Mechanical properties and corrosion resistance of Al-Mg alloys with Cu added by electron beam welding

The changes in the mechanical properties and corrosion resistance of Al-Mg alloys with Cu added by an electron beam welding process were investigated using the measurements of tensile tests, anodic polarization curves and imm ersed corrosion tests of the specimens with or without modification in a di lute HClO4 solution at 303 K. The X-ray diffraction or EDAX analysis of the precipitates, an analysis of the scanning electron micrographs of the spec imens and the surface of the corroded specimens were also performed. The main results obtained are summarized as follows: (1) The grain size of the Al(alpha) phase of the modified Al-Mg alloys decr eased as compared with the untreated specimens, having a size below ca. 10 mu m in diameter. The Al(alpha) + Al2Cu(theta) eutectic structure precipita tes around the grain boundary of the Al(alpha) phase, and the surface area ratios of the eutectic structure increase with increasing Cu content in the modified Al-Mg alloys. However, in the modified 5154 alloy, in addition to the Al2Cu compound, the Al2CuMg compound was also detected. (2) The mechan ical properties such as 0.2% proof stress, U.T.S. and Vickers hardness of t he Al-Mg alloys increased due to the modification. On the contrary, the elo ngation of all the modified specimens is reduced below ca. 5%. (3) The corr osion resistance of the modified specimens significantly decreased with inc reasing Cu content in the alloys. The deterioration of the corrosion resist ance is caused by an electrochemical interaction of galvanic coupling produ ced between the grain of the Al(alpha) phase and the eutectic structure.