Micromechanistic analysis of fatigue in aluminium silicon casting alloys

Abstract

Due to increasingly stringent environmental legislation, there is a requirement for lower emissions and greater overall efficiency of light vehicle diesel (LVD) engines. This continues to be achieved through the optimisation of design and careful selection of the materials used in key LVD engine components, for example pistons, so that they are lighter and can operate at higher temperatures. Pistons are non-serviceable parts and so must be able to withstand the fatigue and high temperature environment of the car engine. It is therefore important to understand the mechanisms of fatigue in these alloys to help inform alloy development for the next generation of pistons.Pistons are typically produced from multi-component Al-Si casting alloys. These alloys exhibit a complex, multiphase microstructure comprising α-aluminium as the matrix with silicon particles and several intermetallic phases. Previous research on Al-Si casting alloys has demonstrated that porosity is detrimental to fatigue life as cracks initiate freely at pores. However, with improved casting techniques porosity can be greatly reduced and other microstructural features influence fatigue life. In particular, Si particles have been shown to play an important role in the initiation and subsequent propagation of fatigue cracks. This study assesses the role of Si content and other microstructural features on fatigue behaviour by testing a set of well-characterised multi-component, Al-Si casting alloys with varying Si content. Fatigue initiation behaviour was investigated at room temperature using S-N and short fatigue crack growth experiments. Pores, Si particles and intermetallic phases were shown to cause fatigue crack initiation. In a 0.67wt% Si containing alloy, large-scale porosity was observed and was the foremost reason for fatigue initiation. In two alloys the Al₉FeNi phase was observed to be the most detrimental hard particle causing fatigue crack initiation. Nanoindentation results showed that Al₉FeNi had a lower hardness and higher modulus than Si and so Al₉FeNi may be expected to fracture preferentially, consistent with the fatigue results. X-ray computed tomography demonstrated that all the alloys investigated contained a complex, interconnected, intermetallic sub-structure. As a result, the micromechanisms of fatigue are different to those in conventional particulate Al-Si alloys because particle fracture is required to ensure a level of crack continuity. At room temperature and 350ºC, and at low and high crack growth rates, the crack tip may be described as a diffuse region of micro-damage and intact ligaments. It is the extent of this damage in the alloys that controls the crack growth rates exhibited and simple trends between the Si content and roughness, reported for particulate systems, do not hold true in the alloys investigated in this study. The balance of the micromechanisms of fatigue was shown to be dependent on temperature. This highlights the importance of fatigue studies at temperatures that are characteristic of those experienced in service.

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ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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