Microstructure and Phase Evolution in Laser Cladding of Ni/Cu/Al Multilayer on Magnesium Substrates
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years, much interest has been generated in applying magnesium alloys to stress-bearing applications, especially in the aerospace and automotive industries,[1,2] because it is well known that magnesium alloys have low densities and are characterized by high strength-to-weight ratios. However, magnesium is high in the electrochemical series, and this makes the metal highly susceptible to galvanic corrosion when contact is made with other metals. Moreover, metallic impurities in magnesium alloys can cause severe pitting corrosion when exposed to moist conditions, especially in the presence of chloride ions. It is generally accepted that it is necessary to apply a surface protective coating to magnesium components or products if they are to survive in a harsh working environment. Indeed, over the years, intensive research efforts have been devoted to developing better coatings in order to meet the challenges demanded by industry. Gray and Luan[3] have written an excellent review article on this subject. Notwithstanding the advancements that have been made in alloy development, heat treatment, and surface T.M. YUE, Professor, is with the Advanced Manufacturing Technology Research Centre, Department of Industrial and Systems Engineering, the Hong Kong Polytechnic University, Hung Hom, Hong Kong. Contact e-mail: [email protected] T. LI, formerly Research Associate, Department of Industrial and Systems Engineering, the Hong Kong Polytechnic University, is Engineering Manager with Kayex-An SPX Brand, Shanghai 200052, People’s Republic of China. X. LIN, formerly Research Fellow, Department of Industrial and Systems Engineering, the Hong Kong Polytechnic University, is Professor, State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China. Manuscript submitted February 4, 2009. Article published online October 30, 2009 212—VOLUME 41A, JANUARY 2010
engineering, it is still difficult to find a single coating technology capable of providing adequate protection to magnesium alloys from corrosion and abrasion in harsh service environments. Yet the need for lightweight and enduring alloys is becoming more urgent with our growing environmental awareness. The market for a successful material is likely to be very large. High-power laser cladding, a surface engineering technique, has shown great potential for improving the surface properties of magnesium alloys. Its main advantage over other processes is its ability to form relatively thick protective coatings on selected areas in which improved properties are desired; in addition, it is a ‘‘clean’’ process. Subramanian[4] and Wang[5] have reported that the laser cladding of Mg-Zr or Mg-Al alloys on magnesium increases the corrosion resistance of magnesium. However, their work did not particularly study the evolution of the microstructure. Galun[6] and Majumdar[7] have investigated the feasibility of the laser surface alloying of magnesium-based alloys with various elements using multikilowatt lasers. Depending on
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