Acoustic Emission Measurements During Electron Beam Welding of Nickel-Base Alloys

During EB welding of nickel-base superalloys flaws may occur under unfavorable conditions. In welds of Waspaloy “Cameron” flaws appeared in the heat-affected zone, while flaws in the weld seam were observed in the case of Inconel 713 “Nimocast.” The flaws in Waspaloy could not be detected by conventional NDT methods because of their position at the root of the weld. Therefore, for direct observation of flaw formation in these alloys, acoustic emission measurements were made during EB welding of these alloys. Two different types of acoustic emission were found. In nickel-base superalloys and in austenitic steel background noise occurs as so-called burst signals which follow one another at short time intervals; these signals correlate exactly with markings (grooves) on the surface of the weld seam. This type of acoustic emission depends on the local heat transfer (beam current per unit time and area). Apparently this background noise is caused by weld solidification. Flaw formation, on the other hand, is indicated by single burst signals having a much higher amplitude than those due to solidification. In the case of Waspaloy this second type of acoustic emission was observed only at fairly high speeds of welding (i.e., higher than 25 mm/sec). This result is consistent with evaluations of micrographs.