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Chapter 3 Flow
3.1 Effect of surface films on filling
Chapter 3
Flow 71
This was a careful study of several aluminium alloys, over a wide range of filling speeds. It seems conclusive that a rolling surface wave to cause an oxide lap does not exist in most situations of interest to the casting engineer. Although Loper and Newby (1994) do appear to claim that they observe a rolling wave in their experiments on steel the description of their work is not clear on this point. It does seem that they observed unzipping waves (see below). A repeat of this work would be useful.
The absence of the rolling wave at the melt surface of aluminium alloys is strong evidence that the kind of laps shown in Figure 2.25 must be cold laps (the old name ‘cold shut’ is an unhelpful piece of jargon, and is not recommended). Rolling waves that form cold laps in aluminium alloy castings can probably only form when the metal surface has developed sufficient strength by solidification to support the weight of the wave. Whether this is a general rule for all cast metals is not yet clear. It does seem to be true for steels, and possibly aluminium alloys, continuously cast into direct chill moulds as described in the following section.
pressurized, the thin sections require an additional tension in their surfaces to overcome the tensile strength of the film before the metal can burst through. For this reason fluidity tests with multiple sections from a single runner are always found to give an effective surface tension typical of a stationary surface, being two or three times greater than the surface tension of the liquid. Results of such tests are described in section 3.3.4.
Turning now to the dynamic situation where the front of the melt is moving, new surface is continuously being created as the old surface is pinned against the mould wall by friction, becoming the outer skin of the casting (as in an unzipping type of propagation as described below). The film on the advancing surface continuously splits, and is continuously replaced. Thus any tension in the surface of the melt will now be supported by a strong chain (the surface film) but with weak links (the fresh liquid metal) in series. The expansion of the surface is therefore controlled by the weak link, the surface tension of the liquid, in this instance.
The strong solid film merely rides as pieces of loose floating debris on the surface. Thus normal surface tension applies in the case of a dynamically expanding surface, as applies, for instance, to the front of an advancing liquid.
During the turbulent filling of a casting the dynamic surface tension is the one that is applicable, since a new casting surface is being created with great rapidity. It is clear that the critical velocities for liquid metals calculated using the dynamic surface tension actually agree accurately with experimental determinations, lending confidence to the use of surface tension of the liquid for expanding liquid surfaces.
3.1.2 The rolling wave
Lap type defects are rather commonly observed on castings that have been filled slowly (Figure 2.25).
It was expected therefore that a lap type defect would be caused by the melt rolling over the horizontal, oxidized liquid surface, creating an extensive horizontal double film defect (Figure 3.1 b).
Interestingly, an experiment set up to investigate the effect (Evans et al. 1997) proved the expectation wrong.
As a background to the thinking behind this search, notice the difference between the target of the work and various similar defects. The authors were not looking for (i) a cold lap, otherwise known as a cold shut, since no freezing had necessarily occurred. They were not searching for (ii) a randomly incorporated film as generated by surface turbulence, nor (iii) a rolling backwards wave seen in runners, where the tumbling of the melt over a fast underjet causes much turbulent entrainment of air and oxides.
3.1.3 The unzipping wave
Continuing our review of the experiment by Evans et al. (1997) to investigate surface waves, as the meniscus slowed on approach towards the top of the mould, an unexpected discontinuous filling behaviour was recorded. The front was observed to be generally horizontal and stationary, and its upward advance occurred by the propagation of a transverse wave that started at the up-runner, and propagated across the width of the plate (Figure 3.1) until reaching the most distant point. The speed of propagation of the waves was of the order of 100 mms-’. Reflecting waves were observed to bounce back from the end wall. Waves coming and going appeared to cross without difficulty, simply adding their height as they passed.
What was unexpected was the character of the waves. Instead of breaking through and rolling over the top of the surface, the wave broke through from underneath, and propagated by splitting the surface oxide as though opening a zip (Figure 3 . 1 ~ ) .
