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Metal Casting Technologies : March 2011
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32 www.metals.rala.com.au TECHNICAL FEATURE to the area of each defect as measured on the fracture surface as shown in Figure 8. The surface defects are seen to be more damaging than internal defects. Likewise in a range of Al alloys containing 7-17%Si surface porosity was observed to be the most damaging defect in reducing fatigue resistance with fatigue life decreasing with increase in pore size [12]. Internal defects such as oxide films and inclusions [13] also provide sites and pathways for the nucleation and growth of fatigue cracks. Many of the recent process developments in casting of light alloys have involved improved melt preparation to provide cleaner liquid metal and reduced turbulence during liquid metal handling and mould filling to reduce the incidence of damaging folded over oxide films and bubble trails [1,14-16]. The tensile properties and fatigue resistance of Al base castings filled in a turbulent manner can be as variable as ceramic materials with Weibull Modulus values as low as 10. Campbell [1] has comprehensively reviewed and summarized the effects of oxide films on fatigue reliability (Figure 9). As well as significantly damaging mechanical properties folded over oxide films also encourage the formation of gas and shrinkage defects and reduce the pressure tightness of castings. Moulds and dies must being filled with clean, suitably degassed metal with minimum turbulence and the castings must solidify with the correct microstructure. For example, as shown in Figure 10, in Al-Si alloys the eutectic Si must be effectively modified since fatigue life is reduced if the Si particles are large [17]. In Al cylinder heads where there is interaction between mechanical and thermal fatigue it has been found [6] that to minimize fatigue failure the key microstructural factors are: correct modification of eutectic Si, sufficiently high solidification rates to ensure a small secondary dendrite arm spacing, avoidance of coarse Al-Fe-Si compounds by limiting Fe level and using Mn for refinement. All air-melted steels will contain inclusions as a result of deoxidation and re-oxidation during pouring. These inclusions will lower ductility, fatigue resistance and fracture Figure 9. The effect of oxide bifilms and filling on fatigue lives of turbulently filled (containing young oxides) and better filled (old oxides) Al-7Si-0.4Mg alloy [1]. Figure 10. The effect of silicon particle size on fatigue life of Al-Si alloy [17]. Figure 8. The evolution of the fatigue limit with defect size for both surface and internal defects in ductile iron [3]
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