High speed thermal imaging and modeling of laser powder bed fusion manufactured WC-Ni cemented carbides

High Speed Thermal Imaging and Modeling of Laser Powder Bed Fusion Manufactured Wc-Ni Cemented Carbides

HDRC (high dynamic range CMOS) allows for more than 120 dB signal range in image processing. Scene details with both very high and extremely low radiant flux may thus appear within the same image. Color constancy over the entire signal range and good high speed performance are further aspects of this logarithmic imager technology. These features qualify HDRC cameras for thermography, since the signal range of Planck's temperature radiation in a two dimensional array is comparable to HDRC's intensity range. Especially in material welding and laser cutting processes, in high power light sources and in high temperature material processing, fast monitoring of the spacial and dynamic temperature distributions present a challenge to conventional thermal imaging and thus call for innovative concepts. A particular challenge is in the compensation of the emissivity of the radiating surface. Here, we present a new concept based on a modified HDRC VGA color camera, allowing for visualization and measurement of temperatures from about 800 °C up to 2300 °C. The modifications include an optical filter for minimizing UV and IR straylight and a notch filter for clipping off the green optical range in order to separate the blue and red RGB regions. An enhanced and adapted software provides a division of the neighboured red and blue pixel signals by means of simply subtracting the HDRC signals. As a result the local temperature information of the visualized scene spot is independent of emissivity. This is, to our knowledge, the first demonstration of a high speed thermal imager to date.

Ballistic impact experiments were performed on triaxially braided polymer matrix composites to study the heat generated in the material due to projectile velocity and penetration damage. Quantifying the heat generation phenomenon is crucial for attaining a better understanding of composite behavior and failure under impact loading. The knowledge gained can also be used to improve physics-based models which can numerically simulate impact of composites. Triaxially braided (0/+60/-60) composite panels were manufactured with T700S standard modulus carbon fiber and two epoxy resins. The PR520 (toughened) and 3502 (untoughened) resin systems were used to make different panels to study the effects of resin properties on temperature rise. Ballistic impact tests were conducted on these composite panels using a gas gun, and different projectile velocities were applied to study the effect on the temperature results. Temperature contours were obtained from the rear surface of the panel during the test through a high speed, infrared (IR) thermal imaging system. The contours show that high temperatures were locally generated and more pronounced along the axial tows for the T700S/PR520 composite specimens; whereas, tests performed on T700S/3502 composite panels using similar impact velocities demonstrated a widespread area of lower temperature rises. Nondestructive, ultrasonic C-scan analyses were performed to observe and verify the failure patterns in the impacted panels. Overall, the impact experimentation showed temperatures exceeding 525 K (485degF) in both composites which is well above the respective glass transition temperatures for the polymer constituents. This expresses the need for further high strain rate testing and measurement of the temperature and deformation fields to fully understand the complex behavior and failure of the material in order to improve the confidence in designing aerospace components with these materials.

This paper considers the calorimetric measurements of losses and the semiconductor thermal cycling monitoring of high power resonant converters. For the tests a single phase resonant converter rated at 1 kV, 250 A (250 kW peak power, duty ratio 10%, 25 kW average power, pulse length 1 ms) has been developed. This represents one phase of a multi-phase resonant power supply designed for long-pulse modulation (typically 1 ms-2 ms) of RF tubes when equipped with a suitable output transformer. Pulsed operation is obtained by direct modulation of the high frequency power supply. The main aim of the work reported here is to monitor semiconductor losses of the IGBT modules through calorimetry and the device temperature using high speed thermal imaging, during the pulse, to identify the limitations and reliability of the modulator technology proposed. The paper provides an overview of the technology and design of the prototype test rig studied. Experimental results, showing semiconductor losses obtained through calorimetry and high quality chip thermal images are provided to validate the effectiveness of the proposed approaches. (5 pages)

This paper considers the semiconductor thermal cycling monitoring of high power resonant converters. For the tests a single phase resonant converter rated at 1 kV, 250 A (250 kW peak power, duty ratio 10%, 25 kW average power, pulse length 1 ms) has been developed. This represents one phase of a multi-phase resonant power supply designed for long-pulse modulation (typically 1 ms-2 ms) when equipped with a suitable output transformer. Pulsed operation is obtained by direct modulation of the high frequency power supply. The main aim of the work reported here is developed a physic-based multi-chip IGBT structures and experimentally monitoring the chip temperature using high speed thermal imaging, during the pulse, to identify the limitations and reliability of the modulator technology proposed. The paper provides a brief overview of the prototype test rig. Simulations including multi-chip structures, experimental results and high quality chip thermal images are provided to validate the effectiveness of the proposed approaches.

