Effect of Laser Power on Formation and Microstructure of Composite Coatings on Zirconium Alloy by Laser Cladding

ZrB₂+ZrC in-situ reinforced coating was fabricated by laser cladding Ni60A+WC powder on zirconium alloy.The effect of laser power on the formation, microstructure, reinforcements and microhardness of coating was investigated.The results indicate that good appearance of the coating is obtained and no defects such as pores and cracks are found.As the laser power increases, the width and height of the coating increase significantly. Reinforcements in the coating are mainly composed of fine α-Zr dendrites, granular and block ZrC and Zr (B, C)₂. ZrB₂+ZrC lamellar eutectic is formed inside the unmolten WC.The similar values of Gibbs free energy for forming both ZrB₂ and ZrC at elevated temperature make it easy to form lamellar eutectic. With laser power increasing, the growth of the secondary arm of dendrites is suppressed and the granular reinforcements gradually transforme to block reinforcements.The microhardness of the coating increases significantly as laser power increasing.The highest value of microhardness(about 720 HV0.5)is obtained when the laser power is 2.8 kW.