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The microstructures of alloys created via Additive Manufacturing (AM) can vary substantially from those present in cast or wrought products, due to the very rapid solidification associated with AM.While numerous studies have investigated the process-microstructure relationship of alloys created by Selective Laser Melting (SLM), few have investigated the effects of laser rescanning to alter the microstructure or take advantage of the rapid solidification conditions the process provides. This study investigates the effect of single-or multiple pass laser scanning upon the grain structure of Addal-loy™, a new Al-Mg-Zr alloy strengthened via L1 2 Al 3 Zr precipitates. The bottom of the melt pools consisted of fine equiaxed grains (1.3 µm) that nucleated from primary Al 3 Zr (100-400 nm) precipitates. The top of the melt pool consists of columnar grains (up to 40 µm long), consistent with lack of Al 3 Zr nucleants due to Zr solute trapping from increased solidification velocities. Additional laser scanning (a second or third scan) reduces the amounts of columnar grains and increased the number equiaxed grains. The change is attributed to a shallower melt pool remelting the columnar grain region upon rescanning, due to reduced laser energy absorption and increased heat losses in the solid.

The relative electrochemical properties of second phases compared to the surrounding matrix gives rise to localization of corrosion on magnesium (Mg) alloys. Localized corrosion and its subsequent propagation in Mg alloys is largely driven by so-called ‘microgalvanic coupling’ of microstructural constituents within the alloy microstructure. In the present work, atomic force microscopy (AFM) imaging coupled with scanning Kelvin probe force microscopy (SKPFM) were used to generate surface Volta potential maps of a range of Mg alloys. In this manner, the relative Volta potential difference(s) between the respective alloy matrix phase and the microconstituent phase(s) of each sample were determined. Correlations between relative Volta potentials and phase composition were then inferred based on comparison of AFM optical and topographical images with corresponding scanning electron microscopy (SEM) images and energy dispersive x-ray spectroscopy (EDS) maps of the same or similar features. Sample preparation technique, testing conditions, and proper calibration of the SKPFM were all seen to influence the Volta potentials acquired. Because the relative Volta potential difference is known to serve as an index for local corrosion—particularly under thin electrolyte layers and in chloride solutions—a review of published SKPFM data was conducted to provide a critical assessment of the surface Volta potential differences between different microconstituent phases in a variety of Mg alloys to aid in understanding and in the future improvement of the atmospheric corrosion of Mg alloys

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