The propagation of these meniscus-unzipping waves was observed to be the origin of faint lines on the surface of the casting that indicated the level of the meniscus from time to time during the filling process. They probably occurred by the transverse wave causing the thickened oxide on the meniscus to be split, and subsequently displaced to lie flat against the surface of the casting. The overlapping and tangling of these striations appeared to be the result of the interference between waves and out- of-phase reflections of earlier waves.
The surface markings are, in general, quite clear to the unaided eye, but are too faint to be captured
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(Evans et al. 1997); ( b ) rolling wave thar may only occur on a partially frozen surface; and ( c ) a n unzipping wave that Surface
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striationson a photograph. A general impression is given by that speed the advance of the liquid changes from the sketch in Figure 3.1. The first appearance of being smooth and steady t o a n unstable the striations seems to occur when the velocity of discontinuous mode. Most of the surface of the rise of the advancing meniscus in liquid 99.8 per melt is pinned in place by its surface oxide, and its cent purity A1 falls to 60 f 20 mm/s or below. At vertical progress occurs only by the passage of
Flow 7 3
powder in the crack. The problem is most noticeable with microalloyed grades containing niobium. The fracture surfaces of laboratory samples of this material are found to be faceted by grain boundaries, and often contain mixtures of AlN, NbN and sulphides.
Other typical early researches are those by Tomono (1983) and Saucedo (1983). The problems of the solidifying meniscus are considered by Takeuchi and Brimacombe (1984). Later work is typified by that of Thomas (1995) who has considered the complexities introduced by the addition of flux powder (which, when molten, acts as a lubricant) to the mould, and the effects of the thermal and mechanical distortion of the solidified shell.
All this makes for considerable complication.
However, the role of the non-metallic surface film on the metal being entrained to form a crack seems in general to have been overlooked as a potential key defect-forming mechanism. In addition, the liquid steel melt in the mould cannot be seen under its cover of molten flux, so that any wave travelling around the inside of the mould, if present, is perfectly concealed. It may be that we are still some way from fully understanding the surface features of continuously cast steels.
successive horizontal transverse waves. At the wave front the surface oxide on the meniscus is split, being opened out and laid against the surface of the casting, where it is faintly visible as a transverse striation.
Since this early work, the author has seen the unzipping wave travelling in a constant direction around the circumference of a cylindrical feeder, spiralling its way to the top. Even more recently, the surface of continuously cast cylindrical ingots of 3 0 0 m m diameter have been observed to be covered with spirals. Some of these are grouped, showing that there were several waves travelling helically around the circumference, leaving the trace of a ‘multi-start thread’. Clearly, different alloys produced different numbers of waves, indicating a different strength of the oxide film. The cylindrical geometry represents an ideal way of studying the character of the wave in different alloys. Such work has yet to be carried out.
In the meantime, it is to be noted that there are a great many experimental and theoretical studies of the meniscus marks on steels. Particularly fascinating are the historical observations by Thornton (1956). He records the high luminosity surface oxide promoting a jerky motion to the meniscus, and the radiant heat of the melt causing the boiling of volatiles from the mould dressing, creating a wind that seemed to blow the oxide away from the mould interface. The oxide and the interfacial boiling are also noted by Loper and Newby (1994).
Much more work has been carried out recently on the surface ripples on continuously cast steels.
Here the surface striations are not merely superficial.
They often take the form of cracks, and have to be removed by scalping the ingot before any working operations can be carried out. It seems that in at least some cases, as a result of the presence of the direct chill mould, the meniscus does freeze, promoting a rolling wave, and a rolled-in double oxide crack. Thus the defect is a special kind of cold lap.
An example is presented from some microalloyed steels that are continuously cast. Cracking during the straightening of the cast strand has been observed by Mintz and co-workers (1991). The straightening process occurs in the temperature range 1000 down to 700°C, which coincides with a ductility minimum in laboratory hot tensile tests. The crack appears to initiate from the oscillation marks on the surface of the casting, and extends along the grain boundaries of the prior austenite to a distance of at least 5 to 8 mm beneath the surface. The entrainment event in this case is the rolling over of the (lightly oxidized) meniscus on to the heavily oxidized and probably partly solidified meniscus in contact with the mould. Evidence that entrainment occurs at this point is provided by the inclusion of traces of mould