Laser cladding with CWCO2 was performed onto Al alloy substrate with feeding Cu alloy powder, and mechanism of clad formation and dilution was analyzed. High speed photographs indicated that the laser beam irradiated at location a little ahead of the center of the molten pool of the clad layer. It is shown that downward molten flow developed by the surface tension gradient caused by steep temperature gradients between the laser irradiated portion and the front edge of the molten pool plays an important role in clad layer formation. At optimum powder feeding rate, intermediate layer is produced by the interfacial diffusion between clad-substrate melt boundary. When the powder feed rates is too small, the downward surface tension flow becomes so large that the molten interface between the substrate and clad layer is broken to produce heavy dilution. At excess powder feed rate, the surface tension flow becomes too small to transfer the molten powder up to the height of the clad layer so that lumps of molten metal are left to produce uneven clad layer.

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