Improvement in cavitation erosion resistance of a copper-based propeller alloy by laser surface melting

Laser surface melting (LSM) of manganese-nickel-aluminium bronze (MAB), a common marine propeller alloy, was performed with the aim of improving the cavitation erosion resistance. Melting was achieved using a 2-kW continuous wave Nd:YAG laser with different scanning speeds and beam diameters, yielding different values of laser fluence. LSM resulted in a melt layer with a thickness of a few hundred micrometer thick, with the microhardness value at the surface increased to more than twice that of as-received MAB. The microstructure of the melt layer is highly refined and homogenized and has a single-phase b.c.c. structure (β phase), in contrast to the complex and heterogeneous microstructure of as-received MAB. With optimum laser parameters (power=1 kW; scanning velocity=35 mm/s; spot diameter=2 mm), the cavitation erosion resistance in 3.5 wt.% NaCl solution was improved by 5.8 and 2.2 times compared with that of as-received MAB and nickel-aluminium bronze (NAB), respectively. The improvement in cavitation erosion resistance is attributable to increased hardness and also to a much more homogeneous microstructure. Detailed analysis of the evolution of the morphology of the cavitated surface by SEM revealed totally different damage mechanisms for untreated and laser surface-melted MAB. For untreated MAB, the cavitation attack started at the κIphase, followed by an attack at the α/β phase boundary during the initial stage and eventually developed into ductile tearing of the matrix. However, the laser surface-melted samples only exhibited slight grain boundary attack at the initial stage, being initiating from triple junctions. In addition, the damaged surface of the laser-treated samples showed fracture of a more brittle nature.