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An Article from the June 2002 JOM: A Hypertext-Enhanced Article

The author of this article organized the symposium Imaging of Dynamic Processes and is senior metallurgist at Ames Laboratory, Iowa State University.
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Conference Review: Feature

The Symposium Imaging of Dynamic Processes: Multimedia Hightlights

Iver E. Anderson

SYMPOSIUM PRESENTATIONS

Of the ten presentations delivered to meeting attendees, five are spotlighted via multimedia enhancement on the JOM web site as a supplement to the June 2002 issue. The featured clips provide excellent examples of some of the information that can be gathered and applied in understanding a variety of processes.

Featured Presentations

Thermal Imaging of Solidification,” William Hofmeister (Vanderbilt University)

Visualization of Primary Austenite and Primary Ferrite Solidification Modes in Fe-Ni-Cr Gas Tungsten Arc Welds,” Aaron C. Hall, Charles V. Robino, John Brooks, Mark Reece, and Danny O. MacCallum (Sandia National Laboratories)

Schlieren Imaging in Materials Processing,” Steven P. Mates (National Institute of Standards and Technology)

Increased Understanding of Gas Atomization from Gas Flow Imaging and High Speed Cinematography,” I.E. Anderson and R.L. Terpstra (Iowa State University, Ames Laboratory) and S. Rau (University of Bremem) and R. Figliola (Clemson University)

Studying Changes in Surface Topography by White Light Interferometry,” Borge Holme (SINTEF)

Other Presentations

“Three Dimensional Microstructural Evolution in Succinonitrile,”
Mark A. Palmer (Kettering University) and Martin E. Glicksman and Krishna Rajan (Rensselaer Polytechnic Institute)

“Investigation of Bubble Nucleation Site Density during Quenching Heat Treatment Process Using Video Imaging,” M. Maniruzzaman, S.H. Ma, R.D. Sisson (Worcester Polytechnic Institute)

“Imaging Spatial Heat Flow and Dynamic Instabilities in Melt Spinning,”
Matthew J. Kramer, Ralph E. Napolitano, Halim Meco, Matthew Sawka, Kevin W. Dennis, and R. William McCallum (Iowa State University, Ames Laboratory)

“The Use of High Speed Imaging for Thermomechanical Characterization of Melt Pool Dynamics during Rapid Solidification,” H. Meco, M.J. Kramer, R.E. Napolitano, M. Sawka, K.W. Dennis, and R.W. McCallum (Iowa State University, Ames Laboratory)

“Imaging and Particle Image Velocimetry of Granular Flows,” Daniel Steingart and James W. Evans (University of California at Berkeley)

High-resolution imaging is critical to enhancing our understanding of the many processing techniques that enable the manufacture of materials both advanced and mundane. If it is true that “one picture is worth a thousand words,” then it is reasonable to extrapolate that this value can be increased by orders of magnitude if a correlated series of images is collected into a movie or video recording. The effect can be more pronounced if the images can slow down or speed up (by use of time-lapse photography) the action of a dynamic process.

Exploration of this concept was the subject of the two-session symposium Imaging of Dynamic Processes, which was held during the 2002 TMS Annual Meeting, February 17–21, in Seattle, Washington. The symposium’s participating authors and the titles of their presentations are listed in the sidebar. The program was conducted under the auspices of the joint Processing Modeling and Control Committee of the TMS Extraction & Processing Division and the TMS Materials Processing & Manufacturing Division. Owing to the multimedia aspects of the symposium, the presentations were not collected for presentation in a traditional conference proceedings volume. Instead, highlights from the symposium are underscored in this conference review, and multimedia highlights have been selected and embedded into the JOM web version of this article for your further study and analysis.

As the on-line presentations illustrate, visualization can provide in-situ process information on either a global or local scale, enabling pieces of sensor data to mesh as a coherent process description. Certainly, materials scientists oftentimes require an enhanced understanding of a phase or physical transformation that cannot be thoroughly characterized via indirect or direct sensing and/or post-process analysis. The high level of process information presented by imaging is invaluable for both developing new techniques and gaining fundamental insights into the operation of existing processes. Eventually, such information can be used to develop process control techniques featuring fully closed-loop logic—something that can be enhanced by artificial intelligence and enable high-level materials manufacturing concepts.

