The microstructure of the surface layer of magnesium laser alloyed with aluminum and silicon

• A CO2 laser was used for surface alloying of Mg with AlSi20.
• Before alloying, an AlSi20 plate was diffusion bonded with the Mg substrate.
• The process parameters affected the alloyed layer microstructure and properties.
• With melting limited to AlSi20, the layer had a structure of rapidly solidified AlSi20.
• Mg–Al and Mg–Si phases were present when both the substrate and the plate were melted.

The surface layer under analysis was formed as a result of diffusion bonding of a thin AlSi20 plate to a magnesium substrate followed by laser melting. Depending on the process parameters, the laser beam melted the AlSi20 plate only or the AlSi20 plate and a layer of the magnesium surface adjacent to it. Two types of microstructure of the remelted layer were thus analyzed. If the melting zone was limited to the AlSi20 plate, the microstructure of the surface layer was typical of a rapidly solidified hypereutectic Al–Si alloy. Since, however, the liquid AlSi20 reacted with the magnesium substrate, the following intermetallic phases formed: Al3Mg2, Mg17Al12 and Mg2Si. The microstructure of the modified surface layer of magnesium was examined using optical, scanning electron and transmission electron microscopy. The analysis of the surface properties of the laser modified magnesium revealed that the thin layer has a microstructure of a rapidly solidified Al–Si alloy offering good protection against corrosion. By contrast, the surface layer containing particles of intermetallic phases was more resistant to abrasion but had lower corrosion resistance than the silumin type layer.