In-situ detection of stochastic spatter-driven lack of fusion: Application of optical tomography and validation via ex-situ X-ray computed tomography

The presence of random defects in laser powder bed fusion (LPBF) parts is an issue that challenges the reliability of this manufacturing process and hinders its employment in structural, defect-sensitive components. A potential solution to increase the reliability of LPBF is employing in-process monitoring targeting defect detection. This study aims to detect stochastic defects driven by spatter particles via in-situ monitoring and validate the detection method ex-situ via X-ray computed tomography (XCT). By means of in-situ optical tomography (OT), monitoring images were registered layerwise during the manufacturing of Hastelloy X specimens. The images were analyzed to detect spatters landing within specimen boundaries, and the spatial coordinates of the detections were obtained. The specimens were also measured ex-situ by means of XCT, from which key features and coordinates of internal defects were obtained. The in-situ spatter detection method was then compared to the XCT measurements. It was found that 79 % of lack of fusion defects were detected in OT images. The detection was particularly successful for large defects. Spatter-induced lack of fusion defects were present in the specimens manufactured with optimized processing parameters in different degrees, depending on the robustness of the processing conditions to spatters. This study demonstrates the applicability of optical tomography in-situ monitoring for indirect detection of stochastic lack of fusion, whose presence is inferred from spatter redeposits on the powder bed

Photothermal excitation of microcantilevers in liquid: effect of the excitation laser position on temperature and vibrational amplitude

Demands to improve the sensitivity and measurement speed of dynamic scanning force microscopy and cantilever sensing applications necessitate the development of smaller cantilever sensors. As a result, methods to directly drive cantilevers, such as photothermal or magnetic excitation, are gaining in importance. Presented is a report on the effect of photothermal excitation of microcantilevers on the increase in steady-state temperature and the dynamics of higher mode vibrations. First, the local temperature increase upon continuous irradiation with laser light at different positions along the cantilever was measured and compared with finite element analysis data. The temperature increase was highest when the heating laser was positioned at the free end of the cantilever. Next, the laser intensity was modulated to drive higher flexural modes to resonance. The dependence of the cantilever dynamics on the excitation laser position was assessed and was in good agreement with the analytical expressions. An optimal position to simultaneously excite all flexural modes of vibration with negligible heating was found at the clamped end of the cantilever. The reports findings are essential for optimisation of the excitation efficiency to minimise the rise in temperature and avoid damaging delicate samples or functionalisation layers

Array based real-time measurement of fluid viscosities and mass-densities to monitor biological filament formation

Liquid mass density and viscosity are fundamental characteristics of fluids. Their quantification by means of classical viscosity and density meters has several drawbacks: (i) the liquid-density and the viscosity cannot be measured simultaneously, (ii) sample volumes in the mL-range are consumed, (iii) the measurements cannot be multiplexed, and, (iv) the quantifications are time-consuming (minutes). Nano-mechanical transducers promise to overcome these limitations. We use fully clamped, gold coated silicon-nitride membranes with a thickness of 200 nm to measure liquid viscosity and density of samples of 1 L volumes residing above the membrane in a miniature well. Photo-thermal actuation is used to excite the membrane, and an optical deflection system measures the response. From the response spectra, the eigenfrequency (f) and the quality (Q) factor are extracted and used to determine liquid density and viscosity by applying a three-point calibrated, simplified lumped model. We tested the system using calibrated solutions with viscosities in the range of 1-219 mPa s and mass densities between 998 kg m(-3) and 1235 kg m(-3). Real-time measurements were performed that characterize the polymerization of G-actin to F-actin filaments. The method presented promises to overcome the aforementioned limitations and thereby enables the real-time characterization of sub-L sample volumes in a multiplexed manner

Predicting laser powder bed fusion defects through in-process monitoring data and machine learning

