Microstructural study of the interface in laser-clad Ni-Al bronze on Al alloy AA333 and its relation to cracking

Abstract

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-AI 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 μm. A compositional transition from the Cu-rich clad (83 wt pct Cu) to the Al-rich substrate (3.2 wt pct Cu) occurs across this interface. Three phases in the interface are identified in TEM: Al solid solution, θ phase, and γ₁ phase, as described in the Cu-Al binary phase diagram. In a good clad track, the θ and γ₁ 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 μm to the whole clad region. Large areas of θ and γ₁ phases are observed. The mechanism of cracking at the interface is related to the formation of a twophase region of θ and γ₁ 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 η₁η₂, ζ₁, ζ₂, ε₁, ε₂, γ₀, β₀, and β, 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, θ phase, and γ₁, phase) showed significant extension of solubility.