3 ,',& ,
2.8 The significance of bifilms
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
Entrainment vent by surface
turbulence We > 1
Pouring
I
QuiescenceI
Casting defect
Compacting Solidification
(unfurling mechanisms) mechanism by Liquid at rest
bulk turbulence R e > 1
Film-free
-
Coalesced-
Rapid flotation No defects (Small possibility ofsurface matrix liquid of bubbles
.
minute retained bubbles)Coalesced
Rapid flotation Few defects (Residual small
Liquid film -+droplets of -+ * droplets in casting.) (Locally
surface liquid of large droplets thickened slag layer on cope.)
Furled bifilm Rapid flotation of
.
Complex macroinclusions on -partly welded -compact bifilmsPartially
molten film closed cope surface
Gas (air) bubbles (solid film gives detrainment problems)
7
Bubble trailsGas precipitation + Gas porosity out of solution
Hydrostatic strain + Shrinkage porosity Buoyant bubbles -W rapid flotation
r) Hot tears Solid film
* L I U I I I
L Cold cracks Crystalline
precipitate Centreline cracks in precipitates straightening Matrix 'decohesion' cracks Dendrite pushing lnterdendritic cracks and hot tears
Grain boundary cracks and hot tears
this is done in Figure 2.47 for metals (i) without films, such as gold, (ii) with films that are liquid, (iii) with films that are partially liquid, and (iv) with films that are fully solid.
Note that the defects on the right of Figure 2.47 cannot, in general, be generated without starting from the bifilm defect on the left. The necessity for the bifilm initiator follows f r o m the near impossibility of generating volume defects by other mechanisms in liquid metals, as will be discussed in sections 6.1 and 7.2. The classical approach using nucleation theory predicts that nucleation of any type (homogeneous or heterogeneous) is almost certainly impossible. Only surface-initiated porosity appears to be possible without the action of bifilms.
In contrast to the difficulty of homogeneous or heterogeneous nucleation of defects, the initiation of defects by the simple mechanical action of the opening of bifilms requires nearly zero driving force;
it is so easy that in all practical situations it is the only initiating mechanism to be expected.
We are therefore forced to the fascinating and enormously significant conclusion that in the absence of bifilms castings cannot generate defects
Figure 2.47 Framework of logic linking surface conditions, flow and solidification conditions to f n a l defects.
that reduce strength or ductility.
(This hugely ihportant facthas to be tempered only very slightly, since porosity can also be generated easily by surface initiation if a moderate pressurization of the interior of the casting is not provided by adequate feeders. However, of course, adequate feeding of the casting is normally accepted as a necessary condition for soundness. This is the one technique that is widely applied, and we can therefore assume its application here.)
The author has the pleasant memories of the early days (circa 1980) of the development of the Cosworth process, when the melt in the holding furnace had the benefit of days to settle since production at that time did not occupy more than a few shifts per week. The melt was therefore unusually free from oxides, and the castings were found to be completely free from porosity. As the production rate increased during the early years the settling time was progressively reduced to only a few hours, causing a disappointing reappearance of microporosity. This link between melt cleanness and freedom from porosity is well known. One of the first demonstrations of this fact was the simple
Entrainment 69 such as a pore or a hot tear actually is the bifilm, but simply opened up. In the latter case no growth of area of the subsequent defect is involved, only separation of the two halves of the bifilm. Both situations seem possible in castings.
Standing back for a moment to view the larger scene of the commercial supply of castings, it is particularly sobering that there is a proliferation of standards and procedures throughout the world to control the observable defects such as gas porosity and shrinkage porosity in castings. Although once widely known as ‘Quality Control’ (QC) the practice is now more accurately named ‘Quality Assurance’
(QA). However, as we have seen, the observable porosity and shrinkage defects are often negligible compared to the likely presence of bifilms, which are difficult, if not impossible, to detect with any degree of reliability. They are likely to be more numerous, more extensive in size, and have more serious consequences.
The significance of bifilms is clear and worth repeating. They are often not visible to normal detection techniques, but can be more important than observable defects. They are often so numerous and/or so large that they can control the properties of castings, sometimes outweighing the effects of alloying and heat treatment.
T h e conclusion is inescapable: it is more important to specify and control the casting process to avoid the formation of bifilms than to employ apparently rigorous Q A procedures, searching retrospectively (and possibly without success) for any defects they may or may not have caused.
Table 2.2 Possible bifilm defects in different alloy systems
Alloy type Porsible deject type
AI-Si alloys Centreline and matrix decohesion cracks in plate-like intermetallics (Si particles, Fe-rich precipitates, etc.) Planar hot tears with dendrite raft morphology of fracture surface.
Plate fracture (spiking) defect Flake cast irons Nitrogen fissures
Ductile irons
Steels Rock-candy fractures on A1N at grain boundaries
Type I1 sulphide phenomena Intergranular facets on fracture surface Initiation of stray grains and high- angle grain boundaries in single crystals
Ni-base superalloys (vacuum cast)
N.B. The causes of defects in the cases of the higher temperature alloys, irons, steels and Ni-based alloys are based only on circumstantial (although strong) evidence at the time of writing.
and classic experiment by Brondyke and Hess (1964) that showed that filtered metal exhibited reduced porosity.
An important point t o note is that the subsequently generated defect, which may be large in extent, may be simply initiated by and grow from a small bifilm. On the other hand, the bifilm itself may be large, so that any consequential defect