High-speed IR Camera Research Articles

In situ melt pool measurements for laser powder bed fusion using multi sensing and correlation analysis

Laser powder bed fusion is a promising technology for local deposition and microstructure control, but it suffers from defects such as delamination and porosity due to the lack of understanding of melt pool dynamics. To study the fundamental behavior of the melt pool, both geometric and thermal sensing with high spatial and temporal resolutions are necessary. This work applies and integrates three advanced sensing technologies: synchrotron X-ray imaging, high-speed IR camera, and high-spatial-resolution IR camera to characterize the evolution of the melt pool shape, keyhole, vapor plume, and thermal evolution in Ti–6Al–4V and 410 stainless steel spot melt cases. Aside from presenting the sensing capability, this paper develops an effective algorithm for high-speed X-ray imaging data to identify melt pool geometries accurately. Preprocessing methods are also implemented for the IR data to estimate the emissivity value and extrapolate the saturated pixels. Quantifications on boundary velocities, melt pool dimensions, thermal gradients, and cooling rates are performed, enabling future comprehensive melt pool dynamics and microstructure analysis. The study discovers a strong correlation between the thermal and X-ray data, demonstrating the feasibility of using relatively cheap IR cameras to predict features that currently can only be captured using costly synchrotron X-ray imaging. Such correlation can be used for future thermal-based melt pool control and model validation.

Pool-Boiling Performance on Thin Metal Foils with Graphene-Oxide-Nanoflake Deposit.

The pool-boiling performance of water on thin metal foils with graphene-oxide deposition was studied. The boiling performance was evaluated both on fully coated surfaces, achieved by spin-coating, and surfaces with a laser-textured nucleation site, into which graphene oxide was added via drop-casting. During the experiments, a high-speed IR camera was used to obtain the transient temperature and heat-flux distribution. At the same time, a high-speed video camera was used to acquire synchronized bubble-growth recordings. In addition, a surface-wettability analysis was conducted for all the samples. In the case of fully coated samples, graphene-oxide deposition resulted in an increased number of active nucleation sites and an increase in the nucleation temperature, leading to a lowered nucleation frequency. Meanwhile, samples with a single laser-textured nucleation site enabled the analysis of isolated vapor bubbles, confirming that graphene-oxide deposition leads to a higher nucleation temperature, consequently resulting in a larger bubble-departure diameter and longer growth time. Two explanations for the results are proposed: the wettability of graphene-oxide deposition and the filling of surface microcavities with graphene-oxide nanoflakes.

A Thermographic Method to Evaluate Different Processes and Assembly Effects on Magnetic Steels

Ferromagnetic materials may be affected by the presence of local losses due to defects or magnetic anomalies caused by machining processes. To highlight such anomalies is not easy; a noninvasive thermographic method has been refined to allow a proper comparison of different machining processes’ impact on the iron losses. Specimens obtained with punching, wire erosion, and laser cut have been analyzed using a high-speed IR camera when subjected to alternate magnetization at different frequencies. Also, the same technique has been adopted to the assembled stacks to investigate more simultaneous phenomena. The possibility to point out localized anomalies should be exploited to foresee and avoid electrical machines core faults.

A Study on Reduction of In-Cylinder Heat Loss Using High-Speed IR Camera

A Study on Reduction of In-Cylinder Heat Loss Using High-Speed IR Camera

Development and validation of an inverse method for identification of thermal characteristics of a short laser pulse

The paper presents development and validation of an inverse method for the identification of thermal characteristics of a short super-Gaussian laser pulse interacting with a metal sample. The method was applied to find unknown power of the laser pulse, dimensionless shape parameter of the super-Gaussian function describing the spatial energy distribution of the beam as well as beginning and end times of its interaction with the heated body. The proposed inverse method is based on the Levenberg-Marquardt technique and utilizes temporal and spatial distributions of temperature on the rear surface of the sample, i.e., the opposite to the irradiated one. The temperature profiles were registered by a high-speed IR camera. During the experiments some of the laser beam parameters, i.e., the power, the laser beam spatial profile, beginning and end times of the exposition as well as thermophysical and optical parameters of the aluminum sample were known. Therefore, the measured data were used for both validation of the numerical model which described thermal interaction of a laser pulse with the sample as well as the assessment of correctness and accuracy of the inverse method.At the first step, numerical model of the forward problem, i.e., heat transfer in the aluminum sample irritated by the laser pulse, was developed and validated based on experimentally-determined temperature distributions. The validation was performed for both single and multiple laser pulses. The numerical model of the forward problem was implemented in the commercial software ANSYS Fluent. Then an inverse algorithm was developed and implemented with the aid of the GNU Octave environment. Subsequently, series of numerical tests were carried out. During these numerical simulations, sensitivity analysis as well as initial calibration and verification of the developed algorithm were performed. Parallel to the modeling tasks, the experimental stand was built and series of experiments were performed which allowed to assess the performance of the method using physical temperature data. Performed investigations showed that the problem is ill-conditioned. Nevertheless, relatively good accuracy of the retrieval has been obtained. It was revealed that the sensitivity of the objective function to the end time of the laser pulse is relatively low and that two of the parameters affect measured temperatures in a similar way. These two properties contributed to ill-conditioning of the inverse problem. Dependence of inverse problem solutions on the initial guess has been observed and methods allowing to minimize its influence have been identified. The accuracy of the method was affected by relatively low temporal resolution of the IR camera (500 Hz, with the exposure time approximately from 0.2 to 1 ms). Despite aforementioned problems, the method was abled to retrieve unknown laser pulse parameters with 20–25% accuracy.

