Abstract
Magnesium alloys are used in the motorsport and aerospace fields because of their high specific strength. However, due to their low corrosion resistance, protective surface treatments, such as conversion coating or electroless plating, are necessary when they are used in humid or corrosive environments. The present study aimed at evaluating the effect of plasma electrolytic oxidation (PEO), followed by the deposition of a polymeric layer by powder coating, on the rotating bending fatigue behaviour of the wrought magnesium alloy ZK60A-T5. The specimens were extracted from forged wheels of racing motorbikes and were PEO treated and powder coated. Microstructural characterization was carried out by optical (OM) and scanning electron microscopy (SEM) to analyse both the bulk material and the multilayer, consisting of the anodic oxide interlayer with the powder coating top layer (about 40 µm total thickness). Rotating bending fatigue tests were carried out to obtain the S–N curve of PEO-treated specimens. The results of the rotating bending tests evidenced fatigue strength equal to 104 MPa at 106 cycles and 90 MPa at 107 cycles. The results of the investigation pointed out that PEO led to a reduction in fatigue strength between 14% and 17% in comparison to the untreated alloy. Fracture surface analyses of the fatigue specimens, carried out by SEM and by 3D digital microscopy, highlighted multiple crack initiation sites at the interface between the PEO layer and substrate, induced by the concurrent effects of coating defects, local tensile stresses in the substrate, and increased roughness at the substrate–coating interface.
Highlights
Magnesium alloys, because of their low density, high specific strength, high damping capacity, and good castability [1,2,3], are attractive for lightweight applications in the automotive and aerospace industries, such as transmission housing, engine blocks, steering components, and wheels
Representative optical micrographs of the ZK60A-T5 forged alloy are reported in Figure 3, showing the presence of both large un-recrystallized dendrites oriented along the plastic flow (Figure 3a) and zones with fine and equiaxed recrystallized grains (Figure 3b) [8,26,27,28,29]
The present study investigated behaviour of of thethe
Summary
Because of their low density, high specific strength, high damping capacity, and good castability [1,2,3], are attractive for lightweight applications in the automotive and aerospace industries, such as transmission housing, engine blocks, steering components, and wheels. Compared to other Mg alloys, ZK series show high strength and formability, mainly due to the presence of Zr, which acts as a grain refiner and leads to the development of a homogeneous equiaxed grain structure, in extruded and in forged and cast-forged components [8]. Even if Zn is added to produce age hardening by precipitation of intermetallic compounds, because of the moderate age hardening response of ZK alloys, the contribution of grain size strengthening is predominant [5]. For this reason, in recent years, some papers focused on the effects of the forging process on the ZK alloys’ grain structure, widely used for the production of complex components.
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