Laser cladding of quasi-crystal-forming Al-Cu-Fe-Bi on an Al-Si alloy substrate

Abstract

We report here the results of an investigation aimed at producing coatings containing phases closely related to the quasi-crystalline phase with dispersions of soft Bi particles using an Al-Cu-Fe-Bi elemental powder mixture on Al-10.5 at. pct Si substrates. A two-step process of cladding followed by remelting is used to fine-tune the alloying, phase distribution, and microstructure. A powder mix Al₆₄CU_22.3Fe_11.7Bi₂ of has been used to form the clads. The basic reason for choosing Bi lies in the fact that it is immiscible with each of the constituent elements. Therefore, it is expected that Bi will solidify in the form of dispersoids during the rapid solidification. A detailed microstructural analysis has been carried out by using the backscattered imaging mode in a scanning electron microscope (SEM) and transmission electron microscope (TEM). The microstructural features are described in terms of layers of different phases. Contrary to our expectation, the quasi-crystalline phase could not form on the Al-Sisubstrate. The bottom of the clad and remelted layers shows there growth of aluminum. The formation of phases such as blocky hexagonal Al-Fe-Si and a ternary eutectic (Al + CuAl₂ + Si) have been found in this layer. The middle layer shows the formation of long plate-shaped Al₁₃Fe₄ along with hexagonal Al-Fe-Si phase growing at the periphery of the former. The formation of metastable Al-Al₆Fe eutectic has also been found in this layer. The top layer, in the case of the as-clad track, shows the presence of plate-shaped Al₁₃Fe₄along with a 1/1 cubicrational approximant of a quasi-crystal. The top layer of the remelted track shows the presence of a significant amount of a 1/1 cubicrational approximant. In addition, the as-clad and remelted microstructures show a fine-scale dispersion of Bi particles of different sizes formed during monotectic solidification. The remelting is found to have a strong effect on the size and distribution of Bi particles. The dry-sliding wear properties of the samples show the improvement of wear properties for Bi-containing clads. The best tribological properties are observed in the as-clad state, and remelting deteriorates the wear properties. The low coefficient offriction of the as-clad and remelted track is due to the presence of approximant phases. There is evidence of severe subsurface deformation during the wear process leading to cracking of hard phases and a change in the size and shape of soft Bi particles. Using these observations,we have rationalized possible wear mechanisms in the Bi-containing surface-alloyed layers.