Objective　Inconel　718　(IN718)　superalloy　is　widely　used　in　aerospace　engines　and　other　high-temperature　components.　Improving　its　fatigue　properties　is　crucial　for　ensuring　the　long-term　stability　of　the　components　in　service.　Selective　laser　melting　(SLM)　technology　is　one　of　the　best　choices　for　the　manufacturing　of　complex　aerospace　components　owing　to　its　high　molding　rate,　high　design　freedom,　and　short　production　cycle　time.　However,　the　rapid　melting　and　solidification　of　IN718　powder　during　SLM　leads　to　the　precipitation　of　a　brittle　Laves　phase　instead　of　a　strengthening　phase,　inside　the　component.　The　characteristics　of　layer-by-layer　scanning　of　SLM　lead　to　the　existence　of　high　residual　stress　inside　SLM-fabricated　parts.　Therefore,　heat　treatment　is　essential.　This　study　investigates　the　influence　of　different　heat　treatment　processes　on　the　microstructure,　phase　distribution,　and　fatigue　properties　of　SLM-fabricated　IN718　alloy.　Methods　In　this　study,　an　IN718　alloy　powder　was　prepared　using　a　gas　atomization　method　with　particle　size　distributions　between　15　and　55　μm.　The　IN718　specimens　were　prepared　by　EOSINT　M280　from　EOS,　Germany　and　then　cut　along　the　plane　of　the　substrate　via　wire　cutting　and　removed.　Subsequently,　the　IN718　specimens　were　subjected　to　three　different　heat　treatments,　as　shown　in　Fig.　2.　The　three　heat　treatments　is:　1)　solution　+　double　aging　(SA);　2)　homogenization　+　double　aging　(HA);　3)　homogenization　+　solution　+　double　aging　(HSA).　After　heat　treatment,　the　specimens　were　processed　into　fatigue　specimens　and　subjected　to　a　constant　stress-controlled　low-cycle　fatigue　test,　at　room　temperature.　Finally,　after　sample　preparation　and　polishing,　scanning　electron　microscopy　(SEM)　and　electron　back-scattered　diffraction　(EBSD)　photographic　analyses　were　performed.　Results　and　Discussions　The　distribution　of　precipitated　phases　differs　significantly　after　different　heat　treatments　(Fig.　5).　After　SA　treatment,　micron-level　γ″and　γ′strengthening　phases　and　a　small　number　of　distributed　δ　phases　exist　in　the　matrix,　whereas　a　large　number　of　δ　phases　distribute　at　the　grain　boundaries.　After　HA　treatment,　nano-sized　γ″and　γ′strengthening　phases　exist　in　the　matrix,　and　a　small　number　of　δ　phases　distribute　at　the　grain　boundaries.　After　HSA　treatment,　nano-sized　γ″and　γ′　strengthening　phases　exist　in　the　matrix,　and　δ　phases　exist　at　the　grain　boundaries.　Differences　in　the　distribution　of　the　resolved　phases　leads　to　differences　in　fatigue　performance　(Fig.　6).　Among　them,　the　fatigue　performance　of　the　HSA　treated　IN718　specimen　is　the　best,　and　the　fatigue　performance　reaches　98.6%　of　the　fatigue　performance　of　the　forged　part.　Subsequently,　the　specimens　were　prepared　by　wire　cutting　and　the　fracture　morphologies　were　observed,　and　the　fatigue　morphologies　of　the　specimens　with　different　heat　treatments　were　basically　same　(Fig.　7),　that　is,　there　are　multiple　fatigue　source　areas,　obvious　fatigue　glow　lines,　and　fatigue　transient　fracture　areas　with　dimples　and　secondary　cracks.　The　EBSD　results　(Fig.　8)　show　that　the　stress　concentration　is　mainly　in　the　δ　-phase　and　grain　boundary　regions.　Via　analysis,　it　is　found　that　dislocations　slide　freely　inside　the　grain.　When　dislocations　slide　to　the　punctate　γ′phase,　dislocations　bypass　the　γ′phase　based　on　the　Orowan　strengthening　mechanism　and　hinder　the　subsequent　dislocation　sliding.　When　dislocations　slide　to　the　flat　elliptical　γ″phase,　the　γ″phase　hinders　the　dislocation　movement　and　then　γ″phase　will　be　cut　with　the　accumulation　of　dislocations.　Because　the　δ　phase　and　matrix　γ　phase　are　non-conglomerative,　dislocations　accumulate　around　the　δ　phase.　When　dislocations　slide　to　the　grain　boundary,　the　grain　boundary　can　hinder　the　dislocation　sliding　and　crack　expansion.　At　the　same　time,　the　δ　phase　at　the　grain　boundary　can　nail　the　grain　boundary　and　delay　the　expansion　of　fatigue　crack,　thus　improving　the　fatigue　performance.　Conclusions　In　this　study,　the　microstructure　and　phase　distribution　of　the　IN718　alloy　fabricated　using　SLM　are　regulated　via　heat　treatment,　to　further　analyze　the　effect　of　heat　treatment　on　the　low-cycle　fatigue　properties,　at　room　temperature.　The　experimental　results　indicate　that　the　alloy　exhibited　precipitation　of　the　internal　δ　phase,　as　well　as　γ″and　γ′strengthening　phases　precipitate　inside　the　alloy　following　heat　treatment,　as　opposed　to　the　as-built　conditions.　The　presence　of　these　phases　contributed　to　the　alleviation　of　internal　stresses　within　the　alloy　and　led　to　a　significant　improvement　in　its　fatigue　performance.　Based　on　the　Orowan　strengthening　mechanism,　the　diffusely　distributed　γ″and　γ′strengthening　phases　within　the　grain　prevent　the　dislocations　from　sliding　within　the　grain.　The　δ　phase　precipitated　at　the　grain　boundary　can　enhance　the　strength　of　the　grain　boundary,　thus　retarding　microcrack　extension　in　the　matrix　and　increasing　the　fatigue　cycles.　Therefore,　after　HSA　treatment,　the　IN718　specimen　has　optimized　fatigue　performance.　The　improvement　of　the　microstructure　and　mechanical　properties　via　heat　treatment　processes　presented　in　this　study　provides　a　reference　for　the　application　of　the　SLM-fabricated　IN718　components.　©　2023　Science　Press.　All　rights　reserved.