The Effect of Fe-Rich Intermetallics on the Weibull Distribution of Tensile Properties in a Cast Al-5 Pct Si-3 Pct Cu-1
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ONE of the most widely used family of cast aluminum alloys is that based on the 319 specification (nominally Al-6 wt pct Si-3.5 wt pct Cu-0.1 wt pct Mg). These alloys have found widespread applications in the automotive industry because of their characteristics of good castability, modest corrosion resistance, strength-to-weight ratio, and relatively low cost, significantly aided by the relatively high Fe content. The tensile properties and fracture behavior of cast Al-Si alloys are strongly influenced by the shape and distribution of the eutectic Si phase and Fe-rich intermetallics in the microstructure, in addition to the common structural defects such as porosity and oxide inclusions. The effect of microstructure and structural defects on mechanical properties such as tensile strength (rTS), elongation (eF), and fatigue life have been characterized by Weibull statistics, more specifically, by the two-parameter Weibull modulus.[1,2] In their early study on A356-type castings, Green and H. ZAHEDI, Master’s Student, M. EMAMY, Associate Professor, and M. MAHTA, Research Assistant, are with the School of Metallurgy and Materials, University of Tehran, Tehran, Iran. A. RAZAGHIAN, Assistant Professor, is with the International University of Imam Khomeiny, Qazvin, Iran. J. CAMPBELL, Professor Emeritus, is with the Department of Metallurgy and Materials, University of Birmingham, Birmingham, West Midlands B15 2TT, United Kingdom. M. TIRYAKIOG˘LU, Professor of Engineering, is with the Department of Engineering, Robert Morris University, Moon Township, PA 15108, USA. Contact e-mail: [email protected] Manuscript submitted: April 25, 2006. Article published online March 29, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
Campbell[1] used two-parameter Weibull statistics and found that the Weibull modulus appeared to be a useful measure of the reliability of the casting process. Since then, a two-parameter Weibull modulus has been used extensively to characterize the tensile properties, especially tensile strength. Important contributions have been made to understanding the effect of composition on Fe-rich intermetallics,[3,4] porosity formation,[4,5] and tensile properties.[4,5] However, in these studies, the effects of microstructure on tensile properties were reported as averages. This study is the first attempt to characterize the effect of Mn and Sr additions on the microstructure and consequent tensile properties using Weibull statistics. For this purpose, the use of three-parameter Weibull statistics has been explored to illustrate its superior analytical potential over the traditional two-parameter approach.
II.
BACKGROUND
The mechanical properties of 319 castings are influenced by melt quality, the gating system design,[6] and the Fe content.[5] During melt preparation and mold filling, significant damage to the liquid metal may take place from the entrainment of the oxide layer on the surface of the melt, folding it in to the liquid with its upper dry side facing a similar area of dry side of the oxide, and so acting as
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