1 Introduction
Additive manufacturing (AM), commonly known as 3D printing, is a rapidly growing group of processing technologies comprehensively reviewed by Horn and Harrysson.[
1] In AM, it is possible to produce geometrically complex components directly from computer-aided design (CAD) models.[
2] The ease of manufacturing, compared to the traditional production routes, makes AM a promising emerging technology. Electron beam melting (EBM) and laser powder bed fusion (LPBF) are two commonly used AM technologies based on powder bed fusion, as broadly reviewed by Frazier.[
3] EBM makes use of a high-energy electron beam to selectively melt and consolidate appropriate regions of each layer of powder raked to build a component in layer-by-layer fashion.[
4] EBM processing of nickel-based superalloys (example: Alloy 718) appears particularly promising to the aircraft engine industry,[
5] which demands complex parts manufactured from difficult-to-machine materials. During EBM processing, individual layers undergo various processing steps, with the typical melting steps involving contour and hatch scanning as detailed in Reference
6. For every layer, the perimeter of the component(s), also known as contour, is typically melted by a ‘multi-spot’ melting strategy. During this step, the spot melting pattern is used to create a ‘frame’ of the component according to a pre-defined CAD geometry as elaborated in Reference
7. This step is followed by hatch melting, during which the beam typically scans in a raster pattern to consolidate the region(s) contained within the contour(s). In a word, the hatch melting melts the bulk material while the contour scanning provides adherence to component geometry.[
8] The thickness of the contour region is thus fixed, but the extent of the hatch region depends on the overall component dimensions.[
9] In case of thinner sections, the microstructure of the contour thus becomes critical. It is worth mentioning that the order of contour and hatch scanning can be modified, and typically for EBM manufacturing of Alloy 718, the contour is melted before the hatch.
Alloy 718 is a precipitation-hardened Ni-based superalloy, which is used in varied operating environments, such as for high temperature as well as cryogenic applications in diverse fields such as aerospace, oil and gas, nuclear industries,
etc. Such extensive usage is attributed to its high strength and excellent corrosion resistance combined with good weldability, creep and fatigue properties.[
10] Alloy 718 derives strength mainly by the coherent metastable
γ″(Ni
3Nb) and
γ′(Ni
3(Al,Ti)) precipitates present in the
γ-FCC matrix. Other phases present in Alloy 718 impart additional properties. For instance, the thermodynamically stable
δ phase (Ni
3Nb) present at the grain boundaries is known to control grain size during heat treatment and thermo-mechanical working. While the
γ″,
γ′ and
δ phases precipitate during solid-state transformation, the primary carbides form during solidification as stated in the solidification path proposed by Knorovsky
et al.[
11] based on the differential thermal analysis. The MC-type primary carbides (
M = Nb, Ti) are also known to pin grain boundaries at high temperatures as observed by Kirka
et al.,[
12] who attributed the lack of grain growth after hot isostatic pressing (HIP) (at 1200 °C) of EBM Alloy 718 to Zener pinning by these carbides present at the grain boundaries. The precipitation of
γ″,
δ, primary NbC carbides is related mainly to the Nb segregation, Nb being a key alloying element in Alloy 718. During solidification, Nb segregates in the interdendritic regions because of its larger atomic radius.[
13] The degree of segregation depends on the solidification parameters, such as the cooling rate. Zhang
et al.[
14] simulated the cooling rates for Alloy 718 during LBPF and casting and found it to be significantly higher in the former (10
3 to 10
6 K/s) compared to the latter (3.8 K/s). In EBM Alloy 718, prior work by Kirka
et al.[
15] has experimentally approximated (through primary dendrite arm spacing) the cooling rate to be 10
3 K/s. In this context it is worth mentioning that, in cast alloy 718, both Mitchell[
16] and Patel
et al.[
17] have observed that an increase in cooling rate caused a decrease in carbide size.
There have been significant research efforts on EBM processing of Alloy 718, such as tailoring the grain structure through both experiments and modeling by Helmer
et al.,[
18] Raghavan
et al.,[
19] Balachandramurthi
et al.[
20] and others. For instance, Helmer
et al.[
18] have shown that the microstructure in the EBM-built Alloy 718 can be tailored by changing the scanning strategy and thus the thermal gradient during solidification. If the thermal gradient is aligned in the preferred direction, columnar grains grow; otherwise, equiaxed grains can form. Raghavan
et al.[
19] found that the preheating temperature (one of the indirect control parameters in the Arcam process) has a significant effect on the volume fraction of the formed equiaxed grains. Balachandramurthi
et al.[
20] have shown that for the ‘multi-spot’ meting strategy the microstructure is affected by the relationship among processing parameters, melting patterns and part geometry and also by the control of energy input. Such increased understanding of the process-structure correlation has not been complemented by understanding the influence of thermal post-treatments on the microstructure. The latter can potentially suppress the inevitable defects (gas porosity, shrinkage porosity and lack-of-fusion), undesirable micro-segregation,
etc., present in the microstructure. The commonly applied post-treatments include HIP and heat treatment (HT). The latter typically involves solution treatment and aging. The majority of past research efforts on the application of post-treatments focused on the hatch region of EBM Alloy 718 as previously detailed by Deng
et al.,[
21] Nandwana
et al.[
22] and others. The hatch region typically consists of elongated columnar
γ grains having strong 〈001〉 texture in the build direction. The contour region, on the other hand, usually exhibits a mix of fine equiaxed grains, curved thin columnar grains and wide columnar grains.[
21] Deng
et al.[
21] attributed the observed difference in the grain structures of the hatch
vs. contour regions to the different melting strategies. Given the significant differences in the microstructure, there is a need for investigating the effect of post-treatments, between these regions, of the EBM-built Alloy 718. This has been an important motivation behind this study. Although prior work by the authors has involved investigation of aspects related to uniformity in a typical EBM Alloy 718 build,[
23] inclusions and precipitates in as-built and post-treated material,[
24] extent of defect closure during HIPing in builds with intentionally introduced defects,[
25]
etc., the response of hatch and contour regions to identical post-treatments remains not fully investigated.
In the present study, detailed microstructural analysis of EBM-built Alloy 718 was carried out and followed with different post-treatment cycles. The two post-treatments carried out were: HIP and HIP + HT. The latter involved solution treatment and two-step aging preceded by HIP and was carried out as a single cycle inside the HIP vessel. The texture, grain morphology and grain size were comprehensively investigated using scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Moreover, the defects, carbides, δ phase and γ″/γ′ phases in the material and the microhardness were investigated.
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