Nondestructive Quantitative Evaluation of Porosity Volume Distribution in Aluminum Alloy Die Castings by Fractal Analysi
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h-pressure die casting is used to fabricate components of automotive and electrical appliances as well as building materials, because of its capability of near-netshape manufacturing of complex shapes with a high production rate. In the manufacturing process, the formation of pores in the die castings is inevitable. Porosity adversely affects the pressure tightness and mechanical properties of components.[1–4] Pores are primarily caused by solidification shrinkage and by gas entrapment.[5–8] To eliminate pores in components, when the new design of die is used and process control is determined, it is absolutely critical to accurately identify the predominant cause of porosity so that die casters can take corrective action.[9] In practice, however, it is frequently difficult to identify the cause of porosity accurately only from observing die castings, because most pores result from a combination of these effects.[5,6,8] At present, the identification of porosity at the foundry floor is conducted on the basis of the appearance of pores, which requires sectioning and polishing of the domains of interest. However, such YOSHIHIKO HANGAI, Assistant Professor, and SHOTA MARUHASHI, Undergraduate Student, are with the Faculty of Engineering, Gunma University, Kiryuu 376-8515, Japan. Contact e-mail: [email protected] SOICHIRO KITAHARA, Chief Executive, is with Gundai Co., Ltd., Isesaki 372-0854, Japan. OSAMU KUWAZURU, Assistant Professor, is with the Faculty of Engineering, University of Fukui, Fukui 910-8507, Japan. NOBUHIRO YOSHIKAWA, Professor, is with the Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan. Manuscript submitted May 28, 2009. Article published online October 2, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
qualitative judgments made by humans are prone to error, and many trial-and-error processes are necessary until die casters can successfully reduce the formation of pores. In this study, the quantitative evaluation of pores is proposed for the accurate identification of the predominant cause of porosity. This evaluation can be conducted by a convenient nondestructive technique such as X-ray computed tomography (CT). X-ray CT allows us to observe the components of interest three-dimensionally, and thus is section independent. Also, it does not require time-consuming sectioning and polishing of the components of interest to observe the pores. If such an evaluation can be performed easily at the foundry floor to identify the predominant cause of porosity, it is expected that the burden of die casters will be substantially reduced. Also, quantitative evaluation based on nondestructive inspection opens the possibility of automated 100 pct inspection on a production line with a high detection rate of die castings using a computeraided system. Additionally, the morphology of pores may strongly affect the mechanical properties of components. It is of interest to develop such a quantitative technique to accurately quantify the correlations between morphology and mechanical properties. So
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