Microstructure and character of laser remelting of plasma sprayed coating (Ni-Cr-B-Si) on Al-Si alloy

In this paper, a plasma sprayed coating (Ni-Cr-B-Si) on an Al-Si alloy surface was remelted by a 5 kFV CO2 laser. Structure and chemical composition of different parts of the laser melted zone were investigated. Hardness distribution in the laser melted zone was measured. Experimental results showed that chemical composition of the sample is not a well-distributed gradient because there is compositional segregation in the laser melted zone. In the surface of the laser-melted zone, a concentration of aluminium exists; and there is a concentration of Fe, Cr and Ni in the sub-surface of this zone. Corresponding to chemical compositional segregation, a aluminium-rich phase Al3Ni emerges in the surface and the nickel-rich phases AlNi and AlNi3 are found in the sub-surface. There are some amorphous regions with super-hardness in the sub-surface of the laser-melted zone. Owing to the tempering effect of laser scanning heat transfer, many nano-crystallites of AlNi3 separate out from the amorphous structure. The hardness distribution in the laser-melted zone corresponds with the structure and solidification conditions. The highest hardness is in the amorphous and nano-crystallite regions. The higher hardness is in the Al+Al3Ni eutectic structure region that is in the rapidly solidified surface and is in the fine needle-like Al3Ni2 structure region that is adjacent to the amorphous region. The thick needle-like Al3Ni2 structure has lower hardness.