Laser fabrication of surface metal matrix composites on aluminum alloy

Abstract

The creation of a wear resistant surface of metal matrix composite on AA6061 aluminum alloy by laser surface cladding of pre-pasted metal/ceramic powder was successfully achieved using a 2 kW CW Nd-YAG laser. Different ratios of metal and ceramic powder were investigated. Powder mixtures of different compositions of Mo/WC, Mo/TiC, Mo/Si3N4 and Si3N4/TiC were pre-placed on the aluminum alloy AA6061 and then irradiated by the defocused laser beam. For the Mo/WC laser cladding system, the processing parameters were 17 mm/s beam scanning speed, 3 mm beam diameter and 1.4 kW laser power at the workpiece and 0.3 mm pre-placed layer thickness. For the Mo/TiC laser cladding system, the optimum processing parameters were 3 mm laser beam size, 17 mm/s beam scanning speed, 1.3 kW laser power at the workpiece and 0.25 mm pre-placed layer thickness. Both of 2 systems used 50% overlapping on every single track to produce a surface. Mo/Si3N4 and Si3N4/TiC systems have a preliminary studied and proved that they are difficult to produce a good clad layer. The chemical composition, microstructure and surface morphology of the clad layer were analyzed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD). High quality metal matrix composite (MMC) coatings of Mo/WC and Mo/TiC on the Al alloy were obtained. The clad layers were free of porosity and crack. Excellent bonding between the coating and the aluminum alloy substrate was ensured by the strong metallurgical interface. The values of surface hardness of the clad specimens were increased significantly. The samples have been characterized by abrasive pin-on-disc wear test. The results show that all clad specimens have extremely good abrasive wear resistance. Mostly, fine dendritic structures were observed in the MMC clad layer. Depending on the process parameters, unmelted or partially melted WC particles were also observed. These unmelted particles were found to contribute to the improvement of the wear resistance. The distributions of the dendrites and unmelted or partially melted ceramic phases were found to depend on the process parameters. For the Mo/WC and Mo/TiC MMC cladding systems, the matrix of the MMC layer consisted of alloys of Mo-W-Al and Mo-Ti-Al respectively, which are harder than the Al alloy. Clad layers using Mo/Si3N4 and TiC/Si3N4 systems were mostly porous and had cracks. Interfacial bonding between the clad surface and the substrate was poor. It was unsuccessful to fabricate a surface large enough for wear test.