3 ,',& ,
2.3 Furling and unfurling
Throughout its life, the bifilm undergoes a series of geometrical rearrangements. An understanding of these different f o r m s is essential t o the understanding of the properties of castings. The stages in the life cycle of the bifilm are:
I . entrainment by surface turbulence;
2. furling by bulk turbulence; and
3. unfurling in the stillness of the liquid after the filling of the casting is complete.
The verb ‘to furl’ is used in the same sense that a sailing boat willfurl its sails, gathering them in; or it may unfurl them, letting them out.
We have seen earlier that if the surface of the liquid suffers turbulence (Le. the Weber number well in excess of unity) the liquid is likely to enfold our familiar bifilm defects. Their size will probably depend on the strength @e. the tear resistance) of the film. For some high duty stainless steels the size might be 100 mm across. For AI-Si alloys containing low levels of Mg the bifilms seem to be more usually from 1 mm up to 10 or 15 mm diameter.
Once submerged, however, the bifilms will be subjected to the conditions of bulk turbulence beneath the surface of the liquid. This is because Reynold’s number, Re, is nearly always over the critical value of 2000 in liquid metal-filling systems as is clear from Figure 2.22. (Exceptions may be counter-gravity and tilt-pouring systems.) The launching of our delicate, gossamer-thin double film into a maelstrom of vortices in this dense liquid will ensure that it is pummelled and ravelled into a compact, convoluted form almost immediately. In this form it will be effectively reduced in shape and size from a planar crack of diameter 1 to IO mm to a small ragged ball of diameter in the range 0.1 to 1 mm. These are the sizes of crumpled dross- like defects seen by Fox (2000) and commonly seen o n polished metallographic sections of aluminium alloy castings.
It is worth emphasizing that although the bifilm now has a highly contorted form - crumpled, convoluted and ravelled in an untidy random manner
- it has not lost its crack-like character. However, its form contrasts sharply with the types of crack that the metallurgist normally associates with stress.
It seems also worth underlining the fact that the double film is unlikely to become separated once again into single films. This is because where the two halves are in contact, interfacial forces will be important, and inter-film friction will ensure that both leaves will move as one. In addition, the continued oxidation, thickening the leaves, will reduce the volume of air in the inter-film gap, reducing the pressure between the films. The exterior atmospheric and metallostatic pressures will thereby pressurize the two halves together, ensuring that the films continue their association as a pseudo- single entity, despite the absence, or near absence, of bonding between the halves.
In this compact form, bifilms are able to pass through filters of normal pore size in the region of 1 mm diameter. Some will be arrested as a result of untidy trails of film that may become caught by the filter, since the compact forms will be winding and unwinding continuously in a random manner in the turbulence. Inside the filter the constraint of the narrow channels promotes laminar flow (Figure 2.22). It is possible that many compact bifilms will be unravelled and flattened against the internal surfaces of the filter if they become hooked up somewhere in a laminar flow stream. This seems likely to b e a potentially important filtration mechanism for compacted bifilrns.
At practically all other locations, during the whole of the transfer into the mould, the bulk turbulence will ensure that the bifilms are continuously tumbled, entering the mould in a relatively compact form.
Thus the casting initially finds itself with a distribution of compact and convoluted bifilms. Their tensile strength will, on average, be about half that of a pore of the same average diameter as the bifilm ball because of the effect of the mechanical interlinking of the crumpled crack as seen in Figure 2.39. If the convoluted crack is subject to a tensile stress it cannot come apart easily. There is much interlocking of sound material, so that much plastic deformation and shearing will be involved in the failure.
Because of the combination of this small strength with their small size, the compact bifilms are rendered as harmless as possible at this stage.
However, once the filling of the casting is complete, the bifilm now finds itself in a new, quiescent environment. If there is any movement of liquid because of solute or thermally driven convection, such movements are relatively gentle, occurring at low values of Re.
Conditions are now right for the bifilm to unfurl, opening, growing by about a factor of 10 on average, to its original size in the range of 1 to 10 mm or so.
By this action the defect now re-establishes itself as a planar crack, and so can impair the strength and ductility of the metal to the maximum extent.
This astonishing ability of the bifilm to open like a
Entrainment SS
(a) 100% strength
(b) Approximately
50% strength
(c) 0% strength Figure 2.39 Relative strength
of
( a ) solid matrix, ( b ) convoluted bijilms and ( c ) pores.flower explains many of the problems of castings, as will become clear throughout this book.
What causes the bifilm to open again?
Interestingly, there are several potential driving forces. Although they will be considered in detail in later sections of the book, they are listed briefly here from recent studies of aluminium alloys where the effects have been most thoroughly studied to date. They include:
1. The precipitation by hydrogen in the gas film between the oxide interfaces, thus inflating the defect. The inflation is likely to take place in two main stages. T h e first w e may call microinflation, with short lengths (defined and separated from adjacent lengths by the sites of folds) inflating individually. When sufficiently inflated, these short lengths will start to exert an unfolding pressure on adjacent lengths. As the inflation proceeds, the bifilm will unfold from fold to fold, length by length. Thus the unfurling action will be jerky and irregular, until the whole bifilm is unfolded. At this stage, of course, if enough hydrogen continues to precipitate, the bifilm will eventually grow into a spherical pore.
