Improvement of Wear Resistance of Magnesium by Laser-Alloying with Silicon

Abstract

The surface modification of magnesium by laser alloying using a powder injection method was carried out for the purpose of improving its wear resistance. Silicon powder was used as the feeding powder. The applied CO2 laser power conditions were 2 kW-1pass and 2 kW-2pass, while the moving speed of the substrate was 8.3 mms � 1 in all the cases. The silicon powder reacts easily with molten magnesium to form fine Mg2Si compound in the modified layer. Under both laser power conditions of 2 kW-1pass and 2 kW-2pass, the modified layer becomes thick as the powder feeding rate increases, and the modified layer obtained using 2 kW-2pass is thicker than that of 2 kW-1pass at all powder feeding rates. Furthermore, fine and homogeneously distributed Mg2Si compound crystallizes on the whole modified layer in the 2 kW-2pass. At the laser power condition of 2 kW-1pass, the area fraction of Mg2Si compound increases with an increase in the powder feeding rate, while at 2 kW- 2pass, the area fraction of Mg2Si compound is almost the same regardless of the powder feeding rate. Wear resistance of the modified layer improves with increasing area fraction of Mg2Si compound held in the soft matrix of magnesium solid solution, while there is almost no wear of the pin. Coefficient of friction rapidly decreases within a few minutes and then remains virtually constant during sliding wear test. Microstructure of the wear-tested surface reveals that the soft substrate of magnesium is worn by pin at the initial stage and pits which subsequently serve as oil pockets are developed, resulting in the remarkable improvement of wear resistance.