Defect detection in additively manufactured AlSi10Mg and Ti6Al4V samples using laser ultrasonics and phase shift migration

Laser ultrasonics (LU) is a non-contact and non-destructive method with a high data acquisition rate, making it a promising candidate for in-situ monitoring of defects in different additive manufacturing (AM) processes, including laser powder bed fusion (LPBF) and directed energy deposition, as well as final part inspection. In order to see the effect of various artificial defect types on an LU sub-surface reconstruction, AlSi10Mg samples with side through-holes, as well as Ti6Al4V samples with bottom blind holes and trapped powder were printed using LPBF, and then ultrasound B-scans of the samples were obtained using an LU system. The resulting scan data was processed using a custom frequency domain phase shift migration (PSM) algorithm, to reconstruct the defects and their locations. Novel ways of pre-processing the B-scan, used as an input to PSM, and taking advantage of its frequency representation, are demonstrated. Newton's method was used to find a stationary phase approximation, used to account in the frequency domain for the fixed offset emitter-receiver arrangement within the PSM calculation. The Newton's method calculation time was reduced by 33%, by using an approximation of the phase function to find an initial guess. The smallest defects that were detected using this method were in the size range between 200 to 300μm for the bottom hole defects, using an 8 ns laser pulse duration. The effect of the laser on the surface of a part being built, and the challenges and further work needed to integrate LU in a LPBF machine for in-situ inspection are discussed.

On the application of sound radiation force for focusing of powder stream in directed energy deposition

One of the challenges in directed energy deposition via powder feeding (DED-PF) is the powder stream divergence that results in low catchment efficiency (i.e., the fraction of particles added to the melt pool). This article introduces a new ultrasound-based powder focusing method referred to as ultrasound particle lensing (UPL), tailored for powder used in DED-PF. The method uses an ultrasound phased array to produce a small volume of high-intensity ultrasound with the required period averaged sound intensity profile. UPL was used to acoustically focus streams of Ti64 and SS 316L particles with an average size of 89μm and a particle speed of 0.6 m/s, exiting from a DED-PF nozzle analog. The e^-1 powder stream widths downstream of the resulting force fields for both materials were reduced by 30%. The experimental results closely match Lagrangian and Eulerian simulations of the process. This novel setup offers the possibility of fast control of the powder stream divergence angle and effective diameter in the process zone during the DED-PF process. This will in turn improve the feature resolution and catchment efficiency of the process.