Solidification behaviour and the evolution of microstructure in the laser cladding of aluminium on magnesium substrate

Laser cladding of aluminium powders was performed on pure magnesium substrates using the blown powder method. The microstructure across the laser-clad track was studied. Starting from the bottom of the laser-clad to the top surface of the track, a series of microstructures was observed: a planar growth interface of the substrate, a narrow band of (β + γ) eutectic, a narrow band of β cellular structure, and finally a relatively thick layer of α-Al and (α + β) eutectic structure. The evolution of the various phases and microstructures is discussed in terms of the distribution of solute in conjunction with the Al-Mg phase diagram, temperature gradient and solidification rate, constitutional supercooling theory, and the condition for columnar-to-equiaxed crystal transition.