Industry application of additive manufacturing demands strict in-process quality control procedures and high product quality. Feedback loop control is a reasonable solution and a necessary tool. This paper demonstrated our preliminary work on the laser powder-bed fusion feedback loop: predict local porosity through in-process monitoring images and machine learning. 3D models were rebuilt from in-situ optical tomography monitoring images and post-build X-ray CT images. They were registered to the original CAD. Dataset for machine learning was assembled from those registered 3D models. The trained machine learning model can precisely predict local porosity caused by lack of fusion and keyhole with multi-layer monitoring images. It also indicates the optimal processing window. It is impossible to be sure about the occurrence of defects in a layer based only on the abnormality of a single layer, and vice versa. Defects in a layer can be caused by improper parameters or anomalies in current layer or subsequent layers; defects in one layer can also be eliminated by proper parameters in the following layers. The work laid the basis for the next step feedback loop control of pore defect

Fluid characterization by resonant nanomechanical sensing

Microfluidic technologies allow handling and characterizing liquid samples on the micro- to picoliter scale. Thereby, the viscosity and mass density are key properties of such samples, because they characterize their flow behavior. The viscosity of a liquid indicates its resistance to flow, whereas the density quantifies the mass per volume. Molecular transformations, such as chemical polymerization, protein folding and aggregation, or nucleic acid hybridization, influence both properties. Therefore, measuring them is fundamental for basic research, quality control, and process monitoring. Since many, especially biological, samples are only available in small quantities and/or expensive, reducing sample consumption is essential. Furthermore, the acquisition time of viscosity measurements nowadays is on the order of minutes, limiting the characterization of large numbers of samples. Hence, increasing the time resolution and the throughput is another significant requirement. It was early noticed that the dynamics of nanomechanical resonators are strongly influenced by the surrounding fluid. This effect can be utilized to measure the fluid properties, specifically the viscosity and mass density. In this thesis, resonant nanomechanical cantilevers were, therefore, employed with focus on the application of higher modes of vibration. First, a suitable method to excite and detect the strongly damped cantilever resonances encountered in liquid was realized: Photothermal excitation uses an intensity-modulated laser to induce cantilever vibration. Its direct and local energy transfer avoids distortions arising in prevalent excitation methods, such as piezo-acoustic excitation, and results in spurious-free resonance spectra. To detect the nanometer vibrations of the cantilevers, a second laser was used in an optical beam deflection configuration. Such optical excitation/detection method is accurate and robust, however, it is only suitable for transparent liquids. Technical details about the developed setup are provided in the appendix of this thesis. Due to the small dimensions of the microfluidic channel containing the cantilever sensors, the influence of proximate surfaces was investigated. Placing a vibrating cantilever below a critical distance to a surface induces squeeze-film damping. The magnitude and range of this undesirable effect on higher mode vibrations was characterized and incorporated in the fluid channel design. The above findings are generally applicable to atomic force microscopy and nanomechanical sensing in liquid. Next, the ability of the sensor to measure viscosity and mass density of liquids was assessed. Dynamic properties of the cantilever resonator were derived from resonance spectra and converted into the surrounding liquid properties, using adapted hydrodynamic models. Multiple modes of vibration covered a broad frequency range in the order of kHz to MHz. A stringent temperature control was implemented, due to the high temperature dependency of the measured parameters. To investigate time-resolved processes, free-radical polymerization reactions were tracked and characterized. The shear-thinning behavior of the polymer solutions, i.e., the non-Newtonian effect of decreasing viscosity with increasing frequency, was resolved by the instrument. The time to characterize a 5 µL sample was on the order of 1 min. Finally, the setup was optimized for automated high-throughput screening of microliter sample droplets. The droplets were generated by an automated sampler and separated by fluorinated oil. To achieve the required time resolution, a higher vibrational mode was tracked using two phase-locked loop demodulators. This allowed to derive the viscosity and mass density of the liquid surrounding the resonator with a temporal resolution of about 1 ms. The instrument was able to detect ~1 µL droplets at a rate on the order of 1 s per droplet. The developed viscosity and mass density sensor opens several possibilities. We recently initiated the study of stimulus-responsive polymers for glucose sensing and the unfolding behavior of proteins. This, by solely measuring changes in viscosity after introducing the analyte or inducing denaturation. Future work could involve monitoring of RNA hybridization and protein aggregation into fibrils

