Current technologies for aluminum castings and their machinability

Manufacturing methods

In conventional sand casting metal is poured into a ‘pouring basin’ and enters the casting via a ‘sprue’ or ‘downgate’, a runner bar and a number of ‘ingates’ (see Figure 16.129). The objective is to introduce metal as smoothly and as free from turbulence as possible, so that it fills up and then solidifies uniformly and simultaneously upwards from the bottom of the casting over as wide an area as possible thus minimizing differential contractions. It solidifies last in the feeding heads which therefore contain all the shrinkage cavities. The ‘pouring basin’ helps to equalize the flow into the downgate and, it is hoped, traps slag or dross.

The ‘sprue’ or ‘downgate’ is a component of the runner system necessary to connect the pouring basin (which must be at the top of the cope) to the runner bar (which must usually be at the level of the parting line). It may be tapered from the top to the bottom to allow for the effect of gravity increasing the rate of flow downwards. Pouring basin and sprue are milled into the mould after moulding is completed.
The 'runner bar' is usually moulded into the face of the cope and the drag, and should extend almost all the way round the casting. It is both larger in section than the sprue (to promote a smooth flow) and the total section of the ingates (to ensure as far as possible that metal flows through them all at the same time). Filters, weirs and spinners are provided for preference between the sprue and the start of the runner bar to trap slag and dross. The correct size and position of the ingates is vital to the success of the casting and may vary according to the metal cast. A recommended design for magnesium is shown in Figure 16.130⁹⁴. This metal has a low heat capacity and solidifies quickly. For this reason, the rule that the cross-section of the ingates is less than that of the runner does not hold for this metal. Filling must be rapid enough to ensure that the casting fills while the metal is molten without resorting to unreasonably high temperature.

Ideally, for all materials (with the exception of a few alloys, which include tin bronze) the aim is to get the whole of the casting to fill with metal while it is liquid. It will then solidify progressively from the thin sections remote from the feeders and the metal will remain liquid last in the feeder heads at the top.
This can be achieved best by placing the ingates into or next to the base of the feeder heads (as in Figure 16.129). If a flange has been placed in between a feeder and an intermediate section its roots should be chamfered. Otherwise a well of liquid metal heated from the other side of the flange will persist until the metal round it has solidified to form a shrinkage cavity. Feeder heads should be larger than the sections they have to feed, and if they are internal to the mould they should be equipped with exothermic devices to keep them hot.
The majority of castings are poured from small solidification range alloys which tend to form skins of solid metal and shrinkage cavities which have to be 'chased' into the feeders. Those alloys which have large solidification ranges form dispersed porosity. This must be overcome by modifying the temperature gradient by control of the casting temperature, by gating and by the use of chills to produce directional solidification.
However, with some alloys (including tin bronzes) the skin of the casting solidifies with such a steep temperature gradient that it is free from shrinkage, and the microporosity goes to the centre of the section and cannot be chased into a feeder. In this case the foundry worker avoids directional solidification which might concentrate porosity in one part of the casting and encourages an even rate of solidification throughout the casting. In all other cases the design engineer should do his or her best to assist the foundry worker to achieve directional solidification. If possible, section thickness should increase smoothly to some point (probably a flange) which is conveniently fed. Thick sections, isolated in the centre of thinner sections, will cause difficulties. Sharp corners and re-entrant angles will cause shrinkage, puncturing and cracks, and this is accentuated by variation in section.
The way in which various types of junction give rise to increases in mass is illustrated in Figure 16.131,⁹⁵ and the way in which they can be lightened and made more flexible by staggering is shown in Figure 16.132.⁹⁵ The development of the type of filter shown in Figure 16.106 has raised the possibility that the entire gating system may be dispensed with. It is claimed that if a ceramic foam system is placed in a convenient riser as close as possible to the mould cavity, gravity, die and sand castings in aluminium, magnesium and some copper alloys may be direct poured to produce higher-quality casting and save substantial quantities of metal.Figure 16.133 compares an aluminium alloy manifold poured through a conventional running system and through a ‘Dypur’ filter unit.