The stages of growth are illustrated in Figure 2.40a for a simply folded bifilm, and 2.40b illustrates the stages for a more convoluted bifilm.
If much gas precipitates, either because there is much gas in solution, or because there is plenty of time for diffusion, thereby aiding the collection of gas from a wide region, the gas pore may eventually outgrow its original bifilm to become a large spherical pore as shown in Figure 2 . 4 0 ~ if growing freely in the liquid, or as in Figure 2.40d if growing later during freezing, appearing now as an interdendritic pore. On sectioning the pore it is probable that the originating fragment of bifilm will never be found. Interestingly, at the stage at which the pore starts to outgrow the boundaries of its originating bifilm, it will extend its own oxide film using its residual oxygen gas
(probably derived from the original bifilm). The new area of the defect will then consist of very new thin film. It is possible to imagine that, if all the oxygen were consumed in this way, eventually the pore would grow by adding clean, virgin metal surface. The many round pores in cast metals indicate that the linear dimensions of many bifilms are smaller than the diameters of such pores. It is a reminder of the wide spread of bifilm sizes to be expected.
2. Shrinkage may pull the two films apart. If this happened at an early stage of freezing the defect would grow no differently to a hydrogen pore, inflating from fold to fold, and the final fold eventually growing to be more or less spherical, as seen in Figures 2.40a and c. If the opening took place at a late stage of freezing the defect would attempt to open when surrounded by dendrites as shown in Figure 2.40d. Both of these modes of opening could be driven by gas or shrinkage or both. Thus the rounded and interdendritic forms are not reliable indicators of the driving force for the growth of pores. The round form is a reliable indicator that the pore grew prior to the arrival of dendrites, while the dendritic pore is a reliable indicator that this was a late arrival. No more can be concluded. In the case of the dendritic morphology, withdrawal of the residual liquid (being either pushed by gas from inside the pore, or pulled by reduced pressure outside the pore) would stretch the film over surrounding dendrites, and pull the bifilm halves into the interdendritic regions. The originating films might eventually be sucked out of sight deep into the dendrite mesh.
3. Iron rich phases, particularly the PFe particles, Al,FeSi, nucleate and grow on the outer, wetted surfaces of the bifilm. Initially, when the PFe crystals are no more than a few nanometres thick, the crystals can follow the curvature and creases of the crumpled double film. However, as the crystals grow in thickness, because the rigidity
(4 Figure 2.40 ( a to d ) Stages of unfurling and infation of bifilms.
of a beam in bending mode is proportional to its thickness to the third power, they quickly develop rigidity. Thus as they thicken and rigidize, they force the straightening of the bifilm. The result
is the familiar PFe plate, straight as a needle on a polished section. Occasional curved pFe plates are probably the result of restraint at the two ends of the plate, so that the plate has been
Entrainment 57 unable to straighten fully, remaining stressed
like an archer’s bow. In any case, pFe plates often exhibit a crack along their centreline (if pFe has precipitated on both sides of the bifilm) or between the pFe and the matrix, showing apparent decohesion from the matrix (if the pFe has precipitated only on one side of the bifilm).
It seems likely that all pFe particles are cracked or decohered in this way because of the presence of the originating bifilm, but the cracks may not be easily seen on a polished section.
4. Oxides are pushed ahead of growing dendrites, with the result that bifilms are automatically unravelled and flattened, effectively organized into planar areas among dendrite arrays, and are pushed into interdendritic and grain boundary regions. This is an important mechanism since it can occur in all cast alloys that have solid surface films. The effect is illustrated schematically in Figure 2.4 I .
Such a room-temperature fracture surface is seen in Figure 2.42 for an A l 4 . 5 C u alloy (Mi 2000) in which the fracture surface is covered in a thin alumina film. The heaps of excess film pushed ahead of the dendrites is clearly seen in the central regions of the cast test bar. Similar flattening of bifilms is seen to be driven by a twinned ‘feather’ dendrite (Figure 2.43a) and a conventional cubic symmetry dendrite (Figure 2.4313) in an Al-Si-Cu alloy. Again,
the piled-up bifilm pushed ahead of the growing crystals is clear. In Figure 2.44 an A1N or A1,O3 film is probably the cause of the planar boundaries seen in the fracture surface of a vacuum cast and HIPped Ni-base alloy IN939 (Cox et al. 2000). In the case of both the AI-Cu alloy and the Ni-base vacuum-cast alloy the castings had suffered surface turbulence during filling, but, significantly, the planar boundary features on the fracture surfaces were not observed in control castings made without such turbulence. The planar features were also observed to be associated with poor tensile ductility. In section 5.5.2 further planar defects are discussed for cast irons, and in section 5.6.2 probable examples in steels are proposed.
