Microstructural Analysis of Fatigue Initiation in Al-Si Casting Alloys

Abstract

Fatigue initiation behaviour in three multi-component Al-Si casting alloys with varying Si content is compared using a range of microscopy and analytical techniques. A higher proportion of stiffer secondary phases leads to load transfer effects reducing particle cracking and particle/matrix debonding. Si appears stronger than the Al9FeNi phase, which cracks and debonds to form initiation sites preferentially over Si. Reducing Si content results in clusters of intermetallics forming, and increased porosity. The effect of porosity, combined with mesoscopic load transfer effects to the high volume fraction intermetallic regions make these potent crack initiation sites in low silicon alloys.

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

“…Previous studies [1,3,7,8] on model piston alloys with high Cu and Ni levels (3.9 wt.% Cu and 2.8 wt.%Ni) confirmed that fatigue crack initiation occurs at Si particles and intermetallic particles such as Al 9 FeNi and Al 3 (CuNi) 2 . Some of the model alloys investigated contained reduced Si levels of 6.9 wt.% and 0.67 wt.% as shown in Table 1.…”

“…Light vehicle engine pistons are commonly cast from near-eutectic multicomponent Al-Si alloys typically containing high levels of Cu and Ni [1]. The rapid increase in performance requirements in diesel engines together with light-weighting and strict emission regulations has necessitated development of new piston designs and materials that can withstand the increasingly harsh combination of fluctuating thermal stresses and cylinder pressure [2,3].…”

“…The rapid increase in performance requirements in diesel engines together with light-weighting and strict emission regulations has necessitated development of new piston designs and materials that can withstand the increasingly harsh combination of fluctuating thermal stresses and cylinder pressure [2,3]. As a result, new alloys have been developed with increasingly higher levels of various alloying elements such as Cu to upwards of 4-5 wt.% and Ni to ~3 wt.% [3,4].…”

“…The rapid increase in performance requirements in diesel engines together with light-weighting and strict emission regulations has necessitated development of new piston designs and materials that can withstand the increasingly harsh combination of fluctuating thermal stresses and cylinder pressure [2,3]. As a result, new alloys have been developed with increasingly higher levels of various alloying elements such as Cu to upwards of 4-5 wt.% and Ni to ~3 wt.% [3,4]. Hypereutectic silicon levels have been suggested [4], but low Si alloys have also been proposed [5] to mitigate against fatigue crack initiation at primary Si particles as often observed in the near-eutectic Al-Si piston alloys [6].…”

“…Hypereutectic silicon levels have been suggested [4], but low Si alloys have also been proposed [5] to mitigate against fatigue crack initiation at primary Si particles as often observed in the near-eutectic Al-Si piston alloys [6]. These new alloys have been shown to have complex and threedimensionally networked microstructure [1]. The successful application of these alloys in modern pistons requires a thorough characterization of their microstructure and the micromechanics of fatigue failure.…”