This study evaluates physical and chemical changes induced by high thermal gradients on the formation and their impact to the stability. The heat sources that effect the formation’s stability are varied, including drilling (due to drilling bit friction), perforation, electromagnetic heating (laser or microwave), and thermal recovery or stimulation (steam, resistive heating, combustion, microwave, etc.). This study uses an integrated approach to characterize rock heterogeneity and mapping heat propagation from different heat sources. The information obtained from the study is vital to accurately design and enhance well completion and stimulation This is an integrated analysis approach combining different advanced characterization and visualization techniques to map heat propagation in the formation. Advanced statistical analysis is also used to determine the key parameters and build fundamental prediction algorithms. Characterization on the samples was performed before, during, and after the exposure to thermal sources; it comprised thin-section, high speed infrared thermography (IR), differential thermal analysis and thermogravimetric analyzer (DTA/TGA), scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray fluorescence (XRF), uniaxial stress, and autoscan (provide hardness, composition, velocity, and spectral absorption). The results are integrated, and machine learning is used to derive a predictive algorithm of heat propagation and mapping in the formation with reference to the key formation variables and heterogeneity distribution. Rock heterogeneity affects the rate and patterns of heat propagation into the formation. Within the rock sample, minerals, laminations, and cementations lead to a heterogeneous, and sometimes anisotropic, distribution of thermal properties (thermal conductivity, heat capacity, diffusivity, etc.). These properties are also affected by the rock structure (porosity, micro-cracks, and fractures) and saturation distribution. The results showed the impact of heat on the mechanical properties of the rocks are due to clays dehydration, mineral dissociations, and micro cracks. High speed thermal imaging provides a unique visualization of heat propagation in heterogeneous rocks. Statistical analysis identified key parameters and their impact on thermal propagation; the output was used to build a machine learning algorithm to predict heat distributions in core samples and near-wellbore. Characterizing rock properties and understanding how heterogeneity modifies heat propagation in rocks enables the design of optimal completion and stimulation strategies. This paper discusses how advanced characterization and analysis, combined with novel algorithms, can improve this understanding, and unleash innovation and optimization. The data and information gathered are critical to develop numerical models for field-scale applications.

An experimental apparatus for measuring the dynamic behavior of materials subjected to strain rates on the order of 103 s-1 and temperatures up to 800°C with a unique triple actuation system is developed in this work. This system is based on the traditional Kolsky (or split-Hopkinson pressure) bar design, with the addition of an external furnace used to heat the specimen to the desired temperature. A synchronized triple pneumatic actuation system is used to control the motion and timing of the the sample, incident, and transmitted bars. The cold contact time (CCT), or the time during which the heated sample is in contact with the room temperature bars before compression, is measured experimentally and carefully controlled to minimize the development of a temperature gradient across the sample and avoid heating of the bars. Experiments are performed in conjunction with ultra high speed imaging and 2D digital image correlation (DIC), as well as high speed thermal imaging. To verify the viability of the proposed system, experiments were conducted on Ti-6Al-4V (wt.%) at temperatures from 25°C up to and 800°C at an average strain rate of approximately 1200 s-1.

To investigate the complex coupling between surface heat transfer and local fluid velocity in convective heat transfer, advanced techniques are required to measure the surface heat flux at high spatial and temporal resolution. Several established flow velocity techniques such as laser Doppler anemometry, particle image velocimetry and hot wire anemometry can measure fluid velocities at high spatial resolution (µm) and have a high-frequency response (up to 100 kHz) characteristic. Equivalent advanced surface heat transfer measurement techniques, however, are not available; even the latest advances in high speed thermal imaging do not offer equivalent data capture rates. The current research presents a method of measuring point surface heat flux with a hot film that is flush mounted on a heated flat surface. The film works in conjunction with a constant temperature anemometer which has a bandwidth of 100 kHz. The bandwidth of this technique therefore is likely to be in excess of more established surface heat flux measurement techniques. Although the frequency response of the sensor is not reported here, it is expected to be significantly less than 100 kHz due to its physical size and capacitance. To demonstrate the efficacy of the technique, a cooling impinging air jet is directed at the heated surface, and the power required to maintain the hot-film temperature is related to the local heat flux to the fluid air flow. The technique is validated experimentally using a more established surface heat flux measurement technique. The thermal performance of the sensor is also investigated numerically. It has been shown that, with some limitations, the measurement technique accurately measures the surface heat transfer to an impinging air jet with improved spatial resolution for a wide range of experimental parameters.

This paper considers the design of a modulator supply for RF tube applications. The supply is based on direct modulation of a series resonant parallel loaded power supply. The main aim of this work is to monitor semiconductor losses of the IGBT modules through calorimetry and the device temperature using high speed thermal imaging, during the pulse. The paper provides an overview of the technology and design used on the test rig prototype under study. The final experimental rig layout designed for internal device temperature and loss measurement will be presented. The overall goal of the work is to identify the limitations and reliability of the modulator technology proposed.

An innovative combination of concepts, namely microphotoluminescence (μPL) mapping, focused ion beam (FIB) microscopy, micro-Raman spectroscopy, and high-speed thermal imaging, was employed to reveal the physics behind catastrophic optical damage (COD), its related temperature dynamics, as well as associated defect and near-field patterns. μPL mapping showed that COD-related defects are composed of highly nonradiative complex dislocations, which start from the output facet and propagate deep inside the cavity. Moreover, FIB analysis confirmed that those dark line defects are confined to the active region, including the quantum wells and partially the waveguide. In addition, the COD dependence on temperature and power was analyzed in detail by micro-Raman spectroscopy and real-time thermal imaging. For AlGaInP lasers in the whole spectral range of 635 to 650 nm, it was revealed that absorption of stimulated photons at the laser output facet is the major source of facet heating, and that a critical facet temperature must be reached in order for COD to occur. A linear relationship between facet temperature and near-field intensity has also been established. This understanding of the semiconductor physics behind COD is a key element for further improvement in output power of AlGaInP diode lasers.

