Strength and corrosion resistance of Al-alloying layer on AZ31B magnesium alloy fabricated in situ by reactive friction stir processing

Highlights: • Al-alloying layer on AZ31B Mg alloy was successfully fabricated in-situ by RFSP using Al powder. • RFSP resulted in the formation of Mg₁₇Al₁₂ and the increase in Al content in α-Mg matrix. • Strength and corrosion resistance of AZ31B were improved through the formation of Al-alloying layer. • Subsequent aging treatment further improved corrosion resistance due to aging-induced Mg₁₇Al₁₂. In this study, reactive friction stir processing (RFSP) was applied for AZ31B Mg alloy to improve their inferior strength and corrosion resistance. A high-Al alloying layer was successfully fabricated on rolled AZ31B plate surface through multipass RFSP reacting the additive pure Al powder (3 μm diameter) along its groove line. During RFSP, the Al powder was completely dissolved into the α-Mg matrix and reacted in situ with the α-Mg matrix to produce Mg₁₇Al₁₂ precipitates. With the strengthening effects by the solid solution of supersaturated Al and in situ Mg₁₇Al₁₂ precipitates, RFSP improved the hardness, yield strength, and ultimate tensile strength of the as-received AZ31B by 58%, 34%, and 15%, respectively. Furthermore, aging treatment was conducted after RFSP, inducing a lamellar-shaped Mg₁₇Al₁₂ in the RFSPed layer. The additional strengthening effect by the aging-induced Mg₁₇Al₁₂ resulted in further improvement in the hardness (15%, 97 Hv) and yield strength (13%, 102 MPa) compared to the RFSPed layer. However, the ultimate tensile strength deteriorated due to the too high fraction of aging-induced Mg₁₇Al₁₂ precipitates that caused a premature fracture. The results of the hydrogen evolution measurements and electrochemical impedance spectroscopy showed that RFSP and subsequent aging treatment enhanced the corrosion resistance due to the corrosion barrier effect of the in situ and aging-induced Mg₁₇Al₁₂.