The effect of cold sprayed coatings on the mechanical properties of AZ91D magnesium alloys

Although many previous studies have confirmed that cold sprayed Al + xAl2O3 (x = 0-75 vol.%) composite coatings on magnesium alloys can effectively improve wear and corrosion resistance, the effect of such coatings on tensile and fatigue properties is still unclear. The present work aims to evaluate the tensile properties and fatigue resistance of an AZ91D alloy with cold sprayed Al + xAl2O3 (x = 0, 30, 50 vol.%) coatings. The experimental results showed that although the cold spray coating leads to an increase in yield strength and fatigue limit, the tensile strength is reduced. In-situ examination of the macro-morphology of surfaces of the coated specimens during tensile testing was carried out through video recording. It indicated that once the tensile specimens have yielded, horizontal cracks that were perpendicular to the tensile axis formed on the surface of the specimens. Fractographic analysis of the fracture surfaces of the tensile specimens in a scanning electron microscope revealed that all the cracks within the cold sprayed coatings were suspended at the coating/substrate interface. Based on these experimental observations, it is considered that the improved yield strength is attributed to the constraint effect of the cold sprayed composite coatings on the magnesium substrate. The enhanced fatigue limit is a result of the higher yield strength and the coating/substrate interface barrier to crack propagation. The decrease in tensile strength is attributed to the brittle fracture of the cold sprayed coatings when yielding of the specimens.