Features of the Microstructure of Welded Joints of Single Crystals of Heat-Resistant Nickel Alloys

Abstract

The peculiarities of the microstructure of welded joints of single crystals of heat-resistant nickel alloys made by electron beam welding are considered. The main structural areas of the welded joint are identified: the base metal, the heat effect zone (HAZ), the fusion area, the epitaxial growth zone, the weld sections corresponding to different deviations of the maximum heat dissipation from the orientation of the predominant crystal growth. The dependences of the structure of separate areas, their sizes from the initial crystallographic orientation of the single crystal, the brand of the studied alloy, temperature-temporal and temperature-spatial parameters of the process, determined mainly by welding modes, are established. HAZ is dominated by changes (γ + γ′)-structures consisting in complete or partial dissolution of the primary γ′-phase, large globular eutectic formations with subsequent (upon cooling) separation of the dispersed nanosized secondary γ′-phase. The weld metal is characterized by more noticeable changes in the dendritic structure-variables in the cross section of the weld, its parameters and morphology, a decrease in chemical inhomogeneity compared to the original metal.

Crystallographic and dimensional characteristics of structure elements in welds of high-temperature nickel alloy single crystals

“…the fusion line zone is a transition area from the haZ to the zone of weld metal epitaxial growth. Its structure is characterized by presence of partially melted dendrites and interdendritic gaps [17]. partially melted dendrites take the shape of cells, a change in the crystallographic orientation being hardly noticeable.…”

“…The zone of epitaxial growth was defined from the very beginning of investigations as an area, which in metallographic presentation consists of orthogonal fusion lines of fine dendrites located in parallel [17]. In the case of a symmetrical welded joint, its structure points to preservation of the initial crystallographic orientation and presence of individual fine (5-40 μm) grains of [111] orientation (Figure 4, a) or [101] orientation (figure 4, b).…”

“…Comparison of the results of EBSD-analysis and metallographic examination [9,14,17] points to the nature of grains of a different orientation, which can form in welding of HTNA single crystals: solidification and deformation-induced. The solidification grains boundaries form during metal solidification at abutment of weld dendrite blocks of different orientation, those of deformation-induced grains form as a result of stress relaxation at metal cooling by the mechanism of, most probably, polygonization and dynamic recrystallization.…”

“…Figure 1 illustrates the area studied by EBSD analysis on the welded joints. The characteristic areas of single crystal welded joints revealed during analysis [16,17] were considered: base metal, haZ, fusion line, epitaxial growth zone and weld areas with different deviations of the crystallographic orientation from the initial one.…”

Modern Techniques for Automated Acquiring and Processing Data of Diffraction Electron Microscopy for Nano-Materials and Single-Crystals

“…It allows leveling a sharp gradient of mechanical properties usually observed in dissimilar joints. Recently, electron beam welding has become the main technological process in the manufacture of singlecrystal blades for aircraft engines [7] and turbines [8,9]. Surface treatment of various metals (steel, copper, aluminum, titanium) by the method of detonation and other types of spraying of coatings allows to obtain composite surface layers with high mechanical properties [10,11].…”