Effect of electron beam accelerating voltage on the melt zone area, secondary-dendrite arm spacing and fusion line microstructure of bead-on-plate welded Hastelloy C-276 sheet

Abstract

Effects of beam current and welding speed on weld morphology (e.g., bead geometry and microstructure) of electron beam welded joints for different types of alloys have been widely analyzed, however, the study of the role of accelerating voltage on weld morphology is limited. In the present investigation, bead-on-plate welding of 2.6-mm-thick Hastelloy C-276 sheet was carried out using the electron beam to study the effect of accelerating voltage on melt zone area and secondary-dendrite arm spacing. Real-time photographic images showing variation in the shape of the plasma plume (generated due to the interaction of electron beam with the substrate) with accelerating voltage were collected. Increase in interaction volume of the substrate with the electron beam at higher accelerating voltage might have increased the melt zone area. The possible phenomenon of increase in the secondary-dendrite arm spacing with accelerating voltage has been analyzed with the help of an index defined as the ratio of ‘segregation ratio of element’ to ‘segregation ratio of nickel’. The index showed that the increase in molybdenum segregation with accelerating voltage might have caused under-cooling and increased the secondary-dendrite arm spacing. The temperature gradient of the melt pool normal to the scanning direction of the electron beam was derived (from Rosenthal's equation for three-dimensional heat flow) to study the variation in microstructure at the fusion line. As the temperature gradient decreases with increase in accelerating voltage, the grain morphology was found to change from cellular structure to cellule-like structure at the fusion line of the weld bead.