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In laser powder bed fusion (LPBF) additive manufacturing, melt pool characterization is one of the potential approaches toward rapid process qualification and efficient non-destructive evaluation of printed parts. Especially melt pool width measurement is crucial for understanding the print process regimes, estimating the solidified melt pool depth, and identifying any process anomalies, among other attributes of interest. While existing works focus on monitoring melt pools of single scan tracks or single layer prints, melt pool characterization for a multi-track multi-layer (MTML) LPBF print has not been extensively studied. In this work, we employ our lab-designed coaxial single-camera two-wavelength imaging pyrometry (STWIP) system to monitor in-situ melt pool properties during a MTML LPBF process. The STWIP-measured melt pool widths are validated using a serial sectioning machine (Robo-Met, UES). The in-situ STWIP and ex-situ Robo-Met measurement data are in close agreement with each other, having a mean absolute error and root mean squared error of 9.83 μm and 16.53 μm, respectively. Furthermore, we demonstrate the successful mapping of melt pool location and melt pool size on the printed MTML part. In sum, this work demonstrates the capability and the applicability of STWIP for accurate large-scale melt pool monitoring during LPBF processing of practical parts, thereby facilitating the development of LPBF process models and control strategies. [ABSTRACT FROM AUTHOR]

The adhesion properties of powder particles could profoundly influence the quality of parts made by Additive Manufacturing (AM) processes. Accurate experimental characterization of adhesion and the spatial surface adhesion distribution of a single microparticle has been a significant challenge, due mainly to difficulties associated with the micro-scale handling/manipulation in a controllable manner and uncertainty in the nature of micro/nano-scale contacts. In current work, an approach for determining the spatial energy/adhesion distribution on the surfaces of single microparticles used in AM is introduced and demonstrated using Molybdenum (Mo) metal particles as model particles. Both ultrasonic base and Surface Acoustic Wave (SAW)-based excitation techniques coupled with laser Doppler vibrometry are utilized to excite and acquire the vibrational rocking motion and to drive the rolling motion of single Mo particles on a Silicon (Si) substrate in a controllable manner and, thereby determining the spatial adhesion distribution on the particle surfaces. Spatial surface energy distribution data of microparticles could be utilized in Discrete Element Method (DEM) simulations as a statistical input for simulating local powder bed dynamics with increased accuracy, which would potentially lead to more predictable AM processes. [ABSTRACT FROM AUTHOR]

Graphical abstract Abstract Capping is a common mechanical defect in tablet manufacturing, exhibited during or after the compression process. Predicting tablet capping in terms of process variables (e.g. compaction pressure and speed) and formulation properties is essential in pharmaceutical industry. In current work, a non-destructive contact ultrasonic approach for detecting capping risk in the pharmaceutical compacts prepared under various compression forces and speeds is presented. It is shown that the extracted mechanical properties can be used as early indicators for invisible capping (prior to visible damage). Based on the analysis of X-ray cross-section images and a large set of waveform data, it is demonstrated that the mechanical properties and acoustic wave propagation characteristics is significantly modulated by the tablet's internal cracks and capping at higher compaction speeds and pressures. In addition, the experimentally extracted properties were correlated to the directly-measured porosity and tensile strength of compacts of Pearlitol®, Anhydrous Mannitol and LubriTose® Mannitol, produced at two compaction speeds and at three pressure levels. The effect compaction speed and pressure on the porosity and tensile strength of the resulting compacts is quantified, and related to the compact acoustic characteristics and mechanical properties. The detailed experimental approach and reported wave propagation data could find key applications in determining the bounds of manufacturing design spaces in the development phase, predicting capping during (continuous) tablet manufacturing, as well as online monitoring of tablet mechanical integrity and reducing batch-to-batch end-product quality variations. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) is rapidly becoming a local manufacturing modality in fabricating complex, custom-designed parts, providing an unprecedented form-free flexibility for custom products. However, significant variability in part geometric quality and mechanical strength due to the shortcomings of AM processes has often been reported. Presently, AM generally lacks in situ quality inspection capability, which seriously hampers the realization of its full potential in delivering qualified practical parts. Here, we present a monitoring approach and a periodic structure design for developing test artifacts for in situ real-time monitoring of the material and bonding properties of a part at fiber/bond-scale. While the production method used in current work is filament based, the proposed approach is generic as defects are always due to materials in a bonding zone and their local bonding attributes in any production modality. The artifact design detailed here is based on ultrasonic wave propagation in phononic coupons consisting of repeating substructures to monitor and eventually to assess the bond quality and placement uniformity--not only for geometry but also for defect states. Periodicity in a structure leads to the dispersion of waves, which is sensitive to geometric/materials properties and irregularities. In this proof-of-concept study, an experimental setup and basic artifact designs are described and off-line/real-time monitoring data are presented. As a model problem, the effects of printing speed on the formation of stop bands, wave propagation speeds and fiber placement accuracy in samples are detected and reported. [ABSTRACT FROM AUTHOR]

