Thermal stabilization of additively manufactured superalloys through defect engineering and precipitate interactions

Abstract

Laser peening (LP) is a mechanical surface modification technique capable of enhancing a treated material's resistance to surface-related failures through the introduction of deep, high-magnitude compressive residual stresses. Conventional LP enables material improvement in systems operating in ambient or low temperature (<0.5Tm where Tm is the melting temperature) conditions. At high temperature (>0.5Tm), LP-induced property enhancements and microstructural modifications encounter thermal degradation through dislocation annihilation, stress relaxation, and grain coarsening. Motivated by the need to retain critical material enhancements, a novel, modified LP technique was introduced and employed in this work, coined laser peening plus thermal microstructure engineering (LP + TME). This approach incorporates cyclic LP events with the inclusion of intermittent, 600 °C (0.55Tm) heat treatment steps in order to impart highly thermally stable microstructural modifications in additively manufactured (AM) Inconel 718. Microstructural and mechanical property evolution was evaluated through instrumented indentation and transmission electron microscopy (TEM). Surface-level compressive residual stresses were discovered to increase by as much as 84% from 167.13 MPa to over 300 MPa following LP + TME and a 350-h, 600 °C exposure. Surface and near-surface microhardness of samples subjected to LP + TME was observed to increase by 33% (to 630 HV) compared to untreated, as-built specimens even after exposure to 600 °C. Hardened zones were characterized by the presence of highly dense dislocation networks and precipitated strengthening phases. Both property enhancement and thermal stability were determined to be the result of various mechanisms at work, stemming from the synergistic combination of cyclic strain and thermal input. These include the facilitated nucleation of γ’’ and δ phases which aid in stabilizing LP-induced microstructures and provide material enhancements at high temperatures; precipitate-dislocation interactions including Orowan bypassing, Friedel cutting; and dislocation pinning effects. The results from this work are of significant interest as they are in contrast with what is typically expected at temperatures exceeding 0.5Tm meaning the technique outlined herein could offer a viable solution to overcoming obstacles preventing the widespread use of LP in critical, high temperature applications.