Improvement of Cavitation Erosion Resistance and Corrosion Resistance of Brass by Laser Surface Modification
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Improvement of Cavitation Erosion Resistance and Corrosion Resistance of Brass by Laser Surface Modification K. F. Tam 1 , 1. T. Cheng', H. C. Man 2 IDepartment of Applied Physics, 2 Department of Manufacturing Engineering, The Hong Kong Polytechnic University, Hong Kong, People's Republic of China ABSTRACT Laser surface modification of brass (Cu-38Zn-1.5Pb) using AISiFe and NiCrSiB alloy was achieved by using a 2kW continuous wave Nd-YAG laser with the aim of improving the cavitation erosion resistance and corrosion resistance. The alloying powder was preplaced on the brass substrate by thermal spraying to a thickness of 350ltm, followed by laser beam scanning to effect melting, mixing and alloying. A modified surface was achieved by overlapping of adjacent tracks. The cavitation erosion resistance and the anodic polarization characteristics of the laser surface modified specimens in 3.5% NaCI solution at 23°C were studied by means of a 20kHz ultrasonic vibrator at a peak to peak amplitude of 60jim and a potentiostat respectively. The cavitation erosion resistance of the specimens modified with AlSiFe and NiCrSiB was improved by a factor of 3 and 7 respectively, compared with that of the brass substrate. Potentiodynamic test, however, indicated that the corrosion resistance of specimens modified with AlSiFe deteriorated, as reflected by a shift of the polarization curve towards higher current densities. On the other hand, the corrosion resistance of specimens modified with NiCrSiB was significantly improved, as evidenced by the presence of a passive region (from -175 mV to -112 mV) and a reduction in the anodic current density by at least an order of magnitude compared with the substrate at the same anodic potential. The hardness profile and the compositional profile were measured using a Vickers hardness tester and EDX respectively. The microstructure and the surface morphology of the specimens were investigated with the aid of SEM and optical microscopy. INTRODUCTION Damage due to cavitation erosion is a common problem in engineering parts exposed to a fast flowing or vibrating liquid, where cavities or bubbles are generated and then collapse due to pressure fluctuations. The shock wave or micro-jet generated during bubble collapse repeatedly exert stress pulses on the parts near by, causing fatigue failure and erosion Brass, a copper-zinc alloy with a zinc content varying from 10% to 40%, is a major copper alloy in engineering applications. The addition of zinc improves the mechanical strength and reduces the cost of the alloy, but at the same time lowers its corrosion resistance[2'. In fact, owing to the large difference in electrochemical activity of zinc and copper, brass with zinc content > 15% is susceptible to dezincification. Brass is also less resistant to cavitation erosion than the more expensive copper alloys such as cupro-nickel. The cavitation erosion resistance and corrosion resistance of brass may be improved by laser surface modification, a process which retains the bulk properties of brass, uses
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