Influence of squeeze-film damping on higher-mode microcantilever vibrations in liquid

The functionality of atomic force microscopy (AFM) and nanomechanical sensing can be enhanced using higher-mode microcantilever vibrations. Both methods require a resonating microcantilever to be placed close to a surface, either a sample or the boundary of a microfluidic channel. Below a certain cantilever-surface separation, the confined fluid induces squeeze-film damping. Since damping changes the dynamic properties of the cantilever and decreases its sensitivity, it should be considered and minimized. Although squeeze-film damping in gases is comprehensively described, little experimental data is available in liquids, especially for higher-mode vibrations. We have measured the flexural higher-mode response of photothermally driven microcantilevers vibrating in water, close to a parallel surface with gaps ranging from ~200 μm to ~1 μm. A modified model based on harmonic oscillator theory was used to determine the modal eigenfrequencies and quality factors, which can be converted into co-moving fluid mass and dissipation coefficients. The range of squeeze-film damping between the cantilever and surface decreased for eigenfrequencies (inertial forces) and increased for quality factors (dissipative forces) with higher mode number. The results can be employed to improve the quantitative analysis of AFM measurements, design miniaturized sensor fluid cells, or benchmark theoretical models

Automated high-throughput viscosity and density sensor using nanomechanical resonators

Most methods used to determine the viscosity and mass density of liquids have two major drawbacks: relatively high sample consumption (similar to milliliters) and long measurement time (minutes). Resonant nanomechanical cantilevers promise to overcome these limitations. Although sample consumption has already been significantly reduced, the time resolution was rarely addressed to date. We present a method to decrease the time and user interaction required for such measurements. It features (i) a droplet-generating automatic sampler using fluorinated oil to separate microliter sample plugs, (ii) a PDMS-based microfluidic measurement cell containing the resonant microcantilever sensors driven by photothermal excitation, (iii) dual phase-locked loop frequency tracking of a higher-mode resonance to achieve millisecond time resolution, and (iv) signal processing to extract the resonance parameters, namely the eigenfrequency and quality factor. The principle was validated by screening series of 3 pi, droplets of glycerol solutions separated by fluorinated oil at a rate of similar to 6 s per sample. An analytical hydrodynamic model (Van Eysden and Sader, 2007 [6]) and a reduced order model (Heinisch et al., 2014 [161) were employed to calculate the viscosity and mass density of the sample liquids in a viscosity range of 1-10.5 mPa s and a density range of 998-1154 kg m(-3)

Real-Time Viscosity and Mass Density Sensors Requiring Microliter Sample Volume Based on Nanomechanical Resonators

A microcantilever based method for fluid viscosity and mass d. measurements with high temporal resoln. and microliter sample consumption is presented. Nanomech. cantilever vibration is driven by photothermal excitation and detected by an optical beam deflection system using two laser beams of different wavelengths. The theor. framework relating cantilever response to the viscosity and mass d. of the surrounding fluid was extended to consider higher flexural modes vibrating at high Reynolds nos. The performance of the developed sensor and extended theory was validated over a viscosity range of 1-20 mPa·s and a corresponding mass d. range of 998-1176 kg/m3 using ref. fluids. Sepg. sample plugs from the carrier fluid by a two-phase configuration in combination with a microfluidic flow cell, allowed samples of 5 μL to be sequentially measured under continuous flow, opening the method to fast and reliable screening applications. To demonstrate the study of dynamic processes, the viscosity and mass d. changes occurring during the free radical polymn. of acrylamide were monitored and compared to published data. Shear-thinning was obsd. in the viscosity data at higher flexural modes, which vibrate at elevated frequencies. Rheokinetic models allowed the monomer-to-polymer conversion to be tracked in spite of the shear-thinning behavior, and could be applied to study the kinetics of unknown processes. [on SciFinder(R)