Weldability and hot cracking in thorium-doped iridium alloys

Abstract

Modified iridium alloys containing ≥ 100ppm Th were found to be very susceptible to cracking during gas tungsten-arc and electron-beam welding. However, the electron-beam welding process showed greater promise of success in welding these alloys, in particular Ir–0·3W doped with 200ppm Th and 50ppm Al. The weld ability of this particular alloy was extremely sensitive to the welding parameters, such as beam-focus condition and welding speed, and the resulting fusion-zone structure. At low speeds successful electron-beam welds were made over a narrow range of beam-focus conditions. However, at high speeds successful welds can be made over an extended range of focus conditions. The fusion-zone grain structure is also a strong function of welding speed and focus condition. Weld-metal cracking was found to be caused by the combined effects of heat-affected-zone liquation cracking and solidification cracking. Scanning electron microscopic analysis of the fractured surface revealed patches of low-melting eutectic. Also, in the welds that cracked, a region of positive segregation of thorium was identified near the fusion boundary. This highly thorium segregated region may also act as a source for the nucleation of a liquation crack.