CMSE2015

Laser Surface Modification of Engineering Alloys for Combating Cavitation Erosion-Corrosion
Prof. Chi Tat Kwok
Department of Electromechanical Engineering Institute of Applied Physics and Materials Engineering University of Macau
Cavitation erosion-corrosion (CEC) is a nausea for engineering components such as ship propellers, pump impellers, valves, ultrasonic cleaners, etc, which are exposed to the high-speed flowing or vibratory corrosive fluids. The generation of cavities and followed by their collapse can damage the surface of materials by shock wave and high-speed liquid microjet impingement. Shock waves are formed by the rapid collapse of the cavities and repetitively generates high pressures leading to a fatigue type loading on the surface. Such pressures are strong enough to cause plastic deformation in most engineering alloys, enhance mass transport and electrochemical reactions and thereby accelerate corrosion rates of their surfaces. With synergistic effect of a corrosive medium, the overall damage induced by CEC involves the interaction of hydrodynamic, mechanical, metallurgical, and chemical factors. The presence of oxygen and aggressive ions in the electrolyte is important in controlling CEC rates. The resistance of engineering alloys to CEC depends on hardness, yield strength, fatigue strength, strain hardening rate, passivity, microstructure and imperfections of the materials. CEC is a surface phenomenon, laser surface modification has been applied to avoid and reduce erosion damage. With the merits of refined microstructure, strong metallurgical bond, minimum thermal distortion, high processing speed, precision and versatility, the problem of CEC have been tackled by enhancing the surface properties of the base materials (ferrous and non-ferrous alloys) with various laser surface modification techniques including laser transformation hardening (LTH), laser surface melting (LSM), laser surface alloying (LSA), laser cladding (LC), laser shock peening (LSP) and laser plasma hybrid spraying (LPHS). A review of published data on correlation between CEC resistances of the laser-surface modified alloys, defined as reciprocal of mean depth of erosion rate with the properties of materials is presented.