Role of Double Oxide Film Defects in the Formation of Gas Porosity in Commercial Purity and Sr-containing Al Alloys

doi:10.1016/j.jmst.2013.09.001

Journal of Materials Science & Technology

Volume 30, Issue 2, February 2014, Pages 154-162

https://doi.org/10.1016/j.jmst.2013.09.001 Get rights and content

The role of double oxide film (bifilm) defects in the formation of gas porosity in commercial purity and Sr-containing Al alloys was investigated by means of a reduced pressure test (RPT) technique. The liquid metal was poured from a height into a crucible to introduce oxide defects into the melt. The melt was then subjected to different “hydrogen addition” and “holding in liquid state” regimes before RPT samples were taken. The RPT samples were then characterized by determining their porosity parameters and examining the internal surfaces of the pores formed in them by scanning electron microscopy. The results indicated oxide defects as the initiation sites for the growth of gas porosity, both in commercial purity and Sr-containing Al alloys. The results also rejected reduction of the surface tension of the melt, increase in the volumetric shrinkage and reduction in interdendritic feeding as the possible causes of an increase in the porosity content of the Al castings modified with strontium. The change in the composition of the oxide layers of double oxide film defects was suggested to be responsible for this behaviour.

Introduction

Porosity is considered to be one of the major factors responsible for the failure of cast aluminium alloy products^[1]. Two major effects are well recognized as contributing to the formation of porosity in solidifying metals, and they are: 1) shrinkage of metal during solidification, and 2) gas evolution resulted from a decrease in solubility in the solid metal compared to the liquid^[2]. These phenomena can occur simultaneously and act synergistically to develop porosity in solidifying metal. A pore, however, as a new separate phase in the liquid metal, principally needs to nucleate before it can grow.

Campbell^[3] calculated the pressure required for the homogeneous nucleation of a H bubble in liquid aluminium melt; it is extremely high (i.e., about 31,000 atm (3.14 × 10⁹ Pa)), and certainly not attainable. For heterogeneous nucleation, this pressure was estimated to be about 360 atm (36.5 × 10⁶ Pa), which is a pressure highly unlikely to be attained in liquid Al alloys.

Campbell^[3] suggested that the existence of crack-like double oxide film (bifilm) defects in the liquid Al would eliminate the need for the nucleation stage to occur during the formation of a H bubble, and hence, greatly facilitate its formation. This defect, which consists of two oxide layers and a volume of gas (presumably predominantly air) trapped between them, can be produced when the surface of the metal folds upon itself and becomes submerged in the bulk liquid. The defect, therefore, necessarily resembles and acts as a crack in the liquid metal. More information about the double oxide film defects can be found in literature[3], [4], [5], [6], [7], [8], [9].

Raiszadeh and Griffiths[4], [5] showed that H atoms can penetrate into the trap atmosphere within a double oxide film defect through the cracks that formed on the oxide layers when the defect is subjected to deformations in the liquid flow, and expand the defect to a gas bubble. Dispinar and Campbell^[1] provided some evidence that porosity cannot form in reduced pressure test (RPT) samples of a clean melt (presumably double oxide film free), even when the H level of the melt is increased to the saturation point (i.e., about 0.6 mL/100 g Al).

The formation of bonding between the two layers of a double oxide film defect and the elimination of its deleterious effect as a porosity initiator was first suggested by Nyahumwa et al.[10], [11], and was recently confirmed by this research team[6], [7], [8], [12]. The two criteria for the formation of bonding between the two layers of an oxide film defect determined in these studies to be: 1) complete consumption of O and N within the defect through the reaction with the surrounding melt, and 2) the occurrence of a transformation involving the rearrangement of atoms on the internal surfaces of the oxide layers. The two layers of an oxide film defect were observed to begin bonding to each other after being held in commercial purity Al melt for 5 h^[8]. This was attributed to the transformation of γ- to α-Al₂O₃. The bonding also occurred between the layers of the defect during holding in Al–0.3 wt% Mg melt^[6]. This time, the bonding was suggested to occur as a result of the transformation of Al₂O₃ to spinel (MgAl₂O₄) and spinel to MgO in short and long holding time, respectively.

