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Local microstructure control in electron beam powder bed fusion (EB-PBF) is of great interest to the additive manufacturing community to realize complex part geometry with targeted performance. The local microstructure control relies on having a detailed understanding of local melt pool physics (e.g., 3-D melt pool shape as well as spatial and temporal variations of thermal gradient (G) and solidification rate (R)). In this research, a new scan strategy referred to as ghost beam is numerically evaluated as a candidate to achieve the targeted G and R of IN718 alloy. The boundary conditions for simulations, including the speed (490 mm/s) and spatial locations of the beam within a given layer, are obtained by using series of snapshot images, recorded at 12,000 frames per second, using a high-speed camera. The heat transfer simulations were performed using TRUCHAS an open-source software deployed within a high-performance computational infrastructure. The simulation results showed that reheating at short beam on-time and time delay decreases both G and R. Local variation of R at the center of the melt pool trailing edge showed periodic temporal fluctuations. Finally, the ghost beam scan strategy was compared to other existing raster and spot scan strategies.

Innovative designs for turbines can be achieved by advances in nickel-based superalloys and manufacturing methods, including the adoption of additive manufacturing. In this regard, selective electron beam melting (SEBM) and selective laser melting (SLM) of nickel-based superalloys do provide distinct advantages. Furthermore, the direct energy deposition (DED) processes can be used for repair and reclamation of nickel alloy components. The current paper explores opportunities for innovation and qualification challenges with respect to deployment of AM as a disruptive manufacturing technology. In the first part of the paper, fundamental correlations of processing parameters to defect tendency and microstructure evolution will be explored using DED process. In the second part of the paper, opportunities for innovation in terms of site-specific control of microstructure during processing will be discussed. In the third part of the paper, challenges in qualification of AM parts for service will be discussed and potential methods to alleviate these issues through in situ process monitoring, and big data analytics are proposed.

High performance mid-wave infrared (IR) cameras are used for in-situ electron beam melt process monitoring and temperature measurements. Since standard factory calibrations are insufficient due to very low transmissions of the leaded glass window required for X-ray absorption, two techniques for temperature calibrations are compared. In-situ measurement of emittance will also be discussed. Ultimately, these imaging systems have the potential for routine use for online quality assurance and feedback control.

The laser flash method as a means of measuring thermal diffusivity is well established and several manufacturers produce equipment for performing these types of experiments. Most analysis methods for analyzing the data from these experiments assume one-dimensional transient conduction with insulated surfaces during the time subsequent to the flash. More recently, models of grater sophistication have been applied to flash diffusivity experiments using nonlinear regression. These models assume an instantaneous flash and are highly accurate for most samples of moderate diffusivity and sample thickness. As samples become thinner and more highly conductive, the duration of the experiments becomes very short. Since the duration of the flash is typically on the order of several milliseconds, the assumption that this period of time is instantaneous becomes less valid for very short experiments. A model accounting for the duration of the flash is applied to three samples of stainless steel of varying thicknesses in this research and analyzed with two different mathematical models. One model accounts for the duration of the flash and the other does not. The model accounting for the flash duration generates results that are much more consistent between samples than the model assuming an instantaneous flash. Moreover, the conformancemore » of the mathematical model accounting for flash duration is much closer to the measured data than the model which assumes an instantaneous flash. As part of the finite flash duration model, the length of the flash is estimated by non-linear regression, optimizing the conformance of the model to the measured data. Additionally, the starting time of the flash is treated as a parameter and is estimated simultaneously with flash duration, thermal diffusivity and flash intensity. Statistical methods are also used for showing the validity of the added level of sophistication of the more advanced mathematical model. Research sponsored by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, as part of the High Temperature Materials Laboratory User Program, Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract number DE-AC05-00OR22725.« less

Advanced Li-ion batteries with high energy and power density are fast approaching compatibility with automotive demands. While the mechanism of operation of these batteries is well understood, the aging mechanisms are still under investigation. Investigation of aging mechanisms in Li-ion batteries becomes very challenging, as aging does not occur due to a single process, but because of multiple physical processes occurring at the same time in a cascading manner. As the current characterization techniques such as Raman spectroscopy, X-ray diffraction, and atomic force microscopy are used independent of each other they do not provide a comprehensive understanding of material degradation at different length (nm2 to m2) scales. Thus to relate the damage mechanisms of the cathode at mm length scale to micro/nanoscale, data at an intermediate length scale is needed. As such, we demonstrate here the use of thermal diffusivity analysis by flash method to bridge the gap between different length scales. In this paper we present the thermal diffusivity analysis of an unaged and aged cell. Thermal diffusivity analysis maps the damage to the cathode samples at millimeter scale lengths. Based on these maps we also propose a mechanism leading to the increase of the thermal diffusivity as the cells are aged.

