Porosity And Mechanical Properties Of Inconel 718 Superalloy Produced By Selective Laser Melting

Selective laser melting(SLM)technology has achieved rapid development in the past ten years because it can achieve high-precision rapid prototyping of complex structural parts,and has been gradually applied in the field of aviation.Due to the current state of the technology,it is still inevitable that there were micro defects such as micro porosity in materials produced by SLM.However,there is rare research on the microstructure and mechanical properties of SLMed parts with pore defects currently.This paper discusses the influence of forming parameters in SLMed samples on the density of Inconel 718 nickel-based superalloy and the cause of pore defects formation.The static and tensile properties and the behavior of fatigue crack growth rate for SLMed IN 718 parts with some pore defects are studied.The main research results are as follows.The reason for the formation of pore defects in Inconel 718 superalloy produced by SLM was studied.When the laser power was 400W,the formation of pore defects at low scanning speeds was mainly circular pore defects with a size of 20-50?mwith“Key-hole”mode.The defects transform from“Key-hole”mode to“Conduction”mode with the scanning speed increasing,and small circular pore defects with a size smaller than 20?m were formed.At the same time,irregular pore defects of small size formed again along with the scanning speed increasing.With a scanning speed of 1800 mm/s,at a lower power,the irregular pore defects of 168?m formed across multiple cladding layers caused by lap lack of fusion.The pores were transformed into circular pore defects which are less than 20?m in size under the mode of“conduction”with the increase of laser power.The microstructure and mechanical properties studies of the SLMed Inconel 718alloy samples show that the as-deposited structure consists of coarse epitaxially grown columnar crystals and the growth direction of columnar crystals is perpendicular to the laser scanning direction and points to the deposition direction.Microscopically,it shows a fine dendrite substructure with dendrite spacing of only 1-2?m,and microsegregation of alloying elements is not obvious.The heat treatment of the SLMed Inconel 718 alloy samples show that the columnar crystals grown epitaxially and continuously are replaced by fine recrystallized grains after heat treatment at 1080°C/1h.After explored at 1030?for 1h,the brittle Laves phase can be completely eliminated.The microhardness and tensile strength of samples reached a maximum of 521.23 HV and1463.07 MPa after the heat treatment procedure of 1080?/1h+?phase aging+double aging.The Microhadness and tensile properties of Inconel 718 alloy samples with different densities were tested.It was found that the microhardness of the sample after the stress relief heat treatment was 350±15 HV.After solution+aging heat treatment,the microhardness is obviously improved and reaches to 510±20 HV,which does not change with the change of density.The test results of tensile property show that the strength of the samples with different densities after heat treatment shows a trend of first increase and then decrease with increasing density.It is found that different types of pore defects have different effects on tensile strength.Large-sized circular pore defects formed in the Key-hole mode have the greatest impact on the strength of the sample,followed by irregular small-sized pore defects and the small size circular pore defects formed under the“conduction”mode.Fatigue crack growth tests were performed on three different density samples with densities of 98.85%,99.26%,and 99.84%.It was found that the threshold value of stress intensity factor of three different density samples was 14.15 MPa·m^1/2?15.08MPa·m^1/2?14.25 MPa·m^1/2.A linear fit of the Paris area yields the following results:da/d N=6.51E-9(?K)^2.7at a density of 98.84%,da/d N=5.46E-9(?K)^2.74at a density of99.26%and da/d N=8.67E-9(?K)^2.61when the density is 99.84%.There was no significant difference between the values of C and m in engineering test.It shows that under the three density conditions selected in this experiment,the density and the type of pore defects have no significant effect on the fatigue crack growth ability of SLM formed Inconel 718 alloy.