Propagation of short fatigue cracks in permanent and semi-solid mold 357 aluminum alloy

Abstract

The high cycle fatigue strengths of aluminum-silicon-magnesium 357 alloy prepared by permanent molding and semi-solid molding are compared at stress ratios R = -1 and R = 0.1. Results show that precipitation hardened permanent mold (PM) and semi-solid mold (SSM) materials have fatigue strengths at R = -1 and 10 7 cycles of 82 and 113 MPa respectively while those at R = 0.1 are 47 MPa and 78 MPa respectively. Part of this difference is explained by the fact that the SSM specimens contain fewer and smaller defects than the PM specimens. Defects reduce the duration of the short crack propagation period, which significantly affects the overall specimen fatigue life. A comparative analysis of the long crack behaviour indicates that the PM microstructure is more resistant to the propagation of long cracks than the SSM microstructure because crack closure is more significant. However, the long crack behaviour cannot be used to predict the specimen fatigue life, which mainly consists of short crack propagation. Observation of many short cracks using the replication technique indicates that grain boundaries are the most 2 important microstructural barrier to short crack propagation in these materials. The results also show that crack growth decelerations occur at a crack depth comparable in size to the average grain radius of the materials. The difference in the distance of these important microstructural barriers can explain the fatigue strength difference measured. It is concluded that the average fatigue strength is inversely proportional to the square root of the average grain size.

Multiaxial fatigue design of cast parts: Influence of complex defect on cast AS7G06-T6

“…They will be used in the following as base for comparing multiaxial criteria and also as the reference ''defect free'' material on the Kitagawa diagram to study the influence of the defect. These results are in agreement with others on identical or very similar materials [23,[25][26][27][28]. Fig.…”

Fatigue Crack Initiation on Semi-Solid Al–7Si–Mg Castings

“…SSM Al-7Si-Mg castings typically have a longer fatigue life than permanent mould castings of the same alloy in high cycle fatigue regimes (>10 4 cycles) [12]. The lower defect content of the SSM castings compared to the permanent mould castings delays the fatigue crack initiation [12,13]. The smaller grain size of SSM castings also hinders the fatigue crack propagation due to the higher density of the grain boundaries, which results in the longer fatigue life of SSM castings compared to permanent mould castings [12,13].…”

Location, location & size: defects close to surfaces dominate fatigue crack initiation

“…For the case of cast Al alloys, literature shows that fatigue cracks are principally nucleated at surface and/or subsurface casting defects181920212223. When no such defects are present, crack nucleation is induced by other microstructural parameters such as favourable grain orientations or Si particles2425.…”