A Novel Thermographic Method and Its Improvement to Evaluate Defects in Laminated and Soft Magnetic Composites Devices

Electromagnetic devices may be affected by the presence of local losses due to material defects or magnetic anomalies caused by mechanical processing. The localization of such defects is the main goal of this article; a noninvasive method has been pursued to perform the inspection and detection of imperfections or defects. The adopted approach is based on the observation of the device under test with a high-speed IR camera; no limitations in size and shapes devices are considered and the method can be widely adopted. Examples of defects detection in the magnetic circuit realization are proposed, both for traditional ferromagnetic laminated sheets and for soft magnetic composites.

Wavelength-modulated planar laser-induced fluorescence for imaging gases.

This work presents the development of wavelength-modulated planar laser-induced fluorescence (WM-PLIF) and its initial application to infrared imaging of carbon monoxide in a laminar flame. A continuous-wave quantum-cascade laser producing 50mW near 4.8μm was injection-current modulated at 1kHz and scanned across the P(20) transition of CO at 20Hz. The corresponding infrared-laser-induced fluorescence from 2065 cm-1 to 2155 cm-1 was imaged orthogonal to the laser sheet using a high-speed IR camera, and digital lock-in detection of the WM-PLIF first-harmonic signal (SF,1f) was performed to provide high-fidelity, background-free imaging of CO. Images of the peak-SF,1f signal are presented for a laminar CO-H2 diffusion flame in air at atmospheric pressure. We demonstrate that this technique is sensitive enough to image nascent CO in flames and present a strategy for simulating the WM-PLIF harmonic signals.

Full-Field Measurement of Strain and Temperature in Quasi-Static and Dynamic Tensile Tests on Stainless Steel 316L

Abstract Tensile tests at low and high strain rates in which full-field strain and full-field temperature are measured simultaneously on the surface of the specimen during the test are presented. Various testing methods are used (including the tensile Split Hopkinson Bar for testing at high strain rates) for loading the specimens. Strain measurements are done using the Digital Image Correlation technique and temperatures are measured with a high-speed IR camera. The experiments provide details of the strain and temperature during the uniform deformation before the initiation of necking and in the necking region during the necking. Results from testing 316L stainless steel show a uniform temperature rise during the uniform deformation which is higher with increasing strain rate and a significant increase in the strain rate and temperature during the necking in the necking region. The strain rate in the necking area can be three times the nominal strain rate and, at a high strain rate test, the temperature in the necking area can increase by 300°C.

High-speed digital holography for neutral gas and electron density imaging.

An instrument was developed using digital holographic reconstruction of the wavefront from a CO2 laser imaged on a high-speed commercial IR camera. An acousto-optic modulator is used to generate 1-25 μs pulses from a continuous-wave CO2 laser, both to limit the average power at the detector and also to freeze motion from sub-interframe time scales. Extensive effort was made to characterize and eliminate noise from vibrations and second-surface reflections. Mismatch of the reference and object beam curvature initially contributed substantially to vibrational noise, but was mitigated through careful positioning of identical imaging lenses. Vibrational mode amplitudes were successfully reduced to ≲1 nm for frequencies ≳50 Hz, and the inter-frame noise across the 128 × 128 pixel window which is typically used is ≲2.5 nm. To demonstrate the capabilities of the system, a piezo-electric valve and a reducing-expanding nozzle were used to generate a super-sonic gas jet which was imaged with high spatial resolution (better than 0.8 lp/mm) at high speed. Abel inversions were performed on the phase images to produce 2-D images of localized gas density. This system could also be used for high spatial and temporal resolution measurements of plasma electron density or surface deformations.