Within this context, the symposium was organized to attract researchers having wide-ranging interests yet common problems in collecting visual information on materials-processing techniques, grouped according to processing approach.

One category concerns solidification processing, and the initial lecture focused on using thermal imaging at Vanderbilt University to study the solidification of a levitated droplet and a surface deposition layer. Thermal imaging allowed the researchers to investigate the relationship between bulk undercooling and solidification velocity in levitated droplets of pure metallic materials and alloys. Researchers from Sandia National Laboratories presented an intriguing collection of high-speed, high-magnification videos of solidification during the gas-tungsten-arc welding of Fe-Ni-Cr alloys. Detailed analysis of the digitized images enabled the researchers to measure solid-liquid interface velocity in the solidifying welds. In work performed at Rensselaer Polytechnic Institute, video imaging was used to study solid-state grain coarsening in thin films of optically transparent succinonitrile. The visualization results permitted the development of an alternative explanation of the phenomena.

An alternative visualization tool, white-light interferometry, was applied to a series of studies conducted at SINTEF Materials Technology. Surface topology changes extending to the nanometer range were studied in processes as diverse as paint drying, surface etching, corrosion, and mechanical strain (Video 3b). At Worcester Polytechnic Institute, bubble nucleation during the transparent-fluid quenching of high-temperature metal samples was studied by high-speed videography. The researchers employed a fully instrumented sample to gain a detailed understanding of the heat-transfer process during quenching.

Researchers at Ames Laboratory, Iowa State University, reported on using stop-action digital photography to analyze the time-temperature evolution of the melt jet and melt pool in a single-roller free-jet melt spinner. Computer-aided combinations of spatial and thermal images were used to provide the boundary conditions necessary for heat-transfer and solidification modeling. In a complementary presentation, results from the melt-spinning study were reported as pertain to the rapid solidification of several rare-earth-containing permanent-magnet alloys. The process parameters that influence melt-pool stability and shape were discussed in detail, particularly with regard to the ribbon-solidification process.

Work from the National Institute of Standards and Technology was outlined using schlieren optical techniques to study supersonic gas flows in both gas atomization and thermal-spray processes (Video 5b). Results were presented on schlieren optical arrangements for optimizing image quality in the study of gas-only flows and in-situ process observations. High-speed cinematography and schlieren imaging were applied to a fundamental study of gasatomization processing by researchers at Ames Lab. The high-speed movies allowed in-situ observation of atomization process dynamics and evidence of progress toward ideal primary atomization dominance.

In the symposium’s final presentation, researchers from the University of California at Berkeley reported the use of high-speed videography to study the flow of dense, large particles from two-dimensional hoppers. Detailed changes in the velocity and spatial distribution of uniform spheres undergoing various discharge flow processes were studied.


MULTIMEDIA HIGHLIGHTS
Of the ten presentations delivered to meeting attendees, five are spotlighted via multimedia enhancement on the JOM web site as a supplement to the June 2002 issue. The featured clips provide excellent examples of some of the information that can be gathered and applied in understanding a variety of processes. The featured presentations are:

"Thermal Imaging of Solidification," William Hofmeister (Vanderbilt University)

Thermal imaging has been useful in determining solidification kinetics from undercooled melts and in the process control of solidification in direct metal deposition. In undercooled melts, solidification is accompanied by recalescence-the release of the latent heat of fusion. This release of sensible heat raises the temperature of the solid such that the solidification front can be tracked on the surface of a levitated drop. At Vanderbilt, we have used various high-speed and ultrahigh-speed thermal imaging techniques to determine the relationship between bulk undercooling and solidification velocity in pure metallic materials and alloys. In the area of direct metal deposition, high-speed thermal imaging has been used to study cooling rates in solidification. In the laser-engineered net-shaping process developed at Sandia National Laboratories, the information from thermal imaging is used to provide feedback control for the process, insuring that the desired cooling rates are produced, regardless of the part geometry.

 

Video 1a. A superheated nickel splat. This temperature-corrected, colorized movie shows a sample impacted on a plate with 175 K superheat. Temperatures range: 1,550 K (black), 1,650 K (blue), 1,730 K (red), and 1,800 K (white).
   
