Detecting Selective Laser Melting Beam Power from Ultrasonic Temporal and Spectral Responses of Phononic Crystal Artifacts Toward In-Situ Real-Time Quality Monitoring

Abstract

Unlike many conventional manufacturing techniques, 3D Printing/Additive Manufacturing (3DP/AM) fabrication creates builds with unprecedented degrees of structural and geometrical complexities. However, uncertainties in 3DP/AM processes and material attributes could cause geometric and structural quality issues in resulting builds and products. Evaluating the sensitivity of process parameters and material properties for process optimization, quality assessment, and closed-loop control is crucial in practice. This study presents a framework for a nondestructive in situ real-time ultrasonic monitoring approach based on the temporal and spectral dispersion analyses of specially designed artifacts with periodic internal structures called Phononic Crystal Artifacts (PCAs). The framework's effectiveness for in-situ monitoring of laser beam power in a Selective Laser Melting (SLM) process is experimentally demonstrated. A PCA is significantly simpler and/or smaller than the actual build, but it represents a specific subset of its geometric and mechanical features and complexities, which are relevant to the objectives of a quality monitoring program. Specifically, the influence of the SLM printer laser beam power on the ultrasonic responses and dispersion properties of stainless steel 316L PCAs is evaluated. Two sensing strategies based on cross-correlation and spectral dispersion analysis of ultrasonic waves transmitted in the artifacts are presented and utilized for evaluating the effect of laser power level on the mechanical and microgeometric properties of fabricated PCAs. The reported novel framework's potential in critical quality monitoring applications for in-situ real-time quality assessment of 3DP/AM processes is also discussed.

Keywords: in situ real-time quality assessment; phononic crystal artifacts; real-time laser power monitoring; selective laser melting (SLM); spectral methods; stainless steel 316L (SS316L); ultrasonic elastic waves.

FIG. 1.

Schematics of the cross-section of a mPCA_SS316L sample with its specified dimensions (a), the experimental set-up operated in the PCM, (b) and PEM (c) configurations with the ray tracing diagrams. Images of the top view (d) and side view (e) of the mPCA_SS316L set on the build plate. PCM, pitch–catch mode; PEM, pulse-echo mode.

FIG. 1.

Schematics of the cross-section of a mPCA_SS316L sample with its specified dimensions (a), the experimental set-up operated in the PCM, (b) and PEM (c) configurations with the ray tracing diagrams. Images of the top view (d) and side view (e) of the mPCA_SS316L set on the build plate. PCM, pitch–catch mode; PEM, pulse-echo mode.

FIG. 2.

Microscopic close-up images of the PCA samples' top surfaces with a scale bar of 800 μm at the laser beam power levels of L_P = 200 W (a), L_P = 225 W (b), and L_P = 250 W (c). No visible differences are observed. PCA, Phononic Crystal Artifact.

FIG. 3.

Normalized (a) pressure and (b) shear waveforms of the samples (solid lines) with the build plate waveforms (dotted lines) at three levels of L_P obtained in the PCM configuration. The ToA values of pressure and shear waves in the build plate and samples are indicated by dark vertical dashed lines. ToA, time-of-arrival.

FIG. 4.

Normalized (a) pressure and (b) shear waveforms of the mPCA_SS316L set (solid lines) with the build plate waveforms (dotted lines) obtained in PEM configuration at three levels of L_P. The W₁^P|_ij and W₁^S|_ij windows (shaded in gray) extend from the first reflection up to the second reflection of interface I₁ in the pressure and shear waves, respectively, for each waveform. The windows W₂^P|_A1 from the waveform mPCA_SS316L_P_A1 and W₂^S|_A1 from the waveform mPCA_SS316L_P_A1 (shaded in yellow) consist of reflections from interface I_L1 mixed with the reflections from interface I₁ of the sample mPCA_SS316L_A1.

FIG. 5.

Maximum cross-correlation coefficients (r_P^max and r_S^max) between the window W₂^P|_A1 from the waveform mPCA_SS316L_P_A1 and W₁^P|_ij from the pressure waveforms (a) and between window W₂^S|_A1 from the waveform mPCA_SS316L_S_A1 and W₁^S|_A1 from shear waveforms of each sample (b).

FIG. 6.

The spectral responses of the build plate (dashed line) and the sample set mPCA_SS316L (solid lines) of the acquired (a) pressure and (b) shear waveforms in the window W₁^P|_ij and W₁^S|_ij in the PEM configuration within the transducer's bandwidth.

FIG. 7.

The normalized attenuation coefficients of (a) the pressure [${\alpha }_P^n\left(f\right)$] and (b) shear [${\alpha }_S^n\left(f\right)$] waveforms and the normalized real wavenumber of (c) pressure [${\beta }_P^n\left(f\right)$] and (d) shear [${\beta }_S^n\left(f\right)$] waveforms as a function of frequency (f) in the transducer's bandwidth. The yellow shading indicates the laser power-sensitive region, which extends over the frequency range [f₁, f₂] or the wavelength range [λ₁, λ₂]. The wavelength Λ^PCA indicated by vertical dash-dotted lines.

FIG. 7.

The normalized attenuation coefficients of (a) the pressure [${\alpha }_P^n\left(f\right)$] and (b) shear [${\alpha }_S^n\left(f\right)$] waveforms and the normalized real wavenumber of (c) pressure [${\beta }_P^n\left(f\right)$] and (d) shear [${\beta }_S^n\left(f\right)$] waveforms as a function of frequency (f) in the transducer's bandwidth. The yellow shading indicates the laser power-sensitive region, which extends over the frequency range [f₁, f₂] or the wavelength range [λ₁, λ₂]. The wavelength Λ^PCA indicated by vertical dash-dotted lines.

FIG. 8.

The average normalized attenuation coefficient values for (a) pressure (${\alpha }_P^{avg}$) and (b) shear (${\alpha }_S^{avg}$) waveforms within the sensitive region ([f₁, f₂]) as a function of L_P along with the least square regression (dash-dotted) line for the data points.