Effect of solution annealing on microstructures and corrosion behavior of wire and arc additive manufactured AZ91 magnesium alloy in sodium chloride solution

Recently, wire and arc additive manufacturing (WAAM) technique has been used to fabricate the Mg alloy parts, and the solution annealing was generally employed to eliminate the residual stress and homogenize the microstructure of WAAM Mg alloy. Due to the microstructure differences between the WAAM and cast Mg alloys, the effect of solution annealing on the microstructures and corrosion behavior of cast and WAAM Mg alloy was theoretically different. Unfortunately, there is no research on the effect of solution annealing on the corrosion behavior of WAAM Mg alloys. In this study, the effect of solution annealing (415 °C for 8 h) on microstructures and corrosion behavior of wire and arc additive manufactured (WAAM) AZ91 magnesium (Mg) alloy in 0.1 M NaCl was investigated by microstructure analysis and electrochemical measurements. It was found that solution annealing coarsened the grain, eliminated non-basal grain orientation and dissolved the Mg17Al12 phase of the as-received WAAM AZ91 Mg alloy. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) results revealed that solution annealing improved the corrosion resistance of WAAM AZ91 Mg alloy evidenced by the decreased corrosion current density, increased the pitting potential and impedance. EN results indicated that the WAAM AZ91 Mg alloy in two states underwent totally different corrosion mechanisms in 0.1 M NaCl, and the heat treated WAAM AZ91 Mg alloy had lower pit initiation rate and lower pit growth ability. The relationship between the microstructures and the improved corrosion resistance after solution annealing is discussed.