<p>In metal directed energy deposition (DED), defects such as porosity, lack-of-fusion and cracking often occur during its material melting-solidification process. These defects may deteriorate the mechanical properties and quality of the manufactured component, and the formation mechanism of these defects is complicatedly affected by various DED parameters. In order to manufacture defect-free and structurally sound objects, both offline inspection and online monitoring techniques are in demand. In this research, a femtosecond laser based&nbsp;transient thermoreflectance (TTR) technique is developed for porosity inspection in metal DED. The major contributions and advantages of the proposed technique include the following:&nbsp;(1) A femtosecond laser TTR measurement system is developed for fully noncontact measurement of thermoreflectance from a newly deposited layer in&nbsp;DED.&nbsp;The TTR measurement is realized in a pump-probe setup, in which a modulated pump laser pulse excites the deposited layer, and a time-delayed probe laser pulse measures the variations in the optical properties induced by the pump pulse;&nbsp;(2) Porosity is inspected by comparing the&nbsp;thermoreflectance&nbsp;measured at different pump modulation frequencies.&nbsp;The acquired&nbsp;thermoreflectance&nbsp;will vary with the modulation frequency if pores exist in&nbsp;the deposited layer but will remain constant in the absence of porosity regardless of the modulation frequency.&nbsp;By modulating the pump excitation at multiple frequencies, the proposed technique is made sensitive to porosity and insensitive to&nbsp;the local thermal property alteration during the DED process; and (3) Due to the noncontact nature and scanning capability, the proposed porosity inspection technique can be readily applied to in-situ porosity monitoring during the DED process.&nbsp;Together with instantaneous correction actions, enhanced quality of the manufactured objects can be achieved.&nbsp;Validation tests were performed on Ti-6Al-4V samples manufactured with&nbsp;different DED printing laser power and scan speed. When comparing the test results with the conventional microscopic inspection results, the developed technique achieved a success rate of 93.3%.</p>

Porosity Inspection in Metal Directed Energy Deposition Using Femtosecond Laser Based Transient Thermoreflectance Measurement

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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