Detection and Influence of Shrinkage Pores and Nonmetallic Inclusions on Fatigue Life of Cast Aluminum Alloys
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RODUCTION
MOST of the aluminum castings are used in automotive applications that involve high mechanical cyclic loads. However, during the production and processing of aluminum alloy melt, several kinds of defects, in various forms, are generated. The defects lead to microstructural discontinuities and adversely influence the structural durability of the produced castings.[1,2] Typical example of cast defects is that of shrinkage pores which are often associated with the release of dissolved gas in the solidifying melt.[3,4] Another type of defects is that of nonmetallic inclusions (in most cases oxide skins) which are generated and introduced into the melt whenever the protecting Al2O3 layer is broken because of surface turbulences by, e.g., degassing or during mold filling.[5] Several other nonmetallic inclusions such as spinel, MgO, carbides, and Al2O3 needles are formed because of the alloy chemistry.[6] In general, bifilms have been reported to be the source of many casting defects.[7,8] The presence of shrinkage pores and nonmetallic inclusions in castings results in unacceptable levels of defective product. There are several techniques and processes to control and prevent pores and inclusions in aluminum castings.[6] However, these defects YAKUB TIJANI and ANDRE´ HEINRIETZ, Researchers, are with the Fraunhofer-Institute for Structural Durability and System Reliability LBF, Bartningstr. 47, 64289 Darmstadt, Germany. Contact e-mail: [email protected] WOLFRAM STETS, Director, is with the Institute of Casting Technology IfG, Sohnstraße 70, 40237 Du¨sseldorf, Germany. PATRICK VOIGT, formerly Project Manager (Researcher) with the Institute of Casting Technology IfG, is now Head with the Research & Development, Hanseatische Waren Handelsgesellschaft, Am Wall 127, 28195 Bremen, Germany. Manuscript submitted September 17, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
cannot be completely avoided during production without incurring additional costs. The fatigue life distribution of A356-T6 specimens where crack initiated at pores has been investigated by Wang et al.[9] Similarly, the effect of the entrained oxide film on the fatigue of an Al-7Si-Mg alloy has been investigated by Nyahumwa et al.[10] The authors observed a slightly decreased scatter in the oxide film investigation in comparison with when crack initiated at pores. However, a correlation of the scatter in fatigue life with the original casting defects was not presented in either investigation. However, in the study of Yi et al.,[11] a correlation was presented between the scatter in fatigue life and the casting porosity in a cast A356-T6 aluminum–silicon alloy. Those authors showed that the fatigue life is influenced by the mean value and standard deviation of pore size, as well as the pore number density. Thus, a quantitative characterization of the defect parameters is required to predict the fatigue life of cast aluminum alloy components. Li et al.[12] used X-ray computer tomography (CT) to characterize the 3D morphology of porosity in A356-T6
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