Clearly, the driving forces for the unfurling of bifilms are all those factors that are already known to the casting metallurgist as precisely those factors that impair ductility; including hydrogen in solution, shrinkage, iron contamination in A1 alloys, and large grain size.
However, we need to bear in mind that it will take time for the unfurling processes to occur. In a casting that is frozen quickly, the bifilms are frozen into the casting in their compact form, with the result that most casting alloys that are chilled rapidly are strong and ductile, as all foundry engineers know.
Conversely, castings that have suffered lengthy solidification times exhibit shrinkage problems,
Figure 2.41 Schematic action of advancing dendrites to straighten a b@m.
higher levels of gas porosity, or large grain size. In addition, all exhibit reduced properties, particularly reduced ductility. The beneficial action of rapid freezing to enhance ductility is, once again, well known to the casting metallurgist.
In summary, it is clear that all the factors that refine DAS (dendrite arm spacing) improve the mechanical properties of cast aluminium alloys.
This is not primarily the result of action of the DAS alone, as has been commonly supposed. The
DAS is merely a measure of the local freezing time, which is the time available for the inflation and unfurling of defects. If the growth of area of the defect was not bad enough, its inflation, causing its surfaces to separate, transforming it into a pore, makes the defect even more damaging. Thus its strength falls further as seen in Figure 2.39. It is both (i) the growth of area and (ii) the growth of volume of the defects that combine to inflict so much damage to the mechanical properties as
Entrainment 59
(b)
Figure 2.42 ( a ) Tensile fracture surface of an A 1 4 . 5 C u alloy that had suffered an entrainment effect observed by X-ray video radiography, showing large new hifilms straightened by large grains, and heaps of excess hifilms pushed into central areas, and a close-up showing a thin doubly folded area (arrowed). (b) Elsewhere in the same casting showing a fracture surface of quietlyfilled material and an area of ductile dimpled fracture (courtesy Mi 2000).
solidification time increases. Increases in DAS are not the cause of the loss of ductility, the DAS is merely an indicator of the time involved.
Returning to the pivotal role of the entrainment defects, it seems clear from practical work in foundries that although the design of the filling system is extremely important, efforts to control the surface turbulence in the mould cavity itself
are the most important factor to yield reliable properties.
This seems likely to be the result of the less severe surface bulk turbulence in the mould cavity.
Reynold’s numbers in a nicely filled mould are in the region of lo3, so that little energy is available to make the films more compact. Thus any bifilm entrained in the mould will probably remain
substantially open, maximizing its area. This in itself will cause maximum damaging effect, but will be additionally enhanced by the hydrogen diffusion to the enlarged area, inflating the defect, and so further reducing any load carrying capacity it may once have enjoyed. The highest velocities and maximum turbulence is in the sprue, runner and gates, where Reynold’s numbers of lo4 to lo5 are easily attained, indicating severe bulk turbulence.
Bifilms entrained in the running system will
Figure 2.43 Fracture s u ~ a c e of fatigued A319 alloy showing straightened oxides by the growth of
( a ) a twinned ‘feather’ crystal and ( b ) a conventional cubic syrnmerp dendrite (courtesy J. Boileau, Ford Labs 2000).
therefore be highly compacted, and, at least initially, have less damaging consequences for mechanical properties
.
It is almost certain that in some circumstance the bifilms will be unable to unfurl despite the action of some or all of the above driving forces.
This is because they will sometimes be glued shut.
Such glues will include liquid fluxes. If present on the surface of the melt, they will find themselves folded into the bifilm, and so form a sticky centre
Entrainment 61
] t T r
*
W / '1
to the sandwich, holding the bifilm closed, but the excess flux weeping or squeezed to its outer edges will aid the pinning of folds. The extreme thinness of bifilms will mean that even weak glues such as molten fluxes will be effective welds. Such action may explain part of the beneficial effect of fluxes.
The melt may not be significantly cleaned by the flux, but its inclusions merely rendered less damaging by being unable to unfurl.
A similar action may explain the effect of low melting point additions such as bismuth to aluminium alloys. Although Papworth and Fox (1998) attribute the benefits of the Bi addition to the disruption of the integrity of alumina films, it seems more likely that a low melting point Bi-rich liquid will be inert towards the oxide, and that its action is passive, being merely an adhesive.
While bifilms remain a feature of our processing techniques for aluminium alloys, fluxes or low melting point metals might be a useful ploy, even if it is effectively 'papering over the cracks' of our current metallurgical inadequacies.
For the future, the very best cast material will be made free from bifilms in foundries specifically designed to deliver perfect quality metal. Such concepts are already on the drawing board and may be realized soon. When this utopia is achieved we shall be able to make castings with any DAS (as will be explained later), solidified fast or slow, all with perfect properties. It is a day to look forward to.
I
!
IFigure 2.44 Tensile fracture surface of a
,
vacuum-cast and HlPped Ni-base alloy, showing apparently brittle grain boundaries after having been~ cast turbulently.