| Abstract Scope |
The microstructures of welded and additively manufactured precipitation-hardened nickel superalloys typically contain elemental segregation, secondary phases, and oriented grain morphologies due to the rapid solidification process. As a result, these materials often do not achieve acceptable microstructures and properties when subjected to standard heat treatment protocols specified for wrought material. For example, standardized solutionization heat treatments are inadequate to fully eradicate the solidification structure in laser powder bed fusion nickel alloy 718, which leads to undesirable delta phase precipitation upon aging. Appropriate homogenization temperatures and holding times would effectively eliminate elemental segregation, dissolve unwanted solidification products, such as Laves phase, and remove solidification cells while limiting extensive grain growth and avoiding incipient melting. In this work, the heat treatment of alloy 718 is investigated for the γ”-strengthened condition with a particular emphasis placed on the homogenization/solutionization step prior to aging. It is shown that although measured tensile properties are within specifications when as-printed material is subjected to current heat treatment protocols, networks of unwanted delta phase form around remnant solidification structures that degrade resistance to hydrogen embrittlement. Increasing the solutionization temperature removes solidification artifacts, recrystallizes grains, and leads to acceptable aged microstructures and tensile properties. However, incipient melting, leading to the formation of eutectic Laves phase, and rampant grain growth occur as temperatures approach 1200 °C. Comparisons are made to welded and cast microstructures where appropriate, and variations in heat treatment response owing to orders-of-magnitude differences in microstructural scales are discussed. The data and conclusions of this work highlight the need for standardization efforts specifically tailored for additively manufactured materials. |