Fatigue life enhancement of cast Mg alloy by surface modification in cold spray process

Abstract

To improve the fatigue life of Mg alloy, high strength AA7075 spherical powder was deposited on AZ31B samples in nitrogen carrier gas environment at 400°C temperature in cold spray process followed by electrostatic painting with zinc phosphate. The fully reverse four-point rotating-bending fatigue tests were conducted on the coated and uncoated samples in different environmental conditions. It is seen that the cold sprayed AA7075 improved the yield strength of the AZ31B cast alloy. Similarly, a significant fatigue enchantment was observed in the coated samples, reaching a fatigue strength of 90 MPa compared to as-cast fatigue strength of 70 MPa at 107 cycles when tested in air, and fatigue strength of 80 MPa under a 3.5%NaCl testing environment. The SEM analysis at the interface of the tensile tested sample exhibited the interfacial fracture followed by delamination of the coating. Similarly, delamination of coating was merely detected in the fatigue fracture sample tested in the air, while the sample tested in the corrosive environment showed pits which allowed the solution to penetrate in the AZ31B substrate results delamination and premature failure. However, the presence of e-paint forms a passive layer which is hindering the pit formation and extending the fatigue life.