The lost foam casting (LFC) process is emerging as a reliable casting method. The metal-fill profile in LFC plays an important role among several factors that affect casting quality. The metal-fill profile is in turn affected by several factors. Several casting defects may result due to an improper metal-fill process. Hence, it becomes essential to characterize and control, if possible, the metal-fill process in LFC. This research presents instrumentation and a technique to monitor and characterize the metal-fill process. The characterization included the determination of the position of the metal front and the profile in which the metal fills up the foam pattern. The instrumentation included capacitive sensors. Each sensor is comprised of two electrodes whose capacitive coupling changes as the metal fills the foam pattern. Foundry tests were conducted to obtain the sensors' responses to the metal fill. Two such sensors were used in the foundry tests. Data representing the responses of these sensors during the metal-fill process were collected using a data acquisition system. A number of finite element electrostatic simulations were carried out to study the metal-fill process under conditions similar to those experienced in foundry tests. An artificial neural network was trained using the simulation data as inputs and the corresponding metal-fill profiles as outputs. The neural network was then used to infer the profile of the metal-fill during foundry tests. The results were verified by comparing the metal-fill profile inferred from the neural network to the actual metal-fill profile captured by an infrared camera used during the foundry tests. The match up between the inferred profiles and the infrared camera measurements was satisfactory, indicating that the developed technique provides a reliable and cost effective method to monitor the metal-fill profile in LFC.

Both mid-wave and long-wave IR cameras are used to measure various temperature profiles in thermoplastic parts as they are printed. Two significantly different 3D-printers are used in this study. The first is a small scale commercially available Solidoodle 3 printer, which prints parts with layer thicknesses on the order of 125μm. The second printer used is a “Big Area Additive Manufacturing” (BAAM) 3D-printer developed at Oak Ridge National Laboratory. The BAAM prints parts with a layer thicknesses of 4.06 mm. Of particular interest is the temperature of the previously deposited layer as the new hot layer is about to be extruded onto it. The two layers are expected have a stronger bond if the temperature of the substrate layer is above the glass transition temperature. This paper describes the measurement technique and results for a study of temperature decay and substrate layer temperature for ABS thermoplastic with and without the addition of chopped carbon fibers.

The thermal diffusivity of free standing thermal barrier coatings (TBCs) was measured by the laser flash technique. The combination of low thermal conductivity (1 to 2 W/m K) and small TBC thickness (300 to 600 µm thick) can cause errors in the measurements. Back surface (opposite the laser) temperatures of free standing plasma-sprayed TBCs were measured as a function of time and laser power. The front surface temperatures were calculated using thermal transport equations. In the high power region, thermal diffusivity decreased significantly with increasing laser power. In the moderate power region, thermal diffusivity remained constant. In the low power region, measurement became unreliable because of noise. The detector nonlinearity was believed to be a possible cause of deviation in the high power region. Measurements at different laser power levels should be conducted in order to obtain reliable thermal diffusivity values for TBCs.

Oak Ridge National Laboratory (ORNL) has been utilizing the ARCAM electron beam melting technology to additively manufacture complex geometric structures directly from powder. Although the technology has demonstrated the ability to decrease costs, decrease manufacturing lead-time and fabricate complex structures that are impossible to fabricate through conventional processing techniques, certification of the component quality can be challenging. Because the process involves the continuous deposition of successive layers of material, each layer can be examined without destructively testing the component. However, in-situ process monitoring is difficult due to metallization on inside surfaces caused by evaporation and condensation of metal from the melt pool. This work describes a solution to one of the challenges to continuously imaging inside of the chamber during the EBM process. Here, the utilization of a continuously moving Mylar film canister is described. Results will be presented related to in-situ process monitoring and how this technique results in improved mechanical properties and reliability of the process.

An extended-range IR camera was used to make temperature measurements of samples as they are being manufactured. The objective is to quantify the temperature variation of the parts as they are being fabricated. The IR camera was also used to map the temperature within the build volume of the oven. The development of the temperature map of the oven provides insight into the global temperature variation within the oven that may lead to understanding variations in the properties of parts as a function of build location within the oven. The observation of the temperature variation of a part during construction provides insight into how the deposition process itself creates temperature distributions, which can lead to failure.

An infrared microscope was used to study the surface temperature profiles of power transistor arrays in integrated circuits (IC) during operation. Each transistor array was set to conduct current for 20-50 microseconds. The integration time of the IR camera is adjusted to be between 2 and 10 microseconds. A thorough study of the camera's timing characteristics allows its precise synchronization to transient thermal events in the transistor arrays. Progressively adding incremental delay times to the synchronization pulses allows the complete characterization of the thermal transients as a function of time and location. The IR microscope timing characteristics were determined by imaging an incandescent lamp filament during pulsed operation. Examples of heat pulses in a lamp filament and power transistors are given.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Thermal transients caused by electrical switching devices and fast optical or electrical pulses only exist for a very short time (less than 1 millisecond). If the transient happens in a high thermal conductivity material, e.g. silicon wafer, the thermal transient will disappear in a few milliseconds or faster. Most modern IR cameras have external triggering capability. However, the IR camera often runs on its own clock. If the triggering of data acquisition is not synchronized with the camera, there is often an uncertainty of timing, e.g. 16.7 ms for camera running at 60Hz. Even when the IR camera is running at 500 Hz, there is 2 ms uncertainty. Depending on when you push/click the button, the high-speed thermal transients can be missed and they will disappear in the next image. We developed a technique to synchronize the IR camera to capture the thermal transients. By using a delay function, we can control the data acquisition very precisely to study the thermal transients.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

