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2.2.8 The bubble trail
A bubble trail is the name coined in Castings (199 1) to describe the defect that was predicted to remain in a film-forming alloy after the passage of a bubble through the melt.
Since air, water vapour and other core gases are normally all highly oxidizing to the liquid metal, a bubble of any of these gases will react aggressively, oxidizing the metal as it progresses, and leaving in its wake the collapsed tube of oxide like an old sack. (In the case of graphite film-forming gases, the bubble trail is, of course, expected to be a collapsed graphitic tube.) The inner walls of the trail will come together dry side to dry side, and so be non-adherent, once again constituting a classic
Entrainment 47 progress of the bubble causes the film to fold and crease. Any spiralling motion of the bubble will additionally tighten the rope-like trail.
Figure 2.32b further illustrates the different sections to be expected along the length of the bubble and its trail, showing the gradual collapsing process that creates the trail.
Divandari (1999) was the first to observe the formation of bubble trails in aluminium castings by X-ray video technique. He introduced air bubbles artificially into a casting, and was subsequently able to pinpoint the location of the trails and fracture the casting to reveal the defect. Figure 2.33a shows the inside of a trail in A1-7Si-0.4Mg alloy. The longitudinally folded film is clear, as is the presence of shrinkage cavities that have expanded away from the defect because the casting was not provided with a feeder. The small amount of shrinkage has sucked back the residual liquid, stretching the film over the dendrites as seen in Figure 2.33b. The form of a double film defect. This particular bifilm
has its special characteristic features, as do the other major bifilms, the random defects arising from surface turbulence, and the geometrical defects that result from the various oxide laps.
The mechanism of the expansion of the film forming the crown of the bubble is schematically illustrated in Figure 2.32. The bubble forces its way upwards while splitting the film on its crown that is attempting to hold it back. Only large bubbles have sufficient buoyancy to overcome the resistance to its motion provided by the strength of the film.
The film exerts its restraint because it is effectively tethered to the point, often located in the early part of the filling system, where the bubble was first entrained. The expanding region of film on the crown effectively slides around the surface of the bubble, continuing to expand until the equator of the bubble is reached. At this point the area of the film is a maximum. Since the film cannot contract, further
(a)
Figure 2.32 ( a ) Schematic illustrations of rising bubbles an, progressive collapse of the bubble trail.
I
(b)
d associated trails; ( b ) cros y-sections illustrating the
(C)
Figure 2.33 ( a ) SEM fractograph o f a bubble trail in AI-7Si-O.4Mg alloy; ( b ) a close-up, including areas of shrinkage probably grown from the trail: ( c ) the oxide film of the trail draped over dendrites, on the point of being sucked into the mesh because of a shrinkage problem in the casting (Divandari 2000).
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thicker, older part of the film is seen pulled in tight creases, and where torn is seen to be replaced by newer, thinner film.
Later he observed open trails in zinc pressure die castings (Figure 2.34). In this case the trail was almost certainly re-opened by high internal pressure (up to 100 bar) in the bubble and its trail when the die was opened slightly prematurely, releasing the support of the die before solidification was complete.
The pressurized bubbles have effectively expanded, opening like powerful springs, while the casting was still in a plastic state.
-
Figure 2.34 SEM imuge of u fracture surjiuce of u zinc pressure die cast component revealing a bubble with an open trail (Divandari 2000).
A cross-section of a group of bubble trails was observed by Divandari ( I 999) on a metallographic section of an aluminium alloy (Figure 2.35). The trails were nowhere near the trajectory of the bubbles, illustrating how internal circulations in the casting had transported the damage, ravelling and tumbling the trails to a distant location. (Other sources of large oxides were excluded from this experiment.) The defects strongly resembled interdendritic shrinkage porosity as a result of their concave, cuspoid outlines. However, the dendrite arm size was an order of magnitude smaller than the average repeat distance of the cusps, indicating that dendrites had not defined the outline of this porosity. It is clear that much porosity in castings has been wrongly attributed to shrinkage, and that the bubble trail is actually a common defect, to be expected in most gravity poured castings and many other types of casting.
