Microstructure and static immersion corrosion behavior of AA7020-O Al plates joined by friction stir welding

Abstract

In the present investigation, the influence of tool pin profile and postweld heat treatment on microstructure, hardness, and static immersion corrosion behavior of AA7020-O Al plates joined by friction stir welding was investigated. Friction stir welding was conducted using two tools having different pin profiles, typically, a tool with tapered cylindrical pin and a tool with two flat-sided cylindrical pin. Postweld heat treatment was carried out using a solution heat treatment temperature of 540 ± 1℃ for 12 h followed by aging at 155 ± 1℃ for 6 h. Corrosion tests were carried out by immersing the welds in an aqueous solution containing NaCl and H2O2 for 6 h according to ASTM-G110. The results revealed that the two flat-sided cylindrical pin tool produces finer α-Al grains, at the center of dynamically recrystallized zones, than the tapered cylindrical pin tool. The postweld heat treatment slightly increases the size of the α-Al grains at the center of dynamically recrystallized zones. In the as-welded conditions, the friction stir welded regions exhibited lower hardness values than the base alloy. However, the regions friction stir welded using two flat-sided cylindrical pin tool showed relatively higher hardness values than those regions friction stir welded using the tapered cylindrical pin tool. The postweld heat treatment slightly increases the hardness of the welded regions to values that are still lower than the base alloy. Generally, the dynamically recrystallized zones are more susceptible to corrosion than the base alloy. In both AW and postweld heat treatment conditions, the thermomechanically affected zones showed the lowest corrosion resistance when compared with dynamically recrystallized zones and heat affected zone regions. In the as-welded conditions, regions friction stir welded using tapered cylindrical pin tool exhibited better corrosion resistance than those friction stir welded using two flat-sided cylindrical pin. The postweld heat treatment improves the corrosion resistance of the dynamically recrystallized zones friction stir welded using two flat-sided cylindrical pin tool.