Detection and location of microdefects during selective laser melting by wireless acoustic emission measurement

Microdefects sometimes form in workpieces fabricated by selective laser melting (SLM). It is difficult to understand the mechanism of generation of microdefects because the occurrence time during processing cannot be easily identified by conventional monitoring methods. We have therefore developed a convenient method for real-time monitoring of microdefects generation during SLM by using proprietary battery-powered equipment capable of continuous recording and wireless transmission of acoustic emission (AE) waveforms. To clarify the detection capabilities of the proposed method and to evaluate the factors that cause microcrack generation, AE monitoring was conducted for single-layer modeling tests with single-track or multi-track. We could successfully detect AE signals during the tests. The times of occurrence and locations of AE events were identified, and the results were compared with observations of the specimen cross-sections by the X-ray computed tomography (CT). At first in single-track tests, burst type AE events were detected during processing and their origin was confirmed to be pores and microcracks in the specimen. Then in multi-track tests, defects slightly after laser irradiation and defects due to the turning back of the irradiation were detected. The effectiveness and potential of our novel AE monitoring technique for the SLM process have been demonstrated.