Characterization of Coextruded Rods with Al-4,5Mg as the Base Alloy and Rapidly Solidified Al-4,5Mg-1Ag as the Surface Alloy
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Coextruded rods with Al-4,5wt%Mg as the base alloy and rapidly solidified Al-4,5wt%Mg-1wt%Ag as the surface alloy has been characterized. Precipitation hardening, light microscopyand scanning electron microscopy (SEM) investigations, tensile testing, deformation analysisand corrosion testing have been performed.Aging at 160 °C gave a hardness increase in the surface material from 77 HV to 129 HV after236 hours, after an initial solution treatment for 30 minutes at 560 °C. The base material ex-perienced a slight hardness increase, with a maximum hardness of 91 HV. A fine distributionof Mg-Ag, and possibly Al-Mg-Ag phases, was observed in the surface material, and most ofthese phases were dissolved during the solution treatment. Surface blisters, caused by gas inthe rapidly solidified surface material, occurred on the rod surfaces during solution treatment at560 °C.Tensile testing showed a nearly linear increase in the ultimate tensile strength of the rods withincreasing thickness of the surface layer, which implies that the strength is higher in the surfacealloy than in the base alloy. Except for a slightly higher ductility in the base material, the twomaterials have the same deformation behaviour, and the separation of the two materials is duc-tile.Electron backscatter diffraction (EBSD) was conducted in SEM, revealing a finer grain struc-ture in the rapidly solidified surface material than in the base material. In addition, the surfacematerial proved to have a much better resistance to grain growth during exposure at 560 °C.Through corrosion testing in 5wt% NaCl solution, it has been found that the corrosion resistanceof the surface material decrease with heat treatment. Pitting corrosion was observed in all thetested conditions: (i) as-coextruded, (ii) solutionized and (iii) aged, while corrosion along grainboundaries was observed only in the aged condition.