Inconel 718 is reliable to be used in a high-temperature environment providing protection from creep, corrosion and thermal shock as desired. Forming geometries made of this superalloy is very challenging and expensive due to high strength, toughness and work hardening at room temperature. Wire Arc Additive Manufacturing (WAAM) technology is highly efficient to fabricate Inconel 718 with complex configurations and demanded by high volume production. The microstructural evolution during the WAAM process is the reason that the ultimate material may not fully reflect the mechanical properties of wrought material. The finer structure and formation of secondary phases are the AM materials characteristics that are attributed to rapid cooling in this process. Hence, settings of processing parameters and part configuration affect the thermal history during the process and the ultimate physical and material properties.
This study investigates the microstructure, solidification structure and mechanical properties of WAAM Inconel 718 thin structures. These parts include four walls and one tube built by the MIG welding equipment. The part’s configuration, shielding gas, deposition strategy and interlayer idle time (IIT) are the effective parameters considered in this parametric study. The results will show the influence of these parameters on the ultimate geometry of the WAAM products and resultant mechanical properties.
ERNiFeCr2 Inconel 718 wire with a diameter of 0.035-inch (0.9 mm) was used as the MIG consumable. A semi-automatic method was used to perform the WAAM operation. In this setup, a motor controls the torch travel speed and the height is adjusted by a threaded adjustment system.
For the parametric study, one tube and four linear thin-wall components were deposited with 30 layers for each. The IIT was only implemented for manufacturing the walls; however, the tube layers were deposited continuously in a counterclockwise direction without IIT. Argon was selected as the shielding gas for the fabrication of the tube and three walls and Helium was used for the manufacturing of one wall. The IIT was lowered from 60 s to 20 s for one of the walls. In another trial, the deposition strategy was altered from the bi-directional deposition path to the unidirectional one.
For microstructural evaluation, the central sections of the walls and tube along the layer direction were sectioned. The microstructure was observed by optical microscopy followed by scanning electron microscopy. The chemical composition was measured by energy dispersive spectroscopy analysis.
For mechanical testing, a Vickers hardness tester was used to evaluate the microhardness. The tension test was performed on miniature tensile samples designed based on the ASTM E8 standard. To avoid stress concentration, an FE analysis was performed on the miniature configuration using COMSOL Multi-physics. During the analysis, different sample dimensions were examined until the optimum geometry was achieved. Five vertical samples (along build direction) were extracted out of each wall and tube, and five horizontal samples (along the layer direction) were cut from each wall. The samples’ surfaces were mirrored polished and etched to trace the fracture site. An in-situ optical microscope was used to monitor the deformation during the tensile test.
Results & Discussion:
The ultimate geometries’ sizes of WAAM walls are shown in Figure 1. It is seen that using helium as the shielding gas led to a thin part with an increase in height. Furthermore, different IIT influenced the ultimate geometry indicating a smaller interlayer idle time resulted in a shorter and wider wall. Also, the alteration of the path strategy did not change the size of the WAAM part. To compare the effect of process parameters on the solidification morphology, the microstructures of longitudinal sections will be shown. The microstructure of the inter-layer and inner-layer regions of the WAAM components will be compared. A finer solidification morphology was observed in the inter-layer region. In addition, the dispersed laves phases have a larger size in the inter-layer region of the tube compared to the corresponding zone in the wall due to different thermal evolution in these WAAM structures. The observation showed the microscale carbides and nanoscale γ^" scattered all over the matrix.
The WAAM products exhibit an anisotropic mechanical performance in the layer and build directions. It should be noted that not only the tensile properties are direction-dependent, but also the horizontal samples are very location dependent on the walls due to various thermal history. In-situ monitoring of tensile test samples will present the deformation of the vertical samples from the test initiation to the fracture. The etched surface of the samples will indicate the fracture regions of the walls and tube. The introduced parameters impact on the mechanical properties of the 718 WAAM materials will be discussed. Among the mentioned processing parameters, Helium shielding gas improved the elongation for the WAAM part. The performed fractography revealed that the laves phase is associated with loss of strength for WAAM Inconel 718.
This research studies the processing parameters and geometry impacts on physical characteristics, microstructure and mechanical properties of Inconel 718 WAAM parts. Tube and walls were manufactured with different sets of parameters. The produced walls using Argon shielding gas were found to be wider resulting in a reduction in the part’s height. The overall tensile strengths lie within a narrow range using either of these shielding gases; however, the ductility was obtained higher when using He due to a higher cooling rate. The shorter IIT increased the layer width and the variation of IIT showed an insignificant impact on the microstructure and mechanical properties. Since the WAAM method is a high-power deposition process, the grains grow vertically and independent of the deposition direction; hence, the path strategy did not affect either physical geometry or mechanical properties. Different heat distribution was obtained for tube structure during the manufacturing process resulted in a slight variation in microstructural evolution. Overall, the mechanical properties of the WAAM products were found to be lower than the wrought material particularly the ductility. The WAAM products were found weaker in the layer direction due to a higher laves phase density at the inter-dendrite regions.