Optical Monitoring and Numerical Simulation of Temperature Distribution at Selective Laser Melting of Ti6Al4V Alloy

Selective laser melting (SLM) is a modern method for producing objects with complex shape and fine structures in one working cycle from metal powders. Combination of the advanced technology of SLM with unique properties of Ti6Al4V alloy allows creating complex 3D objects for medicine or aerospace industry. Since properties of SLM parts depend on the geometrical characteristics of tracks and their cohesion, optical monitoring is actually used to for control the process. Temperature gradient determines the microstructure and mechanical properties of the SLM part, so studies about temperature fields are primarily important. On-line monitoring during laser scanning of Ti6Al4V alloy and formation of a single track in real-time with high-speed IR camera was studied. Numerical simulation allowed estimation the temperature distribution during processing. Conclusion regarding control system based on the online monitoring of deviations of the signal from IR camera during the SLM process was done.

Thermocapillary instabilities in a falling liquid film at small Reynolds numbers

The formation of thermocapillary structures on the surface of falling liquid film of 68% water solution of glycerol has been experimentally studied using a high-speed IR camera under different conditions on the heating surface. The longitudinal temperature gradient is found to affect significantly the formation of rivulets. It is shown that external perturbations cause a transition between structures of different types.

물리 모델 분석을 통한 상호 작용형 스키시뮬레이터 동작인식 시스템 개발

본 연구에서는 스키 시뮬레이터의 사용자 감응형 제어를 위한 물리모델과 동작인식 시스템의 개발되었으며, 스키 선수의 자세 변화에 따른 스키 슬로프 상에서의 거동과의 부합성 확보를 위하여 실제 현장 실험데이터의 회귀분석을 통해 동작인식 시스템에 사용될 파라미터 및 관계식을 도출하였다. 개발된 물리모델 기반 스키 동작 인식 시뮬레이터는 실시간으로 Kinect 장치를 사용하여 사용자의 관절별 질량을 분석하여, 정확한 체중심을 추정하고, 시뮬레이터 하드웨어에서 적용할 수 있도록 힘, 속도, 가속도에 대한 피드백을 전달하도록 구성되었다. 본 연구결과는 스키시뮬레이터의 인식모듈로 사용되었으며, 물리모델 기반 가상 스포츠 시뮬레이터 제작에 응용 할 수 있는 자료로 활용될 것이다.

Study of non-axisymmetric divertor footprints using 2-D IR and visible cameras and a 3-D heat conduction solver in NSTX

Toroidally non-axisymmetric divertor profiles during the 3-D field application and for ELMs are studied with simultaneous observation by a new wide angle visible camera and a high speed IR camera. A newly implemented 3-D heat conduction code, TACO, is used to obtain divertor heat flux. The wide angle camera data confirmed the previously reported result on the validity of vacuum field line tracing on the prediction of split strike point pattern by 3-D fields as well as the phase locking of ELM heat flux to the 3-D fields. TACO calculates the 2-D heat flux distribution allowing assessment of toroidal asymmetry of peak heat flux and heat flux width. The degree of asymmetry (εDA) is defined to quantify the asymmetric heat deposition on the divertor surface and is found to have a strong positive dependence on peak heat flux.

Thermal entry length in falling liquid films at high Reynolds numbers

A behavior of the length of thermal entry was studied experimentally in the vertically falling wavy liquid film at high Reynolds numbers with application of the field methods for thickness and temperature measurements. According to data, obtained by a high-speed IR camera, the zones, whose temperatures exceed the inlet temperature of the film, are formed at 7–10mm from upper edge of the heater in the interrivulet area between the crests of developed 3D synchronous waves even at low heat fluxes. It was determined that in the area of rivulets, formed on the crests of 3D synchronous waves, the length of thermal entry is higher, but it decreases drastically with a rise of heat flux. Analysis of data on the film thickness, obtained by the fluorescent method, has shown that between the synchronous 3D waves in spanwise direction the average film thickness is lower. Respectively, the values of local Reynolds numbers of the film and distances before the appearance of temperature nonuniformity decrease. In these regions the effect of thermocapillary forces is obvious even at relatively low heat fluxes, what finally leads to a considerable decrease in the length of thermal entry.

