Casting defects induced fatigue damage in aircraft frames of ZL205A aluminum alloy – A failure analysis

Two different types of aircraft frame components, which had collapsed respectively in their former vibration-fatigue performance tests, were submitted for failure analysis. The two failed frames were both made of aerial material ZL205A, a high-strength cast Al–Cu–Mn–Ti alloy. According to a series of experimental procedures including visual observations, X-ray detections, fractography inspections, microstructure examinations, mechanical tensile tests, hardness measurements and fluorescent penetrating inspections, it was indicated that the fracture was attributed to fatigue cracks which were induced by casting porosity defects at the external surfaces of frames. Numerous fine fatigue striations presented in the vicinity of casting porosities. Especially, it was observed of a special appearance of latitude–longitude crossed fatigue striations on the fracture surface due to the coupled stresses supplied by the former multi-directional vibration tests. The overload fast-rupture regions on fracture surfaces suggested the typical cleavage fracture mode, which was characterized by a number of river patterns and cleavage steps. The intergranular spatial dendrite-shaped casting porosities largely contributed to the local stress-concentrations in matrix materials. Triangular grain boundaries induced by the former casting burnt implied that the intergranular melting phenomenon had occurred. Furthermore, the effect of groove-shaped structure at roots of spatial convex-bodies on the edge of casting porosity was especially analyzed. And the influence of the casting porosity size on fatigue cracks was briefly discussed.