Study on the solidification microstructure in AZ91D Mg alloy after laser surface melting

Laser-induced microstructural development and phase evolution in magnesium alloy

“…Based on our previous work [11,12], solidification occurs spontaneously by epitaxial growth on the partially melted b-Mg 17 Al 12 grains, and conditions for the growth of cellular/dendrite structure as well as line-shape structure are optimum when one of the easy growth directions coincides with the heat-flow direction. This is due to heat-flow control rather than interface control, as shown in Fig.…”

“…In laser-melt zone of AZ91D Mg alloy, the content of Al element increases significantly due to rapid solidification and selective vaporization. On one hand, under laser irradiation where rapid solidification occurs, the solute is ejected by the a-Mg matrix at the liquid/solid interface when a-Mg phase propagates into the liquid from the original interface into the molten pool, resulting in a supersaturation of Al in the a-Mg matrix duo to very high cooling rates [11,23,24]. On the other hand, selective vaporization of Mg and Al elements takes place, causing Al content increasing in the melted zone due to the relatively stronger evaporation of Mg element [12,25,26].…”

“…Such refined cellular/dendrite and lineshape structure of b-Mg 17 Al 12 secondary phase plays a crucial role in improving degradation behavior of laser-melt AZ91D Mg alloy in modified-simulated body fluid, and increasing polar component of surface energy thus enhancing wetting and adhesion of laser-melt surface significantly. The details of b-Mg 17 Al 12 phase evolution and surface properties change of laser-melt AZ91D Mg alloy were analyzed in our previous work [11,12].…”

“…Typical cellular/dendrite structure was observed in the melt zone, and line-shape structure was also found. XRD analyses indicated that the solidification microstructure of laser melted AZ91D Mg alloy consisted of refined a-Mg matrix phase and b-Mg 17 Al 12 secondary phase [11,12].…”

“…As one of the most widely used Mg alloys AZ91D, previous studies show that wear and corrosion resistance of laser-melt AZ91D Mg alloy is improved significantly due to refined b-Mg 17 Al 12 secondary phase and enriched alloying element concentration in the laser melt zone [9][10][11][12]. However, direct observation of laser irradiation on microstructural evolution of Mg alloy, in terms of phase transformation during rapid solidification, has not been fully reported, although it is well known that faster cooling rates produce finer microstructures.…”