In situ laser synthesis of Co/Cu composite coating on copper substrate and its microstructural evolution

Abstract

The Co-rich particulate composite coatings have been in situ synthesized by injecting the Co-based alloy powders into the molten pool of the pure copper substrate using a CW CO 2 laser at a relatively low powder flow rate of 2.5 mg/mm. Microstructure in the alloyed zone consists of fine irregular polygons and Co-rich elliptic particles embedded at or near the grain boundaries. At 2.5 mg/mm, there is a single alloyed zone; but on increasing the relative power flow rate up to 5 mg/mm, two alloyed zones of the Cu-rich zone and the packed Co-rich zone are observed in the laser pool, suggesting that the powder flow rate has an important effect on the in situ synthesis of the particulate composite coatings. Since the melt could enter the metastable miscibility gap, the liquid phase separation (LPS) occurs prior to the crystallization due to the high cooling rate. The Cu-rich elliptic particles are embedded in the Co-rich matrix, while the Co-rich elliptic particles are embedded at or near the Cu grain boundaries. On the other hand, a bright halo is formed around the Cu-rich particles due to the remelting of a layer of the Co-rich matrix corresponding to LPS and the solidification sequence of the matrix and the embedded particles. In addition, the contents of the alloying elements in the Cu-rich or Co-rich elliptic particles strongly depend on their diameter ( d < 20 μm).