Study On Microstructure Evolution Of Inconel 718 Superalloy Fabricated By Selective Laser Melting Following Different Heat Treatements

Incoel718（IN718）superalloy,having a high yield strength,high tensile strength,excellent creep resistance and good plasticity characteristics,it is widely used in aerospace,nuclear power industry,petroleum industry,and other fields.In recent years,Selective laser melting（SLM）technology has gradually become an important method of IN718 alloy forming,which solves the problem of difficult forming of complex parts.The results show that there are high density dislocations,composition segregation and topologically dense phases in the formed IN718 alloy due to the rapid solidification process of SLM.In order to obtain better properties of IN718 alloy,it is necessary to study the heat treatment process and optimize the process parameters.In this paper,the IN718 alloy was prepared by SLM forming technology.The precipitation law and precipitation strengthening mechanism of precipitation phase were studied,and the effects of solution temperature,double-stage aging and single-stage aging on the microstructure and mechanical properties of the alloy were systematically studied.The main research contents and conclusions are as follows:（1）The precipitation behavior and strengthening mechanism of IN718 alloy prepared by SLM after standard heat treatment（980℃_?1h（AC）/720℃_?8h（FC_?2h）+620℃_?8h（AC））were studied.The results show that the alloy IN718 prepared by SLM can precipitate and precipitate micron-sizedγ″phase andδphase,as well as nano-sizedγ’/γ"composite phase after standard heat treatment of forgings.Under the action of external stress,theδphase at the grain boundary breaks and dissolves,and theγ″phase of micron-sized changes toδphase and dissolves.Theγ′/γ″composite phase of nano-sized grows to about 25nm without any change.Some of theγ″phase of micron-sized grows to micron size,and the micron-sizedγ″phase accelerates to a delta phase transformation under high-temperature stress.The strength of SLMed IN718 alloy after heat treatment is significantly improved,due to the precipitation ofγ″Phase andδphases can effectively pin dislocations,nano-sizedγ′/γ″composite phase can produce Orowan strengthening,and agglomerated nano-sizedγ′/γ″composite phase and micron-sizedγ″phase can hinder stacking faults At the same time,the growth of the nano-sizedγ′/γ″composite phase can delay the transformation of the micro-sizedγ″phase toδphase.（2）The microstructure evolution and mechanical properties of IN718 alloy prepared by SLM after different solution temperatures（940¹140°C_?1h（AC））and two-stage aging（720℃_?8h（FC_?2h）+620℃_?8h（AC））were studied.The results indicate that the effect of solution temperature on the alloy grain structure were:as the solution temperature,the higher the degree of recrystallization of SLMed-IN718;grow significantly and at a temperature above 1020℃solution crystal grains,a columnar shows a tendency to grain equiaxed transition;alloy for complete recrystallization at 1140℃solution temperature,presence of annealing twins generated coherent twin boundaries.The effects of solution temperature on precipitation are as follows:at solution temperature of 940℃,the alloy precipitated micron-sizedγ″phase andδphase;at solution temperature of 980℃,the number of micron-sizedγ″phase decreased,and a large number of nano-sizedγ′/γ″composite phase precipitated.The nano-sizedγ″phase precipitated in the alloy at a solution temperature of 1020-1140°C,and the number ofδphases at the grain boundaries decreased sharply.At the solution temperature of 1020℃,the nano-sizedγ″phase is the largest.With the increase of solution temperature,the size of the nano-sizedγ″phase gradually decreases.The effect of solution temperature on mechanical properties of the alloys are:alloy having a highest yield strength（1332 MPa）,ultimate tensile strength（1512 MPa）and the highest microhardness(499⁵14 HV_0.2)at 1020℃solution temperature.This is possible due to the larger size of the nano-sized precipitated phase to achieve the strengthening effect.When the solution temperature reaches 1140℃,the elongation of the alloy increases to 23.7%,and the tensile strength only decreases to 1451MPa.This is because the improvement of the strength of the alloy is mainly affected by the type and size of the precipitate phase,which is less affected by the grain size,while the elongation of the alloy is greatly affected by the grain size.（3）After solution treatment of SLMed-IN718 alloy（1020℃_?1h（AC））,it was observed that a large number ofγ″phase nucleated uniformly in the matrix.On this basis,the influence of single-stage aging（650⁷00℃_?20h（AC））on the microstructure evolution and mechanical properties of the alloy was studied.The results show that the effects of single-stage aging on the microstructure of the alloy are as follows:the single-stage aged nano-sizedγ′/γ″composite phase grows with the increase of single-stage aging temperature from 650℃to 700℃,the largest nano-sizedγ″phase（about 25 nm）is found in the single-stage aged alloy structure at700℃,and some of the single-stage aged alloy structures grow abnormally to micro-size at725℃.Compared with the two-stage aging,the single-stage aging reduces the content ofγ′phase and forms a large number of dislocation networks.The effect of single-stage aging on the mechanical properties of the alloy is that under 700℃single-stage aging,the alloy has the highest yield strength（1352 MPa）,ultimate tensile strength（1522 MPa）and microhardness(501 HV_0.2),which is close to the mechanical properties of double-stage aging.This is because the number and size distribution of nano-sizedγ′/γ″composite phases play a major role in strengthening.