Beam focusing characteristics and alloying element effects on high-intensity electron beam welding
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INTRODUCTION
W E L D I N G or cutting with a high-intensity beam is characterized by and benefits from large depth-to-width ratios of 5 to 10 or greater. Concentrated energy transfer is essentially required to achieve such results. Although effects of beam power, welding speed, and thermal properties have been studied extensively, tl 4] the overall process is still not well understood. Recently, an additional important factor affecting high-intensity beam welding process was found to be the beam focus location. I~l A volatile alloying element in the workpiece can also exhibit a pronounced influence. I5,61 Since quantitative and theoretical confirmation of these two important parameters is very limited, a detailed investigation was considered to be worthwhile. Adams t7'81 measured depths and widths of the fusion zone for different top and bottom lens focus currents for low- and high-voltage electron-beam welders. At low penetrations and low power, there is a tendency for the fusion zone to broaden with nonoptimum beam focus and for penetration to decrease relatively more than for deep penetration and high power. The sensitivity of the depth/ width ratio was found to increase with increasing distance from the focus coil to the workpiece. These trends are consistent with results presented by Engquist 131 who emphasized that a small error in the focus current can result in a significant change in the focal location and that it was best to operate at the shortest possible focus coil-to-workpiece distance. In view of the importance of the focal location, Sanderson I91 used a high-speed rotating probe technique to delineate the beam diameter and energy flux distribution of an electron beam. However, a complete description of the defocused energy beam was
P.S. WEI, Professor, is with the Institute of Mechanical Engineering, National Sun Yat-Sen University, Kaohsiung, Taiwan, China. Y.T. CHOW, formerly Graduate Student, Institute of Mechanical Engineering, is Assistant Researcher with the National Chung-Shan Institute of Science and Technology, Chung-Li, Taiwan, China. Manuscript submitted May 29, 1990. METALLURGICAL TRANSACTIONS B
not provided. Konkol e t a l . t~~ also measured variations of the fusion zone with beam focal location. Precisely speaking, beam-focusing characteristics are determined by the desired focal location, spot size, and convergence angle (Figure 1). All of these characteristics vary simultaneously as the focus current is changed. Therefore, one objective of this study was to systematically determine separately the variation of the cavity depth or fusion zone with different focal locations, spot sizes, and convergence angles. The present work also examines effects of a highly volatile alloying element. Shintaku e t a l . t51 and Schauer e t a/. 161 w e r e the first to measure cavity surface temperatures and found that when very small amounts of highly volatile elements were present in the material being welded, a substantial decrease in cavity surface temperature occurred. The peak temperature
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