This study explores the interfacial friction in ultrasonic micro-injection moulding by using different polymer feedstock shapes, characterisation of micromoulding melts through thermal imaging and assessing microneedle feature replication. Industry standard polypropylene pellets and discs with different thicknesses were used for varying the amount of interfacial friction during sonication. High-speed thermal imaging and tooling containing sapphire windows were used to visualise the melt characteristics. Moulded products were characterised using laser-scanning confocal microscopy to quantify microneedle replication. The study demonstrates that (i) the interfacial area for the different feedstock shapes affects the heating in ultrasonic micro-injection moulding significantly, (ii) disc-shaped feedstocks result in initially higher flow front velocities and exhibit dominance of viscoelastic heating over interfacial friction and (iii) industrial pellet feedstocks provide a good combination interfacial friction and viscoelastic heating and more viscosity reduction in overall leading to better microreplication efficiency. The results presented could have a significant impact on the process development of ultrasonic micro-injection moulding where process repeatability can be improved by controlling the interfacial friction. The research provides an essential contribution to the development of this process, where interfacial frictional heating can be tailored specifically for miniature functional components, offering improved precision and reduced energy use when compared with conventional methods.

This paper considers monitoring of semiconductor thermal cycling in high-power resonant converters. For the experiments, a dedicated single-phase resonant converter rated at 1 kV, 250 A (250-kW peak power, duty ratio 10%, 25-kW average power, AND pulse length 1 ms) was been developed. This converter represents one phase of a multiphase resonant power supply designed for long-pulse modulation (typically 1-2 ms) when equipped with a suitable output transformer. Pulsed operation is obtained by direct modulation of the high-frequency power supply. The main aim of the study reported here is to develop a methodology to assess performance and reliability issues related to the use of standard commercially available power switch technology, relying on a physics-based multichip insulated gate bipolar transistor (IGBT) structure model, and to experimentally monitor the chip temperature using high-speed thermal imaging, during the pulse, to identify any limitations of the proposed modulator technology. First, an overview of the converter, including its nominal electrical design, is provided. Optimization of the turn-OFF snubber capacitance is performed through a series of experiments, employing calorimetrically measured losses, to determine a value, which minimizes the overall power losses. Accurate calorimetric measurements of the switching losses and infrared measurements of the IGBT surface temperatures during transient operation are presented. Simulations including multichip structures, experimental results, and high-quality chip thermal images are provided to validate the effectiveness of the proposed approaches.

PurposeThe purpose of this paper is to develop, apply and validate a mesh-free graph theory–based approach for rapid thermal modeling of the directed energy deposition (DED) additive manufacturing (AM) process.Design/methodology/approachIn this study, the authors develop a novel mesh-free graph theory–based approach to predict the thermal history of the DED process. Subsequently, the authors validated the graph theory predicted temperature trends using experimental temperature data for DED of titanium alloy parts (Ti-6Al-4V). Temperature trends were tracked by embedding thermocouples in the substrate. The DED process was simulated using the graph theory approach, and the thermal history predictions were validated based on the data from the thermocouples.FindingsThe temperature trends predicted by the graph theory approach have mean absolute percentage error of approximately 11% and root mean square error of 23°C when compared to the experimental data. Moreover, the graph theory simulation was obtained within 4 min using desktop computing resources, which is less than the build time of 25 min. By comparison, a finite element–based model required 136 min to converge to similar level of error.Research limitations/implicationsThis study uses data from fixed thermocouples when printing thin-wall DED parts. In the future, the authors will incorporate infrared thermal camera data from large parts.Practical implicationsThe DED process is particularly valuable for near-net shape manufacturing, repair and remanufacturing applications. However, DED parts are often afflicted with flaws, such as cracking and distortion. In DED, flaw formation is largely governed by the intensity and spatial distribution of heat in the part during the process, often referred to as the thermal history. Accordingly, fast and accurate thermal models to predict the thermal history are necessary to understand and preclude flaw formation.Originality/valueThis paper presents a new mesh-free computational thermal modeling approach based on graph theory (network science) and applies it to DED. The approach eschews the tedious and computationally demanding meshing aspect of finite element modeling and allows rapid simulation of the thermal history in additive manufacturing. Although the graph theory has been applied to thermal modeling of laser powder bed fusion (LPBF), there are distinct phenomenological differences between DED and LPBF that necessitate substantial modifications to the graph theory approach.

Thermal behavior and morphology evolution of polyamide 12 in laser powder bed fusion process: Experimental characterization and numerical simulation

High-Speed Thermal Imaging Can Resolve Short RF Pulse Effects in Tissue Models