At nano/micro-scales, the resistance of a particle to rolling is a critical factor in many applications and biological phenomena as it affects particle adhesion, motion and flow as well as particle manipulation (e.g. picking and placing of microparticles). In present work, a non-contact non-invasive experimental method is detailed and applied to determine the pre-rolling critical leaning angle (CLA) of single microparticles. Transient Rayleigh surface acoustic waves (SAW) are utilized as the excitation mechanism and interferometry and image processing as detection/monitoring and analysis techniques for capturing the micro/nano-scale dynamics of the microparticles. The CLA values for a set of 30 PSL (polystyrene latex) microparticles with a diameter of 14.9 ± 0.6 μm on a soda-lime glass substrate are reported. The CLA of the studied particles are determined to be between 0.9 and 7.8°. It is also observed that during a SAW field burst cycle, microparticles could change their drifting (rolling and/or sliding) directions, speeds and accelerations. The trajectory of a particle is often found to be non-linear. This nonlinear behavior is attributed to the inhomogeneity of the surface properties of the particles and the substrate as well as possible electric charge density, chemistry variations and potential contamination on surfaces. Moreover, the effect of electric charge (developed due to the triboelectric effect) on the particle drifting motion is investigated. It is found that the percentage of drifting particles is not only a function of the amplitude of the excitation field but also a function of possible electrostatic charges developed on the particles due to the drifting motion on the substrate. In addition to their potential uses in particle manipulation, removal and adhesion characterization, the reported results could be utilized in numerical simulations of microparticle motion and deposition. [ABSTRACT FROM AUTHOR]

Toner microparticles are coated with silica nanoparticles for modulating their adhesiveness to optimize two competing requirements in the electrophotographic process: low adhesion for facilitating particle transport and high adhesion with deposited substrates prior to high temperature fusing. In addition to the adhesion effect of nanoparticle coating, nanoparticle-coated toner microparticles are subjected to various thermal fields during utilization. Understanding the effect of temperature on particle adhesion properties has the potential to improve energy efficiency, process predictability and print resolution. In current work, the effect of surface temperature on the adhesion bond between single nanoparticle-coated toner microparticles and a silicon substrate is investigated in a non-contact/non-invasive manner by monitoring the rocking dynamics of acoustically excited single microparticles on a silicon wafer surface. In the reported spectral experiments, at a range of substrate temperature levels from 15 to 30 °C, the work-of-adhesion values of the surface-particle bond are evaluated from the acquired rocking resonance frequencies of the single toner microparticles. Rayleigh Surface Acoustic Waves (SAW) are employed to generate highly controllable surface motion to excite the vibrational modes of deposited microparticles, through which the out-of-plane transient responses of their apexes are acquired using a laser vibrometer. In addition to surface temperature, the effect of coating nanoparticle spatial distribution on the adhesion properties of the microparticle is observed and explained by proposing possible contact configurations, depending upon how silica nanoparticles are accumulated in and around the particle-surface contact zone. Finally, it is observed and reported that the relative humidity (RH) can play a significant role in microparticles rocking motion dynamics and adhesion. [ABSTRACT FROM AUTHOR]

In laser powder bed fusion (LPBF), the in-situ process signatures are known to have a direct correlation with the microstructural properties of the solidified melt pool (MP). It is known that the MP cooling and heating rates, and laser processing parameters can critically determine the grain structure and thereby affect the part properties. The objective of this work is to study the feasibility of using in-process, high-speed imaging pyrometry for evaluating the solidified MP properties "below" the surface, such as depth and microstructural properties. To accomplish this, we employ an in-house single camera-based two-wavelength imaging pyrometry (STWIP) system for monitoring the printing of single-scan tracks with Inconel 718 on a commercial LPBF printer (EOS M290). The lab designed STWIP system is a coaxial high-speed (>10,000 fps) imaging system capable of monitoring MP temperature, morphology, and intensity profiles. The temperature measurements from STWIP are emissivity independent. The STWIP measured MP signatures of the printed tracks are correlated with the ex-situ microscopy characterized MP depth and the average grain lengths. From the data analysis, using support vector machine (SVM)-based regression models, we found that the MP temperature signatures are crucial for an accurate prediction of MP depth and the grain length, thus validating the novelty and necessity of the developed in-situ monitoring methods and analysis. • In-situ melt pool (MP) monitoring using a single camera based pyrometry system. • In-situ MP temperature and width measurements of single track prints. • Correlating in process MP signatures to ex-situ characterized MP depth and grain lengths. • Evaluation of solidified MP properties "below" the surface, using in-situ MP signatures. • MP temperature signatures are crucial for predicting MP depth and the grain lengths. [ABSTRACT FROM AUTHOR]

