Chapter Fifteen - Prospects of additively manufactured nickel aluminum bronzes for marine applications

Abstract

As a step forward to develop the next generation of marine parts, the wire-arc additive manufacturing (WAAM) technique was employed to deposit nickel aluminum bronze (NAB) on a dissimilar 316L stainless steel. The typical microstructural features of cast-NAB are primarily Cu-rich α-matrix and various κ-intermetallic phases designated as κI, κII, κIII, and κIV. The size, distribution, morphology, and volume fraction of κ-precipitates play a vital role in influencing NAB’s mechanical properties and corrosion behavior. This work characterized microstructures of the cast-NAB, WAAM-NAB (i.e., as-built), heat-treated NAB (under three different cycles), and the interface between WAAM-NAB and 316L substrate using optical and multiscale electron microscopy techniques (SEM, EDS, EBSD, TEM, APT). The differences in the phase formation and the nanoindentation and tensile behavior of WAAM-NAB were compared with cast-NAB. In the WAAM-NAB, high cooling rates suppressed the precipitation of κI (rosette Fe3Al) and produced finer microstructures with significantly lower volume fractions of intermetallic particles. κII (globular Fe3Al) and κIII (lamellar NiAl) phases were formed in the interdendritic regions, whereas α-matrix dendrites consist of uniform precipitation of numerous fine (5–10nm) Fe-rich κIV particles. A metallurgically well-bonded interface without pores and cracks was formed between NAB and 316L, and the thickness of the interdiffusion region (i.e., intermetallic layer) is 2μm. However, occasional liquation cracks were formed on the grain boundaries of 316L in the heat-affected zone. Possible solutions to address this problem were suggested. The findings on the effect of various heat treatments (350°C for 2h, i.e., HT-1; 550°C for 4h, i.e., HT-2; and 675°C for 6h, i.e., HT-3) on the tensile properties were discussed in-depth from the context of microstructural and phase transformations.