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2.7 Evidence for bifilms
The evidence for bifilms actually constitutes the entire theme of this book. Nevertheless, it seems worthwhile to devote some time to highlight some of the more direct evidence.
It is important to bear in mind that the double oxide film defects are everywhere in metals. We are not describing occasional single ‘dross’ or ‘slag’
defects or other occasional accidental exogenous types of inclusions. The bifilm defect occurs naturally, and in copious amounts, every time a metal is poured. Many metals are crammed with bifilm defects. The fact that they are usually so thin has allowed them to evade detection for so long. Until recently, the ubiquitous presence of these very thin double films has not been widely accepted
Entrainment 65
because no single metal quality test has been able to resolve such thin but extensive defects.
Even so, over the years there have been many significant observations.
A clear example is seen in the use of the reduced pressure test for aluminium alloys. The technique is also known, with slight variations in operating procedure, as the Straube Pfeiffer test (Germany), Foseco Porotec test (UK) and IDECO test (Germany). At low gas contents many operators have been puzzled by the appearance of hairline cracks, often extending over the whole section of the test casting. They have problems in understanding the cracks since the test is commonly viewed as a check of hydrogen porosity. However, as gas content rises, the defects expand to become lens-shaped, and finally, if expansion continues, become completely spherical, fulfilling at last the expectation of their appearance as hydrogen pores.
The effect is almost exactly that shown in Figure 2.40. Such an effect has been widely observed by many foundry people many times. An example is presented by Rooy ( 1 992).
In a variant of the test to determine the quantity known as the density index, two small samples of a melt are solidified in thin-walled steel crucibles in air and under a partial vacuum respectively. A comparison of the densities of the samples solidified in air and vacuum gives the so-called density index.
However, in this simple form the quantity is not particularly reproducible. The comparison is complicated as a result of the development of shrinkage porosity in the sample solidified in air.
A better comparison is found by taking the lower half of the air-solidified sample and discarding the top half containing the shrinkage porosity. The sound base is then compared to the sample frozen under vacuum. This gives an unambiguous assessment of the porosity due to the combined effect of gas and bifilms. Without bifilms the hydrogen cannot precipitate, leading to a sound test casting, and giving the curious (and of course misleading) impression that the hydrogen can be ‘filtered’ out of liquid aluminium.
A recent novel development of the reduced pressure test has been made that allows direct observation of bifilms (Fox and Campbell 2000).
The rationale behind the use of this test is as follows.
The bifilms are normally impossible to see by X- ray radiography when solidified under 1 atmosphere pressure. If, however, the melt is subjected to a reduced pressure of only 0.1 atmosphere, the entrained layer of air should expand by ten times.
Under 0.01 atmosphere the layer should expand 100 times, etc. In this way it should be possible to see the entrained bifilms by radiography. A result is shown in Figure 2.46.
In this work a novel reduced pressure test machine was constructed so that tests could also be carried
Figure 2.46 Radiograph of reduced pressure test sump1e.s of as-melted AC7Si-O.4Mg alloy solidified under pressures from ( a ) I atmosphere and (b) 0.01 atmosphere (Fox and Campbell2000).
out using chemically bonded sand moulds to make test castings as small slabs with overall dimensions approximately 50 mm high, 40 mm wide and 15 mm thick. The parallel faces of the slabs allowed X-ray examination without further preparation.
Figure 2.46 shows radiographs of plate castings from a series of tests that were carried out on metal from a large gas-fired melting furnace in a commercial foundry. Figure 2.46a shows a sample that was solidified in air indicating evidence of fine-scale porosity appearing as dark, faint compact images of the order of 1 millimetre in diameter. At progressively lower test pressures the compact
‘pores’ unfold and grow into progressively longer and thicker streaks, finally reaching 10 to 15 mm in length at 0.01 atmosphere (Figure 2.46b).
The ‘streak-like’ appearance of the porosity is due to an edge-on view of an essentially planar defect (although residual creases of the original folds are still clear in some images). The fact that these defects are shown in such high contrast at the lowest test pressure suggests that they almost completely penetrate the full 15 mm thickness of the casting, and may only be limited in size by the 15 mm thickness of the test mould. The more extensive areas of lower density porosity are a result of defects lying at different angles to the major plane of the casting.
At 0.01 atmosphere the thickness of bifilms as measured on the radiographs for those defects lying in the line of sight of the radiation was in the range 0.1 to 0.5 mm. This indicates that the original thickness of bifilms at 1 atmosphere was approximately 1 to 5 pm.
These samples containing large bifilms are shown here for clarity. They contrast with more usual samples in which the bifilms appear to be often less than I mm in size, and are barely visible on radiographs at 1 atmosphere pressure. The work by Fox and Campbell (2000) on increasing the hydrogen content of such melts in the RPT at a constant reduced pressure typically reveals the inflation of clouds of bifilms, first becoming unfurled and slightly expanded by the internal pressure of hydrogen gas, and finally resulting in the complete inflation of the defects into expanded spheres at high hydrogen levels.
