Effect of Cr addition on microstructure and welding solidification cracking susceptibility of Co-Al-W based superalloys

Abstract

The novel γ′-strengthened cobalt-base superalloys are promising candidates for high temperature materials. However, the weldability has rarely been studied. In this work, the effect of Cr addition on microstructure and solidification cracking susceptibility during tungsten inert gas welding of the Co-Al-W-xCr-0.5C (where x = 2.5, 5 and 7.5 at%, named as 2.5Cr, 5Cr and 7.5Cr) alloys were investigated. Results show that there are M6C carbide, β phase and γ/γ′ phase existed in all alloys. Cr addition promotes the precipitation of M6C carbides and changes the morphology of γ′ phases from cubic to round-cornered, which is due to the occupied lattice of W atoms by Cr atoms. In addition, the welding solidification cracks length declines significantly with increasing Cr content. Further studies by differential thermal analysis and electron back scattering diffraction show that the relative low solidification cracking susceptibility of 7.5Cr is mainly attributed to the narrow solidification temperature range, sufficient liquid phases backfilling the grain boundary in partially melted zone, serrated grain boundaries and large numbers of solidification sub-grain boundaries.