A Study on the In-Situ Melt Pool Size Estimation Method for Directed-Energy Additive Manufacturing Based on Modal Parameters

Additive manufacturing technology is gaining significant attention in many industries such as automobile, aerospace, and medical. In particular, directed-energy additive manufacturing using a laser source is of central interest, thanks to its geometric precision and higher quality products compared with other heat sources. To produce high-quality components, in-situ melt pool monitoring technology is required. This article deals with an in-situ melt pool size estimation method for directed-energy additive manufacturing using the measured resonant frequency and damping ratio. The test set-up is composed of an impact hammer and two laser Doppler vibrometers (LDV) connected to a Siemens LMS SCADAS FFT analyzer. The LDV was used to measure the velocity of the melt pool and an Al beam, whereas the impulse signal is given by the impact hammer. The melt pool shape was machined by drilling into the top surface of an Al beam that is 50 mm wide and 100 mm long. The diameter (D) of the melt pool was machined to 1 and 2 mm, and the depth of melt pool was machined from 0.5 to 1.5 D. A low-melting-temperature metal that transitions a 70 degrees C was used to readily achieve the liquid state of the melt pool in a laboratory environment. The frequency response function results were obtained from the Siemens Test Laboratory Software. Then the modal parameters were calculated by using the rational fraction polynomial method to extract the natural frequency and damping ratio according to the Al beam length, melt pool diameter, and melt pool depth. As a result, we confirmed that the difference in the resonant frequency and a damping ratio of the melt pool was a function of the Al beam length, melt pool diameter, and melt pool depth.