ORIGINAL ARTICLE

Bifilm defects and porosity in Al cast alloys

M. A. El-Sayed1& Hany Hassanin2

& Khamis Essa3

Received: 4 September 2015 /Accepted: 14 December 2015 /Published online: 5 January 2016# The Author(s) 2016. This article is published with open access at Springerlink.com

Abstract Liquid Al and Mg-base alloys are so reactive that itis reasonable to assume that the surface layer is always oxi-dized. If liquid aluminium entered a mould cavity with a ve-locity greater than a critical value, the surface skin of the liquidmetal would fold over onto itself and be submerged into thebulk liquid with a volume of air entrapped within it, creatingwhat is called a bifilm defect. This defect not only acts as acrack but also it is recognized to initiate hydrogen porosity inthe solidified casting, which has been found to have detrimen-tal effects on the tensile and fatigue properties of the castingsproduced. Previous research suggested that during solidifica-tion, the hydrogen, in excess of the solubility limit, comes outof the solution and diffuses into the bifilm gap, expanding itinto a pore. Also, placing liquid metal in a vacuum may causeits entrained bifilms to expand, enhancing their buoyancy andtherefore their floatation to the surface of the melt. In thiswork, a casting from an A356 Al alloy was allowed to solidifyunder vacuum. The solidified casting was sectioned into twohalves, and the internal surfaces of the pores were investigated

using an SEM to determine their relationship with doubleoxide film defects.

Keywords Entrained inclusions . Aluminium casting .

Porosity . Hydrogen

1 Introduction

The transfer of liquid Al alloys and the action of pouring into amould are often accompanied by entrainment of the oxidizedsurface skin of the alloy. The very high reactivity of liquid Alwith oxygen in the air means that the surface of the liquidmetal can be considered to be permanently coated with anoxide layer. Therefore, as such layer is ruptured by movementof the liquid metal underneath, the rupture must be almostinstantaneously sealed by the reaction between the Al andoxygen. Pouring or transfer of the liquid metal results in con-siderable splashing of the liquid metal (which has a viscositysimilar to that of water), and as the splashes coalesce andmerge with the bulk liquid, a doubled-over oxide film “abifilm” can be carried into the liquid metal. The sides of thebifilm defect are unbonded and would capture a portion of thelocal atmosphere to be carried into the melt, fromwhere it maybe swept into the mould cavity (see Fig. 1) [1].

Bifilm defects therefore consist of a crevice in the solidmetal, formed by the unbonded oxide films, and contained alayer of gas. They form a defect in the final solidified castingof variable size, and with variable orientation with respect toany applied load experienced by the casting in service. It istherefore thought that bifilms play a major role in influencingthe variability of mechanical properties in Al and Mg alloys[2].

Dispinar and Campbell [3] used a reduced pressure test(RPT) to expand the internal atmosphere of bifilm defects

* Khamis Essak.e.a.essa@bham.ac.uk

M. A. El-Sayedm_elsayed@aast.edu

Hany Hassaninh.hassanin@kingston.ac.uk

1 Department of Industrial and Management Engineering, ArabAcademy for Science and Technology and Maritime Transport, POBox 1029, Abu Qir, Alexandria 21599, Egypt

2 School of Mechanical and Automotive Engineering, KingstonUniversity, London, UK

3 School of Mechanical Engineering, University of Birmingham,Birmingham, UK

Int J Adv Manuf Technol (2016) 86:1173–1179DOI 10.1007/s00170-015-8240-6

during solidification of Al-7Si-0.4Mg alloy. The authors com-pared two castings (with different amounts of bifilms intro-duced by different pouring techniques) that were solidifiedunder different reduced pressures and then examined usingx-ray radiography. It was noticed that when following an ap-propriate casting practice that tended to minimize the possi-bility of bifilm entrainment, the amount of pores in the castingwould be a minimum. In contrast for careless casting proce-dures that allowed the introduction of double oxide films tothe casting, the amount of pores would be much higher. Inboth cases, lowering the pressure under which the castingswere solidified resulted in an enlargement of the pores.

