Cellular/dendritic arrays and intermetallic phases affecting corrosion and mechanical resistances of an Al–Mg–Si alloy

Abstract

Outdoor applications of Al–Mg–Si alloys have been specified due to their very good corrosion resistance when compared with those of other aluminum alloys. Nevertheless, these alloys still have corrosion problems. One of the interests consists in characterizing the microstructure evolution, which is supposed to have important role on the final properties. In many aluminum alloys, the effects of intermetallics on both the corrosion and mechanical behavior is of industrial interest. Particularly concerning Al–Mg–Si alloys, hardly anything is known about the influence of the Mg2Si phase. This work aims to encompass such analyses on an Al-3.0 wt% Mg-1.0 wt% Si alloy directionally solidified under a wide range of cooling rates. Experimental results include primary dendritic and cellular spacings, nature and distribution of intermetallics associated with corrosion potential, pitting potential, current density, ultimate tensile strength and elongation. A high-cooling rate cellular region has been identified, followed by a dendritic region that occurred for cooling rates lower than 0.8 K/s. The cellular spacing varied from about 16 μm to 38 μm whereas the primary dendritic arm spacing varied from 120 μm to 270 μm. The α-Al cellular region is shown to be characterized by finely dispersed Mg2Si and Fe-bearing particles, which allowed better mechanical properties (strength and elongation) and better corrosion resistance to be attained. Both mechanical strength and corrosion resistance (for 0.15 M and 0.5 M NaCl electrolytes) is shown to be unaffected by the scale of λ1 within the dendritic region.