Influence of processing on the microstructure of nickel aluminum bronze (NAB)

Nickel-aluminum bronze (NAB) alloys are commonly used for marine applications such as propellers by the U.S. Navy. These NAB components are conventionally manufactured using casting techniques, but recent interest has shifted to the possibility of additive manufacturing (AM) processes. The following literature review discusses the microstructural evolution of (nominally) Cu-9Al-5Ni-5Fe-1Mn NAB alloys in the cast, wrought, friction stir processed, arc-welded, electron beam welded and laser-welded conditions. NAB alloys exhibit a complex microstructure, consisting of a mixture of five κ precipitates distributed in an α or β’ martensitic matrix. The size, morphology, and distribution of these phases are sensitive to changes in cooling rate and thermal history such as those imposed by AM and welding. Although the microstructure is well documented under casting conditions, no continuous cooling diagrams have been developed to describe phase transformations under arc and laser processing conditions for NAB. As such, this review investigates recent reports related to arc, electron beam and laser-based additive manufacturing processes and the associated microstructures and properties. Studies have reported additively manufactured NAB alloys with improved properties compared to cast material, associated with differences in microstructure. At moderate cooling rates, such as those associated with arc-based processing, the microstructure consists of Widmanstätten α and various κ phases, which become more refined with increasing cooling rate. This leads to an improvement in the tensile properties of wire-arc direct energy deposited NAB, with > 17% increase in yield strength and > 90% increase in ductility when compared to cast material. At rapid cooling rates representative of laser-based and electron beam processing, the microstructure consists primarily of a twin-related β’ martensite with nanoscale κ phases. This leads to a significant increase in yield strength (>60–100%) at the expense of ductility (<−30%) when compared to cast material. Additionally, previous studies have reported that the refined microstructure from these high cooling rate processes leads to an improvement in the corrosion resistance of as deposited NAB. There is also evidence that the conventional heat treatments for cast NAB alloys need to be optimized for additively manufactured materials. The report includes the methods and challenges associated with characterizing NAB alloys under these processing conditions and provides recommendations for future research.