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We report on the implementation of a high frequency beam oscillation (wobbling) strategy for improving process control during laser powder bed fusion of single weld tracks on Inconel 625. Oscillation frequencies ranging from ~ 600 Hz to 7000 Hz, and different oscillation trajectories (circular, parallel or perpendicular to the direction of scanning) were explored. Highspeed imaging was used to elucidate the dynamics of the melt pool induced by the wobble beams, along with in situabsorptivity measurements to substantiate our hypothesis that the dynamic nature of wobble beams reduces absorptive losses due to laser-vapor interactions and results in improved coupling at the melt pool. OperandoX-ray radiography was carried out to visualize sub-surface melt flow dynamics, correlate to spatter mechanisms and optimize the window for improving process stability. Our observations indicate that wobble beams increase the aspect ratio of the melt pool by up to 4×, depending on the oscillation frequency and energy input. Highspeed imaging and X-ray radiography reveal an optimized process parameter window for reducing spatter, improving absorptivity and creating a stable melt pool at high (several kHz) oscillation frequencies.

Large thermal gradients, complex melt pool instabilities and poor absorptivity are application-limiting detriments in laser powder bed fusion-enabled metal additive manufacturing (AM), ultimately resulting in poor mechanical properties. Current approaches to improving the mechanical robustness of structures printed using metal additive manufacturing (AM) come at the expense of print reliability, consistency cost and integrability. Here, we employ laser beams shaped in amplitude, phase and polarization, to deliver controlled optothermal profiles on the powder bed. Unique optical properties of such beams (such as non-diffractive propagation) result in an improved combination of reduced porosity, increased part density, lower surface roughness and robust tensile properties across a broad scan parameter range, compared to conventional beams. We employ Multiphysics simulations and highspeed imaging for feedback and validation for printing SS 316L.

Management and control of damage initiation and growth on high-value National Ignition Facility (NIF) optics are critical to its operation. Cone-shaped features are currently being used on the input surfaces to arrest growth of exit surface damage by creating “shadows.” Light refracted from the walls of the cones interferes with the incident beam at the exit surface to create an intensified annulus. Significant exit surface damage was observed at the locations of these annuli. Analysis showed that this damage is consistent with a recently reported new mechanism that degrades damage resistance by dispersing damage precursors over the surface upon exposure to UV laser radiation. This has significant implications for the design and operation of high-energy and power laser systems such as the NIF.

The quality of metal objects fabricated via laser powder bed fusion are highly affected by process parameters, and their influence on final products is yet to be fully explored. In this work, pyrometry signals of the melt pool were collected from a set of stainless-steel samples during manufacturing and the effect of laser power on porosity and roughness of final printed parts was analyzed. Results show that the melt pool pyrometry signal of contours increases with higher laser power, whereas it is lower and decreases for the infilled part. Post-built X-ray computed tomography imaging reveals that porosity decreases while sample roughness increases upon increasing laser power. The decrease in porosity with increasing laser power is attributed to the larger size of the contour welds that were printed first, leading to an increase in dimension of the final products.

In situ ablation of thin metal films on fused silica substrates by picosecond class lasers was investigated as a method of characterizing the beam at the sample plane. The technique involved plotting the areas enclosed by constant fluence contours identified in optical microscope images of the ablation sites versus the logs of the pulse energies. Inconel films on commercially available neutral density filters as well as magnetron sputtered gold films were used. It was also shown that this technique could be used to calibrate real-time beam profile diagnostics against the beam at the sample plane. The contours were shown to correspond to the boundary where part or all of the film was ablated.

Typical powder bed fusion processes spread each layer of powder by pushing it across the build area with a spreader bar or roller. While simple, these methods require good flowing powder and do not have the ability to place more than one powder type per layer. This naturally limits the ability to incorporate more than one material onto each powder layer in powder bed additive manufacturing processes. Here we demonstrate electrostatic powder spreading, a method to deposit a powder layer without mechanical contact with the previous layers. Electrostatic powder spreading (ESPS) uses an electric field formed between a powder container and a counter electrode to move powder particles from the container to the powder bed. We show that the rate at which powder is deposited can be controlled by adjusting the strength of the electric field. Correspondingly, the deposited layer thickness is controlled by the strength of the electric field and the recoating speed. To demonstrate the feasibility of ESPS for laser powder bed fusion powder spreading, nearly fully dense (> 99.8%) coupons were built in a commercial L-PBF machine using the ESPS setup. Finally, we achieve 2D patterning of a single layer by using an array of electrodes and switching the applied electric field on and off. ESPS represents a method to produce an arbitrary patterned multi-material powder bed without complicated powder removal steps and easy implementation into existing L-PBF machines. Furthermore, ESPS presents an opportunity to build parts with 3D gradients in material properties.

