CHARACTERIZING THE MICROSTRUCTURE OF A GTA WELD IN-PROCESS USING HIGH-SPEED, HIGH-MAGNIFICATION, DIGITAL IMAGING A.C. Hall*, G. A. Knorovsky, C. V. Robino", J. Brooks*, D. O. Maccallum, M. Reece, G. Poulter ABSTRACT A high quality zoom lens and a high-speed CCD camera have been used to image gas tungsten arc (GTA) welds in stainless steel. Both the trailing (solidifying) edge and the leading (melting) edge of the weld pool have been observed. A number of solidification phenomena have been captured including: dendrite growth, melting, and weld ripple formation. Significant information about the evolution and structure of the solid-liquid interface can be extracted from these videos using computerized image analysis techniques. This information can be directly related to the microstructure of the finished weld. Video clips will be presented, techniques for extracting microstructural information from those clips will be discussed, and the extracted information will be related to the microstructure of the finished weld. KEYWORDS WELD VISUALIZATION, MACHINE VISION, WELD RIPPLE, SOLIDIFICATION, MELTING INTRODUCTION Over the last decade, significant advances have been made in digital photography and related computer technology. Combination of these technologies has given rise to a field known as machine vision. It is now relatively straightforward to capture and process images using a computer. Robust software exists that allows the user to extract quantitative measurements from digital images. This technology has found widespread application in the world's manufacturing industries [1-3]. Parts are routinely inspected using machine vision technology to see that they have been assembled properly or manufactured to desired tolerances. Surprisingly, this technology has not been widely adopted in the scientific community. It affords the researcher with a powerful new tool set that allows large amounts of information about dynamic physical processes to be accessed and extracted in an automated fashion. Solidification of metals is a field that is ripe for the exploitation of such technology. Conventional techniques for inspecting the solid-liquid interface in a metal typically involve rapidly quenching a sample and then polishing, etching, and examining the quenched sample [46]. Experimental techniques based on such schemes are very useful but have a significant limitation. They only provide the researcher with a "snapshot" of the solid-liquid interface. No direct information about the motion or the evolution of the solid-liquid interface is available from a quenching experiment. The only experimental schemes that do provide information about the motion and the evolution of the solid-liquid interface are based on visualization of transparent materials (like succinonitrile) that solidify in a manner analogous to metals [7, 8]. These experiments have provided a wealth of information about dendrite growth and the evolution of the solid-liquid interface. However, these experiments have a significant limitation. Transparent materials often do not exhibit the complex phenomena that occur during the solidification of multi-component engineering alloys. Solidification mode transformations, which are commonly observed in stainless steels [9], are an example of complex phenomena that are not exhibited by transparent materials. Complex phenomena like this demand new experimental techniques that can provide information about the dynamic behavior of solid-liquid interfaces in engineering materials. In this paper, we describe a technique for using machine vision technology to characterize the behavior of the solid-liquid interface in a gas tungsten arc (GTA) weld. Dendrite growth, melting, and weld ripple formation have all been observed at high spatial and temporal resolution. A large amount of quantitative data can be extracted from these experiments in a relatively straightforward manner. An example showing how data is extracted and related to the microstructure of the finished weld is discussed in detail. EXPERIMENTAL PROCEDURE The solid-liquid interface in a GTA weld was filmed using a Kodak EktaPro camera equipped with a NavitarR® Zoom 6000 lens. The Kodak EktaPro camera is capable of capturing digital images at up to 12,000 frames/second. In practice, we have found that filming at 2000 frames / second provides temporal resolution that is more than adequate for a GTA weld. The Navitar® Zoom 6000 lens is designed specifically for machine vision. It is a parafocal zoom lens system capable of imaging objects within fields of view ranging from 146.6mm to 0.03mm on each side. |