CORROSION and corrosion-fatigue of AZ31 MAGNESIUM weldments

Variable polarity, gas-tungsten arc welds were made on roll-cast AZ31 magnesium sheet using AZ61 filler metal. Weld and base metal coupons were evaluated in air and buffered saline solution to produce S-N fatigue curves. When tested in air, both welds and base metal demonstrated a fatigue limit, with the weld failing in the heat-affected zone (HAZ) at a reduced fatigue life. In saline solution at low cycle-high stress, there was a notable reduction in fatigue life attributed to crack initiation at corrosion pits. At high cycle-low stress, failure of welds shifted to the base metal where there was severe corrosive attack and reduced load bearing cross-section. Controlled immersion tests confirmed this shift in attack from the HAZ and weld metal to the base metal at long times. Although polarization curve measurements showed the weld metal to be the most reactive (i.e. more negative corrosion potential), the gradual formation of a stable passive film eventually protects the weld metal and HAZ from further attack. By alloying more Al in the anodic weld metal (e.g. increasing AZ61 filler dilution), its corrosion potential is shifted closer to that of the base metal and corrosive attack is minimized. Higher Al content also results in higher amounts of eutectic beta, and smaller grain size and higher hardness in the weld metal, matching that of the base metal.