Electrostatic charge significantly affects the adhesion of microparticles. Presently the nature of surface charge distribution on a single microparticle and the effect of electrostatic charge as a contributor to its adhesion are not well understood. As a result of the strong discrepancies between the experimental adhesion measurements data and theoretical predictions over the years, some questions regarding adhesion force contributors in charged microparticles still remain unresolved. One reason for the current state of knowledge is difficulties associated with accurate measurement of particle charge. In current work, a non-contact and non-destructive opto-ultrasonic method is presented and employed to predict the equivalent bulk charge on a single charged micro-scale toner particle and to provide insight into the nature of charge distribution on its surface. From the vibrational spectral response of the particle to the ultrasonic excitation of the substrate, irregular shifting patterns of the vibrational (in-plane rocking) resonance frequencies of the particle are observed for the applied levels of substrate surface voltage (0–1500 V), implying non-uniform (patch-charges) surface charge distribution on the microparticle. For predicting the equivalent bulk charge on a single patch-charged toner particle from this resonance frequency shift, a mathematical model is developed and employed. In conclusion, it is demonstrated that, in a non-invasive/non-contact manner, the total charge on a single microparticle can be predicted using the presented experimental approach, and evidence for the non-uniform charge distribution on a single particle is observed and reported. [ABSTRACT FROM AUTHOR]

• Established comprehensive framework of fringe projection profilometry (FPP) for LPBF-AM. • Enhanced FPP measurement performance in the application scenario of LPBF. • Improved phase wrapping and unwrapping by pixel-wise correction and applying DFT. • Comprehensive measurement uncertainty calculations for the LPBF-specific FPP. • Quantitative comparison & validation of FPP measurement against standard equipment. Fringe Projection Profilometry (FPP) is a cost-effective and non-invasive technology that has been shown to measure finer features. In this work, we developed an in-situ FPP method to measure the dynamic topography of powder bed and printed layer during Laser Powder Bed Fusion (LPBF) additive manufacturing (AM) process. A systematic study towards developing a comprehensive framework of LPBF-specific FPP is demonstrated to enhance and evaluate the performance of applying FPP for in-situ LPBF monitoring, including 1) a modified sensor model with localized correction; 2) improved phase unwrapping with FFT filtering 3) quantitative uncertainty analysis; and 4) experimental validation with ex-situ characterization. The developed LPBF-specific FPP system and methods are implemented on a commercial LPBF-AM machine, achieving better accuracy, more robustness, and increased field of view while maintaining sufficient measurement range and decent resolution, in contrast to literature methods. The established FPP framework will facilitate the development of closed-loop control strategies for advancing LPBF based AM. [ABSTRACT FROM AUTHOR]

This paper details the design and operation of a testbed to evaluate the concept of autonomous manufacturing to achieve a desired manufactured part performance specification. This testbed, the autonomous manufacturing system for phononic crystals (AMSPnC), is composed of additive manufacturing, material transport, ultrasonic testing, and cognition subsystems. Critically, the AMSPnC exhibits common manufacturing deficiencies such as process operating window limits, process uncertainty, and probabilistic failure. A case study illustrates the AMSPnC function using a standard supervised learning model trained by printing and testing an array of 48 unique designs that span the allowable design space. Using this model, three separate performance specifications are defined and an optimization algorithm is applied to autonomously select three corresponding design sets to achieve the specified performance. Validation manufacturing and testing confirms that two of the three optimal designs, as defined by an objective function, achieve the desired performance, with the third being outside the design window in which a distinct bandpass is achieved in phononic crystals (PnCs). Furthermore, across all samples, there is a marked difference between the observed bandpass characteristics and predictions from finite elements method computation, highlighting the importance of autonomous manufacturing for complex manufacturing objectives. [ABSTRACT FROM AUTHOR]

Currently, the compressed tablet and its oral administration is the most popular drug delivery modality in medicine. The accurate porosity and tensile strength characterization of a tablet design is vital for predicting its performance such as disintegration, dissolution, and drug-release efficiency upon administration as well as ensuring its mechanical integrity. In current work, a non-destructive contact ultrasonic approach and an associated testing procedure are presented and employed to quantify and relate the acoustically extracted mechanical properties of pharmaceutical compacts to direct porosity and tensile strength measurements. Based on a comprehensive set of experimental data, it is demonstrated how strongly the acoustic wave propagation is modulated and correlated to the tablet porosity and tensile strength of a compact made using spray-dried lactose and microcrystalline cellulose with varying mixture ratios. The effect of mixing ratio on the porosity and tensile strength on the resulting compacts is quantified and, with the acoustic experimental data, mixing ratio is related to the compact ultrasonic characteristics. The ultrasonic techniques provide a rapid, non-destructive means for evaluating compacts in formulation development and manufacturing. The presented approach and data could find critical applications in continuous tablet manufacturing, its real-time quality monitoring, as well as minimizing batch-to-batch quality variations. [ABSTRACT FROM AUTHOR]