Video 1b. A movie of Ti-6Al-4V under laser-engineered net-shape processing. Temperatures range: 1,733 K (black), 1,833 K (blue), 1,933 K (red), and 2,183 K (white).

 
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"Visualization of Primary Austenite and Primary Ferrite Solidification Modes in Fe-Ni-Cr Gas Tungsten Arc Welds," Aaron C. Hall, Charles V. Robino, John Brooks, Mark Reece, and Danny O. MacCallum (Sandia National Laboratories)

A technique for imaging the solid-liquid interface in gas-tungsten-arc welds at high-speed and high-magnification has been developed. At high magnification, the dendritic structure of the solid-liquid interface can be clearly seen. Computer image-analysis techniques have been developed that allow solid-liquid interface velocity and secondary dendrite arm spacings to be extracted from the video images. This technique has been used to image two Fe-Ni-Cr alloys: austenite solidifier and ferrite solidifier.

 
 

Video 2a. A welded austenite Fe-Ni-Cr alloy (Fe-10.16 Cr-30 Ni-0.025 Si-0.003 C-0.003 N). Multiple grains are visible in the video, but very little ferrite is present in the microstructure.
   
Video 2b. A welded austenite Fe-Ni-Cr alloy (Fe-21.07 Cr-14.18 Ni-0.01 Si-0.006 C-0.001 N). Multiple grains are visible in the video, but very little ferrite is present in the microstructure.
   
Video 2c. A welded ferrite/austenite Fe-Ni-Cr alloy (Fe-22.81 Cr-11.92 Ni-0.1 Si-0.01 C-0.004 N). Two phases are clearly visible at the surface of the weld pool. Significant surface relief is associated with these phases, and both phases appear to grow at the same rate.

 
 
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"Schlieren Imaging in Materials Processing," Steven P. Mates (National Institute of Standards and Technology)

Since before Ernst Mach visualized shock waves cast by a fired bullet in 1888, the schlieren optical technique has been a valuable diagnostic tool in fluid dynamics. The schlieren effect renders visible density gradients in fluid flows, which can occur due to thermal or high-speed effects. Joined with advanced high-speed, high-resolution, charge-coupled device (CCD) imaging technology, schlieren imaging yields a unique perspective on the dynamic behavior of a variety of materials processing applications that involve variable density fluid flows, such as thermal spray. This presentation described the use of the schlieren technique to visualize the behavior of twin-wire and plasma thermal sprays by use of a commercial high-speed CCD camera. Schlieren movies of these electric-arc processes reveal how their inherently unsteady nature manifests dramatically in the behavior of the thermal plumes they generate.

 

Video 3a. Schlieren movie (30 frames/s, 300 ms exposure) revealing the unsteady behavior of the twin-wire arc thermal spraying of a zinc alloy. The schlieren effect reveals the shock wave pattern of the supersonic atomizing air stream (close to the nozzle) as well as the billowing thermal eddies of entrained air that has been heated by the molten droplet spray.
   
Video 3b. Schlieren movie (30 frames/s, 300 ms exposure) of the plasma spraying of zirconia, showing the unsteady behavior of the hot turbulent gas plume generated by the plasma torch.

 
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"Increased Understanding of Gas Atomization from Gas Flow Imaging and High Speed Cinematography," I.E. Anderson and R.L. Terpstra (Iowa State University, Ames Laboratory) and S. Rau (University of Bremem) and R. Figliola (Clemson University)

Schlieren visualization of gas-only flows and high-speed photography of gas atomization have provided critical information in the quest to understand melt-breakup mechanisms and develop a robust model of this widely practiced metal powder production technique. These imaging techniques add valuable insight toward developing a succession of high-pressure gas-atomization (HPGA) nozzles for effective powder size control. HPGA is a close-coupled, discrete, jet-atomization method that has proven one of the most effective methods of producing rapidly solidified fine metal and alloy powders with high yields less than 20 micrometers using Ar, N2, or He gas. The full presentation compared the gas-flow characteristics and the atomization results of HPGA nozzles fitted with discrete jets of either convergent-divergent or cylindrical (convergent) designs used for HPGA. However, the video clips included here focus only on the atomization spray observations made with high-speed cinematography. The-high speed movies were produced with diffuse backlighting from a 15 kW copper-vapor pulsed laser (30 ns/pulse). It was obtained from Oxford Laser (Acton, Massachusetts) and synchronized to a rotating prism film camera (10,000 frames/s, max.) obtained from Photec (Wayne, New Jersey), which recorded 16 mm film images during the initial 4-6 seconds of each atomization experiment. The spray images enable determination of the primary atomization breakup length and estimation of the uniformity of melt-flow distribution around the circumference of the melt feed tube tip. Also, the melt-flow path during the gas-flow onset may be observed, along with the characteristics of the subsequent equilibrated droplet spray generation process.

