An Investigation Of Laser Alloying And Laser Cladding On Magnesium Alloys

Magnesium alloys are important light weight materials finding increased applications in aerospace and automotive industries because of their low densities,high specific strength,strong electromagnetic shielding and anti-radiation,easy cutting and recycling. However, the poor corrosion performance of magnesium alloys restricts their expanded applications. Therefore, it is great significant to enhance the corrosion-resistance property of Mg alloys by using the surface modification technology. In order to improve corrosion-resistance of the Mg alloys, the Blown-powder laser surface alloying and two steps of laser cladding were respectively performed on magnesium alloy AZ91D with Al-Si eutectic powder in this investigation. And laser processing characteristics and feasibility of high melting point materials on the surface of magnesium alloy were studied with Ni-Cr-B-Si powders。The study of Al-Si laser alloying showed that the width, height and melt depth of alloyed layers decrease with increasing laser translation speed at a given laser power. And the width and depth increase with the increasing laser power, but the height decreases, at a given translation speed. Intermetallic compounds (IMCs) of Mg2Si, Al12Mg17 and Al3Mg2 were formed in the matrix of mixedα-Mg and Al solid solutions in the Al-Si alloyed layers. The surface hardness of Al-Si alloyed layers with homogeneous distribution is higher than that of the base metal of AZ91D. The alloyed layer exhibited a lower corrosion rate and higher polarization resistance than that of AZ91D alloy. The polarization corrosion initiated at interface between IMCs and the alloyed matrix. As to the Al-Si laser cladding, laser parameters also influenced morphology and microstructures of clad layers. The metallurgical bonding was obtained using optimized laser parameters with less Mg dilution into clad layer. The clad layer is composed of dendrites ofα-Al solid solution and the matrix Al-Si eutectics (α-Al+β-Si). The surface hardness of Al-Si clad layers is higher than that of the base metal of AZ91D, but lower than Al-Si alloyed layer's hardness. The clad layer also exhibited a lower corrosion rate and higher polarization resistance than that of AZ91D alloy. The polarization corrosion preferentially occurred onα-Al dendrite as it was the anodic. The salt fog experiment indicated that the Al-Si alloyed layer has the lowest corrosion rate, which it was identical to the result of the anodic polarization test.The Ni-Cr-B-Si laser surface alloyed layer showed a non-uniform stratum structure with a distinctly different hardness distributing in the layer. The alloyed layer exhibited a higher polarization resistance than that of AZ91D alloy, but an increased corrosion current, as the result of the non-uniform stratum structure. As to the Ni-Cr-B-Si laser clad layer, the laser cladding was not successful in a wide range of laser parameters, which resulted in either insufficient melting or over melting of the thermally-sprayed layer due to the large melting point difference between AZ91D and Ni-Cr-B-Si alloyed layer.