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shown that in commercial aluminum castings the fatigue life is controlled primarily by the size of the largest pore, or sometimes by the largest oxide film. Significant improve- ments in fatigue life appear to be possible with improved melt treatment and casting practices.


Solidification rate has an overwhelming effect on the amount and distribution of porosity in a casting, and a significant ef- fect on pore size. This is the primary reason why the fatigue and tensile properties of aluminum alloy castings depend so strongly on freezing rate. Proper grain refinement and modi- fication also improve mechanical properties.


There are a number of recent casting process developments that have the potential to deliver castings of consistently higher quality. Particularly noteworthy are improved melt treatment practices: rotary impeller degassing, metal filtra- tion and use of fluxes to remove oxides. Level pour metal transfer and improved mold filling practices are also being employed. Some casting processes apply pressure to so- lidifying castings to reduce porosity. An innovative ‘abla- tion’ process applies water sprays to the semi-solid casting to achieve a rapid, highly directional solidification in sand castings. In this review we have not considered these devel- opments, but instead focused on providing a scientific basis for understanding quality. It is the author’s hope that this understanding will help foundrymen and casting users to im- prove quality in commercial practice.


It should be noted that the focus here has been the deleterious effect of defects on casting quality; with the objective of better understanding the melt treatment, metal handling and casting procedures that must be undertaken to produce the highest qual- ity castings. In other words, this analysis was primarily intend- ed to develop a strong scientific case for improving our foundry practices. Our intent was not to provide an in-depth analysis of the failure mechanisms of metals, or of metal fatigue. The analyses presented were by necessity somewhat simplified. For example, it should be noted that others have taken a different approach to establishing a scientific basis for quality.44


It is pos-


sible that one of these approaches may eventually provide a better understanding of metal failure mechanisms, and how we might reach an ‘ultimate’ level of casting quality.


Finally, the use of Weibull statistics appears to be an ex- tremely powerful and valuable tool to help us evaluate the quality of castings produced by a specific combination of foundry practices and mold design. Further studies along these lines are encouraged.


REFERENCES


1. Miguelucci, E.W. “The Aluminum Association Cast Alloy Test Program: Interim Report,” AFS Transactions, vol. 93, pp. 913-916 (1985).


2. “Special Report on the Mechanical Properties of Permanent Mold Aluminum Alloy Test Castings,”


International Journal of Metalcasting/Winter 11


Jobbing Foundry Division of the Aluminum Association, Washington, D.C., © November, 1990.


3. Caceres, C.H. and Wang, Q.G. “Dendrite Cell Size and Ductility of Al-Si-Mg Casting Alloys: Spear and Gardner Revisited,” Int. J. Cast Metals Research, vol. 9, pp. 157-162 (1996).


4. Whaler, K. R., Stahl Specialty Company, Kingsville, Missouri, private communication (1995).


5. Drouzy, M., Jacob, S., and Richard, M., “Interpretation of Tensile Results by Means of a Quality Index,” AFS International Cast Metals Journal, vol. 5, pp. 43-50 (1980).


6. Sigworth, G.K. and Kuhn, T.A. “Use of ‘Standard’ Molds to Evaluate Metal Quality and Alloy Properties,” AFS Transactions, vol. 117, pp. 55-62 (2009).


7. Fang, Q.T. and Granger, D.A. “Porosity Formation in Modified and Unmodified A356 Alloy Castings,” AFS Transactions, vol. 97, pp. 989-1000 (1989).


8. McLellan, D.L. “Modelling Microstructural Characteristics of Al-Si-Mg Castings to Develop Product Assurance,” AFS Transactions, vol. 90. pp. 173-191 (1982).


9. Vorren, O., Evenson, J.E., and Pedersen, T.B. “Microstructure and Mechanical Properties of Al- Si(Mg) Casting Alloys,” AFS Transactions, vol. 92, pp. 459-466 (1984).


10. Oswalt, K.J. and Misra. M.S., „Dendrite Arm Spacing (DAS): A Nondestructive Test to Evaluate Tensile Properties of Premium Quality Aluminum Alloy (Al- Si-Mg) Castings,“ AFS Transactions, vol. 88, pp. 845- 862 (1980).


11. Cáceres, C.H., “A Rational for the Quality Index of Al- Si-Mg Casting Alloys,” Int. J. Cast Metals Research, 10 (1998), pp. 293-299.


12. Cáceres, C.H., “A Phenomenological Approach to the Quality Index,” Int. J. Cast Metals Research, vol. 12, pp. 367-375 (2000).


13. Edelson, B.I. and Baldwin, Jr., W.M. “The Effect of Second Phases on the Mechanical Properties of Alloys,” Trans. ASM, 55 (1962), pp. 230-250.


14. Cáceres, C.H. and Selling, B.I. “Casting Defects and Tensile Properties of an Al-Si-Mg Alloy,” Mat. Sci. Eng., vol. A220, pp. 109-116 (1996).


15. Sigworth, G.K. and Caceres, C.H. “Quality Issues in Aluminum Net Shape Castings,” AFS Transactions, vol. 112, pp. 373-386 (2004).


16. Wang, Q.G., Apelian, D., and Lados, D.A. “Fatigue Behavior of A356-T6 Aluminum Cast Alloys-Part 1. Effect of Casting Defects,” J. Light Metals, vol. 1, pp. 73-84 (2001).


17. Davidson, C.J., Griffiths, J.R., Badiali, M. and Zanada, A. “Fatigue Properties of a Semi-Solid Cast Al-7Si-0.3Mg-T6 Alloy,” Met. Sci. Tech., vol. 18(2), pp. 27-31 (2000).


18. Wang, Q.G., Crepeau, P.N., Griffiths, J.R., and Davidson, C.J. “The Effect of Oxide Films and Porosity on Fatigue of Cast Aluminum Alloys, Shape Casting: The John Campbell Symposium, 2005, pp. 205-214, The Metals Soc., Warrendale, PA © 2005.


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