 

Video 4a. This side-by-side comparison illustrates the effect of atomization gas (N2) supply pressure on the atomization spray from a convergent-divergent nozzle, termed HPGA-IIIc, with 24 discrete jets and a 14° apex angle between jets (7° off the central nozzle axis). Molten copper was atomized at 1,600°C using a melt feed tube with a slotted orifice to stabilize melt delivery around the tube periphery. Although the video comparison shows that elevated pressure results in some extension downstream of the primary melt breakup limit, the measured mean particle size of the resulting powder was essentially unchanged.
   
Video 4b. This side-by-side comparison illustrates the effect of atomization gas (N2) supply pressure on the atomization spray from a cylindrical jet nozzle, termed HPGA-I, with 30 discrete jets and a 14° apex angle between jets (7° off the central nozzle axis). Molten copper was atomized at 1,600°C using a melt feed tube with a slotted orifice to stabilize melt delivery around the tube periphery. The video comparison shows that elevated pressure results in noticeable projection of the melt stream downstream into the lower region of the primary atomization zone, rather than the immediate stream splitting and feeding of the pour tube periphery, seen at the lower pressure. Size analysis of the resulting powders showed that elevated pressure (producing an estimated gas velocity increase from Mach 2.5 to Mach 3.5) resulted in a reduction of nearly 10 µm in the mean particle size.

 
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"Studying Changes in Surface Topography by White Light Interferometry," Borge Holme (SINTEF)

White-light interferometry gives fast and accurate measurements of surface topography. Within ten seconds, one obtains a topographic image with micrometer resolution laterally and nanometer resolution vertically. At SINTEF Materials Technology, we have extended the capabilities of our WYKO NT-2000 White Light Interferometer to include studies of changes in surfaces. We make repeated images of a surface that undergoes a change in topography (e.g., through etching, polishing, melting, drying, corrosion, or mechanical strain). The sequence of images is made into a video clip, which visualizes the topographic changes in an intuitive manner. Some processes, like drying of paint, can be done in-situ with imaging in real time. Most chemical processes, like etching and corrosion, have to be done in a stepwise manner where the sample is removed and etched in the lab. The technique is well suited for metallic surfaces, but also works for semiconductors, liquid surfaces, and even quite dark surfaces with reflectance as low as two percent.

 
 

Video 5a. A silicon membrane with a square shape and 800 µm edge length was etched on a silicon wafer. The membrane thickness was 10 µm. Changing the pressure on one side of the membrane made it bulge in and out. The square shape of the membrane influences the shape of the bulging surface, making it non-circular. The video provides quantitative data of the mechanical strains in the membrane as a function of pressure difference.
   
Video 5b. A white, oil-based paint was applied on a fibrous paper. While the paint dried, topographic images were made every five minutes. The paper fibers appear when the paint surface retracts. In this video, we have added one interpolated image between each pair of data files in order to give a smoother visual appearance. This example shows that the technique works even on liquid surfaces. Few other methods would give valuable information from such a smooth, white surface.
   
Video 5c. The video focuses on the upper surface on a piece of dark chocolate solidified in air. The sample was heated slowly to the melting temperature of 45ºC. The smooth hills may be "volcanoes" of cocoa butter that flowed out to the surface during the original solidification. The rougher, low lying areas consist mainly of finely ground cocoa mass. During heating, the height differences are reduced as the "volcanoes" melt and float out to fill the lower regions. This example shows that quite dark surfaces can be studied. The minimum required reflectivity is about 2%.

 
 
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Copyright held by The Minerals, Metals & Materials Society, 2002

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