Thermo-mechanical fatigue influence of copper and silicon on hypo-eutectic Al–Si–Cu and Al–Si–Mg cast alloys used in cylinder heads
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: and SL2: ) 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:
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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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2021, International Journal of FatigueCitation Excerpt :However, the influence of dwell time on the TMF lifetime behavior is controversially discussed in literature: While the TMF lifetime is found to be reduced by dwell times in [11,22] compared to corresponding TMF tests without dwell time, this is not the case according to [45–47] for various materials from the class of cast aluminium alloys. Furthermore, dwell times longer than 60 s (e.g. 120 s and 180 s) have no impact on the TMF lifetime [11,45–47]. Despite the huge experimental database of currently used cylinder head and piston alloys, there is still a lack of understanding regarding the operating damage mechanisms and their temperature dependence under low cycle fatigue and thermomechanical fatigue loading in general and under superimposed HCF loading in particular.
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2021, International Journal of FatigueCitation Excerpt :During TMF loading, both damage mechanisms of particle fracture and matrix decohesion likely interact (see Fig. 8b) and lead to lifetimes indicated by region ⑤ in Fig. 16. Similar findings are mentioned in [64]. Out-of-phase TMF conditions result in a positive mean stress evolution, which is known to be more detrimental and attributed in [85] for the lifetime reduction under TMF compared to isothermal loading.