Laser surface alloying of a marine propeller bronze using aluminium powder Part I: Microstructural analysis and cavitation erosion study

In a previous study laser surface melting (LSM) was employed to improve the cavitation erosion resistance of manganese-nickel-alummium-bronze (MAB)[9,10][C.H. Tang, F.T. Cheng, H.C. Man, Surf. Coat. Technol. 182 (2004) 300; C.H. Tang, F.T. Cheng, H.C. Man, Mater. Sci. Eng. A 373 (2004) 195]. To further enhance the improvement, laser surface alloying (LSA) using fine aluminum powder has been attempted in the present study. By employing appropriate laser processing parameters, a homogeneous alloyed layer of thickness about 1 mm, free of cracks or pores, was obtained. The alloyed layer was composed of a single phase, the bee p-phase, with a Knoop microhardness higher than 300 HK. Cavitation erosion test in deionized water of the alloyed layer recorded a 30-fold improvement in the cavitation erosion resistance compared with as-received MAB. The resistance achieved in LSA was more than 3 times that by LSM. The relatively low-cavitation erosion resistance of as-received MAB was attributable to its heterogeneous and multi-phased structure. Surface-alloyed MAB, on the other hand, was characterized by a homogeneous microstructure which was single-phased. Apart from microstructural homogenization, the enhancement in cavitation erosion was also related to the increase in microhardness. Morphological evolution monitored over a period of cavitation erosion test revealed that brittle fracture mode prevailed, with material being chipped away from weak triple-junctions and grain boundaries. Such a mode of erosion damage was similar to the case in laser surface-melted MAB, but at a much milder degree, consistent with a higher erosion resistance in the case of LSA. The higher Al content in the LSA samples which resulted in a harder phase could be the major reason for the higher resistance. In addition, the relatively larger grains in the Al-alloyed samples resulted in less grain boundaries, which were vulnerable sites for erosion initiation, hence also contributing to higher cavitation erosion resistance compared with the lasermelted samples. (c) 2005 Elsevier B.V. All rights reserved.