Analysis of microstructure and fatigue micromechanisms in cast aluminium piston alloys
Mbuya, Thomas O
MetadataShow full item record
Light vehicle engine pistons have traditionally been cast from near eutectic Al-Si cast alloys due to several favourable functional and processing attributes. The increasingly demanding engine performance requirements have necessitated the need for the development of multicomponent alloys with high alloy content and highly complex microstructure. In this regard, recent trends in new piston alloy development have been to increase the level of various alloying elements such as Cu, Ni and even Si. However, low Si compositions of ∼7 wt% Si and ∼0.7 wt% Si have also been proposed largely due to observations that the large blocky primary Si particles found in the near-eutectic alloys are potent fatigue crack initiators. Nonetheless, previous research on these low Si piston alloys has demonstrated that their fatigue performance is significantly impaired by porosity which increases with decreasing Si content. With improved processing techniques, porosity can be reduced to levels that make it impotent in fatigue failure processes. The aim of this work was therefore to characterise the microstructure and fatigue micromechanisms of the low Si piston alloys after hot isostatic pressing (hipping) to reduce porosity. This was achieved using a combination of various imaging tools and fatigue testing to establish the role of microstructure on initiation and growth of fatigue cracks. It has been demonstrated using X-ray microtomography that hipping significantly reduces porosity, especially in the 0.7 wt% Si alloy, while the intermetallic structures remain largely unaffected. The eutectic Si particles in the 7 wt% Si alloy are however transformed from a fine fibrous interconnected structure to coarse, spheroidised and discrete particles. Hipping has also been observed to improve the fatigue performance of the 0.7 wt% Si alloy due to the significant reduction in porosity. Fatigue crack initiation has been observed to occur mainly at intermetallic particles in both alloys after hipping and, consistent with previous work, the most frequent crack initiating phase is found to be Al9 FeNi. Analysis of short fatigue crack growth profiles has shown that intermetallics and eutectic Si particles preferentially debond, thus providing a weak path for crack propagation along their interfaces with the α-Al matrix. However, grain boundaries as well as these hard particles have also been shown to frequently act as effective barriers to crack growth. On the other hand, long fatigue crack growth analysis has shown that fatigue cracks tend to avoid Si and/or intermetallic particles at low ΔK levels (up to ΔK∼7 MPa√m). At higher levels of ΔK, the cracks increasingly seek out these hard particles up to a ΔK of ∼9 MPa√m after which the crack preferentially propagates through them. It has also been observed that crack interaction with intermetallics causes significant crack deflection which may result in roughness related closure mechanisms to be activated.