Damage of the surface layer of Inconel 718 alloy by pulsed beam-plasma flows

Abstract

The damage of the surface layer of the Inconel 718 alloy, prepared by additive technology by selective laser melting with subsequent heat treatment, is studied under the conditions of repeated pulsed exposure to flows of helium ions and helium plasma in two modes of irradiation in the Plasma Focus “Vikhr” facility: in soft mode with energy flux density q0 = 2·108 W/cm2 at pulse duration τ = 50 ns and in hard mode (q0 = 1.5·109 W/cm2, τ = 25 ns). The number of pulsed actions in the experiments was N = 10 and 20. Under the implemented conditions of pulsed irradiation, the processes of sputtering and evaporation of the surface layer of the alloy, as well as its melting and crystallization at a high rate, took place. In this case, the original flat surface of the alloy sample was transformed into a wavy relief containing in some areas a thin film wrinkled in the form of ripples. In the soft irradiation regime, the surface microstructure contained pores, while under more severe energy impacts, surface microcracks and blisters with destroyed shells were also observed. With an increase in the energy flux density q, the intensity of surface erosion (mass loss per pulse) increased. The nature of this phenomenon was influenced by the processes of purification of the alloy surface from elements adsorbed from the external environment before irradiation, as well as deposition of elements of functional materials and impurities of the working gas on the irradiated surface. Features of the formation of the cellular microstructure of the surface layer of the investigated alloy under the realized conditions of beam-plasma impacts were revealed. Using numerical simulation, the redistribution of the fractions of the energy absorbed by the material spent on the evaporation and melting of the irradiated surface layer was established in comparable irradiation modes.