Influence of Additive Friction Stir Deposition on Microstructure and Corrosion Properties of Magnesium Alloy AZ31B

In the present study, a non-beam based solid-state additive manufacturing (AM) process was investigated for processing AZ31B Mg alloy for use as bioactive coating for orthopedic alloys to impede biofilm formation. This solid-state AM process, additive friction deposition (AFSD) achieves layer-by-layer metallurgical deposition by pushing a solid rectangular feedstock rod through a hollow, rotating tool. As the feedstock is pushed through the rotating tool, frictional heat is generated from the shoulder that softens the feedstock and promotes severe plastic deformation resulting in dynamic recrystallization of the deposited material with refined and equiaxed microstructure. Microstructure, secondary phases and grain orientation were analyzed by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and electron backscatter diffraction, along with X-ray diffraction. While feedstock materials exhibited texture in only the extrusion direction, AFSD samples exhibited texture in all three directions. An increase in Al₈Mn₅ precipitate size was observed in AFSD build direction, as well as change in shape from needle to globular. Corrosion properties of AFSD samples performed comparably to feedstock samples in PBS solution, due to formation of robust Mg₃(PO₄)₂ film, although in PO₄³⁻ absent solution, AFSD samples corroded nearly three times faster than feedstock material. This was attributed to residual stress in AFSD samples affecting Mg(OH)₂ formation. As AFSD continues to attract attention as a low-cost coating technique, this work provides insight into the corrosion performance of AZ31B post AFSD processing in both passivating and non-passivating solutions.