Solidification structure
5.1 Heat transfer
5.1.3 Convection
Convection is the bulk movement of the liquid under the driving force of density differences in the liquid.
In section 5.3.4 we shall consider the problems raised by convection driven by solutes; heavy solutes cause the liquid to sink, and the lighter solutes cause flotation. In this section we shall confine our discussion simply to the effects of temperature:
hot liquid will expand, becoming less dense, and will rise; cool liquid will contract, becoming denser, and so will sink.
The existence of convection has been cited as important because it affects the columnar to equiaxed transition (Smith et al. 1990). There may be some truth in this. However, in most castings, grain structure is much less important than soundness, and it seems to be little known that convection can give severe soundness problems.
The problems of convective flow create serious problems in counter-gravity filling systems. Figures 5.1 1 and 5.12 illustrate how, after the mould cavity is completely filled, the temperature gradient in the mould is as wrong as it could be: the hot metal is at the bottom and the cold metal at the top. As the casting starts to solidify, the cold liquid metal drifts downward, draining into the riser tube. Here it is replaced by hot metal flowing up the heated riser tube and into the casting. This freshly reheated metal can remelt a channel through the pasty zone.
If the heat input to the furnace at the base of the riser tube is sufficient then a circulation is set up which can become infinitely perpetuating; the rate
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Figure 5.10 Acceleration of the freezing front in
0 0.5
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compact castings as a result of 3-0 extraction ofCasting heat (sphere and cylinder curves calculated from Santos and Garcia 1998).
Mould
wall Relative distance to centre (dimensionless) centre
Solidification structure I17 Hot, upward flow
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Heat
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Runner
Figure 5.12 Rernnants of the convective plumes in n casting.
defining regions of c.oar.se .strucfure and porosity.
of heat input from below equals the rate of heat loss from the casting. Flow therefore continues indeflnitely, long after the casting should have completely frozen. The result is that when the casting is removed from the casting machine, two things can happen:
1 . In the worst case the liquid can drain completely from the flow path, leaving a hollow tunnel through the casting.
2. In the best case, perhaps because of a cranked delivery system or other impediment to the free
Figure 5.11 Convection-driven ,flow wifhin u solidifiing low-pressure casting.
flow of liquid out of the casting (such as a filter), the flow path then freezes, but without the benefit of applied pressure or any extra feeding liquid.
Thus it becomes a porous region of the casting, appearing to be a region of shrinkage porosity (Figure 5.12). The casting engineer will then increase the size of the feed path from the riser tube to the poorly fed region in an attempt to increase the feeding. With the enhanced ease of convection, and enhanced ease of subsequent emptying of the flow path, the problem merely gets worse!
This was the nightmare problem that blighted the Cosworth development in its early years, almost causing the company to fail. At the time the problem was baffling since many castings could be cast perfectly, but certain not-so-different designs could not be made without severe porosity. (The problem was completely solved some years later by the development of the rollover technique following casting. This is dealt with in Volume 11.)
Thermal convection is not only a problem in low-pressure, uphill-filling systems. It is probably common in any casting that takes a long time to freeze. This is because the circulation pattern takes time to build up and time to carve out a significant flow channel.
Thus it is common in investment castings of steels and nickel-based alloys, especially when these are cast into hot moulds at temperatures near 1000°C, and even more when these moulds are backed by insulating material, all at this high temperature.
Figure 5.13a shows a typical problem casting where the side feeder constitutes a heat source and a circulation path. The result is that the casting becomes too hot at the top, gaining for itself an effectively higher modulus and extended freezing time. A shrinkage-type defect in the top of the heavy section of the casting follows, even though the feeder appears to be correctly sized to feed the casting.
flow
(c)
Figure 5.13 Encouragement of thermal convection by ( a ) side feeding; ( 6 ) bottom feeding; ( c ) its elimination by top feeding.
Turning the casting on its back and feeding from underneath (Figure 5.13b), pressurizing via an auxiliary feeder, is similarly problematical since the sprue will freeze early and thus not continue to pressurize. This system is similar to the low-pressure case shown in Figure 5.12. The choice of ingatel feeder through which the metal decides to rise or fall is probably random, being sometimes at one gate and sometimes at the other in the absence of other influences. The situation of cold dense metal overlying hot lighter liquid is simply unstable and can ‘flip’ over in either direction. The direction of flip is, of course, highly sensitive t o initial perturbations such as the residual effect of the flow induced during filling, or the presence of the heat centre in the heavier runner nearer the sprue, or the fact that the runner may not be perfectly balanced so that more flow has occurred via the far ingate, heating that ingate preferentially. In a metastable density regime a bubble blowing off a core can be a powerful trigger, precipitating a rapid slide into instability.
As we have seen, the counter-gravity geometries can sometimes continue to convect indefinitely. In comparison, the convective flows inside gravity- filled castings are usually not so serious, since without the external heat source, they only continue until the feeder finally solidifies. However, even this may greatly prolong the local solidification time of the casting with the result that, at best, properties are locally impaired, and localized gas porosity will have had increased time to develop.
At worst, shrinkage porosity may occur because of the transfer of the remaining solidifying liquid out of the casting and into the feeding system.
The only reliable solution to avoid convection is to place the heavy sections at the top and feed downwards using gravity. This is a stable feeding orientation. Thus the casting shown in Figure 5 . 1 3 ~ will have enjoyed optimum conditions of filling uphill and feeding downhill. This is a universally applicable condition for reliable castings.
The optional provision of additional gates x and y to provide some hot feed metal directly below the feeders is attractive, but raises the potential for convective problems, if x and y allow convective paths to form. If x and y are narrowed, so as to freeze off early, convection may be avoided, and this mode of filling may become quite efficient.
In general, however, filling the feeders by flow through the casting has the double disadvantage of (i) heating the casting and (ii) cooling the liquid that finally reaches the feeders. The feeding system is therefore necessarily inefficient. This is a problem from which there is often no escape for static casting processes. (An upspruelfeeder system is possible for some products. This solution is described in Volume 11.)
The solution to this problem is the inversion of
Solidification structure 129
the local washing away of the solidification front, as a curving river can erode its outer bank.
The existence of continuous fluidity is a widely seen effect resulting directly from the remelting of the solid material that has formed in the filling system, keeping the metal flowing despite an unfavourable modulus. Without the benefits of this phenomenon it would be difficult to make castings at all!
Other convective flows produced by solute density gradients in the freezing zone take time to get established. Thus channels are formed by the remelting action of low-melting-point liquid flowing at a late stage of the freezing process. The A and V segregated channels in steel ingots, and freckle defects in nickel- and cobalt-based alloys, are good examples of this kind of defect.
the casting immediately after pouring. The filling system is preheated by the flow of metal, and, after inversion, becomes the feeding system. This is an ultimate and powerful solution, universally recommended if completely reliable castings are required.
Finally, the casting engineer needs to be constantly vigilant against problems caused by convection. Convection problems require a trained eye on the lookout for circulation paths that contain hot (or heated) and cool (or freezing) regions. Uphill filling systems are sometimes impaired, whereas uphill feeding systems are usually greatly troubled, often to the point of being insoluble.