Background
Additively manufactured (AM) components are being adopted broadly among many industries and used in an array of applications. AM parts and structures are attractive to these industries because very complex geometries that otherwise cannot be manufactured using traditional methods can be printed and parts can be fabricated as needed. A stock of replacement parts is not required and delivery times can be significantly reduced. Two of the primary industries currently utilizing AM parts are aerospace and oil and gas (O&G). Within these industries, AM 718 is of specific interest due to the alloy’s excellent mechanical properties over a wide range of service temperatures.
Figure 1: Microstructure of wrought Alloy 718. Several companies in the aerospace sector are currently exploring the feasibility of AM parts in rocket propulsion systems, and others have already used AM to fabricate critical rocket components. Guidelines are currently being developed in the O&G industry for the qualification and certification of AM parts for offshore applications. However, in many of these applications alloy 718 is susceptible to hydrogen embrittlement (HE). Microstructural characterization and mechanical properties of AM Alloy 718 are reported in the literature, but the performance of AM 718 in gaseous hydrogen environments has not been thoroughly investigated.
Approach
In order to understand the underlying mechanisms governing the susceptibility of AM alloy 718 to HE under HPHT conditions, mechanical testing, metallurgical evaluations, and hydrogen-alloy interaction experiments will be completed on wrought material and three different AM build configurations. Mechanical testing will be performed on wrought and AM test specimens in high-pressure and high-temperature gaseous hydrogen. Detailed microstructural evaluations will be completed to understand the effects of AM parameters and post-build thermal processing parameters on the porosity, grain size, phase constituents, and hardness. Hydrogen-alloy interactions will be characterized using thermal desorption spectroscopy (TDS) and scanning Kelvin probe force microscopy (SKPFM). TDS will be utilized to determine the hydrogen binding energies and total concentrations within the materials, and SKPFM will be utilized to spatially resolve atomic hydrogen within the metallurgical features of the various alloy configurations.
Figure 2: Microstructure of AM Alloy 718 (left) and remnant columnar structures in RED from the AM and post-build thermal processes (right).
Accomplishments
The initial metallographic evaluations have revealed significant variations in microstructure between the wrought and AM materials. The wrought microstructure has a clearly defined grain structure with expected annealing twins. Two of the three AM material configurations contained regions with cellular morphology consistent with remnant columnar structures that were not recrystallized upon solidification of the AM laser melt pool. These features are artifacts of the AM build process and post-process thermal treatment.
Fatigue crack growth rate testing and characterization of the hydrogen-alloy interactions are currently underway. Post-test fractography will be coupled with the mechanical testing results and microscopy to further understand the effects of microstructural features on the susceptibility of AM alloy 718 to HE. Additionally, the total hydrogen concentrations, the internal distributions of hydrogen, and the binding energies of hydrogen populations will be used to understand how the microstructure effects these material properties.