The authors argued that bifilms would work as ini-tiators for the gas porosity in Al castings while thehydrogen acted only as a contributor to the porosityformation process [3]. They suggested that, during so-lidification, the hydrogen, in excess of the solubilitylimit, comes out of solution and diffuses into thebifilm gap, expanding it into a pore. Therefore, asmore bifilms are incorporated into the melt duringpouring and handling, more pores would be expectedin the solidified casting. The application of vacuumduring solidification helped the pores to expand furtherbecoming more visible in the x-ray image. However, inthis study, the authors did not present an evidence tosupport their suggestion that all the pores observedcame from oxide films.

Recent researches by Griffiths and Raiszadeh [4], El-Sayedet al. [5–7], and El-Sayed and Griffiths [8] have suggested thatH dissolved in an Al melt could diffuse into its entrainedbifilms, increasing their size and forming oxide-related hydro-gen containing porosity, which was found to decrease theWeibull moduli of the tensile properties of castings [8].

The aim of this work was to study the effect the solidifica-tion of Al melt under vacuum on entrained bifilms and to

examine the role played by such defects in initiating porosityin Al casting. The research would allow a more detailed un-derstanding of the behaviour of bifilm defects in commercialalloys and might point towards methods of eliminating thedefects once they are formed.

2 Experimental work

In order to determine the effect of holding an Al alloy meltunder a vacuum on the entrained bifilm inclusions, about 6 kgof A356 alloy, with a chemical composition given in Table 1,was melted in an induction furnace. The liquid metal was thenstirred in an induction furnace (using a power setting of7.5 kW and frequency of 2350 Hz, for 1 min) to create newoxide films and introduce them into the melt, which mightalready contain some old oxide films from the chargematerial.

The crucible containing the melt was then placed in thevacuum chamber which had been initially preheated to800 °C, and the chamber internal pressure decreased to0.2 bar. Subsequently, the heaters of the chamber were turnedoff to allow the melt to solidify to the room temperature underthe reduced pressure. Finally, the chamber was returned toatmospheric pressure and the solidified ingot removed, cutinto two halves and the pores on the machined surface of eachhalf investigated using SEM.

3 Results

Holding the melt under vacuum would be expected to causethe expansion of any atmosphere within the entrained bifilms.In this experiment, the holding process was interrupted andthe liquidmetal was allowed to solidify under vacuum in orderto determine the nature of the porosity inside the casting.

Figure 2a shows a pore, with an average diameter of1.5 mm, on the machined surface of one half of the casting.Another pore was found at the same location on the surface ofthe other half of the casting, which is shown in Fig. 2b.Figure 3a–d shows the results of the energy dispersive x-ray(EDX) analysis that revealed the presence of oxide inside thepores and confirmed its identity as MgAl2O4 spinel.

Fig. 1 Sketch of the formation of an entrained double oxide film defect

Table 1 Chemical composition of A356 alloy

Element Si Fe Cu Mg Al

Percentage 7.1 0.07 0.15 0.4 Bal.

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Another example of a pore containing fragments of oxidesthat was found lying upon the dendrite tips has been illustratedin Fig. 4a, confirmed to be oxide by EDX analysis, as shownin the corresponding EDX spectrum. Figure 4b shows a SEMimage of an oxide film that was found to cover almost all theentire surface of a pore supporting the suggestion that porositywould be formed by expansion of double oxide film defects. Itshould be noted that the presence of a relatively thick oxidelayer inside a pore, such as in Fig. 4a, could be suggestive ofthe presence of an old bifilm defect coming from the materialcharge rather than a new bifilm entrained during the stirring

action. The thickness of the oxide film may perhaps be anindication that it had relatively long time to absorb some Mginto its structure by diffusion. This was also suggested by therelatively stronger Mg peak in the corresponding EDXspectrum.

