Quantification of Entrainment Damage in A356 Aluminum Alloy Castings
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ODUCTION
ALUMINUM alloy castings have been used in automotive and aerospace applications for many decades, mainly due to their good combination of mechanical properties and high strength-to-density ratio. However, the use of castings in safety critical applications has been limited because of their propensity to contain structural defects, mainly pores and bifilm inclusions.[1] These structural defects reduce fatigue life[2–8] and tensile properties, such as tensile strength and elongation.[9–15] Because most aluminum castings have an abundance of pores, some researchers have stated that pores are intrinsic defects[16,17] and therefore cannot be eliminated in a casting. However, recent research[18–21] has shown that bifilms are the root cause for pores in aluminum castings. It has been demonstrated through calculations[20] that it is impossible for a pore to nucleate either homogeneously or heterogeneously in solidifying aluminum. The only mechanism available for pore formation is, therefore, bifilms entrained into the casting ‘‘opening up’’ as a result of negative pressures due to solidification and/or diffusion of hydrogen into the bifilms.[20] Hence, the traditionally MURAT TIRYAKIOG˘LU, PEDRAM YOUSEFIAN, and PAUL D. EASON are with the School of Engineering, University of North Florida, Jacksonville, FL 32224. Contact e-mail: [email protected] Manuscript submitted June 18, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
believed nucleation stage is completely bypassed when bifilms are present in aluminum castings, and pores grow freely, consistent with in situ observations summarized in Reference 20. Therefore, it is of the utmost importance to characterize the formation of entrainment defects, such as oxide bifilms during filling of castings. This study is motivated by this need.
II.
BACKGROUND
Entrainment defects form as a result of surface turbulence during handling of melts and/or mold filling, in which a continuous surface oxide film is folded over and subsequently immersed in the bulk of the melt.[18,22,23] Because the dry, or atmospheric, sides of the oxide film are in contact after entrainment and there is often air between the two layers, there is no bonding between the two sides of the defect, i.e., the bifilm. Campbell[22] showed that there is a critical velocity, vc, above which surface turbulence will occur in unconstrained flows. This critical velocity can be estimated from[24]; 14 rg ; ½1 vc ¼ 2 q where g is gravitational acceleration, r is surface tension, and q is density of liquid metal. For liquid aluminum at melting temperature, r = 0.914 J/m2 and q = 2385 kg/m3.[25] Campbell[22,24] also demonstrated
that due to the fourth power relation, all metals practically have the same critical velocity between 0.35 and 0.5 m/s. For aluminum, Eq. [1] yields a critical velocity close to 0.5 m/s. It is noteworthy that under gravity, a fall of only 12 mm is required to reach this velocity. Runyoro et al.[26] investigated how the three-point bending strength of pure aluminum plate castings changed with i
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