A scenario such as that shown in Figure 2.36a is common. The bubbles are entrained early in the filling system, arriving as a welter of defects, boiling up with the liquid as it fills the mould cavity. Many of the early bubbles will have sufficient buoyancy
to escape. When the casting is finally solidified the appearance on a radiograph is expected to be something like that shown in Figure 2.36b. Porosity and cracks may be detected in regions above the ingate. The large number of remaining trails is likely to be invisible. This mix of residual bubbles and oxides is usefully termed bubble dumuge. In the gate itself, the trails will have been pushed by dendrites into the centre of the section, and will resemble centreline shrinkage porosity. Close examination, however, will confirm its identity as an assembly of close-packed double films. For many resin-bonded sand castings the internal surfaces of the leaves will have a slightly discoloured appearance, stained by the mould gases.
Figure 2.37a shows a bronze casting that has suffered a highly turbulent filling system. The bubbles entrained by this turbulence can be seen to be grouped along a horizontal line (a site of a poor joint between cope and drag, allowing a cooling fin to create a line of dendrites that had caught many of the bubbles) and towards the top of the casting in the ‘skeins of geese’ mode that characterizes these defects in bronzes. The skeins are probably the directionality to the visible defects provided by the arrays of invisible oxide bifilms, streaming like tattered banners in the wind. Figure 2.37b is a close-up of the top of the same casting after defects have been revealed by machining.
A radiograph of a nearly pure copper casting that has also suffered bubble entrainment in its filling system is shown in Figure 2.38. This casting was thin-walled and extensive, so that it continued to receive bubbles late into pouring, with the result that they have become trapped in the solidifying casting. The author has seen similar extensive arrays of bubbles in aluminium alloy oil pan castings.
Figure 2.32 shows a bubble that has been torn free from its trail. Such bubbles, with the stump of their trail showing the bubble to be tumbling irregularly as it rises, have been directly observed by X-ray video. The work by Divandari (Figure 2.35) confirms that bubble trails, in filtered melts known to be free from oxide tangles, can detach and float freely, finally appearing at distant locations.
Clearly, the trail, whether still attached to its bubble or not, is a serious threat to the mechanical strength and integrity of the casting.
Bubble trails are known in low-pressure casting if the riser tube is not pressure-tight, allowing bubbles of the pressurizing gas to leak through, rising up the riser tube and so directly entering the casting. The author has observed such a trail in a radiograph of an aluminium alloy casting. In the quiescent conditions after filling, the resulting trail was smooth and straight, rising through the complete height of the casting. When concentrating on the examination of the radiograph for minute traces of porosity or inclusions such extensive geometric
Entrainment 5 1
Figure 2.35 Optical metallographic section of a number of bubble trails (Divandari 2000).
features are easy to overlook, appearing to be the shadows of integral structural parts of the casting!
A rather serious form of bubble trail is also commonly observed to form from core blows. This will be dealt with in section 6.4.
Finally, it is worth considering what length a bubble trail might reach. If we assume a bubble of 10 mm diameter rising through liquid aluminium, and if aluminium oxide grows to a thickness of 20 nm, it is not difficult to estimate that the 20 per cent oxygen in the air is used up after creating a trail of about 0.5 m. This is quite sufficient to cause a major problem in most castings.
For castings poured in vacuum (actually dilute air of course!) vacuum bubbles are still to be
expected to form, although the situation is a little more complicated. The entrained atmosphere in this case will be at a pressure somewhere between torr). (The local vacuum inside the mould at the instant of pouring is likely to be much higher as a result of mould outgassing than the pressure indicated on the vacuum gauge of the furnace.) Thus, considering the case of the vacuum casting of Ni-base superalloys, at a nominal depth of 100 mm in the liquid of density close to 8000 kgm-3, a bubble of 20 mm diameter will collapse down to somewhere in the region of 5 to 0.5 mm diameter before its internal pressure rises to equal that of the surrounding melt. (We are neglecting the somewhat
and lo4 atmosphere (1 torr to
Entrainment
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53
(a) (b)
Figure 2.37 Bronze bush casting lifter machining, revealing entrained air bubbles on ( a ) the front face and ( b ) on the top edge of the flange.
Figure 2.38 Radiograph of a thin-wall copper statue showing extensive bubble damage.
balancing effects of (i) a small reduction in size because of surface tension, and (ii) a small increase in size from the expansion of the mould gases, since moulds for investment casting are already at 1000°C or more.) The bubbles will be smaller at greater depths, and larger towards the top of the casting of course. Their considerably reduced concentration of oxygen will reduce the potential lengths of trails, or result in a thickness of oxide much less than 20nm. It is not easy to predict what form a bubble trail may take in these circumstances, if the bubble is able to rise at all.
There seems no shortage of research to do yet.