In-situ process monitoring of additively manufactured parts has become a topic of increasing interest to the manufacturing community. In this work, acoustic measurements recorded during laser powder-bed fusion (L-PBF) were used to detect the onset of keyhole pores induced by the lasing process. Post-build radiography was used to identify the locations of keyhole pores in the build. The pore locations were spatially and temporally registered with the recorded time-series of laser position and acoustic pressure to identify specific partitions of the acoustic signals which correspond to pore formation. Ensemble empirical mode decomposition, traditional Fourier decomposition, and statistical measures of the time-series and corresponding frequency spectra were used to extract feature vectors which correlate to keyhole pore formation. Sequential feature selection revealed that measures associated with the acoustic spectra were most useful for identifying pore formations in L-PBF. A subset of the most informative data features was used to train a support vector machine model to predict pore formation with up to 97% accuracy.

Optical coherence tomography (OCT) is explored as a method to image laser-damage sites located on the surface of large aperture fused silica optics during postprocessing via CO_2 laser ablation. The signal analysis for image acquisition was adapted to meet the sensitivity requirements for this application. A long-working-distance geometry was employed to allow imaging through the opposite surface of the 5 cm thick optic. The experimental results demonstrate the potential of OCT for remote monitoring of transparent material processing applications.

Laser powder bed fusion (LPBF) is a powerful tool for additive manufacturing (AM) of metal components. However, fabricating components with overhanging features using LPBF remains a challenge. Overhangs suffer from dimensional inaccuracies, high surface roughness, and agglomerated material or dross. These parts often do not meet engineering requirements and are discarded and reprinted until requirements are met, or redesigned to avoid overhangs. Printing a flat overhang is especially challenging due to the long, unsupported powder bed that fosters material agglomeration and inhibits heat flux. Commercial LPBF machines use distinct laser parameters for the downskin layers in overhang regions. Downskin parameters for a given material are often selected by trial and error: printing many parts, changing the laser power and velocity parameters, as well as the number of layers to apply them. The impact of changing the laser parameters on the thermal history of a given layer and multiple layers is not well characterized or understood. This research uses thermal simulations, infrared (IR) imaging, and high-speed optical imaging to study the fabrication of flat overhang structures in 316L stainless steel. The thermal evolution of different downskin strategies on multiple layers is simulated with high fidelity thermal simulations and experimentally measured as the part is printed with carefully calibrated forward looking infrared (FLIR) imaging. Emissivity measurements are performed in a custom chamber under a range of conditions to accurately convert FLIR measurements to temperatures. The layer construction is also recorded with high-speed video to observe the powder bed, melt pool formation, and effects of material agglomeration. Parameter strategies that minimize layer-to-layer variations of the peak temperature, heating rate, and cooling rate are investigated to inform print conditions for overhang features.

As a fundamentally different process for creation of complex parts, laser powder bed fusion (LPBF) additive manufacturing exhibits complex behavior at the melt pool that has a direct influence on the macroscopic properties of the final material. Powder denudation, a previously studied deleterious phenomenon, is the displacement of powder due to entrainment in a jet of rapidly vaporized metal created by the process laser. To minimize the effect of denudation and inspired by methods in adjacent fields, a laser pre-sintering (LPS) method was studied in an effort to reduce denudation. LPS was found to be most effective in the 1–4 J/mm3volumetric energy density processing regimes, minimizing the common experimentally observed problems with the LPBF process. Scanning electron microscopy of the LPS-treated powder bed revealed insight into the accelerating effect morphological irregularities along the powder particle surface have in the pre-sintering process. To further understand the process, an estimation of the sintered neck sizes necessary to eliminate denudation in LPS is made, qualitatively affirming the low energy densities needed to prevent denudation. The results presented in this work represent an initial step towards understanding and implementing LPS as a quality-enhancing step in LPBF.

•In situhigh-speed thermal monitoring of melt-pool during laser powder bed fusion.•Probabilistic prediction of pore formation based on in situpyrometry monitoring.•Detection of conduction-to-keyhole transition using high-speed pyrometry.