A new quality index for characterizing aluminum cast alloys with regard to aircraft structure design requirements
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I.
INTRODUCTION
IN recent years, precision casting of aluminum components has found considerable attention in aeronautical applications due to the cost efficiency of the produced components. For improving the competitiveness of cast components against components manufactured from wrought aluminum alloys, several efforts have been undertaken or are still in progress, e.g., References 1 through 3. The investigations focus on the improvement of the existing casting techniques aiming to higher mechanical properties and lower scatter, the development of advanced near-net shape aluminum casting processes to reduce the component manufacturing costs, as well as the optimization of the aluminum cast alloys in order to increase tensile ductility and fracture toughness, without essential drawbacks on tensile and yield strength. To facilitate the assessment of the effects of changes in chemical composition, solidification conditions, and heat treatment on the quality of Al-Si-Mg cast aluminum alloys, an empirical, quantitative quality index Q, in conjunction with a relation to assess the expected probable yield strength of the alloy, has been introduced by Drouzy et al. in 1980.[4] The index Q balances the properties ultimate tensile strength and elongation to fracture in one expression, which is given by Q ⫽ Rm ⫹ d # log (Af)
[1]
In Eq. [1], Rm stands for the ultimate tensile strength in MPa, Af stands for the elongation to fracture in pct, and d is an empirical coefficient in MPa chosen such as to make Q practically independent of the aging condition. For the investigated Al-7Si-Mg, the coefficient d has been determined to be 150 MPa. The probable yield strength Rp of an Al-Si-Mg alloy may be assessed by the equation Rp ⫽ a # Rm ⫺ b # log (Af) ⫹ c
[2]
where a, b, and c are alloy depended, empirically determined coefficients. In a diagram of the ultimate tensile strength vs
N.D. ALEXOPOULOS, Dipl. Mech. Eng., Ph.D. Candidate, and S.G. PANTELAKIS, Professor, are with the Laboratory of Technology and Strength of Materials, Department of Mechanical and Aeronautical Engineering, University of Patras, 26500 Patras, Greece. Contact e-mail: pantelak @mech.upatras.gr or [email protected] Manuscript submitted June 30, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
the logarithm of the elongation to fracture, Eq. [1] and [2] represent sets of parallel lines called “iso-quality index” and “iso-yield strength” lines, respectively; they fit the experimentally obtained Q and Rp values resulting from variations in chemical composition, solidification conditions, and heat treatment of Al-Si-Mg aluminum alloys with a good approximation. Hence, the preceding diagram called “quality index chart” provides a very useful tool to reduce the experimental effort for developing or optimizing Al-Si-Mg cast alloys essentially. The quality index Q has been widely accepted due to its simplicity and has been involved in a variety of situations, e.g., (References 1, 3, and 5 through 7). Recently, in Reference 8, the iso-quality index and iso-y
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