Quality Indices for Aluminum Alloy Castings: A Critical Review
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I.
INTRODCUTION
IN cast aluminum, the mechanical properties of alloys are affected to a great extent by initial melt quality and process design. This strong sensitivity to melt quality and filling system design in aluminum castings often makes it necessary to measure mechanical properties for quality-assurance purposes. Because most nondestructive evaluation techniques are not sensitive enough to detect most serious defects in the metal,[1] namely oxide bifilms, the structural quality of aluminum castings are measured almost always in a destructive manner. Structural defects, i.e., pores and oxide bifilms, significantly degrade mechanical properties. They cause premature fracture in tension[2] and fatigue,[3] which results in low ductility, tensile strength, and fatigue life.[4] Moreover, the presence of major structural defects increases variability in mechanical properties.[5,6] To resolve quality concerns in aluminum castings, such as low ductility, a method that has been tried often is changing the heat treatment procedure, assuming that ductility can be increased by trading off strength (an example is provided by Drouzy et al.[7] on p. 49). These efforts usually are ineffective unless the root cause of low ductility, i.e., structural defects, is addressed. Therefore, when oxide bifilms and porosity are present in castings, it is at best inefficient and at worst fruitless to approach the problem from a ductility–strength compromise point of view. Foundry engineers need a metric to measure structural quality as well as how improvements in the process design and/or melt quality are reflected in the MURAT TIRYAKIOG˘LU, University Professor of Engineering, is with the Department of Engineering, Robert Morris University, Moon Township, PA 15108. Contact e-mail: [email protected] JOHN CAMPBELL, Professor Emeritus of Casting Technology, is with the Department of Materials and Metallurgy, University of Birmingham, Edgbaston B15 2TT, UK. NIKOLAOS D. ALEXOPOULOS, Assistant Professor, is with the Department of Financial and Management Engineering, University of the Aegean, 821 00 Chios, Greece. This article is based on a presentation given in the ‘‘3rd Shape Casting Symposium,’’ which occurred during the TMS Spring Meeting in San Francisco, CA, February 15–19, 2009, under the auspices of TMS, the TMS Light Metals Division, the TMS Solidification Committee, and the TMS Aluminum Processing Committee. Article published online October 2, 2009. 802—VOLUME 40B, DECEMBER 2009
mechanical properties and, therefore, final quality of the castings. The so-called quality indices developed in the past are intended to serve this need. This study builds on a recent review,[8] critically evaluates the quality indices developed in the past, and assesses the advantages and disadvantages of each of them.
II.
QUALITY INDICES IN THE LITERATURE
Traditionally tensile properties, occasionally accompanied by some nondestructive testing, have been used to evaluate the structural quality of cast Al alloys. Although stress–strain data usually are observed t
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