Technique for Simultaneous Real-Time Measurements of Weld Pool Surface Geometry and Arc Force

This paper describes a new technique for simultaneous real-time measurement of weld pool surface geometry and arc force. The weld pool surface profile is measured using real-time radiography by vertical x-ray transmission through the weld. The information is received at a rate of 30 frames per second in the form of two-dimensional images which are digitized and processed in real time by computer. The surface topography is determined from the images using the experimentally found relation between image brightness and material thickness wi th an accuracy of 0.2 mm (0.008 in.). Pool depression and humping on the pool periphery have been reconstructed from radiographic images. The volumes of the depressed and humped areas were calculated and conservation of pool mass was verified. The thickness of the liquid layer between the arc plasma-liquid interface and the l iquid-solid interface was determined by measuring the pool depression and weld penetration. Computer simulations have been performed to determine the brightness correction to the radiographic measurement which arises from temperaturerelated material density changes. It was found that for most cases this correction is negligible. The arc force was measured using a precision digital scale. To eliminate the effect of x-ray beam shielding by the force-measuring cell, the apparatus was designed in the form of a balance, where the cell was on the opposite side from A. C. GUU and S. I. ROKHLIN are with The Ohio State University, Department of Welding Engineering, Columbus, Ohio. the x-ray beam. The accuracy and repeatability of the force measurement apparatus have been found to be about 0.05 g. This has been achieved by special design of the balance beam and by using a floating electrical contact between the welding part and the ground. A linear relation between arc force and current squared was found experimentally, which is consistent with theory.