Bubble growth and horizontal coalescence in saturated pool boiling on a titanium foil, investigated by high-speed IR thermography

Abstract Growth of an isolated bubble and horizontal coalescence events between bubbles of dissimilar size were examined during pool nucleate boiling of water on a horizontal, electrically-heated titanium foil 25 μm thick. Wall temperature measurements on the back of the foil by high-speed IR camera, synchronized with high-speed video camera recordings of the bubble motion, improved the temporal and spatial resolution of previous observations by high-speed liquid crystal thermography to 1 ms and 40 μm, respectively, leading to better detailed maps of the transient distributions of wall heat flux. The observations revealed complex behaviour that disagreed with some other observations and current modelling assumptions for the mechanisms of heat transfer over the wall contact areas of bubbles and interactions between bubbles. Heat transfer occurred from the entire contact area and was not confined to a narrow peripheral triple-contact zone. There was evidence of an asymmetrical interaction between bubbles before coalescence. It was hypothesised that a fast-growing bubble pushed superheated liquid under a slow-growing bubble. Contact of this liquid with regions of the wall that had been pre-cooled during bubble growth caused local reductions in the wall heat flux. During coalescence, movement of liquid under both bubbles caused further changes in the wall heat flux that also depended on pre-cooling. Contraction of the contact area caused a peripheral reduction in the heat flux and there was no evidence of a large increase in heat flux during detachment. Boiling on very thin foils imposes special conditions. Sensitivity to the thermal history of the wall must be taken into account when applying the observations and hypotheses to other conditions.

Visualisation of jet fires from hydrogen release

Hydrogen release tests with defined ignition represent one important aspect of hydrogen combustion in a potential accident. This paper shows the application of high-speed cinematography in combination with image processing techniques, the background-oriented-schlieren (BOS) and a difference method to visualise the shape of hydrogen jet. In addition, these methods were also used to observe the ignition and combustion zone on defined initiation. In addition the combustion zone was recorded by a high-speed IR camera. The IR camera was synchronized with a rotating filter wheel to generate four different motion pictures at 100Hz each on a defined spectral range. The results provide some detailed information that might be used for improving model predictions.

Real-Time Compact Thermal Models for Health Management of Power Electronics

Implementation of real-time health assessment and thermal management of power electronic devices require real-time electrothermal models that can be used to predict the temperatures of device junctions, interfaces, etc., which cannot ordinarily be measured during service. This paper presents a real-time reduced-order compact thermal model, which is incorporated in a pulsewidth modulation and current controlled full bridge. An accurate representation of the dynamic thermal behavior was obtained experimentally and converted into a simplified multiexponential form then combined with lookup tables that provide estimates of the device losses based on measured values of the phase current. For interfaces away from the surface, such as solder layers, a validated Flotherm model is used to predict the temperatures of the hidden layers. Comparison of the real-time temperature estimates with IR measured values obtained from a high-speed IR camera showed that the reduced-order model was capable in estimating the module's temperatures over a range of modulation conditions. This real-time model is well-suited to the continuous monitoring of the internal behavior of the electrothermal effects within power electronic modules and can thus be used as part of a prognostic tool to provide knowledge through thermal cycling by combining with thermomechanical wear out models for health management of power electronics.

Effect of Substrate Temperature on the Formation Mechanism of Cold-Sprayed Aluminum, Zinc and Tin Coatings

When describing the cold-spray process, one of the most widely used concepts is the critical velocity. Current models predicting critical velocities take the temperature of the sprayed particles explicitly into account, but not the surface temperature (substrate or already deposited layers) on which the particle impacts. This surface temperature is expected to play an important role, since the deformation process leading to particle bonding and coating formation takes place both on the particle and the substrate side. The aim of this work is to investigate the effect of the substrate temperature on the coating formation process. Experiments were performed using aluminum, zinc, and tin powders as coating materials. These materials have a rather large difference in critical velocities that gives the possibility to cover a broad range of deposition velocity to critical velocity ratio using commercial low-pressure cold-spray system. The sample surface was heated and the temperature was varied from room temperature to a high fraction of the melting point of the coating material for all three materials. The change in temperature of the substrate during the deposition process was measured by means of a high speed IR camera. The coating formation was investigated as a function of (1) the measured surface temperature of the substrate during deposition, (2) the gun transverse speed, and (3) the particle velocity. Both single particle impact samples and thick coatings were produced and characterized. Both the particle-substrate and interparticle bonding were evaluated by scanning electron microscopy (SEM) and confocal microscopy.

Characterization of Crack Initiation Behavior in Ceramics under Thermal Shock

A new experimental technique, Disc-on-Rod test, was developed for evaluating the thermal shock fracture characteristics of ceramic materials. The specimen is disc shaped with a diameter of 20 mm and a thickness of 0.6 mm, and the 2-dimensional thermal stress field was obtained. The specimen was heated to a high temperature and the center region was quenched by means of contact with a cool metal rod with a diameter of 4 mm. The temperature distribution of the specimen was measured by a high speed IR camera and used for the determination of the thermal stress field in the specimen using FEM calculations. In order to evaluate the fracture process, AE signals during thermal shock fracture were detected. It was observed that the maincrack was initiated at the center region, which was subjected to the maximum balanced biaxial stress. The critical stress for maincrack formation was evaluated and compared with that under mechanical loading, considering the volume effect of strength based on Weibull statistics.