For most of the pores analysed, a single oxide film wasdetected inside each pore, as shown in Fig. 4. However, in afew cases, many oxide fragments were found dispersed overthe inside of the pore, as shown in Fig. 5a. Figure 5b, c, showsthe EDX analysis of the oxide fragments detected inside thepore. This could be a suggestion of a bifilm that was expanded

Fig. 2 Two pores found atsymmetrical locations on the twoopposite sides of the casting

Fig. 3 a, b EDX spectra of the fragments shown in Fig. 2a at locations X1 and X2, respectively. c, d EDX spectra of the fragments shown in Fig. 2b atlocations X1 and X2, respectively

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and then torn apart leaving such oxide fragments within thepore.

Figure 6 shows SEM images for two pores on themachined surfaces of the casting. No oxides were visi-ble on the internal surface of the pore, and the EDXspectrum taken at different locations within the pore didnot reveal any oxygen peaks, suggesting this pore maynot have originated from an inflated bifilm. Investiga-tion of porosity in this experiment confirmed a relation-ship for most, but perhaps not all, pores with oxides.This suggests that a possible cause for the formation ofporosity in this Al alloy casting was bifilm defects thatwere expanded due to the application of vacuum duringsolidification.

4 Discussion

It was suggested by Fox and Campbell [9] and Dispinarand Campbell [3] that placing liquid metal under vacu-um may cause its entrained bifilms to expand to formpores. In both works, the authors presented x-ray im-ages of the solidified castings that showed a significantincrease in the size of porosity associated with the

reduction in the pressure. However, no EDX analysiswas carried out inside these pores to verify thishypothesis.

SEM images (presented in Figs. 2, 4, and 5) showedmany fragments of oxides inside pores in the castingsolidified under vacuum, which were confirmed byEDX analysis to be MgAl2O4. This suggests that theobserved pores were initially bifilm defects that expand-ed due to the application of a vacuum and the decreasedmelt pressure around them, which might result in tear-ing the films apart.

If the liquid aluminium was held under vacuum for asufficient time, this might allow double oxide film de-fects to expand and float to the melt surface [10, 11]. Inthe experiment performed here, the holding treatmentwas suspended at an intermediate stage to trap somepores during their presumed journey to the surface ofthe melt, which were subsequently analysed for evi-dence of oxide films.

However, SEM images (presented in Fig. 6) alsoshowed a pore that did not reveal oxygen by EDXanalysis and therefore may not contain any oxides. Thiscould be because a bifilm that was also expanded undervacuum to form a pore. This pore was then torn openunder the reduced pressure around it allowing fresh melt

Fig. 4 SEM images with the corresponding EDX analysis of spinel inside pores on the machined surfaces of a casting of A356 Al alloy solidified undervacuum

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to seal its entire atmosphere and reform a thin oxide layer(about 20 nm), which was undetectable by EDX analysis.

The significance of this experiment was that since most ofthe pores were confirmed to be originated from bifilm defects,

Fig. 5 a An SEM image of oxidefragments within a pore on themachined surface of a casting ofA356 Al alloy solidified undervacuum. b, c EDX analysis atlocations X1 and X2, respectively

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Fig. 6 aAn SEM image of a poreon the machined surface of acasting of A356 Al alloysolidified under vacuum, b and cEDX analysis at locations shownin a (named spectrum 2 andspectrum 1, respectively)

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it is rational to suggest that following proper casting practice(intended to minimize melt surface turbulence during mouldfilling) would lead to the elimination of bifilm entrainmentand associated porosity formation. Doing this would allowAl casting producers to obtain substantially defect-free cast-ings at reasonable cost that might be more reliable than manyaerospace forgings.

5 Conclusions

1. SEM investigation detected many spinel fragments insideporosity in the casting solidified under vacuum, suggest-ing that such pores were initially bifilm defects that ex-panded due to the application of the vacuum and then tornapart leaving only some oxide bits within the pores.

2. Entrained bifilms not only can cause premature failure byacting as cracks in the Al casting but also have been as-sociated with other defects, such as hydrogen porosity.

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