The effect of solidification rate on the growth of small fatigue cracks in a cast 319-type aluminum alloy

Abstract

A study was conducted to investigate the effect of solidification rate on the growth behavior of small fatigue cracks in a 319-type aluminum alloy, a common Al-Si-Cu alloy used in automotive castings. Fatigue specimens were taken from cast material that underwent a hot isostatic pressing (HIP) process in order to eliminate shrinkage pores and to facilitate the observation of surface-initiated cracks by replication. Naturally initiated surface cracks ranging in length from 17 m to 2 mm were measured using a replication technique. Growth rates of the small cracks were calculated as a function of the elastic stress-intensity-factor range (⌬K ). Long-crack growth-rate data (10 mm Յ length Յ 25 mm) were obtained from compact-tension (CT) specimens, and comparison to the small-crack data indicates the existence of a significant small-crack effect in this alloy. The solidification rate is shown to have a significant influence on small-crack growth behavior, with faster solidification rates resulting in slower growth rates at equivalent ⌬K levels. A stress-level effect is also observed for both solidification rates, with faster growth rates occurring at higher applied-stress amplitudes at a given ⌬K. A crackgrowth relation proposed by Nisitani and others is modified to give reasonable correlation of smallcrack growth data to different solidification rates and stress levels.

Micromechanisms of short fatigue crack growth in an Al–Si piston alloy

“…The stress effect on short crack growth has also been observed in the low Si piston alloy (Al0.7Si) [9]. Similar observations have also been reported in a 319 type alloy by Caton et al [11] and in A356 by Plumtree and Schaefer [12]. The loading conditions used in this work result in a plastically deformed layer near the surface of the specimen right from the first loading cycle.…”

“…The magnitude of this surface layer deformation naturally depends on the applied stress. Previous reports [11,13,14] suggest that this stress dependent global and local crack tip plasticity can accelerate the growth of short cracks and is therefore be the cause of the stress dependent results observed in this alloy.…”

Residual stresses and fatigue performance

“…Correlation of small crack growth rate relationships for the various specimen conditions using an expression based on crack tip opening displacement and yield strength. This has previously proved useful in obtaining a single curve that defined crack growth rate behaviour for a range of microstructures in an aluminium casting alloy [10]. 3.…”

High cycle fatigue strength of permanent mold and rheocast aluminum 357 alloy

“…Nevertheless, in defect free specimens, the microstructural constituents play a role in fatigue crack nucleation and long crack propagation. Crack nucleation has been observed at the surface of eutectic silicon particles [2,14,16,17], or at intermetallic particles [2] and at persistent slip bands (PSB) [6,16,18]. The morphology of the alpha phase and of the eutectic silicon particles was also found to influence the crack propagation threshold in a study carried out on PM and SSM cast specimens [19].…”