Campbell's hypothesis about the role of double oxide film defects as initiators of the growth of gas porosity, however, requires more systematic investigation for support. This was the aim of the work reported here. In this research, the role of double oxide film defects in the formation of gas porosity in commercial purity and Al–0.05 wt% Sr alloys has been investigated by means of a reduced pressure test (RPT) technique. This technique is widely known and already used throughout the industry, usually to assess H content of the melt. The test has been used recently for the evaluation of double oxide film content of liquid Al melts. The test consists of solidification under reduced pressure that serves to expand the entrapped gas between the halves of the oxide defects, rendering the defects more visible[1], [13], [14], [15].

Sr-containing Al alloy has also been selected for investigation besides commercial purity Al melt, since this element is known to increase the porosity content of hypoeutectic Al–Si alloys when it is added to these alloys as a Si eutectic modifying agent. A tremendous amount of work has been carried out to study this effect^[16], with no agreement achieved among the researchers. The main causes of an increase in porosity in Sr-treated Al–Si alloys reported in literature include:

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an increase in both the inclusion content of the melt and the amount of H absorbed into the oxides;
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an increase in the H content of the melt;
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a reduction of the surface tension of the liquid;
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an increase in the volumetric shrinkage;
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a reduction in interdendritic feeding;
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an increase in the time for H diffusion and porosity growth due to an increase in the freezing range by depressing the eutectic solidification temperature.

The influence of adding 0.05 wt% Sr to commercial purity Al melt on the behaviour of double oxide film defects was investigated recently by Nateghian et al.^[12]. Their results showed that in the presence of Sr, the A1₂O₃ layers of a newly formed oxide defect started to transform to SrO gradually from the moment that the layers submerged into the melt, through the replacement of Al atoms by Sr. The rate of transformation in the oxide layers was relatively high during the first minutes after the entrainment of the defect such that 20% of the transformation took about 12 min, while the whole transformation took about 50 h to complete. It was suggested that since the composition of the oxide layers of a double oxide film defect submerged in Sr-treated melt is different from that of pure Al, the mechanical properties and the behaviour of the defect in these two melts would also be significantly different.

Section snippets

Experimental

Commercial purity Al melt of 3.5 kg, with the composition shown in Table 1, was prepared in an electric furnace and then poured at a temperature of 750 °C from a height of 500 mm into a second crucible with a pouring rate of 0.4 kg s⁻¹ to introduce double oxide film defects into the melt. RPT samples were taken from the melt using a stainless steel conical mould of 60 mm in height, with the radii of the small and large bases 35 and 45 mm, respectively. The samples solidified at the reduced

Results

The results obtained from the experiments indicated that among the porosity parameters, only the total porosity area and the Bifilm Index could be used to indicate the level of double oxide film defects present in the melt. The results obtained for the number of pores and the mean pore roundness were scattered and did not follow any meaningful trends. This was probably due to the movement of the inflated oxide defects in the RPT samples and the possibility of two or more defects joining each

Discussion

Raiszadeh and Griffiths^[18] studied the effect of holding an Al melt containing double oxide film defects under 1000 and 80 mbar (100 × 10³ and 8 × 10³ Pa) for extended periods of time up to 60 min. Their results suggested that the number of oxide film defects in the melt would gradually decrease due to their continuous migration towards the upper surface, and that holding the melt under a vacuum would increase this removal rate due to enhanced floatation. In the present study, therefore, it

Conclusions

(1)
Increasing the H content of the melt before holding it for 24 h eliminated all the porosity from the cross-section of the RPT samples, in both commercial purity and Al–0.05 wt% Sr alloys. It is suggested that this procedure caused all the oxide film defects to be removed from the melt by expansion and floatation to the melt surface. This observation supports the proposition of double oxide film defects as the initiators of the gas pores in Al melts.
(2)
The lack of formation of pores at the

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Role of Double Oxide Film Defects in the Formation of Gas Porosity in Commercial Purity and Sr-containing Al Alloys

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

Scr. Mater.

Int. J. Cast Met. Res.

AFS Trans.

Complete Casting Handbook

Metall. Mater. Trans. B

Metall. Mater. Trans. B

J. Mater. Sci.

J. Mater. Sci.

Metall. Mater. Trans. B

J. Mater. Eng. Perform.

AFS Trans.

J. Mech. Behav. Mater.