Fatigue tests under reversed flexure have been conducted on samples of 7075-T6 Al-alloy with the application of electroless Ni-P deposits of thickness ranging from 5 to 50 mm after salt spray corrosion (100, 500 and 1000 hours). The corrosion damage of the coated and uncoated samples was determined through the measurement of the mass loss and surface corroded morphology. The mean microhardness of the high phosphorous (Hi-P) deposits in 5, 25 and 50 mm thickness were found to be 334, 457 and 606 Hv 50 , and the mean surface roughness (Ra) were measured at 0.197, 0.217 and 0.223 mm for 5, 25 and 50 mm thickness, respectively. The salt spray tests revealed that the corrosion damage increases as the exposure time increases for the 5 mm coating. The corrosion damage literally stopped when the coating thickness was increased to 25 mm. The fatigue strength of the coated samples was found decrease slightly as the deposit thickness increases to 25 mm, whereas the reduction on the fatigue strength for the 50 mm deposit is significant. It has been observed that the reduction in the fatigue strength is less or to none for the thicker coating system. For the 5 mm Hi-P deposit sample, the fatigue crack initiation started at the surface corrosion pitting then extending to the substrate and onto failure, this phenomenon reduced the fatigue strength as compared to the uncoated samples. The fatigue strength is decreased slightly as the thickness increases to 25 mm and 50 mm. With or without corrosion damage, the fatigue crack initiation all started from the coating and extended to the substrate through the bonding between the coating and the substrate. Balancing between the need for the corrosion resistance and the fact on the fatigue strength reduction, the optimum coating thickness seems to be greater than 5 mm but less than 25 mm for the aerospace alloy 7075-T6 Al-alloy.

During the casting process of thin-wall parts, due to fast cooling rate or chemical composition of the ductile iron, iron carbide (cementite) may exist in the microstructure that retards the mechanical properties of the iron greatly. In this study we discuss the effects of eutectic carbide contents in relation to the cooling parameters and the erosion behavior of the erosion incident angle on the casting specimens. Different content of eutectic carbide was produced by varying the cooling rate and silicon content. Solid particle erosion tests were performed using a shot-blasting machine with angular Al 2 O 3 particles. The results show that, the largest amount of eutectic carbon (plate-like) exists in the faster cooling rate and lower silicon content. The amount of pearlite and nodules counts produced at lower cooling rates also decreased accordingly. Erosion rate also was found in the same trend that higher carbide content yielded better erosion resistance and lower erosion rates, but the amount of reduction is lower than the amount of increase on the hardness level. The maximum erosion rate occurred at the incident angle equaling to =4:5, and the deepest erosion penetration occurred at an incident angle of =3:6 for four comparison specimens. The impinged surface morphologies of four specimens exhibit cutting and plowed furrows at lower incident angles, but the overlapped chippings at higher incident angles. The erosion mechanisms, at medium and high incident angle erosion, show fatigue crack and subsurface lateral crack propagation in higher eutectic carbide specimen, but platelet peeling-off and internal crack and deformation in lower eutectic carbide specimen. The test also revealed that the failure mechanism of the casting thin plates tend to be ductile-brittle fracture.