热处理对激光选区熔化IN718合金室温低周疲劳性能的影响

Effect of Heat Treatment on Low⁃Cycle Fatigue Properties of Selective Laser Melted IN718 at Room Temperature

Inconel 718高温合金被广泛用于制造航天发动机等的热端零部件,提高其疲劳性能对于零部件的长期稳定服役意义重大。研究了不同热处理制度对激光选区熔化成形Inconel 718合金微观组织、相分布及疲劳性能的影响。采用电镜和电子背散射衍射仪分析了热处理试样的断口形貌、断口纵剖面应力分布及微观特征,详细阐述了热处理合金的低周疲劳断裂机理。结果表明:相较于成形态,热处理合金内部析出了δ相和γ″、γ′强化相,内部应力得以释放,疲劳性能显著提升。经过均匀化+固溶+双时效热处理后,Inconel 718合金的疲劳循环周次能够达到31990次。基于Orowan强化机制,晶粒内弥散分布的γ″、γ′强化相以及晶界上析出的δ相会阻碍位错滑移,从而延缓基体中微裂纹的扩展,增加疲劳过程中的循环周次。本次试验采用的热处理制度为激光选区熔化成形Inconel 718零部件提供了参考。

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 55μm.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-levelγ″andγ′strengthening phases and a small number of distributedδphases exist in the matrix,whereas a large number ofδphases distribute at the grain boundaries.After HA treatment,nano-sizedγ″andγ′strengthening phases exist in the matrix,and a small number ofδphases distribute at the grain boundaries.After HSA treatment,nano-sizedγ″andγ′strengthening phases exist in the matrix,andδphases 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 theδ-phase and grain boundary regions.Via analysis,it is found that dislocations slide freely inside the grain.When dislocations slide to the punctateγ′phase,dislocations bypass theγ′phase based on the Orowan strengthening mechanism and hinder the subsequent dislocation sliding.When dislocations slide to the flat ellipticalγ″phase,theγ″phase hinders the dislocation movement and thenγ″phase will be cut with the accumulation of dislocations.Because theδphase and matrixγphase are non-conglomerative,dislocations accumulate around theδphase.When dislocations slide to the grain boundary,the grain boundary can hinder the dislocation sliding and crack expansion.At the same time,theδphase 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 internalδphase,as well asγ″andγ′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 distributedγ″andγ′strengthening phases within the grain prevent the dislocations from sliding within the grain.Theδphase 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.