The impact of AA7075 cold spray coating on the fatigue life of AZ31B cast alloy

To improve the fatigue resistance of magnesium alloys, AA7075 powder, which has high fatigue strength and high adhesion strength, was successfully coated on as-cast AZ31B substrate. The coating proved to be dense, with a porosity of < 4% within an average build-up of 282 gm, and showing adhesion strength of 42 MPa. Residual stress distribution due to the peening effect of cold spray was measured in the coat and substrate by a hole drilling method, showing a maximum compressive residual stress of - 122 MPa. Coated and un-coated samples were then tested in load-control mode under fully-reversed loading conditions. Fatigue strength improvement of 25% at 10 million cycles was observed for the coated samples. Casting defects, such as oxides and porosity in un-coated cast AZ31B was found to be the primary source of crack initiation at multiple locations. In the coated samples, while cracks were still initiated from the substrate's pores near the interface, the coating remained intact and crack propagation was delayed considerably. Delamination of coating was solely observed along the final fracture zone, and was due to a high load and mismatch of coat-substrate properties. The longer lives of the coated samples were attributed to the higher fatigue strength of AA7075 relative to AZ31B and coating's high adhesion strength.