MICROSTRUCTURAL STUDY OF THE INTERFACE IN LASER-CLAD NI-AL BRONZE ON AL-ALLOY AA333 AND ITS RELATION TO CRACKING

The interface toughness between a laser clad and the substrate determines whether the cladding is useful for engineering application. The objective of this investigation is to correlate the interface properties of laser-clad Ni-Al bronze on Al alloy AA333 with the microstructure and crystal structure of the interface. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX) are used to examine the interface. In a good clad track, the interface is an irregular curved zone with a varying width (occasionally keyholing structure) from 30 to 150 mu m. A compositional transition from the Cu-rich clad (83 wt pet Cu) to the Al-rich substrate (3.2 wt pet Cu) occurs across this interface. Three phases in the interface are identified in TEM: Al solid solution, theta phase, and gamma(1) phase, as described in the Cu-Al binary phase diagram. In a good clad track, the theta and gamma(1) phases are distributed in the Al solid solution. In a clad track with cracks, the interface structure spreads to a much larger scale from 300 mu m to the whole clad region. Large areas of theta and gamma(1) phases are observed, The mechanism of cracking at the interface is related to the formation of a two-phase region of theta and gamma(1) phases. To understand the microstructure, a nonequilibrium quasibinary Cu-Al phase diagram is proposed and compared with the equilibrium binary Cu-Al phase diagram. It is found that the occurrence of many phases such as eta(1), eta(2), zeta(1), zeta(2), epsilon(1), epsilon(2), gamma(0), beta(0), and beta, as described in the equilibrium binary Cu-Al phase diagram, is suppressed by either the cladding process or by the alloying elements. The three identified phases (Al solid solution, theta phase, and gamma(1) phase) showed significant extension of solubility.