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A multilayer coating of Ni/Cu/Al was fabricated on magnesium substrates using laser cladding. The solidification behavior and the phase evolution of the compositionally graded coating were studied. The results of the X-ray diffraction (XRD) analysis together with the metallographic study showed that a series of phase evolutions had occurred along the gradient (Mg) fi (Mg) + Al 12 Mg 17 fi (Mg) + Q + k 2 fi k 1 fi k 1 + c 1 fi c 1 + (Cu) + k 1 fi (Cu) + k 1 fi (Cu) fi (CuNi) fi (Ni). The rapid solidification condition had suppressed the invariant reactions that existed in the ternary Mg-Al-Cu alloy system. As a result, many of the predicted Al-rich brittle intermetallic compounds, which are detrimental to the performance of the coating, were not produced. The solidification path during the laser cladding of the Al and Cu layers was determined and the various phases, as predicted by the corresponding phase diagram, agreed well with the experimental results. Finally, the primary arm spacing (PAS) and the solidification morphology of the dendrites in the Cu and Ni layers were analyzed in relation to the solidification conditions.

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 ( þ ) eutectic, a narrow band of cellular structure, and finally a relatively thick layer of -Al and ( þ ) 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.

bearing coatings were synthesized onto nickel substrates by laser induced thermite reactions. The oxide coatings were produced via thermite reactions between NiO and Al powders with and without the addition of Y 2 O 3 -stabilised ZrO 2 (YSZ), respectively. In the case where no YSZ was added, a graded coating with a top layer of Al 2 O 3 was produced. Whereas, when YSZ was added, the coating essentially comprised ZrO 2 dendrites with the interdendritic regions filled with the ZrO 2 -Al 2 O 3 eutectic phase. The formation process of the coatings is described.

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