Summary

The results of this research were the characterization of Fatigue Crack Growth Rate (FCGR)’s for As-Built (AB), Conventional Heat Treatment (CHT), and Modi-fied Heat Treatment (MHT) conditions of Inconel 718 (IN718) manufactured by Laser Powder Bed Fusion (LPBF) and the demonstration of anisotropy in the FCGR be-tween specimen build orientation. Testing methodology was based on the methods laid out in American Society for Testing and Materials E647-15 (ASTM E647-15) and American Society for Testing and Materials E1820-17a (ASTM E1820-17a) and deviations were made to meet the unique needs of this research. Compact Tension (C(T)) specimens were manufactured by LPBF and tested in either constant am-plitude fatigue or in tension to fracture. Both of these tests rely on measuring a growing crack and the associated force which was accomplished with a load cell on a hydraulic tension tester and by use of a clip gauge to measure the Crack Mouth Opening Displacement (CMOD). Due to observed peculiarities in the fracture sur-faces, a brief examination of these surfaces was conducted utilizing surface roughness measurements. The examination indicates that fracture surfaces exhibited differences based on the specimen build orientation and that the MHT produced a much rougher surface due to grain growth.

The hypothesized questions for this research were:

• What degree of difference does specimen build orientation have on the Fracture Toughness (FT) and FCGR?

• Does a modified heat treatment remove the specimen build orientation depen-dence of FT and FCGR?

• Does the MHT provide improvements to FT and FCGR in all specimen build orientations?

Regarding the hypothesized questions in this research the following conclusions can be reached:

• specimen build orientation was seen to have effects on the FCGR of all heat treatments tested. AB specimens showed least deviation in fatigue crack growth life between specimen build orientations 1.1 - 1.3 times difference in fatigue crack growth life between specimen build orientations. Specimens subjected to the CHT showed showed the greatest differences of 1.4 - 2.5 times difference in fatigue crack growth life between specimen build orientations. MHT speci-mens showed a difference in fatigue crack growth life between specimen build orientations of 1.3 - 1.9 times.

• The MHT was not able to remove the anisotropic effects of specimen build orientation. MHT specimens showed anisotropic behavior between specimen build orientations despite the re-crystallization of the grain structure.

• The MHT showed a significant decrease in the FCGR of all specimen build orien-tations compared to the AB and CHT. The MHT decreases the FCGR of IN718 manufactured by LPBF and increases the number of cycles to initiate a crack.

In the initial precracking procedure it was noted that the MHT specimens took approximately 3 - 4 times more cycles than the AB or CHT specimens to grow an appropriately sized precrack. The MHT specimens showed a FCGR that was significantly lower than the AB and CHT specimens. The MHT increased fatigue crack growth life between 1.3 - 6.0 times compared to the CHT. Com-parison of the MHT to the AB condition showed an increase in fatigue crack growth life between 2.5 - 3.6 times, dependent on specimen build orientation.

The Flat(X) specimen build orientation was weakest for the MHT but provided a minimum 1.3 times increase in fatigue crack growth life over any of the other AB or CHT condition. This increase in fatigue crack growth life is attributed to the change in grain size and the growth of γ” precipitates during the MHT.

Fracture toughness testing revealed high levels of plasticity during testing requir-ing that FT tests should be run by the standards of ASTM E1820-17a. Due to extensive crack front curvature, future fracture toughness testing should incorporate side grooves into the design of C(T) specimens as described in ASTM E1820-17a Sec-tion 7.5. Fracture toughness values calculated in this research are invalid according to the requirements of ASTM E1820-17a. Observations of CMOD vs Force plots shows that the MHT specimens required more force to reach the displacement maximum of 1.5mm than did AB specimens. This corresponds with tensile data taken in these two conditions and may indicate an improvement in FT with the MHT but no conclusive statements can be made without further valid testing.

In both FT and FCGR, specimen fracture surfaces showed clear evidence of the specimen build orientation. Grain size increases were readily apparent in the MHT fatigue specimens as seen in Figures 4.17 - 4.19 when compared to AB and CHT samples. This corresponded to surface roughness measurements taken of the fracture surfaces where MHT specimens showed much higher values of surface roughness com-pared to the AB and CHT specimens. A layered texture was visible in the Flat(X) and Vertical(Z) specimens corresponding to the specimen build orientation. Edge(Y) built specimens do not show this feature but evidence of the laser scan pattern can be seen in these fracture surfaces as shown in Sections 4.2.1 and 4.4.3. In the case of the Edge(Y) specimens it appears that the crack propagated between the build layers of the specimen. This indicates that bonding between layers is weaker than the surrounding material. This closely matches the results of tensile tests performed

on LPBF IN718[22]. These features seem to indicate that incomplete fusion of layers is present with the printing parameters used. This effect is seen in all three heat treatments.