Stresses in electron beam weld joints of superalloys. (News Briefs).

Single crystalline turbine blades are the pinnacle of a decade-long development and refinement of nickel-based superalloys. Because of the considerable investment in manufacturing, electron beam welding with a polycrystalline replacement material is used to repair minor structural damage of failed components of these otherwise poorly weldable materials. While both the fusion zone and the heat affected zone are small, about one millimeter, the cooling rates are high so that very high residual stresses are generated.

In order to study the efficiency of stress relief, scientists from the NIST Center for Neutron Research and a private company have investigated by means of neutron diffraction the residual stress distribution both in the as-welded state and after a heat treatment. The heat treatment includes a solution treatment and subsequent aging. The typical grain size of the polycrystal side of the weld joint was approximately one millimeter which is about the size of the incident neutron beam. Thus, stress measurements could be done only on the single crystal side of the joint.

It was found that the as-welded sample exhibits tensile stresses on the order of the yield stress both parallel and perpendicular to the weld. We have also found evidence for solution partitioning and a substantial level of plastic strain both in the fusion zone and in an approximately half-millimeter wide heat affected zone. Both effects are completely removed after the heat treatment. The heat treatment also removes very effectively and completely the detrimental tensile residual stresses in the extended weld zone. These results are the first on this type of system, in which single-crystal and polycrystalline constituents are joined.

CONTACTS: Thomas Gnaupel-Herold, [phone]; [e-mail] or Henry Prask, [phone]; [e-mail].