Análisis efectos de la variabilidad climática sobre los usos del suelo en la

Little research has been published regarding the effect of high temperature on the fatigue behaviour of Al-Si casting alloys. Al-Si casting alloys are widely used in car engines (Ye, 2003) for applications such as the cylinder head (Fischersworring-Bunk

et al. 2006) and pistons (Haqueand Maleque, 1998). Therefore an understanding of the fatigue behaviour up to a temperature of 400˚C is important to ensure the alloys are ‘fit for service’ and to allow for alloy development. As previously mentioned in section 2.3.4, time dependent processes may be expected to occur above 0.4Tm

(Callister, 2000) and so may be significant in Al-Si alloys which operate at temperatures up to approximately 0.8Tm.

Myers and Hurd (1990) investigated the fatigue performance of eutectic Al-Si alloys at 300˚C. Some of the samples were preloaded at the test temperature and a maximum stress of 40 MPa for 70% of the lifetime to rupture. Other samples were aged for the same amount of time but not under load. In addition, pistons composed of the same materials were engine tested, during these tests the materials were thermally and mechanically loaded over a range of temperatures, loads and frequencies. S-N tests were performed on the pre-creep and no-creep samples: at high strains (where lifetimes were shorter than 100,000 cycles) the materials gave equivalent lifetime results, at lower strains the pre-creep samples exhibited shorter lifetimes. Analysis of the no-creep samples showed that the damage (particle fracture and debonding) was confined to a region near the crack. However, in the pre-creep samples damage occurred throughout the specimens and took the form of interdendritic voids and fractured particles; this damage was similar to that observed in the engine test samples around the combustion bowl (this is the maximum stress and temperature region in a piston). Analysis of the initiation sites indicated that the same features caused

initiation in both the pre-creep and no-creep samples and that these features were oxides, pores and Si particles approximately 100-150 µm in size. It was concluded that the damage caused by creep did not affect initiation in Al-Si alloys. Creep damage was thought to provide a weak path for short fatigue cracks in their early stages (where they are most microstructurally sensitive), which resulted in a reduction of fatigue life compared with the no-creep samples.

Joyce et al. (2002; 2003) also investigated the high temperature fatigue performance of two eutectic Al-Si alloys; the compositions are given in Table 2-1. Fatigue tests were performed at 200˚C and 350˚C; these temperatures were identified as being characteristic of those experienced at the gudgeon pin boss and the combustion bowl (identified in Figure 2-5). At elevated temperature the initiation of fatigue cracks was associated with the fracture and debonding of primary Si particles and so was similar to that observed at room temperature. Long fatigue crack propagation tests showed that at a test temperature of 200˚C both alloys exhibited improved fatigue resistance compared with their RT performance. From post-failure analysis it was identified that the critical value of ∆K at which the dominant failure mechanism changes from debonding of the particle/matrix interface to particle fracture (Gall et al., 1999; Joyce

et al., 2002; Chan et al., 2003) was higher at elevated temperature. The improvement in performance at 200˚C was attributed to the increased level of debonding which is the preferable particle failure mode. At 350˚C the low Cu and Ni containing alloy exhibited a frequency effect on fatigue performance, crack propagation rates were faster at 50 Hz compared with those at 15 Hz. This was not observed in the tests on the high Cu and Ni containing alloy. This indicates that the phases formed by the addition of Cu and Ni do make the material more resistant to time-dependent processes, but the exact mechanism(s) by which this occurs is unknown.

2.5

Summary

To optimise the various properties of Al-Si casting alloys, many alloying elements are added and as a result the alloys exhibit complex multiphase microstructures. With improvements in casting techniques it is possible to reduce the levels of porosity and oxides, which are detrimental to fatigue life, it is therefore important to understand how other microstructural features, for example: Si and intermetallic particles (hard

particles), affect fatigue crack initiation and subsequent propagation. Fatigue cracks have been reported to initiate as a result of particle failure and this may be via the mechanism of particle fracture or debonding of the particle/matrix interface. Classic short crack behaviour has been observed in Al-Si alloys and grain boundaries and Si particles retard crack growth. At low values of ∆K and when Si particles are small (~2.5 µm) a fatigue crack is likely to be deflect around intact particles and cause debonding at the particle matrix interface. Due to a size effect, large particles may fracture especially at (relatively) high values of ∆K (>6-8 MPa√m at RT) causing static failure modes ahead of the crack tip and therefore an acceleration in the crack growth rate.

Given their high temperature applications in car engines, few studies were found in the available literature on the high temperature fatigue properties of Al-Si alloys. Initiation appears to occur at the same microstructural features as at RT. Creep causes damage in the interdendritic regions, which can accelerate short fatigue crack growth. At high temperature there are increased levels of debonding, instead of particle fracture, during fatigue propagation and so at 200˚C the alloys exhibited improved fatigue performance. Additions of Cu and Ni resulted in an improved resistance to time dependent mechanisms during fatigue crack propagation at 350˚C.

Alloy Si (wt.%) Cu (wt.%) Ni (wt.%) Mg (wt.%) Fe (wt.%) Mn (wt.%) Ti (wt.%) Zr (wt.%) V (wt.%) P (ppm) AE160 11.22 3.1 2.27 1.05 0.3 0.08 0.17 0.15 0.06 53 AE413 11.15 0.94 0.96 0.88 0.49 0.07 - - 35

Figure 2-1 S-N curve for a material exhibiting no fatigue limit.

Figure 2-3 Schematic diagram showing the estimated plastic zone sizes at a crack tip on a graph of stress versus distance from the crack tip when θ=0. After Anderson (1994).

Figure 2-5 Photograph of a light vehicle diesel engine piston from the die prior to shaping and polishing.

Figure 2-7 Schematic diagram showing crack propagation behaviour for short and long cracks.

Figure 2-9 Weibull statistics plot showing the effect of pores, oxides and slip bands and Si particles on fatigue life from Wang et al. (2001a).

Figure 2-10 Debonded Si particles causing fatigue crack initiation at the high stress region near the concave tip of a shrinkage pore from Buffière et al. (2001).

Figure 2-11 Schematic diagram from Lados et al. (2006) showing the micromechanisms of fatigue near threshold in (a) a low Si alloy (b) a mid Si alloy and (c) a eutectic alloy.