A much earlier result was so many years ahead of its time that it remained unappreciated until recently. In 1959 at Rolls-Royce, Mountford and Calvert observed the echoes of ultrasonic waves that they directed into liquid aluminium alloys held in a crucible. What appeared to be an entrapped layer of air was observed as a mirror-like reflection of ultrasound from floating debris. (Reflections from other fully wetted solid phases would not have been so clear; only a discontinuity like a crack, a layer of air, could have yielded such strong echoes.) Some larger particles could be seen to rotate, reflecting
like a beacon when turning face-on after each revolution. Immediately after stimng, the melts became opaque with a fog of particles. However, after a period of 10 to 20minutes the melt was seen to clear, with the debris forming a layer on the base of the crucible. If the melt was stirred again the phenomenon could be repeated.
Stirred melts were found to give castings containing oxide debris together with associated porosity. It is clear that the macroscopic pores observed on their polished sections appear to have grown from traces of micropores observable along the length of the immersed films. Melts that were allowed to settle and then carefully decanted from their sediment gave castings clear of porosity.
Other interesting features that were observed included the precipitation of higher-melting-point heavy phases, such as those containing iron and titanium, on to the floating oxides as the temperature was lowered. This caused the oxides to drop rapidly to the bottom of the crucible. Such precipitates were not easy to get back into suspension again.
However, they could be poured during the making of a casting if a determined effort was made to disturb the accumulated sludge from the bottom of the container. The resulting defects had a characteristic appearance of large, coarse crystals of the heavy intermetallic phase, together with entrained oxide films and associated porosity. These observations have been confirmed more recently by Cao and Campbell (2000) on other A1 alloys.
It is clear, therefore, from all that has been presented so far that a melt cannot be considered to be merely a liquid metal. In fact, the casting engineer must think of it as a slurry of various kinds of debris, mostly bifilms of various kinds, all with entrained layers of air or other gases.
In a definitive piece of research into the fatigue of filtered and unfiltered A1-7Si-0.4Mg alloy by Nyahumwa et al. (1998 and 2000), test bars were cast by a bottom-filling technique and were sectioned and examined by optical metallography. The filtered bars were relatively sound. However, for unfiltered castings, extensively tangled networks of oxide films were observed to be randomly distributed in almost all polished sections. Figure 2.5 shows an example of such a network of oxide films in which micropores (assumed to be residual air from the chaotic entrainment process) were frequently observed to be present. In these oxide film networks, it was observed that oxide film defects constitute cracks showing no bond developed across the oxideloxide interface. In the higher magnification view of Figure 2.5 the width between the two dry surfaces of folded oxide film is seen to vary between 1 and 10 pm, in confirmation of the low pressure test results described above. However, widths of cracks associated with pores were usually found to be substantially greater than 10 pm, in places
Entrainment 67 tube, a bubble trail constitutes a long bifilm of rather special form. The passage of air bubbles though aluminium alloy melts has been observed by video radiography (Divandari 1998). The bubble trail has been initially invisible on the video radiographic images. However, the prior solidification of the outer edges of the casting imposed a tensile stress on the interior of the casting that increased with time. At a critical stress the bifilm appeared. It flashed into view in a fraction of a second, expanding as a long crack, following the path taken by the bubbles, through what had appeared previously to be featureless solidifying metal.
The evidence for bifilms has been with us all for many years.
approaching 1 mm. Here the crack had opened sufficiently to be considered as a pore.
A polished section of a cast aluminium alloy breaking into a tangled bifilm is presented in Figure 2.6. The top part of the folded film comes close to the sectioned surface in some places, and has peeled away, revealing the inside surface of the underlying remaining half of the bifilm.
The detachment of the top halves of bifilms to reveal the underlying half is a technique used to find bifilms by Huang et al. (2000). They subjected polished surfaces of aluminium alloy castings to ultrasonic vibration in a water bath. Parts of bifilms that were attached only weakly were fatigued off, revealing strips or clouds of glinting marks and patches when observed by reflected light. They found that increasing the Si content of the alloys reduced the lengths of the strips and the size of the clouds, but increased the number of marks. The addition of 0.5 and 1 .O Mg reduced both the number and size of marks. Their fascinating polished sections of the portions of the bifilms that had detached revealed fragmentary remains of the double films of alumina apparently bonded together in extensive patches, appearing to be in the state of partially transforming to spinel (Figure 2.45).
The scanning electron microscope (SEM) has been a powerful tool that has revealed much detail of bifilms in recent years. One such example by Green (1995) is seen in Figure 2.11, revealing a film folded many times on the fracture surface of an A1-7Si-0.4Mg alloy casting. Its composition was confirmed by microanalysis to be alumina.
The thickness of the thinnest part appeared to be close to 20 nm. It was so thin that despite its multiple folds the microstructure of the alloy was clearly visible through the film.
Finally, there are varieties of bifilms in some castings that are clear for all to see. These occur in lost foam castings, and are appropriately known as fold defects. Some of these are clearly pushed by dendrites into interdendritic spaces of the as-cast structure (Tschapp et al. 2000). The advance of the liquid into the foam is usually sufficiently slow that the films grow thick and the defects huge, and are easily visible to the unaided eye. Other clear examples, but on a finer scale, are seen in high pressure die castings. Ghomashchi (1995) has recorded that the solidified structure is quite different on either side of such features. For instance, the jets of metal that have formed the casting are each surrounded by oxides (their ‘oxide flow tubes’ as discussed in section 2.2.6) seen in Figure 2.31.
Between the various flowing jets, each bounded by its film, the boundaries naturally and necessarily come together as double films, or bifilms. They form effective barriers between different regions of the casting.
As an ‘opposite’ or ‘inverted’ defect to a flow