Thermo-mechanical fatigue influence of copper and silicon on hypo-eutectic Al–Si–Cu and Al–Si–Mg cast alloys used in cylinder heads

https://doi.org/10.1016/j.ijfatigue.2016.02.017Get rights and content

Highlights

  • Correlation of thermomechanical fatigue endurance and damage mechanisms.

  • Comprehensive thermo mechanical fatigue testing of 8 Al–Si cast alloys.

  • Identification of characteristic fatigue mechanisms and behaviours.

  • Observation of special damage mechanisms by refinement and iron impurity.

  • Identification of high temperature oxidation under thermo mechanical fatigue.

Abstract

In this paper, different hypo-eutectic Al–Si cast alloys with varying silicon, copper and iron contents were tested under thermo-mechanical fatigue (TMF) conditions to achieve a unified approach to damaging and fatigue endurance behaviour. The cylinder heads from which the specimens were taken were serially produced with T79 heat treatment. This includes homogenisation, quenching and ageing. The alloy used by these investigations are commonly used for automotive cylinder heads. Under operational conditions, a complex interaction of mechanical and thermal cyclic loadings is inherent. Hence, the main purpose here is to distinguish between the influences of the alloying elements of hypo-eutectic Al–Si cast alloys and propose its effects to mechanical and environmental damages.

The results underlined the important role of hard phases for crack nucleation especially in high copper alloys. The hard phases, like eutectic silicon or primary AlCu, AlFeSi phases in general got ruptured, independent of the applied mechanical loading. In contrast, damages in low copper alloys were caused by high plastic matrix deformations and the loss of adhesion of eutectic silicon particles. For both types of alloys, the crack propagation was mostly constrained on the eutectic itself. An improved TMF endurance was achieved by AlSi9Cu1(Sr) and primary AlSi8Cu3. Here, probably the increased matrix strength by precipitation hardening and the manifold crack deflections on the eutectic silicon reduced intrinsically the microstructural damage. Further, in TMF regimes it is recommended to use Al–Si cast alloys with low iron contents because present ternary α-Al15(Fe,Mn)3Si2 and quaternary β-Al5FeSi supported crack nucleation and lowered the fatigue endurance. Additionally, an over-aged AlSi7MgCu alloy in T74 condition significantly lacks in fatigue endurance because of a minor fatigue strength by incoherent θ-precipitations.

However, the results presented here identify the major damages of hypo-eutectic Al–Si cast alloys and the influences of the alloying elements on TMF endurance. This enables further unified fatigue modelling by the automotive designer and additional optimizations of the metallurgical systems by the foundry which underlines the demand of improved knowledge of the cylinder head under real-life operational conditions.

Introduction

Increasing requirements on engines regarding efficiency, engine size reduction, specific power increase have supported complex thermomechanical fatigue investigations and become more and more standard for calculating cylinder head fatigue, especially for high performance engines [1], [2], [3], [4]. As cylinder heads are among the most critical components of an engine it is necessary to be able to predict their fatigue endurance as accurately as possible [5]. Cylinder heads used in personal car vehicles are commonly made of hypoeutectic Al–Si cast alloys, because of their low weight, good castability, thermal conductivity and thermal endurance [6]. Under operating conditions, the component undergoes complex interaction between different cyclic thermomechanical mechanisms [7]. For advanced TMF models, pure fatigue, creep and oxidation are the most common damage mechanisms for fatigue modelling [8], [9], [10], [11].

Commonly, different variants of two hypoeutectic Al–Si cast systems (Al–Si–Mg and Al–Si–Cu) are used for different types of engines, and each has individual damaging mechanisms or damage interferences. The most investigated of these cast alloy groups are A356/A357 (Al–Si–Mg) and B319 (Al–Si–Cu) systems [12], [13], [14]. Copper increases thermal endurance but reduces corrosion resistivity. Magnesium, on the other hand, also acts as a precipitation hardening element and has good corrosion resistance but lacks in high temperature regimes [15], [16]. In addition, heat treatment also influences fatigue behaviour. For automotive application, T5, T6 and T7 are favoured. While T5 means special temperature controlled cooling with additional ageing, T6 and T7 also include solution heat treatments, artificial ageing and improved fatigue endurance by homogenisation. T6 ageing is characterised by maximum yield strength whereas further ageing leads to the T7 condition which shows increased ductility with an ongoing decrease of yield strength [17]. Economical aspects try to push the heat treatment to lower ageing times although TMF endurance gets improved by long ageing times [4], [18].

In this paper, a comprehensive TMF investigation of 8 different Al–Si cast alloys, belonging to the Al–Si–Cu and Al–Si–Mg groups was conducted, with the aim to predict the influence of the alloying elements with similar T79 heat treatment. The treatment condition is defined as very limited over-aged with slightly improved ductility but with no huge loss of mechanical strength [19], [20]. Detailed temperature profile of each alloys can be found in [21]. Not only phenomenological aspects were discussed but also the corresponding microstructural damage processes investigated through semi in situ and post mortem fractography. Thus, it provides an insight of TMF damaging processes in hypoeutectic Al–Si cast alloys.

Section snippets

Experimental methods

In order to determine how the different Al–Si cast alloys behave, a comprehensive TMF test program was conducted. In total, eight hypo-eutectic Al–Si–Cu and Al–Si–Mg cast alloys that are commonly used for automotive cylinder heads were tested. Each material is still in use for serially produced cylinder heads, depending on the engine’s conditions and system. A detailed chemical composition chart is given in Table 1, as measured with an optical emission spectrometer. The characteristic secondary

Results

The ongoing TMF investigations with varying testing conditions and different material influences produced a huge data set. Thus, an identification of the individual effects supports further developments of specific fatigue models. In Fig. 2a, all TMF endurance data of the analysed alloys in T79 condition are charted. For better and direct comparison two strain amplitudes (SL1: m,a=0.26% and SL2: m,a=0.37%) are chosen. Because not all TMF tests were performed at those strain levels, a simple

Discussion

The NEMAK ROTACAST© system gained with no macroporosity, and microporosity was less than 0.01% in metallographic investigations. Hence, the damage analysis confirms prior researches at ambient temperatures [21], [22] that the residual porosity or defects has minor importance in the alloy’s fatigue. In literature published TMF results of the alloys A356, AlSi10Mg AlSi8Cu3 (e.g. [18], [30], [31], [1], [32]) whose casting process was different or porosity was essential underlines the here proposed

Conclusion

In this paper, the different cylinder head materials AlSi6Cu4(-/Sr), p/sAlSi8Cu3(Sr), AlSi7MgCu(Sr), AlSi7Mg(Sr) and AlSi10Mg(Sr) in T79 (and T74) conditions were analysed for TMF endurance behaviour, produced with the ROTACAST© system. The evaluation of the fatigue testing results and its discussion have led to the following conclusions:

  • The silicon and copper contents of hypo-eutectic Al–Si alloys play a major role for the TMF endurance behaviour by particle interactions and not defect based

Acknowledgements

The authors would like to thank the FFG-Austrian Research Promotion Agency for funding of this research work in the frame work of the FFGs BRIDGE programme. The commendable support by NEMAK Linz GmbH is also much valued.

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