High-speed infrared (IR) imaging provides researchers a noncontact and nondestructive tool of studying fatigue behavior of materials. In this study, ULTIMETTM superalloy manufactured by Haynes International, Inc. was used. High-cycle fatigue tests (20 Hz) were conducted at the University of Tennessee using two material test system machines. An IR camera recorded temperature evolution during the fatigue test. Four distinct temperature regions were observed: (1) the initial temperature rise was found to be dependent on the damage condition of the specimen. A pre- damaged specimen showed a slower rate of initial temperature rise than the as-received specimen; (2) the equilibrium temperature of a specimen depends on the applied stress, test frequency, and thermophysical properties of the material. The equilibrium temperatures varied from 23 degree(s)C to 50 degree(s)C; (3) further temperature increase was detected 5,000 cycles before final failure, indicating the onset of a fatal crack; and (4) final temperature drop was found due to the separation of the test sample. High temperature regions were observed at the crack tip as a result of stress concentration. High-speeding IR imaging also revealed the thermoelastic effect during cyclic loading.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper covers the development and comparison of two systems to measure the temperature variation on rotating components. Measurement of hot spots on a rotating brake disk is used for demonstration purposes. The two systems utilized are a high-speed infrared camera and an array of fiber optics connected to multiple high-speed single-point two-color infrared detectors coupled to a high-speed data acquisition system. The advantages and disadvantages of the two systems are discussed relative to real time data and post processing analyses. Results from both systems are presented for comparison.

Wear of coated surfaces tends to progress through a series of stages in which damage accumulates until the coating fails to protect its substrate. Depending on the coating system and the contact conditions, these stages can sometimes be detected as a series of discrete periods of changing frictional behavior, or they can occur quite rapidly, leading to rapid removal of the coating. A new technique has been developed to capture magnified infrared (IR) images of a selected location on a moving wear surface and to synchronize these cycle-by-cycle images with the instantaneous friction force that occurs at the same location. A pin-on-disk tribometer has been used to demonstrate the principle, but other kinds of test geometries can also be used. Contrast in the IR images derives not only from the surface temperatures but also from the emissivity of surface features. A spatial calibration of the system allows the measurement of the width of the wear path as a function of time. By studying a series of captured and friction- synchronized images, it is possible to observe the detailed progression of wear and the corresponding frictional transitions in a limitless variety of materials. Examples of several different materials, including, steel, aluminum, brass, and paint, will be used to illustrate the application of time-resolved microscopic tribo-thermography to coatings research.

Automotive brake systems are energy conversion devices that convert kinetic energy into heat energy. Several mechanisms, mostly related to noise and vibration problems, can occur during brake operation and are often related to non-uniform temperature distribution on the brake disk. These problems are of significant cost to the industry and are a quality concern to automotive companies and brake system vendors. One such problem is thermo-elastic instabilities in brake system. During the occurrence of these instabilities several localized hot spots will form around the circumferential direction of the brake disk. The temperature distribution and the time dependence of these hot spots, a critical factor in analyzing this problem and in developing a fundamental understanding of this phenomenon, were recorded. Other modes of non-uniform temperature distributions which include hot banding and extreme localized heating were also observed. All of these modes of non-uniform temperature distributions were observed on automotive brake systems using a high speed IR camera operating in snap-shot mode. The camera was synchronized with the rotation of the brake disk so that the time evolution of hot regions could be studied. This paper discusses the experimental approach in detail.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The energy distribution and temperature evolution within paintlike systems that are exposed to spectral radiant energy were studied. A complete set of material properties was derived and discussed. Infrared measurements were conducted to obtain experimental data for the temperature in the paint film. The heat flux due to the incident radiation from the plasma arc lamp was measured using a heat flux sensor with a very short response time. A radiative transport model based on spectral four-flux radiation transport equations has been developed for multilayered and semitransparent material systems. Comparisons between the computed and experimental results for temperature show that the energy transport model yields accurate results for a black painted substrate.

The materials studied in this research, having a wide range of thermal conductivities, permit a study of the mechanisms which affect their behavior. Data on the thermal conductivity of several substrate materials, over a wide temperature range, were analyzed using the Klemens model. Parameters in this model, which include crystallite size and impurity concentration, are determined through a nonlinear least squares fitting routine and related, where possible, to values obtained by other techniques. This analysis predicts an achievable upper limit of 290 W/m.K for very high purity sintered AlN with 5 micron crystallites.