Pore Formation During Solidification of Aluminum: Reconciliation of Experimental Observations, Modeling Assumptions, and
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ALUMINUM castings have been used in automotive and aerospace applications for their specific weight which helps with fuel efficiency. However, the mechanical properties and performance of aluminum castings are strongly affected by structural defects, such as pores and entrained oxide films,[1] which degrade tensile strength[2–5] and elongation[5–13] as well as fatigue life.[14–19] Moreover, pores can lead to rejection of the aluminum castings during final non-destructive inspection, such as X-ray. Moreover, pores have been observed[20] in situ to initiate hot tears, which are common in high strength cast aluminum alloys, such as the Al-Cu system. Therefore, understanding pore formation is paramount to lowering production costs, increasing their quality and performance, and consequently their wider use. It is commonly assumed that pores nucleate in the last stages of solidification,[21] by shrinkage and/or rejection of dissolved gas by the solidifying metal. Consequently, pores have been characterized based on their appearance on micrographs, as either shrinkage or gas pores. Two PEDRAM YOUSEFIAN and MURAT TIRYAKIOG˘LU are with the School of Engineering, University of North Florida, Jacksonville, FL 32224. Contact e-mail: [email protected] Manuscript submitted June 22, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
examples are presented in Figure 1,[22] where the pore presented in Figure 1(a) has a tortuous shape because it is surrounded by the tips of dendrites and as a result, is usually interpreted as a shrinkage pore. The pore in Figure 1(b), however, is circular, and consequently, is interpreted as a gas pore. This interpretation is generally but not necessarily correct, as will be addressed later. Because most aluminum castings have an abundance of pores, some researchers have stated that pores are intrinsic defects[23,24] and therefore cannot be eliminated. Whether pores are indeed intrinsic defects is addressed in this study, by reviewing and reinterpreting previous results of pore formation observations, common assumptions made in pore formation models, and the physics of pore formation. The differences between observations and the physics of pore nucleation are addressed via a review of the literature, and a mechanism that bridges the gap between physics and observations is discussed.
II.
A REVIEW OF IN SITU OBSERVATIONS OF PORE FORMATION
Researchers have used a variety of methods to investigate pore formation during solidification, including
Fig. 1—Pores in a low-pressure die cast A356 engine block, commonly interpreted as a (a) shrinkage, and (b) gas pore (From ***Ref. [22] with permission).
metallography of samples from castings after solidifi-
cation,
metallography of castings quenched while partially
solidified,[25] in situ observation pores formed in transparent organic materials[26] with low melting points which behave similarly to metals, such as cyclohexane[27] and succinonitrile–acetone[20] with isothermal[28,29] and gradient[30,31] microscope stages in situ observation of p
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