leads to slower cooling and solidification rates, resulting in delayed crack formation and detection further behind the laser beam's position.